Gerald E. Schuman

IMPACT OF GRAZING MANAGEMENT ON THE CARBON AND NITROGEN BALANCE OF A MIXED-GRASS RANGELAND

Rangeland grazing management strategies have been developed in an effort to sustain efficient use of forage resources by livestock. However, the effects of grazing on the redistribution and cycling of carbon (C) and nitrogen (N) within the plant–soil system are not well understood. We examined the plant–soil C and N balances of a mixed-grass rangeland under three livestock stocking rates using an area that had not been grazed by domestic livestock for more than 40 years. We established nongrazed exclosures and pastures subjected to continuous season-long grazing at either a light stocking rate (20 steer-days/ha) or a heavy stocking rate (59 steer-days/ha, ∼50% utilization of annual production). Twelve years of grazing under these stocking rates did not change the total masses of C and N in the plant–soil (0–60 cm) system but did change the distribution of C and N among the system components, primarily via a significant increase in the masses of C and N in the root zone (0–30 cm) of the soil profile. The m...

Carbon exchange rates in grazed and ungrazed pastures of Wyoming.

The influence of cattle grazing on carbon cycling in the mixed grass prairie was investigated by measuring the CO(2) exchange rate in pastures with a 13 year history of heavy or light grazing and an ungrazed exclosure at the High Plains Grasslands Research Station near Cheyenne, Wyo. In 1995, 1996 and 1997 a closed system chamber, which covered 1 m(2) of ground, was used every 3 weeks from April to October to measure midday CO(2) exchange rate. Green vegetation index (similar to leaf area index), soil respiration rate, species composition, soil water content, soil temperature, and air temperature were also measured to relate to CO(2) exchange rates of the 3 grazing treatments. Treatment differences varied among years, but overall early season (mid April to mid June) CO(2) exchange rates in the grazed pastures were higher (up to 2.5 X) than in the exclosure. Higher early season CO(2) exchange rates were associated with earlier spring green-up in grazed pastures, measured as higher green vegetation index. As the growing season progressed, green vegetation index increased in all pastures, but more so in the ungrazed exclosure, resulting in occasionally higher (up to 2 X) CO(2) exchange rate compared with grazed pastures late in the season. Seasonal treatment differences were not associated with soil temperature, soil respiration rate, or air temperature, nor was there a substantial change in species composition due to grazing. We hypothesize that early spring green-up and higher early season CO(2) exchange rate in grazed pastures may be due to better light penetration and a warmer microclimate near the soil surface because of less litter and standing dead compared to the ungrazed pastures. When all the measurements were averaged over the entire season, there was no difference in CO(2) exchange rate between heavily grazed, lightly grazed and ungrazed pastures in this ecosystem. DOI:10.2458/azu_jrm_v53i2_lecain

Soil bulk density and water infiltration as affected by grazing systems.

The itiuences of continuous, rotationally deferred, and shortduration rotation grazing systems on soil compaction and water infiltration were assessed. Bulk density and water innltration were measured to evaluate the effects of the 3 grazing systems at moderate and heavy stocking rates. Measurements were made in the spring before grazing and at the end of the grazing season in 1983 and 1984. Bulk density was not affected by grazing systems or stocking rate; bulk density was greater in the fall than in spring of 1984, but not in 1983. Infiltration was significantly lower under the heavy stocking rate than under the moderate stocking rate at the end of the grazing season. The average water infiltration was significantly less in the fall than in the spring for the heavy stocking rate but showed no seasonal effect for the moderate stocking rate. Infiltration was significantly greater under continuous grazing than under rotational deferment but no different from that under short-duration grazing in 1983. However, in 1984 the relationship was reversed. The grazing systems evaluated did not affecct soil bulk density and water infiltration in a consistent manner; however, the stocking rate resulted in reduced infiltration during the grazing season.

Carbon exchange and species composition of grazed pastures and exclosures in the shortgrass steppe of Colorado

