Impacts of livestock grazing on soil physical quality and phosphorus and suspended sediment losses in surface runoff

Livestock grazing is considered a major contributor in the loss of phosphorus (P) and suspended sediment (SS) from soil in surface runoff, which in turn can have adverse impacts on aquatic ecosystems. Recent and ongoing increases in the intensity of pastoral farming in New Zealand represent an enhanced risk of P and SS loss. The objective of this research was to assess and quantify the impacts of livestock grazing on soil physical quality (measured by macroporosity, soil bulk density, and saturated hydraulic conductivity Ksat) and associated losses of P and SS in surface runoff. The hypothesis was that livestock grazing would have detrimental impacts of soil physical quality which would increase P and SS loss. A mechanical cattle hoof and rainfall simulator were used to examine the influence of cattle treading and soil type on short-term P and SS losses. Field experiments were also carried out to determine if stock type (sheep, beef cattle and deer) influenced P and SS losses in runoff and the last experiment examined the potential to mitigate losses in runoff via sub-soiling in cattle-grazed pasture. Results revealed that soil type, soil moisture, and land management practices have a major influence on soil physical quality and losses of P and SS in runoff under livestock grazing. Losses of P and SS in runoff were greater from the more compacted Pallic and Recent Gley soils with mean macroporosity values of 12 and 17% v/v, respectively, compared with Brown (23% v/v) and Melanic (37% v/v) soils. Data from field trials at Invermay and Windsor showed that soil physical quality differed between the Brown-Pallic soil at Invermay and the Pallic soil at Windsor. For example, mean macroporosities for cattle grazed plots were 16 and 8% v/v, respectively. At Invermay, significant relationships were found between loads and concentrations of P and SS with changes in macroporosity and Ksat; an increase in compaction increased surface runoff losses. These findings are comparable to results obtained from the rainfall simulation studies with greater P and SS losses occurring from the heavier compacted Pallic and Recent Gley soils than the better-structured Melanic and Brown soils. Soil moisture had a major influence on P fractions lost and runoff processes. Increasing soil moisture from 10% to 90% available water holding capacity increased particulate P concentration from 0.028 mg/L to 0.161 mg/L, while SS concentrations increased from 0.009 g/L to 0.169 g/L. These changes were mainly attributed to deeper hoof penetration and soil disturbance with increased soil moisture. On the other hand, dissolved reactive P concentrations decreased with increased soil moisture from 0.423 mg/L to 0.127 mg/L, respectively, which was mainly attributed to a combination of P release from soil microbial biomass and lack of dilution. Results from the field study carried out at Invermay demonstrated how soil moisture can influence seasonal P dynamics and runoff processes. Winter runoff occurred as a consequence of saturation-excess conditions that accounted for most P and SS loss (mean 0.015 kg TP/ha and 17.61 kg SS/ha). In summer, infiltration-excess runoff dominated and although loads (mean 0.007 kg TP/ha and 1.23 kg SS/ha) were not as great as those occurring in winter, concentrations were greater (summer mean 1.585 mg TP/L versus winter mean 0.015 mg TP/L). The enrichment of P concentrations in summer could pose a significant algal growth risk to surface water quality due to increased light and warmth. Management practices were confirmed as having a major influence on soil physical quality and losses of P and SS in runoff. Results from the rainfall simulator study showed that cattle dung was the main source of TP and SS in runoff (0.511 mg/L and 0.092 g/L, respectively). Field results revealed that P and SS losses in runoff decreased with time since grazing. Treading by cattle had greater negative impacts on all soil physical properties (macroporosity, soil bulk density and Ksat) than sheep or deer treading. For example, mean macroporosity for cattle grazed plots was 16% v/v followed by 22 and 23% v/v for deer and sheep grazed plots, respectively. However, stock type did not affect P and SS losses in runoff. To mitigate runoff losses of P and SS from a Pallic soil at Windsor, the site was sub-soiled (aerated) to 20 cm soil depth. Results revealed an increase in dissolved reactive P (2.24 kg P/ha/yr) for aerated soil compared with non-aerated (control) soil (1.20 kg P/ha/yr), which was attributed to mechanical soil disturbance causing the desorption of P from soil during the last storm event, which had a return period of 40 years. Six months after soil aeration soil physical quality was similar between aerated and control treatments. The collective findings of this research demonstrated how livestock grazing negatively impact soil physical quality and P and SS losses in runoff. It was clear that the best management practice to effectively decrease these losses is to restrict or avoid grazing when soil moisture has reached field capacity, particularly for vulnerable soil types, regardless of stock type.