Forest conversion stimulated deep soil C losses and decreased C recalcitrance through priming effect in subtropical China

Abstract

The mechanisms about high soil organic C (SOC) loss from deep soil when natural forests are converted into forest plantations are poorly understood. In this study, we added 13C-glucose to the top soil (0–10 cm) and deep soil (60–80 cm) in order to investigate the OC mineralization and priming effect (PE) of soil under an evergreen broad-leaved forest (BF) or under Chinese fir (CF) plantation. The PE was quantified from the variation in soil C-CO2 evolution caused by adding 13C-glucose to soil. The conversion of BF into CF decreased the SOC with the soil depth significantly. The cumulative C mineralization of the top soil was significantly higher in BF than in CF (794.2 and 503.6 mg kg−1 soil, respectively), but no significant difference was found in the deep soil layer (145.2 and 150.4 mg kg−1 soil for BF and CF). Specific mineralization rate, expressed as gram C per kilogram SOC, was the highest in the deep soil of CF (36.7 g kg−1 SOC) and the lowest in top soil of BF (21.1 g kg−1 SOC), probably because the SOC was less recalcitrant to microbial decomposition in deeper than top layer of the CF due to its relatively high concentration of labile C. The PE was higher in deep layer than in top layer of BF soil (166.8 and 116.7 mg kg−1, respectively), whereas the opposite was observed in CF (100.1 and 153.9 mg kg−1, respectively). The PE was significantly correlated to the specific C mineralization (r = −0.92, P < 0.001), suggesting that PE increased significantly with SOC recalcitrance, defined as the specific mineralization rate of SOC. Moreover, Gram-negative bacteria (cy17:0), fungi (18:1ω9c), and actinomycete (10Me19:0) phospholipid fatty acids (PLFAs) were positively correlated with PE intensity, indicating that these microbial groups may promote positive SOC priming. We inferred that the priming is mainly controlled by the soil organic matter (SOM) recalcitrance and soil microbial community composition. Probably, the deep soil C losses were due to the increase in available C in the soil profile by forest conversion associated with changes in microbial diversity and increase in recalcitrant C losses in the deep soil layer.