Nitrogen fluxes in chickpea grown in Mediterranean agroforestry systems
Résumé
The insertion of leguminous crops into temperate and Mediterranean agroforestry systems where few di-nitrogen-fixing trees can grow is attempted to improve soil N fertility. However, as the microenvironment created by trees could affect plant development, rates of legume growth and biological nitrogen fixation may prove inadequate. In this study, we analyze the nitrogen benefits of chickpea (Cicer arietinum L.) grown in rainfed conditions as a sole crop (SC) or in the alleys of a 15-year old hybrid walnut-based agroforestry system (AF). The fate of N derived from biological fixation by chickpea was assessed with the(15)N natural abundance method and the amount of N released from roots into the soil by rhizodeposition was quantified for the 0-30 cm horizon using the(15)N cotton-wick labeling method. The measurements were made in the middle of the crop alley, where the total cumulative radiation received by plants was 90 % of full sunlight, so as to focus on N cycling. The total plant N was 33 % higher in AF than in SC whereas plants had similar N concentrations in vegetative above-ground parts and roots in AF and in SC. This difference was due to a very high N concentration in grains and to twice as much seed biomass in AF plants as in SC ones. The percentage of plant N fixed from the atmosphere by chickpea plants grown in AF was half that in SC. The N rhizodeposition was 47.2 +/- A 10.2 mg plant(-1) and 60.4 +/- A 15.3 for SC and AF respectively. Considering the chickpea plant density, we estimated the nitrogen soil supply to chickpea at 2.2 and 28.2 kg N ha(-1) for SC and AF respectively. In conclusion, our results showed that environmental conditions in the middle of the AF crop alley were beneficial for chickpea growth, seed biomass and quality of the seed, probably because of the combination of a higher mineral N availability (higher N turnover) due to better conditions for organic nitrogen mineralization, created by the trees. The organic matter status and water balance modification after 15 years of agroforestry may increase nutrient availability, even for legumes.