Can Shading Affect Nitrogen Fixation Of Forage Legume Swards? An Assessment Of B-Value Through The 15N Natural Abundance Method

Tree-based intercropping systems are gaining pace as a land-use strategy to cope with climate change and provide environmental, economic, and social benefits. The integration of nitrogen-fixing crops between trees can be a solution to increase the land productivity and reduce the reliance on external inputs by increasing nitrogen (N) availability and then both tree and crop growth. Intercropping perennial legumes with trees can also reduce nitrogen losses, due to the higher amount of N accumulated in stable forms in the soil due to biological N₂-fixation and N root compartimentation. On the other hand tree competition for light, water and nutrients eventually could limit legume growth and N₂-fixation. The isotopic method based on ¹⁵N natural abundance is one of the most used methods to assess Biological Nitrogen Fixation (BNF). The B-value, that is defined as the δ¹⁵N value of a legume when completely dependent on N₂-fixation for satisfying its N demand, is of primary importance for BNF estimations. The B-value may vary with species, plant age at harvest and growing conditions, e.g. light availability. Therefore, the B-value found in literature could not be representative for all legumes and environments, in particular for legumes grown intercropped with trees and thus subject to shading conditions. In this pot experiment we assessed the B-value of several forage legumes, as affected by different levels of simulated shading and grown in N-free medium. A greenhouse pot experiment was established in March 2021 at the Department of Agriculture, Food and Environment (DAFE) of the University of Pisa to determine the B-value for ¹⁵N calculations about two forage legume species grown in a field trial located at the Center of Agri-Envirnomental Research “Enrico Avanzi” of the University of Pisa, San Piero a Grado (Pisa) (43°41'6.97"N 10°20'29.22"E), using the same shade treatments.