Counterfactual scenarios reveal historical impact of cropland management on soil organic carbon stocks in the United States.

Natural climate solutions provide opportunities to reduce greenhouse gas emissions and the United States is among a growing number of countries promoting storage of carbon in agricultural soils as part of the climate solution. Historical patterns of soil organic carbon (SOC) stock changes provide context about mitigation potential. Therefore, our objective was to quantify the influence of climate-smart soil practices on SOC stock changes in the top 30 cm of mineral soils for croplands in the United States using the DayCent Ecosystem Model. We estimated that SOC stocks increased annually in US croplands from 1995 to 2015, with the largest increase in 1996 of 16.6 Mt C (95% confidence interval ranging from 6.1 to 28.2 Mt CO2 eq.) and the lowest increase in 2015 of 10.6 Mt C (95% confidence interval ranging from - 1.8 to 22.2 Mt C). Most climate-smart soil practices contributed to increases in SOC stocks except for winter cover crops, which had a negligible impact due to a relatively small area with cover crop adoption. Our study suggests that there is potential for enhancing C sinks in cropland soils of the United States although some of the potential has been realized due to past adoption of climate-smart soil practices.

A gap in nitrous oxide emission reporting complicates long-term climate mitigation.

Nitrous oxide (N2O) is an important greenhouse gas (GHG) that also contributes to depletion of ozone in the stratosphere. Agricultural soils account for about 60% of anthropogenic N2O emissions. Most national GHG reporting to the United Nations Framework Convention on Climate Change assumes nitrogen (N) additions drive emissions during the growing season, but soil freezing and thawing during spring is also an important driver in cold climates. We show that both atmospheric inversions and newly implemented bottom-up modeling approaches exhibit large N2O pulses in the northcentral region of the United States during early spring and this increases annual N2O emissions from croplands and grasslands reported in the national GHG inventory by 6 to 16%. Considering this, emission accounting in cold climate regions is very likely underestimated in most national reporting frameworks. Current commitments related to the Paris Agreement and COP26 emphasize reductions of carbon compounds. Assuming these targets are met, the importance of accurately accounting and mitigating N2O increases once CO2 and CH4 are phased out. Hence, the N2O emission underestimate introduces additional risks into meeting long-term climate goals.

Higher than expected N2O emissions from soybean crops in the Pampas Region of Argentina: Estimates from DayCent simulations and field measurements.

In developing countries, agriculture generally represents a large fraction of GHG emissions reported in National Inventories, and emissions are typically estimated using Tier 1 IPCC guidelines. However, field data and locally adapted simulation models can improve the accuracy of IPCC estimations. In this study we aimed to quantify anthropogenic N2O emissions from croplands of Argentina through field measurements, model simulations and IPCC guidelines. We measured N2O emissions and their controlling factors in 62 plots of the Pampas Region with corn, soybean and wheat/soybean crops and in unmanaged grasslands. We accounted for gross emissions from crops and background emissions from unmanaged grasslands to calculate net anthropogenic emissions from crops as the difference between them. We calibrated and evaluated the DayCent model and then simulated different weather and management scenarios. Finally, we applied IPCC guidelines to estimate anthropogenic N2O emissions at the same plots. The DayCent model accurately simulated annual N2O emission for all crops as compared to measured data (RMSE = 1.4 g N ha-1 day-1). Measured and simulated emissions in soybean crops were higher than in corn and wheat/soybean crops. Gross N2O emissions ranged from 1.4 to 5.1 kg N ha-1 yr-1 for current environmental (soil and weather) and management (crops and fertilizer doses) conditions. Background emissions ranged between 1.1 and 1.3 kg N ha-1 yr-1, and therefore net anthropogenic emissions ranged from 0.3 to 4.0 kg N ha-1 yr-1. IPCC Tier 1 emission factors underestimated N2O releases from soybean, that were on average 4.87 times greater when estimated with DayCent and observations (0.53 vs 2.47 and 2.69 kg N ha-1 yr-1, respectively). On the contrary, IPCC estimates for corn and wheat/soybean crops were similar to modeled and measured values. Our results suggest that N2O emissions from the vast 15 million ha of soybean croplands in the Pampas Region may be substantially underestimated.

