Upon reviewing the list of practice changes that growers can implement to enroll in a carbon credit program one that seems to generate confusion is nitrogen efficiency. What does the efficient usage of nitrogen have to do with sequestering carbon? It is important to first explain what the term “nitrogen efficiency” means in terms of a practice change.
The adoption of Nitrogen (N) efficiency practices typically involves a switch from a single N application to a split application. Including a N inhibitor also qualifies as a N efficiency practice. Essentially, any practice change that results in less N loss from denitrification and/or leaching can qualify as a practice change to enroll in a carbon program.
The basic strategy for carbon sequestration is to (1) increase the amount of C entering the system through an increase in net primary productivity, which essentially means increasing plant biomass production within a land area, and (2) increase the storage of the biomass C into the soil. At a high level it is easy to understand how cover cropping and reducing tillage improve a soil’s capacity to act as a C sink – cover crops add additional biomass C, which can potentially be stored in soil, and reducing tillage prevents the loss of this C through improved soil structure and a reduction in erosion and/or mineralization. So where does N factor into the sequestration equation?
It starts with soil microorganisms, which are the main drivers of C transformations in the soil. As these bacteria and fungi decompose plant residues they utilize a portion of the C in the residue for their own growth and maintenance while respiring some as CO2. This process is called mineralization and results in the transformation of plant nutrients that were formerly bound in the plant residue into a form which can be taken up by plants or other microorganisms. The C that is allocated for growth and maintenance is referred to as microbial biomass C and is the main precursor to stable soil organic carbon (SOC). The majority of C (60-80%) in stable SOC pools comes from microbial biomass C with the remainder deriving from unprocessed plant residue that is physically trapped within soil aggregates, making it unavailable for degradation by microbes.
Nitrogen is an important component in the mineralization process because the C/N ratio of microorganisms is typically one to two order of magnitude lower than that of plant biomass, meaning that microbes need additional N or higher quality residue (C/N < 24) to completely degrade most plant residues efficiently. Low quality residue is used less efficiently by the microbes, meaning that less of the C in the plant residue will eventually become stable SOC. For example, legume (high quality residue) cover crops often result in greater increases in SOC compared to non-legumes (low quality residue) even when the legume cover crops produce less biomass.
When decomposing low quality residues soil microbes are forced to utilize plant available N (immobilization) in the soil and if plant available N is unavailable they will begin to mine native SOM to free up N to meet their nutritional requirements (priming effect), which consequently lowers SOC stocks. Remember, SOM is around 58% C but also contains plant nutrients such as N, P, and K and microbes also utilize these nutrients for growth and maintenance when decomposing residue. Preventing N loss prevents the loss of SOC by inhibiting the priming effect and encourages C sequestration by increasing biomass production of the cash crop.
If you are transitioning to a carbon positive practice that does not include a legume cover crop it is not wise to immediately cut back N rates but rather take steps to maximize the efficiency of applied N. If applying anhydrous ammonia in the fall prior to corn a nitrification inhibitor will result in more available N come spring when soil microbes are waking up and considering mining the carbon in your soil. For systems using synthetic N fertilizer a split application with one application at or prior to planting followed by a sidedress application is best practice for optimal C sequestration and soil health. If you are considering a legume to replace synthetic N fertilizer it is critical that an overwintering species is used (e.g. hairy vetch, clover, and some winter pea species).
It is unlikely that a legume will provide a N credit sufficient to meet the economic optimum N rate so be prepared to sidedress some N. Any practice change that aims to increase stocks of soil C cannot do so without an accompanying utilization of the 4Rs of nutrient stewardship (right rate, right time, right source, right place). Ensuring that your increasingly expensive N fertilizer is used in the most efficient way possible is a critical step in maximizing income from a carbon credit contract.