Quantification of Nitrogen Lost to Greenhouse Gas Emissions in Cover Crop Systems

View the project final report

Commodities

• Agronomic: corn, wheat

Practices

• Crop Production: cover crops, nutrient cycling

Nitrous oxide (N₂O) emissions were measured in a cropping system which included fall-planted oilseed radish, annual ryegrass, and a radish + ryegrass mixture. No fertilizer was applied to the cover crops or the following corn crop. Cover crop fall and spring biomass production varied by site and year, as did N₂O emissions. Although cover crop C:N ratios were large enough to cause soil N immobilization at one site, there was no effect on N₂O emissions. Cumulative N₂O-N emissions were lower than would be expected. Cover crop treatments had minimal effect on corn growth and yield.

Introduction:

Cover crop use provides multiple environmental and economic benefits (Snapp et al. 2005). Among other management strategies, Robertson and Vitousek (2009) advocate their use to improve nitrogen (N) fertilizer uptake into cropping systems. However, in order to increase N use efficiency (NUE), the correct amount of N must be applied at peak cash crop N demand while minimizing N loss to the environment (Ribaudo et al. 2011). In this regard fall-planted cover crops may be problematic because some cover crops which winter-kill (e.g., radish) may decompose and release N before the cash crop needs it while others which do not winter-kill (e.g., annual ryegrass) may cause N to be immobilized during peak cash crop N demand. Furthermore, the early release of N from winter-killed cover crops may provide an opportunity for N to leave the system. Research has focused primarily on the potential for this N to leach. N lost via the emission of greenhouse gases such as nitrous oxide has been far less researched.

A search of the SARE projects database reveals a similar lack of research in the area of greenhouse gas emissions and cover crops. Some SARE regions have received grants for “train the trainer” activities related to climate change (e.g. ENE05-091 “Climate change and agriculture: Preparing educators to promote practical and profitable responses”). This project complemented those efforts by providing data on which to base recommendations.

The SARE projects database includes two projects with direct and immediate relevance to this project. One is student project GNE10-005 “Balancing nitrogen sinks and sources using cover crops on manured fields”. My project differed in that: a) it included a mix of winter-hardy and non winter-hardy cover crops as well as pure stands of each cover crop type, b) gas sampling dates were chosen specifically to elucidate greenhouse gas emissions during the peak cover crop decomposition period, c) no fertilizer or manure was applied, and d) it also investigated the impact of the cover crops on the following cash crop.

The other relevant SARE project is GNC12-158 “Improving resource use efficiency through strip tillage, cover cropping, and deep fertilizer placement”. My project differed in two regards: its primary focus was on cover crops and greenhouse gas emissions, and the work was in an agronomic production system while GNC12-158 was in a vegetable production system. My project aimed to provide data on greenhouse gas (nitrous oxide) emissions in agricultural systems which incorporate fall-planted cover crops, and to characterize the growth of the cover crops and quantify their impact on the following corn crop.

This project had three objectives:

1. Quantify the N lost to greenhouse gas emissions in fall-planted cover crops.
2. Compare the greenhouse gas emissions in a system using a cover crop which winter-kills with one using a cover crop which over-winters and one with a mixture of the two cover crop types.
3. Determine cover crop impact on the following cash crop.