Deficit irrigation impacts on greenhouse gas emissions under drip-fertigated maize in the Great Plains of Colorado.

Precise water and fertilizer application can increase crop water productivity and reduce agricultural contributions to greenhouse gas (GHG) emissions. Regulated deficit irrigation (DI) and drip fertigation control the amount, location, and timing of water and nutrient application. Yet, few studies have measured GHG emissions under these practices, especially for maize (Zea mays L.). The objective was to quantify N2 O and CO2 emission from DI and full irrigation (FI) within a drip-fertigated maize system in northeastern Colorado. During two growing seasons of measurement, treatments consisted of mild, moderate, and extreme DI and FI. Deficit irrigation was managed based on growth stage so that full evapotranspiration (ET) was met during the yield-sensitive reproductive stage, but less than full crop ET was applied during the late vegetative and maturation growth stages. In the first year, mild DI (90% ET) reduced N2 O emissions by 50% compared with FI. In the second year, compared with FI, moderate DI (69-80% ET) reduced N2 O emissions by 15%, and extreme DI (54-68% ET) reduced N2 O emissions by 40%. Only extreme DI in the second year significantly reduced CO2 emissions (by 30%) compared with FI. Mild DI reduced yield-scaled emissions in the first year, but moderate and extreme DI had similar yield-scaled emissions as FI in the second year. The surface drip fertigation resulted in total GHG emissions that were one-tenth of literature-based measurements from sprinkler-irrigated maize systems. This study illustrates the potential of DI and drip fertigation to reduce N2 O and CO2 emissions in irrigated cropping systems.

Modeling nitrous oxide mitigation potential of enhanced efficiency nitrogen fertilizers from agricultural systems.

Agriculture soils are responsible for a large proportion of global nitrous oxide (N2O) emissions-a potent greenhouse gas and ozone depleting substance. Enhanced-efficiency nitrogen (N) fertilizers (EENFs) can reduce N2O emission from N-fertilized soils, but their effect varies considerably due to a combination of factors, including climatic conditions, edaphic characteristics and management practices. In this study, we further developed the DayCent ecosystem model to simulate two EENFs: controlled-release N fertilizers (CRNFs) and nitrification inhibitors (NIs) and evaluated their N2O mitigation potentials. We implemented a Bayesian calibration method using the sampling importance resampling (SIR) algorithm to derive a joint posterior distribution of model parameters that was informed by N2O flux measurements from corn production systems a network of experimental sites within the GRACEnet program. The joint posterior distribution can be applied to estimate predictions of N2O reduction factors when EENFs are adopted in place of conventional urea-based N fertilizer. The resulting median reduction factors were - 11.9% for CRNFs (ranging from -51.7% and 0.58%) and - 26.7% for NIs (ranging from -61.8% to 3.1%), which is comparable to the measured reduction factors in the dataset. By incorporating EENFs, the DayCent ecosystem model is able to simulate a broader suite of options to identify best management practices for reducing N2O emissions.

Weakened growth of cropland-N2 O emissions in China associated with nationwide policy interventions.

China has experienced rapid agricultural development over recent decades, accompanied by increased fertilizer consumption in croplands; yet, the trend and drivers of the associated nitrous oxide (N2 O) emissions remain uncertain. The primary sources of this uncertainty are the coarse spatial variation of activity data and the incomplete model representation of N2 O emissions in response to agricultural management. Here, we provide new data-driven estimates of cropland-N2 O emissions across China in 1990-2014, compiled using a global cropland-N2 O flux observation dataset, nationwide survey-based reconstruction of N-fertilization and irrigation, and an updated nonlinear model. In addition, we have evaluated the drivers behind changing cropland-N2 O patterns using an index decomposition analysis approach. We find that China's annual cropland-N2 O emissions increased on average by 11.2 Gg N/year2 (p < .001) from 1990 to 2003, after which emissions plateaued until 2014 (2.8 Gg N/year2 , p = .02), consistent with the output from an ensemble of process-based terrestrial biosphere models. The slowdown of the increase in cropland-N2 O emissions after 2003 was pervasive across two thirds of China's sowing areas. This change was mainly driven by the nationwide reduction in N-fertilizer applied per area, partially due to the prevalence of nationwide technological adoptions. This reduction has almost offset the N2 O emissions induced by policy-driven expansion of sowing areas, particularly in the Northeast Plain and the lower Yangtze River Basin. Our results underline the importance of high-resolution activity data and adoption of nonlinear model of N2 O emission for capturing cropland-N2 O emission changes. Improving the representation of policy interventions is also recommended for future projections.

