A SIMPLE-MODEL FOR ESTIMATION OF ATMOSPHERICALLY-DERIVED NITROGEN IN GRASS-CLOVER SYSTEMS

Atmospherically-derived nitrogen can be a major source of nitrogen in organic or other low-input farming systems. lt is, however, in practice difficult to quantify the contribution of nitrogen from the atmosphere especially when dealing with grass-clover leys in the crop rotation. In this study a simple model based on knowledge about the content of clover in herbage and the time after establishment is presented and calibrated to a range of production conditions in Denmark. A statistical analysis of three experiments showed that the content of clover made a satisfactory description of the variation in atmospherically-derived nitrogen above stubble height (harvested fixed-N-2). The variation in harvested fixed-N-2 was considerable (38-208 kg N ha(-1) year(-1)) due to experimental manipulation by rate of N-fertilizer, cutting frequency, soil type and irrigation. It was, however, also found that the time after establishment of the grass-clover leys has a significant effect in that way, that at a given content of clover harvested fixed-N-2 in the 3rd, 4th and 5th cropping year was only 2/3 of that in the 1st and 2nd cropping year. Mineralization and immobilization of nitrogen are discussed to predict accumulation of atmospherically-derived nitrogen below stubble height. It is concluded that within the first two years after establishment there is a net accumulation of fixed-N-2 below stubble height corresponding to 24-29% of harvested fixed-N-2. After the 2nd year the total fixed-N-2 is assumed to correspond directly to harvested fixed-N-2. Based on three classes of visual estimates of clover content (10-29%, 30-49% or above 49%), the atmospherically-derived nitrogen was estimated to be (kg ha(-1) year(-1)) 80, 157 or 248 in the first two years and 47, 84 or 128 in the 3rd, 4th and 5th years.