Evaluation of soil nitrogen availability by growing tufts of nitrophilic species in an intensively grazed biodiverse legume-rich pasture

Biodiverse legume-rich pastures (BLRP) have been recommended for extensive animal production since they can improve productivity and pasture quality. However, the consequences for the N balance within the agro-system, due to the increase in biological N2 fixation, must be monitored. A field trial was carried out to evaluate the soil N availability in a BLRP in comparison with an adjacent unsown pasture. The field experiment consisted of growing tufts of nitrophilic species (turnip, Brassica campestris and rye, Secale cereale) in the pastures rounded by PVC rings. Soil inorganic-N levels were monitored during a period of one year. The potentially available soil N was determined by growing ryegrass (Lolium multiflorum) in a pot experiment and carrying out several chemical extraction methods. The mean values of N recovered by field-grown turnip and rye were, respectively, 30.6 and 31.1 kg ha-1 in BLRP, not statistically higher than that recovered in the unsown pasture. This is consistent with the very low levels of soil inorganic-N observed both in BLRP and the unsown pasture. Nitrogen recovered by ryegrass grown in pots was significantly higher in the soil collected from the BLRP than in soil from the unsown pasture. In this study, plant-available inorganic-N appeared as a strong limiting factor for the growth of the non-legume component. The BLRP seems to be currently environmentally sound, since the risk of N loss is practically non-existent. However, the potentially mineralisable organic N is increasing, which requires a further monitoring of the soil N dynamic as the pasture ages.