Dr Fabien Ferchaud3, Dr Alain Mollier2, Dr Isabelle Bertrand1
1INRA UMR Eco&Sols, Montpellier, France, 2INRA UMR ISPA, Bordeaux, France, 3INRA UR AGROIMPACT, Laon, France
Carbon (C), nitrogen (N) and phosphorus (P) cycles are intimately linked in ecosystems through key processes such as primary production and litter decomposition. Ecological stoichiometry has become a common approach for exploring relationships between biogeochemical cycles and ecosystems functions in ecological science. In agronomy, the concept of stoichiometry is far less utilized, probably because the addition of fertilizer reduced biotic interactions between the C, N and P cycles. Surprisingly, little is known about the long-term impact of agricultural practices on soil stoichiometry. Within the context of agro-ecology, however, alternative agricultural practices aim to increase nutrient recycling from plant residues, soil organic matter and inorganic reserves (e.g. legacy P), while reducing tillage or mineral fertilizer input. The success of such practices relies on the increase of soil biotic interactions and may impact C storage in soils on the long term, if soil organic matter stoichiometry is constrained.
We aimed at determining the long-term impacts of alternative agricultural practices on soil stoichiometry. To do so, we compiled and completed a dataset of long-term (8-49 yr) field experiments in France in which P or N fertilization rates or tillage intensity was strongly reduced.
The agricultural soils studied presented C:N and C:P ratios ranging from 8 to 14.5 and from 15 to 28 respectively, and N:P ratios ranging from 1.5 to 2.8 (total P). The site effect was significant on the soil CNP contents and ratios (one-way ANOVA, P < 0.05). Interestingly, whereas the soil C:N ratios were constrained and not influenced by the different agricultural practices, the C:P and N:P were more flexible. The C, N and P balance were calculated at each site and related to the soil stoichiometry.
Isabelle BERTRAND is a senior scientist working on soil organic matter dynamic, its interactions with soil microorganisms and fauna. She is working on simple and complex agrosystems such as monocrops and agroforestry systems. Her focussed is in soil functional ecology and the soil C, N and P cycling.