Positive feedback relationship between shrub encroachment and arbuscular mycorrhizal fungi in the Inner Mongolia grassland of northern China

Abstract

Arbuscular mycorrhizal fungi (AMF) are soil microorganisms symbiotic with plants and play critical roles in the growth and survival of their hosts. However, the influence of shrub encroachment on AMF remains unclear. We analysed the soil microbial biomass of fungi, especially AMF, under interspace grass patches and shrub patches during the process of shrub encroachment using phospholipid fatty acid (PLFA) analysis. The results illustrated that the soil microbial biomass of AMF gradually increased under the shrub patches and gradually decreased under the interspace grass patches during the process of shrub encroachment. The most obvious change in the soil microbial biomass of AMF was found from moderately disturbed grassland to severely disturbed grassland under interspace grass patches and shrub patches. Soil nutrients, especially soil organic carbon (SOC), were found in greater amounts under the shrub patches than under the interspace grass patches. Shrub encroachment positively affected the microbial biomass of AMF by increasing the SOC content, and the increase in the microbial biomass of AMF increased the soil available phosphorus (AP) content and further promoted the development of shrub encroachment. There was a positive feedback relationship between shrub encroachment and AMF. This feedback mechanism was only applicable to AMF and was not found for other fungi. Our results highlight that there is a linkage between shrub encroachment and the microbial biomass of AMF, which explains the potential irreversibility of the transition from grass dominance to shrub dominance in the case of shrub encroachment.