Changes in rhizosphere soil nitrogen fractions associated with enzyme activities are linked to the microbial community following intercropping combined with nitrogen fertilization

Cereal/legume intercropping is effective for reducing chemical fertilizer inputs and achieving sustainable development. However, the effects of intercropping combined with nitrogen (N) fertilization on rhizosphere soil microbial characteristics and N dynamics–enzyme activity relationships remain unclear. A field experiment was performed to analyze rhizosphere soil N fractions, related enzyme activities, and microbial communities on the Chinese Loess Plateau. The results showed that proso millet/mung bean intercropping (PM) combined with suitable N fertilization increased the total nitrogen (TN), ammonium (NH4+), nitrate (NO3−), and microbial biomass nitrogen (MBN) contents and enzyme activities, whereas the high N input (especially for N3) showed a decreasing trend. MBN was more closely associated with enzyme activities than were other N fractions. Intercropping and N fertilization strongly affected the bacterial and fungal diversity and community composition, as shown by a shift from K– to r–strategist dominance. Proteobacteria was the dominant bacterial phylum and was more abundant under intercropping than under sole cropping (SP), whereas Acidobacteria and Chloroflexi abundances were lower under PM than under SP. Ascomycota and Mortierellomycota were dominant fungal phyla whose abundances were significantly associated with rhizosphere soil N fractions and enzyme activities, although a high N input may inhibit microbial growth under intercropping systems. Overall, the observed changes in rhizosphere soil enzyme activities were most strongly affected by MBN driven by soil bacteria and fungi. These results provide scientific guidance for using rational fertilizer and further studying the mechanisms allowing reduced fertilizer use and increased efficiency under intercropping combined with N management.