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.

During vegetative growth maize can accumulate luxury nitrogen (N) in excess of what is required for biomass accumulation. When post-silking N uptake is restricted, this luxury N may mitigate N stress by acting as an N reserve that buffers grain yield and maintains plant function. The objective of this study was to determine if and how luxury accumulation of N prior to silking can buffer yield against post-silking N and/or water stress in maize. In a greenhouse experiment, maize was grown in high (N veg ) and low (n veg ) N conditions during vegetative growth. The n veg treatment did not affect biomass accumulation or leaf area by silking but did accumulate less total N compared to the N veg treatment. The N veg treatment generated a reserve of 1.1 g N plant -1 . Plants in both treatments were then subjected to water and/or N stress after silking. 15 N isotope tracers were delivered during either vegetative or reproductive growth to measure N remobilization and the partitioning of post-silking N uptake with and without a luxury N reserve. Under post-silking N and/or water stress, yield was consistently greater in N veg compared to n veg due to a reduction in kernel abortion. The N veg treatment resulted in greater kernel numbers and increased N remobilization to meet grain N demand under post-silking N stress. Luxury N uptake at silking also improved leaf area longevity in N veg plants compared to n veg under post-silking N stress, leading to greater biomass production. While post-silking N uptake was similar across N veg and n veg , N veg plants partitioned a greater proportion of post-silking N to vegetative organs, which may have assisted with the maintenance of leaf function and root N uptake capacity. These results indicate that N uptake at silking in excess of vegetative growth requirements can minimize the effect of N and/or water stress during grain-fill.

Corn producers often overapply nitrogen (N) fertilizer to minimize risk of yield loss because of uncertainty regarding actual corn N requirements. On-farm N trials, which are simple to deploy and interpret, may enable producers to better understand their corn N fertilizer requirements. We analyzed a database of corn yield response to N trials conducted in Ontario, Canada to determine if delta yield (dY) N trials can reliably estimate economic optimum N rates (EONRs) and to assess the financial liability of dY-EONR estimates. Delta yield is calculated as the yield difference between nonlimiting and very low (starter only) N rates. The dY-EONR estimation relationship is derived from a rectangular hyperbolic relationship between agronomic efficiency and dY. The derived dY-EONR relationship has a rapid initial estimated EONR rate of increase that diminishes with increasing dY and that approaches a constant increase rate of 16.1 kg N ha -1 . At a N-corn price ratio of 7, the dY-EONR estimation model has RMSE = 29.1 kg N ha -1 (R 2 = .64). Within range of recent corn and N prices, 64-80% of dY-EONR estimates in combined calibration and validation data (n = 746) had return losses less than $25 ha -1 relative to the actual EONR.Return losses exceeding $50 ha -1 occurred for 8-18% of the dY-EONR estimates, most of which (83-98%) were due to overestimation. Delta yield trials provide an easily implemented method for corn producers to conduct on-farm yield response trials repeatedly over years in order to obtain a better idea of their N requirements. INTRODUCTIONNitrogen (N) fertilizer contributes significantly to corn (Zea mays L.) production costs, but the actual requirement varies substantially among years and fields and spatially within fields (Scharf et al., 2005;Shanahan, Kitchen et al., 2008). Ransom et al. (2020) reported that corn producer-selected Abbreviations: AE, agronomic efficiency; dEY, grain yield increase associated with applying EONR; dY, delta yield or grain yield increase associated with applying nonlimiting N rates; EONR, economic optimum nitrogen rate; OMAFRA, Ontario Ministry of Agriculture, Food and Rural Affairs; PR, ratio of N cost and corn price.

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