Agroforestry Trees for Nutrient Cycling and Sustainable Management

Abstract

The integration of trees with crops can influence both the supply and availability of nutrients in the soil. Trees can increase the supply of nutrients within the rooting zone of crops through (i) input of nitrogen (N) by biological N2 fixation, (ii) capture and "pumping up" of nutrients from below the rooting zone of crops and (iii) reduction in nutrient losses by leaching and erosion. Trees can increase the availability of nutrients through increased release of Nutrients from soil organic matter (SOM) and recycled organic materials. Roots of trees frequently extend beyond the rooting depth of crops. The potential of deep-rooted trees to capture subsoil nutrients is (i) greatest when trees have a high demand for nutrients, (ii) greatest when high amounts of plant-available nutrient are present in the subsoil and (iii) greater for mobile nutrients like nitrate, than for less mobile nutrients, like phosphate. Nitrate can accumulate below the rooting depth of annual crops when (i) production of the crops is limited by pests and nutrients other than N and (ii) subsoils contain anion exchange sites to sorb the nitrate. Recent research showed that Sesbama sesban is very effective in taking up accumulated subsoil nitrate. Sesbania is also more effective than natural grass fallows in extracting subsoil water, which suggests that the potential for leaching loss of nutrients is less under sesbania than natural fallows. Other research showed that fast-growing trees with high N demand, such as Calliandra calothyrsus, S. sesban and Eucalyptus grandis, are much more effective in taking up subsoil nitrate than slower-growing trees, like Grevillea robusta and Markhamw lutea. The mineralization of SOM is a source of plant-available N and phosphorus (P) Nutrient release from SOM is normally more dependent on the portion of the SOM in biologically active fractions than on total quantity of SOM. Recent research indicated that the amount of N in the sand-associated fraction of SOM that floats in a dense liquid (referred to as light fraction N) is directly related to the release of N from SOM. Yield of unfertilized maize on a N-limiting soil has been shown to be strongly related to both inorganic soil N before maize planting and light fraction N. Inorganic soil N, N mineralization, and aI110unt of light fraction N were higher after 2 and 3-yr tree fallows that continuous unfertilized maize in Zambia and Kenya Among six tree fallows, inorganic N, N mineralization and light fraction N were (i) higher for the mean of the five N-fixing trees than the one non-fixing tree. (ii) higher for the mean of the two trees with lowest (lignin + polyphenol)/N ratios leaflitter than the two trees with highest ratios in leaf litter, and (iii) higher for S. sesban than the mean of the other trees. Some agroforestry trees have potential to provide sufficient N to sustain moderate crop yields through (i) increased N inputs from biological N fixation and deep capture of nitrate and (ii) effective recycling of N from plant residues and manures. Agroforestry trees, on the other hand, are not likely to provide sufficient P for sustained crop yields. Phosphorus is not fixed from the air like N, and the capture of P from subsoil is typically small. The recycling of P from organic materials is normally