ReVIeW ARtIcle t Research connects hydrology and stream water chemistry in California oak woodlands

by Anthony T. O’Geen, Randy A. Dahlgren, Alexandre Swarowsky, Kenneth W. Tate, David J. Lewis and Michael J. Singer

The UC Sierra Foothill Research and Extension Center (SFREC) is located in the heart of typical California blue oak and live oak woodlands within metavolcanic terrain of the Sierra Ne- vada foothills. These types of wood- lands often exist at the interface between urban, wild and agricultural lands and are used extensively for livestock grazing, wildlife habitat and surface water supply. Soil surveys for this region and within SFREC depict Claypan relatively few soil types compared to areas that support more-intensive agricultural land uses. Despite this s The bracket shows an inferred homogeneity, our study abrupt clay increase showed that the biogeochemical associated with the upper boundary of the and physical properties of vary claypan in a typical soil profi le at SFREC. sharply over short distances of less than 10 feet and also experience changes by season and as a result of storm events. An understanding of soil variability in this setting is impor- tant to assess rangeland productivity, perennial grass and oak restoration potential, carbon sequestration, stream fl ow generation and stream Uc Davis undergraduate tony Orozco installs a sensor in the Lewis-1 watershed at the UC Sierra Foothill Research and water chemistry. Extension Center.

ak woodlands are a patchy array quality. Physical (bulk density, water such as California, due to the timing of of open grassland and oak canopy. infi ltration rates), chemical (nutrient precipitation. Summers are warm and TheO presence or absence of trees is one enrichment, pH, cation exchange capac- dry, and most of the rain falls in winter factor that imposes spatial variability in ity) and biological (microbial biomass, months when vegetation is dormant soil characteristics. Oak trees change the ) properties are enhanced and the temperature is low. Most bio- properties of soils beneath their canopies in soils under oak canopies (Dahlgren geochemical processes require hospita- through a variety of nutrient-cycling et al. 1997). In contrast, soils forming ble temperatures and an adequate water processes, such as annual inputs of lit- under annual grasses in the absence of supply. Interactions between the eco- ter fall (Dahlgren et al. 2003). As leaves oak canopy are less fertile, have higher system services that soils provide, such drop and decompose at the soil surface, bulk density and are more susceptible to as nutrient cycling and the regulation of organic matter and nutrients are resup- surface-water runoff. water quality and quantity, are strongly plied to the upper soil horizons, creating Soil processes exhibit unique pat- infl uenced by the seasonality of climate islands of fertility and enhanced soil terns in Mediterranean ecosystems and individual storm events.

78 CALIFORNIA AGRICULTURE • VOLUME 64, NUMBER 2 Together, (1) the Mediterranean cli- lands. The ability to co-locate these chosen because they are similar in el- mate, (2) the inferred spatial homoge- projects is rare and would not be pos- evation, topography, geology, soils and neity of soils and (3) oak tree–induced sible without the support of the SFREC vegetation (type and canopy coverage). differences in soil properties, pose an infrastructure and staff. The main difference between Lewis-1 interesting challenge for understand- and Schubert is size, Lewis-1 being 90 A natural laboratory ing the ecosystem services that soils acres and Schubert, 255 acres. As part of provide and subsequent management SFREC has served as a natural a project beginning in 2002, Lewis-1 has implications. laboratory to study soils, ecology, hy- been ungrazed for about 10 years, and Our primary objective was to ex- drological processes and land manage- Schubert has been heavily grazed. amine the linkages between nitrogen ment effects for many years. There is a In the experimental Lewis-1 water- cycling and soil hydrology in a manner nearly 30-year record of water flow and shed, we installed a weather station, 400 that considers how the temporal and water quality for its Schubert water- soil-moisture sensors, a perched water spatial variability of soil properties gov- shed (Lewis et al. 2000, 2006). Schubert monitoring infrastructure and a stream- ern these processes and ultimately the watershed is 255 acres in size, with flow monitoring network. A perched soil’s ability to regulate stream water tree coverage of approximately 50%, water table occurs during the rainy sea- quantity and quality in oak woodlands. predominantly blue oak. A similar ex- son above a relatively impermeable clay- We present results from multiple long- perimental 90-acre watershed at SFREC, rich horizon situated approximately 10 term monitoring projects at the UC called Lewis-1, was instrumented in inches below the soil surface. This infra- Sierra Foothill Research and Extension 2002. These paired watersheds are structure has allowed us to document Center (SFREC) that integrate soil, ecol- within 3 miles of each other, and are the relationships between rainfall, soil ogy and water-resources data from the two of four watersheds at SFREC that moisture storage, lateral flow through area’s California blue oak (Quercus doug- we have been monitoring for stream soils and stream flow at 15-minute inter- lasii) and live oak (Q. wislizeni) wood- flow and water quality. They were vals since summer 2006.

