No-till production is widely considered an appropriate crop production system for sequestering carbon — the process of storing carbon in plants and the soil so that the buildup of carbon dioxide in the atmosphere is reduced or slowed. But the extent of its benefits depends on soil type and soil depth, Ohio State University soil scientists have discovered.
Humberto Blanco, a research scientist with the School of Environment and Natural Resources, said that it is well documented that no-till systems effectively store carbon within the plow layer — up to 10 inches of the soil surface. Little is known, however, about carbon storage in deeper soil profiles.
"Most of the previous studies about soil organic carbon accrual in no-till soils have primarily focused on the surface layer, and not for the whole soil profile," said Blanco. "The lack of adequate data on the soil organic carbon profile is a hindrance to conclusively ascertain the effects of no-till farming on carbon sequestration and off-setting carbon dioxide emissions."
Blanco and Rattan Lal, a soil scientist with the Ohio Agricultural Research and Development Center, set out to study carbon sequestration in deeper soil profiles, up to 25 inches deep, and compare the amount of stored carbon between no-till and tilled fields. The data collected on 11 soils in Kentucky, Ohio and Pennsylvania revealed that concentrations of carbon in the sub-soil layers of no-till fields were low, and in some cases stored carbon in tilled fields was higher than in no-till fields.
"The data shows that no-till farming increases soil organic carbon in the upper layers of some soils, but does not store carbon more than tilled soils for the entire soil profile," said Blanco. "This suggests that no-till farming impacts on carbon sequestration depend on soil type, sampling depth, drainage and numerous other factors."
According to the results of the study, the soil organic carbon pools in no-till exceeded those of tilled fields in five out of 11 soils, but only within the surface layer. Below 4 inches in depth, no-till soils had equal to or even lower soil organic carbon than tilled soils. Total soil organic carbon up to 25 inches deep in no-till was similar to those of tilled soils, and in some cases, the total carbon pool in tilled land was about 30 percent higher than in no-till soils.
Results of the study will be published in the May-June issue of Soil Science Society of America Journal.
Researchers hypothesize that the reason why more carbon was found in deeper soil profiles in tilled fields compared to no-till fields was because the plowing process mixes residue in lower depths, creating a greater pool of carbon storage. In some cases, the plowing process may loosen compacted soil enough to allow plant roots to extend deeper into the soil.
"With no-till the residue stays on the soil surface, but is absent in deeper soil profiles. Also with no-till, plant roots tend to be confined to the surface," said Blanco. "In tillage systems, roots have the ability to grow to lower depths. For example, corn roots can extend as far down as 15 inches. The differences are probably also related to differences in soil profile characteristics." The research indicated that silt-loam and loam soil types captured the highest amounts of carbon.
Blanco also indicated that tillage systems that store carbon in deeper soil profiles might provide more long-term benefits to sequestering carbon.
"Carbon stored near the surface is not stable. Most carbon dioxide emissions occur from the soil surface," said Blanco. "Carbon stored deeper in the soil profile is locked in at those depths and may contribute to long-term carbon sequestration. That is, carbon at those depths could remain there for years, compared to carbon found on the soil surface. Benefits of no-till technology to sequester carbon must neither be relegated to just the soil surface, nor be generalized to all soils."
Blanco added that it is important to recognize that no-till production systems have a myriad of other ancillary benefits that tillage does not provide, including reduction in labor, reduction in machinery wear, decreased fossil fuel consumption, reduced soil erosion, improved soil productivity, increased wildlife habitats and a better method of maintaining and conserving soil water.
The research is funded by the U.S. Department of Energy-National Energy Technology Laboratory as a part of the Midwestern Regional Carbon Sequestration Partnership. The purpose of the long-term research is to measure the level of carbon sequestration in various soil types under different crop production systems, as each soil type and each system have different soil carbon sink capacities.