Explore the vast range of titles available.

Management of rangelands can aid in the mitigation of rising atmospheric carbon dioxide concentrations via carbon storage in biomass and soil organic matter, a process termed carbon sequestration. Here we provide a review of current knowledge on the effects of land management practices (grazing, nitrogen inputs, and restoration) and precipitation on carbon sequestration in rangelands. Although there was no statistical relationship between change in soil carbon with longevity of the grazing management practice in native rangelands of the North American Great Plains, the general trend seems to suggest a decrease in carbon sequestration with longevity of the grazing management practice across stocking rates. The relationship of carbon sequestration to mean annual precipitation is negative for both the 0 to 10 cm (0 to 3.9 in) and 0 to 30 cm (0 to 11.8 in) soil depths across stocking rates. The threshold from positive to negative carbon change occurs at approximately 440 mm (17.3 in) of precipitation for the 0 to 10 cm soil depth and at 600 mm (23.6 in) for the 0 to 30 cm soil depth. We acknowledge that largely unexplored is the arena of management-environment interactions needed to increase our understanding of climate-plant-soil-microbial interactions as factors affecting nutrient cycling. Continued refinement of estimates of terrestrial carbon storage in rangelands will assist in the development of greenhouse gas emissions and carbon credit marketing policies, as well as potentially modifying government natural resource conservation programs to emphasize land management practices that increase carbon sequestration.

Good management of rangelands promotes C sequestration and reduces the likelihood of these ecosystems becoming net sources of CO2. As part of an ongoing study, soil was sampled in 2003 to investigate the long-term effects of different livestock grazing treatments on soil organic carbon (SOC), total nitrogen (TN), and microbial communities. The three treatments studied (no grazing, EX; continuously, lightly grazed [10% utilization], CL; and continuously, heavily grazed [50% utilization], CH) have been imposed on a northern mixed-grass prairie near Cheyenne, WY, for 21 yr. In the 10 yr since treatments were last sampled in 1993, the study area has been subject to several years of drought. In the 0 to 60 cm depth there was little change in SOC in the EX or CL treatments between 1993 and 2003, whereas there was a 30% loss of SOC in the CH treatment. This loss is attributed to plant community changes (from a cool-season [C3] to a warm-season [C4] plant dominated community) resulting in organic C accumulating nearer the soil surface, making it more vulnerable to loss. Soil TN increased in the EX and CL treatments between 1993 and 2003, but declined in the CH treatment. Differences in plant community composition and subsequent changes in SOC and TN may have contributed to microbial biomass, respiration, and N-mineralization rates generally being greatest in CL and least in the CH treatment. Although no significant differences were observed in any specific microbial group based on concentrations of phospholipid fatty acid (PLFA) biomarkers, multivariate analysis of PLFA data revealed that microbial community structure differed among treatments. The CH grazing rate during a drought period altered plant community and microbial composition which subsequently impacted biogeochemical C and N cycles.

Sustainable intensification is an emerging model for agriculture designed to reconcile accelerating global demand for agricultural products with long-term environmental stewardship. Defined here as increasing agricultural production while maintaining or improving environmental quality, sustainable intensification hinges upon decision-making by agricultural producers, consumers, and policy-makers. The Long-Term Agroecosystem Research (LTAR) network was established to inform these decisions. Here we introduce the LTAR Common Experiment, through which scientists and partnering producers in US croplands, rangelands, and pasturelands are conducting 21 independent but coordinated experiments. Each local effort compares the outcomes of a predominant, conventional production system in the region ('business as usual') with a system hypothesized to advance sustainable intensification ('aspirational'). Following the logic of a conceptual model of interactions between agriculture, economics, society, and the environment, we identified commonalities among the 21 experiments in terms of (a) concerns about business-as-usual production, (b) 'aspirational outcomes' motivating research into alternatives, (c) strategies for achieving the outcomes, (d) practices that support the strategies, and (e) relationships between practice outreach and adoption. Network-wide, concerns about business as usual include the costs of inputs, opportunities lost to uniform management approaches, and vulnerability to accelerating environmental changes. Motivated by environmental, economic, and societal outcomes, scientists and partnering producers are investigating 15 practices in aspirational treatments to sustainably intensify agriculture, from crop diversification to ecological restoration. Collectively, the aspirational treatments reveal four general strategies for sustainable intensification: (1) reducing reliance on inputs through ecological intensification, (2) diversifying management to match land and economic potential, (3) building adaptive capacity to accelerating environmental changes, and (4) managing agricultural landscapes for multiple ecosystem services. Key to understanding the potential of these practices and strategies are informational, economic, and social factors-and trade-offs among them-that limit their adoption. LTAR is evaluating several actions for overcoming these barriers, including finding financial mechanisms to make aspirational production systems more profitable, resolving uncertainties about trade-offs, and building collaborative capacity among agricultural producers, stakeholders, and scientists from a broad range of disciplines.

