Polyaspartic calcium improved soil quality and altered nitrification process in saline-sodic paddy soils

Saline-sodic paddy soils in the Songnen Plain suffer from nitrogen loss due to nitrification. The purpose of the study is to explore soil saline improvement and nitrification mitigation effects of polyaspartic calcium (PASP-Ca) by evaluating changes of soil quality, nitrification, and microbial communities. Four PASP-Ca application treatments (additions of 0, 500, 1000, and 1500 kg hm⁻²) were studied in an experiment in saline-sodic paddy soils of the Songnen Plain, China. Results showed that PASP-Ca application significantly decreased soil pH, electrical conductivity (EC), and water-soluble salt ions, and significantly increased soil total carbon (TC), total nitrogen (TN), urease activity (UA), and sucrase activity (SA). PASP-Ca application significantly slowed down soil nitrification, which was manifested in a significant increase in ammonium nitrogen (${\mathrm{NH}}_4^{+}-N$) and a significant decrease in nitrate nitrogen (${\mathrm{NO}}_3^{-}-N$) and ammonia monooxygenase activity (AMO). The composition and distribution of soil nitrifying microbial communities were affected by soil salinity, nutrient, and enzyme activities. Ammonia-oxidizing bacteria (AOB) plays an important role in the nitrification process of saline-sodic paddy soils, while PASP-Ca application significantly inhibited nitrification by suppressing AOB amoA gene abundance. This study shows that PASP-Ca, as an effective amendment, can improve soil salinization and slow down nitrification, which has an important role and significance in improving nitrogen utilization and reducing nitrogen loss of saline-sodic soils.