Small-scale, high-intensity rainfall simulation under-estimates natural runoff P concentrations from pastures on hill-slopes

Rainfall simulation is a widely used technique for studying the processes, and quantifying the mobilisation, of phosphorus (P) from soil/pasture systems into surface runoff. There are conflicting reports in the literature of the effects of rainfall simulation on runoff P concentrations and forms of P compared to those under natural rainfall runoff conditions. Furthermore, there is a dearth of information on how rainfall simulation studies relate to hill-slope and landscape scale processes and measures. In this study we compare P mobilisation by examining P forms and concentrations in runoff from small-scale, high-intensity (SH, 1.5 m2, 80 mm/h) rainfall simulation and large-scale, low-intensity (LL, 1250 m2, 8 mm/h) simulations that have previously been shown to approximate natural runoff on hill-slopes. We also examined the effect of soil P status on this comparison. The SH methodology resulted in lower (average 56%) runoff P concentrations than those measured under the LL methodology. The interaction method × soil P status was highly significant (P < 0.001). There was no significant effect of method (SH v. LL) and soil P status on P forms (%).The hydrological characteristics were very different between the 2 methods, runoff rates being c. 42 and 3 mm/h for the SH and LL methods, respectively. We hypothesise that the lower runoff P concentrations from the SH method are the result of a combination of (i) the P mobilisation being a rate-limited process, and (ii) the relatively high runoff rates and short runoff path-lengths of the SH method allowing for relatively incomplete attainment of equilibrium between P in the soil/pasture system and runoff. We conclude that small-scale, high-intensity rainfall simulation provides a useful tool for studying treatment effects and processes of mobilisation in pastures, but concentration and load data should not be inferred for natural conditions at larger scales without a clear understanding of the effects of the rainfall simulation methodology on the results for the system being studied.