| Corn (Zea mays L.), a major grain crop in Ontario, Canada, is grown using varied crop rotations and tillage systems. Corn growth and yield varies across these production systems and it is hypothesized that yield differences are, in part, due to direct effects of soil aggregate size distribution, and/or indirectly, by affecting other soil characteristics such as availability of nutrients, water, oxygen, and soil temperature.; To evaluate the effect of aggregates on early corn growth, a series of experiments were carried out with objectives; (1) to evaluate the effect of crop rotation and tillage practices on soil aggregate distribution, and its relationship to corn biomass production; (2) to characterize corn root morphological responses to aggregate size under conditions where indirect effects of aggregates are removed; and (3) to determine possible mechanisms involved in corn response to aggregates.; In a field experiment, rotation and tillage system independently affected corn dry matter production. In soils of continuous corn, 54% to 59% of the soil mass occurred in the largest aggregate size class (>8mm) at planting and this was significantly higher when compared to soils of other rotations. Corn plant biomass was negatively correlated to the percentage of >8mm soil aggregates [r=-0.34 (p=0.009) and r=-0.79 (<0.0001) in year 2004 and 2005, respectively]. Similarly in a growth room experiment, corn plant biomass was reduced by aggregate size when grown in four size classes of turface (0.2mm, 0.6mm, 1.5mm, and 4.5mm) under controlled environmental conditions. Corn response to both direct and indirect effects of aggregate size occurred as early as 12 days after emergence (DAE). Reductions of shoot biomass production, leaf area and root characteristics, were also significant at very early stages.; To isolate the effect of aggregate size, an experiment was conducted by developing and utilizing a hydroponics system with controlled environmental condition, wherein corn was grown in Turface (0.2mm and 4.5mm) or in a nutrient solution. Soil aggregates directly affected corn root and shoot growth at very early stages of growth (7-15 DAE) even at non-limiting nutrients and water status. Coarse aggregate reduced shoot biomass by 50% and 25% at 7 and 15 DAE, respectively, compared to fine and no aggregate treatments. This was not observed at later stages due to plant adaptation to aggregates when resources are non-limiting. Similar trend was observed in root characteristics. Early season suppression due to direct effect of coarse aggregates in the seed bed may not be overcome by providing additional nutrients or irrigation.; The early season direct effect of coarse aggregate was observed even when only a part of the root system was grown in coarse aggregates while another part of root system was in fine or no-aggregate stress media. High level of carbon loss as exudates and/or respiration by roots grown in coarse aggregate may not explain the effect of aggregates on early corn growth. Preferential root growth was not observed in fine aggregates or no-aggregate medium. Root originating sensing and signaling mechanisms may be involved in governing the effect of aggregates observed in early corn growth; however this idea could not be conclusively evaluated due to greater root pruning stress observed. |
