Daniel T. Walters

Agroecosystems, nitrogen-use efficiency, and nitrogen management.

Abstract The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield increases on the worlds limited supply of productive farm land.

Improvements in Life Cycle Energy Efficiency and Greenhouse Gas Emissions of Corn‐Ethanol

Corn-ethanol production is expanding rapidly with the adoption of improved technologies to increase energy efficiency and profitability in crop production, ethanol conversion, and coproduct use. Life cycle assessment can evaluate the impact of these changes on environmental performance metrics. To this end, we analyzed the life cycles of corn-ethanol systems accounting for the majority of U.S. capacity to estimate greenhouse gas (GHG) emissions and energy efficiencies on the basis of updated values for crop management and yields, biorefinery operation, and coproduct utilization. Direct-effect GHG emissions were estimated to be equivalent to a 48% to 59% reduction compared to gasoline, a twofold to threefold greater reduction than reported in previous studies. Ethanol-to-petroleum output/input ratios ranged from 10:1 to 13:1 but could be increased to 19:1 if farmers adopted high-yield progressive crop and soil management practices. An advanced closed-loop biorefinery with anaerobic digestion reduced GHG emissions by 67% and increased the net energy ratio to 2.2, from 1.5 to 1.8 for the most common systems. Such improved technologies have the potential to move corn-ethanol closer to the hypothetical performance of cellulosic biofuels. Likewise, the larger GHG reductions estimated in this study allow a greater buffer for inclusion of indirect-effect land-use change emissions while still meeting regulatory GHG reduction targets. These results suggest that corn-ethanol systems have substantially greater potential to mitigate GHG emissions and reduce dependence on imported petroleum for transportation fuels than reported previously.

Maize plant contributions to root zone available carbon and microbial transformations of nitrogen

Root-derived C influences soil microbial activities that regulate N transformations and cycling in soil. The change in 13C abundance of soil microbial biomass was used to quantify contributions from maize (Zea mays L.), a C4 plant, to root zone-available C during growth in soil with a long history of C3 vegetation. Effects of root-derived available C on microbial transformations of N were also evaluated using a 15NH415NO3 fertilizer tracer. Root-released C (microbial respired C4C + soil residue C4C) accounted for 12% (210 kg C ha−1) of measured C fixed by maize at 4 wk and 5% at maturity when root-released C totaled 1135 kg C ha−1. Of the C4C remaining in soil, only 18–23% was found in microbial biomass, indicating either a rapid turnover rate of biomass or a lower availability of C4 substrates. Average daily production of root-derived available C was greatest during 4–8 wk maize growth (7 kg C ha−1 d−1) when 4–11% of the soil microbial biomass came from this C source. At maize maturity, 15% of the microbial biomass (161 kg C ha−1) came from root-derived available C, which totaled 402 kg ha−1. Of the 15N remaining in bare and cropped soils, averages of 23 and 16% (10 and 2 kg N ha−1) were found in microbial biomass, and 64 and 2% (28 and 0.2 kg N ha−1) were in inorganic 15N form, leaving 13 and 82% (6 and 10 kg N ha−1) as non-biomass organic N, respectively; this suggests that N cycling through microbial biomass was enhanced by root-derived C. Denitrification and N2O losses from planted soils were low (1–136 g N ha−1 d−1) when soil water-filled pore space (WFPS) was 2–3 mg kg−1) was present in the soil. The presence of maize plants increased denitrification losses from soil by 19 to 57% (average of 29%) during early growth stages when the release of root-derived C was greatest.

