Influence of cultivation methods on suspended solids and phosphorus concentrations in surface runoff on clayey sloped fields in boreal climate

Methods for decreasing agricultural phosphorus (P) loadings to surface waters are needed to achieve good water quality. To reliably find out the efficiency of different methods in changing hydrological conditions, long-term experiments are of invaluable importance. Here the effects of various cultivation methods on total suspended solids (TSS) and P concentrations in – and the volume of – plough layer runoff (PLR) were studied in a sloping experimental field in southwest Finland during a 9-year period. Yearly means of PLR and flow-weighted mean concentrations from the treatment plots were compared with a control, winter wheat (Triticum aestivum) treatment (WW), of which the values assigned were equal to 1. No major differences were found in PLR for cultivation methods up and down slope, except for the cross-ploughing treatment (0.49). Normal ploughing and cultivation treatments produced the highest TSS concentrations (1.38 and 1.18, respectively), whereas values between 0.44 and 0.53 were measured for three treatments with reduced (or no) tillage. Particle-bound P (PP) concentrations closely followed those of TSS. Dissolved reactive P (DRP) showed contrasting behaviour, with the greatest treatment effects from the three reduced tillage treatments (1.58–2.29). This study showed that by changing over from intensive autumn tillage to permanent vegetation cover, erosion and PP loss were markedly decreased. Methods employing no autumn tillage and leaving an undisturbed soil surface during winter showed particularly high reductions in loading. The simultaneous increase in DRP loss is an undeniable drawback that should be resolved separately. This study, as a part of a wider complex of integrated studies, will be utilized in a decision-making tool for the assessments of loading effects and load reduction possibilities in terms of cultivation methods.