Conversion of native grassland to coniferous forests decreased stocks of soil organic carbon and microbial biomass

Aims Encroachment of woody species into grasslands is a global phenomenon that affects ecosystem services, including soil organic carbon (SOC) storage and microbial community structure. We determined stocks of SOC and soil microbial biomass as affected by conversion of grasslands to coniferous forests.Methods We examined SOC and soil d(13)C signatures under three vegetation covers: native grasslands, eastern redcedar (Juniperus virginiana), and ponderosa pine (Pinus ponderosa), at six soil depths (0 - 10, 10 - 30, 30 - 100, 100 - 170, 170 - 240, 240 - 300 cm). Fatty acid methyl esters (FAMEs) were used to quantify soil microbial biomass (MB) and as biomarkers for bacteria, arbuscular mycorrhizal fungi (AMF), and saprophytic fungi (SF) which includes ectomycorrhizal fungi (ECM).Results Native grasslands had higher overall SOC and MB stocks in the top 300 cm soil depth (7.39 kg SOC m(- 2) and 65.6 mmol MB m(- 2)) than converted cedar (6.80 kg SOC m(- 2) and 38.4 mmol MB m(- 2)) or pine (5.87 kg SOC m(- 2) and 39.9 mmol MB m(- 2)) forest. Losses of AMF biomass occurred on conversion of native grasslands to pine forest. Stocks of SF biomass only differed with vegetation type in topsoil.Conclusions Conversion of native grasslands to cedar or pine forest increased topsoil (0 - 10 cm) SOC storage due to needle litter accumulation. Conversion of native grasslands to forests decreased subsoil (10 - 300 cm) SOC stocks and had replaced, by increasing depth, 30 to 19% and 45 to 26% of grassland C with cedar and pine C, respectively. Changes in soil pH, nutrients, and soil microbial communities accelerated SOC turnover.