Soil microbial biomass response to woody plant invasion of grassland
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Woody plant proliferation in grasslands and savannas has been documented worldwide in recent history. To better understand the consequences of this vegetation change for the C-cycle, we measured soil microbial biomass carbon (Cmic) in remnant grasslands (time 0) and woody plant stands ranging in age from 10 to 130 years in a subtropical ecosystem undergoing succession from grassland to woodlands dominated by N-fixing trees. We also determined the ratio of SMB-C to soil organic carbon (Cmic/Corg) as an indicator of soil organic matter quality or availability, and the metabolic quotient (qCO2) as a measure of microbial efficiency. Soil organic carbon (Corg) and soil total nitrogen (STN) increased up to 200% in the 0-15 cm depth increment following woody plant invasion of grassland, but changed little at 15-30 cm. Cmic at 0-15 cm increased linearly with time following woody plant encroachment and ranged from 400 mg C kg-1 soil in remnant grasslands up to 600-1000 mg C kg-1 soil in older (>60 years) woody plant stands. Cmic at 15-30 cm also increased linearly with time, ranging from 100 mg C kg-1 soil in remnant grasslands to 400-700 mg C kg-1 soil in older wooded areas. These changes in Cmic in wooded areas were correlated with concurrent changes in stores of C and N in soils, roots, and litter. The Cmic/Corg ratio at 0-15 cm decreased with increasing woody plant stand age from 6% in grasslands to <4% in older woodlands suggesting that woody litter may be less suitable as a microbial substrate compared with grassland litter. In addition, higher qCO2 values in woodlands (0.8 mg CO2-C g-1 Cmic h-1) relative to remnant grasslands (0.4 mg CO2-C g-1 Cmic h-1) indicated that more respiration was required per unit of Cmic in wooded areas than in grasslands. Observed increases in Corg and STN following woody plant encroachment in this ecosystem may be a function of both greater inputs of poor quality C that is relatively resistant to decay, and the decreased ability of soil microbes to decompose this organic matter. We suggest that increases in the size and activity of Cmic following woody plant encroachment may result in: (a) alterations in competitive interactions and successional processes due to changes in nutrient dynamics, (b) enhanced formation and maintenance of soil physical structures that promote Corg sequestration, and/or (c) increased trace gas fluxes that have the potential to influence atmospheric chemistry and the climate system at regional to global scales. 2007 Elsevier Ltd. All rights reserved.
