Greenhouse Gas Emissions Under Winter Wheat-Based Organic and Conventional Crop Productions

Project Director: Upendra Sainju, USDA-ARS

Project Overview

The agriculture sector is one of the largest contributors to greenhouse gas (GHG) emissions in the United States, contributing an estimated 10% of total emissions in 2022. Comparing agricultural greenhouse gas emissions between cropping systems (i.e., organic, conventional, integrated crop-livestock) can be an effective way of identifying GHG ‘hotspots’ and developing targeted reduction strategies.

This study, conducted over several consecutive growing seasons in Bozeman, MT, evaluated how cropping system (organic vs. conventional) and cropping sequence impacted agricultural greenhouse gas emissions (CO2, N2O, and CH4) in a winter wheat-based crop rotation.

Farmer Takeaways

Cumulative carbon dioxide (CO2) and nitrous oxide (N2O) fluxes were greater in conventional cropping systems for winter wheat-based crop phases during at least one growing season, resulting in overall higher global warming potentials (GWP) for conventional systems. Organic management practices may reduce the global warming potential for winter wheat crop phases in multi-crop rotations.
Growing a cereal crop (i.e., winter wheat) after a legume crop (i.e., lentil, sweetclover) may result in increased CO2 emissions due to enhanced microbial activity and root growth associated with amplified soil nitrogen.
Grazing sheep during fallow periods may provide soil health benefits in organic cropping systems, but may also increase N2O emissions; as such, livestock should be managed carefully.

Project Objectives and Approach

To evaluate how cropping system (organic vs. conventional) and cropping sequence impact agricultural greenhouse gas emissions (CO2, N2O, and CH4) over several consecutive growing seasons in a winter wheat-based crop rotation

A crop production experiment was conducted in Bozeman, MT from 2013-2016. Treatments included two cropping systems (organic and conventional) and three cropping sequences (see below).
- - The three cropping sequences included: (1) L-W, lentil after winter wheat; (2) W-C, winter wheat after sweetclover cover crop; and (3) W-L, winter wheat after lentil.
  - The cropping sequences were part of a 5-year crop rotation of safflower & sweetclover cover crop / sweetclover cover crop / winter wheat / lentil / winter wheat. Treatment plots were established such that each phase of the crop rotation was present in every year.
In the organic cropping system, weeds and pests were managed with a combination of sheep grazing (during fallow periods in the spring and fall, as well as during cover crop growth in summer) and minimal tillage, and crops were grown without N fertilizer or pesticides/herbicides. In the conventional cropping system, weeds and pests were managed with minimal tillage and herbicide/pesticide application, and an estimated 300kg N/ha was provided to the winter wheat phase.
Two static greenhouse gas chambers were installed in each plot, and were only removed during tillage, planting, fertilization, and harvest events. Gases were sampled and data was analyzed on CO2, N2O, and CH4 fluxes.

Key Findings

Carbon dioxide (CO2) flux* varied significantly by cropping sequence, with higher emissions for the crop phases where winter wheat was grown immediately after a legume

CO2 flux: The rate of exchange of CO2 from an ecosystem to the atmosphere. In an agricultural context, CO2 flux measures the balance of CO2 uptake via photosynthesis and CO2 release via microbial respiration and decomposition, with high fluxes corresponding to high CO2 release.
When averaged across cropping systems, CO2 flux tended to be greater between May and September for the W-C (winter wheat after clover) and W-L (winter wheat after lentil) cropping sequences than L-W (lentil after winter wheat), suggesting that growing a legume crop prior to a cereal crop may result in higher CO2 emissions. The researchers postulated that this could be due in part to the increased availability of nitrogen following a legume crop, which may have enhanced winter wheat root growth and microbial activity.

Conventional cropping systems had a greater cumulative GHG fluxes than organic cropping systems during both winter wheat cropping sequences

Cumulative CO2 and N2O fluxes were greater for the conventional cropping system than the organic cropping system for the winter wheat after clover (W-C) cropping sequence during the 2014/2015 growing season, and cumulative CO2 flux was greater for conventional winter wheat after lentil (W-L) in the 2015/2016 growing season.
- - This is likely due to the application of synthetic N fertilizer during the winter wheat phases of the conventional cropping system, which may have promoted microbial activity (respiration) and root growth or been directly converted to N2O via denitrification.

Nitrous oxide (N2O) flux* varied by cropping sequence and cropping system, with higher emissions for the lentil-after-winter-wheat (L-W) cropping sequence and during the months after sheep grazing in the organic cropping system

N2O flux was significantly greater for the organic cropping system than the conventional cropping system from May-June 2014 and March-April 2015. This was likely due to the denitrification of nitrogen deposited in sheep urine and manure during the spring fallow grazing period, and indicates that livestock – while providing soil health benefits – may serve as a hotspot for GHG emissions and should be managed carefully when integrated into organic crop rotations.
N2O flux was significantly greater for the lentil after winter wheat (L-W) cropping sequence than for either of the winter wheat cropping sequences (W-C, W-L) from May-September 2014 and March-May 2015.

Resources

Sainju, U. M., Hatfield, P. G., & Ragen, D. L. (2021). Greenhouse gas emissions under winter wheat-based organic and conventional crop productions. Soil Science Society of America Journal, 85(5), 1349–1361.

Location

Montana

Collaborators

Patrick Hatfield, Montana State University
Devon Ragen, Montana State University

Region

Northwest

Topic

Soil Health, Climate Solutions, Cropping Systems

Year Published

2020