Nitrogen (N) exchange mediated through common mycorrhizal networks (CMNs) was investigated in pure and mixed systems of Casuarina cunninghamiana and Eucalyptus maculata, and Glycine max (soybean) and Sorghum bicolour (sorghum). Both 15N labeling and 15N natural abundance (δ 15N) studies were performed.
Seeds of all four species were aseptically germinated with or without mycorrhizal fungi or N2-fixing bacteria on agar media in petri dishes. Seedlings were transplanted into three-compartment growth units and paired. 37 μm diameter nylon mesh, RainSaver crystals (high water holding capacity) and N-serve (nitrification inhibitor) prevented direct root contact, soil nutrient flow with water and nitrification, respectively.
At harvest, none of the controls was mycorrhizal or nodulated, while all originally non-mycorrhizal seedlings were colonised. Mycorrhizal hyphae penetration through the nylon mesh was directly observed and demonstrated with an Environmental Scanning Electron Microscope (ESEM). Colonisation of roots as high as 80% confirmed that common ectomycorrhizal or arbuscular mycorrhizal networks were established between pairs in all combinations.
Mycorrhization had significant effects on biomass production in both N2-fixing plants (Casuarina, soybean) and non-N2-fixing ones (Eucalyptus, Sorghum). Dry matter production was highest in both partners when N2-fixing plants were mycorrhizal and nodulated. However, mycorrhization had little impact on N accumulation in eucalypts, but had a major effect in casuarinas, despite eucalypts having nearly double the colonisation rate. Biomass was positively correlated with tissue N content in both species. The nodulated mycorrhizal casuarinas and their companion mycorrhizal eucalypts had the highest tissue N accumulation. Both biomass and total N in all N-receivers equalled those in N-donors, especially when nodulated casuarinas were N-receivers. The above trends were generally true for soybean and sorghum pairs. In addition, δ15N values were negative in nodulated casuarinas, but positive in nodulated soybeans. Biological nitrogen fixation (BNF) contributed up to 50% and 40% of N in nodulated mycorrhizal casuarinas and soybeans, respectively.
From both 15N labelling experiments and δ15N analyses it was established that N-transfer occurred bidirectionally (two-way) between Casuarina and Eucalyptus, and between soybean and Sorghum. The percentages and amounts of N transferred, and the % of N in the receiver derived from the transfer (%NDFT) were generally significantly higher in the nodulated/mycorrhizal pairs than in the nonnodulated/ mycorrhizal pairs. This occurred regardless of whether the nodulated N2-fixing plants were 'N-donors' or 'N-receivers'. However, the %NDFT was always on the same scale regardless of the direction of N-transfer. The % and amount of N-transfer were also significant from non-N2-fixing plants to nodulated N2-fixing plants (with up to 50% biological N2-fixation) rather than the reverse. Significantly higher bidirectional and net N-transfer were also found between the sole mycorrhizal and the nodulated/mycorrhizal pairs. These results indicated a net gain in N by N2-fixing plants, but not by non-N2-fixing ones.
The similar N transferred to non-N2-fixing plants and to N2-fixing ones in the sole mycorrhizal pairs indicated that two-way N-transfer could occur naturally between any mycorrhizal plants, regardless of whether they were N2-fixing plants or non-N2-fixing ones, and that N resources could equally be reallocated between plants through mycorrhizae. The significantly greater intensity of bidirectional N-transfer in the nodulated mycorrhizal pairs showed that more substantial amounts of N could be shuttled between plants because of a generally greater physiological and ecological N demand in low-external-N-input conditions. These results therefore suggest that N2-fixing capacity might not be a prerequisite for, but might affect the intensity of, this two-way N-transfer.
In addition to accessing N from soils directly by roots, the experiments suggest that N2-fixing plants have two further strategies, N2-fixation and mycorrhization, to satisfy their high N-demand, while mycorrhization alone can meet the needs for relatively low N-demand by non-N2-fixing plants. Two 'mycocentric' N-transfer mechanisms are postulated to account for these differences. It seems that any plant that gives more N than it receives is an 'N-donor', while the opposite is true for an 'N-receiver'. If these mechanisms operate as these experiments have demonstrated and prove to be widespread, ideas about mycorrhiza-mediated N exchange and cycling in both agricultural and natural ecosystems may have to be re-evaluated, and concepts about nutrient cycling and energy exchange in plant communities may also have to be reformulated. Bidirectional N-transfer certainly has important implications for the nitrogen economy of N2-fixation-based agricultural and natural ecosystems. In such ecosystems, the magnitude of mycorrhiza-mediated N-transfer and N movement seems to be determined by the dynamic four-way interactions between plant roots, mycorrhizal fungi, N2-fixing bacteria, and N resource availability and requirements.