Using Long-Term Soil Incubations to Determine Drivers of Soil Organic Carbon Accumulation In Response to Woody Encroachment.

Woody encroachment into grasslands and savannas is a global phenomenon that alters the biogeochemical cycling of carbon (C) and nitrogen (N).  In the Rio Grande Plains region of southern Texas, fire suppression and livestock overgrazing have resulted in the progressive encroachment of C₃ woodlands into the native C₄ grasslands, resulting in C and N accrual with woody stand age.  Our previous work has shown this accrual is predominately in particulate soil organic matter fractions, where the abundance and chemistry of lignin and aliphatic biopolymers is altered, suggesting that selective accrual of purportedly more recalcitrant plant molecules is a potential mechanism for C accrual in this region.  To evaluate the importance of changing plant chemistry upon soil organic carbon (SOC) stabilization, we measured the quantity and isotopic composition of respired CO₂ during long term laboratory incubations of whole soil and size (>250 µm) and density (<1.0 g/cm³) soil fractions along a chronosequence of woody encroachment in this region.  During the whole soil incubation a greater proportion of total SOC was respired from older woody stands (34-86 yrs) than younger woody stands (14-23 yrs) and grasslands.  In contrast, older woody stands respired significantly less SOC during the soil fraction incubations.  Together, these data indicate that in these sandy soils, increasing biochemically recalcitrant polymers with woody encroachment results in decreased respiration from physically unprotected soil fractions.  However, in the whole soil this effect is masked by the greater overall allocation of SOC into physically unprotected soil fractions.  δ¹³C values of CO₂ respired from whole soils and soil physical fractions confirm these hypotheses from the SOC loss data.  This work has important implications for understanding interactions between the microbial community and the chemical and physical accessibility of SOC.