Effects of riparian grazing on distinct phosphorus sources

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 (0-10cm) 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 indicate
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
autumn short-term 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.