Stocking methods and soil macronutrient distributions in southern tallgrass paddocks: Are there linkages?

Authors: Northup, Brian; Starks, Patrick; Turner, Kenneth

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2019
Publication Date: 5/31/2019
Citation: Northup, B.K., Starks, P.J., Turner, K.E. 2019. Stocking methods and soil macronutrient distributions in southern tallgrass paddocks: Are there linkages? Agronomy. 9(6):281. https://doi.org/10.3390/agronomy9060281.
DOI: https://doi.org/10.3390/agronomy9060281

Interpretive Summary: Availability of the macronutrients nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), and magnesium (Mg) in soils of rangelands that can be taken up by plants are important to plant health and productivity. Their availability is influenced by growing conditions, type of plant community, location in pastures, and how pastures are managed. Grazing by animals is an important way for macronutrients in plant tissues to be recycled back to the soil. One important issue is whether type of grazing system can influence where plant-available macronutrients are distributed (via urine and feces) within pastures. To test for differences, we looked for changes in macronutrients in soils available for uptake by plants to see if type of grazing system would cause changes close to water sources and near the center in pastures, and sub-pastures of rotational grazing systems, of southern tallgrass prairie. We used paired sets of probes containing membranes that absorb different forms of the 8 macronutrients in ways that were similar to plant roots. We collected measures of flux from pairs of pastures managed under four different stocking methods, twice during the 2015 growing season (mid-March, start of growth by native grasses; early-August, time when peak production occurred). Probes were set in the upper 6 inches of soils at four locations in pastures; near water tanks (2 locations within 20 feet) and near pasture mid-points (75 to 1300 ft. from water, depending on grazing system) to test whether amounts of macronutrients changed among locations. We found that all tested grazing systems, from continuous (yearlong) through different types of rotational grazing, effected amounts of plant-available macronutrients during the growing season and among locations in pastures. We noted that distribution of amounts of available macronutrients were not consistent for any stocking method. Such responses indicated grazing systems may not prevent hot spots of available macronutrients that affect other ecosystem features like water quality, particularly nitrates (NO3) or S. However, results also indicated the overall exposure of landscapes (east vs. west) and how different features (location of water, paddock corners) were organized in these pastures may have also affected the macronutrient distributions we recorded.

Technical Abstract: Broad ranges of factors (parent materials, climate, plant community, landscape position, management) can influence macronutrient availability in rangeland soils. Two important factors in production-scale paddocks are the influences of location in space and grazing regime. This study examined plant-available macronutrients (total mineral and nitrate-N, P, S, K, Ca, and Mg) in soils, with paired sets of probes (anion and cation exchange membranes) that simulate uptake by plant roots. Data were collected from sets of paddocks of southern tallgrass prairie in central Oklahoma, managed under four grazing regimes during the 2015 growing season (mid-March, growth initiation by native grasses; and early-August, peak living plant biomass). Macronutrient availability in the 0-7.5 cm and 7.5-15 cm depths were determined at locations in close proximity to water [water tanks and 25% of distance between tanks and paddock mid-points (PMP)], and distances near mid-points of paddocks [70% of the distance between water and mid-points (0.7 PMP), and PMP]. All tested grazing regimes affected levels of availability of macronutrients at different times of growing season, and among locations in paddocks. Such responses indicated grazing systems may not provide uniform distribution of flux in macronutrients. The overall exposure of landscapes and arrangement of features within paddocks also appeared to influence macronutrient distributions.