Abstract
Purpose Studies about soil carbon (C) and nitrogen (N) dynamics with land use change are urgently needed for urban ecosystems. We used fractionation of soils combined with stable isotopic analysis to examine soil C and N cycles after decadal forest and lawn planting in the Pearl River Delta, China. Methods Soil samples from bare soil (CK) and four land use treatments (55 and 20 years of forest plantation, F-55 and F-20; 55 and 20 years of lawn plantation, L-55 and L-20) were split into different chemical fractions. Then we analyzed the C and N contents, C/N ratio, δ13C and δ15N, C and N recalcitrant indices (RIC, RIN), and a C pool management index (CPMI).Results Forest vegetation substantially enhanced soil organic carbon (SOC) caused by the recalcitrant (RC) and labile C (LC) pools, while the larger soil organic nitrogen (SON) was ascribed to the increased recalcitrant N (RN). Enhanced LC but minor changes in labile N (LN) suggested a decoupled C and N in labile fractions of the forest soils. In contrast, the larger LN, and the enhanced decomposition of SOC, indicated that the lawns may have inhibited N mineralization of labile pools, also suggesting a decoupled C and N turnover and leading to low RIN values. Conclusions Urban forest and lawn plantations significantly changed the soil C and N dynamics, and emphasized the inconsistency between C and N processes, especially in labile pools, which would eventually lead to minor changes in N and limit the ecosystem C sequestration.
