Effects of Soil Conditioners and Organic Fertilizer Application on Enzyme Activity and Rice Yield in an Acid Paddy Soil

The effects of application of soil conditioners and their combination with organic fertilizers at a low addition rate on soil acidity, nutrients, enzyme activity and rice yield following three successive years were investigated in this study, so as to provide a theoretical basis for soil improvement in acid red soil in Southern China.

A field experiment was conducted with eight treatments: a control without fertilization and soil conditioner (CK0), traditional fertilization (CK1), application of mineral soil conditioner (0.75 t·hm^-2) with traditional fertilization (M), organic soil conditioner (1.2 t·hm^-2) with traditional fertilization (O), and combined application of mineral and organic soil conditioners with traditional fertilization (MO), as well as co-application of M, O and MO with organic fertilizer (4.5 t·hm^-2) under traditional fertilization (MF, OF and MOF). The soil conditioners and organic fertilizer were applied once per year. After consecutive application for 3 years, rice yield was measured and soil chemical properties and soil enzyme activity were analyzed.

Compared with CK0 and CK1, all the treatments significantly increased soil pH, respectively, and significantly reduced soil exchangeable H⁺, exchangeable Al³⁺, and exchangeable acid concentrations. Compared with CK1, MF, OF, and MOF treatments significantly increased soil organic matter content, while MOF treatment also increased available phosphorus (P) and potassium (K) content, microbial biomass carbon and nitrogen and rice yield. Compared with the CK1, the application of MF and OF significantly reduced the activity of acid phosphatase, while MOF significantly reduced the vector angle, indicating a decreased microbial phosphorus (P) limitation. Soil exchangeable H⁺, exchangeable Al³⁺, and exchangeable acid concentrations had positive correlations with the activity of acid phosphatase, while soil organic matter, pH, available N, microbial biomass carbon, available P had negative correlations with it. Redundant analysis indicated that decrease in soil potential acidity and increase in soil nutrients were the key drivers of soil enzyme activities. Correlation analysis suggested that rice yield was significantly positively correlated with soil organic matter, available N, and available P, and significantly negatively correlated with exchangeable acids and the vector angle. Structural equation modeling further indicated that variation in soil organic matter and exchangeable acid changes rice yield via microbial biomass carbon and P limitation, which explained 62% variation in rice yield. Soil organic matter had a significant positive effect on rice yield, while the exchangeable acid concentration and vector angle had negative effects on it.

The combined application of mineral or organic soil conditioners with organic fertilizer following three successive years reduced soil acidity, improved soil organic matter and nutrient availability and mitigated microbial P limitation. The combination of the three amendments had the best improvement on rice yield, thereby can be recognized as an effective way for reducing acidity and improving soil fertility in red soil in the south China.