The emergence of markets for mitigation of climate change presents new opportunities for increasing economic and ecological returns to rangelands in developing countries. Improving rangeland management is a potentially significant source of mitigation from sequestration. It is appealing due to the likely links to sustainable agricultural development and poverty reduction. Many of the changes needed to sequester carbon are also associated with improved rangeland productivity and incomes. We provide an overview of the key issues that arise in determining the potential of carbon markets to support improved rangeland management focusing on West Africa, an area where pastoralism is a major economic activity with extensive rangelands that offer considerable potential for sequestering carbon. Estimates of the potential for increasing sequestration through improved rangeland management are summarized. Per hectare amounts are low, but aggregate potential is high. Carbon emission reductions are generated by reducing or avoiding land degradation, rehabilitating degraded lands, and increasing native carbon stocks by increasing aboveground and belowground biomass. Avoiding degradation and rehabilitating lightly degraded lands are the least costly and can generate significant carbon emission reductions. Carbon offsets from agricultural sources are currently limited under regulatory cap and trade regimes, and prices in voluntary markets are relatively low. Low returns to carbon offsets per hectare mean that significant co- benefits in the form of increased rangeland productivity and incomes would be necessary to induce participation. High transactions costs can be a problem in carbon markets and in adopting improved rangeland management practices, highlighting the need for institutions to provide effective coordination, monitoring, and enforcement. Evidence from Burkina Faso suggests the potential for existing local-level institutions to play an important role in future carbon payment programs, should they emerge. 

Rangelands and savannas occupy 70% of the Australian continent and are mainly used for commercial grazing of sheep and cattle. In the center and north, where there are extensive areas of indigenous land ownership and pastoral production is less intensive, savanna burning is frequent. Greenhouse gas emissions from rangelands have been overwhelmingly from land clearing and methane production by livestock. Reductions in the rate of land clearing have substantially reduced Australia’s greenhouse gas emissions, but these have been controversial with the reduced potential pasture production being of concern to many land managers. Reductions in direct livestock emissions are possible through manipulation of the genetics, rumen flora, or diet of animals. However, the greatest potential benefit is a whole-property approach with improved animal husbandry and attention to other aspects of property management such as fossil fuel consumption. Focus on emissions per unit of land area is likely to have other ecological benefits for land condition and to capture the effects of changes in carbon stocks in vegetation and soils. In much of northern and central Australia, changes in settlement patterns have led to more frequent and intense fires than under indigenous management regimes before European settlement. The implementation of more benign regimes of savanna burning has great potential benefit for greenhouse abatement, biodiversity, and livelihoods of indigenous people in remote settlements. 

Nutrient controls of ecosystem pattern and process have been widely studied at the Jasper Ridge Biological Preserve, a well-studied California rangeland ecosystem. Here we review these studies, from leaf to landscape scales, with the intention of developing a deeper understanding of carbon (C)-nutrient interactions in such an ecosystem. At the leaf scale, several studies conducted on diverse plant species have revealed a strong positive relationship between leaf nitrogen (N) concentrations and maximal rates of photosynthesis. This relationship, which has subsequently been observed globally, can be explained by the nutritional requirements of photosynthetic machinery. Consistent with this local physiological constraint, N availability has been shown to limit carbon uptake of California rangeland ecosystems. In some cases phosphorus (P; and N plus P) limits productivity, too—particularly in serpentine soils, pointing to the importance of parent material in regulating CO2 uptake at landscape scales. Nutrient dynamics are also affected by herbivory, which seems to accelerate N and P cycles over the short term (years), but may lead to nutrient limitation of plant production over the longer term (decades). Simulated global change experiments at Jasper Ridge have also provided insight into C-nutrient interactions in grasslands. In particular, several field- based experiments have shown that CO2 doubling does not necessarily simulate productivity of California grasslands; rather, the strength and sign of net primary productivity (NPP) responses to CO2 doubling varies across years and conditions. Although simulated N deposition stimulates NPP, N plus CO2 combinations do not necessarily increase productivity beyond N treatments singly. Poorly understood feedbacks between plants, microbes, and P availability may underlie variation in the response of California grasslands to increasing atmospheric CO2 concentrations. We conclude that interactions between C, N, and P appear especially vital in shaping plant productivity patterns in California rangelands and the capacity of this ecosystem to store additional C in the future. 

Grasslands are one of the most modified biomes on Earth. Land use changes had a large impact on carbon (C) stocks of grasslands. Understanding the impact of land use/land cover changes on C stocks and fluxes is critical to evaluate the potential of rangeland ecosystem as C sinks. In this article we analyze C stocks and fluxes across the environmental gradients of one of the most extensive temperate rangeland areas: the R ́ıo de la Plata Grasslands (RPG) in South America. The analysis summarizes information provided by field studies, remote sensing estimates, and modeling exercises. Average estimates of aboveground net primary production (ANPP) ranged from 240 to 316 g C m-2 yr-1. Estimates of belowground NPP (BNPP) were more variable than ANPP and ranged from 264 to 568 g C m-2 yr-1. Total Carbon ranged from 5 004 to 15 008 g C m-2. Plant biomass contribution to Total Carbon averaged 13% and varied from 9.5% to 27% among sites. The largest plant C stock corresponded to belowground biomass. Aboveground green biomass represented less than 7% of the plant C. Soil organic carbon (SOC) was concentrated in the slow and passive compartments of the organic matter. Active soil pool represented only 6.7% of the SOC. The understanding of C dynamics and stocks in the RPG grasslands is still partial and incomplete. Field estimates of ANPP and BNPP are scarce, and they are not based on a common measurement protocol. Remotely sensed techniques have the potential to generate a coherent and spatially explicit database on ANPP. However, more work is needed to improve estimates of the spatial and temporal variability of radiation use efficiency. The absence of a flux tower network restricts the ability to track seasonal changes in C uptake and to understand fine-scale controls of C dynamics. 