Glossary (4") A

Biogeochemistry: The study of (12") AB chemical, physical, geological and biological processes on Earth. Bulk density: The mass of dry (24") Bt1 soil per unit volume of soil. Genetic : A layer of (43") BC soil often occurring parallel to the soil surface that differs from adja- cent soil layers in physical, chemical and biological properties or mor- phological features such as color, structure or roots. Litter fall: Dead plant residues that accumulate on the soil surface annually. Metavolcanic: Metamorphosed, igneous, extrusive bedrock. Nitrogen cycling: The cyclical transformations and translocations of nitrogen from the atmosphere (N2), to plant and microbial tissues, to inorganic nitrogen (NO3 and NH4). Perched water table: A discon- tinuous saturated zone in soil that occurs when vertically percolating water reaches a slowly permeable soil horizon. Soil profile: An excavation in the Above, the Lewis-1 watershed at SFREC was equipped with instruments in 2002 to sample and soil approximately 3 feet long by 3 monitor the interactions between soil and stream water. Top left, a perched water monitoring feet wide by 5 feet deep used to de- trench uses trays to collect water samples from different soil horizons (A, AB, Bt1 and BC). Top scribe and sample soils. right, tipping buckets are mounted on the blue containers to measure lateral water flow.

http://californiaagriculture.ucanr.org • APRIL–JUNE 2010 79 To understand the temporal and spatial patterns in water content, sen- sors were placed in 100 soil profiles, typically at depths of 4, 12, 20 and 40 inches. Perched water flow was moni- tored continuously from a trench that contained three soil profiles fitted with water-collection trays inserted at four depths: 4 inches (bottom of A horizon), 12 inches (bottom of AB horizon), 24 inches (upper boundary of the clay pan) and 43 inches (below the clay pan). Water flow rates were recorded from each layer using tipping buckets, and water chemistry samples were collected in plastic drums. Stream flow was mon- itored continuously using water-height sensors connected to a combination V-notch weir for measurement at low- flow conditions, and a Parshall flume was used for high-flow measurements. Automated samplers were used to col- lect water samples from streams on an Fig. 1. Nitrogen cycling with major pools of nitrogen (pounds/acre) for an oak hourly basis during storm events, and woodland–grassland ecosystem in the Schubert watershed at SFREC. base flow was collected periodically be- tween storms. Soil properties The soil and ecological data that half of the landscape area in the exper- we collected reflects a variety of ex- Soils at SFREC and throughout the imental watershed and tend to occur periments that have been conducted metavolcanic terrain within the foothill on level or gently sloping hillsides and at SFREC over the last two decades, region are moderately deep to very deep, land forms that accumulate water. Clay including at Schubert watershed. Soil ranging from 20 inches to more than 60 content decreases below the Bt hori- profiles under the oak canopy and inches. Soils possess a distinctive red zons, resulting in a transition horizon open grasslands were sampled by color due to the high iron content of the (BC). The soil interface with bedrock is genetic soil horizon to determine the parent material that is released through described by either Cr (soft bedrock) or effects of oak nutrient cycling on soil weathering. A typical soil