Climatic effects on soil organic nitrogen fractions and amino acid chirality in paddy soils

Soil organic nitrogen (ON) accounts for more than 90% of the total nitrogen (TN) in paddy soils. Inadequate understanding of the different ON fractions in paddy soils and their corresponding bioavailability under different climatic conditions has constrained the development of appropriate nutrient management strategies for rice production. In this study, we applied a modified Bremner method coupled with high-performance liquid chromatography to characterize how soil ON fractions and amino acid chirality varied under different climatic conditions at five typical rice production sites along a latitudinal gradient. According to the results, climate had no obvious influence on TN, nitrogen (N) form, and individual amino acid contents. However, the proportions of various N forms in TN had linear relationships with annual mean temperature (AMT), with high correlation coefficient (r) values. Amino acid components also exhibited similar trends, with r as high as 0.85. Most notably, consistent linear relationships were observed between the D/L ratios of several amino acids and AMT in paddy soils (r = 0.18-0.92). Findings of this study provide insights into ON and amino acid dynamics in paddy soil systems under intensive production along climate gradients.