Typology of nitrogen balances on a farm scale: research of operating indicators.

A farm. survey based. on the method of the nitrogen apparent balance at the farm scale quantified the nitrogen inputs and outputs due to the farm comm ercial transactions. A total of 555 farms, partly situated in France and pa rtly in north-west Italy, was analyzed. This method considers the farm. as a "black box" and also takes the nitrogen fixation into account. However, n itrogen input due to atmospheric deposition, or nitrogen losses due to vola tilisation, denitrification or leaching are disregarded: this is the reason why it is called "apparent balance". The study quantified the excess of ni trogen and its most: important sources of variation in ii important, types of farm production systems including stockless fanning, crop-less intensive rearing, forage crop-livestock integrated farming systems, mixed farming w ith livestock. The surplus of N was smaller in the stockless fanning system (50 kg N ha(-1) y(-1), on average), intermediate in dairy cow or beef bree ding farms (from 30 to 250 kg ha(-1)), and higher in intensive stocking sys tems where forage was not produced in the farm (from 250 to 500 kg-ha(-1), or even more). in general, where the apparent N surplus was small (< 200 kg ha(-1)) the excess of N was mainly caused by the purchase of mineral ferti lizers, while in farms with a high surplus (>200 kg h(-1)) the N surplus wa s highly dependent on N input of purchased animal feedstuffs. Two groups of farming systems were clearly identified: one was oriented to crop producti on and was characterized by a favourable N efficiency or N conversion (72%) ; the other one was oriented to livestock production, with very variable N fluxes, but little outputs, and was characterized by lower values (22-44%). The very large variability in N surplus and in indicators (N efficiency, N conversion and N losses) suggests that in many situations it should be pos sible to reduce the environmental impact of a farm without drastic changes in the production system.