Net Greenhouse Gas Emissions
How Does Net Greenhouse Gas Accounting Work?
When thinking about the greenhouse gas (GHG) implications of vegetable production (or anything else), it’s important to consider nitrous oxide (N2O) and methane (CH4) in addition to carbon dioxide (CO2). This is especially true since N2O is about 300 times as bad as CO2 as a greenhouse gas, and CH4 is about 30 times as bad. When multiple GHGs are emitted, the overall impact can be expressed as an equivalent amount of CO2, or CO2-eq. For example, if 1 ton of CO2, 0.01 tons of CH4, and 0.002 tons of N2O were emitted, this could be expressed as 1.9 tons CO2-eq.
Direct emissions of CH4 during vegetable production are usually minor, so the focus here is on CO2 and N2O.
Sources and Estimates of Emissions for Vegetable Crops
A study modelling all the GHG emissions associated with organic vegetable production in Western Washington found that between 0.045 and 0.623 lb CO2-eq were emitted per lb for crops such as onions, squash, dry bush beans, potatoes, and chard.
Compost application offers an attractive way to increase SOC and build soil health, in part, because the carbon in compost is relatively stable and less likely to decompose and turn back into CO2. This is because as the compost was produced, the portion of the organic matter that would easily decompose did so, releasing some CO2 in the process.
Frequent tillage associated with intensive vegetable production exposes soil organic matter and enables the release of CO2 through soil mineralization thus reducing SOC levels. Essentially, the frequent exposure to oxygen burns off the organic matter and turns it into CO2. (A more nuanced discussion of tillage and reduced tillage is certainly necessary but will be saved for another time.)
There is no clear and definitive answer to whether/how much vegetable crops are contributing to building SOC or even what the net GHG emissions are. This variation is driven by what crop is being grown, what methods (e.g., fertilizer type, tillage intensity, cover cropping) are used, how long those methods have been used, soil type, climate, and more. Some ongoing research seeks to better understand this overall picture while other research seeks to understand the impact of specific management practices on SOC and GHGs.
The Impact of Cover Crops...
On Soil Organic Carbon
There is a little more clarity when considering the impact of a specific management practice—in this case cover cropping—on soil organic carbon and greenhouse gas emissions. One study of cover cropping over 8 years of intensive organic vegetable production showed that increasing cover cropping frequency from 1 in 4 years to every year increased the SOC in the top 12 inches. However, a portion of the increase in SOC was in a form that could easily turn back into CO2. This study showed that planting a legume-rye, mustard, or rye cover crop can increase SOC levels. All three cover crop choices boosted SOC by similar amounts. In addition, this study demonstrated that, regardless of how often cover crops are grown, compost application increased SOC levels.
A 9-year study of cover crop introduction to vegetable production in southern Ontario, Canada showed that cover cropping increased SOC in the top 6” of soil. In fact, at one site, oilseed radish used as cover crop increased SOC from 28 to 36 tons C/ac.
It’s also important to consider the full soil profile when thinking about carbon sequestration. In one study, SOC was measured over a 20-year period in a corn-tomato rotation system with and without cover crops. Addition of cover crops increased SOC in the top foot of soil, however SOC decreased in the top 6 feet. In this case, the cover crops may have been successful if the goal was to improve soil health. However, if the goal was to sequester carbon, then the cover crops were unsuccessful.
On Nitrous Oxide Emissions
One often discussed benefit of cover crops is the potential to scavenge soil nitrogen and—specifically for legume cover crops—to fix atmospheric nitrogen. However, recall that N2O is a very potent greenhouse gas. So, how do cover crops influence the emission of N2O?
A well-established benefit of cover crops is their ability to reduce nitrate in run-off leaving the farm. Because this nitrate could result in N2O emissions later on, reducing nitrate loads in run-off may also reduce GHG emissions.
Several studies which measured N2O emissions over a full year saw little to no net impact from the presence of non-legume cover crops. In general, not incorporating cover crop residues (i.e., leaving them as mulch) and using non-legume species minimizes N2O emissions.