profile has R (hard bedrock) horizons. characteristics. Organic matter con- the following genetic horizon sequence: Nitrogen cycling centrations (total carbon and nitrogen) A-AB-Bt1-Bt2-BC-Cr or R. were determined for each soil horizon, The A and AB horizons have a Nitrogen cycling within the land- and these values were converted to an clay- texture and appreciable soil scape is directly linked with the oak area basis using the soil bulk density, organic-matter enrichment. Under oak canopy distribution and hydrologic horizon thickness and rock content. trees, A horizons are thicker (4 versus cycle. The majority of nitrogen is pres- Two representative blue oaks (70 and 2 inches), with higher organic matter ent as stored in the 92 years old with 11.5- and 19-inch- and lower bulk density compared to A and AB horizons (approximately the diameter breast heights, respectively) those in open grasslands (Dahlgren et upper 15 inches of soil). We found that were destructively sampled, and all al. 1997). The A and AB horizons are the major nitrogen pool is contained in aboveground components were dried porous and extensively burrowed by soil profiles under oak trees at about and weighed to determine the size of pocket gophers. Soils with Bt1 and Bt2 8,373 pounds per acre (fig. 1). The ni- the carbon and nutrient pools. Root horizons represent an accumulation trogen pool contained within the oak biomass and nutrient concentrations of clay (about 35% to 50%, maximum) trees was approximately 14% of the were calculated from root quantifica- transported from overlying horizons total soil nitrogen pool. Nitrogen stored tion studies conducted for these same by percolating water. Bt horizon tex- in blue oak was about 1,138 pounds per sites (Millikin and Bledsoe 1999). To tures range from clay loam to clay acre (931 aboveground plus 207 pounds document temporal changes in soil so- depending on the landscape position. per acre in roots belowground), while lution chemistry, 1.5-to-1 water extracts Claypans (Bt2) are a subset of Bt hori- understory components (e.g., grasses were prepared for each soil horizon zons that have an abrupt clay increase and herbs) added 39 and 53 pounds per monthly to determine the concentra- (more than 20% relative to the over- acre under the oak canopy and in open tions of soluble nutrients in soil hori- lying horizon) over a short vertical grasslands, respectively. The soil nitro- zons beneath the oak canopy and in distance (less than 1 inch). Soil profiles gen pool was nearly 1.6 times greater adjacent grasslands. containing claypans occupy less than (8,373 versus 5,344 pounds per acre) in

80 CALIFORNIA AGRICULTURE • VOLUME 64, NUMBER 2 In contrast, nitrate concentrations are 1.6 60 very low and similar between oak and

1.4 open-grassland soils during the winter 50 months when storage 1.2 Flow (gallons) and are at their peaks, and Nitrate (pounds per day) 40 soil temperatures and microbial activ- 1.0 ity are low. In addition, annual grass growth and 0.8 30 oak foliage production results in strong

0.6 demand for biological nutrients in 20 Flow (million gallons) spring, further depleting the soil nitrate 0.4 pool in spite of favorable moisture and Nitrate flux (pounds per day)

10 temperature conditions for microbial 0.2 mineralization. The increase in soil nitrate in late spring coincides with the 0 0 Dec. 1 Dec. 21 Jan. 5 Jan. 25 Feb. 9 Mar. 1 Mar 21 Apr. 5 Apr. 25 senescence (die off) of annual grasses, which results in a decrease in nitrogen

