Focusing on functional traits of crop residues and their connection to soil carbon storage, Pablo Garcia Palacios comments on recent article, Crop traits drive soil carbon sequestration under organic farming.

Soil organic carbon is a major agricultural resource for two different reasons. First, it is a measurable component of soil organic matter, which has a key influence on the capacity of soils to retain moisture and nutrients, and ultimately on plant growth. Although the relationship between soil organic carbon and crop yield may be not as straightforward as we think, especially if we consider different fractions of the soil organic matter. On the other hand, soil organic carbon represents the largest terrestrial carbon pool, and CO2 released from soils is far larger than anthropogenic emissions. If we consider that croplands comprise a major proportion of the earth’s land surface, the typical decline in soil organic carbon found after agricultural conversion of natural areas may exacerbate climate change. Thus, increasing the sequestration of soil organic carbon in agricultural lands plays an important role in mitigating climate change.

Improved farming practices (crop diversification, reduced tillage, manure addition, cover crops) have been proposed to reduce soil carbon losses or even increase soil carbon storage over time compared with conventional farming. Organic farming is one of the largest existing models of ecological intensification worldwide in terms of surface area, and increases soil organic carbon compared to conventional farming. Although the higher carbon inputs supplied in organic farming via manure certainly contribute to such results, other driving factors may also play a role, as net soil organic carbon also depends on carbon outputs from the soil (via organic matter decomposition).

In our study, we focused on the functional traits (carbon, nitrogen, lignin concentration) of crop residues (leaves and roots), which constitute a major control of soil organic matter decomposition. Our main goal was to assess if crop residue traits consistently determine soil organic carbon sequestration rates under organic farming. To do that, we collected soil carbon and crop residue traits data from studies comparing organic (absence of synthetic fertilizers and pesticides), vs. conventional (presence of synthetic fertilizers and pesticides) farming worldwide, and complemented such literature synthesis with European-wide field measurements across six agricultural sites (Eco-Serve network).

Our research confirmed that organic farming increases soil organic carbon stocks and sequestration rates, even in the organic farms with low manure application rates. Thus, the positive effects of organic farming on soil carbon are not only the product of higher carbon inputs via manure addition. Our analysis revealed that the crop leaf and root nitrogen concentration also played a significant role. Interestingly, our field measurements across the European network of sites suggested a driving mechanism: increases in crop residues nitrogen concentration under conventional farming, and thus in soil organic matter decomposition, promoted the lower soil carbon content compared with organic farming.

The importance of crop traits for agricultural research has been highlighted before in this blog from the functional and nutritional points of view. Our study indicates that the traits of crop residues, especially those related to resource-use strategies (leaf and root nitrogen concentration), are of major importance to predict the effects of organic farming on soil carbon sequestration. In addition to management practices and farm climatic conditions, crop residue traits should be considered when evaluating the potential of organic farming for climate change mitigation.

Read the full article, Crop traits drive soil carbon sequestration under organic farming in Journal of Applied Ecology.