Woodland development and soil carbon and nitrogen dynamics and storage in a subtropical savanna ecosystem

Abstract

Woody plant invasion of grasslands is prevalent worldwide, but the biogeochemical consequences of this vegetation shift remain largely unquantified. In the Rio Grande Plains, TX, grasslands and savannas dominated by C4 grasses have undergone succession over the past century to subtropical thorn woodlands dominated by C3 trees/shrubs. To elucidate mechanisms of soil organic carbon (SOC) and soil total N (STN) storage and dynamics in this ecosystem, I measured the mass and isotopic composition (δ13C, δ15N) of C and N in whole-soil and soil size/density fractions in chronosequences consisting of remnant grasslands (Time 0) and woody plant stands ranging in age from 10-130 years. Rates of SOC and STN storage averaged 10-30 g C m-2yr-1 and 1-3 g N m-2yr-1, respectively. These accumulation rates increased soil C and N pools 80-200% following woody encroachment. Soil microbial biomass (SMB-C) also increased after woody invasion. Decreasing Cmic/C org and higher qCO2 in woodlands relative to grasslands suggests that woody litter is of poorer quality than grassland litter. Greater SOC and STN following woody invasion may also be due to increased protection of organic matter by stable soil structure. Soil aggregation increased following woody encroachment; however, most of the C and N accumulated in free particulate organic matter (POM) fractions not protected within aggregates. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ13C with time after woody encroachment. Free POM had the shortest average MRTs (30 years) and silt+clay the longest (360 years). Fine POM had MRTs of about 60 years, reflecting protection by location within aggregates. δ15N values of soil fractions were positively correlated with their MRTs, suggesting that higher δ15N values reflect an increased degree of humification. Increases in SOC and STN are probably being sustained by greater inputs, slower turnover of POM (some biochemical recalcitrance), and protection of organic matter in aggregates and association with silt and clay. Grassland-to-woodland conversion during the past century has been geographically extensive in grassland ecosystems worldwide, suggesting that changes in soil C and N dynamics and storage documented here could have significance for global C and N cycles.