Nitrous Oxide and Ammonia Emissions from Cattle Excreta on Shortgrass Steppe.

Grazing cattle redistribute nitrogen (N) consumed in forage through urine and feces patches. The high concentration of N in these patches often exceeds the uptake demands of the local plant community, thereby providing ideal conditions for losses of reactive N. However, knowledge on nitrous oxide (NO) and ammonia (NH) emissions from excretal patches on shortgrass steppe grassland is limited. We studied the effect of cattle urine (1002 kg N ha) and feces (1021 kg N ha) patches on NO and NH emissions in two sites with contrasting vegetation: (i) cool-season (C3) 'Bozoisky-Select' Russian wildrye [ (Fisch.) Nevski], pasture (C3Past) and (ii) C4-dominated native shortgrass steppe rangeland (C4SS). Nitrous oxide and NH were measured using semi-static and semi-open chambers, respectively. Cumulative NO emissions were 217 and 173% greater and cumulative volatile NH emissions were 339 and 157% greater on C3Past compared with C4SS from the urine and feces treatments, respectively. Nitrous oxide emission factors were 0.20 and 0.05% for urine and 0.07 and 0.03% for feces on C3Past and C4SS, respectively. Our findings suggest that using the IPCC Tier 1 default emission factor (2%, 95% CI = 0.7-6%) to estimate NO emissions from cattle excretal patches on shortgrass steppe grassland would result in a significant overestimation for these dryland systems. Ammonia emission factors were 35 and 10% for urine and 7 and 5% for feces on C3Past and C4SS, respectively. With the exception of the urine treatment on C3Past, observed NH emissions were consistent with the IPCC Tier 1 default assumption that 20% (95% CI = 5-50%) of excretal N is volatilized as NH+NO.

Nitrogen Management Affects Nitrous Oxide Emissions under Varying Cotton Irrigation Systems in the Desert Southwest, USA.

Irrigation of food and fiber crops worldwide continues to increase. Nitrogen (N) from fertilizers is a major source of the potent greenhouse gas nitrous oxide (NO) in irrigated cropping systems. Nitrous oxide emissions data are scarce for crops in the arid western United States. The objective of these studies was to assess the effect of N fertilizer management on NO emissions from furrow-irrigated, overhead sprinkler-irrigated, and subsurface drip-irrigated cotton ( L.) in Maricopa, AZ, on Trix and Casa Grande sandy clay loam soils. Soil test- and canopy-reflectance-based N fertilizer management were compared. In the furrow- and overhead sprinkler-irrigated fields, we also tested the enhanced efficiency N fertilizer additive Agrotain Plus as a NO mitigation tool. Nitrogen fertilizer rates as liquid urea ammonium nitrate ranged from 0 to 233 kg N ha. Two applications of N fertilizer were made with furrow irrigation, three applications under overhead sprinkler irrigation, and 24 fertigations with subsurface drip irrigation. Emissions were measured weekly from May through August with 1-L vented chambers. NO emissions were not agronomically significant, but increased as much as 16-fold following N fertilizer addition compared to zero-N controls. Emission factors ranged from 0.10 to 0.54% of added N fertilizer emitted as NO-N with furrow irrigation, 0.15 to 1.1% with overhead sprinkler irrigation, and <0.1% with subsurface drip irrigation. The reduction of NO emissions due to addition of Agrotain Plus to urea ammonium nitrate was inconsistent. This study provides unique data on NO emissions in arid-land irrigated cotton and illustrates the advantage of subsurface drip irrigation as a low NO source system.

Greenhouse Gas Emissions from Solid and Liquid Organic Fertilizers Applied to Lettuce.

