Catena 170 (2018) 108–118

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Catena

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Alpine catena response to nitrogen deposition and its eﬀect on the aquatic system T ⁎ M. Iggy Litaora, , K. Sudingb,c, S.P. Andersonb,d, G. Lituse, N. Caineb,d a MIGAL – Galilee Research Institute and Tel Hai College, 1220800, Israel b Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, USA c Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA d Department of Geography, University of Colorado, Boulder, CO, 80309-0260, USA e Western Colorado Research Center, Colorado State University, Grand Junction, CO 81503, USA

ARTICLE INFO ABSTRACT

Keywords: Alpine areas are vulnerable to nitrogen (N) deposition because of low N-buffering capacity and limited ability to Nitrogen (N) deposition resist change. The objective of the study was to assess if > 30 years of N deposition have resulted in a decline in Catena exchangeable base cations (CB) coupled with an increase of exchangeable aluminum (Al). We used soil and Exchangeable base cations (CB) stream data sampled between 1982 and 2015 at Green Lakes Valley, Colorado Front Range to evaluate the Exchangeable aluminum (Al) change in an alpine catena. The CB in the surface horizons of the summit position decreased significantly from −1 −1 −1 45 cmolc kg in 1982 to 2.3 cmolc kg in 2015, while the exchangeable Al increased from 0.09 cmolc kg to −1 −1 −1 0.7 cmolc kg in the summit and from 8.9 cmolc kg to 10.5 cmolc kg in the toeslope. Spatiotemporal distribution of soil moisture along the catena exhibited the lowest values during the winter months because the temperature was below freezing. The soil moisture increased in early spring as soil-temperature rose. A Seasonal − − Kendall test (SK) showed that the soil moisture decreased along the catena by −0.007 m3 m 3 yr 1(P < 0.001). − The soil moisture trend coincided with a soil temperature increase from the summit to toeslope of 0.68° yr 1. Segmental SK analysis of acid-neutralizing capacity (ANC) measured in the outlet of the lake below the catena − − showed a decrease of −2.15 μeq L 1 yr 1 for the first monitoring period from 1982 to 1995, and an increase by − − 0.51 μeq L 1 yr 1 during the second monitoring period from 1995 to 2014. These trend analyses attest to the limited influence of the alpine soil system on the overall aquatic chemistry. Hence, a clear distinction should be made between the alpine soils and the terrestrial system (e.g., rock glacier, taluses, and screes). Most of the soils have little impact on the aquatic system, whereas other terrestrial features are more important in this regard.

1. Introduction such as the Southern Alps, high N wet deposition was observed with − − respect to critical loads (60–70 meq m 2 y 1), computed as the sum of Acid deposition of reactive N has affected a variety of environments ammonium and nitrate (Rogora et al., 2016). N deposition has also been globally, in particular pristine mountain areas (Burns et al., 2016; found in Asian alpine terrains such as the Qinghai-Tibetan Plateau Pannatier et al., 2011; Rice and Herman, 2012; Williams et al., 2015). studied by Zhao et al. (2017) and the base of Mt. Everest, which is Monitoring programs and experimental studies indicated that in re- exposed to N deposition from fossil fuel combustion (Balestrini et al., sponse to N deposition, many complex biogeochemical processes have 2016). caused shifts in chemical species, soil base-cation gains or losses, and The Long-Term Ecological Research site in Niwot Ridge (NWT changes in plant biodiversity (Bowman et al., 2014). Other studies LTER), Colorado Front Range provides an excellent opportunity to as- suggested that remote regions in western North America are exhibiting sess the impact of N deposition over time, since long-term compre- symptoms of ecological sensitivity because of N deposition (Fenn et al., hensive data sets have been collected there for over 35 years. Data 2003; Lieb et al., 2011). In particular, alpine zones are vulnerable to N collected by the National Atmospheric Deposition Program (NADP) deposition because of shallow soil cover with low N buffering capacity, from the Saddle grid, Niwot Ridge, Colorado (Fig. 1) suggest that N − + −1 causing insufficient biotic sequestration of N deposition. This, in turn, loading (NO3 +NH4 ) increased from 6.4 kg N ha in 1984 to a − results in acidification and soil base-cation loss. In other alpine areas, peak of 31.5 kg N ha 1 in 2000, followed by a steady decrease to

⁎ Corresponding author. E-mail address: [email protected] (M. Iggy Litaor). https://doi.org/10.1016/j.catena.2018.06.004 Received 19 October 2017; Received in revised form 27 April 2018; Accepted 4 June 2018 Available online 09 June 2018 0341-8162/ © 2018 Elsevier B.V. All rights reserved. M. Iggy Litaor et al. Catena 170 (2018) 108–118

Fig. 1. Green Lakes Valley and Niwot Ridge LTER site. The two study sites are the catena adjacent to Green Lake 4 and the area known as the Saddle grid. The D1 meteorological station is located on the ridge above Green Lake 4 at 3743 m. Climate and weather data have been collected since 1964.

− 9.5 kg ha 1 in 2016 (http://nadp.sws.uiuc.edu/NADP/). The N loading on Niwot Ridge at elevation of 3500 m is significantly higher than another alpine NADP site (3112 m) located approximately 35 km away in Loch Vale, Rocky Mountain National Park (Fig. 2). The origin of these N − species is mostly summer upslope rain events carrying NO3 from the + urban corridor between Denver and Fort Collins and NH4 from agri- cultural activities on the plains east of the Front Range (Burns, 2003). The difference between the two sites is probably the proximity of Niwot Ridge to the Denver metropolitan area. Under such loading rates, the pristine alpine ecosystems of the Colorado Front Range have already shown evidence of N saturation, which has led to a change from non- exporting to exporting N (Williams et al., 1996; Williams and Tonnessen, 2000). On the other hand, a study by Mast et al. (2014) suggested that stream nitrate concentrations in Loch Vale just north of Niwot Ridge have declined by over 40% since the mid-2000s in response to a decrease in N emissions. A conceptual model developed by Bowman et al. (2014) suggests that elevated N deposition in an alpine ecosystem will change biotic composition and chemistry, which, in turn, will increase rates of N − − cycling and expand soil NO3 pools. The expansion of NO3 pools should decrease exchangeable base cation pools, increase exchangeable Al and soil acidity, and decrease soil pH and buffering capacity. The latter two processes would then cause a decrease in the net primary production rate of the alpine ecosystem. From this conceptual model, Bowman et al. (2014) developed a prediction for a worst-case scenario for alpine ecosystem change under N deposition, increasing from a − + −1 −1 −1 −1 Fig. 2. Total N deposition (NO3 +NH4 )kgha yr in Niwot Ridge and background rate of 0.2 kg N ha yr to a high loading of − − Rocky Mountain National Park. Data was extracted from the NADP-NTN ar- 40 kg N ha 1 yr 1. Their model suggests that significant leaching of chive. − NO3 will occur from an alpine ecosystem experiencing a deposition − − rate of 10–15 kg N ha 1 yr 1, and if N deposition rate were to reach

