Nitrous oxide emission and soil carbon sequestration from herbaceous perennial biofuel feedstocks

Authors: Johnson, Jane; Barbour, Nancy

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2016
Publication Date: 7/7/2016
Publication URL: http://handle.nal.usda.gov/10113/63287
Citation: Johnson, J.M., Barbour, N.W. 2016. Nitrous oxide emission and soil carbon sequestration from herbaceous perennial biofuel feedstocks. Soil Science Society of America Journal. 80:1057-1070.

Interpretive Summary: Burning fossil fuels has contributed to measured increase in atmospheric carbon dioxide and other gases, which are causing global climate change. Plants can take carbon dioxide released when burning fossil fuels, transfer it into the biological carbon cycle to produce food, feed, fiber and biofuel and build soil quality by increasing soil organic matter. As a result, this helps reduce the concentration of atmospheric carbon dioxide and provides other benefits. The bioenergy industry is starting to using non-food, cellulosic feedstock to make liquid fuels like ethanol. Before ethanol was mostly made from starch grains like corn. Unlike starch, cellulose cannot be digested by humans. Plants contain large amounts of this energy-rich material. The objective of this study was to measure the release of a potent greenhouse gas: nitrous oxide associated with fertilizing crops. When grasses are harvested, they need to be fertilized or yield will decline very quickly. In this study, growing perennial grasses were managed as a biofuel feedstock. Perennial grasses can protect soil from erosion and help build soil organic matter and soil health. Plants use carbon dioxide from the air to grow, some of carbon dioxide that is converted by photosynthesis is moved through the roots to the soil and eventually becomes the backbone of soil organic matter – called soil organic carbon. In this study we found that the benefits of increasing soil organic matter by growing and harvesting the perennial grasses outweighed the small increase in the release of nitrous oxide. Also, growing corn and soybean without tillage and returning all of their non-grain material also had environmental benefits. We suggest that perennials grasses be grown on the landscape together with conservation management of crops to achieve the environmental and agronomic benefits. This work will aid producers, energy industry, and action agencies determine if perennial grasses will meet the Renewable Fuel Standard 2 – criteria of a renewable fuel.

Technical Abstract: Greenhouse gas (GHG) mitigation and renewable, domestic fuels are needed in the United States. Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman) are potential bioenergy feedstocks that may meet this need. However, managing perennial grasses for feedstock requires nitrogenous inputs, which may alter their effectiveness to mitigate GHG emission since N fertilizer can cause N2O emission. Managing annual crops without tillage may also sequester carbon (C) and offset GHG emission. The objectives of this study were to compare nitrous oxide (N2O) flux and soil C storage parameters between 1) grasses with legume companion crop or with nitrogenous fertilizer, 2) two grass harvest times (autumn, spring); and 3) perennial systems and corn/soybean (C/S) rotation without tillage on a long-term study site. Nitrous oxide flux was measured from May 2009 to May 2012 and soil C parameters were measured (total soil C to 100 cm and active fraction to 30 cm). Cumulative N2O emission was 14 to 40% greater when feedstock was harvested in the spring compared to autumn for big bluestem and switchgrass, respectively. Including a legume rather than applying urea to perennial grasses reduced annual emission to near background at 3 kg N2O-N ha-1 yr-1. Average cumulative emission of all six perennial treatments (16.0 kg N2O-N ha-1) exceeded that of C/S rotation (12.7 kg N2O-N ha-1). All treatments sequestered soil C and provided offsetting C equivalents from N2O. Thus, C/S rotation without tillage or perennial systems managed for bioenergy feedstock are strategies with potential for GHG mitigation.