Frankiasymbiosis as a source of nitrogen in forestry: a case study of symbiotic nitrogen-fixation in a mixed Alnus-Picea plantation in Scotland

Abstract

SynopsisAlnus rubragrowing in the field in Scotland is nodulated effectively by local strains ofFrankia. Strains which have been isolated fromAlnus rubraat Lennox Forest show different growth medium requirements and colony morphology compared with isolates fromAlnus glutinosaand several different strains have been isolated from the same locality. Preliminary evidence suggests that some spore negative, northwest AmericanFrankiastrains may be more effective than local spore positive crushed nodule inoculum for nitrogen-fixation inAlnus rubra.On the moderately fertile clay soil of Lennox Forest, a mixed plantation of alternatePicea silchensisandAlnus rubrashowed no improvement in growth of spruce compared with pure spruce plots. However, the presence of alder increased upper soil nitrogen status by 585 kg ha ' which approximates the standard 150 kg N ha 'of fertiliser nitrogen applied in practice to nitrogen deficient stands at about 5-year intervals. Although the alder had penetrated the subsoil, there was no apparent effect on spruce rooting depth in the mixed stands in this high clay soil. In addition to nitrogen content, the total phosphorus of the upper soil horizons was improved in the mixed plots by an estimated 3–6 kg ha-1y1and it is suggested that this phosphorus may be brought from the subsoil by the deeper rooting alder and deposited on the surface in its litter.Although improved growth of spruce in mixture withAlnus rubrais only likely where pure spruce stands are stressed for nutrients, the potential benefits of symbiotic nitrogen-fixation in silvicultural practice make it desirable to investigate other species and provenances ofAlnussuited to British conditions, to achieve maximum symbiotic fixation of nitrogen by selection and inoculation with superior strains ofFrankiaand to include such plants in trials of mixtures on sites where nitrogen-availability may be critical.