Release of mineral nitrogen from plant root nodules

Root nodules from a variety of nodulated leguminous and non-leguminous plants were added to soil samples and the net release of mineral N was monitored periodically for 56 or 98 days. Over these periods, various nodules demonstrated either net N immobilisation, mineralisation, or immobilisation followed by mineralisation. No nodular tissue mineralised > 30% of its organic N content. Mineral N release from the nodules was not significantly related to their C to N ratios or organic N content (r = 0.410). Nitrogen fractionation data from these nodules indicated that the main forms of N present in combined form were α-amino N and hydrolysable unidentified N(HUN). Attempts to relate mineral N release to the forms of N present in the nodules indicated that nodule N mineralisation was significantly related to nodule concentration of NH4-N and hexosamine-N in combination (r =0.741). It was concluded that these two components are probably highly important in regulating mineral N release from decomposing root nodules.