Improved nodulation has been observed on tree roots in close contact with the roots of non-nodulating trees and shrubs. In a 1981 study using the leguminous tree Inga jinicuil in a coffee plantation, the nodules and fine roots of the tree were found to concentrate around the trunks of coffee trees, within or just below the lower layer (Roskoski, 1981). In many cases the association of the nodulating and non-nodulating roots has been so close, it is difficult to discern which plant is nodulating.
When Leucaena leucocephala was planted in a hedgerow-intercropping experiment in Nigeria, Leucaena roots were observed in direct contact with cassava roots about 2 meters from the Leucaena hedge, occupying the same soil layers as the cassava crop (Hairiah and van Noordwijk, 1986). This finding contradicts conventional wisdom which dictates that in agroforestry systems, tree roots should be in deeper soil layers than crop roots.
In this experiment the direct contact of tree nodules with the cassava roots suggests that a direct transfer of nitrogen is possible. Reports of increased nodulation in association with annual crops are more scarce.
There are two possible mechanisms underlying the hypothesis. Firstly, nodulation is inhibited in soils of high mineral nitrogen content. In well-rooted zones there is a depletion of mineral nitrogen, therefore nodulation would be stimulated in a well-rooted zone. The second possibility involves root exudates of the non-nitrogen- fixing plant. In certain cases, rhizobia have been found to thrive in the rhizosphere of non-nodulating plants, increasing the potential of the soil to stimulate nodule formation in the vicinity of their roots. This second scenario is closely linked to a hypothesis involving the role of the tree root in the maintenance of soil fertility: that the potential to increase nutrient use efficiently arises through the capacity of tree root and associated mycorrhizal systems to take up nutrients from the soil solution. However, as only the Roskoski data is sufficiently quantitative to support the new hypothesis, more study is needed.
Roskoski, J.P. 1981. Nodulation and N2 Fixation by Inga jinicuil, a woody legume in coffee plantations. 1. Measurements of nodule biomass and field C2H2 reduction rates. Plant Soil. 59: 201-6 pp.
van Noordwijk, M., Dommergues, Y.R. 1990. Root Nodulation: the twelfth hypothesis. Agroforestry Today. 3(3): 9-10 pp.