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Accurate assessment of crop water uptake (WU) and water use efficiency (WUE) is not easy under field conditions. Carbon isotope discrimination (Δ13C) has been used as a surrogate of WUE to examine crop yield responses to drought and its relationship with WU and WUE. A 2‐yr study was conducted (i) to characterize genotypic variation in Δ13C, grain yield, and other physiological parameters in common bean (Phaseolus vulgaris L.) parental lines, and (ii) to examine the relationships between grain Δ13C, shoot Δ13C, and grain yield under well‐watered and terminal drought stress conditions. All measured plant traits were strongly influenced by water availability, and genotypic differences in grain yield, shoot Δ13C, and grain Δ13C were found in both watered and terminal drought stress environments. The parental lines were classified into two drought adaptation groups, drought resistant and drought sensitive, based on a yield drought index. High yields under drought conditions were related to (i) greater water uptake, as indicated by high Δ13C in genotypes previously shown to have deeper roots (e.g., SEA 5 and BAT 477), and (ii) increased WUE, denoted by lower Δ13C and greater pod harvest index (PHI) (e.g., SER 16). Coupling of Δ13C measurements with measured yield and yield components analyses, such as PHI, provided an avenue to distinguish different physiological traits among drought resistant genotypes underlying adaptation to water deficit stress.

Core Ideas Two‐year study evaluating high biomass sorghum yield response to a broad range of N treatments. A minimal N fertilizer rate of 56 kg ha−1 is needed to increase average ethanol yield to 5519 L ha−1, with no additional yield benefit at greater N rates. Yields were limited by varying environmental conditions and delayed planting. Nitrogen recovery efficiency were greatest at low N rates, but N use efficiency more stable across N rates, but greatly impacted by environment. High biomass sorghum [Sorghum bicolor (L.) Moench] could potentially produce high yields in the midwestern United States with minimal fertilizer inputs. However, little is known about the yield response, N uptake, nitrogen use efficiency (NUE), and nitrogen recovery efficiency (NRE) of high biomass sorghum (HBS) at varying N fertilization rates when grown for lignocellulosic ethanol. The objectives of this 2‐yr study were to determine the effect of five N rates (0, 56, 112, 168, 224 kg N ha−1) on the dry matter (DM) yield and lignocellulosic ethanol yield (LEY), and the NRE and NUE of two HBS varieties in the midwestern United States. The two varieties responded similarly for most measured parameters. Total DM yield averaged 13.1 Mg ha−1 and estimated LEY averaged 5519 L ha−1 across N rates when grown in central Missouri. Nitrogen rates between 56 and 224 kg N ha−1 resulted in similar DM yield and LEY in 2010, and there were no yield differences among N rates ranging from 0 to 224 kg N ha−1 in 2011. The greatest N removal in DM was at or above 56 kg N ha−1. Nitrogen recovery efficiency and NUE differed by year, where low rainfall and a shortened growing season in the second year resulted in reduced NRE and NUE. Lack of NUE and DM yield differences at N rates above 56 kg N ha−1 indicates the potential for producing high biomass sorghum with minimal amounts of fertilizer N in the Midwest.

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Core Ideas Five‐year study examined soil C changes from three annual biofuel crops. Soil organic C and a labile C fraction (active C) were measured. High biomass sorghum has greater potential than corn to maintain or increase soil C. Emerging annual biofuel feedstock systems could impact the midwestern US marginal soils through soil C sequestration. Yet, the short‐term impact by these potential cropping systems on soil organic carbon (SOC) concentrations is largely unknown. A two‐site, 5‐yr study was conducted to determine the effects on SOC pools by converting land use from a minimally managed perennial grassland on marginal lands to annual bioenergy feedstock production. Maize (Zea mays L.), sweet sorghum [Sorghum bicolor (L.) Moench], and high biomass sorghum (HBS) were each grown in rotation with soybean (Glycine max L.) and soil samples were collected in the first, second, and fifth years, then analyzed for differences among the cropping systems in SOC and active carbon fraction (AC) concentrations. Additionally, in the fifth year, soil from the adjoining undisturbed grassland was analyzed against the cropping systems and C fraction stocks determined. Concentrations of SOC and AC decreased over the first 2 yr at both sites, but SOC concentrations rebounded near to initial levels after 5 yr. The central Missouri site maintained greater SOC and AC concentrations than the southwestern Missouri site. A slight increase in AC concentration occurred from year 2 to 5 at the southwestern Missouri site, suggesting beneficial effects from the biofuel feedstocks. In contrast, AC concentrations continued to decrease at the central Missouri site. Crop rotation treatments revealed that continued HBS–soybean rotation beyond the 5th yr has the potential to increase C concentrations and stocks at depth (10–20 cm) compared to a maize–soybean rotation.

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