Welcome to the Willamette Valley Regenerative Landscape Coalition Soil Carbon Blog series!
The opinions, beliefs and viewpoints expressed by the author of this five-part blog series do not necessarily reflect the opinions, beliefs and viewpoints of Benton Soil & Water Conservation District representatives.
Part 3: Making organic matter stay in the soil
In Part 1 we talked about the fact that all organic matter (OM) decomposes. If we want to improve soil and slow climate change, we need to make more OM stay in the soil. Decomposer organisms in the soil break down OM over time, eventually releasing the carbon into the atmosphere. This decomposition is necessary, as it makes nutrients available to plants and other soil life. This cycling of carbon is important. We need to think of soil as a dynamic balance between storing and releasing carbon. Unfortunately, the way soil is commonly treated, think of tilled, bare, eroded soil, has tipped the balance to the point that OM levels are decreased.
The balance can be tipped back toward more OM storage. This can be done by improving soil biology. One sign of improved biology is the presence of aggregates. These little soil structures are built as a result of root exudates that feed microbes and fungi. Microbial glues and fungal nets hold together clumps of soil. Organic matter is more protected from decomposers inside the aggregates.
Healthy, functional soil has stable aggregates and good structure with pore space for holding water and nutrients. You can easily check aggregate stability. Fill a clear container with water and place a basket (hardware cloth or sink drain) at top, as shown in the photo. Drop an air-dried clump of soil into the basket. Stable aggregates hold together. Unstable aggregates are pushed apart from the force of the water entering soil pores. When soil falls apart in water, think erosion, water contamination, loss of nutrients, loss of top soil, loss of function, loss of money.
Since aggregates are formed by soil life, it suggests that biology is crucial to increasing OM. In fact, fairly recent research has revealed an important reason why soil life is so important:
Most long-term OM in the soil is actually made up of dead microbes.
Soil disturbances, such as tilling, break apart fungal structures, damage soil life, and encourage decomposer microbes that release CO2. Many chemical fertilizers and herbicides also damage soil life to where the system is unbalanced and can’t take in as much OM. Now, we should point out, people have developed systems where they use disturbances judiciously and are still able to increase OM levels. Occasional tilling or chemical use does not mean soil is instantly destroyed.
To build soil and get carbon out of the air, we need to focus on adding OM by increasing photosynthesis and root exudates. By doing so, we can make the soil biology even more vibrant, which will create a positive spiral of increased OM and increased plant health.
Read the other posts in this series
Part 1 Introduction to soil carbon sequestration
Part 2 Root exudates
Part 4 Increasing organic matter levels more quickly
Part 5 Limits to soil carbon sequestration