Can minor compaction increase soil carbon sequestration? A case study in a soil under a wheel-track in an orchard

In an organic apple (Malus domestica Borkh., Braeburn on MM.106 rootstock) orchard in New Zealand we observed that the soil's carbon concentration in 0–0.1m depth directly below a wheel-track in the grassed alley-way was significantly higher than under the grassed tree-row. By using a literature review and direct measurements, we systematically exclude two explanations for the possible accumulation of soil carbon under the wheel-track. Firstly, we discard the possibility of higher carbon inputs to the wheel-track compared to the tree-row area. The tree-row received more carbon inputs by the regular application of compost, and the growth of roots was not impeded by compaction. Secondly, we directly measured the monthly dynamics of microbial biomass and respiration rates per unit of soil carbon over a year, and based on 3D X-ray computed tomography (CT) images we modelled the gas exchange close to water saturation. A restricted gas exchange under wet conditions would be an indication that soil carbon mineralization is reduced under wet conditions. We found that both the microbial carbon decomposition dynamics and the gas exchange under wet conditions were similar in the tree-row and under the wheel-track. The most probable explanation for the enhanced carbon sequestration under the wheel-track is a reduction in carbon losses. The water infiltration rates into the wheel-tracks were significantly smaller than in the tree-row. This means that the carbon loss with water flow in the form of dissolved carbon is probably smaller under the wheel-track. Additionally, a reduction in meso- and macro-faunal activities exporting particulate organic matter from the soil volume is probable under minor compaction. We conclude that a minor compaction like under a wheel-track can be a physical protection mechanism for soil carbon.