Respiration dynamics of size-separated soil fractions

Soil fractionation schemes aim to isolate conceptually-defined soil organic carbon (SOC) pools with varying turnover times and stabilization potentials that can be represented in SOC models. Among these different fractions, mechanisms for SOC stabilization may vary in their importance. In order to examine the microbial response to SOC of varying chemistry and accessibility, laboratory incubations were performed on soil physical fractions obtained from a chronosequence of progressive C3 woody plant encroachment (WPE) into C4-dominated grasslands at the Texas AgriLife La Copita Research Area in southern Texas. The quantity and stable isotopic composition of respired CO2 were measured during long-term incubations of the free light fraction (FLF, density <1.0 g/cm3), water stable macroaggregate-sized fraction (size >250 μm), and whole soil. In contrast to the whole soil, both the FLF and macroaggregate fraction from younger woody stands (ages 14-23 yrs in the chronosequence) are losing a greater proportion of SOC than older woody stands (ages 34-86 years). The grassland FLF and macroaggregate fraction also display greater variability in the proportion of SOC respired than whole soil. The isotopic composition of respired CO2 relative to the macroaggregate fraction displays a strong trend with age for the first 20 days of the incubation. Specifically, older woody stands initially release a highly enriched pulse of CO2 (~6‰). After 20 days the isotopic composition of CO2 from all landscape elements resembles the macroaggregate fraction. The isotopic composition of CO2 respired from the FLF is enriched relative to FLF, however the extent of this enrichment is variable and displays no relationship to woody stand age or landscape element. Combined, these data suggest that the factors controlling respiration patterns in the two least physically protected soil fractions are substantially different than those of the bulk soil. It appears that within the macroaggregate fraction the source and quantity of SOC degraded by microorganisms are influenced by SOC chemistry, where grassland-derived carbon is more easily degraded relative to woody plant carbon. The heterogeneity in the respiration data from the FLF reflects the variability in the input and structure of SOC that may be driven predominately by small changes due to differences plant community structure, belowground input rates, micronutrient availability, or other environmental factors at each specific site. This work highlights the importance of understanding the differential responses of SOC pools to environmental perturbations and helps to identify some of the controls upon SOC degradation within sandy soils.