Canopy Carbon Dioxide Profile in Relation to Row Spacing of ‘Essex’ Soybean

Abstract

AbstractLittle field work has been done on how row spacing in soybean [Glycine max (L.) Merr.] alters the canopy microclimate. Canopy CO2 profiles were measured for ‘Essex’ soybean planted in 0.25‐, 0.38‐, and 0.51‐m row spacings with or without a wheat (Triticum aestivum L.) straw mulch (4.5 Mg ha−1) in 1983, and in 0.13‐, 0.25‐, 0.51‐, and 0.76‐m row spacings with no mulch treatment in 1984, under irrigation on Sequatchie loam (a fine‐loamy, siliceous, thermic Typic Hapudult). Canopy CO2 for individual treatments and days was modeled as a function of time of day and height, with R2 values ranging from 0.52 to 0.97. Analysis of covariance for pooled data during reproductive growth indicated significant differences in CO2 among row spacings in the upper three fourths of the canopy. Upper canopy level CO2 was 5 to 15 µL L−1 lower after midday but slightly higher at dusk in the narrower versus wider row spacings and seemed related to a higher leaf density. Mulching did not influence canopy CO2. After midday, the CO2 in the row middles was 5 to 10 µL L−1 lower than over the drill for the 0.76‐m row spacing, which had incomplete canopy closure. Horizontal deviations in CO2 were not evident in the treatments that had complete canopy closure. Carbon dioxide profile variations observed among row spacings seemed to be highly related to leaf area density.