Legume residue and soil water effects on denitrification in soils of different textures

Abstract Legume cover crops commonly used to supply additional N and reduce potential for over-winter N leaching losses may also influence denitrification depending upon soil water status and soil type. Interrelationships between incorporated hairy vetch (Vicia villosa) residue and soil water status on denitrification in coarse, medium and fine textured soils were investigated in the laboratory. Repacked soil cores were incubated, 10, 20 and 30 d with and without acetylene (C2H2). Denitrification losses were 20–200 μg N kg−1 from each soil when 60% of the soil pore space was filled with water and increased to from 14.0 to 18.6mg N kg−1 at 90% water-filled-pore space (WFPS). Incorporation of vetch residue (2.5 g kg−1) greatly stimulated denitrification (51.1–99.5 mg N kg−1), probably due to greater availability of organic C as indicated by higher CO2 emissions. The major denitrification losses occurred during the first 10 days and more so in residue-amended soils. The supply of C from incorporated legume crop residue was a major factor influencing denitritication especially when soil wetness restricted aeration and adequate nitrate was present. At similar water contents, rates of denitrification differed greatly in soils of varying texture, but when varying water holding capacity and bulk density were accounted for using WFPS. all soils behaved very similarly. Use of WFPS as an index of aeration status enabled identification that differences in denitrification losses in vetch-amended soils of varying texture resulted in part from varying capacity to supply NO3− and metabolize organic matter. These results illustrate the utility of WFPS, compared with soil water content, and its reliability as an indicator of reduced aeration dependent denitrification for soils of varying texture.

Optimizing nitrogen management in food and energy production and environmental protection: summary statement from the Second International Nitrogen Conference.

Human efforts to produce food and energy are changing the nitrogen (N) cycle of the Earth. Many of these changes are highly beneficial for humans, while others are detrimental to people and the environment. These changes transcend scientific disciplines, geographical boundaries, and political structures. They challenge the creative minds of natural and social scientists, economists, engineers, business leaders, and decision makers. The Second International Nitrogen Conference was designed to facilitate communications among all stakeholders in the “nitrogen community” of the world. The Conference participants’ goal in the years and decades ahead is to encourage every country to make optimal choices about N management in food production and consumption, energy production and use, and environmental protection. Scientific findings and recommendations for decision makers that emerged from the Conference are presented.

Nitrogen supply affects root:shoot ratio in corn and velvetleaf (Abutilon theophrasti )

Abstract Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited. Nomenclature: Velvetleaf, Abutilon theophrasti Medic., ABUTH; corn, Zea mays L.

Standardized precipitation index and nitrogen rate effects on crop yields and risk distribution in maize.

Crop performance in rainfed cropping systems generally is dependent on rainfall amount and distribution. The objective of this study was to analyze the long-term consequences of rainfall expressed as a standardized precipitation index (SPI) and fertilizer nitrogen (N) on yields and risk probabilities of maize in the udic-ustic moisture regimes in the Great Plains in Nebraska. The SPI is a precipitation index for classifying drought stress conditions. The study was conducted on a Kennebec silt loam (Cumulic Hapludoll) over an 11-year period, 1986‐1996, using monoculture maize (Zea mays L.) and maize in rotation with soybean (Glycine max.(L.) Merr.) in combination with N fertilizer levels between 0 and 160 kg ha 1 . Maize yields in monoculture ranged from 4.8 to 5.7 Mg ha 1 , and from 6.4 to 6.8 Mg ha 1 in rotation. The differences in yields between monoculture and rotation were larger at low N rates and decreased as N fertilizer increased above 40 kg ha 1 . Current year’s maize yields either exhibited a weak or no response to N fertilizer in years when the preceding preseason (October‐April) and the previous growing season (May‐August) were dry (negative SPI value). Regression of yield as the dependent variable and the 12-month April SPI as the independent variable explained up to 64% of yield variability in a curvilinear relationship. Optimum SPI values were in the range of 1.0 to 1.0, substantiating the adaptability and performance of crops under mild stress as proposed by other scientists. Prediction of subsequent yields using past SPI data was relatively better in rotations (R 2 D41‐50%) than in monoculture (R 2 D15‐40%). Risk, calculated as the lower confidence limit of maize returns over variable cost of fertilizer, was less in rotations than in monoculture, and in both cropping systems returns were maximized with the application of N fertilizer at 40 kg ha 1 . Used with other criteria, the SPI can be a practical guide to choice of crops, N levels, and management decisions to conserve water in rainfed systems.