109 M. Iggy Litaor et al. Catena 170 (2018) 108–118

− − 28 kg N ha 1 yr 1, the alpine soil system will undergo major acid- The alpine catena study site slopes west towards Green Lake 4 on a ification coupled with an increase in aluminum toxicity. It should be till-mantled section of the valley floor and is 125 m long by 15 to 25 m noted, however, that in addition to anthropogenic loading, the ob- wide. The thickness of the till over the bedrock has not been measured served increase of stream nitrate concentrations in some alpine catch- but rough field estimation suggests 2 to 3 m. The GLV was deglaciated ments is also attributed to climate change impact on N transport and between 18 and 12 ka (Dühnforth and Anderson, 2011); hence the ca- internal changes in catchment hydrology (Baron et al., 2013). tena soils have begun developing sometimes after 12 ka. The Green Lake Many studies were conducted to ascertain the impact of N deposi- 4 (GL4) catchment extends to the Continental Divide at an elevation tion on the alpine ecosystem of the Colorado Front Range, in particular slightly above 4000 m and drains an area of 2.25 km2 above an eleva- in the NWT LTER site. For example, the impact of N deposition on tion of 3515 m (Fig. 1). The watershed is typical of the alpine en- vegetation composition, diversity, and productivity was clearly identi- vironment of the Colorado Front Range with long, cool winters and a fied by Bowman et al. (2006, 2012), Seastedt and Vaccaro (2001), and short growing season of one to three months. The soils along the catena Suding et al. (2008) among others. Alpine soil microbial community are classified as Typic Cryumbrepts (Litaor, 1987a). To further assess structure, function, and nutrient cycling were evaluated by Nemergut the impact of N deposition on alpine soil chemistry, additional soil et al. (2008), who found marked changes in the assemblage of bacterial samples were collected from an experimental site in an area known as communities and minimal changes in fungi. The composition and the Saddle grid located on Niwot Ridge, which forms the northern abundance of phytoplankton in Green Lake 4 showed a shift from an N- boundary of GLV (Fig. 1). Moreover, since the Saddle has been an ex- deficient system to a P-limited system in alpine lakes receiving N de- tensive experimental site since the 1960s, it provides an important position (Elser et al., 2009; Gardner et al., 2008). This finding is con- addition to temporal data that are scarcer in the valley. The Saddle is sistent with the assertions made by Williams et al. (1996) and Williams located at an elevation of about 3525 m where a research grid of and Tonnessen (2000) that the Green Lakes Valley has transformed 17.5 ha (550 m × 400 m) was established. Soil development across the from an N-limited environment to an N-saturated ecosystem. Somewhat experimental grid is affected by the edaphic-topographic-snow cover similar results were obtained for alpine lakes in the Canadian Rocky relationship. The most important site characteristic in this relationship Mountains, where almost 75% of the lakes have become P-limited due is the number of snow-free days per year. Soil pedogenesis was ex- to the addition of atmospheric N deposition (Murphy et al., 2010). Fi- plained by six snow-cover sites: 1) extremely windblown sites are nally, Bowman et al. (2006, 2012) and Lieb et al. (2011) suggested that characterized by > 300 snow-free days; 2) wind-blown sites by alpine soil acidification triggered by enhanced nitrate leaching would 225–300 days; 3) minimal snow cover sites by 150–200 days; 4) early eventually cause a decrease in the ecosystem buffering capacity. melting sites by 100–150 days; 5) late-melting sites by 50–100 days; Although the predictive model of Bowman et al. (2014) suggests an and 6) wet meadow sites by ~100 days. Soil classification varied from increase in exchangeable Al pools coupled with a decrease in ex- Dystric Cryochrept in the wind blown site, Pergelic Cryumbrept in the changeable base cations and soil buffering capacity, to date there has minimal snow cover site, Typic Cryumbrept in the early snow melting been no confirmation of this prediction from direct evaluation of these site, Dystric Cryochrept in the late melting snow site, and various pools over time. Since future impact of N deposition on the alpine Cryaquepts and Cryaquolls in the wet meadow sites (Burns and Tonkin, aquatic system is highly dependent on the ability of the terrestrial/soil 1982). system to sequester and accumulate N as a means to minimize alpine The vegetation cover along the catena consists of 75% graminoids lake change (Miller and McKnight, 2015), it is imperative to evaluate and 25% forbs in the summit position, 50% graminoids and 50% forbs the hydro-geochemical linkage between the alpine soil system and in the backslope position, and < 10% graminoids and 90% forbs in the nearby lakes. In this context, we hypothesized that under the current N toeslope position. The dominant species along the catena are Carex deposition rate, a decline in exchangeable base cations coupled with an elynoides, Trifolium parryii, Geum rossii ssp., Bistorta vivipara, increase in exchangeable Al should be noticeable in the soils and in the Deschampsia caespitosa, and Bistorta bistortoides. Several species increase chemistry of the aquatic system. The objectives of this study were: 1) to significantly down slope (e.g., Castilleja occidentalis and Geum rossii). test whether a change in the chemistry of alpine soil catena during the The toeslope position is characterized by higher abundance of species last three decades has transpired and 2) to assess the impact of the including Lloydia serotina, Artemisia scopulorum, Castilleja occidentalis, alpine soil catena on an adjacent lake using trend analysis of selected Minuartia obtusiloba, Silene acaulis, and woody plant such as Salix glauca physical and chemical attributes. (Litaor, 1992). The vegetation cover and detailed species composition survey of the Saddle site was conducted by May and Webber (1982).In 2. Methods the context of the current work, suffice it to state that the dominant plant species in the dry, moist, and wet meadows are the graminoid 2.1. Study sites Kobresia myosuroides, the forb Acomastylis rossii, and the graminoid Deschampsia caespitosa, respectively. The main study site is an alpine geochemical catena located in the Green Lakes Valley (GLV) watershed, which is 35 km west of Boulder, 2.2. Field and laboratory methods Colorado, on the eastern side of the Continental Divide (Fig. 1 and Supplemental information). The watershed has a drainage area of The study site was divided into three slope positions: summit at an 5.46 km2 at the spillway of Lake Albion of which 0.4 km2 consists of six elevation of 3550 m, backslope at 3525 m, and toeslope at 3500 m. The permanent lakes. These lakes are part of the water supply system for the slope position terminology relates to geomorphic and pedologic pro- city of Boulder. The watershed is characterized by steep slopes, glacial cesses and follows Birkeland (1999). Summit positions normally exhibit cirque, semi-permanent snowfields, exposed bedrock, and talus out- greater evaporation, backslope positions may show signs of erosion and crops. The bedrock is composed of Precambrian schists and gneisses, the toeslope is the depositional zone. Originally, two soil pits were Silver Plume quartz monzonite and Audubon–Albion stock (Gable and sampled at each position to investigate the physicochemical char- Madole, 1976). At the northern side of the valley is Niwot Ridge, an acteristics of this alpine soil catena (Litaor, 1987a, 1987b, 1992). The NADP site where the Saddle grid is located and environmental research length of the catena is approximately 125 m and the sampling locations has been conducted since the early 1950s (Fig. 1). The continental, are about 40 m apart. Soils were collected during the summer after high-mountain climate of GLV has been recorded continuously since the complete snow melt in the three slope positions according to diagnostic 1950s at the alpine D1 (3700 m) and subalpine C1 (3021 m) meteor- horizons (Oe, 2Bw, and Ab) in 1982, 2008, and 2015. Additional ological stations on Niwot Ridge, logging a mean annual temperature of sampling was conducted in 1988 to assess the chemistry of the soil −4.1 °C at D1 (Greenland and Losleben, 2001). interstitial water, thus the soil analyses were limited to soil-pH only.

