Poor availability of nutrients commonly constrains crop production in marginal environments of developing countries. Degraded soils and poor access to fertilizers limit the yields that can be produced by resource-poor farmers. In such circumstances, farmers need to use management techniques that maximize nutrient use efficiency of their crops. There are various techniques available, such as the use of legumes in crop mixtures or rotations, or careful placement and timing of fertilizer applications. In low-input agriculture the direct uptake of dissolved organic N (DON) by plants may be extremely important. DON represents a significant pool of soluble N in most ecosystems. Some plants may possess a greater capacity to take up DON rather than dissolved in organic N (DIN). DON is composed of many compounds which enter soil from a range of sources (e.g. litterfall, root and microbial exudation, turnover of roots and organisms, urine and faeces, organic fertilizers). My aim was to investigate the impact of plant residues on DON cycling when incorporated into soils and to study the uptake of DON in comparison with DIN by the plant root system. In addition, the secondary aim was to investigate the influence of the rhizosphere on the transformation DON in soil. In the first trial, three experimental treatments were used to alter
organic inputs: (1) Soil amended with straw (high C/N ratio), (2) Soil amended with grass residues (low C/N ratio), and (3) Non-amended (control).
Results indicated that soil solution NO3-and NH4 + accumulated in the grass-
amended soil in contrast to that amended with straw or in the unamended
control soil. Overall, straw immobilized DIN in solution. DON in the grass amended soil increased from day 14 to 21 and sharply decreased thereafter whilst the straw amended soil and control remained relatively constant. Contrary to expectation, the results indicate that addition of organic matter did not cause a large rise in DON relative to that of DIN. This suggests that in this high fertility agricultural soil the microorganisms rapidly break down DON contained in N rich organic residues to DIN. For N poor residues DON appears to represent a more important source of N, however, its availability to plants remains as yet unknown.