Fig. 2. Stream flow and nitrate-nitrogen (NO3-N) load in the 1992–1993 water year, uptake demands coupled with the Dec. 1 through April 30, at the Schubert watershed. release of nitrogen from dead annual grass roots by microbial decomposition. Dry conditions in the upper soil profile soils beneath the oak canopy compared ticulate matter, as well as from root up- throughout summer result in low to open grasslands (fig. 1). take with subsequent foliar leaching. nitrogen uptake by oak roots. Organic matter. The ability of oaks to Roots. In addition, oaks are rooted Instead of continuous nitrogen alter their soil environment occurs pri- considerably deeper than annual feedback among senescing plants, marily through the addition of organic grasses, which reduces deep leaching their soils and new growth, nitrogen is matter and nutrient cycling. During a and the loss of nutrients beneath the mineralized and accumulates in soils 3-year (1990–1992) study period, oaks oak canopy. About 70% of the oak root during the dry summer and fall months returned about 4,008 and 75 pounds biomass occurs within the upper 20 (Hart et al. 1993). Throughout summer, per acre annually of organic carbon inches of the soil profile (Millikin and organic and inorganic nitrogen and nitrogen, respectively, to the soil Bledsoe 1999). Oak roots also extend accumulates in the soil, and there is surface through aboveground litter fall. well beyond the edge of the canopy, little plant growth to sequester these Belowground carbon inputs through where they can sequester nutrients nutrients. Early fall rains (September oak and annual grass root turnover from grassland soils. Over time, they to October) cause the leaching of also contributed an appreciable amount concentrate these nutrients beneath the ammonium, nitrate and dissolved of organic matter (Jackson et al. 1990; oak canopy in the form of litter fall and organic nitrogen (DON) from annual Millikin and Bledsoe 1999). An addi- canopy throughfall. Given the organic- grass residues (more than 50% of total tional 5.3 pounds per acre of nitrogen matter enrichment in soils and oak veg- DON) while senescent oak foliage was added annually to the soil surface etation, there is considerable potential accounts for more than 70% of total by canopy throughfall (canopy drip) for carbon sequestration (about 89 tons DON after the first storm (Chow et al. and stem flow (water flowing down the per acre) associated with the conver- 2009). High concentrations of DON tree trunk). Nutrient flux in the canopy sion of grasslands to oak woodlands. To combined with water availability and throughfall originates from the capture ensure long-term carbon sequestration, warm residual soil temperatures lead to of atmospheric aerosols, gases and par- oak trees must be protected. Organic an explosion of microbial activity that carbon and many of the enhanced soil results in rapid nitrogen mineralization properties are lost within 20 to 30 years and the accumulation of nitrate in the 40 following an oak’s removal (Dahlgren upper soil horizons. Oak - A horizon Oak - AB horizon et al. 2003). An asynchrony within nutrient 30 Grass - A horizon Climate and seasons. The Mediter- cycles occurs in the Mediterranean Grass - AB horizon ranean climate contributes to strong climate, causing a marked nitrate spike 20 spatial and seasonal patterns in soil- in stream water during the onset of the solution and stream nutrient con- rainy season (December to January) Nitrate (ppm) 10 centrations, as illustrated by nitrate (Holloway and Dahlgren 2001). Nitrate concentrations during the winter/ leaching from oak woodland water- 0 spring rainy season (figs. 2 and 3). sheds at SFREC is strongly linked to the Dec Jan Feb Mar Apr May Jun Peak soil-solution nitrate concentra- hydrologic cycle at time scales ranging tions occur in fall and late spring and Fig. 3. Nitrate-nitrogen (NO3-N, parts per from storm events (hours) to years. million) concentrations in soil pore water under are considerably higher in soils under Pulses in stream nitrate concentrations oak canopy and open grassland. oak canopy during these times (fig. 3). are observed that match those observed