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The last soil sampling campaign in 2015 was limited to only surface order to collect freely flowing interstitial waters and to evaluate the Al horizons (Oe and Ab), because time-domain reflectometry (TDR) probes speciation and flux across the catena (Litaor, 1987a, 1988, 1992). The and temperature sensors were installed at 15 cm depth in these soil pits zero-tension samplers were constructed of halved 400-mm long sections in 2008 and deeper sampling collection would have disturbed in-situ of PVC pipe 250 mm in diameter, capped on one end, and plumbed to long-term monitoring. In all three sampling campaigns, soil samples drain into a 1-L storage bottle connected to the surface with Tygon® from each diagnostic horizon were collected in triplicate across the tubing. Soil interstitial waters were collected again from 1996 to 1999 entire pit face to minimize the inherent local spatial soil variability. by Liu et al. (2004) and analyzed for major cations, anions, and stable This enabled changes in a given soil characteristic to be attributed with isotopes. The collection started when the catena became snow free and more confidence to temporal change. The soil samples were inserted continued at weekly to biweekly intervals; however, by the end of snow into zip lock bags and air dried in the laboratory on the same day of melt, the zero-tension soil samplers rarely contained water except after sampling. Next, the soil samples were gently crushed and sieved in major rain events (Liu et al., 2004). 2 mm mesh. Since soil digging in the alpine catena is highly destructive and the recovery of the alpine turf is extremely slow, we limited sub- 2.4. Stream water sequent soil sampling campaigns to the original two soil pits at each catena location for a total of six pits. Stream samples during the summer (June–October) were taken Additional soil samples were collected from the experimental grid weekly from 1982 to present from the outlet of Green Lake 4, and located at the Saddle site on Niwot Ridge in 1986, 1999, 2001, 2003, monthly to bimonthly from the lake beneath the ice cover during the 2008, and 2015. These additional soil sampling sites allowed us to winter months. Stream samples for inorganic constituent determina- evaluate the impact of N deposition on several locations across the al- tions were collected in acid-washed polypropylene 125 ml bottles. pine ecosystem, while doubling the sampling dates from the mid-1980s Stream pH was measured in the field and the stream samples were kept to 2015 and minimizing the disturbance across the sensitive alpine refrigerated until analysis was done in the laboratory, usually within catena. In 1986, three surface soil samples from each of the six snow 5 days of sample collection. Samples were not filtered prior to analysis, controlled sites described above, for a total of 18 samples, were col- because suspended material concentrations in these samples are low lected to establish general soil characteristics for the Saddle grid site. (Caine and Thurman, 1990). The acid neutralizing capacity (ANC) was The grid location and other pertinent field information of the Saddle calculated by summation of the base cations (Ca2+,Mg2+,K+, and + − − 2− site are given in Litaor et al. (2002). Between 1999 and 2008, 110 Na ) minus the sum of the anions (NO3 ,Cl and SO4 ) in equiva- surface soil samples were collected with a hand held spade from the dry lent units. The cations were determined by atomic absorption spectro- (minimal snow cover), moist (early melting) and wet (late melting) photometry, while anions were estimated by ion chromatography using meadows located in the Saddle site. The meadow locations, type of triplicates of each sample. A description of the sampling handling, in- vegetation, and other pertinent field information are described in Litaor strumentation, and relevant quality assurance and quality control in the et al. (2005). In 2015, 21surface soil samples from the dry, moist, and first 7 years of sampling is detailed by Caine and Thurman (1990). All wet meadows and 73 surface soil samples from the Saddle grid were later-date sampling procedures, instrumentation, modifications, and resampled. All surface samples representing the Oe horizon (0 to 15 cm) changes are recorded on the Niwot Ridge data website and in Caine were inserted into zip lock bags and handled as described above. (2010). Stream water data were obtained from the Niwot Ridge data- Detailed particle-size distributions of the soil samples collected in base (http://niwot.colorado.edu/data). Volume-weighted mean con- 2015 were determined using a Malvern Mastersizer 3000. centrations for individual species were calculated as annual mass flux Exchangeable cations in all three soil campaigns were determined using divided by annual discharge. the widely common ammonium acetate method (Hendershoot et al., 2008, after Lavkulich, 1981). The exchangeable Al and acidity in soils 2.5. Statistical methods collected in 1982 and 2015 were determined using 1 M KCl as the displacing salt solution (Thomas, 1982). Due to logistical considera- Trend analysis was performed on soil moisture and soil temperature tions, the exchangeable Al in the soils collected in 2008 were analyzed data along the catena and on selected chemical attributes measured in in the CSU soil testing laboratory using ammonium acetate as the dis- water samples collected from the outlet of Green Lake 4. Chemical at- placing cation. The pH of the samples collected in 2008 was also de- tributes with a high number of missing values were excluded from the + termined by the CSU soil testing laboratory using a 1:10 soil to water analysis (e.g., NH4 ). The purpose of the trend analysis was to de- ratio, while soil pH of the surface horizons sampled in 2015 was termine if there is a significant trend in the long-term data set, as well as measured in saturated paste (see below for detailed discussion). its magnitude, direction, and confidence bounds. The trend analysis can also be run with a covariate (e.g., precipitation and stream discharge) to 2.3. Soil moisture, temperature, and interstitial waters assess the influence of the covariate on the overall trend of soil moisture, temperature, and chemical attribute under consideration. Campbell Scientific volumetric water content probes (CS 616) and There are a number of procedures to evaluate trends in long-term temperature sensors (T107) were installed in 2008 in 5 soil pits along environmental data. Time series analysis is most commonly used to the Green Lake 4 alpine catena. The probes were installed as part of the build predictive models, whereas the Mann-Kendall test (MK), the Boulder Creek Critical Zone Observatory (CZO) project (Anderson et al., seasonal Kendall test (SK) developed by Hirsch et al. (1982), and the 2008; Anderson and Rock, 2015). The probes were installed at 12 to Kendall-Theil Robust line (KTRL) written by Granato (2006) have been 15 cm depth in the summit (1 pit), backslope (2 pits), and toeslope (2 used extensively to assess trends in time series with multiple missing pits) positions. The location and number of probes installed across the data, values below detection limits, and strong seasonality (e.g., Burns, catena were dictated by logistical considerations, stoniness nature of 2003; Nilles and Conley, 2001; Prestbo and Gay, 2009; Waller et al., the summit position, and a belated decision made by the LTER scientists 2012). In this study, we employed the ‘rkt’ package installed in R by to prevent further disturbance to this site by additional soil sampling. Marchetto et al. (2013). This package contains functions that compute The data have been collected continuously from 2009 to present at a the Mann-Kendall test (MK), the seasonal Kendall test (SK), the regional 10 min frequency, and can be retrieved from the Boulder Creek CZO Kendall test (RK), and the Theil-Sen's slope estimator (Sen, 1968). database (http://czo.colorado.edu/query/slmotsen_glv_cat.shtml# These functions use time as the independent variable to determine the description). trend regardless of whether the trend is linear or if the time series is In each of the soil pits originally dug and described in 1981, two normally distributed. The MK and SK are tests for monotonic (single zero-tension soil samplers were installed in the Oe and 2Bw horizons in direction) trend in a time series based on the Kendall rank correlation.

