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 practic e difficult to quantify the contribution of nitrogen from the atmosphe re 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 c alibrated to a range of production conditions in Denmark. A statistica l analysis of three experiments showed that the content of clover made a satisfactory description of the variation in atmospherically-derive d nitrogen above stubble height (harvested fixed-N-2). The variation i n harvested fixed-N-2 was considerable (38-208 kg N ha(-1) year(-1)) d ue to experimental manipulation by rate of N-fertilizer, cutting frequ ency, soil type and irrigation. It was, however, also found that the t ime after establishment of the grass-clover leys has a significant eff ect 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 nitro gen below stubble height. It is concluded that within the first two ye ars 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 clo ver 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 t he first two years and 47, 84 or 128 in the 3rd, 4th and 5th years.