Dynamics of mineral nitrogen in topsoil, during regrowth of pasture in two contrasting grassland systems

Abstract

Changes in soil mineral nitrogen(N) were monitored during regrowth of pasture between consecutive grazings in two contrasting grassland systems; Grass-clover (the norm in NZ) and a more intensive system, Grass+N400 (pure grass + 400 kg fertiliser N/ha/year). The experiment was carried out during autumn at DSIR Grasslands.Palmerston North. Net mineralisation of N under field conditions was estimate_d- i~n- an ancillary experiment, using soil samples from undisturbed soil cores contained in PVC tubes. The dynamics of mineral N in soil were dominated by a 'pulse' of ammonium, observable soon after grazing. Nitrification proceeded rapidly thereafter. Mineral N in soil then progressively declined, much of it going into organic combination presumably through uptake by plants. Since nitrate formation in the soil is minimised by maximising the residence time of N in plant (organic) form, differentmanagementoptions(varyinginfrequency and intensity of defoliation) may have important influences, not only on pasture utilisation and production, but also on the management of mineral N in the soil-plant-animal complex. Tubes embedded in soil and incubated in the field have provided some additional, useful perspectives. There was only limitedevidence for significant net mineralisation of organic N throughout the period of regrowth. Analyses of individual soil cores demonstrated a sharp contrast between the pasture at large and the 10 - 15% of total area influenced by urine from the previous grazing, in terms of mineral N content. 'Averaging' these by bulking numerous cores into a composite sample can provide an accurate quantitative estimate of mineral N, which can be related to herbage uptake of N over the whole area. But if losses of N (by leaching or volatilisation) are disproportionate to the concentration of mineral N in affected and unaffected volumes of soil, then bulking samples and averaging will not be the most appropriate way to estimate these losses. The results of this study point to the importance of the urine of grazing ruminants as a N substrate for pasture regrowth in the absence of fertiliser N. At the same time, urine patches provide the main avenue for Nescape to the wider environment from developed pastures. Keywords mineral N, N in pastures, N cycling by animals