http://californiaagriculture.ucanr.org • APRIL–JUNE 2010 81 in soil pore water in the fall. Each storm rapidly delivering water as lateral fl ow to conduct water rapidly. While the progressively fl ushes this nitrogen pool, to streams. permeability of the AB horizon is so that by March there is little nitrate Storm events. The hydrologic moni- similar to that of the A horizon, the found in soils and stream water (fi gs. 2 toring infrastructure within the experi- AB horizon supplied more water and 3). mental watershed demonstrated the because it was saturated longer due to connectivity of lateral fl ow in soils with its proximity to the underlying water- Hydrologic fl ow paths stream fl ow during a typical storm (fi g. restrictive clay layer. The delay in fl ow Stream fl ow and water quality are 4). The timing of stream and perched of the A relative to the AB horizon related to storage and hy- water fl ow during the course of a storm represents the time it took for the drologic fl ow paths through soils. Our was similar. Peak stream fl ow was perched water table to extend upward conceptual model of the dominant sustained for approximately 3 hours into the A horizon. The AB horizon hydrologic fl ow path in soils of this from 7 p.m. to 10 p.m. during a storm is also thicker than the A horizon (8 area is lateral, perched water fl ow in January 2007 and corresponded with versus 4 inches), resulting in a greater through the upper soil layers (A and the initiation of lateral fl ow through the cross-sectional area to deliver water. AB horizons) following watershed AB horizon located above the claypan. Water quality. The hydrologic fl ow priming. Approximately 7 to 10 inches Perched water fl ow through the AB paths through soils at SFREC directly of precipitation are required to bring horizon was the dominant fl ow path. affect water quality. With their perme- the watershed soils to their maximum During this storm event the AB horizon able nature, surface horizons convey water-holding capacity before stream delivered 60% of the total water vol- water rapidly to streams in large vol- fl ow generation begins (Lewis et ume, measured from the perched water umes. Infi ltration rates range from 1 to 4 al. 2000). With the onset of winter monitoring infrastructure. The other inches per hour, and as a result, surface storms, water infi ltrates and percolates horizons contributed relatively equal runoff is rare when soils are not satu- through the highly permeable A and water volumes ranging from 10% to 17% rated (Lewis et al. 2000). Because plants AB horizons. The vertical percolation (fi g. 5). An additional line of evidence is are not actively transpiring during the of water is impeded by the presence apparent from the slope characteristics rainy season, the soil water-storage of the claypan approximately 24 associated with the rising limb of the capacity is frequently exceeded, which inches below the soil surface. The hydrographs, where the stream and AB results in the transport of water via claypan is slowly permeable to water, horizon have nearly identical slopes, and as a result saturated conditions suggesting that the rapid increase in develop at the upper boundary of the stream fl ow is a result of water trans- claypan during storms. A saturation port through the AB horizon (fi g. 4). zone (perched water table) forms at The AB horizon supplied more water Rain the claypan boundary and extends to the stream for a variety of reasons. upward into the A and AB horizons as The AB horizon is more permeable precipitation continues. Once they are than the Bt and BC horizons, because it saturated, water moves with the force contains less clay, better of gravity (down slope) through the and more large pores (e.g., gopher Gopher burrow highly permeable A and AB horizons, burrows and decayed root channels)

0.9 A (Stream) A

/sec) 13% 3 0.6 AB

(ft Rapid increase Stream flow 60% 0.3 Bt1 Peak Decreasing flow 17% 0.0 Bt2

8 BC B (Perched) 4 in. (A) Claypan 6 12 in. (AB) 10% /hr)

3 24 in. (Bt1, above claypan)

(ft 4 43 in. (BC) lateralflow

2

0 noon 3 pm 6 pm 9 pm 12 pm 3 am 6 am 9 am noon Storm event 2007 Fig. 5. Contribution of hydrologic fl ow paths in soils at SFREC. Percentages refer to the total Fig. 4. (A) Stream and (B) perched water hydrographs during the fi rst stream fl ow event lateral fl ow contributed by each soil layer of 2007. during a typical storm event.

82 CALIFORNIA AGRICULTURE • VOLUME 64, NUMBER 2 More than two-thirds of california’s drinking-water supply A passes through or is stored in oak woodlands. lateral fl ow through the nutrient-rich A distinct decrease in concentration dur- and AB horizons, directly to the stream. ing storms as the hydrologic fl ow path The nitrate accumulated in the A and is short-circuited through the upper soil Volumetric AB horizons is susceptible to leach- horizons (fi g. 6). water content ing because little plant uptake occurs Nitrate variability. There is con- (%) 40 during the winter months. The shift to siderable year-to-year variability in laterally dominated fl ow paths during nitrate leaching from oak woodland 35 (saturated) storms results in elevated nitrogen con- watersheds (fi g. 7). The mean annual 30 B centrations in stream water as nitrate, nitrate-nitrogen (NO3-N) export from which is leached directly to the stream the Schubert watershed was 1.4 pounds 25 via lateral subsurface fl ow through the per acre annually (ranging from 0.16 to 20 A and AB horizons (fi gs. 4, 5 and 6). In 3.2 pounds per acre annually) (Lewis et contrast, elements that are associated al. 2006). While annual nitrate-nitrogen with groundwater and chemical weath- fl ux at Schubert reached values as high ering in the lower Bt and BC horizons as 3.2 pounds per acre annually, the loss (e.g., calcium and sodium) display a of nitrate-nitrogen from the watershed was substantially less than atmospheric

