Objectives Nitrogen mineralization is an important ecological process that determines ability of soil nitrogen supply. Nutrient elements addition and soil water content play important roles in regulating soil nitrogen transformation. Effect of nitrogen application and soil water content on soil nitrogen mineralization under ryegrass returning is beneficial to further optimize the management of water and fertilizer in upland red soil.

Methods A laboratory experiment was conducted to study the effects of nitrogen rates and soil water contents on the soil net nitrification, ammonification and nitrogen mineralization under ryegrass (Lolium perenne L.) retuning into red soil.

Results The results showed that under the 15% soil water content, the nitrogen application improved soil net nitrification during the early stage of ryegrass returning, while in the late stage of ryegrass returning, the 120 mg/kg nitrogen application inhibited the soil net nitrification. Under the soil water content 30%, the 120 mg/kg nitrogen application significantly inhibited soil nitrification during the late stage of ryegrass returning. The soil water content of 45% inhibited the soil net nitrification at different nitrogen rates in the early stage of ryegrass returning, but increased the soil net nitrification at 91 d, meanwhile, the soil net nitrification under the nitrogen application of 60 mg/kg was significantly higher than that under 120 mg/kg. The soil net ammonium was positive in the whole process of ryegrass returning, and was in a dynamic change of rise- fall reciprocating. Under the three soil water contents, the soil net ammonium under the nitrogen application was significantly higher than that without nitrogen application. The increase of soil water content was beneficial to increase the soil ammoniation during the early stage of ryegrass returning under the 120 mg/kg nitrogen application, but decrease the soil net ammonia content during the later stage of ryegrass returning. Compared with soil net nitrogen mineralization without nitrogen application, it was increased with the nitrogen application under the three water contents during the most process of ryegrass returning. The 30% soil water content had the greatest change of soil net nitrogen mineralization. Compared with the soil water content of 15%, the 30% water content promoted soil net nitrogen mineralization during the middle stage of ryegrass returning (13–57 d), but the 45% water content inhibited soil net nitrogen mineralization during the late stage of ryegrass returning (73–91 d).

Conclusions Reasonable chemical nitrogen fertilizer (60 mg/kg) should be applied during the ryegrass returning into red upland soil. Keeping high soil water content could inhibit soil nitrogen mineralization at the early stage of ryegrass returning. Reducing soil water content (30%) could increase soil nitrogen mineralization at the middle and late stages of ryegrass returning.