Effects of the conversion of cropland to grassland on soil microbial and nematode communities in the Danjiangkou water source conservation area

Studying the changes in characteristics and mechanisms of interaction between the soil microbial and nematode communities resulting from the conversion of cropland to grassland in the Danjiangkou water conservation area can provide primary data for comprehensive analysis and evaluation of the ecological effect of soil. Additionally, this can offer a scientific basis for the regional ecological environmental protection and sustainable land use. In September 2017, we chose three representative Alfalfa (Medicago sativa) grasslands (three years) for the conversion of cropland to grassland. The plots are located upstream of the Danjiangkou water conservation area. We also chose an uncultivated maize (Zea mays) field as the control. In this study, we examined the effects of the conversion of farmland to grassland on the microbial organisms present in the soil and the interactions between the soil microbial and nematode communities. The results showed that the soil microbial community structure changed greatly after the cropland was converted into grassland in the Danjiangkou water source conservation area. The amount of total microbial phospholipid fatty acids in the 0 – 10 and 10 – 20 cm soil layers increased significantly by 59.13% and 62.58%, respectively (P < 0.05). The soil organic carbon, total nitrogen, and nitrate nitrogen contents had a positive correlation with the total phospholipid fatty acids (PLFAs) (P < 0.01), whereas, the soil pH and C/N negatively correlated with the total PLFAs (P < 0.05). As the main components of the soil micro-food web, there is an interaction between the nematodes and the microbial community. The soil G^– bacteria had a negative relationship with the abundance of bacterivores (P < 0.05), and a positive correlation with the abundance of fungivores (P < 0.05). Moreover, a significant positive correlation was observed between the G^– bacteria and nematode ecological index (EI and SI) (P < 0.05). This study indicated that the conversion of cropland to grassland changed the micro-food web structure, and soil nematode contributed more to the stability of the soil micro-food web.