Agroforestry promotes soybean yield stability and N-2-fixation under water stress

Agroforestry systems can buffer crop performance against the impacts of climate change, particularly decreases in the availability of soil water. Nonetheless, farmers are reluctant to adopt agroforestry systems due to concerns of yield losses, predominantly in the tree-crop competitive zone. Yet little is known about crop performance in the tree-crop competitive zone under water limited conditions. We therefore studied the effect of a full season water deficit on soybean N-2-fixation, nodulation, and yield in the tree-crop zone of a mature agroforestry system. We hypothesized higher N-2-fixation yet lower but stable yield in the tree-crop zone. Rainfall reduction shelters were used to reduce available soil moisture throughout the growing season in the tree-crop competitive zone of a 27-year-old tree-based intercropping agroforestry system and a paired monoculture control plot in southern Ontario, Canada. Results show that soybean yields in the tree-crop zone were lower compared to monoculture. However, soybean yields were stable in agroforestry and only in monoculture did the rainfall reduction induce a significant decline in soybean yields. Soybeans in the tree-crop zone relied heavily on N-2-fixation to meet N demand, with a percentage of N derived from atmosphere (%Ndfa) of 91 % versus 63 % in monoculture. However, total fixed N declined significantly under rainfall reduction in both the tree-crop zone and in monoculture. Of note, soybean nodulation patterns adapted to soil moisture availability, allocating a larger proportion of nodules lower on the rooting system under water limitation. Our results demonstrate that important N pathways may be altered under water limitation. We can also expect that in areas where growing conditions are predicted to become drier in the future, yields in the tree-crop competitive zone will not be reduced further, thus increasing the viability of adopting agroforestry systems in areas affected by climatic change.