Effects of Cropping Systems on Soil Physicochemical Properties and Abundances and Spatial Distributions of Nitrogen-Cycling Bacteria

Soil nitrogen (N) is a common limiting factor where soil N-cycling is a key component of agroecosystems. Soil N transformation processes are largely mediated by microbes, and understanding bacteria involvement in soil N-cycling in agricultural systems has both agronomic and environmental importance. This 2 yr field-scale study examined the abundances and spatial distributions of the total bacterial community (16S rRNA), bacteria involved in nitrification (amoA) and denitrification (narG, nirK, and nosZ), and soil physicochemical properties of winter wheat (Triticum aestivum L.)-soybean (Glycine max L.) double-crop with 2-3 weeks of spring grazing (WGS) and without grazing (WS) and tall fescue (Festuca arundinacea (L.) Schreb.) pasture (TF) managed to near-natural conditions with similar grazing. The TF soil had a significantly higher abundance of 16S rRNA, amoA, narG, nirK, and nosZ genes than the WS and WGS soils, which had similar levels between themselves. Soil organic matter (OM) and soil pH had stronger effects on the N-cycling bacteria gene abundance. All bacterial gene concentrations and soil pH showed nonrandom distribution patterns with a 141-186 m range autocorrelation. These results indicate that biological N transformation processes are more important in natural agricultural systems and the abundance of N-cycling bacteria can be manipulated by field-scale management strategies.