Response of the rhizosphere soil microbial diversity to different nitrogen and phosphorus application rates in a hulless barley and pea mixed-cropping system

Hulless barley (Hordeum vulgare) is a major agricultural resource in the Tibetan region. When planted in monoculture, practices such as overuse of fertilizer have led to declines in soil microbial diversity and eventually to decreased barley yield. When barley is planted in a mixed-cropping system with peas (Pisum sativum), previous studies have shown that this can ameliorate the deleterious effects of monoculture. We investigated the effects of various fertilizer addition rates on the bacterial and fungal communities in both monoculture and mixed-culture systems. Our results indicated that with increasing nitrogen and phosphorus application, the bacterial alpha-diversity in the rhizosphere of the mixed-cropping system initially increased and then decreased after reaching a maximum. The alpha-diversity of soil bacteria was higher under the mixed-cropping mode than under the monocropping mode, while the diversity of rhizosphere soil fungi in the mixed-cropping system initially increased and then decreased. We suggest that these results for mixed-cropping mode were due to changes in soil organic matter, available phosphorus, and alkaline hydrolyzable nitrogen.