Effect of nitrogen addition on the comparative productivity of corn and velvetleaf (Abutilon theophrasti )

Abstract Weeds that respond more to nitrogen fertilizer than crops may be more competitive under high nitrogen (N) conditions. Therefore, understanding the effects of nitrogen on crop and weed growth and competition is critical. Field experiments were conducted at two locations in 1999 and 2000 to determine the influence of varying levels of N addition on corn and velvetleaf height, leaf area, biomass accumulation, and yield. Nitrogen addition increased corn and velvetleaf height by a maximum of 15 and 68%, respectively. N addition increased corn and velvetleaf maximum leaf area index (LAI) by up to 51 and 90%. Corn and velvetleaf maximum biomass increased by up to 68 and 89% with N addition. Competition from corn had the greatest effect on velvetleaf growth, reducing its biomass by up to 90% compared with monoculture velvetleaf. Corn response to N addition was less than that of velvetleaf, indicating that velvetleaf may be most competitive at high levels of nitrogen and least competitive when nitrogen le...

Large-scale on-farm implementation of soil moisture-based irrigation management strategies for increasing maize water productivity

Irrigated maize is produced on about 3.5 Mha in the U.S. Great Plains and western Corn Belt. Most irrigation water comes from groundwater. Persistent drought and increased competition for water resources threaten long-term viability of groundwater resources, which motivated our research to develop strategies to increase water productivity without noticeable reduction in maize yield. Results from previous research at the University of Nebraska-Lincoln (UNL) experiment stations in 2005 and 2006 found that it was possible to substantially reduce irrigation amounts and increase irrigation water use efficiency (IWUE) and crop water use efficiency (CWUE) (or crop water productivity) with little or no reduction in yield using an irrigation regime that applies less water during growth stages that are less sensitive to water stress. Our hypothesis was that a soil moisture-based irrigation management approach in research fields would give similar results in large production-scale, center-pivot irrigated fields in Nebraska. To test this hypothesis, IWUE, CWUE, and grain yields were compared in extensive on-farm research located at eight locations over two years (16 site-years), representing more than 600 ha of irrigated maize area. In each site-year, two contiguous center-pivot irrigated maize fields with similar topography, soil properties, and crop management practices received different irrigation regimes: one was managed by UNL researchers, and the other was managed by the farmer at each site. Irrigation management in farmer-managed fields relied on the farmers’ traditional visual observations and personal expertise, whereas irrigation timing in the UNL-managed fields was based on pre-determined soil water depletion thresholds measured using soil moisture sensors, as well as crop phenology predicted by a crop simulation model using a combination of real-time (in-season) and historical weather data. The soil moisture-based irrigation regime resulted in greater soil water depletion, which decreased irrigation requirements and enabled more timely irrigation management in the UNL-managed fields in both years (34% and 32% less irrigation application compared with farmer-managed fields in 2007 and 2008, respectively). The average actual crop evapotranspiration (ETC) for the UNL- and farmer-managed fields for all sites in 2007 was 487 and 504 mm, respectively. In 2008, the average UNL and average farmer-managed field had seasonal ETC of 511 and 548 mm, respectively. Thus, when the average of all sites is considered, the UNL-managed fields had 3% and 7% less ETC than the farmer-managed fields in 2007 and 2008, respectively, although the percentage was much higher for some of the farmer-managed fields. In both years, differences in grain yield between the UNL and farmer-managed fields were not statistically significant (p = 0.75). On-farm implementation of irrigation management strategies resulted in a 38% and 30% increase in IWUE in the UNL-managed fields in 2007 and 2008, respectively. On average, the CWUE value for the UNL-managed fields was 4% higher than those in the farmer-managed fields in both years. Reduction in irrigation water withdrawal in UNL-managed fields resulted in

Emissions savings in the corn-ethanol life cycle from feeding coproducts to livestock.

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