Organic matter-mediated leaching of alkalinity in limed acid soils is affected by dissolved organic carbon adsorption and soil structure

Abstract. Subsurface soil acidity severely limits crop growth and is challenging to adjust by surface liming. There have been several proposals for subsurface liming using the combination of lime and an organic amendment, as organic anions may migrate deeper in acid subsoil than carbonates. This study aimed to identify mechanisms of subsurface liming, postulating that it is hindered by dissolved organic carbon (DOC) adsorption but enhanced in structured compared to sieved soils due to preferential flow in macropores. Column leaching experiments were set up using three sieved acid soils with contrasting properties, of which one was additionally sampled as undisturbed soil cores. The upper layer of each soil was treated with lime, compost, or a combination of both, in addition to an untreated control and columns were leached with artificial rainwater. Deeper subsurface liming in the lime+compost treatment than in the lime treatment was detected in only one of the three soils. The effect of compost on the migration of alkalinity was explained by differences in DOC sorption among soils, the lowest sorption leading to deepest subsurface liming. Imaging of in situ pH using a planar optode showed evidence of preferential alkalinity flow in the structured soil, however destructive sampling of bulk soil layers did not confirm this. We conclude that combining lime with an organic amendment can effectively ameliorate subsoil acidity but this requires weakly DOC adsorbing subsoils. The role of soil structure on this process needs to be corroborated with plant responses to identify benefits of liming the macropores.