Effects of cultivation on soil and soil water under different fertiliser regimes

Nutrient exports from agriculture are an important environmental issue in many countries. In some production systems phosphorus (P) and nitrogen (N) accumulate at or near the soil surface and cultivation is one option for addressing the legacies of previous nutrient applications. This study aimed to compare the effects of cultivation (mouldboard ploughing), pasture type (ryegrass or ryegrass–clover) and P fertiliser application rate (10, 35 or 100 kg P/ha per annum) on P and N in soil and soil water from dairy pastures over a three year period. The results are generally consistent with inversion of organic matter and P rich surface soil (i.e. 0–140 mm). Cultivation lowered surface soil (0–20 mm) P and N concentrations. Surface soil P concentrations also increased with increasing P fertiliser rate but organic matter and P buffering did not. The effects of vegetation were more equivocal. Ploughing lowered most P and N species in surface soil water. The concentrations of P in soil water increased with increasing fertiliser rates as did ammonia concentrations where pastures included clovers. Analyses of trends with time suggests that, for pasture re-establishment, cultivation reduced the risk of N exports through surface pathways. Moreover, analyses of P export risk measured as the dissolved reactive P concentration in surface soil water (0–20 mm) suggests that where there is no predisposition to erosion, mouldboard ploughing not only reduces the risks of P exports, but that effect is likely to be more enduring where paddocks are in positive P balance. It follows that the use of cultivation as part of pasture renovation activities may reduce catchment-scale P exports, particularly in critical source areas where the risks of erosion and subsurface exports of N are low.