Using nitrogen budgets to indicate nitrogen use efficiency and losses from whole farm systems: a comparison of three methodological approaches

Three approaches to nitrogen budgeting were developed and their ability to quantitatively describe nitrogen cycling in a fertilizer based and a grass-clover based beef system tested. Budgets ranged in complexity from the Economic Input:Output (EIO) budget, which accounted simply for purchases and sales of nitrogen over the farmgate, through the Biological Input:Output (BIO) budget, which included estimates of biological nitrogen fixation and attempted to partition losses into leaching and gaseous forms, to the Transfer:Recycle:Input:Output (TRIO) budget, which also accounted for key soil processes. Nitrogen unaccounted for in the fertilized system decreased with increasing budget complexity (285, 212 and 188 kg ha(-1) yr(-1) unaccounted for by the EIO, BIO and TRIO budgets, respectively). In the legume based grass-clover system, the EIO budget did not accurately describe total nitrogen inputs as it did not include 146 kg ha(-1) yr(-1) from symbiotic nitrogen fixation. In the grass-clover system, nitrogen unaccounted for was again greater using the BIO than the TRIO budget (103 and 79 kg ha(-1) yr(-1), respectively). In conclusion, the most complex budgeting approach (TRIO) was able to account for the fate of a greater proportion of nitrogen inputs than the simpler approaches. However, the perceived success of the different approaches was strongly dependent on the precise objective.