When it comes to carbon storage, a new peer reviewed publication shows the benefits of digging a little deeper. Researchers from University of California, Davis, found that compost is a key to storing carbon in semi-arid cropland soils. The key to this study is the significant carbon storage benefits found below 30cm.
A lesser publicised aspect of this study is the organic dimension: not only is the beneficial practice – using compost – from the organic handbook; the best performing agronomic system studied was the certified organic one. For their 19-year study, published in Global Change Biology, scientists dug two metres down, at five depth increments, to compare soil carbon changes in different farming systems.
The systems were maize-tomato and wheat-fallow systems, and three input methodologies were assessed: conventional, cover-cropped and compost-added. The maize-tomato system, using both cover crops and composted poultry manure, was certified organic and showed by far the best results. When both compost and cover crops were added in the organic-certified system, soil carbon content increased 12.6 percent over the length of the study, and “across the 2-m profile”. This is an increase of about 0.7 percent annually.
As UC Davis press pointed out, “that’s more than the international “4 per 1000” initiative, which calls for an increase of 0.4 percent of soil carbon per year. It is also far more carbon stored than would be calculated if only the surface layer was measured.” Carbon storage is seen by many as a key strategy for offsetting GHG Emissions.
The researchers were interested in three questions:
(1) How do long-term inputs of different sources of carbon and management affect soil C sequestration in row crops?
(2) Do patterns of C sequestration across different depths vary between crops and management practices, and
(3) Can patterns of C sequestration observed in the top 30 cm of soil predict C sequestration throughout the deeper soil profile.
Of the two systems, the maize-tomato system is the system with the certified organic component. The maize-based systems compared conventional and organic approaches to crop and soil management, and consisted of
1) conventional maize–tomato with synthetic fertilizer, pesticides, and winter fallow crops;
2) certified organic maize–tomato with composted poultry manure and winter cover crops and
3) a hybrid system with synthetic fertilizer, pesticides, and winter cover crops. (The organic system is called ORG, while soil organic carbon is called SOC.)
To quote from the paper itself:
• “The greatest increases in SOC were observed in the ORG, especially in surface layers…
• No changes in SOC concentrations or stocks were observed in the 30 to 60 cm layer except in the ORG…
• In the ORG…average SOC concentrations and stocks increased across the entire soil profile….
• Change in SOC was positively correlated with cumulative poultry manure compost C inputs for the ORG at 15 to 30 cm…30 to 60 cm…and 100 to 200 cm”.
In cover-crop systems, when SOC appeared to increase, this only occurred when the top 30cm was analysed. Taking into account the deeper layers, only the organic system increased SOC. “The researchers did not compare composted systems without cover crops, but suspect the compost helped sequester carbon despite the cover crop, a notion they intend to investigate further” according to their University’s press centre. Unlike cover crops, compost adds nutrients as well as carbon for microbes, the researchers suspect. Microbes process carbon to stabilize it.
Irish Dutch research, supervised by Gary Lanigan and Rachel Creamer and published in 2017, also found the benefits of digging deeper: soils with high clay content in deeper layers lock away carbon for much longer, and at much greater depth than previously known, it was found. Whereas carbon in topsoil is released quickly, carbon at greater depths is bound to increasingly smaller soil particles, typically for decades. The paper can be found here, while the UC Davis press release can be found here.
A version of this article also appeared in the Irish Examiner newspaper’s farming supplement