Growth and nitrogen status of young walnuts as affected by intercropped legumes in a Mediterranean climate

Walnut trees (Juglans nigra L. and Juglans nigra x regia NG23) were intercropped with alfalfa (Medicago sativa L.) or sainfoin (Onobrychis sativa L.) forage legumes compared to a grass (Festuca arundinacea Schr., fescue), or to spontaneous weeds as a control in two Mediterranean sites (Castries and Notre-Dame de Londres) near Montpellier (France). Tree growth, soil water depletion and nitrogen content of the tree leaves were monitored to assess the impact of both water competition and possible facilitation resulting from fixed nitrogen transfer from the leguminous crops to the trees. At Castries, where alfalfa and fescue were compared, they were found to have the same impact on tree growth. At Notre-Dame de Londres where sainfoin was compared with a spontaneous grassing treatment, the sainfoin crop was more competitive to the walnuts. The nitrogen content of walnut leaves was enhanced when intercropped with nitrogen fixing species at both sites. Perennial leguminous intercrops were more competitive for soil water resources than fescue or weeds, but in the long term this may be compensated by the improved nitrogen status of the trees. This compensating effect was observed during a rather rainy year. The overall impact of leguminous intercrops on tree growth may depend on the frequency of dry (competitive) and wet (non-competitive) years.