Improper application of nitrogen (N) fertilizer and environmental factors can cause the loss of nitrous oxide (NO) to the environment. Different types of fertilizers with different C/N ratios may have different effects on the environment. The focus of this study was to evaluate the effects of environmental factors and four organic fertilizers (feather meal, blood meal, fish emulsion, and cyano-fertilizer) applied at different rates (0, 28, 56, and 112 kg N ha) on NO emissions and to track CO emissions from a lettuce field ( L.). The study was conducted in 2013 and 2014 and compared preplant-applied solid fertilizers (feather meal and blood meal) and multiple applications of liquid fertilizers (fish emulsion and cyano-fertilizer). Three days a week, NO and CO emissions were measured twice per day in 2013 and once per day in 2014 using a closed-static chamber, and gas samples were analyzed by gas chromatography. Preplant-applied solid fertilizers significantly increased cumulative NO emissions as compared with control, but multiple applications of liquid fertilizers did not. Emission factors for NO ranged from 0 to 0.1% for multiple applications of liquid fertilizers and 0.6 to 11% for preplant-applied solid fertilizers, which could be overestimated due to chamber placement over fertilizer bands. In 2014, solid fertilizers with higher C/N ratios (3.3-3.5) resulted in higher CO emissions than liquid fertilizers (C/N ratio, 0.9-1.5). Therefore, organic farmers should consider the use of multiple applications of liquid fertilizers as a means to reduce soil greenhouse gas emissions while maintaining high yields.

Nitrous Oxide Emissions from a Golf Course Fairway and Rough after Application of Different Nitrogen Fertilizers.

Few studies have quantified nitrous oxide (NO) emissions from intensively managed turfgrass systems on golf courses. Fertilizer treatments consisting of urea with inhibitors of nitrification and urease (INU), polymer-coated urea (PCU), and uncoated balanced methylene urea (BMU) chain, which use different mechanisms to control the release of N substrate, were applied to a golf course fairway and rough three times during the 2011 growing season at a rate of 50 kg N ha per application. The vented chamber method was used to measure turf-soil-atmospheric NO exchange. Cumulative emissions from fairway INU, PCU, and BMU treatments totaled 6.5, 1.9, and 7.6 kg NO-N ha yr, representing a 4.02, 1.25, and 4.75% loss of total N applied, respectively. Summer INU and BMU fertilization to the fairway produced the greatest NO fluxes. Rapid fluxes during the summer were likely related to low physiological activity in cool-season turfgrass and to warm, wet soil conditions that increased denitrification rates. However, PCU applied to the fairway was more resistant to NO losses than other fertilizer treatments. Fertilizer treatments applied to the rough had cumulative emissions of 2.4, 1.50, and 1.49 kg NO-N ha yr from INU, PCU, and BMU treatments, corresponding to a 1.21, 0.62, and 0.61% loss of total N applied, respectively. The lower NO emission on roughs was likely associated with greater carbon pools, lower soil moisture, and lower temperatures. This study supports the effectiveness of PCU to reduce NO emission from cool-season turfgrass fairways when soil conditions favored denitrification during warm periods. Applying INU and BMU when soil was cool and dry was effective in moderating NO losses.

Manure and Inorganic Nitrogen Affect Trace Gas Emissions under Semi-Arid Irrigated Corn.

Dairy manure is often applied to cropped soils as a substitute for inorganic N fertilizers, but the impacts of manure on soil trace gas fluxes, yields, and soil N are uncertain in the semiarid western United States. Soil carbon dioxide (CO-C), methane (CH-C), nitrous oxide (NO-N), and ammonia (NH-N) emissions were monitored using surface chambers from five N treatments: (i) partially composted solid dairy manure (DM) (412 kg N ha), (ii) DM + AgrotainPlus (DM+AP), (iii) enhanced efficiency N fertilizer (SuperU [SU]) (179 kg N ha), (iv) urea (179 kg N ha), and (v) check (no N applied), to determine their effect on growing season (GS) and nongrowing season emissions from a tilled clay loam soil under irrigated, continuous corn production for 3 yr. SuperU and AgrotainPlus contain urease and nitrification inhibitors. Averaged over years, GS soil CO-C emissions were greater for DM and DM+AP than for urea, SU, and check treatments due to the large amount of C added with the manure; CH-C emissions did not vary among N treatments; and NO-N emissions decreased in the order urea = DM = DM+AP > SU > check. AgrotainPlus added to the DM did not reduce NO-N emissions compared with DM. Cumulative NH-N emissions after manure application decreased in the order urea > SU > check, with no significant differences between SU, DM, and DM+AP. Dairy manure provided slow-release N with nitrate intensities lower than urea and NO-N emissions similar to urea. These results highlight the importance of best-management practices such as immediate irrigation after N application and use of urease and nitrification inhibitors to minimize N losses.