111 M. Iggy Litaor et al. Catena 170 (2018) 108–118

−1 −1 The SK is an intra-block test in which test statistics are computed for a 45 cmolc kg in the 1982 soil sampling campaign to 2.3 cmolc kg in period of choice. When the data showed more than one possible trend, the 2015 soil sampling campaign. Significant decrease in CB temporal then the Kendall-Theil Robust line (KTRL) was used (Granato, 2006)to pattern was also observed in the surface horizons of the backslope −1 −1 evaluate the slope and the significance of each segment of the time (29 cmolc kg ) and toeslope (7 cmolc kg ) positions measured in −1 −1 series. 1982, to only 2.3 cmolc kg and 0.8 cmolc kg in these two slope The seasonal Kendall test accounts for periodicity by computing the positions, respectively, in 2015. The large difference in CB concentra- Mann-Kendall test separately for each of the selected periods (e.g., tions between surface and subsurface horizons in the study area was weeks, months, or seasons), and then combining the results. No com- explained by eolian deposition of minerals from Western Colorado parisons are conducted across period boundaries. Hence, the seasonal providing base cations to alpine soils (Litaor, 1987a; Muhs and

Kendall statistic (SK) is the sum of individual S statistics for a selected Benedict, 2006; Neﬀ et al., 2008). On the other hand, most of the CB in period. The SK is standardized and a continuity correction is added so the subsurface horizons originated from in-situ weathering of plagio- that the ZSk statistics can be evaluated from a standard normal dis- clase, biotite, and pyroxene minerals (Litaor, 1987b). Lieb et al. (2011) tribution (Helsel and Frans, 2006). asserted that dust deposition from Western Colorado has declined in the Other experimental and monitoring results were evaluated using last several decades, but more recently Clow et al. (2016) found that SPSS version 21. Comparisons of diﬀerences of various chemical attri- alkaline dust deposition increased by 81% in the Southern Rockies from butes across the slope positions at time of sampling and at other soil 1993 to 2014. Hence, the level of CB in the surface horizons of the locations were conducted using the univariate ANOVA routine in con- alpine catena most likely resulted from two opposing processes: N de- junction with Tukey post-hoc analysis. Data transformations were position from the urban corridor east of the Colorado Front Range re- conducted to improve the normality of the distribution and reduce its duced the level of CB, mostly during the 1980s and 1990s, while al- variance. kaline dust from the Colorado plateau contributing calcium to the soil

mitigated somewhat the CB reduction. Changes in CB in response to 3. Results and discussion decrease in acidic emissions is a slow process as was shown by Likens et al. (1996) for Ca2+ in stemwood in Hubbard Brook Experimental 3.1. Catena characterization Forest. Hence, a lag in the soil response to the observed decrease in N deposition is to be expected.