Grasslands comprise approximately 40% of the world’s terrestrial surface. Consequently, grassland ecosystems are a significant component of the global carbon cycle. In order to better understand how grazing affects the carbon cycle of grasslands, this study measured CO2 exchange rate (CER) and soil respiration rate (SRR) on 130 ha pastures with a 56-year history of heavy (60% removal) and light (20% removal) grazing, and their accompanying 0.8 ha exclosures, on the shortgrass steppe of northeastern Colorado, USA. A CER chamber that covered 1 m 2 of native grassland was used on five plots in each of the four areas. Mid-day CER and SRR were measured during the growing seasons of 1995–1997, along with green vegetation index (GVI, similar to leaf area index) and plant species composition. When averaged over each growing season, there was no significant difference in CER of grazed pastures versus exclosures. However, there were seasonal differences in CER, which varied over the 3 years. Differences in CER between grazed pastures and exclosures were not related to GVI, which rarely differed between treatments. Grazing treatment differences in CER were driven by climate variability and species composition differences resulting from long-term grazing and exclusion from grazing. Exclosures had more cool-season (C3) grasses and forbs than grazed plots, which contained more warm-season (C4) grasses (primarily Bouteloua gracilis(H.B.K.) Lag. Ex Steud.). The somewhat unique, cool spring of 1995 was favorable to cool-season plant metabolism and resulted in higher CER in exclosures compared with grazed pastures. Warm, dry conditions in spring of 1996 favored warm-season species, resulting in higher CER in the heavily-grazed pasture. In 1997, there was little difference in CER between grazed pastures and exclosures. There were very few sampling dates when SRR was different in grazed pastures and exclosures. This study suggests that these intensities of cattle grazing do not alter the photosynthetic and soil respiration components of the carbon cycle of the US shortgrass prairie. It appears that cattle grazing can be a sustainable component of managing this ecosystem for maximum global carbon sequestration. Published by Elsevier Science B.V.

Carbon fluxes on North American rangelands

Abstract Rangelands account for almost half of the earths land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Influence of Topsoil Depth on Plant and Soil Attributes of 24-Year Old Reclaimed Mined Lands

ABSTRACT Topsoil replacement on reclaimed mine lands is vital for improved infiltration, plant rooting media, enhanced nutrient cycling, and as a potential source of plant propagules to increase plant community diversity. Varying topsoil depth may influence reclamation success. This study assessed the long-term (24 years) effects of four topsoil replacement depths (0, 20, 40, and 60 cm) on plant community attributes (species richness, diversity, canopy cover, and production) and soil characteristics [organic carbon (C), total nitrogen (N), available phosphorus (P), pH, soluble cations, electrical conductivity (EC), and cumulative water infiltration]. Species richness and diversity were highest at the 0 cm topsoil depth and lowest at the 60 cm topsoil depth. Percent canopy cover of grasses was highest (25%) at 60 cm and lowest (15%) at 0 cm topsoil depth. Percent forb cover was highest (6%) at the 0 cm depth and lowest (2%) at 60 cm topsoil depth. Seeded species cover was highest (12%) at the 40 cm depth, but was not significantly different from the other depths. Aboveground biomass was similar between the 40 (727 kg ha−1) and 60 cm (787 kg ha−1) topsoil depths and higher than the 0 (512 kg ha−1) and 20 cm (506 kg ha−1) replacement depths. Plant species richness and diversity decreased with increasing topsoil depth, while biomass increased. Organic C mass in the soil profile (75 cm) was greatest in the 60 cm topsoil replacement (18.7 Mg C ha−1) and lowest in the 0 and 20 cm treatments (11.3 and 10.5 Mg C ha−1, respectively). N mass (75 cm profile) exhibited a similar pattern with 60 cm of topsoil having the highest (1.9 Mg N ha−1) and the 0 and 20 cm the lowest (0.8 Mg N ha−1and 0.9 Mg N ha−1, respectively). Cumulative water infiltration was highest (134 mm) for the 40 cm topsoil depth followed by 60 cm (116 mm), and lowest (61 mm) for the 0 cm treatment. Soil N, organic C, and infiltration data indicate topsoil replacement depths of 40 and 60 cm provide the best nutrient status and water storage potential for sustainable reclamation. Placement of shallow topsoil replacement depths should be carefully planned to ensure topsoil thickness is adequate to sustain a vegetative community capable of protecting the soil surface against erosion. Variable topsoil replacement depths can be used in reclamation to manipulate plant community characteristics and create a mosaic of vegetation types. However, the reduced vegetation cover observed at the shallower topsoil depths may not protect against soil erosion; therefore, using variable topsoil depth replacement as a reclamation practice will require careful planning and implementation.

Functional Group and Species Responses to Precipitation in Three Semi-Arid Rangeland Ecosystems

The objective of this study was to compare forage production and foliar and basal cover responses of plant communities, plant functional groups, and individual species between years with below average (2004) and well above-average (2005) spring precipitation in three semi-arid rangeland ecosystems (shortgrass steppe, northern mixed-grass prairie, and sagebrush grassland). Foliar and basal cover at the time of a peak standing crop were visually estimated using modified Daubenmire cover categories, and forage production by species was harvested from areas that had been excluded from large herbivores. Responses of forage production to precipitation, but not foliar and basal cover, were similar for the three semi-arid ecosystems. Total forage production was more responsive (75–159%) than basal (8–35%) or foliar (2–29%) cover to increasing precipitation. Absolute (1016 kg·ha−1) and relative (159%) increases in total forage production from 2004 to 2005 were greatest for the shortgrass steppe. Forage production increases were largely attributable to greater production by C3 perennial graminoids in each ecosystem; increases in basal and foliar cover for this plant functional group were observed in shortgrass steppe and sagebrush grassland, but not in northern mixed-grass prairie. Fine-scale inputs of species and plant functional group responses to precipitation will further the accuracy of forage prediction models in predicting both total biomass production and relative proportions of plant biomass.

