Climate change models predict that future precipitation patterns will entail lower-frequency but larger rainfall events, increasing the duration of dry soil conditions. Resulting shifts in microbial C cycling activity could affect soil C storage. Further, microbial response to rainfall events may be constrained by the physiological or nutrient limitation stress of extended drought periods; thus seasonal or multiannual precipitation regimes may influence microbial activity following soil wet-up. We quantified rainfall-driven dynamics of microbial processes that affect soil C loss and retention, and microbial community composition, in soils from a long-term (14-year) field experiment contrasting "Ambient" and "Altered" (extended intervals between rainfalls) precipitation regimes. We collected soil before, the day following, and five days following 2.5-cm rainfall events during both moist and dry periods (June and September 2011; soil water potential = -0.01 and -0.83 MPa, respectively), and measured microbial respiration, microbial biomass, organic matter decomposition potential (extracellular enzyme activities), and microbial community composition (phospholipid fatty acids). The equivalent rainfall events caused equivalent microbial respiration responses in both treatments. In contrast, microbial biomass was higher and increased after rainfall in the Altered treatment soils only, thus microbial C use efficiency (CUE) was higher in Altered than Ambient treatments (0.70 +/- 0.03 > 0.46 +/- 0.10). CUE was also higher in dry (September) soils. C-acquiring enzyme activities (beta-glucosidase, cellobiohydrolase, and phenol oxidase) increased after rainfall in moist (June), but not dry (September) soils. Both microbial biomass C:N ratios and fungal:bacterial ratios were higher at lower soil water contents, suggesting a functional and/or population-level shift in the microbiota at low soil water contents, and microbial community composition also differed following wet-up and between seasons and treatments. Overall, microbial activity may directly (C respiration) and indirectly (enzyme potential) reduce soil organic matter pools less in drier soils, and soil C sequestration potential (CUE) may be higher in soils with a history of extended dry periods between rainfall events. The implications include that soil C loss may be reduced or compensated for via different mechanisms at varying time scales, and that microbial taxa with better stress tolerance or growth efficiency may be associated with these functional shifts.

Core Ideas Removal of corn and sorghum residues did not affect soil carbon under no‐tillage. Soil carbon (0–15 cm) increased in 10‐yr stands of switchgrass and miscanthus. Soil carbon increased with root biomass, fungi abundance, and soil aggregate size. Perennial crops improved soil health while providing feedstock for biofuels. The substitution of cellulosic biofuel in place of conventional fuels could reduce greenhouse gas (GHG) emissions from transportation. However, changes in soil organic carbon (SOC) and soil health during biofuel crop production could have a major impact on the GHG balance of biofuels. We assessed temporal changes (10 yr) in SOC stocks to a 90 cm depth in Cumulic Hapludolls from central Kansas under perennial and annual cropping systems. The perennial crops were miscanthus (Miscanthus sacchariflorus) and switchgrass (Panicum virgatum L.). The annual cropping systems were continuous corn (Zea mays L.), and corn, dual purpose–grain sorghum (Sorghum bicolor (L.) Moench), sweet sorghum, and photoperiod‐sensitive sorghum (PS) in rotation with soybean [Glycine max (L.) Merr.]. All standing aboveground biomass was removed at harvest of corn, sorghum, and perennial crops. Stocks of SOC increased in the 0–15 cm depth under switchgrass and miscanthus by 0.8 and 1.3 Mg C ha−1 yr−1, respectively. The SOC stocks did not change at the other depths or at any depth in the annual cropping systems nor throughout the soil profile under any crops. Root biomass measured in the seventh year of the study was 3.7 to 7.8 times greater in perennials than in annual crops. Increases on SOC were correlated with greater root biomass, abundance of arbuscular mycorrhizae and saprophytic fungi, and soil aggregate diameter. These results demonstrate the potential for perennial biofuel crops to enhance C sequestration and improve soil quality while providing feedstock for production of cellulosic biofuel.