16 inputs of mineral nitrogen (ammonium Stream flow [NH4] plus nitrate [NO3]) in the bulk Sodium (Na) precipitation (mean 8.4 pounds per acre Fig. 8. Extent of saturated conditions in the 12 Calcium (Ca)

/sec.) experimental watershed within the AB horizon 3 Nitrate (NO −N) annually). As a result, there was a net 3 during peak fl ow of two storm events: (A) Jan. retention of mineral nitrogen within the 4 and (B) Feb. 27, 2008. Saturated conditions 8 Schubert watershed, assuming no loss are depicted in green where water content is of nitrogen through denitrifi cation or 35% ± 3%. 4 volatilization. The annual nitrate- Stream flow (ft. nitrogen load was positively and signif- 0 icantly (P < 0.05) related to both annual 40 precipitation and stream fl ow (Lewis et AB horizon as indicated by saturated al. 2006). Nitrate fl uxes were a function conditions (water content greater than 30 of total stream fl ow volume and the 35%) during peak stream fl ow for two timing of storm-fl ow events throughout storms: (A) Jan. 4, with a large stream 20 the season. Years with high rainfall nitrate concentration and (B) Feb. 27, and runoff, combined with high runoff with a low stream nitrate concentration. 10 during the early portion of the rainy The spatial extent of the perched water Concentration (ppm) season — when nitrate concentrations table was 59% of the watershed area 0 are highest in the soil profi le — resulted 0 24 48 72 96 120 during the fi rst storm (fi g. 8A) com- Time (hours) in the largest fl uxes of nitrate-nitrogen pared to 36% during the second storm Fig. 6. changes in stream water chemistry from the watershed. (fi g. 8B). The perched water table was during an early-season (Jan. 2007) storm event. Differences in stream nitrate fl ux connected to the stream, as indicated by (fi gs. 2 and 6) can also be explained by the overlap of regions shaded in green the extent of the perched water table with blue fl ow lines, which were gener- 0.8 within the watershed and its connectiv- ated from a digital elevation model to 0.6 0.4 ity to the stream. The observed variation predict the routing of water by topog- 0.2 in stream nitrate fl ux is a result of fl ush- raphy, using the slope and curvature of 0.0 ing of the A and AB horizons by the the landscape (fi g. 8). −0.2 lateral fl ow path, because these horizons The perched water table during the −0.4 (lbs./acre/yr.) mineral N export contain most of the nitrogen. The degree second storm occupied isolated areas 10 −0.6 of fl ushing is a function of the amount with less connectivity to the stream, as Log −0.8 and intensity of rainfall, the seasonal indicated by areas of yellow (unsatu- −1.0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 distribution of rainfall, the spatial extent rated) and green (saturated) intersect-

Log10 stream flow (inches/yr) of the claypan, and the soil moisture ing the blue fl ow lines (fi g. 8B). This content prior to a storm event. suggests that the dominant source area Fig. 7. Relationship of stream nitrate fl uxes and Perched water table. stream fl ow volume (water fl ux) for a 20-year Figure 8 of stream fl ow was limited to the cen- period (1981–2000) in the Schubert watershed shows the extent and connectivity of ter of the watershed during the Feb. 24 at SFREC; r2 = 0.52; P < 0.001. the perched water table within the storm, when saturated conditions coin-

http://californiaagriculture.ucanr.org • APRIL–JUNE 2010 83 s Two-thirds of California’s drinking water passes through or is stored in oak woodlands. Detailed knowledge about soil properties can help rangeland managers to assess the impacts of grazing on water supply and quality.

