Nitrogen and phosphorus budgets for the sub-tropical Richmond River catchment, Australia

Nitrogen and phosphorus budgets were developed for four sub-catchments in the Richmond River catchment for two study years. The catchment is used for a variety of farming pursuits including dairying, beef, cropping, fruit, nuts, forestry, and sugar cane. Each sub-catchment varies in hydrology, the proportion of each land use, and the population density which enabled a unique opportunity to study fluxes and storage associated with a variety of environmental factors. Total loadings entering each sub-catchment varied from 12 to 57 kg ha(-1)yr(-1) for nitrogen and 0.25 to 6.6 kg ha(-1)yr(-1) for phosphorus with little inter-annual variation. Averaged across the whole catchment, nitrogen fixation (47%) dominated the inputs; fertiliser (26%) and rainfall (21%) made up the next largest inputs. Fertiliser inputs dominated the phosphorus budget (65.5%); rainfall and manures making up 13% and 12% respectively. Produce dominated the outputs of both nitrogen and phosphorus from the four sub-catchments being greater than the riverine export. The delivery of nitrogen to catchment streams ranged from < 1 to 24% of the total inputs and the delivery of phosphorus to catchment streams ranged from < 1 to 39%. Storage of phosphorus in catchment soils varied between -0.32 and 4.46 kg ha(-1)yr(-1). When denitrification and volatilisation were estimated using data from other studies, storage of nitrogen ranged from 1 to 24 kg ha(-1)yr(-1). Despite the episodic nature of runoff in the sub-tropical Richmond River catchment, the magnitude of nutrient fluxes and storage appear similar to other catchments of the world which have mixed land use and relatively low catchment nutrient loadings.