Integrating biorefinery and farm biogeochemical cycles offsets fossil energy and mitigates soil carbon losses.

Crop residues are potentially significant sources of feedstock for biofuel production in the United States. However, there are concerns with maintaining the environmental functions of these residues while also serving as a feedstock for biofuel production. Maintaining soil organic carbon (SOC) along with its functional benefits is considered a greater constraint than maintaining soil erosion losses to an acceptable level. We used the biogeochemical model DayCent to evaluate the effect of residue removal, corn stover, and wheat and barley straw in three diverse locations in the USA. We evaluated residue removal with and without N replacement, along with application of a high-lignin fermentation byproduct (HLFB), the residue by-product comprised of lignin and small quantities of nutrients from cellulosic ethanol production. SOC always decreased with residue harvest, but the decrease was greater in colder climates when expressed on a life cycle basis. The effect of residue harvest on soil N2O emissions varied with N addition and climate. With N addition, N2O emissions always increased, but the increase was greater in colder climates. Without N addition, N2O emissions increased in Iowa, but decreased in Maryland and North Carolina with crop residue harvest. Although SOC was lower with residue harvest when HLFB was used for power production instead of being applied to land, the avoidance of fossil fuel emissions to the atmosphere by utilizing the cellulose and hemicellulose fractions of crop residue to produce ethanol (offsets) reduced the overall greenhouse gas (GHG) emissions because most of this residue carbon would normally be lost during microbial respiration. Losses of SOC and reduced N mineralization could both be mitigated with the application of HLFB to the land. Therefore, by returning the high-lignin fraction of crop residue to the land after production of ethanol at the biorefinery, soil carbon levels could be maintained along with the functional benefit of increased mineralized N, and more GHG emissions could be offset compared to leaving the crop residues on the land.

Modeling nitrous oxide emissions from irrigated agriculture: testing DayCent with high-frequency measurements.

A unique high temporal frequency data set from an irrigated cotton-wheat rotation was used to test the agroecosystem model DayCent to simulate daily N20 emissions from subtropical vertisols under different irrigation intensities. DayCent was able to simulate the effect of different irrigation intensities on N20 fluxes and yield, although it tended to overestimate seasonal fluxes during the cotton season. DayCent accurately predicted soil moisture dynamics and the timing and magnitude of high fluxes associated with fertilizer additions and irrigation events. At the daily scale we found a good correlation of predicted vs. measured N20 fluxes (r2 = 0.52), confirming that DayCent can be used to test agricultural practices for mitigating N20 emission from irrigated cropping systems. A 25-year scenario analysis indicated that N20 losses from irrigated cotton-wheat rotations on black vertisols in Australia can be substantially reduced by an optimized fertilizer and irrigation management system (i.e., frequent irrigation, avoidance of excessive fertilizer application), while sustaining maximum yield potentials.

Patterns of new versus recycled primary production in the terrestrial biosphere.

Nitrogen (N) and phosphorus (P) availability regulate plant productivity throughout the terrestrial biosphere, influencing the patterns and magnitude of net primary production (NPP) by land plants both now and into the future. These nutrients enter ecosystems via geologic and atmospheric pathways and are recycled to varying degrees through the plant-soil-microbe system via organic matter decay processes. However, the proportion of global NPP that can be attributed to new nutrient inputs versus recycled nutrients is unresolved, as are the large-scale patterns of variation across terrestrial ecosystems. Here, we combined satellite imagery, biogeochemical modeling, and empirical observations to identify previously unrecognized patterns of new versus recycled nutrient (N and P) productivity on land. Our analysis points to tropical forests as a hotspot of new NPP fueled by new N (accounting for 45% of total new NPP globally), much higher than previous estimates from temperate and high-latitude regions. The large fraction of tropical forest NPP resulting from new N is driven by the high capacity for N fixation, although this varies considerably within this diverse biome; N deposition explains a much smaller proportion of new NPP. By contrast, the contribution of new N to primary productivity is lower outside the tropics, and worldwide, new P inputs are uniformly low relative to plant demands. These results imply that new N inputs have the greatest capacity to fuel additional NPP by terrestrial plants, whereas low P availability may ultimately constrain NPP across much of the terrestrial biosphere.

