Amino acid nitrogen trends in paddy soils under long-term rice cultivation in southeast coast of China

Abstract

In China, tidal flats in some coastal areas have been converted into paddy fields. Such reclamation and prolonged rice cultivation activities could influence soil amino acid nitrogen (N) (AAN) contents and N cycling. However, the effects of long-term rice cultivation on the contents and proportions of AAN in paddy soils remain poorly understood. Paddy soil samples were collected in a chronosequence cultivated for 50 (P50), 100 (P100), 300 (P300), and 700 (P700) years to elucidate the responses of AAN to long-term rice cultivation. Soils from a neighboring tidal flat (T0) and salt marsh (10 years after diking, S10) used as controls. AAN, total amino acid (AA), and free amino acid (FAA) contents and proportions, as well as N-related enzyme activity and soil properties were analyzed. AAN content significantly (p < 0.05) increased from 0.12 g kg−1 in T0 to 0.54 g kg−1 in P300, then slightly decreased to 0.47 g kg−1 in P700. The enantiomer L type-AA and L type-FAA contents were higher than D type-AA and D type-FAA contents in all soils. Urease and protease activities increased sharply within 100 years and then gradually declined, while urease activity remained steady over time. Regression analysis revealed linear relationships between AAN and pH, soil organic carbon, total N, microbial biomass carbon, microbial biomass N, and urease and protease activities. Overall, AAN was accumulated under long-term rice cultivation, and was closely linked to the soil properties. However, AAN accumulation was limited to 300 years, highlighting the potential effects of long-term rice cultivation on the stability of soil N cycling and sustainable N-use.