Soil organic nitrogen composition and mineralization of paddy soils in a cultivation chronosequence in China

Soil organic nitrogen (SON) mineralization is a crucial process of nutrient dynamics for plant growth. However, impacts of long-term rice cultivation on SON composition and mineralization in paddy soils are few. This study examines the influence of long-term rice cultivation on SON composition and mineralization. The results provide useful data for long-term soil N fertility development and N biogeochemistry turnover in paddy soils. A chronosequence of rice paddy soils (50, 100, 300, and 700 years) was selected from uniform marine deposits with identical landscape and climate conditions. The paddy soil samples were obtained from Cixi, Zhejiang Province, China. SON of paddy soils, salt marsh soils (young soil), and tidal flat soils (control) was fractionized by the modified Bremner method. N mineralization was measured using a waterlogged incubation method. Soil total nitrogen (TN) increased from 0.47 to 2.00 g kg(-1) with cultivation time, whereas soil pH dropped from 8.5 to 6.8. SON and its fractions (amino acid N, ammonium N, amino sugar N, and hydrolyzable unknown N) exhibited an exponential growth with cultivation time and stabilized rapidly at approximately 100 years, which might be caused by the paddy management such as fertilization, flooding, plowing in practice. However, the proportion of various fractionized N to TN did not follow a uniform trend with the cultivation time, suggesting the local paddy environment exerted an important impact on SON composition. Soil microbial C and N showed the same pattern with TN. The soil net mineralization rate of paddy soils was higher than those of salt marine and tidal flat soils. However, no significant difference was observed among the paddy soils with various cultivation times. These results demonstrate that reclamation and rice cultivation could improve soil N accumulation and availability. N stabilization was achieved within 100 years. Rice cultivation could maintain the sustainable long-term use of soil N.