The particle size distribution (PSD) in samples collected in 2015 The spatiotemporal pattern of CB concentrations decreased sig- along the GL4 alpine catena showed that the average clay fraction was nificantly over time and across the geochemical catena (F = 44.3, rather small (2.3%) compared with the silt (44%) and the fine sand P < 0.001, F = 17.7, P < 0.001, respectively). A Tukey post hoc test (41%) fractions. In the summit and backslope positions, there was a showed that CB in the mid-1980s was significantly higher than in later fi small increase with depth in clay but mostly in fine silt content. Silt sampling dates (P < 0.001) and that the CB in 2015 was signi cantly translocation is typical of coarse-grained soils in alpine and arctic en- lower than in the mid-2000s (P < 0.001). A Tukey post hoc test also fi vironments. Frost-heave sorting cycles, silt movement during snowmelt, showed that CB in the summit and backslope positions was signi cantly and eolian deposition (Muhs and Benedict, 2006) are possible me- higher than in the toeslope position (P < 0.001), but no significant chanisms leading to this cold-climate soil feature (Birkeland, 1999). difference in CB concentrations was observed between the summit and Soil developed in the toeslope exhibited a limited silt increase with backslope positions. A simulated N deposition experiment on Niwot depth, most likely due to gelifluction (Litaor, 1992). The detailed PSD Ridge soils showed that Ca and Mg concentrations in ambient plots − − and mineralogical attributes across the catena are given in the Sup- decreased by 20 to 30% under a 20 kg N ha 1 yr 1 N addition to retain plementary information. electrical neutrality (Lieb et al., 2011). These simulated outcomes are more modest than the monitored results presented in Table 1; however,

3.2. Sum of base cations (CB), aluminum and acidity pattern along the the current work represents over 30 years of continuous N deposition catena with a peak loading of over 26 kg N ha (Fig. 2). Indeed, Lieb et al. (2011) acknowledged that their short-term experimental additions may The alpine geochemical catena is a well-defined hydrological site underestimate the deposition threshold in which acidification could because the shallow soils are laid on bedrock, which serves as a semi- occur. impermeable barrier. Thus, water and material flow along the catena is The exchangeable Al concentrations in the surface horizons of the −1 mostly towards the lake, with little water loss other than evapo- summit position in the alpine catena increased from 0.09 cmolc kg in −1 fi transpiration. The sum of base cations (CB) and exchangeable Al pat- 1982, to 0.7 cmolc kg in 2015. An even more signi cant increase in terns over time of sampling and along the geochemical catena are exchangeable Al concentrations was observed in the surface horizons of summarized in Table 1. The decrease in CB in the surface horizons of the the backslope and toeslope in 2015 compared with 1982 (Table 1). The summit position in the alpine catena are rather remarkable, from results obtained in 2008 were lower than expected; however, the

Table 1

Mean and standard error values of CB and exchangeable Al in the alpine soils across time and space of sampling. The mean and SE were calculated from triplicate samples collected from surface (Oe and A) horizons and subsurface horizons (2Bw) at the diﬀerent locations across the alpine catena in 1982, 2008 and 2015.

Year CB cmolc/kg Al cmolc/kg

1982 2008 2015 1982 2008 2015

Surface Horizons Summit 45 ± 2.3 15.2 ± 2.2 2.3 ± 1.6 0.09 ± 2.4 0.15 ± 2.0 0.7 ± 1.7 Backslope 29 ± 2.2 21.9 ± 2.1 2.3 ± 1.4 1.3 ± 2.6 0.1 ± 1.9 4.2 ± 1.8 Toeslope 7 ± 2.1 9.9 ± 1.1 0.8 ± 1.3 8.9 ± 2.1 1.4 ± 1.2 10.5 ± 1.4 Subsurface Horizons Summit 9.5 ± 2.2 4.6 ± 2.2 NDa 1.8 ± 2.3 2.1 ± 2.5 ND Backslope 6.6 ± 2.1 6.9 ± 2.1 ND 5.1 ± 2.5 0.7 ± 2.4 ND Toeslope 4.6 ± 2.0 2.9 ± 1.1 ND 8.1 ± 3.1 2.3 ± 1.1 ND

a ND – not determined (see Methods for explanation).

112 M. Iggy Litaor et al. Catena 170 (2018) 108–118 apparent decrease in exchangeable Al in the backslope and toeslope is quite comparable to the toeslope position on the GL4 catena in terms positions may have been the result of the displacing salt used (NH4OAc) of snow cover, plant species, and exchangeable Al concentrations. and the neutral pH of this procedure. Simulated N deposition experi- The pH of the surface horizons along the catena decreased sig- ments on alpine dry meadow soils collected at Niwot Ridge suggested nificantly from summit to toeslope position (F = 43.0, P < 0.001) in that the threshold at which acidification occurred was all the years of sampling (Table 3). No common effect was noted be- − − 28 kg N ha 1 yr 1 (Lieb et al., 2011). However, at this threshold they tween the time and location of sampling along the catena. The ANOVA also observed a decrease in extractable Al3+ compared to their control test showed that the pH measured in soils collected in 2015 was sig- site, whereas a significant increase in extractable Al was observed only nificantly lower than at the previous sampling times, while no sig- − − above 40 kg N ha 1 yr 1. Hence, short-term simulation experiments nificant pH change was observed between the 1980s and 2008. This provide a general conceptual model of N additions versus loss of soil lack of significant change is explained by the method of measurement. buffering capacity, but they may not necessarily simulate accurately The pH in soils collected in 2008 was measured in the soil laboratory of continuous, long-term (> 30 yr) N deposition at variable rate (Fig. 2). Colorado State University where the protocol called for a fixed water to The overall exchangeable Al spatiotemporal pattern exhibited an soil ratio. The pH of the soils collected at other times was measured increased trend over time and location, albeit not statistically sig- using the saturated paste method. The latter method takes into account nificant at the commonly upper level (F = 3.1, P < 0.07), but with a the high organic matter content of the soils and their hydrophobic highly significant difference between the summit and the toeslope nature (Litaor, 1992). The high acidity of alpine soils (Table 3) and the (F = 12.4, P < 0.001). A Tukey post hoc test showed that exchange- significant increase in acidity along the catena (F = 9.7, P < 0.001) able Al was significantly lower in the mid-1980s than at later sampling support the interpretation that these soils exhibit higher proton level in dates (P < 0.001). No significant change in exchangeable Al con- their humic substances as was demonstrated by Litaor and Thurman centrations was observed between the summit and backslope positions, (1988). Hence, pH determination in saturated paste better reflects the but both sites were significantly lower than the toeslope position intensity status of acidity in these soils than the common agronomic pH (P < 0.001). Field observations confirmed that the toeslope exhibits test of a fixed soil to water ratio. Moreover, the pH of soil interstitial higher snow accumulation, followed by more significant snowmelt and waters collected from 1983 to 1987 varied around 5.78 (Litaor, 1988), higher soil moisture content (see below). Lieb et al. (2011) suggested which is in agreement with the pH determination using the fixed water that sites with higher snow accumulation in Niwot Ridge get up to to soil ratio but does not necessarily reflect the pools of acidity stored in − 15 kg N ha 1 during snow thaw. The higher N input may increase the the exchangeable sites. The significant increase in acidity along this potential of acidification by loss of buffering capacity and increase of alpine catena, coupled with the significant decrease in CB and the ap- exchangeable Al concentrations in the toeslope position. These findings parent increase in exchangeable Al, especially in the toeslope position, support our initial premise that under the current ambient N deposition raised the possibility of adverse effects on the water quality of Green the change in CB would be noticeable but the temporal change in ex- Lake 4. The increased leaching of aluminum and acidity from the soil to changeable Al would be somewhat less conclusive. the aquatic system may enhance the potential of aluminum toxicity and

