Characteristics and influencing factors of soil nitrogen mineralization and nitrification in saline-sodic paddy fields

Objectives Nitrogen mineralization and nitrification under different saline conditions and the relationships between the two processes and soil salinity and nutrient content were studied to clarify the effect of soil salinization on nitrogen transformation, to provide a theoretical basis and data support for the scientific management of soil nutrients.

Methods Thirty paddy soil samples from at 0–20 cm depth with different degrees of salinization were randomly collected. According to the degree of salinization, the collected soil samples were divided into light (salt 0.1%–0.3%, exchange sodium percentage 5%–15%), moderate (salt 0.3%–0.5%, exchange sodium percentage 15%–30%) and severe (salt 0.5%–0.7%, exchange sodium percentage 30%–45%) saline-sodic soils. The incubation experiments for N mineralization and nitrification were carried out in the laboratory (25℃, 24 hours illumination). The NH₄⁺-N and NO₃^–-N contents and urease and alkaline protease activities were measured on 0, 3, 6, 9, 15 and 21 days of incubation. The correlation between soil indexes and nitrogen mineralization and nitrification was studied by correlation analysis. The main influencing factors of soil nitrogen mineralization and nitrification were screened by stepwise regression analysis.

Results With the intensification of soil salinity, nitrogen mineralization and nitrification decreased significantly (P<0.05). The maximum net mineralization rate, cumulative mineralization, maximum net nitrification rate, cumulative nitrification in moderate and severe saline-sodic soils were 12.7% and 29.8%, 15.7% and 25.2%, 15.4% and 23.1%, 15.4% and 23.1% lower than those of light saline-sodic soils, respectively (P<0.05). The maximum activities of urease and alkaline protease were 16.0% and 34.8%, 6.0% and 15.6% lower than those of light saline-sodic soils, respectively. Stepwise regression analysis showed that soil electrical conductivity (EC), pH, CO₃^2–, Na⁺, total N and organic matter contents were the main factors affecting soil nitrogen mineralization, andEC, pH, CO₃^2–, Na⁺ and organic matter contents were the main factors affecting soil nitrogen nitrification.

Conclusions With the increase in soil salinization, soil net nitrogen mineralization rate, net nitrification rate, cumulative mineralized nitrogen, cumulative nitrification nitrogen, urease and alkaline protease activities decrease, and soil salinization significantly inhibits nitrogen mineralization and nitrification.