Long-term incubations of size and density separated soil fractions to inform soil organic carbon decay dynamics

Soil organic matter in coarse-textured soils is more vulnerable to environmental disturbances due to reduced potential for soil organic carbon (SOC) stabilization in aggregates or organo-mineral complexes. In sandy loam soils from the Rio Grande Plains region of southern Texas, woody encroachment has resulted in the rapid accrual of root and leaf tissues derived from trees and shrubs into poorly physically protected (macroaggregate >250 μm) and non-mineral associated (free light fraction <1.0 g cm−3) soil fractions. To determine the impact of changing plant input chemistry on potential degradability of accumulating SOC fractions, we measured the quantity and isotopic composition of respired CO2 from year-long incubations of the macroaggregate and free light soil fractions along a grassland to woodland successional chronosequence. During incubation of both fractions, the proportion of SOC respired from older woody stand soils (∼40–90 yrs) relative to recently established woody stands (<40 yrs) and remnant grassland soils decreased. We interpreted this decrease with woody stand age to result from a change to plant input chemistry with more lignin and aliphatic structures combined with a progressive shift to more non-hydrolyzable, poorly accessible forms of soil organic nitrogen, resulting in a system with slower short-term decay dynamics. The δ13C values of respired CO2 from all landscape elements indicated a selective release of older grassland-derived SOC in the first month of the macroaggregate incubation, possibly due to the disruption and rapid microbial utilization of grassland SOC after the soil fractionation process. Due to the sensitivity of these rapidly-cycling soil fractions to environmental disturbance and their capacity to influence longer-term SOC dynamics, understanding their decay dynamics is essential for understanding mechanisms of SOC stabilization. This is especially important in coarse-textured soils where large SOC stocks may be present in physical fractions that are relatively unprotected from decomposition.