Soil texture controls added organic matter mineralization by regulating soil moisture-evidence from a field experiment in a maritime climate

Soil organic carbon (SOC) levels generally increase with increasing clay and silt content under similar climatic conditions because of increased physicochemical protection. Surprisingly though, many silt loam soils in Western Europe have low top SOC levels compared to coarser textured soils. Soil texture also strongly controls soil moisture with consequent indirect impact on heterotrophic activity. Especially during periods of prolonged drought, which seem to be occurring more frequently throughout Europe, it could be expected that this soil textural control of moisture retention decisively affects added organic matter (OM) decomposition. We hypothesized that, contrary to expected textural effects on physicochemical protection of OM, the higher moisture availability in silty soils strongly favors added OM mineralization during periodic drought than sandy soils. Moreover, as a secondary objective, we investigated if during prolonged drought, capillary moisture supply could significantly alleviate the water stress on added OM decomposition in sand with rather shallow groundwater table. Therefore, we established a one-year field trial in which the upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low SOC. In a fourth treatment, a gravel layer was added beneath the sand layer to exclude capillary rise. Maize residues were mixed homogenously with soil in top 25 cm in all plots. Changes in soil moisture and maize-C mineralization (C-maize-min) were measured. We found that soil texture did not affect C-maize-min until after about five months and thereafter C-maize-min rates were higher in the silt loam than in the sandy soils. Moisture content correlated positively with the C-maize-min rate for the sand-textured soils only. Capillary rise did not result in a significant increase in C-maize-min in the sandy soil. After one year, a larger share of added C-maize was mineralized in the silt loam soil (81 +/- 6%) than in the sandy soil (56 +/- 7%). These results clearly highlight that soil texture controlled C-maize-min indirectly through regulating moisture under the field conditions when the study area faced a period of unusual drought. Moreover, our results imply that, under future climate scenarios, more frequent droughts might lead to a lesser SOC depletion in sandy soils compared to in finer textured soils under similar management.