The temporal trend of CB and exchangeable Al and acidity was also pH reduction of the lake water resulting in adverse effects upon aquatic examined in the surface soil horizons of the Saddle site on Niwot Ridge life (Baldigo et al., 2016). (Fig. 1), with the assumption that the N deposition would not differ significantly between the ridge and the valley (Williams and Tonnessen, 3.3. Hydrological connectivity along the alpine catena

2000). The CB decreased signiﬁcantly (F = 30.1, P < 0.001) from a −1 −1 mean of 17.6 cmolc kg in 1986 to a mean of 7.0 cmolc kg in 2015 The increase in exchangeable Al and acidity over time and along the (Table 2). A Tukey post hoc test showed that the CB in the mid-1980s catena is only one factor in the ability of the terrestrial/soil system to was signiﬁcantly higher than in later sampling dates (mid-2000s and induce change to alpine lake biogeochemistry. The physicochemical

2015; P < 0.001), while the CB in 2015 was significantly lower than in attributes along the catena attest to some movement downslope, how- the mid-2000s (P < 0.001). Exchangeable Al and total acidity in the ever, the question is whether we can also demonstrate the viability of mid-1980s were significantly lower than subsequent years (F = 3.6, hydrological connectivity between the catena and the lake. To test the P < 0.01 and F = 8.7, P < 0.001, respectively), however a Tukey post possibility of Al transport from the catena to Green Lake 4 we analyzed hoc test showed that exchangeable Al and total acidity did not change the TDR data along the catena (Fig. 3). The soil moisture fluctuations significantly between the mid-2000s and 2015. The reason for this throughout the six years of monitoring exhibited a very clear pattern apparent inconsistency in exchangeable Al concentrations over time reflecting soil temperature change with season. When the soil was (i.e., mid 2000s to 2015) is the significant differences across the Saddle frozen, the soil moisture was extremely low. As the soil temperature grid between dry, moist, and wet meadows (0.85 ± 0.1, 1.2 ± 0.2, rose, snow cover and the ice within the soil began to melt and the and 1.8 ± 0.2 cmolc, respectively), which hinder somewhat the tem- volumetric soil moisture increased. The thawing of soil ice coincided poral changes. Partial support for this assertion was given by Freppaz with snow melt especially in the backslope and toeslope locations, et al. (2012), who evaluated the response of alpine soils in the Saddle whereas the summit, a wind-blown site, had a low snow depth and a grid to 16 years of N-fertilization additions. They found that the highest short duration of snow cover. The recession of soil moisture was also exchangeable Al concentrations and the lowest CB were in N-addition rather steep, suggesting very quick drying of the soil after ice thaw and plots of moderate snow cover (100 cm). Their moderate snow cover site snow melt. The effect of summer rain events was also quite evident in the TDR record, which shows a fast increase followed by a relatively Table 2 quick decrease of the soil moisture content (Fig. 3). The huge number of Means and standard errors of exchangeable ions in surface horizons of soils TDR and temperature measurements (> 290,000) has masked more collected in the Saddle site. subtle seasonal trends in the data sets such as those depicted in Fig. 3 (produced from over 2000 daily averages). Hence, for better under- C Al Acidity n B standing of the soil moisture and temperature distribution pattern, we −1 cmolc kg focused on the measurements taken in 2012 because of relatively fewer missing values compared with other years of monitoring (Fig. 4). 1986 17.6 ± 3.7 1.5 ± 0.23 1.8 ± 0.7 18 The 2012 measurements reveal that the soil moisture content in- 1999–2008a 9.2 ± 0.3 1.3 ± 0.13 4.5 ± 0.4 110 2015 7.0 ± 0.4 1.8 ± 0.15 3.2 ± 0.4 98 creased in early spring in response to a temperature rise. Once the soil temperature rose above freezing, the soil moisture content climbed − a Soil campaigns were conducted in 1999, 2001, 2003 and 2008. from relatively dry conditions of 0.06 m3 m 3 to wet conditions of

113 M. Iggy Litaor et al. Catena 170 (2018) 108–118

Table 3 pH and acidity values measured across the alpine catena and over time. The mean and SE were calculated from triplicate samples collected from surface (Oe and A) horizons and subsurface horizons (2Bw) at the diﬀerent locations across the alpine catena in 1982, 1988, 2008 and 2015.

−1 Year pH Acidity cmolc kg

1982 1988a 2008 2015 1982 2008 2015

Surface Horizons Summit 5.2 ± 0.2 5.0 ± 0.2 5.5 ± 0.17 4.7 ± 0.15 ND 1.8 ± 2.0 4.3 ± 1.7 Backslope 4.9 ± 0.2 4.7 ± 0.2 5.5 ± 0.16 4.48 ± 0.1 ND 2.9 ± 1.9 21.5 ± 6.1 Toeslope 4.4 ± 0.19 4.3 ± 0.2 4.4 ± 0.09 3.8 ± 0.1 ND 10.1 ± 1.2 34.4 ± 9.7 Subsurface Horizons Summit 5.0 ± 0.17 4.8 ± 0.1 4.9 ± 0.18 ND ND 3.1 ± 1.5 ND Backslope 4.7 ± 0.16 4.5 ± 0.2 4.7 ± 0.13 ND ND 5.7 ± 2.4 ND Toeslope 4.1 ± 0.15 4.0 ± 0.2 4.2 ± 0.09 ND ND 11.1 ± 4.1 ND

a The data were taken from Litaor (1992).