Forage Production and Quality of a Mixed-Grass Rangeland Interseeded With Medicago sativa ssp. falcata

Abstract Interseeding alfalfa into rangelands has been assessed for decades as a method of range improvement to increase forage production and forage quality for livestock. Research was initiated in 2001 to examine the long term effects of interseeding yellow-flowered alfalfa (Medicago sativa ssp. falcata) on northern mixed-grass rangelands. Forage production and forage quality parameters were assessed on sites interseeded in 1965, 1987, and 1998 and compared to adjacent native rangelands. Live aboveground biomass for the 1965, 1987, and 1998 interseeded sites was 68, 143, and 42% higher, respectively, compared to their native control areas. Alfalfa aboveground biomass accounted for 1 489 of the 2 969 kg·ha−1 live biomass harvested from the 1965 interseeded site, 1 940 of the 2 744 kg·ha−1 on the 1987 interseeded site, and 796 of the 2 322 kg·ha−1 on the 1998 interseeded site. Increased soil N resulting from N fixation by the alfalfa significantly increased the crude protein (CP) content of several native species, whereas the alfalfa itself provided forage with 16 to 18% CP. Alfalfa had higher protein degradability and provided higher concentrations of calcium (Ca), potassium (K), and magnesium (Mg) than the native rangeland grasses. This research has shown that the practice of interseeding yellow-flowering alfalfa into rangelands is sustainable over decades and will increase forage production and improve nutritive value of forage in the northern Great Plains.

ACCUMULATION OF ORGANIC CARBON IN RECLAIMED COAL MINE SOILS OF WYOMING

The potential to sequester carbon and increase organic nutrient storage in disturbed soils, such as those reclaimed after surface coal mining, appears to be significant. Quantification of organic carbon accumulation is complicated, however, by the presence of coal and coal dust in these soils. Our preliminary data on organic matter content of reclaimed soils at surface coal mines in Wyoming suggest they are sequestering carbon at a rapid rate. Data from a surface mine reclamation site near Hanna, WY indicate that surface (0-15 cm) soil organic carbon content has increased from a low of 10.9 g C kg soil in 1983 to 18.6 g C kg soil in 1998 and to 20.5 g C kg soil in 2002. Undisturbed soil directly adjacent to the reclaimed site has a mean organic carbon content of 15.1 g kg soil. At a mine near Glenrock, WY, soil organic carbon at a site reclaimed in 1979 increased from an estimated low of 5.8 g C kg soil to a current level of 18.4 g C kg soil. Organic carbon content of undisturbed soils adjacent to the reclaimed area range from 9.9 to 15.7 g C kg soil. In contrast to the elevated organic carbon content, amounts of microbial biomass in reclaimed soils at both mines are lower than in nearby undisturbed soils (ca. 60% or less). We have collected similar data from a number of other surface coal mines in Wyoming. We hypothesize that decomposition rates are slow in reclaimed mine soils due to low microbial activity relative to that in undisturbed soils. Additional

Canopy Growth and Density of Wyoming Big Sagebrush Sown with Cool-Season Perennial Grasses

Post-mining revegetation efforts often require grass seeding and mulch applications to stabilize the soils at the same time as shrub seeding, creating intraspecific competition between seeded shrubs and grasses that is not well understood. Artemisia tridentata Nutt. ssp. wyomingensis (Beetle and Young) (Wyoming big sagebrush) is the dominant premining shrub on many Wyoming mines. The Wyoming Department of Environmental Quality, Land Quality Division requires reestablishment of 1 shrub m−2on 20% of post-mined lands in Wyoming. Reclamationists seldom document the impacts of grass competition on shrub canopy size after reclamation plantings become established even though shrub canopy development is important to vegetative structural diversity. In 1999, we initiated a study at the Belle Ayr Coal Mine near Gillette, Wyoming, to evaluate the influence of grass competition on establishment and growth of Wyoming big sagebrush. Combinations of three sagebrush seeding rates (1, 2, and 4 kg pls ha−1) and seven cool-season perennial grass mixture seeding rates (0, 2, 4, 6, 8, 10, and 14 kg pls ha−1) were seeded during winter 1998–1999. Shrub density and grass cover were assessed from 1999 to 2004. We monitored sagebrush canopy size in 2001, 2002, and 2004. All sagebrush seeding rates provided shrub densities ≥1 shrub m−2 after six growing seasons. Grass production ≥75 g m−2 was achieved by seeding grasses at 6 to 8 kg pls ha−1. Canopy growth of individual sagebrush plants was least in the heaviest grass seeding rate. Reduced grass seeding rates can aid in achieving Wyoming big sagebrush density standards and enhance shrub canopy growth.