Soil pH and C/N ratio determines spatial variations in soil microbial communities and enzymatic activities of the agricultural ecosystems in Northeast China: Jilin Province case

Soil microbes and their functions play important roles in the sustainable development of agricultural ecosystems but are vulnerable to climate change and human activity. We measured the contents of microbial phospholipid fatty acids (PLFAs) and enzymatic activities in various soil types (dark-brown soil, albic soils, black soil, chestnut soil, and chernozem) in three climatic zones (humid zone, subhumid zone, and semi-arid zone) of the Songnen Plain in northeastern China. The objectives were to investigate the spatial variations in community structure of soil microbes and to evaluate the functional potential of the microbes in croplands in northeastern China. The C-acquiring activities were highest in the chernozem soil of the semi-arid zone, but N-acquiring and P-acquiring activities and peroxidase were highest in the albic soil of the humid and subhumid zone. Phosphorus deficiencies were indicated by the reduced ratios of N- relative to P-acquiring activity and C-relative to P-acquiring activity in the croplands compared with the global average. Different groups of PLFA contents were highest in the dark-brown soil of the humid zone and the chestnut soil of the semi-arid zone. The community compositions of the microbes varied amongst the soil types and climatic zones, with the ratios of fungi/bacteria and Gram-positive/Gram-negative bacteria highest in chernozemic soil of the semi-arid zone. The variations in soil enzyme activities and microbial community structure were more dependent on climate conditions than soil type. Soil pH, soil organic C content, and the C/N ratio were the most important factors affecting the spatial patterns of the soil enzyme activities and microbial-community compositions across soil types and climatic zones. Mean annual precipitation and clay content strongly influenced enzymatic activities and microbial-community compositions, respectively, when each variable was used as the only factor. Their effects, however, were negligible when combined with other factors. Balanced chemical fertilizer with scientific N-P-K ratio should be applied instead of only N fertilizers. The other combination of different conservational agricultural practices, such as application of organic fertilizer, not tilling, crop rotation, and intercropping, would be more beneficial for restoring the community compositions and functions of soil microbes and achieving sustainable agricultural development.