Gas exchanges and growth of soybean as affected by irrigation depths and soil compaction

The availability of water above compacted soil layer improves the performance of soybean ( Glycine max (L.) Merr.) gas exchanges. However, studies do not address the responses of the photosynthetic process of soybean in compacted soil. The objective of this study was to evaluate the dynamics of gas exchange, growth and performance of soybean in compacted soil with poor irrigation. The experiment was conducted in a greenhouse with a completely randomized design in a 3 × 4 split plot for three soil densities (1.0 ± 0.013, 1.15 ± 0.022 and 1.30 ± 0.035 Mg m -3 ) and four daily water depths (4.0, 5.0, 6.0 and 7.0 mm d -1 ) with three replicates. Net photosynthesis rate (A), transpiration rate (E), stomatal conductance (g s ), water use efficiency (WUE), instantaneous water use efficiency (iWUE), net assimilation rate, growth rate of shoot and root, relative growth rate of shoot and root, shoot biomass, root biomass at different layers (upper, compacted and lower), total root biomass and pod biomass were evaluated. The 4.0 and 5.0 mm d -1 water depths provided higher WUE and iWUE, but not soybean biomass accumulation; with these water depths and 1.15 Mg·dm -3 , gas exchange and accumulation of shoot biomass improved. From 45 to 65 d after emergence (DAE), shoot growth rate had a maximum peak at 7.0 mm d -1 in compacted soil. The growth rate and relative growth rate of soybean roots had smaller increases in biomass at 65 DAE. Daily water depths of 4.0 and 5.0 mm d -1 provided the best results in terms of WUE, however, it did not reflect an accumulation in soybean biomass. In these depths, soil compaction at 1.15 Mg m -3 provided an increase in g s , E, and A.