Recipient Organization
RESEARCH FOUNDATION OF THE CITY UNIVERSITY OF NEW YORK
85 ST NICHOLAS TERRACE
NEW YORK,NY 100311246
Performing Department
Environmental Sciences
Non Technical Summary
Nitrogen can be lost from agricultural soils through multiple pathways, including leaching of nitrate in groundwater, and emissions of the greenhouse gas nitrous oxide (N2O). Nitric oxide is a trace gas that is produced by soil microbes and emitted to the atmosphere, where it is is important in the formation of ozone and particulate matter pollution. NO emissions are an important loss pathway for nitrogen from agricultural systems, and these emissions can increase substantially with the addition of fertilizer. Winter NOx (NO + nitrogen dioxide (NO2)) emissions from non-urban sources have been suggested to be an important precursor to particulate matter pollution over the midwestern U.S. Unfortunately, agricultural NOx emissions in the midwestern U.S. are poorly constrained over during winter. In particular, the effects of freeze-thaw events on soil NO emissions are not well understood, even though they are globally important sources of N2O.We will conduct a series of linked activities to improve our understanding of soil NO emissions during winter as well as the rest of the growing season: 1) We will use the eddy covariance technique at the Platte River - High Plains Aquifer Long Term Agricultural Research site (PR-HPA) to conduct continuous, high-frequency measurements NOx fluxes for one year--the first such high frequency observations of NOx from an agricultural system in the United States. 2) We will use high resolution satellite-based observations of NO2 to evaluate the impact of freeze-thaw events on regional atmospheric composition to determine if they are an important source of NOx in the midwestern U.S. 3) We will conduct laboratory incubations of soils from conventional agricultural management and management that incorporates a greater conservation approach at PR-HPA to evaluate biogeochemical and microbial responses to freeze-thaw events, and to identify the physical and biological factors that are responsible for freeze-thaw emissions of NO. The outcomes of this project will address goals a) and b) of the Soil Health Program Area Priority, as well as both suggested topics for FY2020: the effects of management practices on soil microbial community function and the assessment of approaches for enhancing understanding of biogeochemical processes contributing to agricultural sustainability.
Animal Health Component
20%
Research Effort Categories
Basic
80%
Applied
20%
Developmental
(N/A)
Goals / Objectives
Nitrogen (N) transformations are a critical component of biogeochemical cycling in agricultural soils. In many ecosystems, state changes in the chemical or physical conditions of soils can trigger short-term pulses of biogeochemical activity that play a large role in annual N budgets. For example, work from our group shows that when substrate such as N fertilizer is added to soils, when dry soils are rewetted, or when soils are frozen and subsequently thaw, microbially-mediated N transformations often proceed at rates higher than at any other time of year, leading to substantial losses of N to the environment. In temperate agricultural ecosystems, freeze-thaw events--and winter emissions more generally--have long been known to be important sources of the greenhouse gas nitrous oxide (N2O; Figure 1). Even though freeze-thaw emissions of N2O were known to be larger in agricultural than natural ecosystems, it is only in the last few years--through the implementation of continuous, high-frequency measurements--that it has become clear just how important these emissions are: Freeze-thaw emissions are estimated to be responsible for roughly half of all N2O emissions from temperate agricultural systems, and one quarter of all agricultural N2O emissions globally.While many studies have investigated the importance of freeze-thaw dynamics on N2O emissions, very few have addressed more reactive N gases such as nitric oxide (NO) even though N2O and NO are produced by the same processes: nitrification and denitrification. In order to constrain N loss pathways from agriculture and to understand agricultural influences on air quality, it is essential to understand winter soil NO emissions, particularly during freeze-thaw events. We will leverage advances in trace gas measurements, detailed molecular analyses, and