Data on soil characteristics and dominant grass and soil chemical composition gathered on active rangeland, livestock exclosures on active rangeland, and the Sevilleta NWR.
This study compares grazed grassland (treatment G), grassland within livestock exclosures (treatment X) and undisturbed (by livestock) grassland (treatment S - for Sevilleta). During the winter of 1992/1993, three 300m x 300m plots were delineated for each of these three treatments. Plots are referred to as G1, G2, G3, X1, X2, X3, S1, S2, and S3 (number increasing from west to east). Each of these nine plots are separated by at least 300 m. Exclosure plots are surrounded by wire fencing, and the Sevilleta plots are located within the NWR boundary. A rodent trapping web is centered within each plot. Within the framework of this mammal trapping web, twelve 3x4 m quadrats were randomly placed. Corners of plots are marked by aluminum fence posts and locations were recorded with GPS. Centers and lines of mammal trapping webs are marked by 1m high (centers and end of lines) and .25m high (lines) rebar and aluminum numbered tags. Corners of quadrats are marked by short rebar. The location of mammal trapping webs and quadrats is the same within each plot.
For this particular study a subsample of four quadrats, chosen randomly for each plot, within each plot was sampled. Data collected include soil texture, field water content, 50% water holding capacity, organic matter content, percent C, percent N, percent P, and dominant grass (usually Bouteloua eriopoda) percent C, percent N, and percent P. Soil samples were collected as cores to 10cm depth, with 5-10 cores taken along a diagonal from one corner of the quadrat to the other corner, then pooled in a plastic bag as one sample from each quadrat. Grass samples of green above-ground tissue (growth from summer/fall 03) include tissue from 5-10 individual plants within the quadrat. All analyses were conducted for each quadrat excepting soil texture, for which equal weights of soil from each quadrat were pooled into one sample per plot. Soil samples were collected on 8/10/03 and plant samples were collected on 10/18/03, before and after the most significant rains of the season.
Soil texture was analyzed using the hydrometer method, and data are recorded as sand, silt, and clay. Soil field water content was measured as the difference in weight before and after the drying of a subsample of soil. Soil organic matter was measured as the difference in weight before and after combustion of a subsample of dry soil. Soil 50% water holding capacity was measured using standard Sevilleta techniques (after White). Soil and plant percent C and N were measured using a Carlo Erba AutoAnalyzer. Soil and plant percent P was measured as extractable P, converted to PO4-P, using a Technicon AutoAnalyzer.
Additional field crew member: Ben Zimmerman
Additional Study Area Information
Study Area Name: Pino Gate
Study Area Location: The study site was located near the base of the Los Pinos mountains and directly adjacent to the nothern fencline of the SNWR at Pino Gate
Elevation: 1600 m
Vegetation: Burrograss (Scleropogon brevifolius), sand dropseed (Sporobolus ryptandrus), and black grama (Bouteloua eriopoda) were the dominant vegetation.
Soils: Deep clayey loam soils
Geology: On an upper bajada slope, in a broad swale
Climate: Long-term mean annual precipitation is 243 mm, about 60% of which occurs during the summer. Long-term mean monthly temperatures for January and July are 1.5°C and 25.1°C, respectively.
Site history: Historically, prairie dogs were common throughout the area, but were exterminated by the early 1970’s (John Ford, United States Department of Agriculture Wildlife Services, personal communication). Gunnison’s prairie dogs began to re-colonize the study site from adjacent private land in 1998. During our study, the colony occurred within a 5 ha area, near the base of the Los Piños Mountains in an area with deep clayey loam soils. The site has been long inhabited by kangaroo rats, and represents typical northern Chihuahuan Desert grassland.
Animal grazers affect grassland disturbance patterns via changes in plant community composition and structure, nutrient cycling, and soil structure. Human-managed livestock grazing is a significant grassland disturbance worldwide.
a. Is livestock grazing related to changes in arid grassland soil structure and chemical composition or plant nutrient composition? If so, how?
(hypothesis) Grazed rangeland will have more compact soil (lower water holding capacity), lower levels of nutrients (C, N, P) and lower organic matter content than grassland unaffected by livestock grazing. Grazed rangeland will produce grasses of lower nutritional quality (i.e. higher C:N and C:P ratios) than grassland unaffected by livestock grazing.
b. When livestock are exclosed from a rangeland, does soil structure and chemical composition or plant nutrient composition `recover', exhibit resilience, align more closely with values characteristic of grassland unaffected by livestock grazing?
(hypothesis) Within livestock exclosures (free of livestock for 9 years) soil water holding capacity will be higher, levels of nutrients (C, N, P) and organic matter content will be higher, and plant nutritional quality will be higher (C:N and C:P ratios will be lower) than on grazed rangeland. These structural and chemical characteristics will be intermediate between those of grazed rangeland and grassland unaffected by livestock grazing.