Do aggregation, treading, and dung deposition affect phosphorus and suspended sediment losses in surface runoff?

Abstract

Cattle grazing can enhance the loss of phosphorus (P) and suspended sediment (SS) in surface runoff, which can impair receiving water bodies. We tested the hypothesis that much of the particulate-bound P in surface runoff is carried in light, low-density aggregates that may enhance the transfer of P and SS to surface water. Four contrasting soil types were treated with combinations with or without treading by an artificial hoof to simulate a 24-h grazing event, and with or without dung return. A rainfall simulator was used to generate surface runoff. Dung application enriched P and SS losses much more than treading, while losses were greater from Recent Gley and Pallic soils (New Zealand Soil Classification), which were defined as vulnerable to physical damage and slaking, as opposed to well-structured Melanic and Brown soils. Allowing runoff samples to settle showed that, according to Stoke’s law, particles that remained floating after 19.1 min (termed ‘lighter’ particles; <10 µm) accounted for a larger proportion of the P load than particles that sank after 19.1 min (termed ‘heavy’ particles; >10 µm), despite only contributing <30% of runoff volume. Furthermore, dispersion of the different sized particles showed that aggregation was significant for the lighter (<10 µm) sample, especially if treading occurred or dung was applied to the Pallic or Recent Gley soils. Caution should be practiced when grazing Recent Gley and Pallic soils because of the greater potential for P and SS loss particularly when these soils are wet and surface runoff is likely.