Nitrogen budgets in pluricompartmental systems
Increasing concern over the adverse environmental impact of intensive agriculture has led to pressure to develop more sustainable, integrated farming systems which have the potential to minimise the loss of nitrogen to the environment whilst maintaining productivity. Alternative systems include pluricompartmental systems, which combine more than one enterprise or species in a formalised design, such as a crop rotation or spatial arrangement e.g. agroforestry. Three approaches to nitrogen budgeting were developed and their ability to quantitatively describe nitrogen cycling in pluricompartmental systems tested. Budgets ranged in complexity from the EIO Budget, which accounted simply for purchases and sales of nitrogen over the farmgate, through the BIO Budget which included estimation of biological nitrogen fixation and attempted to partition losses into leaching and gaseous forms, to the TRIO Budget which also accounted for key soil processes. Unaccounted for N was attributed to leaching, however it is recognised that this is a predictor of leaching potential rather than actual loss. These approaches were tested in a range of systems; a mixed organic farm, a silvopastoral agroforestry system, organically and conventionally managed beef systems and a series of ley/arable rotations containing different proportions of ley. A simple model for predicting nitrogen fixation from yield was also developed and tested. The success of the different approaches was strongly dependent on the precise objective; whilst all the approaches were able to predict N use efficiency in economic terms, the TRIO Budget was likely to give the best estimate of potential N loss by leaching. The TRIO Budget was the only approach which quantified intercompartmental N fluxes, and it is suggested that this parameter may be a useful indicator of sustainability. Intercompartmental fluxes were particularly difficult to quantify in spatially pluricompartmental systems due to the importance of belowground processes in these systems.