- Author: Jordon Wade
“Soil health”, as both a term and a concept, has been gaining traction in the last few years. The National Resource Conservation Service (NRCS) has defined soil health as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. Generally, soil health is considered to be the intersection of soil physical, chemical, and biological factors. Each soil will have differing levels of health within these three areas. Some of these constraints are inherent to a soil due to differences in the geologic material that it was formed from, but each individual soil can be optimized for agronomic performance by using management strategies that will improve the health of an individual soil.
In recent years, one area of soil health that has received a lot of attention is the biological component. Researchers are diving into the complexity of soil microbiology and looking for ways to translate these complex, micro-scale interactions into reliable, actionable recommendations for growers to use. These recommendations are usually going to be adjusted using results from commercial soil test labs. One biologically-based test that has been increasing in popularity in recent years is respiration, or the production of CO2 by soil microbes, after an air-dried soil is rewetted. Measuring CO2 production rates is a quick, inexpensive method to estimate soil microbial community size and activity level, as well as the availability of carbon to those soil microbes. For these reasons, respiration can serve as one useful tool in the toolbox for assessing soil health. However, translating these respiration measurements into a recommendation for growers has proved difficult.
One proposed use of respiration is to predict the inherent ability of a soil to supply nitrogen to a crop. The breakdown of soil organic matter into plant available forms of nitrogen, a process known as mineralization, is largely controlled by soil microbial activity, so a measure of microbial activity (e.g. respiration) has the potential to predict this process. If nitrogen mineralization can be predicted prior to planting, nitrogen fertilizer recommendations can be reduced, resulting in decreased fertilizer costs for growers and decreasing the potential for adverse environmental effects.
Funded by CDFA's Fertilizer Research and Education Program (FREP), UC Davis researchers Jordon Wade, Martin Burger, and Will Horwath sought to test the ability of respiration to predict nitrogen mineralization in California's diverse cropping systems. The study used soils from four distinct regions—Yolo County, San Joaquin County, Fresno/Kern Counties, and Monterey County—and two differing management strategies—fields receiving a winter cover crop and those not receiving a cover crop. A suite of common commercial test lab indicators were used to predict N mineralization.
The ability of our soil tests to predict a soil's N mineralization potential was very low. Respiration was able to predict nitrogen availability in cover cropped fields better than in non-cover cropped fields, although neither was a particularly strong relationship. Combining biological and chemical tests did improve the predictive ability, but the relationships were still weak. Additionally, they were inconsistent in their accuracy across growing regions. There is a potential to use N fertilizer rate trials to calibrate these tests for an individual field or set of fields, but large-scale recommendations at this time would likely be inaccurate.
While respiration may not be able to predict a specific outcome (such as N availability), it has been shown to be correlated with increased yields in corn (both grain and silage) and processing tomatoes in California agricultural systems. Together, these findings suggest that measuring respiration does have limited agronomic utility, but that its ability to predict specific outcomes is uncertain.