Does an increase in soil organic carbon improve the filtering capacity of aggregated soils for organic pesticides? — A case study

Organic carbon is the backbone of the soils ecosystem service to protect ground and surface water bodies against the contamination with pesticides. Our objective was to study the long-term effects of organic carbon (OC) inputs on the soilʹs generic filtering capacity for organic pesticides. We defined the generic filtering capacity of an aggregated soil for organic pesticides as the capacity of the soil aggregates to take up pesticides from the soil solution, to adsorb them to the soil matrix, and to degrade them. For this purpose, we identified three pesticide filtering indicators. (1) The lack of soil hydrophobicity indicated the intactness of soils to absorb soil solution; (2) the SOC contents indicated the soils capacity to adsorb and (3) the microbial activity the capacity to degrade pesticides. We analyzed how these pesticide filtering indicators changed as a result of long-term OC additions for soils under different land use in a case study. The first land use pair consisted of soils under apple orchards. In one orchard (= ‘organic’) the topsoils in the tree row regularly received compost and were permanently grassed. The tree row of the other neighboring orchard (= ‘integrated’) was vegetation-free. After 12 years the topsoils of the ‘organic’ orchard had 32% more SOC than those of the ‘integrated’ orchard. The second land use was a permanent pasture grazed by sheep. After 20 years the camp sites (‘manured sites’) had 28% more SOC than the non-camp sites of the same paddock. The OC addition, for both land uses, significantly increased the values of the pesticide filtering indicators for sorption and degradation. In the orchards, the OC addition did not affect the intactness of the soilʹs capacity to take up soil solution into aggregates. However, in the pastoral system the OC addition increased the degree of soil hydrophobicity further impairing the soilʹs capacity to absorb soil solution. We conclude that an increase in soil organic carbon improves the soilʹs generic capacity to filter organic pesticides only as long as no soil hydrophobicity occurs.