− 0.35 m3 m 3 in the summit position, much wetter conditions insignificant (P < 0.93). However, it should be noted that the rainfall − − (0.46 m3 m 3) in the backslope, and up to saturation (0.6 m3 m 3)in record at D1 includes numerous days with missing or zero values in the the toeslope in just a few weeks in early April. The wetting conditions modeled period; hence, the real effect of the precipitation on the trend − lasted a few days and began to drop rather rapidly to below 0.1 m3 m 3 is not well established. Next, we ran an SK of soil moisture with catena − in the summit and 0.3 m3 m 3 in the toeslope as the ice stored in the locations as intra-block and found that during the six years of mon- soil was exhausted while the soil temperature continued to rise (Fig. 4). itoring, the soil moisture content decreased along the catena by − − The rapid decline of soil moisture along the entire catena also suggests −0.007 m3 m 3 yr 1 (P < 0.001). This trend coincides with the tem- that the overall volume of interstitial waters flowing through the soil perature trends that increased from the summit to backslope to toeslope − − − system to the lake below is quite limited. By the end of the ice thawing by 0.07° yr 1, 0.16° yr 1 and 0.68° yr 1, respectively. Most of the ob- − period, the soil had dried again to a level below 0.1m3 m 3 in the served warming occurred during late winter months (see Figs. 3 and 4), summit but remained somewhat moister in the backslope and toeslope which triggered a sudden small increase in soil moisture in all catena positions. During the summer months from June to August, the soil locations followed by a temporary decrease of soil moisture before the moisture content was somewhat affected by rainfall (Fig. 5) and occa- onset of complete ice thawing and soil wetting. Although the temporal − − sionally rose to a moist condition (between 0.2 m3 m 3 to 0.45 m3 m 3 series is only six years long, the warming trend of the soil environment depending on the catena location). Since the summer precipitation was coupled with the small but consistent decrease of soil moisture along quite uniform along the catena, the increase in soil moisture from the catena indicate the possibility of overall hydrological changes si- summit to toeslope may indicate the occurrence of throughflow. milar to those detected at an earlier date of spring flow and peak flow However, after each rainfall event a rapid drying began due to evapo- (Caine, 2010). transpiration and no time lag in the drying segments of the soil moisture The large variance in soil moisture during the summer season at all across the catena is evident. Hence, the magnitude of the throughflow, if slope locations (Fig. 6) and especially in the toeslope position attest to it even exists, is very small. The soil moisture content began to drop intermittent drying periods resulting from the intense summer radiation rapidly as the soil temperature fell below freezing in early fall. A similar and high rate of soil water up-take during the short alpine plant pattern was observed in other years of monitoring (not shown for growing season. These processes most likely reduce the magnitude of brevity). water flux along the catena and may have reduced the hydrological Spatiotemporal distribution of the volumetric water content is de- connection between the catena and the aquatic system. Support for this picted in Fig. 6. As stated above, the lowest liquid water content oc- inference is found in zero-tension samplers installed along the catena curred during winter when the temperature was well below freezing. that collected very small quantities of interstitial waters only during The soil water content increased during late spring months as soil- snowmelt and rarely after rain events (Liu et al., 2004). The median temperatures rose. Perhaps the most striking feature of the soil moisture δ18O in these interstitial waters was −12‰ compared with the median distribution pattern is the significant increase of water content in the of −19‰ in snowbank and ice (Williams et al., 2006). There are two toeslope position compared with the upslope positions for all seasons possible fractionation enrichment mechanisms at play here, direct (F = 165, F < 0.001). The greater soil wetness parallels the greater evaporation and re-freezing of interstitial water to ice (Cooper, 2000). exchangeable Al and acidity downslope, supporting the notion that These results explain the high variance in soil moisture content during acidified soil water from the toeslope position may reach the lake the summer, which supports the notion of only intermittent flow from below. However, the large temporal variations of the volumetric soil the catena to the lake below. In this study, we evaluated soil processes moisture in all catena locations and the lack of significant throughflow using dynamic and stationary data sets. The dynamic data of soil may also indicate that the hydrological connection between the catena moisture, soil temperature (Figs. 3 and 4), volume of freely flowing and Green Lake 4 exists only intermittently during the summer, re- interstitial water, and the isotopic data (Liu et al., 2004) suggest rather sulting in lower impact of the interstitial waters on the aquatic system. inconsequential hydrologic connectivity, thus the impact of the alpine The question of the hydrological connectivity and the potential soil system on the lake could be significantly smaller than previously impact of the alpine soil environs on the aquatic system was further suggested (Litaor, 1992), or the impact is somewhat less conclusive due addressed by running an SK test to assess the temporal trends of the soil to the spatiotemporal distribution of exchangeable Al and acidity along moisture and temperature data. Lag-autocorrelation analysis of the data the catena (Tables 2 and 4). exhibited potential serial correlation, thus adjustment was made by employing simulations of seasonally or monthly data as recommended 3.4. Influence of alpine soils on the aquatic system by Hirsch and Slack (1984). These simulations are known as intra-block in the nomenclature of trend analysis, with precipitation measured at The significant reduction in CB along the catena and the concurrent D1 (Fig. 1) serving as a covariate. We found a minimal decreasing trend increase in acidity raised the question of the potential impact of the 3 −3 3 −3 from −0.004 m m in the summit to −0.002 m m in the toeslope, alpine soils on the aquatic system. We hypothesized that the observed and the influence of summer rainfall on the soil moisture trends was temporal changes in the chemistry of alpine soils coupled with

114 M. Iggy Litaor et al. Catena 170 (2018) 108–118

Fig. 3. The temporal distribution of soil moisture (points) and soil temperature (line) at 15 cm depth along the catena. The shoulder in temperature at 0 °C on the spring warming limb reﬂects release of latent heat from melting ice. No shoulder is evident in the fall, suggesting that the ice is snowpack derived. Soil moisture and temperature axis scales across the ﬁgure are varied to better ac- commodate data distribution pattern. Arrows in the backslope position show soil moisture increase following short summer precipitation. Similar soil moisture distribution pattern is evident in the summit and toeslope positions.

