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Agricultural systems are being challenged to decrease water use and increase production while climate becomes more variable and the world's population grows. Low water use efficiency is traditionally characterized by high water use relative to low grain production and usually occurs under dry conditions. However, when a cropping system fails to take advantage of available water during wet conditions, this is also an inefficiency and is often detrimental to the environment. Here, we provide a systems-level definition of water use efficiency (sWUE) that addresses both production and environmental quality goals through incorporating all major system water losses (evapotranspiration, drainage, and runoff). We extensively calibrated and tested the Agricultural Production Systems sIMulator (APSIM) using 6 years of continuous crop and soil measurements in corn- and soybean-based cropping systems in central Iowa, USA. We then used the model to determine water use, loss, and grain production in each system and calculated sWUE in years that experienced drought, flood, or historically average precipitation. Systems water use efficiency was found to be greatest during years with average precipitation. Simulation analysis using 28 years of historical precipitation data, plus the same dataset with ± 15% variation in daily precipitation, showed that in this region, 430 mm of seasonal (planting to harvesting) rainfall resulted in the optimum sWUE for corn, and 317 mm for soybean. Above these precipitation levels, the corn and soybean yields did not increase further, but the water loss from the system via runoff and drainage increased substantially, leading to a high likelihood of soil, nutrient, and pesticide movement from the field to waterways. As the Midwestern United States is predicted to experience more frequent drought and flood, inefficiency of cropping systems water use will also increase. This work provides a framework to concurrently evaluate production and environmental performance of cropping systems.

Using prairie biomass as a renewable source of energy may constitute an important opportunity to improve the environmental sustainability of managed land. To date, assessments of the feasibility of using prairies for bioenergy production have focused on marginal areas with low yield potential. Growing prairies on more fertile soil or with moderate levels of fertilization may be an effective means of increasing yields, but increased fertility often reduces plant community diversity. At a fertile site in central Iowa with high production potential, we tested the hypothesis that nitrogen fertilization would increase aboveground biomass production but would decrease diversity of prairies sown and managed for bioenergy production. Over a 3 year period (years 2-4 after seeding), we measured aboveground biomass after plant senescence and species and functional-group diversity in June and August for multispecies mixtures of prairie plants that received no fertilizer or 84 kg N ha À1 year À1 .We found that nitrogen fertilization increased aboveground biomass production, but with or without fertilization, the prairies produced a substantial amount of biomass: averaging (±SE) 12.2 ± 1.3 and 9.1 ± 1.0 Mg ha À1 in fertilized and unfertilized prairies, respectively. Unfertilized prairies had higher species diversity in June, whereas fertilized prairies had higher species diversity in August at the end of the study period. Functionalgroup diversity was almost always higher in fertilized prairies. Composition of unfertilized prairies was characterized by native C 4 grasses and legumes, whereas fertilized prairies were characterized by native C 3 grasses and forbs. Although most research has found that nitrogen fertilization reduces prairie diversity, our results indicate that early-spring nitrogen fertilization, when used with a postsenescence annual harvest, may increase prairie diversity. Managing prairies for bioenergy production, including the judicious use of fertilization, may be an effective means of increasing the amount of saleable products from managed lands while potentially increasing plant diversity.

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We compare subsurface-drainage NO-N and total reactive phosphorus (TRP) concentrations and yields of select bioenergy cropping systems and their rotational phases. Cropping systems evaluated were grain-harvested corn-soybean rotations, grain- and stover-harvested continuous corn systems with and without a cover crop, and annually harvested reconstructed prairies with and without the addition of N fertilizer in an Iowa field. Drainage was monitored when soils were unfrozen during 2010 through 2013. The corn-soybean rotations without residue removal and continuous corn with residue removal produced similar mean annual flow-weighted NO-N concentrations, ranging from 6 to 18.5 mg N L during the 4-yr study. In contrast, continuous corn with residue removal and with a cover crop had significantly lower NO-N concentrations of 5.6 mg N L when mean annual flow-weighted values were averaged across the 4 yr. Prairies systems with or without N fertilization produced significantly lower concentrations below <1 mg NO-N L than all the row crop systems throughout the study. Mean annual flow-weighted TRP concentrations and annual yields were generally low, with values <0.04 mg TRP L and <0.14 kg TRP ha, and were not significantly affected by any cropping systems or their rotational phases. Bioenergy-based prairies with or without N fertilization and continuous corn with stover removal and a cover crop have the potential to supply bioenergy feedstocks while minimizing NO-N losses to drainage waters. However, subsurface drainage TRP concentrations and yields in bioenergy systems will need further evaluation in areas prone to higher levels of P losses.