Impact of cattle treading on hill land: 2. Soil physical properties and contaminant runoff

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Abstract
A simulated rainfall study carried out as part of a larger grazing experiment was conducted to investigate the effects of a 2-3 day cattle-treading event in winter on soil physical properties and contaminant (sediment and nutrient) runoff in topographically variable hill land. Measurements were made on two land zones: easy contoured ridges and gullies (15-24 degrees slope) and steep inter-track land (28-39 degrees slope). Simulated rainfall was applied to 16 plots (0.5 m(2)) with a range of treading damage for each land zone within 14 days of the treading event. Simulated rainfall was repeated on the same plots in the following summer and winter to assess the recovery of soil from treading damage. Treading damage reduced water infiltration rate, particularly in the steep zone. It also caused a significant increase in the transport of suspended solids (SS), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) from the plots during simulated runoff. On the steep inter-track zone, damaged areas had a 46% lower infiltration rate, and runoff from these areas contained on average 87% more sediment, and 89% more N and 94% more P compared with undamaged areas. These effects had disappeared six months later. The rainfall simulation and soil physical studies show that a 2-3 day winter treading event may increase soil bulk density, reduce soil macroporosity and total porosity, and lead to a decrease in the water infiltration rate and an increase in contaminant runoff.