Although energy crops could eventually supply a growing portion of cellulosic biofuel feedstocks, long-term comparisons of annual and perennial crops are rare. An experiment was established in 2007 near Manhattan, KS, to compare biomass productivity and ethanol yield of perennial and annual crops. Perennial crops included three C4 grasses: switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), and miscanthus (Miscanthus sacchariflorus). Annual C4 crops were corn (Zea mays L.) in two rotations: continuous and rotated with soybean [Glycine max (L.) Merr.]; and five types of sorghum [Sorghum bicolor (L.) Moench]: photoperiod sensitive, sweet, dual purpose (grain and biomass), brown mid-rib, and grain; all rotated with soybean. Annual crops produced 7 Mg ha-1 yr-1 more biomass than perennial crops throughout 11 yr, with sweet sorghum exceeding 22 Mg ha-1 yr-1 , and 12 m 3 ha-1 yr-1 of ethanol. Biomass yield of miscanthus approached 14 Mg ha-1 yr-1 , essentially the same as for several annual crops but with half as much fertilizer nitrogen. Annual ethanol production from miscanthus and switchgrass was 3.6 m 3 ha-1 yr-1 , half as much as that of several annual crops that produced similar biomass yields. Big bluestem consistently produced the least biomass and ethanol, less than 7 Mg ha-1 yr-1 and 1.7 m 3 ha-1 yr-1 , respectively. Rotated corn averaged 7.1 m 3 ha-1 yr-1 of ethanol. Eleven years of results indicate that annual corn and sorghum crops as well as perennial grasses such as miscanthus and switchgrass could play a role as potential bioenergy feedstocks in diversified production systems.

Core Ideas A majority (40–80%) of N2O was emitted in the first 10 wk of the growing season. No consistent significant differences were found in N2O emissions among crops. Most N2O was emitted during large events of short duration after substantial rainfall. Burning of fossil fuels in the transportation sector accounts for 28% of US greenhouse gas (GHG) emissions. Substitution of cellulosic biofuel in place of conventional gasoline or diesel could reduce GHG emissions from transportation; however, the effectiveness of cellulosic biofuel in reducing emissions depends on emissions during the growth of biofuel crops. The objectives of this study were (i) to measure N2O emissions of potential cellulosic biofuel cropping systems, and (ii) to characterize the temporal variations in N2O emissions in these cropping systems. Annual N2O emissions were measured in corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] and photoperiod‐sensitive sorghum [Sorghum bicolor (L.) Moench]–soybean rotations as well as in switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus sacchariflorus) from 2011–2013 in Manhattan, KS, using static chambers. No consistent significant differences were found in N2O emissions among crop species, though miscanthus tended to have the least emissions. Most N2O was emitted during large events of short duration (1–3 d) that occurred after substantial rainfall events with high soil NO3−. In 2011 and 2012, most N2O was emitted during the growing season. In 2013, 30–50% of the N2O emissions were emitted after September which was attributed to freeze–thaw cycles.

Switchgrass ( L.) has been promoted as a potential feedstock for cellulosic biofuel in the United States. Switchgrass is known to respond to N fertilizer, but optimal rates remain unclear. Given the potential nonlinear response of nitrous oxide (NO) emissions to N inputs, N additions to switchgrass above optimal levels could have large impacts on the greenhouse gas balance of switchgrass-based biofuel. Additionally, N additions are likely to have large impacts on switchgrass production costs. Yield, N removal, and net returns were measured in switchgrass receiving 0 to 200 kg N ha in Manhattan, KS, from 2012 to 2014. Emissions of NO were measured in the 0- to 150-kg N ha treatments. Total emissions of NO increased from 0.2 to 3.0 kg NO-N ha as N inputs increased from 0 to 150 kg N ha. The 3-yr averages of fertilizer-induced emission factors were 0.7, 2.1, and 2.6% at 50, 100, and 150 kg N ha, respectively. Removal of N at harvest increased linearly with increasing N rate. Switchgrass yields increased with N inputs up to 100 to 150 kg N ha, but the critical N level for maximum yields decreased each year, suggesting that N was being applied in excess at higher N rates. Net returns were maximized at 100 kg N ha at both a high and low urea cost (US$394.71 and $945.91 ha, respectively). These results demonstrate that N inputs were necessary to increase switchgrass productivity, but rates exceeding optimal levels resulted in excessive NO emissions and increased costs for producers.