Animal treading during wet soil conditions reduces N2 fixation in mixed clover-grass pasture

Abstract

A field experiment was carried out over 12 months to determine the effect of animal treading on N2 fixation in a mixed white clover-ryegrass pasture. The experimental site was defoliated by mowing for the duration of the study. A single treading event of moderate or severe pugging intensity was initiated in plots during wet spring conditions by using dairy cows at varying stocking rates (4.5 cows 100 m−2 for 1.5 or 2.5 h, respectively). Inputs of dung and urine onto the plots was avoided by overnight housing of the cows and interception of excreta during the pugging event. Soil air-filled porosity decreased from 21% in the non-pugged control to 15–16% in pugged treatments by day 3. Bulk density of soil was not significantly affected by pugging. Soil inorganic N concentration increased in pugged treatments, and was 4-fold greater on day 28 in severely pugged plots compared to non-pugged plots. White clover plant density and plant size was markedly lower in pugged treatments (up to 85% and 72% reduction, respectively under severe pugging). White clover growth was most affected during the first 156 days after pugging (up to 90% decrease under severe pugging), leading to an annual clover dry matter production loss of 9% and 52%, respectively. The proportion of clover N derived from atmospheric N2 (%Ndfa; estimated by 15N dilution) was initially reduced (to a lower limit of 43%) by severe pugging (days 28–71) before recovery to control levels (90%) by day 91. Annual N2 fixation in clover herbage decreased significantly from 76 kg N ha−1 yr−1 in the non-pugged control, to 66 and 36 kg N ha−1 yr−1 under moderate and severe pugging, respectively. Most of this difference was evident within the first 156 days after pugging. Our data indicates that the major loss in fixed N2 input under pugging was due to reduced clover growth and production resulting from pugging damage and loss of residual white clover biomass by hoof action.