Soil-based carbon sequestration (CS) can provide a significant, but finite sink for atmospheric CO2.

In recognition that social and economic factors ultimately govern the sustained adoption of land-based CS, strategies have been sought that sequester carbon while providing tangible production benefits to farmers (Ponce-Hernandez et al., 2004). For arable systems, no-till cultivation has been promoted as a "win-win" strategy for achieving net Global Warming Potential (GWP) reductions. Tillage disrupts soil aggregates, making organic matter pools that had been physically protected from microbial degradation more vulnerable to decomposition (Duxbury, 2005). Higher levels of soil organic matter are associated with attributes, such as crop tilth, water holding capacity and fertility that are favorable to crop growth (e.g., Lal, 1997). Although concerns have been raised about the methodologies used to assess soil carbon stocks (Baker et al., 2007), recent synthesis of data from many sites across the United States suggests that adoption of no-till (West and Post, 2002) or conversion of cropland into perennial pastures (Post and Kwon, 2000) generates soil organic carbon increases on the order of 450 kg C ha-1 yr-1. Depending on factors such as soil texture and land use history, maximum rates of C sequestration tend to peak 5-10 yrs after adoption of CS practices and slow markedly within two decades. Hence increasing the organic matter content of soils is as an interim measure for sequestering atmospheric CO2. Estimates from the United States suggest that if all US cropland was converted to no-till, enhanced CS rates would compensate for slightly less than 4% of the annual CO2 emissions from fossil fuels in the U.S. (Jackson and Schlesinger, 2004). On a global scale, carbon sequestration in soils has the potential to offset from 5 to 15% of the total annual CO2 emissions from fossil fuel combustion in the near-term (Lal, 2004).

Improved management of the vast land area in rangelands has led to significant carbon sequestration, but the benefits of carbon credit payments are not currently accessible, particularly in common property systems.

Grazing lands cover 32 million km2 and sequester large quantities of carbon (UNDP/UNEP/WB/WRI, 2000). Processes that reduce carbon sinks in grazing lands include overgrazing, soil degradation, soil and wind erosion, biomass burning, land conversion to cropland; carbon can be improved by shifting species mixes, grazing and degradation management, fire management, fertilization, tree planting (agroforestry), and irrigation (Ojima et al., 1993; Fisher et al., 1994; Paustian et al., 1998). But where land is held in common, mitigation is particularly complex. Mitigation activities are most successful when they build on traditional pastoral institutions and knowledge (excellent communication, strong understanding of ecosystem goods and services) and provide pastoral people with food security benefits at the same time (Reid et al., 2004).

Agroecosystems involving tree-based carbon sequestration can offset greenhouse gas emissions.

Early assessments of national and global terrestrial CO2 sinks reveal two primary benefits of agroforestry systems: direct near-term C storage (decades to centuries) in trees and soils, and, potential to offset immediate greenhouse gas emissions associated with deforestation and shifting agriculture. On a global scale, agroforestry systems could potentially be established on 585-1275 106 ha, and these systems could store 12-228 (median 95) tonnes C ha-1 under current climate and soil conditions (Dixon, 1995). In the tropics, within 20-25 years the rehabilitation of degraded farming systems through the development of tree-based farming systems could result in above-ground carbon sequestration from 5 tonnes C ha-1 for coffee to 60 tonnes C ha-1 for complex agroforestry systems (Palm et al., 2005a). Belowground carbon sequestration is generally lower, with an upper limit of about 1.3 tonnes C ha-1 yr-1 (Palm et al., 2005a). Agroforestry systems with nitrogen-fixing tree species, which are of particular importance in degraded landscapes, may be associated with elevated N2O emissions (Dick et al., 2006). The benefits of tree-based carbon sequestration can have an environmental cost in terms of some soil modification (Jackson et al., 2005) (see 3.2.2.1.7).

The value of increased carbon sequestration in agroecosystems (e.g., from no-till) must be judged against the full lifecycle impact of CS practices on net greenhouse warming potential (GWP).

Increased carbon sequestration is not the only GWP-related change induced by adoption of agronomic practices like no-till. No-till maize systems can be associated with comparatively large emissions of N2O (Smith and Conen, 2004; Duxbury, 2005). Over a 100-yr timeframe, N2O is 310 times more potent in terms of GWP than CO2 (Majumdar, 2003) and higher N2O emissions from no-till systems may negate the GWP benefits derived from increased rates of carbon sequestration. On the other hand, soil structural regeneration and improved drainage may eventually result in a fewer N2O emissions in no-till systems. Nitrogen fertilization is often the surest method for increasing organic matter stocks in degraded agroecosystems, but the benefits of building organic matter with N fertilizer use must be discounted against the substantial CO2 emissions generated in the production of the N fertilizer. By calculating the full lifecycle cost of nitrogen fertilizer, many of the gains in carbon sequestration resulting from N fertilization are negated by CO2 released in the production, distribution, and application of the fertilizer (Schlesinger, 1999; Follett, 2001; West and Marland, 2002).

Climate change is affecting crop-pest relations.