Surface N balance
Often farm-gate or whole system budgets, or balances, are created to study the environmental impact of N cycle acceleration in agricultural systems. These budgets [correct antecedent?] take into consideration the N input, and the output and total loss for the farm or system considered, without specifying where and in which form losses occur. Other studies use a lumped approach to estimate country surface N balance surpluses, without specifying the fate of the surplus N.
Geographically distributed N budgets for agro-ecosystems
PBL researchers focus on the geographic distribution of the balance and fate of nitrogen in the environment (Bouwman et al., 2005c). The surface N balance approach, which considers all relevant input, output and loss terms for a given land area, is therefore more appropriate than farm-gate or system balances. The budgets include both N inputs (N fertiliser, animal manure, biological N fixation and atmospheric N deposition) and outputs (including N removal from the field in harvested crops and grass, and grass consumption by grazing animals, along with ammonia volatilisation, denitrification and leaching). The surface N balance approach has now been incorporated into IMAGE 2.4 for calculating and presenting geographically distributed N budgets for agro-ecosystems (PBL ,then MNP, 2006).
Surface N balancing is based on the distribution of different crop and livestock production systems (mixed, landless and pastoral), as published in various PBL (then MNP) papers articles(Bouwman et al., 2005a; Bouwman et al., 2006b; Eickhout et al., 2006). Pastoral systems depend almost exclusively on grazing, while mixed and landless systems rely on a mix of concentrates (food crops) and roughage, consisting of grass, fodder crops, crop residues and other sources of feedstuffs. A model was developed to describe these two aggregated production systems for different world regions - each system with typical production characteristics, for example, milk production per animal for dairy cattle, and off-take rates and carcass weights for non-dairy cattle, sheep and goats.
