Chapter 3.3. Nitrogen balance in cropping systems that include legume

This session explores the data required to quantify a system's nitrogen balance using crop legumes as an example, with particular emphasis on the methodologies that might be used to quantify the below ground contributions of nitrogen associated with roots and root nodules. There are a number of pathways by which nitrogen (N) inputs via biological N2 fixation (BNF) can improve the N fertility of soils for the benefit of other crops (Table I). However, the actual contribution of a legume-rhizobia symbiosis to the N balance of an ecosystem will primarily be determined by the difference between the total input of fixed N and the amount of N removed in agricultural produce or plant residues. The principles will be the same, regardless of whether the legumes are annuals or perennials grown for grain, as a green manure, or for forage, although, in the case of grazed forage or pasture legumes, estimates of N losses from urine patches should be included in the calculations