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Modeling salt movement and halophytic crop growth on marginal lands with the APEX model
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Saline soils negatively impact crop productivity in nearly 20% of irrigated agricultural lands worldwide. Cultivation of highly salt-tolerant plants, or halophytes, may increase productivity compared to conventional salt-sensitive crops (i.e., glycophytes), thereby increasing the economic potential of these marginal lands affected by salinity. To better understand the long-term impacts of halophyte cultivation on environmental quality, the integrated Agricultural Policy/Environmental Extender (APEX) model was adapted to simulate the growth of the facultative halophyte quinoa (Chenopodium quinoa), along with salt dynamics in the plant-soil-water system. Modifications to the model included salt uptake and salt stress functions that were formulated using data from a greenhouse study. Data from a field experiment conducted in Reno, NV with saline and non-saline soil treatments were used to further parameterize, calibrate, and validate the modified model. While initial simulation results were promising, differences between simulated and observed soil salinity and salt uptake values during the growing season in both the calibration and validation runs suggested that additional modifications to the salt uptake and soil salinity algorithms need to be made to improve the model. Salt uptake was underestimated in the calibration and validation runs, with alternating periods of fast and slow uptake rates that did not follow the observed patterns. Soil salinity peaks following irrigation and precipitation events were overestimated in the calibration and validations runs. To demonstrate the utility of a comprehensive management model like APEX, the calibrated model was used to run six preliminary 15-year scenarios to estimate effects of differences in irrigation amounts and salinity on quinoa biomass production, soils and water quality. Simulated annual biomass typically increased with increasing irrigation amounts. Modeled soil salinity increased in all scenarios, especially when saline irrigation water was used. Soil salt content increased by almost 30 t/ha/y in the high salinity, high irrigation scenario. In the non-saline (420 ppm), moderate irrigation scenario, salt uptake accounted for less than one third of salt inputs. Model results indicated that as soil salinity increased, biomass generally remained steady. Salt uptake increased in each scenario with increasing root zone salinity, but uptake was several orders of magnitude smaller than root zone salt, indicating that the potential for remediation of salt-affected soils by plant uptake may be very limited. Overall, simulation results suggest that quinoa can be successfully cultivated in saline, arid environments as long as soil moisture is sufficient. Still, further experiments are needed to better understand salt uptake dynamics and the stresses for other crops so that future model updates and simulations can better represent salt dynamics in plants and soils in a multitude of agricultural settings.