Nutrient redistribution by grazing cattle drives patterns of topsoil N and P stocks in a low-input pasture ecosystem

Abstract

Nutrient cycles in grassland often involve net transfers from some areas to others. Here, we analyse patterns of N and P transfers by cattle in two grazing periods, and their relationships to soil P and N stocks in an unfertilised old pasture with a history of [50 years grazing. Net transfers were assessed from spatial patterns of nutrient ingestion and excretion. Total soil N and P were determined at 0-5, 5-10, 10-30 and 30-60 cm. All analyses were performed with a spatial resolution of 10 9 10 m 2 or higher. Data were geostatistically interpolated. Nutrients accumulated in the flat crest zone and were depleted in the steeper areas. Nutrient ingestion was less and excretion higher in the accumulation zone (and vice versa in the depletion zone) revealing that both components of grazer-driven net transfers of nutrients promoted the development of accumulation/depletion zones. Topsoil stocks of N and P were closely correlated with excreta density and net transfers of N (P \ 0.001), whereas N and P at 30-60 cm displayed only weak or no correlation. Redistribution involved a small fraction of the soil stocks: nutrients grazed in two periods were equivalent to 0.8% of N and 0.2% of P of the whole pasture (0-60 cm). These factors suggest that topsoil nutrient distribution was modified by long-term stable patterns of net transfers of nutrients. The excess of N in the accumulation zone (N in accumulation zone minus N in depletion zone, in 0-60 cm) was 11 times larger than that of annual (i.e. short-term) net transfers; but for P, it was 30 times larger. This contrast likely derived from leaching/ volatilization losses of N in the accumulation zone and only small inputs of biologically fixed N in the depletion zone. The nitrogen status of vegetation (Nitrogen Nutrition Index) had a strong effect on herbage production throughout the pasture, while the Phosphorus Nutrition Index indicated no limitation, except in urine patches.

Isotopic composition of soil, vegetation or cattle hair no suitable indicator of nitrogen balances in permanent pasture

“…In all these cases, the influences of the management are integrated in the stable isotope composition of the samples, which are stable over time (with some seasonal variation) if the management is stable (e.g. Neilson et al 1998;Schnyder et al 2010;Wittmer et al 2010).…”

The Rengen Grassland Experiment: relationship between soil and biomass chemical properties, amount of elements applied, and their uptake

“…An alternative approach to identifying nutrient limitation is using nutrition indices for the nitrogen (NNI) and phosphorus (PNI; Duru and Ducrocq 1997;Lemaire 1997) used widely for arable crops (Bélanger et al 2001;Ziadi et al 2008) and grasslands (Jouany et al 2004;Garnier et al 2007;Schnyder et al 2010). The major advantage of this approach is that it reflects the typical dilution effect, i.e., the decrease in N and P concentrations during the progressive accumulation of aboveground standing biomass.…”

15N fractionation between vegetation, soil, faeces and wool is not influenced by stocking rate

“…In grassland, such variation concerns smallscale heterogenity caused by excrement patches Augustine and Frank 2001), large-scale redistribution of excrements often associated with topography (Burke 1999;Schnyder et al 2010), or spatio-temporal variation in soil water status (Handley et al 1994). Augustine and Frank (2001) noted that large herbivores altered the distribution of soil N at every spatial scale from that of single plants to that of the landscape.…”