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California continues to show its leadership in addressing environmental issues through the passage of Assembly Bill 32, The California Global Warming Solutions Act of 2006. The most important requirement of this legislation is to establish a statewide greenhouse gas (GHG) emissions cap for 2020 that will reduce emissions to 1990 levels. California is the tenth largest emitter of carbon dioxide (CO2) and other GHG in the world. The state produces 475 million metric tons (MMT) of CO2-equivelent gases annually. Agricultural activities contributed approximately seven percent to these total emissions, which may not seem like much, however, half of the emissions occur as nitrous oxide (N2O), the most potent GHG. A molecule of N2O is 296 times more potent in trapping solar radiation than a molecule of CO2. The N2O emissions are a result of inefficient fertilizer nitrogen (N) use by crops leaving soil bacteria to convert nitrate to N2O through a process called denitrification.
In order for agriculture to provide solutions to address AB 32, it must either sequester GHG out of the atmosphere into a sink or reduce their emission to the atmosphere. During photosynthesis, crops capture CO2 in order to grow. When crops are harvested, the remaining residue (straw, vines, roots) is transformed into organic matter. Storing this C in soil represents sequestration of carbon (C) into a sink. In contrast, reduced tillage approaches such as no till or conservation tillage (CT) can reduce diesel fuel consumption and thus reduce the emission of CO2. Likewise, increasing fertilizer N use efficiency would decrease denitrification and thus the potential to emit N2O, which represents a reduction in the emission of GHG. To date, our studies at the UC Davis Russell Ranch Sustainable Agricultural Facility in the Sustainable Agriculture Farming Systems (SAFS) and Long-term Research on Agricultural Systems (LTRAS) projects show little effect of CT on soil C sequestration. Other reductions in GHG emissions can occur through the use of waste (biosolids, manure, etc.) and biological N fixation from legumes to reduce the fossil fuel required to synthesize chemical fertilizer N.
Our results show that certain practices, such as the use of cover crops, can address both CO2 and N2O emissions. Systems using winter cover crops can store up to three tons of C per hectare in 10 years compared to a traditional winter fallow system. Organic systems can sequester an additional two tons of soil organic matter for a total of five tons per hectare over 10 years. Regardless of the management used to sequester soil C, two things must be considered. First, consistent management is required. Inconsistent management (not practicing annual cover cropping or manure additions) will lead to no or only a small amount of soil C sequestration. Second, there is a finite capacity of soils to sequester C. In California, the limits to soil C sequestration are influenced strongly by climate. The warm Mediterranean climate works to limit the amount of soil C that can be sequestered through maintenance of seasonal microbial activity compared to colder climates.
SAFS and LTRAS research results and results from other studies across the state reveal that 75 to 90 percent of the potential California agriculture’s role in addressing climate change by Will Horwath, SAFS project leader soil C sequestration occurs within five years of implementing these management strategies. This shows that though soils can be significant C sinks, they are a one-time solution or offset. If consistent management for SOM is not practiced, soils will release CO2 back to the atmosphere.
By 2020, the state will need to offset 174 MMT of CO2 to return to 1990 emissions levels. Irrigated row crops represent 3.5 million hectares of land use. Table 1 provides an example of irrigated row crop land’s potential to sequester soil C and address the 2020 AB 32 emissions cap. If 100 percent of growers plant winter cover crops they could sequester 39 MMT of CO2 or almost 25 percent of what’s needed to meet AB 32 requirements. However, practicing cover cropping on all irrigated row crop land is not likely considering the challenges of planting the crops before winter rains. In addition, field entry in the spring to manage (cut and incorporate) the cover crops before planting summer crops is also dependent on the weather. It is more likely that growers could achieve planting cover crops on 25 percent of their land, representing 10 MMT of CO2 sequestration or six percent of the reduction in GHG emission required by AB 32. Also it must be remembered that this represents a one time offset, since the soil has a finite capacity to sequester C.
| Table 1. The range of winter cover crop adoption and corresponding sequestration of soil C over a 10-year period. | ||
| Adoption of winter cover cropping | Total Soil C sequestration (total tons) | MMT CO2 eq |
| 25 | 2,645,466 | 10 |
| 50 | 5,290,932 | 20 |
| 75 | 7,936,398 | 29 |
| 100 | 10,581,864 | 39 |
An increase in fertilizer N-use efficiency could be achieved by reducing N inputs by about 15 percent. Many fertilizer rate studies suggest that reducing N inputs by this amount will not impact yields in most crops. About 10 to 15 percent of fertilizer N is typically denitrified and about 10 to 15 percent of this is released as N2O. If growers reduced fertilizer N inputs by 25 kg per hectare, this amounts to about 1.0 MMT of CO2 equivalents resulting from reduced N2O emission. Though small compared to the 174 MMT CO2 required, it represents an annual offset compared to the one-time result of soil C sequestration. Using CT could potential reduce diesel fuel consumption by 20 to 30 percent, representing an additional yearly 0.5 MMT of CO2 offset that growers could achieve.
As markets for C trading evolve, growers may someday have a financial incentive to sequester soil C and reduce emission of GHG. Overall, California irrigated row crop farmers can contribute significantly to achieving the goals of AB 32 if they adopt the above approaches. In addition, these approaches promote soil productivity—a win-win for growers, the environment and food consumers.