new, high resolution satellite NO2 observations to quantify the magnitude of winter NOx emissions in the U.S. Corn Belt, to gain insight into approaches for controlling these emissions, and to understand their impacts on regional NO2 concentrations. Specifically, we will 1) conduct in situ observations of NO, NOx, and N2O fluxes using the eddy-covariance technique at the USDA Platte River--High Plains Aquifer (PR-HPA) Long Term Agricultural Research (LTAR) site to quantify freeze-thaw NO pulses and obtain the first well-constrained estimate of NOx losses from agricultural soils in North America, 2) evaluate physical, biogeochemical, and microbial contributions to freeze-thaw NO emissions using laboratory incubations of repacked soil cores, and 3) use satellite observations to detect, explain, and statistically analyze the degree to which freeze-thaw events alter atmospheric NO2 concentrations across the Corn Belt.Novel ideas and contributions: This project incorporates several key novel ideas and contributions. As noted above, freeze-thaw emissions of NO have not been widely investigated and are poorly constrained, particularly in North American agro-ecosystems. Our observations and experiments will evaluate several key questions that have not been explored previously, including:1) providing the first long-term eddy covariance measurements of NOx fluxes from an agricultural system in the United States,2) investigating the impact of freeze-thaw events on atmospheric composition at regional scales,3) evaluating the response of nitrifying bacteria and archaea to freeze-thaw events.
Project Methods
The proposed work consists of field experiments, laboratory incubations, and remote sensing analyses:Field experiments:Site information: The USDA Platte River--High Plains Aquifer (PR-HPA) site includes a cropland business as usual experiment and is implementing an aspirational site starting this year. The cropland business as usual experiment consists of corn-corn and corn-soybean rotations. It has field-scale treatments in place that are instrumented with eddy covariance, meteorological, and pheno-cam instrumentation. These sites are located at the Eastern Nebraska Research and Extension Center (ENREC). The field-scale aspirational irrigated site has been identified at ENREC and will incorporate variable rate irrigation technology and cover crops. Crop yields and livestock performance are measured at appropriate times during the year. Vegetation health and seasonal changes in plant traits are assessed using physiological and optical measurements.Eddy covariance flux measurements: We will outfit the existing flux tower at PR-HPA with a high-frequency high-sensitivity chemiluminescence analyzer to measure NO (CLD 780TR Ecophysics, Switzerland), coupled with a blue light convertor (PLC 762 Photolytic Converter), which photolyzes NO2 into NO, to measure NOx. The CLD 780TR is an in-site system that conducts simultaneous measurements of NO and NOx at 10 Hz with a detection limit of 25 ppt at 3 second averaging. The instrument will be calibrated regularly in the field using a 5 ppm NO gas standard diluted in zero air. To investigate the portion of nitrogen oxide emissions emitted as NO, we will simultaneously measure N2O eddy covariance fluxes by co-deploying a Los Gatos Research Off-Axis Integrated Cavity Output Spectroscopy (ICOS) enhanced performance analyzer (LGR 913-0015). The high-sensitivity LGR instrument also samples at a 10 Hz rate. The LGR system will undergo frequent calibrations in the field and characterization of variations in the cell flush time. To collect these measurements, sample air will be brought into the field lab through Teflon PFA inlet tubing at a fast flow rate controlled by Edwards XDS20i scroll pumps. Eddy-covariance fluxes will be calculated using a set of custom MATLAB scripts.Lab analysesSpecifically, we will evaluate the effects of variation in freezing temperature, freezing degree days (FDDs), and freeze-thaw cycling under conventional and aspirational management approaches:Experiment 1. We will assign replicate cores to different freezing temperatures and FDD treatments. Specifically, replicates will be frozen for 15 days at a) -1°C (15 FDDs) , b) -3°C (45 FDDs), and c) -5°C (75 FDDs), after which the cores will be thawed to 5°C for 14 days. Control cores will be maintained at 5°C, and a second set of replicate cores kept at -3°C will be thawed after 5 days (15 FDDs) to