Managing soil carbon for short and long-term benefits: Have we cracked the code?

Reference: Zhang, Z., Kaye, J. P., Bradley, B. A., Amsili, J. P., & Suseela, V. (2022). Cover crop functional types differentially alter the content and composition of soil organic carbon in particulate and mineral‐associated fractions. Global Change Biology.

Soil carbon helps us fight climate change and support crop growth

Many people are familiar with the rising level of carbon dioxide, or CO2, in the atmosphere and its role in global climate change, but fewer are familiar with the role that soil can play in re-capturing, or sequestering, that carbon. In fact, soils are the largest terrestrial reservoir for carbon, meaning they can take carbon out of the atmosphere and trap it in the ground, helping to mitigate climate change. At the same time, soil carbon is crucial for supporting plant growth, because it helps create healthier, more fertile soils. Identifying ways to build soil carbon has consequently become a major research priority for climate sustainability and food security.

While increasing the total amount of soil carbon is beneficial, it’s also important to recognize that there are different types of soil carbon that function in different ways. Specifically, researchers have identified two types of soil carbon with distinct properties: particulate organic matter (POM) and mineral-associated organic matter (MAOM). POM is a relatively short-lived form of soil carbon, often decomposed by soil microbes in a matter of a few years. Although this means that POM does not represent a long-term form of carbon sequestration, the decomposition and cycling of POM helps support plant growth. On the other hand, MAOM is a more persistent form of soil carbon that can remain in the soil for decades or centuries. Increasing MAOM can therefore help keep carbon out of the atmosphere for longer time scales. Scientists thus need to identify strategies that strike a balance between helping plants grow in the short-term and locking away carbon for the long-term.

Compared to carbon-poor soils, carbon-rich soils are darker in color, less susceptible to compaction, and are better able to supply water and nutrients to plants.

Cover crops increase soil carbon, but how and what types?

Given the importance of soil carbon for plant growth and carbon sequestration, researchers are particularly interested in increasing soil carbon in agricultural soils. Many agricultural soils have low levels of carbon due to intensive tillage practices that lead to loss of soil carbon over time. However, this also means that there is now huge potential for re-building soil carbon in agricultural fields. One way to do this is by using cover crops. Farmers can plant a cover crop during periods when they are not growing their primary cash crop, and instead of harvesting the cover crop, they incorporate the entire plant back into the soil to decompose. As the cover crop decomposes, the plant tissues, which are full of carbon acquired through photosynthesis, become part of the soil carbon reservoir.

Previous studies have shown that using cover crops increases total soil carbon, but we still don’t know which types of soil carbon (POM or MAOM) cover crops contribute to. Additionally, different types of cover crops could affect soil carbon in different ways. Farmers can choose from many different types of cover crops, including grasses (like rye), brassicas (like canola), and legumes (like clover). Grasses, brassicas, and legumes have different characteristics that could influence the way they decompose into the soil and become part of the soil carbon reservoir. To better understand how different types of cover crops influence soil carbon, a group of researchers from Clemson University and Pennsylvania State University teamed up to measure POM and MAOM across different cover crop treatments.

From left to right: grass (rye), legume (clover), and brassica (canola) cover crops. Source: University of Nebraska; Hoss Tools; Gardens Alive

Cover crop mixtures offer the best of both worlds

In a long-term experiment at Penn State’s Russell E. Larson Agricultural Research Center, researchers have been planting different cover crop treatments as part of an organic agriculture system for the past decade and studying their environmental impacts. In this study, the researchers focused on four cover crop treatments: a grass, a brassica, a legume, and a mixture of all three. In each treatment, they collected soil samples and measured the amount of POM and MAOM that had accumulated, as well as total carbon. As they had suspected, the different cover crop types produced very different soil carbon outcomes. Compared to soils without cover crops, the grass and brassica treatments had much higher quantities of POM, while the legume treatment had much more MAOM. Importantly, when all three types of cover crops were grown together in a mixture, they increased both POM and MAOM, offering the best of both worlds! These findings demonstrate that including a diversity of crop types on farms is key for increasing both short- and long-term soil carbon. This experiment also provides farmers with an example of how they can build climate-friendly, fertile soils.

An aerial view of the Russel E. Larson Agricultural Research Center in Rock Springs, PA.
Source: Penn State University

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