cided with the main flow lines. Thus, processes (surface runoff, subsurface variation in nitrate flux observed over flow, stream flow) within watersheds. a season (figs. 2 and 3) and during a Currently, California’s soil surveys in References storm (fig. 6) is a result of the sequential rangelands are not mapped at scales Chow AT, Lee S, O’Geen AT. 2009. Litter contri- flushing of nitrate from the soil over fine enough to portray these soil butions to dissolved organic matter and disin- time and the area of the watershed that features across landforms and their fection byproduct precursors in California oak woodland watersheds. J Env Qual. 38:2334–43. is leached, determined by the spatial connectivity to streams. It is important extent of the perched water table and its that stakeholders communicate Dahlgren RA, Horwath WR, Tate KW, Camping connectivity to streams. this concern to the U.S. National TJ. 2003. Blue oak enhance in Califor- Cooperative to focus soil nia oak woodlands. Cal Ag 57(2):42–7. Soils and rangeland management survey updates in these important Dahlgren RA, Singer MJ, Huang X. 1997. Oak More than two-thirds of California’s landscapes. This level of detail is tree grazing impacts on soil properties and nutri- drinking-water supply passes through warranted to better understand ents in a California oak woodland. Biogeochem- or is stored in oak woodlands. Despite patterns in oak regeneration, perennial- istry 39:45–64. the seasonal peaks in nitrate in streams grass restoration, forage productivity, Hart SC, Firestone MK, Eldor PA, Smith JL. 1993. at SFREC, the concentrations are low rainfall-to-runoff characteristics and Flow and fate of soil nitrogen in an annual grass- relative to drinking-water standards water-quality dynamics. land and a young mixed-conifer forest. Soil Biol Biochem 25:431–42. (10 parts per million [ppm] nitrate- nitrogen). However, the temporal and Holloway JM, Dahlgren RA. 2001. Seasonal and spatial patterns between soils and event-scale variations in solute chemistry for four A.T. O’Geen is Associate Soil Resource Specialist Sierra Nevada catchments. J Hydrol 250:106–21. streams observed in this study are in Cooperative Extension, and R.A. Dahlgren is relevant to understanding how soils Professor of Soil Science, Department of Land, Air Jackson LE, Strauss RB, Firestone MK, Bartolome affect water supply and its quality, and Water Resources, UC Davis; A. Swarowsky JW. 1990. Influence of tree canopies on grassland and can help to guide rangeland is Graduate Student, Soils and Biogeochemistry productivity and nitrogen dynamics in deciduous oak savanna. Agric Ecosyst Env 32:89–105. management practices such as the Graduate Group, UC Davis; K.W. Tate is Range- stocking rate, season and location land Watershed Specialist in Cooperative Exten- Lewis D, Singer MJ, Dahlgren RA, Tate KW. 2000. of livestock congregation activities sion, Department of Plant Science, UC Davis; D.J. Hydrology in a California oak woodland water- (supplemental feeding stations). Lewis is UC Cooperative Extension Advisor and shed: A 17-year study. J Hydrol 240:106–17. In some instances, the variability County Director, Marin County; and M.J. Singer is Professor of Soil Science, Department of Land, Air Lewis DJ, Singer MJ, Dahlgren RA, Tate KW. of soil properties at SFREC and in 2006. Nutrient and sediment fluxes from a and Water Resources, UC Davis. We thank all the the Sierra Foothill region exceeds our California rangeland watershed. J Env Qual staff at SFREC who supported this research over 35:2202–11. ability to document them at scales the past 30 years. This research was supported in relevant to rangeland management. part by the UC Integrated Hardwood Range Man- Millikin CS, Bledsoe CS. 1999. Biomass and dis- Key soil properties, however, such as agement Program, UC Kearney Foundation of Soil tribution of fine and coarse roots from blue oak the distribution of claypans, should Science, UC Division of Agriculture and Natural (Quercus douglasii) trees in northern Sierra Ne- vada foothills of California. Plant Soil 214:27–38. be documented by soil surveys in Resources, Russell L. Rustici, USDA, USDA Coop- order to understand the spatial erative State Research, Education, and Extension connectivity of hydrologic-transport Service, and State Water Resource Control Board.

84 CALIFORNIA AGRICULTURE • VOLUME 64, NUMBER 2