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Constraints and capabilities of no-till dryland agroecosystems for bioenergy production




Lloyd, Grace Susanna, author
Hansen, Neil C., advisor
Brummer, Joe, committee member
Paustian, Keith, committee member
Leach, Jan, committee member

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Crop residues are receiving attention as potential feedstocks for lignocellulosic biofuels. Sustainable residue harvest may be limited by soil erosion and the need to maintain soil organic carbon (SOC). Little attention has been given to the potential for residue harvest in the semi-arid Great Plains, largely due to assumptions of low production levels and the strong erosive forces of wind. Due to expanding interest in growing dedicated biofuel crops on marginal lands, these studies examined the capabilities and constraints of harvesting agricultural residues from dryland production systems in the semi-arid Great Plains. The first study examined long-term production levels of grain and stover for wheat (Triticum aestivum), corn (Zea mays), and grain sorghum (Sorghum bicolor) at three no-till dryland cropping sites in wheat-corn-fallow and wheat-sorghum-fallow rotations, and evaluated the impact of stover removal on wind and water erosion, soil organic carbon dynamics, and future productivity. The Revised Universal Soil Loss Equation and the Wind Erosion Equation were used to simulate water and wind erosion under various levels of residue removal. The DAYCENT model was used to estimate changes in soil organic carbon, grain yields, and soil fertility, if 50% of corn and wheat stover were harvested each crop year. Model validation was performed by comparing long-term production rates and measured changes in soil organic carbon to model simulated output. Total aboveground biomass production for corn and sorghum, averaged over site and soil type, was 5550 ± 2810 kg ha-1 yr-1, with average stover production of 2750 ± 1570 kg ha-1 yr-1 and 2800 ± 1570 kg ha-1 yr-1 for grain. The total aboveground annual biomass production of wheat across all sites averaged 5840 ± 2440 kg ha-1. Wheat annual stover yields were 3940 ±1880 kg ha-1 and grain yields averaged 1950 ± 820 kg ha-1. A 50% stover removal rate only slightly increased water erosion from 0.53 Mg ha-1 yr-1 (no removal) to a maximum of 1.4 Mg ha-1 yr-1. Wind erosion was a bigger risk, with rates surpassing the tolerable erosion levels after removing 10 - 30% of corn stover, depending on site and soil landscape position. However, at all three sites, up to 80% of wheat straw could be harvested without surpassing tolerable erosion rates. Soil organic carbon (SOC) declined 6-9% after 96 years of simulating 50% removal of corn and wheat stover. Under 0% removal, SOC levels appeared relatively stable, with maximum declines of 2.0%. As SOC levels are very low in these dryland systems, these declines represent a very small net loss of SOC when compared to losses observed in more humid regions. Under current wheat-corn-fallow management, virtually all stover must remain in order to control wind erosion and maintain soil organic carbon. However, if dedicated non-grain bioenergy crops were grown on an annual basis, there could be 2500-2700 kg ha-1 of harvestable biomass yearly while still retaining enough residues to maintain SOC. Total biomass production of a dedicated non-grain energy crop could be higher than the biomass production of the grain crops examined, namely because energy is not diverted to grain, and non-grain crops are not as sensitive to the timing of water deficits. Replacement of the fallow period with a non-grain biomass crop could lower the amount of residue needed to control erosion. Elimination of the fallow period would likely reduce the amount of residue that must remain to maintain SOC, increasing the amount of biomass available for removal. The second study uses the DAYCENT model to simulate variable responses in fertility, yield, and soil organic carbon within two field landscapes in eastern Colorado. Grain yield, soil fertility, and soil carbon were all impacted by stover harvest, but the magnitude and direction of responses were dependent on soil type. Yield declines as great as 1615 and 1382 kg ha-1 were simulated for corn and wheat, respectively. Declines in annual mineralization rates as high as 13 g N m-2 yr-1 were observed with stover harvest, but simulated changes in mineralization rates were highly variable between soil types, with net mineralization rates increasing with stover removal in some years. The impact of stover harvest on soil organic carbon varied with soil type and landscape position. Results are used to highlight the variable impacts stover harvest could have within one field or management unit, and demonstrate the need for landscape scale predictive models to assess the impact of stover removal on soil fertility and SOM dynamics and transfer processes models. Simulations characterized by an average soil type are not sufficient to account for the complexity of soils or the interactions and feedbacks of sediment, nutrient, and water transport that occur within agricultural fields. In addition to predictive model support systems, management of soil-specific responses to stover harvest will likely require adoption of precision agriculture technologies and practices such as variable rate harvesting and fertilization.


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