Management Practices to Increase Nitrogen Retention and Decrease Losses in Agricultural Systems.

To achieve global food security greatly increased nitrogen (N) inputs to agriculture will be required, especially from fertilizers but also from more efficient recycling of N from manures and organic by-products and through inclusion of legumes in cropping systems. But increased inputs bring increased risks of N losses which cause environmental damage. Global estimates indicate that, on average, 50% of fertilizer N is unaccounted for in crop uptake or soil retention, implying a loss to the environment of about 50 Tg /yr. Timing, form and method of application of fertilizer N and slurries or manures can be adjusted to minimise direct losses, though it is common for decreased loss via one pathway to lead to an increase through another. A range of “precision farming” technologies can assist in improved targeting of N applications in timing, position or quantity. Various sensors are increasingly available, both tractor-mounted and linked to fertilizer delivery or hand-held chlorophyll meters suitable for small farmers. Even leaf color charts can be surprisingly beneficial for managing N timing by small-holder farmers in less developed regions.  In some situations inhibitors of nitrification or urease activity are effective.

Predicting and managing N mineralized from soil organic matter, crop residues or manures is a central factor in improved N utilization, but in practice difficult to achieve. In temperate regions, ¹⁵N-fertilizer studies commonly show that some 20% of applied fertilizer N is retained in soil organic matter at the end of the first growing season; in tropical regions it is often double this. Residual organically-bound N is a source of N through future mineralization, but its release is slow. Intercropping cereals with legumes is useful in some systems. In the long-term inclusion of biological nitrogen fixing activity into cereal crops may be possible, but it is likely improved agronomy based on incremental understanding of N cycle processes and more precise management will be the key approach for many years.