Biogeochemical consequences of establishing a diverse agroforestry system in place of annual row crops: A preliminary analysis

Background/Question/Methods

Annual, monoculture cropping systems, have become the standard agricultural model in the Midwestern US. Unintended consequences of these systems include surface and groundwater pollution, greenhouse gas emissions, loss of biodiversity, and soil erosion. Diverse agroforestry (DA) systems dominated by fruit and nut trees/shrubs have been proposed as an agricultural model for the Midwestern US that can restore ecosystem services while simultaneously providing economically viable and industrially relevant staple food crops. Despite the popularity of these systems among specialty farmers and the wide range of anecdotal data supporting their benefits, no replicated studies have been done to quantify the potential biogeochemical impact of these systems implemented on large scales. A DA system including six species of fruit and nut crops was established on long-time conventional agricultural land at the University of Illinois at Urbana-Champaign in 2012, with the conventional corn-soybean rotation (CSR) as a control. The initial changes of the carbon, nitrogen, and water cycles during the first two years of transition were measured to determine the speed and magnitude of the biogeochemical consequences of this land-use transition. 

Results/Conclusions

Woody biomass increased significantly during the DA system’s first two years. Very low (<5%) mortality led woody biomass to approximately double during the system’s second season. Net annual N₂O flux from the DA system was about half that of the CSR system, indicating a dramatically reduced greenhouse gas impact. Since no external fertilizer inputs were used in the DA system, it is likely that the N₂O flux observed was due to pre-transition residual excesses of soil inorganic nitrogen. Consequently, a continued gradual decline in net annual N₂O flux is expected. Net nitrate leaching through the soil was reduced to a negligible amount in the DA system. Such a rapid decrease in leaching suggests that the transition to DA systems has the potential to all but hault non-point source nutrient loading in the Midwest with very little lag time. Although cumulative ET was very similar between the two systems, the seasonal patterns were quite different. In contrast to the CSR, the DA system exhibited a much longer growing season, but had a lower peak ET. Overall, these preliminary results suggest that the land use transition from CSR to DA systems can have positive biogeochemical consequences soon after establishment.