water-holding capacity, and the physical environment for root development.

Agriculture has accelerated and modified the spatial patterns of nutrient use and cycling, especially the nitrogen cycle..

Nitrogen fertilizer has been a major contributor to improvements in crop production. In 2000, 85 million tonnes of N were used to enhance soil fertility (Figure 3-1). The use of N fertilizers affects the natural N cycle in the following ways:

1. increases the rate of N input into the terrestrial nitrogen cycle;
2. increases concentrations of the potent greenhouse gas N2O globally, and increases concentrations of other N oxides that drive the formation of photochemical smog over large regions of Earth;
3. causes losses of soil nutrients, such as calcium and potassium, that are essential for the long-term maintenance of soil fertility;
4. contributes substantially to the acidification of soils, streams, and lakes; and
5. greatly increases the transfer of N through rivers to estuaries and coastal oceans.

In addition, human alterations of the N cycle have increased the quantity of organic carbon stored within terrestrial ecosystems; accelerated losses of biological diversity, especially the loss of plants adapted to efficient N use, and the loss of the animals and microorganisms that depend on these plants; and caused changes in the composition and functioning of estuarine and near-shore ecosystems, contributing to long-term declines in coastal marine fisheries (Vitousek et al., 1997).

Innovative soil and crop management strategies can increase soil organic matter content, hence maintaining or enhancing crop performance.

The organic matter content of the world's agricultural soils is typically 50-65% of pre-cultivation levels (Lal, 2004). Strategies to increase soil organic matter (carbon) include the integration of crop and livestock production in smallscale mixed systems (Tarawali et al., 2001, 2004); no-till farming; cover crops, manuring and sludge application; improved grazing; water conservation and harvesting; efficient irrigation; and agroforestry. An increase of 1 tonnes in soil carbon on degraded cropland soils may increase crop yield by 20 to 40 kg ha-1 for wheat, 10 to 20 kg ha-1 for maize, and 0.5 to 1 kg ha-1 for cowpeas. The benefits of fertilizers for building soil organic matter through enhanced vegetation growth only accrue when deficiencies of other soil nutrients are not a constraint.

No-tillage and other types of resource-conserving crop production practices can reduce production costs and improve

soil quality while enhancing ecosystem services by diminishing soil erosion, increasing soil carbon storage, and facilitating groundwater recharge.

Low-External Input Sustainable Agriculture (LEISA) is a global initiative aimed at the promotion of more sustainable farming systems (www.leisa.info). In the US, more than 40% of the cultivated cropland uses reduced or minimum tillage. At the global scale, no-till is employed on 5% of all cultivated land (Lal, 2004), reportedly covering between 60 million ha (Harington and Erenstein, 2005; Dumanski et al., 2006; Hobbs, 2006) and 95 million ha (Derpsch, 2005). Minimum tillage is a low-cost system and this drives adoption in many regions. No-till can reduce production costs by 15-20% by eliminating 4-8 tillage operations, with fuel reductions of up to 75% (Landers et al., 2001; McGarry, 2005). Conservation agriculture, which combines no-till with residue retention and crop rotation, has been shown to increase maize and wheat yields in Mexico by 25-30% (Govaerts et al., 2005). In the USA, the adoption of notill increases soil organic carbon by about 450 kg C ha-1 yr-1, but the maximum rates of sequestration peak 5-10 yrs after adoption and slow markedly within two decades (West and Post, 2002). In the tropics soil carbon can increase at even greater rates (Lovato et al., 2004; Landers et al., 2005) and in the Brazilian Amazon integrated zero-till/ crop-livestock-forest management are being developed for grain, meat, milk and fiber production (Embrapa, 2006). On the down-side, no-till systems often have a requirement for increased applications of herbicide and can be vulnerable to pest and disease build-up (e.g., wheat in America in late 1990s).

Short-term improved fallows with nitrogen-fixing trees allow small-scale farmers to restore depleted soil fertility and improve crop yields without buying fertilizers.

Especially in Africa, short-rotation (2-3 years), improved agroforestry fallows with nitrogen-fixing trees/shrubs (e.g., Sesbania sesban and Tephrosia vogelii) can increase maize yield 3-4 fold on severely degraded soils (Cooper et al., 1996; Kwesiga et al., 1999). Unlike hedgerow inter-cropping, which as a high labor demand, these fallows are well adopted (Jama et al., 2006). Similar results can be achieved with legume trees and rice production in marginal, nonirrigated, low yield, conditions. The use of these improved fallows to free small-scale maize farmers from the need to purchase N fertilizers is perhaps one of the greatest benefits derived from agroforestry (Buresh and Cooper, 1999; Sanchez, 2002) and is a component of the Hunger Task Force (Sanchez et al., 2005) and the Millennium Development Project (Sachs, 2005). By substantially increasing maize yields in Africa, these easily-adopted fallows can reduce the gap between potential and achieved yields in maize.