2018-2019

SEAWEED: Soil erosion in agricultural watersheds: how can gullies be potential corridors of biodiversity?

In this project, we wanted to study gully erosion in agricultural watershed and the link to biodiversity. We specifically asked the question: What are the processes and factors that influence seed distribution, soil properties and soil and nutrient losses along a catena of steep slopes agricultural watershed? We covered five main topics as follow: 1) Soil and nutrient losses by runoff along the catena 2) Soil seed bank and seed rain 3) Soil water infiltration and soil macrofauna 4) Seed movement and soil loss by rainfall simulation 5) Seed movement under natural rainfall.

Our main results are exposed below:

• The highest runoff and associated sediment loss were found at the top position of the catena (point 1 on the figure). The soil loss by runoff was significantly reduced by the development of maize growth during the season. Total soil organic carbon, nitrogen and phosphorus losses by sediments were significantly higher at top and upper slope position than the middle and down slope position. We found similar results for nutrients in runoff water. While topography showed an effect on runoff, nutrients, and soil losses, it had no effect on maize yield.
• As expected, the soil seed bank was higher in the downstream part of the catena, while we observed a higher seed rain in the upper and middle part (point 2 on the figure above).
• Few months after the beginning of the experiment, position in the catena and biochar application had no significant effect on hydraulic conductivity Ks nor on sorptivity S. Similar results were found for macrofauna indicating us that more investigations are needed on the longer term for soil water infiltration and soil macrofauna (point 3 on the figure).
• The study of seed movement under rainfall simulation showed that 66 % of beads (imitating seeds) were buried under sediments. Only 7 % of beads have been exported downslope. Biochar had an effect by reducing the distance travelled by the beads. The bead distance was significantly positively correlated to rainfall intensity, soil crust % and soil losses (point 4 on the figure).
• This correlation was confirmed under natural rainfall, between soil losses and seed removal. Moreover, Soil losses were greater at submit hillslope positions. Dominant species are possibly Ageratum conyzoides, Crassocephalum crepidioides, and Hyptis capitata.

In conclusion, clear differences along the catena were observed on soil properties, soil and nutrient losses and seed bank. In order to restore a higher biodiversity and limit soil erosion, landscape management taking into account catena properties would be helpful on steep slopes.