Cover crops are widely accepted as a means of increasing soil organic carbon – and research shows that planting crops featuring different plant types creates greater chemical stability in carbon stored in soils than by growing monocultures.
In the study, sampling soils from a long-term experiment in Pennsylvania, USA, researchers measured two types of organic matter – particulate organic matter (POM) and mineral-associated organic matter (MOAM) – from fields where a legume, a grass and a brassica were grown, as well as one with a mix of the three.
The study proved to be the first ever showing that a mix of cover crop types enhanced organic carbon in both the soil fractions analysed, with the mix offering the greatest balance of short- and long-term persistence of the carbon, the researchers said.
Balancing carbon sources
POM tends to be dominated by compounds from plants, with MOAM by contrast mainly consisting of those from microbial processes.
“Our study shows that the inclusion of cover crops with distinct functional traits has differential influences on the abundance of plant-derived carbon and microbial necromass carbon that are distributed in POM and MAOM fractions,” they explained.
“We found that cover crops characterised by low litter quality (e.g., grass) resulted in a higher accrual of POM which was abundant in plant-derived carbon, while cover crops with high litter quality (e.g., legume) contributed to a greater accumulation of microbial necromass carbon in MAOM.”
Plant species choice key
The study underlined that repeated use of cover crops in rotation by farmers would be an effective means of drawing carbon from the atmosphere, they said, while the use of such crops has also been widely associated with improving soil biodiversity and improving its structure.
While there has been a move to favour cover crops more likely to form MOAM than POM, in order to achieve longer-term carbon capture in soils, short-lived soil carbon should not be neglected, they argued.
“Our study highlights that cover crop mixtures might be a feasible strategy to target both POM and MAOM…Nevertheless, the complementary effect of cover crop mixtures depends largely on the right selection and combination of species,” they concluded.
You can read the full journal article in Global Change Biology.
