Modelling regional-scale mass balances of phosphorus, cadmium and zinc fluxes on arable and dairy farms

Abstract

Accumulation of heavy metals in agricultural soils may cause serious problems for soil quality, groundwater and food chains. Based on the databases of the Swiss agricultural statistics and on data from experimental farms, we modelled regional-scale mass balances of phosphorus (P), cadmium (Cd) and zinc (Zn) and their uncertainties for a large number of arable and dairy farms in the northwest of Switzerland using the model PROTERRA-S. Large differences in P fertilization characteristics between these farm types were found. Crop demand for P on arable farms were primarily met through additions of inorganic fertilizers and sewage sludge, while on dairy farms the largest P input was provided by animal manure. Average model calculations predicted an enrichment between 0.8 and 1.9 g ha −1 per year for Cd and between 76 and 525 g ha −1 per year for Zn for soils of arable and dairy farms, respectively. Hence, according to our analyses, current agricultural land use is not sustainable in terms of soil quality standards. Predicted accumulation of Cd in soil was larger for arable crops than for grassland, while the opposite was true for Zn. Analyzing scenarios of changes in P fertilization management showed that Cd and Zn enrichment in soil was more sensitive to decreasing net P inputs to arable farms than to dairy farms. When uncertainty of input data was taken into account, large variances of the calculated accumulation rates were found. Coefficient of variation of the net fluxes ranged between 56 and 162% for Cd, and between 46 and 82% for Zn, and distribution functions of the net fluxes indicated that in some soils, particularly under grassland, also a depletion of Zn and Cd may occur. Hypothetical steady-state soil concentrations of Cd were estimated to exceed average soil threshold concentration on arable farms, while steady-state Zn concentrations in soils of dairy farms were on average below the corresponding threshold value. However, if the uncertainty of these Zn estimates was considered, a significant part of the distribution exceeded the threshold value. The results of our study demonstrate that information on variation in element balances is beneficial. Average flux balances alone do not provide sufficient information for an adequate assessment of sustainable land use.