Grassland management practices are likely to affect:
- 1) the storage of carbon in the soil. Grasslands accumulate carbon mainly in the form of organic matter in the first thirty centimeters of soil. The carbon stock of a meadow is a function of pedoclimatic conditions, plot history, age, and floristic composition of the canopy. The key factors in the constitution and preservation of the carbon stock are the levels of primary production (plant biomass produced) and organic restitution (livestock manure from pasture or manure spreading), as well as soil disturbances which, by accelerating the mineralization of organic matter, induce destocking.
- 2) N2O emissions related to mineral nitrogen fertilization or manure management. These emissions are all the more important as the nitrogen inputs are in excess of the vegetation absorption capacity; however, recent studies suggest that actual fertilization habits would often exceed effective intakes by a quarter. A reduction in nitrogen inputs thus makes it possible to reduce the direct and indirect emissions of N2O. N2O emissions also come from animal droppings; a change in their diet can affect the amount of nitrogen excreted and therefore N2O emissions. The overturning of the grassland, by accelerating the decomposition of soil organic matter, transforms organic nitrogen into mineral nitrogen, causing direct and indirect N2O emissions.
- 3) CO2 emissions due to fossil fuel consumption during interventions on grasslands; and
- 4) CH4 emissions related to enteric fermentation and manure management. They are influenced by the diet; they are reduced by a ration based on grass. Emissions resulting from the fermentation of manure are more important in the anaerobic conditions that prevail in building and during the storage of effluents, than in the grasslands.
What are the possibilities for optimizing grassland management on these different criteria?
A first action is to lengthen the duration of grazing: put the animals in pasture earlier in the spring, and return them later in the end of autumn. Grazing is likely to increase N2O emissions because soil can be compacted by trampling animals (more anaerobic conditions), and manure can move under less favorable moisture or temperature conditions than when spreading is performed under selected conditions. However, grazing avoids the N2O and CH4 emissions associated with the indoor management of manure and the spreading of effluents, as compared to indoor consumption of harvested grass. Animal load on the plot has an influence on direct and indirect N2O emissions and on CH4 emissions from enteric fermentation and animal waste.
Increasing the duration of temporary grassland is also being studied, reducing the frequency of grassland turnover: less fuel consumed, and a longer carbon storage phase, without increasing temporary grassland areas.
"Desintensify" grasslands would result in lower fertilizer inputs, and would not generally be detrimental as grasslands are currently over-fertilized. A decrease of the contributions of 10 to 14% according to the situation would be easily possible.
At the same time, the small intensification of low productivity permanent grasslands could increase carbon storage by stimulating crop production: moderate grass removal, and increased manure input, by a simple means which is increasing animal loading on these grasslands.