Responses of soil properties, microbial community and crop yields to various rates of nitrogen fertilization in a wheat-maize cropping system in north-central China

Excessive nitrogen (N) fertilization is widespread in intensive double cropping system in China and assessment of changes in soil quality and crop production under various N application rates is important for N fertilizer management. A wheat (Triticum aestivum L.)-maize (Zea mays L.) rotation experiment was conducted from 2009 to 2012 in north-central China to study the effects of high N fertilization rates on soil chemical properties and microbial community, and to evaluate soil sustainability under reduced N inputs. The N rates tested were 0 (N0), 70 (N1), 140 (N2), 210 (N3), 280 (N4) and 350 kg N ha−1 (N5) in the maize season, and 0 (N0), 60 (N1), 120 (N2), 180 (N3), 240 (N4) and 300 kg N ha−1 (N5) in the wheat season, respectively. Soil NO3−-N in the 0-100 cm depth, and soil electrical conductivity (EC) and nitrification potential in the 0-20 cm were significantly increased, whilst pH in the 0-20 cm was decreased with increasing N application rates. In addition, the high rates of N fertilization (N4 and N5) increased soil fungal abundance and the ratio of fungi to bacteria compared with lower N rates and the N control; however, N rates did not influence abundance of soil bacteria and actinomycetes, and total phospholipid fatty acid (PLFA). The application of N at 180 and 210 kg N ha−1 during the wheat and maize seasons, respectively, sustained high yields and enhanced accumulated N recovery efficiency (RENac) compared with higher N rates. Our results indicated that the high N inputs degraded soil quality and changed microbial community structure. A 12.5-40% reduction in the farmers' conventional N application rates was practical to reduce excess N input while maintaining the sustainability of the wheat-maize cropping system in north-central China.