Converting native shrub forests to Chinese chestnut plantations and subsequent intensive management affected soil C and N pools

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Changes in land-use can markedly influence soil carbon (C) and nitrogen (N) pools, and consequently C sequestration and soil fertility. The objective of this study was to investigate the effects of converting native shrub forests (NSF) to Chinese chestnut (Castanea mollissima Blume) plantations (CP) and the subsequent long-term intensive management (including chemical fertilizer application, deep tillage and understory vegetation control) on soil C and N pool sizes and the chemical composition of soil organic C (SOC). We studied SOC and total N (TN) storage, water soluble organic C (WSOC), hot-water soluble organic C (HWSOC), microbial biomass C (MBC), readily oxidizable C (ROC), NH4+–N, NO3-–N, water soluble organic N (WSON), microbial biomass N (MBN), and chemical composition of SOC determined by the 13C-nuclear magnetic resonance (NMR) technique in the soils of NSF and CP with 5, 10, and 20years of intensive management. Our results showed that conversion of NSF to CP decreased SOC storage and concentrations of WSOC, HWOC, MBC, ROC, WSON, and MBN (P<0.05), but increased TN storage and concentrations of NH4+–N and NO3-–N (P<0.05). The NMR results showed that after NSF was converted to CP, alkyl and O-alkyl C contents decreased, while aromatic C content and the aromaticity of organic C increased, suggesting that land-use conversion and subsequent long-term intensive management changed the SOC chemical composition, which may be attributed to changes in litter input, microbial diversity and activity, and the disturbance of soil during land-use change and long-term management. We conclude that conversion of NSF to CP and the subsequent intensive management were beneficial in increasing soil TN stock, but had a negative consequence for soil C storage, since they decreased SOC and labile organic C pools. Therefore, to sustain SOC and consequently develop sustainable CP, management practices such as application of organic fertilizer or biochar that can increase C input into soils should be developed in the future.