Leaching and Uptake of Nitrogen Applied to Annual Grass and Clover‐Grass Mixtures in Lysimeters¹

Abstract

Abstract The annual grassland soils of California are nearly always N deficient. If early winter feed is desired and total production is to be increased, N must be added, either by a legume or through N fertilization. Comparison of these two N sources in terms of forage and protein production and groundwater pollution is important for efficient management of land, water, and N resources. This study was initiated to measure the effect of N fertilization on forage yields, N‐uptake by the plants, and N leached from the soil where grass grew alone or from clover‐grass mixtures. Grass (soft chess, Bromus mollis L.) growing alone, grass plus subclover ( Trifolium subterraneum L.), and grass plus rose clover ( T. hirtum All.) were grown in lysimeters on Josephine soil with and without N applied in the autumn of each of 4 years. Nitrogen fertilization increased winter forage yields and N‐uptake in all species mixtures. However, in the spring harvest, clover‐grass mixtures yielded as much forage and N‐uptake without N as with N applied. Apparent recovery of the applied N in the forage was 25% for the grass, −2% for the subclover‐grass, and 0% for the rose clovergrass. The amount of apparent fertilizer N found in drainage water was 37% from grass, 58% from subclover‐grass, and 50% under rose clover‐grass. Without N fertilization the N leached was about the same from clovergrass as from grass alone. Of total N leached, 94% was in the fall, 6% in winter, and less than 1% in spring, whereas the total drainage water was divided 43% in the fall, 54% in whiter, and 3% in spring. With N fertilization, NO3‐N in the water in the fall was 45 ppm from clover‐grass and 38 ppm from grass. Without applied N, the NO 3 ‐N values were 25 ppm in the fall and were 1/10 as great in winter and about 1/100 in spring. There was no rain in the summer.