Nutrient Cycling in Agroecosystems - a Rhizosphere Biogeochemistry Perspective.

Producing more food with less impact on ecosystem services such as the biogeochemical cycles of nutrients is a major challenge ahead of us. Some solutions to design a sustainable intensification of agroecosystems pertain in the understanding of rhizosphere ecology and biogeochemistry, as the root-soil interface is a major hotspot of biological activities in soils. The aim of this paper is to point some of the key root-mediated biogeochemical processes that could be better promoted to achieve an ecological intensification of agroecosystems. Roots can considerably alter nutrient concentrations and soil pH over short spatial and temporal scales as a result of their uptake and exudation activities. We will briefly review how these root-induced changes of rhizosphere biogeochemical properties impact : the dissolution/precipitation of soil minerals, the adsorption/desorption of ions, and hence the bioavailability of key major nutrients, either anions (e.g. phosphate) or cations (e.g. potassium). While much of the past research has been considering young plants under controlled conditions, there is a need to assess these processes under field conditions for mature plants as well, and to make better use of modelling to fully understand the complex interactions at play. We will also stress the need to get beyond the topsoil, and account for rhizosphere processes occurring at depth in the soil, and show some exciting results obtained at several meter depth. The development of novel tools and approaches will thus be instrumental, as shown through a few examples in this talk. Further understanding the rather unique rhizosphere biogeochemistry is thus pivotal to quantifying the driving role of higher plants in ecosystem services, especially support services such as soil formation and nutrient cycling. It provides exciting perspectives for breeding crops and trees that most efficiently acquire and cycle nutrients for improving the nutrient use efficiency of the agroecosystems of tomorrow.