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.

Using isotope pool dilution to understand how organic carbon additions affect N2 O consumption in diverse soils

Nitrous oxide (N2 O) is a formidable greenhouse gas with a warming potential ~300× greater than CO2 . However, its emissions to the atmosphere have gone largely unchecked because the microbial and environmental controls governing N2 O emissions have proven difficult to manage. The microbial process N2 O consumption is the only know biotic pathway to remove N2 O from soil pores and therefore reduce N2 O emissions. Consequently, manipulating soils to increase N2 O consumption by organic carbon (OC) additions has steadily gained interest. However, the response of N2 O emissions to different OC additions are inconsistent, and it is unclear if lower N2 O emissions are due to increased consumption, decreased production, or both. Simplified and systematic studies are needed to evaluate the efficacy of different OC additions on N2 O consumption. We aimed to manipulate N2 O consumption by amending soils with OC compounds (succinate, acetate, propionate) more directly available to denitrifiers. We hypothesized that N2 O consumption is OC-limited and predicted these denitrifier-targeted additions would lead to enhanced N2 O consumption and increased nosZ gene abundance. We incubated diverse soils in the laboratory and performed a 15 N2 O isotope pool dilution assay to disentangle microbial N2 O emissions from consumption using laser-based spectroscopy. We found that amending soils with OC increased gross N2 O consumption in six of eight soils tested. Furthermore, three of eight soils showed Increased N2 O Consumption and Decreased N2 O Emissions (ICDE), a phenomenon we introduce in this study as an N2 O management ideal. All three ICDE soils had low soil OC content, suggesting ICDE is a response to relaxed C-limitation wherein C additions promote soil anoxia, consequently stimulating the reduction of N2 O via denitrification. We suggest, generally, OC additions to low OC soils will reduce N2 O emissions via ICDE. Future studies should prioritize methodical assessment of different, specific, OC-additions to determine which additions show ICDE in different soils.

An approach for calibrating laser-based N2 O isotopic analyzers for soil biogeochemistry research

RATIONALE

Technological advances have motivated researchers to transition from traditional gas chromatography/isotope ratio mass spectrometry to rapid, high-throughput, laser-based instrumentation for N₂ O isotopic research. However, calibrating laser-based instruments to yield accurate and precise isotope ratios has been an ongoing challenge. To streamline the N₂ O isotope research pipeline, we developed the calibration protocol for laser-based analyzers described here. While our approach is targeted at laboratory soil incubations, we anticipate that it will be broadly applicable for diverse types of stable isotope research.

METHODS

We prepared standards diluted from USGS52 and from a commercial cylinder to develop a calibration curve spanning from 0.3 to 300 ppm N₂ O. To calibrate over this broad range, we binned each isotopocule (N₂ O, N¹⁵ NO, ¹⁵ NNO, and NN¹⁸ O) into low, medium, and high concentration ranges and then used mathematically similar polynomial functions to calibrate the isotopocules within each concentration range. We also assessed the temporal stability of the instrument and the capacity for our calibration approach to work with isotopically enriched gas samples.

RESULTS

Our calibration approach yielded generally accurate and precise data when isotopocules were calibrated in concentration ranges, and the measurements appeared to be temporally stable. For all isotopocules at natural abundance, the residual percentage error was smallest in the medium N₂ O range. There was more noise in the corrected isotopomers and isotopologue at natural abundance in samples with the lowest and highest N₂ O concentrations. Corrected isotopomer results from isotopically enriched samples were very precise.

CONCLUSIONS

Developing our calibration strategy involved learning several key lessons: (1) calibrate isotopocules in distinct concentration ranges, (2) use mathematically similar models to calibrate the isotopocules in each range, (3) calibrated N₂ O concentrations and δ values tend to be most accurate and precise in the medium N₂ O range, and (4) we encourage users to take advantage of isotopic enrichment to capitalize on laser-based instrument strengths.