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

Abstract

During six annual drainage periods (DP O to DP 5), the drainage water, the NO 3 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 (DP 0), N leaching increased significantly with the initial SoilN under the bare soils, the pure grass and the mixture, but was 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 DP 1 and DP 2 and decreased with increasing initial SoilN in DP 3, DP 4 and DP 5. 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.