Forestry and agriculture are important sources of carbon dioxide, methane, and nitrous oxide. The world's forests contain vast quantities of carbon. Some forests act as "sinks" by absorbing carbon from the air, while forests whose carbon flows are in balance act as "reservoirs". At the global level, deforestation and changes in land use make forests a net source of carbon dioxide. As for agriculture, it accounts for about 20% of the human-enhanced greenhouse effect. Intensive agricultural practices such as livestock rearing, wet rice cultivation, and fertilizer use emit 50% of human-related methane and 70% of our nitrous oxide. Fortunately, measures and technologies that are currently available could significantly reduce net emissions from both forests and agriculture - and in many cases cut production costs, increase yields, or offer other socio-economic benefits. Forests will need better protection and management if their carbon dioxide emissions are to be reduced. While legally protected preserves have a role, deforestation should also be tackled through policies that lessen the economic pressures on forest lands. A great deal of forest destruction and degradation is caused by the expansion of farming and grazing. Other forces are the market demand for wood as a commodity and the local demand for fuel-wood and other forest resources for subsistence living. These pressures may be eased by boosting agricultural productivity, slowing the rate of population growth, involving local people in sustainable forest management, adopting policies to ensure that commercial timber is harvested sustainably, and addressing the underlying socio-economic and political forces that spur migration into forest areas. The carbon stored in trees, vegetation, soils, and durable wood products can be maximized through "storage management". When secondary forests and degraded lands are protected, they usually regenerate naturally and start to absorb significant amounts of carbon. Their soils can hold additional carbon if they are deliberately enriched, for example with fertilizers, and new trees can be planted. The amount of carbon stored in wood products can be increased by designing products for the longest possible lifetimes, perhaps even longer than what is normal for living wood. Sustainable forest management can generate forest biomass as a renewable resource. Some of this biomass can be substituted for fossil fuels; this approach has a greater long-term potential for reducing net emissions than does growing trees to store carbon. Establishing forests on degraded or non-forested lands adds to the amount of carbon stored in trees and soils. In addition, the use of sustainably-grown fuel-wood in place of coal or oil can help to preserve the carbon reservoir contained in fossil fuels left unneeded underground. Agricultural soils are a net source of carbon dioxide - but they could be made into a net sink. As much as 400800 million tonnes of carbon could be taken up by agricultural soils every year through improved management practices designed to increase agricultural productivity. Low-tech strategies include the use of composting and low- or no-tillage practices, since carbon is more easily liberated from soil that is turned over or left bare. In the tropics, soil carbon can be increased by returning more crop residues to the soil, introducing perennial (year-round) cropping practices, and reducing periods when fallow fields lie bare. In semi-arid areas, the need for summer fallow could be reduced through better water management or by the introduction of perennial forage crops (which would also eliminate the need for tillage). In temperate regions, soil carbon could be increased by the use of more animal manure. One recent study suggests that reduced tillage practices alone could convert US agricultural soils from an estimated net source of 200 million tonnes of carbon per year to a net sink of 200300 million tonnes by the year 2020, while improving yields for some crops. Methane emissions from livestock could be cut with new feed mixtures. Cattle and buffalo account for an estimated 80% of annual global methane emissions from domestic livestock. Additives can increase the efficiency of animal feed and boost animals' growth rates, leading to a net decrease of 515% in methane emissions per unit of beef produced. In rural development projects in India and Kenya, adding vitamin and mineral supplements to the feed mixture of local dairy cows has significantly increased milk production and decreased methane emissions. Laboratory experiments with bovine somatotropin, a growth hormone for cows, have increased milk production in dairy cows while reducing methane emissions by up to 9%. Methane from wet rice cultivation can be reduced significantly through changes in irrigation and fertilizer use. Some 50% of the total cropland used to grow rice is irrigated. Today's rice farmers can only control flooding and drainage in about one-third of the world's rice paddies, and methane emissions are higher in continually flooded systems. Recent experiments suggest that draining a field at specific times during the crop cycle can reduce methane emissions by up to 50% without decreasing rice yields. Additional technical options for reducing methane emissions are to add sodium sulfate or coated calcium carbide to the urea-based fertilizers now in common use, or to replace urea altogether with ammonium sulfate as a source of nitrogen for rice crops. Nitrous oxide emissions from agriculture can be minimized with new fertilizers and practices. Fertilizing soils with mineral nitrogen and with animal manure releases N2O into the atmosphere. By increasing the efficiency with which crops use nitrogen, it is possible to reduce the amount of nitrogen needed to produce a given quantity of food. Other strategies aim to reduce the amount of nitrous oxide produced as a result of fertilizer use and the amount of N20 that then leaks from the agricultural system into the atmosphere. One approach, for example, is to match the timing and amount of nitrogen supply to a crop's specific demands. Another is to use advanced fertilization techniques such as controlled-release fertilizers and systems that deliver fertilizer to the plant's roots through its leaves rather than through the soil (where most nitrous oxide production occurs). The fertilizer's interactions with local soil and climate conditions can also be influenced by optimizing tillage, irrigation, and drainage systems.