Carbon and nitrogen dynamics in agroforestry systems : temporal patterns of some important soil processes

The ameliorative effects of different tree species on soils in some agroforestry systems were studied. The temporal pattern of nutrient release from tree organic material is important to achieve synchrony with crop uptake. When, as in many tropical agroforestry systems, trees (C3-plants) are planted on C4-carbon dominated soils, the difference of around 12-16%o in natural abundance of l3C between C3 and C4 plants makes the natural abundance of l3C a particularly sensitive indicator of the influence of trees on the soil. An increase of 3-5% of the percentage C was proven to derive from trees by this method, only five years after planting. By using the difference of around 10%o in natural abundance of l3C between the endogenous soil C (mainly C4) and the applied C (C3) in green manure experiments, the contributions of the two C sources to soil respiration can be calculated. The microbial response to the additions of leaves was an immediate increase in respiration. This non-destructive method allows repeated measurements of the actual rate of C mineralisation and facilitates decomposition studies with high temporal resolution in the field. The mineralisation of N was also very rapid and the concentration of NH4+ in the soil correlated well with respiration of added C. In our studies, 3-4% of the added C was respired daily, for the first 10 days after addition of Sesbania sesban leaves. Although respiration rates decline with time, we estimated 70-90% to be respired in as short time as 40 days. Weight losses of around 80% after 52 days, from high quality residues in litterbags, also indicate substantial C losses. Measurable build-up of soil organic matter is, hence, unlikely. For immediate soil fertility, addition of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application, inducing the risk of nutrient losses before the nutrient demand of crops is high. Green manure of poorer quality mineralised slower and may have the potential to build up SOM, but not to meet the short-term nutrient requirements of crops. In improved fallow systems there is a mixture of qualities of organic material added to the soil, as there are differences in quality between, e.g., leaves and roots, thereby reducing the risk of nutrient losses. Sesbania fallows added 280-360 kg N ha'1 by N2-fixation, resulting in a positive N-balance, after wood export and two residual maize crops, of 170-250 kgN ha1. The Sesbania fallows also produce considerable amounts of firewood and, most importantly, increase the residual maize yields