Sheep urine affects soil solution nutrient composition and roots: differences between field and sward box soils and the effects of synthetic and natural sheep urine

Urea-N in sheep urine returned to soil is an important component in the N-cycle of extensively managed grassland soils but relatively little is known of its effects on P and C cycling. Previous studies using sward boxes sown with Agrostis capillaris and soil mixed from the Ah and AB horizon of a brown ranker showed that the application of synthetic sheep urine (SSU) increased soil pH and the soil solution concentration of dissolved organic C (DOC) and organic and inorganic forms of N and P. To extend information to the field, we compared the effects of SSU addition to soil in sward boxes with that in the field using an application rate corresponding to the upper value of the range found in urine patches (500 kg urea-N ha−1). For 50-mm deep soil cores taken at intervals up to 56 d after treatment, the magnitude of the change in soil pH and the soil solution concentrations of the dissolved nutrients were less in the field and lagged behind in time compared with the sward boxes. The difference was attributed to the larger mass of roots, the existence of a discrete Ah horizon and the greater moisture content of the soil in the field that were able to buffer effects. Synthetic sheep urine, prepared according to published formulae, may provide a greater concentration of urea-N than found in natural sheep urine (NSU) from hill sheep and, additionally, does not contain minor components such as hippuric acid that influences nitrification and pH changes associated with the presence of NH3/NH4+. To compare effects of SSU and NSU we used sward boxes and applied equal volumes of SSU (9.97 g l−1 of urea-N) or NSU (4.04 g l−1 of urea-N) from ewes fed on grass. Natural sheep urine increased the concentration of dissolved nutrients in soil solution but the increases were disproportionately less than for SSU. Synthetic sheep urine was toxic to roots in sward boxes and in the field, increasing the proportion of roots that were dead or decomposing, especially in the sward boxes. The study highlights differences in soil solution chemistry and root dynamics between sward box and field soils and between SSU and NSU, demonstrating a need for caution in extrapolating results from sward boxes to the field.