Nitrogen placement and source effects on nitrous oxide emissions and yields of irrigated corn.

Limited information is available on how N fertilizer placement affects soil nitrous oxide (NO) emissions under irrigated conditions in the semiarid western United States. Our objective was to compare surface banding near corn row and broadcasting of three N sources (urea, polymer-coated urea [PCU], and stabilized urea [SU] containing urease and nitrification inhibitors) on NO emissions from a clay loam soil under sprinkler-irrigated continuous corn production. The N fertilizers were applied at a rate of 202 kg N ha to strip-till (2010 and 2011) and no-till (2011) corn at crop emergence, with ∼19 mm irrigation water applied the next day. Band-applied N had a 1.46-fold greater NO emission than broadcast N averaged over N sources and three studies. Soil NO-N emissions from urea were 1.48- and 1.74-fold greater than from PCU and SU, respectively, when averaged over N placement and studies. The N placement × source interaction was not significant. Averaged across studies, grain yield and N uptake did not vary with N placement, whereas grain yields were greater for SU than PCU but were not different from urea. Nitrous oxide emissions per unit of N applied, per unit of grain yield, and per unit N uptake were 59, 49, and 47% greater, respectively, with banded than with broadcast N fertilizer. These studies show that N placement and N source selection are important manageable factors that can affect NO emissions and need to be considered when developing NO mitigation practices in irrigated cropping systems in the semiarid western United States.

Nitrogen source and placement effects on soil nitrous oxide emissions from no-till corn.

A nitrogen (N) source comparison study was conducted to further evaluate the effects of inorganic N source and placement on growing-season and non-crop period soil nitrous oxide (NO). Commercially available controlled-release N fertilizers were evaluated for their potential to reduce NO emissions from a clay loam soil compared with conventionally used granular urea and urea-ammonium nitrate (UAN) fertilizers in an irrigated no-till (NT) corn ( L.) production system. Controlled-release N fertilizers evaluated were: a polymer-coated urea (ESN), stabilized urea (SuperU), and UAN+AgrotainPlus (SuperU and AgrotainPlus contain nitrification and urease inhibitors). Each N source was surface band applied (202 kg N ha) near the corn row at emergence and watered into the soil the next day. Subsurface banded ESN (ESNssb) and check (no N applied) treatments were included. Nitrous oxide fluxes were measured during two growing seasons and after harvest using static, vented chambers. All N sources had significantly lower growing-season NO emissions than granular urea (0.7% of applied N), with UAN+AgrotainPlus (0.2% of applied N) and ESN (0.3% of applied N) having lower emissions than UAN (0.4% of applied N). Similar trends were observed when expressing NO emissions on a grain yield and N uptake basis. Corn grain yields were not different among N sources but were greater than the check. Selection of N fertilizer source can be a mitigation practice for reducing NO emissions in NT, irrigated corn in semiarid areas. In our study, UAN+AgrotainPlus consistently had the lowest level of NO emissions with no yield loss.

Nitrogen source effects on soil nitrous oxide emissions from strip-till corn.

Nitrogen (N) application to crops generally results in increased nitrous oxide (NO) emissions. Commercially available, enhanced-efficiency N fertilizers were evaluated for their potential to reduce NO emissions from a clay loam soil compared with conventionally used granular urea and urea-ammonium nitrate (UAN) fertilizers in an irrigated strip-till (ST) corn ( L.) production system. Enhanced-efficiency N fertilizers evaluated were a controlled-release, polymer-coated urea (ESN), stabilized urea, and UAN products containing nitrification and urease inhibitors (SuperU and UAN+AgrotainPlus), and UAN containing a slow-release N source (Nfusion). Each N source was surface-band applied (202 kg N ha) at corn emergence and watered into the soil the next day. A subsurface-band ESN treatment was included. Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. All N sources had significantly lower growing season NO emissions than granular urea, with UAN+AgrotainPlus and UAN+Nfusion having lower emissions than UAN. Similar trends were observed when expressing NO emissions on a grain yield and N uptake basis. Loss of NO-N per kilogram of N applied was <0.8% for all N sources. Corn grain yields were not different among N sources but greater than treatments with no N applied. Selection of N fertilizer source can be a mitigation practice for reducing NO emissions in strip-till, irrigated corn in semiarid areas.