Isotopic signatures of 13C were used to quantify the relative contributions of C3 and C4 plants to whole-ecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and tallgrass communities) along an east-west environmental gradient in the North American Great Plains. Functional group composition of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent responses to long-term livestock grazing along this gradient. C4 plants [primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated by C3 and C4 species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities. Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared to the ungrazed site (4022 vs. 3236 g C m-2). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m-2; tallgrass: 8273 vs. 8997 g C m-2). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass communities (7476-8280 g C m-2) than the shortgrass community (2517-3307 g C m-2) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass communities (0.06-0.10) compared to the shortgrass community (0.20-0.27) suggesting that variation in root organic matter inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation in the relative proportion of C3 and C4 functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of these grassland ecosystems.

Preparation of fatty acid methyl esters from forages comparing methanolic boron-trifluoride (BF3) to methanolic hydrochloric acid (HCl) as a catalyst in single-step direct transesterification has not been reported. Our objective was to compare 1.09 M methanolic HCl to 1.03 M (7%) methanolic BF3 as catalysts for direct transesterification of fatty acids in freeze-dried forage samples. Thin-layer chromatographic analysis revealed complete conversion of total lipid extracts to fatty acid methyl esters using both catalysts. Additionally, gas-liquid chromatography analysis confirmed similar (P = 0.95) total fatty acid concentrations for both catalysts. Regression analysis indicated that similar values for total concentration would be obtained (P < 0.0001; r2 = 0.96; slope = 0.98 ± 0.03) between the two catalysts. Concentrations of most individual fatty acids were similar (P = 0.17-0.99) for both catalysts. Summed weight percentages of identified fatty acids, as well as sum of unidentified peaks, were not affected (P = 0.27) by catalyst (91.8 and 8.7% vs. 90.8 and 9.2% for HCl and BF3, respectively). We conclude that 1.09 M methanolic HCl is an appropriate substitute for 1.03 M methanolic BF3 for preparation of fatty acid methyl esters from freeze-dried forage samples. This result is of interest because HCL is both less costly and less caustic than BF3.

Establishment of perennial grasses on degraded soils has been suggested as a means to improve soil quality and sequester carbon in the soil. Particulate organic carbon may be an important component in the increased soil carbon content. We measured particulate organic carbon [defined as organic carbon in the 53 to 2000 μm (0.002 to 0.08 in) size fraction] and mineral associated organic carbon (defined as the less than 53 μm (0.002 in) size fraction) at three locations in central Texas. Each location had a never-tilled native grassland site, a long-term agricultural site and a restored grassland on a previously tilled site. Organic carbon pool sizes varied in the surface 40 cm (16 in) of native grassland, restored grasslands and agricultural soils. The native grasslands contained the largest amounts of total organic carbon, while the restored grasslands and agricultural soils contained similar amounts of total organic carbon. Both particulate organic carbon and mineral associated carbon pools were reduced beyond the depth of tillage in the restored grass and agricultural soils compared to the native grassland soils. The restored grassland soils had a larger particulate organic carbon content than the agricultural soils, but the increase in particulate organic carbon was limited to the surface 5 cm (2 in) of soil. Trends in particulate organic carbon accumulation over time from nine to 30 years were not significant in this study.

An experiment was conducted to compare below-ground soil organic carbon and total nitrogen accumulation between caespitose and rhizomatous perennial grasses in long-term (<25 yrs) grazed and ungrazed sites in semiarid and mesic communities in the North American Great Plains. Development of greater nutrient pools beneath than between clones occurred at minimal clone basal areas (<60 cm²) for both caespitose species. Caespitose grasses accumulated substantially greater pools of carbon (20-200 fold) and nitrogen (50-500 fold) in soils to a depth of 10 cm beneath clones than rhizomatous grasses accumulated in rhizomes in both communities. Carbon and nitrogen pools in soils beneath caespitose clones exceeded combined (soil + rhizome) pools for rhizomatous grasses for a majority of the clone basal areas (>90 cm²) in the mesic community. In contrast, both pool sizes were smaller beneath the caespitose grass at all clone basal areas than the combined pools for the rhizomatous grass in the semi-arid community. The occurrence of larger soil nutrient pools beneath the rhizomatous species in the semiarid community was largely a consequence of niche separation for microsites characterized by soils with higher nutrient concentrations, rather than plant-induced increases in nutrient concentrations. Although nutrient islands do not occur beneath rhizomatous grasses, their distribution in the semi-arid community was restricted to microsites characterized by soils with higher SOC and N concentrations. A greater efficiency of nutrient accumulation per unit rhizome mass and the maintenance of rhizome nutrient pools of similar magnitude to those of the rhizomatous grass in the mesic community may also contribute to the distribution of rhizomatous grasses in semi-arid communities. The existence of nutrient islands beneath a wide range of clone sizes in both mesic and semi-arid communities provides circumstantial evidence to suggest that nutrient islands beneath caespitose grasses may contribute to clone fitness in this growth form.

