The productivity of many pastoral lands, particularly in the tropics and arid zones, is restricted by nitrogen and other nutrient limitations and the unsuitability of some native species to high-intensity grazing. Introduction of nitrogen-fixing legumes and high-productivity grasses or additions of fertilizer can increase biomass production and soil carbon pools.
Use and Potential
Native grasses in grasslands, woodland, and shrublands in some regions are sensitive to heavy grazing, and growth is often nutrient-limited. Introduction of grass and legume species to overcome these limitations has a long history. Such introductions, where successful, can significantly increase primary production by up to a factor of three or more (Montes and Masco, 1996), thereby increasing soil and biomass carbon. Legume and grass introductions increase soil carbon by about 0.08 and 3.97 t C ha-1 yr-1, respectively (Conant et al., 2000; Tables 4-4 and 4-6), although the value for grasses is significantly higher in a study by Fisher et al. (1997) that introduced African grasses to South America.
The area of grassland is variously estimated as 1900-4400 Mha, with a median value of 3220 Mha derived from the IGBP-DIS database (Loveland and Belward, 1997). In developed nations, much of this land may be improved, but in these and other regions there is likely to be considerable further potential. For example, Fisher et al. (1994) suggest that there may be 35 Mha of suitable land in South America that could store 100-507 Mt C yr-1 by using deep-rooted African grasses. This estimate is thought to be too high because of economic and management constraints (Fearnside and Barbosa, 1998); carbon storage will likely plateau as decomposition rates match increased carbon inputs (Davidson et al., 1994).
Species characteristics such as resistant root material appear to influence carbon storage (Urquiaga et al., 1998). In some environments, productivity declines over time because of nitrogen immobilization in the root mass (e.g., Robbins et al., 1989; Robertson et al., 1997). Nitrogen can be released by soil disturbance, which can also release soil carbon in the short term. Wills et al. (1996) suggest that there is potential for the establishment of chenopod shrubs over about 6 Mha in New Zealand, with growth of 2-3 t C ha-1. Similar opportunities exist to grow halophyte shrubs in coastal deserts, inland saline soils, or salinized irrigated lands (Table 4-5).
Plant growth on many pastoral lands is limited by lack of nutrients. Additions (particularly phosphorus, nitrogen, and sulfur) can result in large growth responses-which lead to increases in biomass and soil carbon pools, especially in more mesic environments. Normal management practice, however, is to increase the harvesting intensity of the additional biomass produced (e.g., Winter et al., 1989), leading to potentially little change in carbon stocks. In the absence of grazing, increases in aboveground biomass carbon are maximized. In New Zealand, for example, fertilizer application to pasture increased total biomass and soil carbon (0-25 cm) by 7.6 t C ha-1 in ungrazed grasslands, compared with 6.3 t C ha-1 in grazed pasture (McIntosh et al., 1997). Even with grazing, fertilizer application in the semi-arid tropics increased aboveground grass biomass from 0.88 to about 3 t C ha-1 (Cameron and Ross, 1996). In prairie vegetation, nitrogen fertilization (0.224 t N ha-1 yr-1) over 40 years resulted in an increase of about 22 t SOC ha-1 compared with unfertilized plots (Schwab et al., 1990).
Addition of superphosphate to phosphate-poor soil in conjunction with planting of legumes has been shown to more than double soil organic matter levels (Williams and Donald, 1957; Russell, 1960; Barrow, 1969). Dalal and Carter (1999) suggest that phosphorus and sulfur fertilization over 56 Mha of northern Australia could increase carbon sequestration by 28 Mt C yr-1 (280 Mt C over 10 years). Haynes and Williams (1992) reported that SOC (0-4 cm) levels of 37-year-old grazed pastures that received 0, 188, and 376 kg of superphosphate per year were greater than or equal to SOC levels at their "wilderness" site.
Curent Knowledge and Scientific Uncertainties
The basic processes that change carbon stocks under this activity are well understood. The extent of the potential area for this activity and the proportion of potential carbon storage benefits that will result are uncertain. Rates of increase of soil carbon are generally higher in more mesic environments than in drier ones (Table 4-4); these rates vary between ecosystems, with woodlands and native grasslands exhibiting the greatest increases (Conant et al., 2000). Background environmental changes such as climate variability, climate change, and atmospheric CO2 increases may impact on carbon stocks.
Where direct sampling is used, soil carbon pools must be measured throughout the profile down to at least 1 m because of deep carbon storage with some species. In at least some environments, surface soil carbon is correlated with pasture condition (Ash et al., 1996), which could be assessed as described in Fact Sheet 4.6. There is some capacity to model carbon changes, if information about climate, soils, species characteristics, fertilizer applications, and livestock usage is sufficiently well known.
Monitoring, Verifiability, and Transparency
Techniques for repeat direct sampling exist for the biomass and soil components (Chapter 2). There may be opportunities to develop approaches to scale up these results by using the approaches that are briefly described in Fact Sheet 4.6. Agricultural statistics on areas of improved pastures and fertilizer use and associated information on livestock density and characteristics may be needed to make evaluations of likely impacts on system carbon stores.
Soil carbon can continue to accumulate for periods of greater than 40 years (Conant et al., 2000). For example, linear increases in soil carbon over 40- to 50-year periods have been recorded (e.g., Russell, 1960). Carbon accumulation will plateau at some point, however, and this time is likely to vary substantially between systems and with specific activities.
Biomass is removed by livestock and other herbivores, by fire, and through detachment and decomposition. Soil carbon is removed mainly by decomposition and erosion. Both processes are strongly influenced by management.
Cessation of these practices will tend to result in restrictions of further carbon storage. Soil carbon is likely to have varying residence times, extending to millennia. Biomass carbon is vulnerable to disturbance but will recover rapidly in most circumstances, and mean levels of carbon are likely to be higher than in the absence of the activity given suitable grazing regimes.
Increased agricultural productivity is likely, as is some loss of biodiversity from native grass ecosystems. Increased legume components are likely to increase acidification rates in tropical (Noble et al., 1997) and temperate (Helyar et al., 1997) pastures, through increased leaching of nitrate and increased productivity. Productivity may fall if pH is lowered too far. Scanning accessions for lower rates of acid excretion may lead to introduction of new varieties that impose less acidification risk (Tang et al., 1998). Optimization of fertilizer application rates can reduce these risks and reduce off-site impacts from nutrient leaching and pollution of waterways and groundwater.
Increases in legumes may result in more nitrous oxide emissions than from native grass pastures (Veldkamp et al., 1998). Increased digestibility and protein content of improved pastures, however, may reduce livestock methane emissions substantially (Kurihara et al., 1999). The radiative forcing of methane emissions is about an order of magnitude larger than that of nitrous oxide emissions from tropical grasslands (Howden et al., 1994). Addition of nitrogenous fertilizer is associated with nitrous oxide emissions. These emissions and methane emissions from livestock are accounted under the current IPCC Guidelines.
Relationship to IPCC Guidelines
The Reference Manual includes procedures for estimating changes in soil carbon stocks associated with pasture management as it affects productivity. Individual practices (e.g., species replacement, fertilization), however, are not dealt with, and general default values are provided in the Workbook only for improved, unimproved, and degraded pasture soils.
Other reports in this collection