Prescribed burning of aboveground biomass in tallgrass prairie is common and may influence dynamics and magnitudes of carbon (C) movement between the surface and atmosphere. Carbon dioxide (CO2) fluxes were measured for 2 yr using conditional sampling systems on two adjacent watersheds in an ungrazed tallgrass prairie near Manhattan, Kansas. One watershed was burned annually (BA) and the other biennially (BB). Leaf and soil CO2 fluxes were measured in the source area. Net ecosystem exchange (NEE) of CO2 reached a maximum daily gain of 26.4 g CO2 m-2 d-1 (flux toward surface is positive) in July 1998 (year when both sites were burned and precipitation was above normal); gains were similar between sites in 1998. The maximum daily NEE loss of CO2 was 221.8 g CO2 m-2 d-1 from BA in September 1997 (year when only BA was burned and precipitation was below normal). When data were integrated over the two years, both sites were net sources of atmospheric CO2; NEE was –389 g C m-2 2 yr-1 on BA and –195 g C m-2 2 yr-1 on BB. Burning increased canopy size and photosynthesis, but the greater photosynthesis was offset by corresponding increases in respiration (from canopy and soil). Carbon losses from fire represented 6-10% of annual CO2 emissions (bulk came from soil and canopy respiration). Data suggest that annual burning promotes C loss compared to less-frequently burned tallgrass prairie where prairie is not grazed by ungulates. Greater precipitation in 1998 caused large increases in biomass and a more positive growing season NEE, indicating that C sequestration appears more likely when precipitation is high. Because C inputs (photosynthesis) and losses (canopy and soil respiration) were large, small measurement or modeling errors could confound attempts to determine if the ecosystems are long-term CO2 sources or sinks. 

This article presents a new parameterization of the global vegetation organizing carbon and hydrology in dynamic ecosystems (ORCHIDEE) model, modifying the assimilation, allocation, and phenology processes for a steppe ecosystem. It aims 1) to improve the modeled growth primary production (GPP) based on both in situ CO2 flux measurements and remote-sensing data of the fraction of absorbed photosynthetic radiation, and 2) to evaluate how GPP improvement results in better-modeled fluxes for ecosystem respiration, net ecosystem exchange, and latent heat. This new parameterization leads to a realistic annual GPP (comparable to the data within 10%), and a realistic seasonal variability of GPP (R2 = 0.80). Further, we found that improving GPP into ORCHIDEE immediately brings ecosystem respiration and net ecosystem exchange fluxes into better agreement with the eddy-covariance data, both on seasonal but also on interannual time scales. This result suggests that the response of this steppe ecosystem to interannual climate variations can be well reproduced from the response to seasonal variation, and that biotic effects are not interannual. This indicates the potential ability to reproduce climate-induced changes in the carbon balance of steppes with the use of a generic process-oriented vegetation model such as ORCHIDEE. 

Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO2 exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO2 fluxes in a sagebrush-steppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167-183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grünwald, K. Havránková, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J. M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11:1424-1439). Maximum values of the quantum yield (a = 75 mmol mol-1), photosynthetic capacity (Amax = 3.4 mg CO2 m-2 s-1), gross photosynthesis (Pg, max = 116 g CO2 m-2 d-1), and ecological light-use efficiency (eecol = 59 mmol mol-1) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO2. Maximum values of gross primary production (8 600 g CO2 m-2 yr-1), total ecosystem respiration (7 900 g CO2 m-2 yr-1), and net CO2 exchange (2 400 g CO2 m-2 yr-1) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO2, with mean net uptake of 700 g CO2 m-2 yr-1 for intensive grasslands and 933 g CO2 m-2 d-1 for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO2, this does not imply that they are necessarily increasing their carbon stock. 

Grazinglands, vegetated land that is grazed or has the potential to be grazed by wild and/or domestic ungulates, occupy about half of the world’s surface, provide livelihoods to almost one billion people, and constitute a major component of the global carbon stock and cycle. Grazinglands have significant potential for mitigation of climate change in the short term, and management practices that lead to mitigation are fairly well known. However, the degree to which this mitigation potential is realized pivots on our ability to design cost-effective protocols to promote the implementation of such management practices. In order to create efficient protocols we need a deeper and more precise knowledge of the processes and factors that affect greenhouse gas (GHG; mainly CO2, N2O, and CH4) fluxes and carbon (C) stocks in grazinglands.