Discharges of nitrogen, phosphorus (P) and carbon from point sources have been subject of controls for many years but losses from non-point sources are now under scrutiny (Nash et al 2002). In southern Australia, dissolved organic matter (DOC) and P are significant contributors to low water quality in reservoirs. Some streams draining the Mount Lofty Ranges watershed in South Australia, for example, have four times the recommended US Environmental Protection Agency levels of P, and levels of DOC above 20 mg.L-1. Whereas climate and soil type are significant factors in the movement of these components from soils to waterways there is little doubt that diverse human activities in catchments have exacerbated this transport. Soil and land management offer the prospect of significantly reducing the problem.

Calcium carbonate and gypsum have been used to enhance P retention but there are few studies on their efficacy in reducing C movement. Here we report the use of calcium amendments for retention of natural organic matter (NOM) and P in soils (agriculture and silviculture) and reduction in the movement of DOC and P from their source (soils) to streams and reservoirs. In a recent paper (Cox et al 2005) we reported a preliminary experiment of Nelson et al (1991) in which DOC was reduced by 44% in extracts from Alfisols treated with gypsum.

We investigated the effect of gypsum on the movement of NOM and P in catchments by quantifying the amount and pathways of organic carbon (OC) and P movement in paired catchments, on small hillslope plots and in lysimeters. We show that throughflow (as opposed to surface erosion) is an important pathway in some landscapes. The potential for reducing the flux of these components by the application of a surface amendment (gypsum) was studied. Over four seasons, soils in the catchment treated with gypsum attained markedly higher stores of OC compared with the control catchment and surface and soil interflows had much lower concentrations of NOM. Calcium amendments in the form of gypsum can significantly attenuate the transport of NOM from soils and potentially improve the quality of water draining from a catchment. Widespread adoption of amendment technology will be dependent on not only an increase in water quality but also an increase in agricultural productivity.