Preferential Transport in Macropores is Reduced by Soil Organic Carbon

Abstract

Core Ideas Soils of larger SOC content showed weaker preferential transport. These soils also had larger volumes of pores <600 μm in size. Results suggest a threshold SOC content above which effects were negligible. It has been suggested that some management practices and farming systems that promote C sequestration may exacerbate the risk of groundwater pollution due to fast preferential transport in soil macropores. However, soil organic C (SOC) may also impact the soil pore structure at scales smaller than the macropore scale, where complexes of SOC and clay form microaggregates that may increase pore volumes in the micrometer size range. These effects of SOC per se on pore network architecture, water flow, and solute transport have hardly been investigated. Therefore, to investigate this question, we measured tracer transport through soil cores sampled along a transect on a field under grass–clover ley with a natural gradient in SOC content. The strength of preferential transport was characterized at two flow rates (2 and 5 mm h−1) and related to the volume, size distribution, heterogeneity, and connectivity of pore networks quantified by X‐ray tomography. The results showed that soils with a larger SOC content had larger volumes of pores in the smallest imaged size range (200–600 μm) that were also more uniformly distributed. These effects of SOC on the imaged pore networks were only apparent up to a threshold value of the ratio between clay and SOC of 10:1, which is assumed to correspond with the amount of SOC needed for C saturation of the clay fraction. The increased flow capacity of these smaller macropores in soil columns with larger SOC contents prevented flow from being activated in larger pores, which significantly reduced the strength of preferential transport.