Objective This study aims to measure the effects of green manure and chemical nitrogen fertilization on soil nitrogen fractions in winter wheat fields under dry farming conditions, and to explore ways to improve soil nitrogen supply capacity in dryland cropping systems.

Methods The field experiment for winter wheat-summer green manure began in 2017, utilizing a two-factor design involving green manure and nitrogen fertilization. Green manure, as the main treatment, included Sudan grass (SG), Changwu soybean (SB), a mixture of Sudan grass and Changwu soybean (MIX), and a control without green manure (CK). Chemical nitrogen fertilizers were applied to winter wheat at rates of 0, 60, and 120 kg/hm². At wheat harvest in June 2023, soil samples were collected at depths of 0−10, 10−20, and 20−40 cm to measure soil total nitrogen (STN), particulate organic nitrogen (PON), potential nitrogen mineralization (PNM), and microbial biomass nitrogen (MBN) content, respectively.

Results Compared to CK, green manure treatments had no significant effect on STN at all depths but significantly increased PON content by 25.8%−54.8% at 0−10 cm, PNM contents by 9.7%−14.1% and 11.2%−13.7% at 0−10 and 10−20 cm, and MBN contents by 15.1%−24.11%, 18.2%−28.8%, and 22.3%−32.5% at 0−10, 10−20, and 20−40 cm, respectively. Compared to SG, both SB and MIX increased PON content by 23.1% at 0−10 cm, and SB also had higher MBN contents by 7.79%−8.93% at 0−40 cm. No significant differences in PNM contents were found among treatments at all depths. The relative contents of soil nitrogen fractions were in the order of PON>MBN>PNM at all soil depths. Compared to CK, green manure significantly increased the relative contents of PON, PNM, and MBN at 0−10 cm, the relative contents of PNM and MBN at 10−20 cm, and the relative contents of PON and MBN at 20−40 cm. Nitrogen application increased nitrogen fractions and their relative contents at 0−20 cm but had no significant effect at 20−40 cm. Correlation analysis indicated that PON, PNM, and MBN were significantly positively correlated with mean annual nitrogen input, except for STN. The contents of STN, PON, PNM, and MBN were significantly positively correlated with each other, and PON was more capable of indicating changes in soil nitrogen supply capacity than other fractions.

Conclusions Summer green manure and nitrogen application can significantly increase intermediate labile and liable nitrogen fractions and their relative contents at 0-20 cm in the dryland winter wheat system. Among the treatments, the combination of Changwu soybean as green manure and chemical nitrogen fertilization at 120 kg/hm2 is a reliable practice to enhance soil nitrogen supply capacity.