Nitrogen application and intercropping change microbial community diversity and physicochemical characteristics in mulberry and alfalfa rhizosphere soil

Abstract

Intercropping of mulberry (Morus alba L.) and alfalfa (Medicago sativa L.) is a new forestry-grass compound model in China, which can provide high forage yields with high protein. Nitrogen application is one of the important factors determining the production and quality of this system. To elucidate the advantages of intercropping and nitrogen application, we analyzed the changes of physicochemical properties, enzyme activities, and microbial communities in the rhizosphere soil. We used principal components analysis (PCA) and redundancy\xa0discriminators analysis to clarify the relationships among treatments and between treatments and environmental factors, respectively. The results showed that nitrogen application significantly increased pH value, available nitrogen content, soil water content\xa0(SWC), and urea\xa0(URE) activity in rhizosphere soil of monoculture mulberry. In contrast, intercropping and intercropping\u2009+\u2009N significantly decreased pH and SWC in mulberry treatments. Nitrogen, intercropping and intercropping\u2009+\u2009N sharply reduced soil organic matter content and SWC in alfalfa treatments. Nitrogen, intercropping, and intercropping\u2009+\u2009N increased the values of McIntosh diversity (U), Simpson diversity (D), and Shannon–Weaver diversity (H′) in mulberry treatments. However, PCA scatter plots showed clustering of monoculture mulberry with nitrogen (MNE) and intercropping mulberry without nitrogen (M0). Intercropping reduced both H′ and D but nitrogen application showed no effect on diversity of microbial communities in alfalfa. There were obvious differences in using the six types of carbon sources between mulberry and alfalfa treatments. Nitrogen and intercropping increased the numbers of sole carbon substrate in mulberry treatments where the relative use rate exceeded 4%. While the numbers declined in alfalfa with nitrogen and intercropping. RDA indicated that URE was positive when intercropping mulberry was treated with nitrogen, but was negative in monoculture alfalfa treated with nitrogen. Soil pH and SWC were positive with mulberry treatments but were negative with alfalfa treatments. Intercropping with alfalfa benefited mulberry in the absence of nitrogen application. Intercropping with alfalfa and nitrogen application could improve the microbial community function and diversity in rhizosphere soil of mulberry. The microbial community in rhizosphere soil of mulberry and alfalfa is strategically complementary in terms of using carbon sources.