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Research Project: Bridging Project: Integrated Forage Systems for Food and Energy Production in the Southern Great Plains

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Title: Soil macronutrient responses in diverse landscapes of southern tallgrass to two stocking methods

item Northup, Brian
item Starks, Patrick
item Turner, Kenneth

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2019
Publication Date: 6/20/2019
Citation: Northup, B.K., Starks, P.J., Turner, K.E. 2019. Soil macronutrient responses in diverse landscapes of southern tallgrass to two stocking methods. Agronomy. 9(6):329.

Interpretive Summary: Availability of the macronutrients nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), and magnesium (Mg) in rangeland soils, and their distribution within pastures are influenced by many factors. Their availability for uptake by plants is influenced by climate, type of plant community, pasture location, and pasture management. Grazing by animals (one from of management) is one way that macronutrients in plant tissues are recycled back to the soil (in urine and feces). One important issue is whether stocking methods can influence distribution of macronutrients, by redistribution in urine and feces, within the landscapes of larger production-scale pastures. As a test, we examined plant-available macronutrients in soils at 16 different locations within landscapes of tallgrass prairie of central Oklahoma. These pastures were managed during 2009-15 under two contrasting stocking systems (150 acres under continuous yearlong or 200 acres under rotational grazing among 10 sub-pastures). Available macronutrients were defined with paired sets of probes that contained membranes that absorb macronutrients in a fashion similar to plant roots. We collected measures available macronutrients in pastures during mid-March (start of growth by native grasses) and early-August (time of peak production). Probes were set in the upper 6 inches of soils at 13 locations in paddocks along transects from water tanks to the farthest corner from water, plus closest corners to water and locations along the furthest and nearest fence lines. Results showed we could not identify an overall effect related to stocking method in the distribution of macronutrients within pastures. The only significant interaction between stocking method and pasture location (an indicator of stocking effects) occurred for K. All remaining macronutrients displayed effects of pasture location that were common across stocking method. There were some effects that could be related to stocking method in their interactions with soil depths or time of year, but these responses varied. Greater amounts of S, Ca and Mg were available within 80 feet of water, which may be related to grazing. However, the landscapes and locations of water within the pastures were not uniform, which may have been involved. These results show that defining how stocking methods influence soil macronutrients will be difficult in production-scale pastures with mixed landscapes. More attention to landscape features included in paddocks, and more uniform organization of water and pasture features are needed to define how grazing effects macronutrient distributions.

Technical Abstract: Macronutrient (N, P, S, K, Ca, and Mg) availability and distribution in soils of grassland ecosystems are affected by diverse factors, including landscape position, climate, and forms of management. This study examined flux in plant-available macronutrients within landscapes of production-scale (61 to 80 ha) paddocks of southern tallgrass prairie of central Oklahoma, managed (2009-15) under two contrasting grazing (continuous yearlong; rotational grazing) regimes. Distribution of macronutrient availability within the 0-7.5 cm and 7.5-15 cm soil depths were determined with sets of anion-cation exchange membrane probes at 16 locations within paddocks, oriented along transects from water sources to far corners in the spring and summer of the 2015 growing season. No clear overall effect related to grazing regime was recorded in distribution of all macronutrients. The only significant (P=0.01) stocking system X location interaction occurred for K. All other macronutrients displayed significant (P<0.08) location effects. Effects relatable to grazing regime were in interactions with soil depths or times of year (P<0.10); responses of macronutrient flux to grazing regimes in these interactions varied. Higher flux occurred in available S, Ca and Mg in proximity (<24 m) to water sources, which may be related to grazing. However, organization of the landscape within paddocks may also have been involved. More attention to landscape features included within paddocks, and standardized organization of water and other features within paddocks, would improve the potential to define grazing effects on macronutrient distribution.