Abstract
Intercropping and rotation systems involving legumes can be useful means to achieve a sustainable agricultural production. Biological nitrogen fixation (BNF) is widespread among leguminous species thanks to their ability to establish symbiosis with nitrogen (N) fixing microorganisms. Moreover, the application of intercropped legumes increases the agricultural biodiversity and helps farmers to reduce N leaching preventing soil erosion and weed spreading. Little information is available regarding the contribution of N2 (atmospheric nitrogen) from symbiotic fixation of pea and clover when involved in intercropping or in succession systems with other vegetable crops. This study, carried out in a mountain agroecosystem (Venosta Valley, South Tyrol, Italy), has been conceived as part of a more complex research trial on N optimization of cauliflower intercropped and in succession with other vegetables. We have applied the 15N natural abundance technique to field grown clover and dwarf pea, with or without external N supplied, to assess the potential contribution of N2 for intercropping and crop rotation. The technique exploits naturally occurring differences in 15N abundance between plant available N sources in the soil (more enriched in the 15N isotope) and that of N2 in the atmosphere (δ15N=0). When a legume crop fixes N2, its 15N abundance should be lower than that of a no-N fixing crop, exploiting only the soil N pool. Reference plant N derived from atmosphere (%Ndfa) was the white cauliflower (Brassica oleracea var. botrytis), one of the most important vegetables on a world scale belonging to the Brassicaceae family and the first vegetable crop cultivated for fresh consumption in South Tyrol. In our study, clover and pea crop residues contained around 74 and 43 kg N ha-1, respectively. The %Ndfa for the two legumes was similar and ranged from 20% (dwarf pea) to 22% (clover). In presence of external N fertilizers, values of δ15N of the species resulted lower in comparison with the unfertilized control, making the BNF estimation difficult. The study indicated clover and dwarf pea as a potential tool to reduce inorganic fertilizers supply when included in a rotation system. Further trials are needed to confirm our results on BNF of the analysed species in different agroecosystems.