No abstract

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 treecrop 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.

Understanding crop resilience to environmental stress is critical in predicting the consequences of global climate change for agricultural systems worldwide, but to date studies addressing crop resiliency have focused primarily on plant physiological and molecular responses. Arbuscular mycorrhizal fungi (AMF) form mutualisms with many crop species, and these relationships are key in mitigating the effects of abiotic stress in many agricultural systems. However, to date there is little research examining whether (1) fungal community structure in agroecosystems is resistant to changing environmental conditions, specifically water limitation and (2) resilience of fungal community structure is moderated by agricultural management systems, namely the integration of trees into cropping systems. Here, we address these uncertainties through a rainfall reduction field experiment that manipulated short‐term water availability in a soybean‐based (Glycine max L. Merr.) agroforest in Southern Ontario, Canada. We employed terminal restriction fragment length polymorphism analysis to determine the molecular diversity of both general fungal and AMF communities in soybean roots under no stress, stress (rainfall shelters added), and poststress (rainfall shelters removed). We found that general fungal and AMF communities sampled from soybean roots were resistant to rainfall reduction in a monoculture, but not in an agroforest. While AMF communities were unchanged after stress removal, general fungal communities were significantly different poststress in the agroforest, indicating a capacity for resiliency. Our study indicates that generalist fungi and AMF are responsive to changes in environmental conditions and that agroecosystem management plays a key role in the resistance and resilience of fungal communities to water limitation.