The biogeochemistry of bioenergy landscapes: carbon, nitrogen, and water considerations.

The biogeochemical liabilities of grain-based crop production for bioenergy are no different from those of grain-based food production: excessive nitrate leakage, soil carbon and phosphorus loss, nitrous oxide production, and attenuated methane uptake. Contingent problems are well known, increasingly well documented, and recalcitrant: freshwater and coastal marine eutrophication, groundwater pollution, soil organic matter loss, and a warming atmosphere. The conversion of marginal lands not now farmed to annual grain production, including the repatriation of Conservation Reserve Program (CRP) and other conservation set-aside lands, will further exacerbate the biogeochemical imbalance of these landscapes, as could pressure to further simplify crop rotations. The expected emergence of biorefinery and combustion facilities that accept cellulosic materials offers an alternative outcome: agricultural landscapes that accumulate soil carbon, that conserve nitrogen and phosphorus, and that emit relatively small amounts of nitrous oxide to the atmosphere. Fields in these landscapes are planted to perennial crops that require less fertilizer, that retain sediments and nutrients that could otherwise be transported to groundwater and streams, and that accumulate carbon in both soil organic matter and roots. If mixed-species assemblages, they additionally provide biodiversity services. Biogeochemical responses of these systems fall chiefly into two areas: carbon neutrality and water and nutrient conservation. Fluxes must be measured and understood in proposed cropping systems sufficient to inform models that will predict biogeochemical behavior at field, landscape, and regional scales. Because tradeoffs are inherent to these systems, a systems approach is imperative, and because potential biofuel cropping systems and their environmental contexts are complex and cannot be exhaustively tested, modeling will be instructive. Modeling alternative biofuel cropping systems converted from different starting points, for example, suggests that converting CRP to corn ethanol production under conventional tillage results in substantially increased net greenhouse gas (GHG) emissions that can be only partly mitigated with no-till management. Alternatively, conversion of existing cropland or prairie to switchgrass production results in a net GHG sink. Outcomes and policy must be informed by science that adequately quantifies the true biogeochemical costs and advantages of alternative systems.

Nitrogen source effects on nitrous oxide emissions from irrigated no-till corn.

Nitrogen fertilization is essential for optimizing crop yields; however, it may potentially increase nitrous oxide (N2O) emissions. The study objective was to assess the ability of commercially available enhanced-efficiency N fertilizers to reduce N2O emissions following their application in comparison with conventional dry granular urea and liquid urea-ammonium nitrate (UAN) fertilizers in an irrigated no-till (NT) corn (Zea mays L.) production system. Four enhanced-efficiency fertilizers were evaluated: two polymer-coated urea products (ESN and Duration III) and two fertilizers containing nitrification and urease inhibitors (SuperU and UAN+AgrotainPlus). Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. Enhanced-efficiency fertilizers significantly reduced growing-season N2O-N emissions in comparison with urea, including UAN. SuperU and UAN+AgrotainPlus had significantly lower N2O-N emissions than UAN. Compared with urea, SuperU reduced N2O-N emissions 48%, ESN 34%, Duration III 31%, UAN 27%, and UAN+AgrotainPlus 53% averaged over 2 yr. Compared with UAN, UAN+AgrotainPlus reduced N2O emissions 35% and SuperU 29% averaged over 2 yr. The N2O-N loss as a percentage of N applied was 0.3% for urea, with all other N sources having significantly lower losses. Grain production was not reduced by the use of alternative N sources. This work shows that enhanced-efficiency N fertilizers can potentially reduce N2O-N emissions without affecting yields from irrigated NT corn systems in the semiarid central Great Plains.