Mycorrhizas and nutrient cycling in sand dune ecosystems

Abstract

SynopsisThe extent of occurrence, the form and the function of mycorrhizal infection are shown to change with successional development across coastal sand dune systems. The interrelationships between these changes and the prevailing physico-chemical conditions are explored and clear patterns are recognised in terms of both type and function of the infection. The periodically disturbed and nutritionally enriched high tide line is colonised by non-mycorrhizal ruderal species. There follows a sequence of plant communities, each characterised by the presence of a dominant mycorrhizal type and a distinctive nutritional limitation. In the foredunes, pioneer grasses are normally infected with vesicular-arbuscular (VA) mycorrhizal fungi. Plants such as Ammophila, Leymus and Uniola, all of which have extensive root systems, appear, when fully grown, to be only facultatively mycorrhizal. However, experimental evidence suggests that infection is important for the early growth of such plants and it is suggested that in these, as in many other dune species, mycorrhizas may be essential at critical stages in the life cycle, most notably during the phase of seedling establishment. Here, phosphorus (P) is the most important growth-limiting nutrient. The extensive mycelial network of VA hyphae not only facilitates capture of this element but also provides the aggregation of sand grains necessary for dune stabilisation. In semi-fixed dune pastures, as sand inputs are reduced, productivities are low and species diversity increased. Phosphate limitation persists and the majority of the characteristic species show VA infection. Experimental studies using microcosms of dune sand containing an assemblage of species typical of such communities suggest that the maintenance of the species richness is dependent upon mycorrhizal fungi which produce a large absorptive mycelial network into which the roots of germinating seedlings become incorporated as they are infected. Accumulation of organic matter in dune-slacks leads to reduction of pH. Nitrification is inhibited, ammonium becomes the major mineral nitrogen (N) source and N replaces P as the key growth-limiting element. Here plants with ecto-mycorrhizal infection predominate. Salix repens produces a shrub layer enriched with litter in which a guild of species interconnected by a common mycorrhizal mycelium occurs. The functional basis of this guild structure is explored, the ability of some of its mycorrhizal fungi to mobilise nutrients from organic macro-molecules being seen as a vital attribute. Where organic matter accumulation and base depletion are most strongly developed in the oldest parts of the succession, plants with ericoid mycorrhizas become important. The ability of their mycorrhizal fungi to liberate N and P from acidic organic complexes, as well as to assimilate or exclude, and hence detoxify, organic acids and metal ions facilitates vigorous growth of ericaceous species in soil conditions which are inimical to plants important earlier in the succession. Since the attributes of each mycorrhizal type are relevant to a specific suite of edaphic properties the formation of appropriate symbiotic associations is likely to be a prerequisite for successional change. It is concluded that mutualism contributes significantly to fitness in the sand dune ecosystems, the further understanding of which will be dependent upon more effective collaboration between the microbiological and ecological disciplines.