Effect of soil-N and urine-N on nitrate leaching under pure grass, pure clover and mixed grass/clover swards

During six annual drainage periods (DPO to DP5). the drainage water, the NO3 concentration of the: drainage water and the total leached N were compared under bare soils and under ryegrass/white clover, pure ryegrass and pure white clover stands in 80 deep lysimeters with 3m(2) area. For each soil cover, the sensitivity of the variables to the soil N supplying capacity at sowing was measured, using a set up of 32 lysimeters. This initial capacity to supply mineral N (SoilN) varied from 90 to 230 kg N ha(-1) year(-1). The stands were managed in a simulated rotational grazing system, without addition of fertilizer N. During the first drainage period after sowing (DP0), N leaching increased significantly with the initial SoilN under the bare soils, the pure grass and the mixture, but wits not influenced under the pure clover. in the following drainage periods, N leaching increased according to the sequence pure grass (1-5 kg N ha(-1) year(-1)), mixed swards (1-19 kg N), pure white clover(28-140 kg N) and bare soils (84-149 kgN ha(-1) year(-1)). It was only slightly greater under the mixture than under the pure grass. despite the N harvest and the N animal returns were much higher. Under the mixed stands, N leaching became independent of the initial SoilN in DP1 and DP2 and decreased with increasing initial SoilN in DP3, DP4 and DP5. This inversion of the SoilN effect in time and the limited amounts of leached N demonstrated that adaptations in the ecosystem tend to counteract the SoilN effect on the N losses. In the mixed stands, the accumulated N leaching represented 12 and 21% of the accumulated N at harvest for the initially rich and poor soils. respectively and 32% of the accumulated N harvest in the mixed clover, whatever the initial SoilN. N leaching also represented 13%, of the urine-N above 80 kgN ha(-1) year(-1). The low values of N leaching under the mixed swards make them sustainable for environment quality. Mechanisms which regulate the N fluxes are discussed, using published data on the soil and some results concerning the harvests in the same experiment. (C) 2001 Elsevier Science B.V. All rights, reserved.