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.

Methods
A randomized complete block experimental design was used to assess the sources of riparian WEP and investigate how it changes following cattle grazing or mowing treatments. In addition to a control, the three treatments included grazing, high-density grazing, and mowing. Each treatment, including a control, was replicated in riparian areas surrounding four prairie potholes (wetlands). Samples of biomass, litter, organic layer, and Ah horizon, were collected in three locations both pre- and post-treatment.

Four types of samples were collected: 1) biomass, 2) litter, 3) organic layer, and 4) Ah horizon. Using a 0.25 m2 quadrate, biomass was collected by cutting the standing live vegetation and litter by raking the surface and picking up the previous year’s growth. Both the biomass and litter were dried at 40 deg C, weighed, and homogenized using a blade grinder (<1cm). A composite of five soil samples was collected within the same quadrat as the biomass/litter using a 19 mm diameter soil probe and was divided into the organic layer (1 – 2 cm deep) and the top 10 cm of the Ah horizon. The organic layer and Ah soil were air-dried, disaggregated with a mortar and pestle, and passed through a 2-mm sieve.

Water Extractable Phosphorus (WEP), an environmental soil and vegetation P test, was used to infer soil P release into runoff water. Dried and homogenized samples were extracted by shaking (150 RPM) with deionized water for one hour at a mass-to-volume ratio of 1:30 for the biomass and litter samples (1 g) and 1:15 for the organic and Ah samples (2 g). Extractions were gravity filtered through a Whatman 42 filter followed by syringe filtration with a 0.45 micron nylon filter. WEP in the extract was measured spectrophotometrically by the colorimetric molybdate–ascorbic acid method.

The concentration of WEP (mg kg-1) was calculated for all sources of P. In addition, the areal density of WEP was calculated for biomass and litter by combining WEP concentration with the mass of material collected from the quadrat. The vertical profile of WEP within the riparian area assessed from samples collected before treatments were implemented across the 3-year study. For comparison, an approximate estimation of areal density WEP in the organic layer and Ah was calculated using the bulk density and depth measurements.

For additional information see https://github.com/alex-koiter/riparian-grazing-manuscript