Nutrient cycles are part of the main agro-ecosystem support services. During the past fifty years, men have modified the environment to satisfy their increasing needs for food, drinking water, wood, fibres and fuel. These changes have helped to improve the well-being of mankind at the cost of damaging many ecosystem services, in particular water and N and P nutrient cycles. These phenomena are the direct consequence of the significant increase in agricultural inputs: from 1965 to 2000, world cereal production doubled, the use of phosphate fertilizers increased by a factor of 3.5 and the use of nitrogen fertilizers increased by a factor of 6.9. Apart from the effects on eutrophisation, the considerable, increasing energy cost of the synthesis of nitrogen fertilizers as well as the impact in terms of greenhouse gas emission, argue against continuing this frenetic increase in the use of nitrogen fertilizers. For phosphorus, the limitation lies in the rapid exhaustion of world resources of exploitable sources of natural phosphates. Moreover, in developing countries where the increase in population requires an increase in agricultural production, the considerable increase in the cost of artificial fertilizers risks reducing the productivity of the agricultural systems even further. A major scientific challenge is, therefore, to conceive low input cultivation systems that are productive, sustainable and environmentally friendly. As for theme 1, the term used is ecological intensification of agro-systems, systems that reconcile increased productivity with the preservation of ecosystem services related to nutrient cycles.

To meet this challenge of ecological intensification of agro-systems, our research is designed to determine, quantify and hierarchise the biotic and abiotic processes and factors that regulate the fluxes of nutrients in particular in low input agro-ecosystems, from the functional biological regulation domain (rhizosphere, detritusphere and biogenic structure) in collaboration with theme 1 to the agro-system domain in collaboration with theme 3.

We test new agronomic practices to manage nutrient fluxes in agro-ecosystems with a view to ecological intensification.

• the management of organic and artificial inputs,
• the choice of plant species,
• the associated rhizosphere micro-organisms (introduction of strong genotypes),
• the use of complex populations (associated cultures, mixture of genotypes or mixed plantations).

Research issues

The general issue of this theme is what are the biogeochemical processes that determine the efficiency of acquisition, utilisation and recycling of nutrients in tropical and Mediterranean agro-ecosystems with low inputs?

The specific issues concern the stocks of nutrients in the soil, their bioavailability for plants, their take-up by plants and their recycling in the agro-ecosystem.

The aim is to build up a spatial model of nutrient dynamics in the soil-plant system and to apply this model as required. The model is based on the hierarchy of biological processes (activities and biological transformations), geochemical processes (supply of nutrients by dissolution / desorption) and physical processes (transfer of nutrients in the soil) for the nutrient dynamics for the roots, the whole plants and the whole plantation.

• 1. The horizons of the subsoil and deep roots play a decisive role in the absorption of water and nutrients, especially in dry periods. Their role in the acquisition of nutrients by the whole plant and in the allocation of C to the deep root system is real. This hypothesis is tested using quantitative data on the relative contribution of these roots to the nutrition of the whole plant, in relation to the roots that colonise the top horizon where the inputs of fertilizers and litter are concentrated.
• 2. Polyculture, with companion planting of legumes and plants that do not fix nitrogen, appears to provide ideal agro-ecosystems in the context of ecological intensification and adaptation to global change. The increase in productivity of polyculture over monoculture is explained in part
  • by a niche complementarity resulting from differential exploitation of the soil horizons by the companion species
  • by exploitation of different nutrient pools (N2 vs inorganic N, organic P vs inorganic P) reducing competition for a given resource which occurs where all the individuals have the same capacities;
  • by facilitation, or supplying the associated species with a nutrient acquired by the other species.

Given the potential involvement of underground biological systems in such interactions, close contact will be maintained with Theme 1 on this point. The ability of polyculture to increase the efficiency of acquisition and utilisation of natural resources in comparison with monoculture is studied in agro-ecosystems, in close collaboration with Theme 3.

The experimental approach adopted consists in combining experimentation with numerical modelling. In this case, experimentation covers (i) field experiments on a limited number of sites with a large number of instruments for exhaustive measurement of all the fluxes within the agro-ecosystem, (ii) co-ordinated agronomic experiments in agro-ecosystems commonly found in Mediterranean and tropical areas and (iii) laboratory experiments on mesocosms to constrain as many variables as possible and restrict the number of processes.

A second, less costly, observational approach is also used on networks of parcels of land to test the applicability of the results obtained using the above approaches, or to reveal nutritional factors limiting production in low technology farming. In this case, the fluxes measured will be very limited, or even replaced by indicators (e.g. bioavailability estimators, nutritional status of the plant).

Experiments and measurements are undertaken mainly by our two laboratories (Physical-Chemical and Life Science Experimentation) in Montpellier and involve close contact with US 49 (Soil Analyses) of CIRAD.

The modelling will re-use software already developed (e.g. MIN3P for reactive transport in soil and Amap-Sim for root architecture) combined either in the Sol Virtuel project, depending on the degree of progress, or as a stand-alone system.

 

Research will be carried out mainly into annual seed crops (cereals / legumes), tree plantations and perennial crop plantations, usually in agroforestry systems, in each case comparing monoculture and polyculture.

• Perennial tropical ligneous plantations, comparing eucalyptus-acacia companion plantations with corresponding single species plantations. These plantations are in two sites (Brazil and the Congo) with a large number of instruments for measuring all nutrient fluxes within the agro-ecosystem.
• Agroforestry systems associating perennial ligneous legumes with conventional and organic coffee growing (Costa Rica) and annual crops (West Africa and Madagascar);
• Annual cultivation with a low level of inputs and/or organic agriculture in the Mediterranean area, comparing companion crops of durum wheat and chick peas with monocultures (France, Mauguio) and in participative research with two networks of farmers.

Fields of application and targets

• Development of nutrient bioavailability indicators (RHIZOtest to estimate bioavailability for plants of trace elements such as copper: Near Infrared spectroscopy,
• Collaboration with industry or small organisations to explore the possibilities of recycling local mineral or organic resources (natural micronised phosphates, organic waste, fragmented wood, etc).