Quantifying annual variations in field scale element flows and balances is essential for sustainable nutrient management in farming systems

Element (nutrient) balances have proven useful in evaluating the sustainability of farming systems. However, most studies have been carried out over 1 year for single nutrients; thus data on annual variations are scarce, and so is the information on the behaviour of different elements. In addition, most studies have been carried out as farm-gate balances that do not include internal flows such as home-produced fodder and manure, which is crucial to assess the sustainability of management practices, in particular in organic systems. In this study, field level flows were monitored during 3 years and input–output mass balances were calculated for macronutrients (K, P) and trace elements (Cd, Zn) in adjacent organic and conventional dairy systems in Sweden. In addition, nitrogen balances were established for the first year of the study. Variations in element concentrations and mass flows between systems and years were analysed statistically for selected inputs and outputs. The K balances were negative (−39 to −22 kg ha−1 yr−1) all years in both systems, except one year in the conventional system, with less K harvested in organic crops. The organic P balance fluctuated around zero all years, whereas it was slightly positive in the conventional. Cadmium and Zn balances were positive in all years for both systems with lower surplus in the organic (Cd 0.24–0.70; Zn 225–463 g ha−1 yr−1). The inter-annual variation in flows was 10–40% for most elements. Incorporating variations, in addition to mean values, in field (soil surface) balances is recommended in order to assess and combat risks for long-term soil depletion or accumulation of essential and potential toxic elements.