Agricultural Crops

The seasonal course of NEE over agricultural regions is much different than that of the native ecosystems we have examined (Figure 15.8). During a long portion of the year, the surface is respiring because it is bare or covered with detritus. At the start of the growing season, the landscape experiences a long period when the canopy is sparse because crops must grow from seed. During this growth period, rates of net CO2 uptake by crops are relatively low, compared with those of temperate and conifer forests. Maximum rates of carbon uptake occur after the crop achieves closure and are among the highest of all vegetation types, since crops are selected for high productivity, they grow on very fertile soils, they are fertilized, and they tend to have abundant soil moisture due to rainfall or irrigation (Ruimy et al. 1995). The period during which a crop canopy is closed is rather short, limiting the duration of maximum uptake rates. The occurrence of anthesis is another factor limiting carbon uptake rates of crops during the peak growing period. Respiration in wheat jumps distinctly after anthesis, as a substantial amount of assimilated energy is used to produce reproductive organs, which have high respiration costs (Baldocchi 1994; Rochette et al. 1995). During autumn and after harvest, ecosystem respiration peaks again, owing to the new input of fresh decomposable material and the warmth of the soils. Soil respiration during the fallow period is much lower than in native ecosystems, which have more decaying detritus on the ground or are maintaining the catabolism of a substantial overstory of trees and shrubs.

Because crops are fertilized and grow in regions with ample rainfall or are irrigated, sunlight is the main factor limiting NEE of crops. Like grasslands, the light response curve saturates when the canopy is sparse and becomes quasi-linear after full closure (Baldocchi 1994; Rochette et al. 1995; Ruimy et al. 1995). Photosynthetic pathway (C3 or C4) also affects maximum rates of CO2 uptake.

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