Mineral nitrogen in soil profiles under long-term wheat cultivation and alfalfa continuous cropping in the dry farming area

The distribution and accumulation of soil mineral N in soil profiles under different fertilization after 23-year wheat cultivation and alfalfa continuous cropping were studied, and as well as the effects of nitrate leaching in wheat cultivation and alfalfa continuous cropping. The results show that NO-3-N is centralized in 0-60 cm soil layer in the CK and P treatments after 23-year wheat cultivation and alfalfa continuous cropping, and the content of NO-3-N is less than 2 mg/kg below 60 cm-deep of soil profiles. Lots of NO-3-N are accumulated in 20-100 and 140-320 cm soil layers after 23-year wheat continuous cropping under the NPM treatments, and the accumulation rate of NO-3-N can reach to 42.12 kg/(ha·a). While, NO-3-N is centralized in 0-60 cm soil layer and is slightly accumulated in 200-300 cm soil layer after 23-year alfalfa continuous cropping under the NPM treatments, and the accumulation rate of NO-3-N is about 1.01 kg/(ha·a). In terms of residual NO-3-N rate in soil, it is not affected by wheat or alfalfa cultivation under no fertilization or P fertilization alone. However, the residual NO-3-N rate in 0-400 cm soil layer is increased under the NPM fertilization after 23-year wheat continuous cropping, and the differences of the residual NO-3-N rates are significant between the NPM treatment and the treatments CK and P. The residual NO-3-N rate is increased slightly under the NPM fertilization in the alfalfa fields, and the differences of NO-3-N accumulation are not significant between the NPM and CK, P treatments. The NH+4-N contents are about 10-20 mg/kg in all soil layers after 23-year wheat and alfalfa continuous cropping, and the phenomenon of NH+4-N accumulation is not found in soil profiles. Therefore, the residual NH+4-N rates are not affected significantly by the fertilization and cropping.