Glomalin, an Arbuscular-Mycorrhizal Fungal Soil Protein, Responds to Land-Use Change
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Plant and Soil 253: 293–299, 2003. 293 © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Glomalin, an arbuscular-mycorrhizal fungal soil protein, responds to land-use change Matthias C. Rillig1,4, Philip W. Ramsey1, Sherri Morris2 & Eldor A. Paul3 1Microbial Ecology Program, Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA; 2Biology Department, Bradley University, Peoria, IL 61625, USA; 3Natural Resource Ecology Laboratory, Colorado State University, Fort Collins CO, 80523, USA; 4Corresponding author∗ Received 16 May 2002. Accepted in revised form 7 January 2003 Key words: decomposition, forest soil, glycoprotein, soil carbon storage Abstract Glomalin is a soil proteinaceous substance produced by arbuscular mycorrhizal fungi. Most of the information available concerning this protein has been collected in relation to its role in soil aggregation. In this study, we explored the distribution of glomalin across soil horizons, decomposition of glomalin, and relationship with soil C and N in an agricultural field, a native forest, and an afforested system. Glomalin was present in A, B, and C horizons in decreasing concentrations. Land-use type significantly affected glomalin concentrations (mg cm−3), with native forest soils having the highest concentrations of the three land-use types in both A and B horizons. In terms of glomalin stocks (Mg ha−1), calculated based on corrected horizon weights, the agricultural area was significantly lower than both afforested and native forest areas. As measured after a 413 day laboratory soil incubation, glomalin was least persistent in the A horizon of the afforested area.. In agricultural soils and native soils, ca. 50% of glomalin was still remaining after this incubation, indicating that some glomalin may be in the slow or recalcitrant soil C fraction. Comparison of glomalin decomposition with CO2-C respired during incubation indicates that glomalin makes a large contribution to active soil organic C pools. Soil C and N were highly correlated with glomalin across all soils and within each land-use type, indicating that glomalin may be under similar controls as soil C. Our results show that glomalin may be useful as an indicator of land-use change effects on deciduous forest soils. Introduction Anderson, 2000). Glomalin is linked with soil carbon storage via its effect on soil aggregate stabilization Arbuscular mycorrhizal fungi (AMF) are ubiquitous (Rillig et al., 2002b), and it also presents a potentially symbionts in terrestrial ecosystems, colonizing a ma- important soil C pool (Rillig et al., 2001). The protein jority of land plants (Smith and Read, 1997). AMF is quite recalcitrant, being extracted by autoclaving. produce the iron-containing glycoproteinaceous sub- It decomposes slowly in laboratory incubations; in a stance glomalin (Wright et al., 1996), which accumu- prairie soil glomalin concentrations decreased by only lates in soils to concentrations of several mg per cm3 25% after 5 months (Steinberg and Rillig, 2003). We of soil (e.g., Rillig et al., 2001). Pools of glomalin know very little about what controls glomalin stabiliz- are responsive, even in the short term, to ecosystem ation in soil, and these controls may be different from perturbations, such as elevated atmospheric CO2 con- factors stabilizing the bulk of soil carbon. centrations (e.g., Rillig et al., 1999, 2000), warming Glomalin concentrations are consistently highly (Rillig et al., 2002a), and various agricultural manage- correlated with soil aggregate water stability (e.g., ment practices (e.g., Wright et al., 1999; Wright and Wright and Upadhyaya, 1998), and the majority of research on glomalin has focused on this aspect of its ∗ FAX No: 406-243-4184. E-mail: [email protected] importance in the soil system. However, numerous ba- 294 sic questions regarding the distribution of this protein never cleared or plowed, and a current agricultural on the landscape remain unanswered. For example, we field under a corn (Zea mays L.) – soybean (Glycine do not know about the distribution (or even presence) max) rotation. of glomalin across soil horizons, since its concentra- Soils (Morley silt loam; fine, illitic, mesic typic tions have mostly been measured for the top layer Hapludalf) were sampled in three replicate plots of of soil most relevant to assessing soil aggregation. each land-use type. In each forest plot, three maple However, assessing soil C for global change scenarios trees were chosen, and two transects with three requires an understanding of C pools in deeper soil samples each were taken from the base of the tree to horizons (Haile-Mariam et al., 2000). the interspace between the trees (approximately 1.5 While knowledge concerning the responsiveness m). The three samples in each transect were com- of glomalin