Soil water balance and nitrate leaching in winter wheat-summer maize double-cropping systems with different irrigation and N fertilization in the North China Plain

Field experiments over a 3 y period were conducted in a winter wheat-maize double-cropping system at the Dongbeiwang Experimental Station, Beijing, China. Three different treatments of irrigation (sprinkler "suboptimal" and "optimized"; conventional flood irrigation) and N fertilization (none, according to N-min soil tests, conventional) were studied with respect to effects on soil water balance, nitrate leaching, and grain yield. Under sprinkler irrigation, evaporation losses were higher due to a more frequent water application. On the other hand, in this treatment nitrate leaching was smaller as compared to flood irrigation, where abundant seepage fluxes > 10 mm d(-1) along preferential flow paths occurred. For quantifying nitrate leaching, passive samplers filled with ion-exchange resins appeared to be better suited than a method which combined measurements of suction-cup concentrations with model-based soil water fluxes. As a result of the more balanced percolation regime (compared to that under conventional flood irrigation), there was a tendency of higher salt load of the soil solution in the rooting zone. Given a seepage rate of 50mm, a winter wheat grain production of 5-6 t ha(-1) required a total water addition of about 430 mm. Fertilizer treatments > 100 kg N ha(-1) did not result in any additional yield increase. An even balance between withdrawing and recharge of groundwater cannot be achieved with "optimized" irrigation, but with a reduction of evapotranspiration losses, adapted cropping systems, and/or by tapping water resources from reservoirs in more distant areas with surpluses.