Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands

Abstract

Soil carbon sequestration (enhanced sinks) is the mechanism responsible for most of the greenhouse gas (GHG) mitigation potential in the agriculture sector. Carbon sequestration in grasslands can be determined directly by measuring changes in soil organic carbon (SOC) stocks and indirectly by measuring the net balance of C fluxes. A literature search shows that grassland C sequestration reaches on average 5 6 30 g C/m 2 per year according to inventories of SOC stocks and 2231 and 77 g C/m 2 per year for drained organic and mineral soils, respectively, according to C flux balance. Off-site C sequestration occurs whenever more manure C is produced by than returned to a grassland plot. The sum of on-and off-site C sequestration reaches 129, 98 and 71 g C/m 2 per year for grazed, cut and mixed European grasslands on mineral soils, respectively, however with high uncertainty. A range of management practices reduce C losses and increase C sequestration: (i) avoiding soil tillage and the conversion of grasslands to arable use, (ii) moderately intensifying nutrient-poor permanent grasslands, (iii) using light grazing instead of heavy grazing, (iv) increasing the duration of grass leys; (v) converting grass leys to grass-legume mixtures or to permanent grasslands. With nine European sites, direct emissions of N 2 O from soil and of CH 4 from enteric fermentation at grazing, expressed in CO 2 equivalents, compensated 10% and 34% of the on-site grassland C sequestration, respectively. Digestion inside the barn of the harvested herbage leads to further emissions of CH 4 and N 2 O by the production systems, which were estimated at 130 g CO 2 equivalents/m 2 per year. The net balance of on-and off-site C sequestration, CH 4 and N 2 O emissions reached 38 g CO 2 equivalents/m 2 per year, indicating a non-significant net sink activity. This net balance was, however, negative for intensively managed cut sites indicating a source to the atmosphere. In conclusion, this review confirms that grassland C sequestration has a strong potential to partly mitigate the GHG balance of ruminant production systems. However, as soil C sequestration is both reversible and vulnerable to disturbance, biodiversity loss and climate change, CH 4 and N 2 O emissions from the livestock sector need to be reduced and current SOC stocks preserved.

The importance of grasslands for animal production and other functions: a review on management and methodological progress in the tropics

“…Grasslands and potential for GHG reduction Grasslands can potentially offset a major proportion of the global emissions of GHG due to livestock (Soussana et al, 2010). These GHG are derived primarily from emissions of enteric methane by ruminants, to a minor extent from nitrous oxide produced from excreta, indirectly from production of grain crops for animal feedstuffs and from deforestation to create new pastures (FAO, 2009).…”

“…Storage of C is higher under extensive management and low grazing pressures providing that soil nutrients are not limiting (Soussana et al, 2010). In a meta-analysis involving trials from 19 global grazing land regions, Follett and Schuman (2005) observed a general positive relationship between the C sequestration rate and the animal stocking density as livestock per hectare, the latter being obviously directly associated with pasture primary productivity.…”

“…However, further information is needed for tropical grasslands. The range of management practices that can influence the loss of soil C sequestration and increase C (Soussana et al, 2010) includes (i) avoiding soil tillage and the conversion of grasslands to cropping, (ii) moderate intensification of nutrient-poor permanent grasslands, (iii) the use of lower stocking density, (iv) increasing the duration of grass leys and (v) converting grass leys to grass-legume mixtures or to permanent grasslands. Comprehensive assessment of soil organic C sequestration requires net C accounting which also considers the global warming potential of non-CO 2 gas fluxes associated with defined agricultural practices (Bavin et al, 2009).…”

Grassland systems of red meat production: integration between biodiversity, plant nutrient utilisation, greenhouse gas emissions and meat nutritional quality

“…The ability of grassland to sequester carbon has also been put forward as a key strategy to partly mitigate GHG emissions from the livestock sector (Soussana et al, 2010). Jones and Donnelly (2004) noted that carbon sequestration rates for permanent grasslands ranged from 0.2 to 0.6 tonnes carbon/ha per year (equivalent to 0.7 to 2.2 tonnes CO 2 e/ha per year) according to management, while the conversion from arable to grassland significantly increased carbon sequestration to 8 tonnes carbon/ha per year (29 tonnes CO 2 e/ha per year).…”

“…Soussana et al (2010) also argued that soil carbon sequestration is reversible and vulnerable to disturbance, biodiversity loss and climate change.…”