Defining competition vectors in a temperate alley cropping system in the midwestern USA - 2. Competition for water

A study was carried out in an alley cropping system in the Eastern Maize Belt of the United States with the objective of quantifying the competition for water between trees black walnut (Juglans nigra L.) and red oak (Quercus rubra L.) and maize (Zea mays L.). Spatial and temporal variation in soil moisture, tree and maize rooting patterns, tree and maize water uptake, and maize leaf area expansion were determined in three treatments; 'barrier' (polyethylene root barriers separating maize and tree roots), 'trench' (trenching without a polyethylene barrier), and 'no barrier' (control). Significant temporal variation in soil moisture was observed in both black walnut and red oak alley cropping systems. The 'barrier' and 'trench' treatments resulted in higher soil moisture in the 'maize alley' and lower soil moisture in the 'tree row' than the 'no barrier' treatment. Uptake of water by maize was higher in the 'barrier' than the 'no barrier' treatment. However, tree water uptake was higher in the 'no barrier' treatment than in the 'barrier' treatment because tree roots utilized water from the 'maize alley'. Maize leaf area was negatively impacted by water stress in the 'no barrier' treatment. Quantification of rooting patterns revealed that maize and tree roots were concentrated in the top 30 cm soil layer where water fluctuations were greatest. The 'barrier' and 'trench' treatments successfully eliminated the belowground root competition between trees and maize and resulted in greater leaf area and higher grain yields than the 'no barrier' treatment. Thus, competition for water rather than competition for light seems to be critical in defining the productivity and sustainability of this alley cropping system.