Fig. 4. Seasonal variations of soil moisture and temperature along the alpine catena during the 2012 monitoring year. For brevity, the temperature variations are shown only for the summit. Temperature increase commenced at the end of February and intensiﬁed in April 2012, followed by an increase in soil moisture in all locations across the catena.

moderate hydrological connectivity between the soils and the lake would result in temporal decrease of ANC and pH in the outlet of Green Lake 4. However, inspection of over 750 measurements of ANC and pH suggests limited increase of these attributes during the 35 years of monitoring. The results of the SK test for trends support this observation (Table 4). The ANC has increased significantly (P < 0.001) by − − 0.1 μeq L 1 yr 1 when modeling the entire data set of 35 years. Caine (1995) found that the ANC (derived from measured alkalinity) in Green − − Lake 4 declined by 0.0087 μeq L 1 yr 1 from 1982 to 1995. Indeed, an inspection of the data suggests a shift in the ANC trend around the mid- 1990s. Hence, a Kendall-Theil Robust line (KTRL) with segment analysis was conducted to assess the significance of the change in the direction of the trend (Fig. 7). The Kendall-Theil slope exhibits a decrease − − of −2.15 μeq L 1 yr 1 for the first time period (1982 to 1995) with a lower and upper 95th percent confidence interval of −1.82 to − − −2.48 μeq L 1 yr 1. The slope of the second time period from 1995 to − − 2014 was reversed to an increase of 0.51 μeq L 1 yr 1 with a lower and − − upper 95th percent confidence interval of 0.16 to 0.88 μeq L 1 yr 1. These trend analyses attest to the limited influence of the alpine soil system on the overall aquatic chemistry. The ANC trend raised another − question regarding the temporal pattern of NO3 in the lake given the − finding by Mast et al. (2014) that linked the decrease in NO3 concentrations in the outlet of Loch Vale, RMNP to the decrease in nitrogen − oxide emissions. The NO3 in Green Lake 4 has increased significantly (SK = 0.147, P < 0.001) over the 30 years of monitoring (Fig. 8), which suggests that other processes currently poorly understood may also be important and that separate watersheds in the same mountain

115 M. Iggy Litaor et al. Catena 170 (2018) 108–118

Table 4 SKT slopes corrected for seasonal and monthly concentrations of the period 1982 to 2014 with discharge as covariate. ANC, cations, and anions are in − μeq L 1.

Chemical No. SKT 2- 2-sided p- SKT 2- 2-sided P- Attribute sided value sided value P- P- value† value

Season Covariate Monthly Covariate

pH 766 0.01 0.001 0.001 0.008 0.001 0.001 Ca 782 2.66 0.001 0.001 2.4 0.001 0.001 Mg 790 0.44 0.001 0.001 0.40 0.001 0.001 Na 790 0.27 0.001 0.001 0.23 0.001 0.001 K 790 0.06 0.02 0.02 0.045 0.001 0.02

SO4 770 1.96 0.001 0.001 1.69 0.001 0.001

NO3 718 0.26 0.001 0.003 0.22 0.001 0.01 Cl 776 −0.009 0.52 0.61 −0.004 0.57 0.73 ANC 771 0.10 0.001 0.63 −0.01 0.82 0.001

† Two-sided tests were employed, where the null hypothesis is the assumption of no trend against the alternative of either an increasing or decreasing trend.

Fig. 5. Summer precipitation recorded in D1 located just above the alpine catena (see Fig. 1 for location).

Fig. 7. Trend analysis of ANC values in the outlet of Green Lake 4 from 1982 to 2014. A change in the direction of the trend was observed.

and Benedict, 2006; Williams et al., 2006). However, an excess of Ca was calculated in these alpine stream waters above that expected from stoichiometric considerations of weathering of the dominant bedrock minerals (Clow et al., 1997) or from contribution of cation exchange reactions in the alpine soil system. The apparent excess can be ac- Fig. 6. Spatiotemporal distribution pattern of the volumetric water content counted for by an eolian contribution of dust to elevate the base sa- measured by TDR probes across the alpine catena. The box represents the inter- turation and pH in the Colorado Front Range (Litaor, 1987a; Rhoads quartile range that contains 50% of the measured values. The whiskers are lines et al., 2010). Further support for the eolian contribution to base sa- that extend from the box to the highest and lowest values, excluding outliers. turation was recently demonstrated by Clow et al. (2016), who found a The line across the box indicates the median. strong correlation (r2 = 0.94) between Ca2+, alkalinity, and dust concentrations deposited in snow during dust events primarily across the range may respond diﬀerently to environmental stress. central and southern Rockies. 2+ Finally, the highest seasonal volume-weighted mean of ANC was The most predominant cation in the ANC is Ca , which increased − − − − − μ 1 in the outlet of Green Lake 4 by 2.4 μeq L 1 yr 1 to 2.66 μeq L 1 yr 1, found in the winter (98.7 ± 13.6 eq L ), followed by spring μ −1 μ −1 contingent upon the selected intra-block variable. The source of the (68.2 ± 20.1 eq L ) and fall (59.7 ± 13.9 eq L ). The smallest calcium is weathering of minerals such as epidote in the granitic and seasonal volume-weighted mean value was observed in the summer μ −1 biotite gneiss bedrock of the catchment (Gable and Madole, 1976; Muhs (51.4 ± 38.8 eq L ). The high volume-weighted means of ANC in

116 M. Iggy Litaor et al. Catena 170 (2018) 108–118

Acknowledgments

Research in the Green Lakes Valley has been supported by the US National Science Foundation through the Niwot Ridge LTER program (DEB-1027341) and the Boulder Creek Critical Zone Observatory (NSF- 0724960, 1239281, and 1331828). The senior author would like to thank Dr. Reichmann for his help with the data handling, Mr. Ben Hakimi for his analyses of the soils collected in 2015 and Prof. Tim Seastedt for his review of an earlier version of the manuscript. Travel funding for the senior author was given by Tel Hai College and MIGAL, Israel.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2018.06.004.

References

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