Effects of riparian grazing on distinct water-extractable phosphorus sources

Abstract

Riparian areas play an important role in maintaining water quality in agricultural watersheds by buffering sediment, nutrients, and other pollutants. Recent studies have shown that in some cases riparian areas are a net source of phosphorus (P) in cold climates. This study assessed the impact of cattle grazing or harvesting of riparian areas on the spatial and vertical distribution of water-extractable phosphorus (WEP). This study measured the WEP in four distinctive sources: biomass, litter, organic layer, and Ah horizon in three riparian locations extending from the edge of the waterbody to the field edge. In addition to a control, three treatments were examined: (1) grazing; (2) high-density grazing; and (3) mowing. Prior to implementing the treatments, the Ah (0–10 cm) soil was the largest pool of WEP (42.5 mg m⁻², ∼44%); however, the biomass (i.e., standing vegetation) was a considerable proportion of the total (26.3 mg m⁻², ∼25%) WEP pool. The litter and organic layer had median WEP areal densities of 11.1 and 17.7 mg m⁻², respectively. Findings revealed significant reductions in biomass WEP with median reductions of 10.4 and 18.7 mg m⁻² for high-density grazing and mowing treatments, respectively. This reduction was more pronounced in the lower riparian locations where there was more biomass available to be grazed or mowed. There were no detectable changes in the other sources of WEP across all the treatments. Assessment of the control plots (pre- and post-treatment) clearly indicates that there is considerable small-scale spatial variability in P measurements in riparian areas. Overall, the results of this study suggest that management practices that target vegetation, including harvesting and short-term autumn grazing, may be mechanisms to reduce the potential P loss during the snowmelt period. To fully assess the risk of P loss, studies investigating other important riparian processes that also have a demonstrated impact on P mobility, including freeze-thaw cycles and flooding, are needed.