S. De Baets

Estimation of Soil Carbon Input in France: An Inverse Modelling Approach

Development of a quantitative understanding of soil organic carbon (SOC) dynamics is vital for management of soil to sequester carbon (C) and maintain fertility, thereby contributing to food security and climate change mitigation. There are well-established process-based models that can be used to simulate SOC stock evolution; however, there are few plant residue C input values and those that exist represent a limited range of environments. This limitation in a fundamental model component (i.e., C input) constrains the reliability of current SOC stock simulations. This study aimed to estimate crop-specific and environment-specific plant-derived soil C input values for agricultural sites in France based on data from 700 sites selected from a recently established French soil monitoring network (the RMQS database). Measured SOC stock values from this large scale soil database were used to constrain an inverse RothC modelling approach to derive estimated C input values consistent with the stocks. This approach allowed us to estimate significant crop-specific C input values (P -1 year -1 (silage corn) to 5.15 ± 0.12 t C ha -1 year -1 (grassland/pasture). Furthermore, the incorporation of climate variables improved the predictions. C input of 4 crop types could be predicted as a function of temperature and 8 as a function of precipitation. This study offered an approach to meet the urgent need for crop-specific and environment-specific C input values in order to improve the reliability of SOC stock prediction.

Root Strategies for Rill and Gully Erosion Control

Many studies attribute the effects of vegetation in reducing water erosion rates to the effects of the above-ground biomass. The effects of the below-ground biomass on flow erosivity and topsoil resistance to concentrated flow erosion are much less studied. However, roots play an important role in controlling soil erosion rates, especially when the above-ground biomass disappears (e.g. due to fire, drought, harvest, grazing) and particularly when incisive processes are concerned. Roots affect properties of the soil, such as soil roughness, infiltration rate, aggregate stability, moisture content, soil cohesion and organic matter content, all of which control soil erodibility to various degrees. It is generally recognised that plant roots contribute to the overall cohesion of the soil. Moreover, roots were also assumed to have a flow-retarding effect. The main objective of this chapter is therefore to discuss the mechanical and hydraulic effects of plant roots during concentrated flow erosion which allows us to better understand and predict soil erosion rates during concentrated flow erosion. Several empirically based relationships that can be used to predict the erosion-reducing effects of plant root during concentrated flow erosion are presented. The relative contribution of roots versus shoots in preventing soil detachment will also be addressed. Finally, a methodology to evaluate plant traits (both shoots and roots) for gully erosion control is proposed.

Effectiveness of Plants and Vegetation in Erosion Control and Restoration

In this chapter the approaches and methods used to measure plant effectiveness in reducing runoff and erosion are explained and results presented for each of the major land units, hillslopes and channels. Evaluations of the properties of plants required are made to inform plant selection for different sites. For use of cover crops in orchards it is important to assess whether the cover crops would have an effect on orchard tree productivity, whilst also reducing soil erosion. A climatic threshold for their use was identified. Soil moisture measurements from different treatment areas and water balance and runoff modelling exercises showed where use of such crops could be beneficial. Extent of vegetation growth on abandoned lands was shown to have a marked effect on runoff, water repellency and soil crusts. Various root parameters were measured on a range of plants and their relation to soil detachment calculated. Differences in root architecture and in orientation of rows of plants were tested. Plant stem density, stem bending and trapping efficiency effects were also assessed experimentally and plant species growing in the Mediterranean study area were grouped according to their erosion control potential. The effects of vegetation and various plant species on roughness, flow hydraulics and sediment trapping in channels were assessed by field measurements and modelling and their resilience to high flow evaluated from observed flood impacts.