Vegetation cover and soil organic carbon along gradients of cattle grazing intensity in the Jornada Basin, July-August 2016

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t: The goal of this Master’s thesis project, which was carried out in July and August of 2016, was to assess the effect of inferred grazing intensity on 1) vegetation cover type and 2) soil organic carbon (SOC) at the Jornada Experimental Range in southern New Mexico. A sampling transect was established at each of 3 long term cattle water sources (85-106 years old), beginning 5m from the water source and continuing 1500m outward. Soil bulk density, soil organic carbon, soil organic nitrogen, and dominant plant cover type (shrub, grass, and bare soil) were sampled at 20 locations on each transect. Two hypotheses evaluated in this study are: 1) higher grazing pressure near the water source will lead to reduced vegetation cover and C inputs into the soil, leading to higher SOC stocks in soil with far proximity to the water source; and 2) Grazing very close to the water source will exert high disturbance and deposit SOC via defecation, leading to higher SOC stocks in soil with close proximity to the water source.


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This study was located within the Jornada Experimental Range near Las Cruces, NM. The pastures chosen are 12A, 12C, and 15, with the corresponding wells of Headquarters, Middle, and Taylor respectively. These water sources are the oldest in the study site, ranging from 85-106 years old, and serve as a consistent water source for cattle. One transect was established at each of the water sources, originating 5m from cattle water sources and radiating 1500m outward from the water sources. Distances farther from the water source are assumed to experience lower grazing impacts, forming a gradient of grazing intensity. Sampling locations along the transect were set at every 50m until 500m from the water source, then every 100m until the end of the transect at 1500m. Vegetation Sampling Dominant vegetation cover type was measured at each sampling location along the three transects on 30m transects perpendicular to the 1500m transect. A modified version of the line-point intercept method was used. Dominant cover (bare soil, shrub, or grass/forb) was assessed at every 10cm, resulting in a total of 30 points per transect. Percent cover for the 3 cover types were then calculated for each sampling location. These values were then fit to a regression model to see if the cover changed along the grazing gradient. It is important to note that vegetation cover was measured in July 2016, which comes just at the start of the wet monsoon season, before the plants are able to take advantage of the favorable soil moisture levels that come with the monsoons. Sampling vegetation cover took around four weeks, beginning two weeks before the first monsoon rainfall event. All vegetation cover data from Taylor Well and around half of cover data from Middle Well were measured after the first significant rainfall event. This led to lower vegetation cover compared to peak production months (September and October). Soil Sampling Soil cores were extracted at the 20 sampling locations along the 1500m transects. As soil organic carbon (SOC) is likely to vary depending on the above vegetation type, one soil sample was extracted from each cover type (shrub, grass and bare) at each sample location if they were available within 20m. Cores were extracted with a cylindrical core and a spatula to prevent any loose soil in the core from escaping through the bottom. In the lab, soil samples were put through a 2mm sieve to remove root and leaf litter before drying at 105°C for 24 hours in preparation for measuring bulk density (g/cm3) using methods suggested by Throop et al., 2012. Soil organic carbon and total nitrogen concentrations were measured using elemental combustion analysis. First, each sample was ground to a fine powder using a mortar and pestle before placing 30mg of the ground samples into a silver capsule and acid fumigating them to eradicate any inorganic carbon (Harris et al., 2001). They were then sealed in tins and combusted to measure the percent C and N of each capsule using a PE 2400 CHN analyzer.


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