Soil biological fertility evolution in a chronosequence under long-term rice cultivation after land reclamation in China

Land use significantly affects soil biological fertility through impacts on carbon (C) and nitrogen (N) cycling. The present study investigated the effects of long-term rice cultivation after tidal flat reclamation on soil C and N metabolism, microbial biomass and biological fertility. Eighteen composite topsoil (0-20 cm) samples were identified in a chronosequence of coastal reclamation areas (0-700 years old) in subtropical monsoon climate zone, namely tidal flat (T0), salt marsh soil (S10) and paddy soil (P50, P100, P300 and P700). Using ANOVA analysis, mono-exponential regression model, and multiple linear regressions, soil organic matter (SOM), total nitrogen (TN), cumulative C mineralization content (Ct) and N mineralization content (Nt), basal soil respiration (BSR) and microbial biomass C and N (MBC and MBN) in the P50-P700 samples were significantly higher than those in the T0 and S10 samples, whereas C metabolic quotients (qCO2) in the P50-P700 were significantly lower than those in the T0 and S10 samples. The time to steady state for SOM and TN are 357 years and 80 years, respectively; 133 and 221 years for Ct and Nt, respectively; and 318 and 183 years for MBC and MBN, respectively. Also, a soil biological fertility index (SBFI) was calculated on the basis of SOM, BSR, Ct, MBC, qCO2 and qCM. P100-P300 samples had the highest SBFI score (28.7) and ranked in the class V (very high) of biological fertility, achieving steady-state conditions after 146 years. SBFI was significantly positively correlated with SOM, TN, MBC, MBN, BSR, Ct and Nt, whereas it was significantly negatively correlated with pH, qCO2 and C mineralization quotient (qCM). MBC and qCM were two independent variables with considerable positive effects on SBFI. Long-term rice cultivation could facilitate C and N accumulation and enhance biological fertility in soils via microbial activity, especially within 300 years. Our findings demonstrate that rice cultivation has the potential to enhance soil C and N accumulation. Carbon-related SBFI is suitable for assessing soils under long-term rice cultivation, mainly because the rice paddy field is an intensive and conservative system.