Cover Crops and Soil Carbon Sequestration
Soil organic carbon [SOC] accumulation resulting from carbon [C] sequestration in soil is an important attribute of soil health. Use of cover crops [CC] is considered a primary means for increasing SOC by providing residues that can be used to increase soil organic matter. However, it is not known just what cover crop types (grass, legume, brassica) or species mixture will result in the greatest increase in SOC.
Results from research that was conducted to determine the effect of grass, legume, and brassica CC’s on SOC accrual and longevity are reported in an article titled “Cover crop functional types differentially alter the content and composition of soil organic carbon in particulate and mineral-associated fractions” by Zhang et al. (Glob Change Biol 2022:28:5831-5848). Definitions and pertinent points from that article follow.
Definitions.
• SOM–soil organic matter, or biomolecules that can be protected against decomposition by soil aggregates or soil mineralization.
• PFT–plant functional type [grass, legume, or brassica].
• fPOM–free particulate organic matter (not protected by mineral association or aggregate assimilation) that is a relatively short-lived form of soil C.
• oPOM–occluded POM protected by aggregation.
• MAOM–mineral-associated organic matter [organic matter from high-quality litter (low C:N) that is stabilized in the soil].
• C:N–carbon to nitrogen ratio.
Pertinent Points.
• Soils used for crop production are usually depleted of SOC, and integrating CC’s into crop production systems is a strategy that is presumed to increase SOC in these systems.
• Few studies have focused on the SOC fractions where the new C from CC residues is stored.
• The authors hypothesized that 1) CC species producing litter with low quality [high C:N ratio–e.g. grasses] would have a greater proportion of C in the POM fraction, whereas CC species producing litter of high quality [low C:N ratio–e.g. legumes] would have a greater proportion of C in the MAOM fraction, and 2) CC mixtures with all three PFT’s would have an intermediated proportion of C in the POM and MAOM fractions, respectively, compared to soils growing monoculture grass and legume CC’s.
• Soil type at the experimental site was predominantly silt loam, and the study was conducted during the 2012-2019 period.
• CC treatments were monocultures of legume [crimson clover], brassica [canola], and grass [triticale] PFT’s. The CC mixture treatment was a mixture of crimson clover, canola, and triticale.
• The legume and mixture treatments had the lowest average shoot C:N ratios [12.8 to 14.6], whereas the grass and brassica treatments had average shoot C:N ratios of 21.1 to 22.l.
• Total C inputs from the CC’s were the sum of root and shoot C. Contribution of shoot C to total C from the CC’s was about twice that of root C across all CC treatments.
• The SOC contents in the MAOM fraction were 17% and 26% higher in the legume monoculture and mixture CC treatments, respectively, compared to the brassica and grass monocultures.
• The C:N ratios in soil averaged 13.3% higher in all CC treatments than in a fallow treatment.
• Overall, total microbial C in MAOM was 21% higher in soils that had the legume monoculture and mixture CC’s compared to soils that had monoculture grass and brassica monoculture CC’s.
• Results revealed that the CC mixtures with all three PFT’s enhanced SOC in both oPOM and MAOM fractions. The oPOM was comprised mainly of plant-derived compounds, whereas the MAOM was comprised mainly of microbial-derived compounds. The microbial C can be stabilized by its association with soil minerals, thus increasing long-term C sequestration in the soil. This confirms that plant residues of different quality will differentially affect distinct SOC fractions.
• The greater SOC content resulting from use of CC’s vs. not using CC’s supports the use of CC’s in cropping systems to enhance soil C storage.
• Soils with the legume CC had a higher concentration of microbial mass C in MAOM compared to soils with the grass and brassica CC’s. This larger positive effect on soil microbes by the legume CC is primarily attributed to the higher quality of C inputs from legume shoot residues and roots. In essence, bacteria in the soil favor the decomposition of high-quality litter such as that from legume CC’s.
• From these results, it appears that CC’s with low litter quality (e.g. grasses) resulted in a higher accrual of POM, whereas CC’s that produced high-quality residues (e.g. legumes) contributed to a greater accumulation of microbial mass C in the MAOM fraction.
• Finally, these results showed that: 1) CC mixtures could balance the effects of PFT’s on POM and MAOM SOC fractions; 2) PFT’s may affect the dominant SOC pathways–e.g. POM vs. MAOM–which will subsequently impact short- and long-term C stabilization in soil; 3) total C inputs from CC’s alone do not explain the variation in SOC accumulation in soil since the pathways of SOC formation and persistence differ among CC types; and 4) these results underline the importance of knowing how CC PFT will affect long-term SOC stocks and subsequent soil health.
Takeaways.
• Roots of CC’s are a significant contributor to the biomass produced by those CC’s.
• The PFT of a CC is a controlling factor in how the C from a CC is sequestered in soil. This is likely associated with the C:N ratio of the CC.
• These results further underline the importance of identifying why a CC is being inserted into a cropping system–e.g. is it for providing weed control, providing soil cover to prevent erosion during the fallow period between summer crops, increasing water infiltration into the soil, scavenging N/reducing nutrient loss, increasing N supply for a following summer grain crop.
• If the objective of using a CC is to provide residues to increase SOM and/or SOC, then special attention should be paid to the PFT of the CC to ensure its carbon stock is of sufficient quality to promote the microbial activity that will be necessary to accomplish long-term SOC sequestration. In other words, PFT is a determining factor in the accumulation of short- vs. long-term SOC accumulation.
A Cover Crops White Paper on this website provides details about using cover crops in soybean and corn production systems.
Composed by Larry G. Heatherly, Oct. 2022, larryh91746@gmail.com