Shade‐cloth Microclimate of Soybeans1

Abstract

AbstractA shade cloth tent was erected over a crop of soybeans [Glycine max (L.) Merr.], in a micrometeorological CO2 enrichment experiment, to restrict eddy diffusivity and vertical turbulent transport. This barrier was tried because earlier studies had shown that CO2 released in crops under natural conditions was rapidly dispersed by turbulence. The objectives were to evaluate plant response to the modified microclimate and the effectiveness of the shade cloth tent in reducing losses of applied CO2. Profiles of air temperature, CO2 concentration, water vapor concentration, and wind were measured both above and below the shade cloth and in an adjacent open field. Total solar, visible, and net radiation were also measured.Less light reached the shaded crop, but the plants grew larger and gave about the same dry‐matter yield. However the shaded crop lodged severely. Soil water was extracted at a slower rate by the shaded crop for two reasons: a) because less radiation reached the crop to supply energy for evapotranspiration and b) because a “humidity blanket” developed under the cloth barrier giving a smaller leaf‐to‐air vapor pressure gradient. The “humidity blanket” resulted from lower turbulent transport under the shade cloth. The shade cloth wind screen decreased water stress and water use, increased the amount and improved the distribution of diffuse irradiance, and increased the diffuse to direct‐beam irradiance ratio. Therefore shade culture might have potential beyond present‐day tobacco (Nicotiana tubacum L.) culture, for high value crops with low light saturation of photosynthesis, especially in arid climates with high solar radiation.Eddy diffusivity was reduced to about 16% at the height of the shade cloth, but CO2 concentration increases at the crop's top during releases were only about twice as large with the shade cloth in place as they were with the shade cloth removed. Therefore the shade cloth was moderately effective in trapping released CO2 but much still escaped.