Biogeochemical Changes Accompanying Woody Plant Encroachment in a Subtropical Savanna

Ecosystem properties of surficial (0–10 cm) soils in remnant herbaceous patches were compared to those of contrasting woody plant patch types (upland discrete cluster, upland grove, and lowland woodland) where shifting land cover is known to have occurred over the past 50–77 yr. The purpose of this study was to evaluate and quantify the biogeochemical consequences and subsequent developmental rates of woody plant formation on sites formerly dominated by grasses. Clay and water content of woodland soil patches was higher than that of soils associated with upland discrete cluster and grove patches. Even so, lowland woody patches were generally comparable to upland grove and discrete shrub cluster patches with respect to soil organic carbon (SOC), soil N, the ratio of annual N mineralization:total N, annual litterfall, and root biomass. The fact that finer soil texture, enhanced soil moisture, and the more advanced age of lowland woody patches did not translate into greater accumulations of SOC and N relative to upland grove and discrete cluster patches suggests that C and N losses might be higher in recently developed lowland woodland communities. Fluctuations in monthly root biomass standing crop (0–10 cm) far exceeded annual foliar litterfall in upland and lowland woody patch types, suggesting that belowground inputs of organic matter may drive changes in soil physical and chemical properties that occur subsequent to woody plant establishment. The estimated annual mean rates of soil C accretion in the ‘‘islands of fertility’’ that developed subsequent to tree/shrub encroachment were variable and ranged from 8 to 23 g/m2 (in groves and discrete clusters, respectively); N accretion ranged from 0.9 to 2.0 g/ m2 (in groves and discrete clusters, respectively), even though mean annual N mineralization rates were threeto fivefold greater than those measured in remnant herbaceous patches. Woody plant proliferation in grasslands and savannas in recent history has been widely reported around the world. The causes for this shift in vegetation are controversial and center around changes in livestock grazing, fire, climate, and atmospheric CO2. Our data, which are conservative in that they examine only the upper 10 cm of the soil profile, indicate that the rate and extent of soil C and N accumulation associated with this phenomenon can be rapid, substantial, and accompanied by increased N turnover. This geographically extensive vegetation change thus has important implications for understanding how the global carbon and nitrogen cycles may have been altered since Anglo-European settlement of arid and semiarid regions.