SIGNAL will answer the question whether agroforestry systems are more sustainable than conventional agriculture and thus will increase the land equivalent ratio (LER). The two main criteria that we will use to evaluate this are the efficiency with which nutrients and water are used in agroforestry and conventional systems. We expect that in agroforestry systems a higher input of root and leaf litter of crops, grass and trees will positively influence the composition of the soil community, its functions (litter decomposition, N & P mineralization, N, P, K, Ca, Mg retention), and related soil properties (water holding capacity, aggregate stability) and thus ultimately increase crop yields.
Project results from phase 1
Nutrient response efficiency (NRE) describes how efficient plant-available soil nutrients are used (NRE = biomass productivity / unit plant-available nutrient). The combined growth curves and nutrient response efficiency (NRE) curves of agroforestry and conventional systems showed that nutrient availability at studied systems was not limiting plant productivity. From these results, we conclude that presently high fertilization rates that surpass crop demands preclude more optimal NRE values for agroforestry systems. We expect that a significant reduction in applied fertilizer will result in relatively minor reductions in crop yield, but at the same time considerable increases in nutrient response efficiencies.
Project results from phase 2
Once nutrient saturation (nutrient availability in excess of plant demands) is reached in any agroecosystem, productivity will not further increase, and the efficiency of nutrient utilization substantially decreases, posing a high risk of nutrient losses to the environment. In phase 2, despite a 2-year reduction in fertilization, crop yield in agroforestry and monoculture systems did not decrease, with a potential for increased NRE. This maintained yield is due to the prevailing nutrient saturation conditions, supporting our earlier findings. Although agroforestry enhanced three important ecological functions (carbon sequestration, habitat for
biological activity, erosion resistance), the presently high fertilization rates mask its potential to increase efficient nutrient-use and -retention.
Expected results phase 3
We expect that NRE will increase further toward its optimum. Next, results from the 1st and 2nd phases showed
more positive than negative interactions between rows of trees and crops. Based on full tree-rotation (up to 7yrs)-crop evaluation, we expect that 3-4yrs will be the optimal age for facilitating tree-crop interactions in alley-cropping agroforestry; at younger tree age (1-2yrs) these interactions are less pronounced; at higher age (5-7yrs), competition becomes dominant. We also expect nitrate leaching losses to reduce with increasing tree age and in agroforestry compared to monocultures, because of effective soil biological mechanisms such as root distribution, N immobilization, and denitrification.
