HR: 13:40h
AN: B53D-01 INVITED     [Abstracts]
TI: Soil Organic Matter Dynamics Following Land-Cover Change in a Subtropical Savanna: Insights from Soil Physical Fractionation and Stable Isotopes
AU: * Liao, J D
EM: jade@neo.tamu.edu
AF: Texas A&M University, Department of Rangeland Ecology and Management, College Station, TX 77843-2126 United States
AU: Boutton, T W
EM: boutton@neo.tamu.edu
AF: Texas A&M University, Department of Rangeland Ecology and Management, College Station, TX 77843-2126 United States
AU: Jastrow, J D
EM: jdjastrow@anl.gov
AF: Argonne National Laboratory, Environmental Science Division, Argonne, IL 60439 United States
AB: Soil physical structure is an important determinant of soil organic carbon (SOC) storage and turnover due to differential accessibility of SOC to decomposer organisms. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (d13C, d15N) of those soil fractions have been used to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C3 trees and shrubs (d13C = -27 o/oo) have largely replaced C4 grasslands (d13C = -14 o/oo) over the past 100-200 yr. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10-130 yr. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural d13C and d15N values. Rates of whole-soil C and N storage in the upper 15 cm of the soil profile averaged 10-30 g C/m2/yr and 1-3 g N/m2/yr, respectively, over the past 130 yr of woodland development. These rates of accumulation have increased soil C and N pool sizes in older wooded areas by 80-200 o/o relative to remnant grasslands. The relative proportions of the free light fraction (density less than 1.0 g/cc) and macroaggregate fraction (greater than 250 um) increased linearly with time following woody plant invasion of grassland. Conversely, the relative proportions of free microaggregate (53-250 um) and free silt+clay (less than 53 um) fractions decreased linearly with time after woody invasion, likely reflecting incorporation of these fractions into macroaggregates. C and N concentrations in all soil fractions increased with time following woody invasion, but most of the C and N accumulated in light (density less than 1.85 g/cc) particulate organic matter (POM) fractions not protected by stable soil structure. Mean residence times (MRTs) of soil fractions were calculated based on changes in their d13C with time after woody encroachment. The shortest MRTs (mean = 30 yr) were associated with all POM fractions not protected within aggregates. Fine POM (53-250 um) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 yr. All silt+clay fractions had the longest MRTs (mean = 360 yr) regardless of whether they were found inside or outside of aggregate structure. d15N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher d15N values reflect an increased degree of humification. Increases in whole-soil C and N in wooded areas are probably being sustained by greater inputs and relatively slow turnover of POM, perhaps due to the biochemical recalcitrance of POM materials and/or nutrient-water limitations to microbial activity. These results indicate that soil structure may provide a mechanistic explanation for C and N processes and dynamics following land cover changes in terrestrial ecosystems.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1094 Instruments and techniques
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting