Objective: Research on effects of monocropping or rotation of different crops on water and different forms of soil organic nitrogen (N) and mineral N in dryland soils is recognized to be of great significance in determining dryland crop sequences, increasing soil fertility, and optimizing nutrients management. Method: Dryland soil samples were taken from 6 cropping systems: fallowing, continuous wheat monocropping, continuous maize monocropping, continuous alfalfa cropping with no-till, pea-wheat-wheat-millet rotation and maize-wheat-wheat-millet rotation, from a 23-year long-term experimental site on the Loess Plateau, to study the effects of cropping systems on soil water, organic N, light fraction organic N and mineral N. Result: Results obtained showed that continuous long-term fallowing system contained the lowest organic N, light fraction organic N and mineral N. Long-period alfalfa cropping with no-till system could promote water storage in topsoil layers, and accumulation of organic and light fraction organic N in 0-20 cm soil layers, but increased exhaustion of water and mineral N in deep soil layers. Continuous monocropping of wheat or maize could all significantly increase soil organic and light fraction organic N contents, and the organic N were increased more by monocropping of wheat. Organic N contents in soil layers of two rotation systems showed no obvious different to monocropping of wheat. Light fraction organic N contents also were not obvious different between two rotations in 0-20 cm soil layers, but they were all significantly higher than wheat or maize monocropping. The amount of soil mineral N was found to depend on the status of crop growing or N fertilizer application at sampling time. Conclusion: Although long term monocropping of legumes without tillage can enhance organic N accumulation by increase the light fraction organic N in soil, the exhaustion of soil water in deep layers is also increased. Rotations of legumes with cereal crops or the shallow and deep root crop rotations are proved to be feasible measures to optimize soil water utilization, increase organic N accumulation in soil, and synergize soil N supply capacity.