to agricultural management practices is posited for each forest, giving an n = 6 per forested accumulating, we know little about potential changes site. In the agricultural field, three samples were taken in glomalin concentrations with different land-use approximately 0.5 m apart in three locations (no less practices, such as afforestation. Afforestation of than 25 m apart) in the center of the field. Soils were former agricultural soils has been a major factor af- sampled to a depth of 1 m using a soil corer with a fecting landscapes of temperate North America, and diameter of 3.8 cm. The cores were separated by hori- this land-use practice is also considered important in zon and placed into individual sample bags. Soil cores soil C sequestration (e.g., Myneni et al., 2001). were weighed by horizon and dry weight and bulk These gaps in our knowledge about this protein, density were determined. Samples were then sieved together with the general scarcity of information re- and composited. garding glomalin in deciduous forest soils, provided the motivation for this study. Soil incubation Here we asked the following questions: 1. Do glomalin concentrations (per cm3 of soil) and Samples of approximately 80 g were maintained at total stocks (per hectare) differ among land-use 60% of water holding capacity, in sealed jars and in- types (agriculture, native forest, afforested)? cubated at 25 ◦C (Paul et al., 2001) for 413 days for 2. How is glomalin distributed across soil horizons the A horizon samples and 397 days for B and C (A, B, C), and are glomalin levels in different soil horizon samples. Jars were sampled for CO using a horizons affected differentially by land-use? 2 Licor infrared gas analyzer (LI-COR, Lincoln, Neb- 3. Does persistence (as measured by potential de- raska, USA) periodically over the incubation period. composition of glomalin under standardized con- Jars were periodically flushed with CO free air to pre- ditions) differ among the different land manage- 2 clude CO levels inhibitory to microbial respiration. ment types? 2 Soils were dried following incubation and stored in 4. Are glomalin concentrations related to soil chem- plastic vials until analysis. ical characteristics? Glomalin analysis Materials and methods Glomalin is operationally defined as the compound ex- Study sites and soil sampling tracted from soil by autoclaving with citric acid buffer (and detected with a Bradford assay). Since extract The site chosen for this study was part of the Delaware from soil has not been purified, and since biochem- Wildlife Area in Delaware County, Ohio (40◦ 24 N, ical characteristics/ identity of glomalin (as a gene 83◦ 01 W). The mean annual temperature is 10.8 product) are still unknown, we cannot exclude that ◦C and mean annual precipitation 934 mm at the other compounds are part of this extract (Rillig et site. The areas sampled included a successional (ca. al., 2001). However, immunoreactivity of this protein 1950) forest with mixed deciduous species, dominated fraction with an antibody raised against spores of an by maple (Acer spp.), elm (Ulnus spp.) and walnut arbuscular mycorrhizal fungus (MAb32B11; Wright (Juglans spp), a native forest remnant (hickory [Carya et al., 1996) links the origin of this substance to ar- spp.], walnut [Juglans spp.], maple [Acer spp.], oak buscular mycorrhizal fungi (Wright and Upadhyaya, [Quercus spp.] and cherry [Prunus spp.]) that was 1996; Wright et al., 1996). 295 Table 1. Organic C,†CO2 respired after incubation,% C remaining, EEG and TG, and percentage of glomalin pools remaining after incubation for A, B and C horizon of the three land-use types (standard errors of the mean in brackets). Means followed by a different letter differ significantly (Tukey-Kramer HSD; P < 0.05) among land-use types within a horizon. EEG = easily extractable glomalin; TG = total glomalin Organic C CO2-C C remaining EEG EEG TG TG remaining (mg/cm3 respired (%) (mg/cm3 remaining (mg/cm3 (%) soil) (mg/cm3 soil) soil) (%) soil) Aff A 20.56 (1.85)a 2.06 (0.10)b 89.80 (0.45)a 0.44 (0.02)a 36.33 (4.34)a 3.41 (0.39)a 31.58 (5.77)b Agric A 17.87 (0.42)a 1.31 (0.15)a 92.68 (0.89)b 0.44 (0.03)a 22.82 (0.89)b 3.06 (0.13)a 46.58 (4.18)a Nat A 38.52 (1.02)b 2.80 (0.09)c 92.69 (0.42)b 0.46 (0.03)a 35.47 (3.50)a 4.91 (0.10)b 51.84 (1.89)a Aff B 9.23 (0.65)a 0.60 (0.08)b 93.74 (1.01)a 0.33 (0.01)a 39.84 (4.33)a 2.01 (0.25)a 18.56 (3.43)a Agric B 8.72 (0.53)a 0.30 (0.05)a 96.60 (0.40)b 0.31 (0.02)a 37.38 (1.39)a 1.90 (0.27)a 31.83 (7.62)a Nat B 14.47 (0.78)b 0.98 (0.67)c 93.13 (0.57)a 0.42 (0.02)b 42.64 (1.52)a 2.90 (0.27)b 33.07 (4.51)a Aff C 9.46 (0.47)a 0.36 (0.21)a 96.09 (0.38)a 0.20 (0.01)a 48.64 (6.10)a 1.08 (0.06)a 24.25 (6.32)a Agric C 9.42 (0.36)a 0.21 (0.16)a 96.67 (0.63)a 0.19 (0.03)a 12.44 (0.55)b 1.18 (0.10)a 22.88 (18.79)a Nat C 11.39 (0.58)b 0.44 (0.92)a 95.19 (0.40)a 0.21 (0.01)a 23.89 (5.32)c 1.23 (0.07)a 21.95 (7.67)a †From Paul et al.