Aboveground growth of C₄plants responds more strongly to atmospheric CO₂ concentration when soil water is limiting rather than abundant. Whether the same is true of root growth and morphology, however, remains to be evaluated. We investigated interactive effects of CO₂ and soil water on root growth and morphology of two C₄ grasses. Seedlings of the dominant C₄ grasses from tallgrass prairie, Schizachyrium scoparium and Andropogon gerardii, were grown for 8 weeks in an elongated, controlled environment chamber at CO₂ concentrations of 368 (ambient) and 203 (subambient) µmol mol⁻¹. Seedlings were maintained at either high (ca. 90%) or low (ca. 50%) soil relative water holding capacity (RWC). Both root and shoot systems of C₄ grass seedlings responded similarly to CO₂ enrichment irrespective of whether soil water was limiting or abundant. Root growth was affected primarily by increased RWC (40-51% increases) and secondarily by higher CO₂ (15-27% increases). The relative distribution of root surface area, number of root tips and length and volume of roots were significantly affected by CO₂ enrichment with proportional increases of 55-61%, 39-52%, 50-55% and 53-58%, respectively, occurring in very fine (0-0.3 mm) roots. The indirect effect of CO₂ enrichment on conservation of soil water in grasslands may be as important as direct photosynthetic response effects in the CO₂-induced enhancement of whole-plant growth in C₄ grasses.

1. Intraclonal regulation of the C4 caespitose perennial grass Schizachyrium scoparium was evaluated in response to various levels of above-ground (radiation intensity) and below-ground (soil volume) resource availability in the field for three successive growing seasons. We reasoned that the relative plasticity of clonal growth in response to various levels of resource availability may provide insight into the mechanism of intraclonal regulation. 2. Six naturally occurring clones were randomly assigned to each of five treatments: (i) unrestricted soil volume (control); (ii) large soil volume in which roots were confined to a cylinder of soil with a radius four times that of the clone; (iii) small soil volume, twice the clone radius; (iv) large soil volume with radiation intensity reduced to 55% of ambient; and (v) small soil volume with reduced radiation intensity. 3. Ramet recruitment increased within the first season following root confinement in large soil volumes, while ramet and leaf growth exhibited less plasticity. The effects of a reduction in radiation intensity were smaller, and, in contrast, increased ramet and leaf growth, but did not modify ramet recruitment. 4. The pattern and magnitude of ramet mortality in clones confined to the large soil volume were similar to those of clones in unrestricted soil volume, indicating that intraclonal regulation was sufficient to reduce ramet recruitment and thus density to within the carrying capacity established by resource availability. 5. Similar growth and ramet demography of clones in the unrestricted and small soil volumes suggest that unrestricted clones normally access resources from a soil volume similar to that of the small soil volumes. 6. Rapid clonal growth in response to root confinement to large soil volumes, resource acquisition from a relatively small soil volume that contains the zone of resource accumulation, and the capacity for rapid expression of ramet plasticity provide circumstantial evidence that individual caespitose clones may respond to resource accumulation in soils beneath them.

An experiment was conducted to evaluate the influence of long-term (> 25 yrs) grazing on soil organic carbon (SOC) and total soil nitrogen (N) accumulation beneath individual plants of three perennial grasses along an environmental gradient in the North American Great Plains. The zone of maximum SOC and N accumulation was restricted vertically to the upper soil depth (0-5 cm) and horizontally within the basal area occupied by individual caespitose grasses, which contributed to fine-scale patterning of soil heterogeneity. Long-term grazing mediated SOC and N accumulation in the tall-, mid-and shortgrass communities, but the responses were community specific. SOC and N were lower beneath Schizachyrium scoparium plants in long-term grazed sites of the tall-and midgrass communities, but higher beneath Bouteloua gracilis plants in the long-term grazed site of the shortgrass community. SOC, but not N, was greater in soils beneath compared to between S. scoparium plants in an abandoned field seeded in 1941, indicating that this caespitose grass accumulated SOC more rapidly than N. SOC and N were greater in the 0-5 cm soil depth beneath a caespitose grass (S. scoparium) compared to a rhizomatous grass (Panicum virgatum) in the tallgrass community, with no significant accumulation of either SOC or N beneath P. virgatum plants. Grazing appears to indirectly mediate nutrient accumulation beneath caespitose grasses along the environmental gradient by modifying the size class distribution of plants. Populations with a greater proportion of large plants have a greater potential for biomass incorporation into soils and may more effectively capture redistributed organic matter from between plant locations. Contrasting plant responses to grazing at various locations along the environmental gradient conform to the predictions of the generalized grazing model, as the selection pressures of grazing and aridity may have also influenced the ability of caespitose grasses to accumulate nutrients in soils beneath them by mediating grazing resistance, competitive ability and population structure.