On the heels of uncovering a more refined method for calculating soil carbon budgets, Hero Gollany, soil scientist for the US Department of Agriculture’s research arm, caught up with Milling & Grains on the variables that impact carbon sequestration, why fallow periods aren’t always as effective as historically thought and how wheat farmers can mitigate—and even reverse—soil carbon loss.
Milling & Grains: Do certain crops accrue more soil carbon than others?
Hero Gollany: Yes, crops which produce a greater amount of biomass, such as corn, wheat or sorghum, can contribute more carbon to the soil and can boost carbon accretion more than crops such as soybean or pea.
M&G: Why is it so challenging to accurately measure soil carbon sequestration?
Gollany: It is challenging to accurately measure soil carbon sequestration because: 1) soil organic carbon sequestration is a slow process which can take several years to decades; 2) changes in soil organic carbon are currently measured with instrumentation that is not sensitive enough to detect very small changes in soil organic carbon; and 3) spatial and temporal changes in total soil organic carbon which can contribute to errors in estimation of soil carbon sequestration.
M&G: Talk about the range of variables that can affect soil carbon sequestration and how you discovered the role of accrued carbon through the CQESTR program.
Gollany: Precipitation and temperature are major factors that affect the amount of biomass or plant materials produced, which in turn affects soil organic carbon returned to the soil. Temperature and moisture further influence rate of decomposition of plant material and conversion to soil organic carbon. Tillage is another factor; inversion tillage buries organic matter hastening decomposition and reducing carbon sequestration, while ‘no-till’ seems to preserve organic carbon allowing more eventual sequestration. Clay content and clay minerals help carbon sequestration. Soil pH (acidity) also is another variable that can affect carbon sequestration.
While running simulations for three cropping systems in Washington using the process-based carbon balanced mode CQESTR, we were predicting less carbon than the measured values—and carbon values measured later in the spring were greater still. I suspected inclusion of fine crop residue or straw materials that passed through the sieves during sample processing. Organic matter or ‘light-fraction carbon’ in the samples increases measured total soil carbon values. We reevaluated the field samples by looking for the light-fraction carbon—carbon that had accrued in the soil but was not yet sequestered via decomposition. This transitory carbon is still part of the total soil carbon pool and can provide food to soil microbial population and improve soil properties for a short period of time, such as a single growing season.
M&G: What are the implications for cereal growers in particular?
Gollany: In our region, wheat fields are typically losing carbon over the long term. We are examining greater cropping intensity—producing more biomass (and of course more grain) —by reducing or eliminating fallow. Fallow periods have been thought important in conserving soil water, but the bare soil makes this less effective. One advantage of ‘no-till’ is covering the soil, which cools it and reduces evaporative loss and soil erosion.
Also during the fallow period microbial activities increase, which increases decomposition of organic matter and carbon dioxide emissions. So combining ‘no-till’ with planting more frequently seems to be a good strategy. Some fields need even more carbon input. Addition of manure (or other organic waste, e.g. municipal solid waste) is still the best way of reversing loss of soil carbon. Soil organic carbon is a reservoir for carbon and plant nutrients, and increases the potential of soil water holding capacity, and available water for crops and crop yield.
M&G: When will we see this research put into practice?
Gollany: Some producers around the Pacific Northwest are already using direct-seeding or no-till and we are encouraging others to adopt no-till practices to conserve organic matter and store carbon in the soil.