Impact of maintenance operations on the seasonal evolution of ditch properties and functions

Ditch networks were traditionally designed to protect fields from soil erosion or control waterlogging. They are still frequently managed by either mowing, chemical weeding, dredging or burning to ensure their optimal hydraulic capacity. Ditches were recently reported also to improve water quality and sustain biodiversity. These ditch functions are related to specific ditch properties. By contrastingly modifying ditch properties, maintenance operations were supposed to regulate these functions. There is, therefore, a need to re-examine the design and maintenance strategies of ditches to optimize the whole range of ecosystem services that they provide. In this study, we address the innovator question of how maintenance operations affect the yearly evolution of ditch properties, and in turn, the panel of functions that ditches support. During one year, we monitored the vegetation, litter, soil properties, and ash cover of five ditches that were being unmanaged, dredged, mowed, burned, and chemically weeded, respectively, with timing and frequency as generally operated by farmers in the study area. We then used indicators to evaluate the effect that the evolution of these properties has on the ditch water conveyance, herbicide retention and biodiversity conservation functions. We found that the evolution of these properties significantly contrasted among the 5 maintenance strategies. All the maintenance operations cleared the vegetation, which improves the hydraulic capacity by up to 3 times. The optimal hydraulic capacity is maintained longer after chemical weeding and dredging, but these operations have negative impacts on the herbicide retention and biodiversity conservation functions. The litter and ash layers generated by mowing and burning, respectively, improve the herbicide retention by up to 45%. Our results confirm that maintenance can be an efficient tool for optimizing ditch functions. The choice of maintenance operation and timing are key to successfully optimizing most of the functions that ditches can support.