Effect of soil degradation on the carbon concentration and retention of nitrogen and phosphorus across Chinese rice paddy fields

Soil degradation is a multi-factorial process that threatens crop production and the environment. According to the classification method of Pieri et al. (1992), soils are classified into different degradation classes by calculating the soil structural stability index (SSI). Soil nitrogen, phosphorus, labile organic carbon, mineral N and available-P concentration and stoichiometry under different degrees of degradation in rice fields in China were determined and analyzed. The results showed that 47% of the soil in China's rice fields had degradation problems. As the soil gradually degraded, soil N, P, AP concentrations were found to significantly decline (p < 0.05). Soils with lower organic matter levels and higher degrees of degradation were typically found in wetter and hotter climates. Furthermore, soil salinity, pH and bulk density in Chinese paddy fields were 65.7%, 12.3%, and 12.0% higher in the soils with low risk of degradation compared to the degraded soils. Nutrient stoichiometric ratios were found to significantly vary between different stages of soil degradation. Moreover, as the soil degraded, the rate of soil carbon loss was found to be greater than for nitrogen, and alluded a decoupling of soil carbon and nitrogen cycles. Soil degradation also effected other physical and chemical properties, including pH, organic matter, porosity and bulk density. Restraining annual cropping frequencies and adopting sustainable rice farming practises should safeguard against further soil degradation in China.