evaluate the ability of FDDs to predict freeze-thaw NO emissions.Experiment 2. After the 14 days at 5°C, replicate cores at the -5°C treatment will undergo two additional freeze-thaw cycles. In each cycle, the cores will be kept for 28 days at -5°C, followed by two weeks at 5°C.Soil cores will be incubated in gas-tight chambers in He-O2 mixtures ranging from 0 to 20% O2 and the fluxes of N2O, N2, and CO2 are directly measured by gas chromatography; NO fluxes are directly measured with a Sievers Nitric Oxide Analyzer with a chemiluminescence detector. A duplicate set of vessels will also be established for partially destructive measurements of gravimetric soil moisture, NO3-, ammonium (NH4+), net N mineralization, net nitrification, microbial biomass, and denitrification enzyme activity. Concentrations of NO2-, NO3-, and NH4+ will be determined colorimetrically on a Lachat QuikChem autoanalyzer (Loveland, CO) following extraction in 2M KCl solution. Net N mineralization and net nitrification will be determined as the difference in starting and ending total inorganic N (mineralization) or NO2- + NO3- (nitrification). Denitrification enzyme activity will be analyzed using the acetylene inhibition method.Soil samples will be taken prior to thaw and daily for the first five days after thaw in experiment 1, and once before and then daily for the first five days after each subsequent thaw in experiment 2. Soils will be collected, placed in whirl-pak bags, and stored at -80 °C. Pre-thaw soils will be considered Tinitial for soil microbial community analysis. We will conduct extraction of nucleic acids from soil samples. Soil microbial DNA and RNA will be co-extracted from triplicate 2 g of soil using the Qiagen RNeasy PowerSoil Total RNA Kit, followed by a RNeasyPowerSoil DNA Elution Kit (Germantown, MD 20874). A combination of existing and novel primers designed using Primer3 software will be synthesized by IntegratedDNATechnologies, Inc. (Coralville, Iowa 52241). Gene targets will be used to evaluate abundance and dynamics of narG, H & I (nitrate reductase), nirS and nirK (nitrite reductase), norB & C (NO reductase), and nosZ I & II (N2O reductase). Quantitative PCR of denitrification genes in environmental samples will be compared to the abundance of 16S rRNA as well as to linear standards of known amounts of template created from plasmid clones. Amplification will be carried out with a Stratagene Mx3000p qPCR System (Agilent Technologies, Santa Clara, CA) using SYBR green detection. We will conduct a repeated measures analysis using the lmer function in the R programming language.Satellite analysisWe will use TROPOMI NO2 tropospheric column data first to evaluate the ground-level observations of NO fluxes over PR-HPA. Comparisons of this time series to the field observations will allow for evaluation of the satellite observations.We will also conduct a regional assessment of the effects of freeze-thaw events on atmospheric NO2 concentrations, focusing on the Corn Belt states of Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, and Ohio, and restricting our analysis to pixels with >50% agricultural land cover. We will regrid TROPOMI tropospheric NO2 observations to 0.1° x 0.1° resolution using an area-weighted approach. We will also obtain daily minimum and maximum soil temperature data from the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR). We will conduct time series analyses of the NO2 and soil temperature data to determine whether freeze-thaw soil NO emissions can be detected from space, and if so, evaluate their contribution to tropospheric NOx pollution.To observe freeze-thaw emission events, we will evaluate maps of three-day mean tropospheric NO2 VCDs across our study region for periods immediately preceding, during, and following wide-spread soil thaw as indicated by the NARR temperature data. We will also conduct piecewise linear regression analyses of the relationship between the mean soil temperature and mean NO2 VCDs for daily values integrated across the entire study region, focusing on the period before and after spring thaw. We will repeat this analysis but using values for individual pixels as the predictor and response variables.Lastly, we will infer daily surface NOx emissions across the US Corn Belt region using the TROPOMI observations and a simple box model, and will evaluate these modeled fluxes against fluxes calculated from the ground-level measurements.