Above ground nutrient cycling in tropical forest ecosystems:
Nitrogen fixation in the phyllosphere.
An unresolved paradox associated with many evergreen tropical rainforest ecosystems is their high productivity even when existing over nutrient poor soils (Jordan et al. 1980). Although it has been shown that gradients in soil fertility among tropical forests can be as pronounced as those documented in temperate systems, a majority of lowland evergreen rainforests, like those of the Amazon Basin, support high levels of primary productivity over infertile soils (Jordan and Herrera 1981). Above ground tree biomass in central Amazonian evergreen tropical rainforests was estimated at nearly 400 tons per hectare, suggesting that a vast majority of carbon and micro-nutrient capital in these ecosystems is locked in living organisms (Aubert and Tavernier 1972).
Tropical plant species which have adapted means of conserving and competing for above-ground nutrient resources confer a distinct advantage in environments with infertile soils by capitalizing on these alternative nutrient pools (Jordan et al. 1980). Plants are nitrogen-limited in most ecosystems primarily because usable forms of the element must be biologically fixed: an energy-demanding process necessitating a reliable source of organic matter available for nitrogen fixing microbes (Schlesinger 1997). Because tropical soils are characteristically low in organic matter content, nitrogen fixing soil bacteria are potentially limited in their fixation rates. However, surveys estimating litterfall nutrient content from lowland tropical forest ecosystems across the globe point to the inefficiency of within-stand nitrogen recycling, suggesting that nitrogen, more so than the other nutrients, is not as limited in tropical forests as previously expected (Vitousek 1984). Nonetheless, if this lost nitrogen is not readily detectable in the soils of tropical systems and is demonstrably lost from the canopy, what are other replacement sources for nitrogen in tropical rainforests?
photo by Chas Zartman
Nitrogen availability in tropical forests is often limited due to the large amount retained within living biomass and the rapid uptake of nitrogen from the soil. It has been estimated that 70% or more of the nitrogen in tropical systems is retained within living tissue (Bentley and Carpenter 1987). Further nitrogen losses occur through percolation of soil water to ground and stream water, volatilization and denitrification. Runoff loss has been estimated at 29 kg N per ha year and gaseous loss at 73 kg N per ha year for a Puerto Rican rain forest (Edmisten 1970). Data on denitrification rates are limited, but have been estimated at 1-2 kg per ha year for an Amazonian forest (Vitousek and Sanford 1986).
Although plants primarily acquire nitrogen through organic decomposition, a significant component of nitrogen utilized by plants comes from the biological fixation of atmospheric nitrogen (Bentley 1987). It has been estimated that nearly 45% of transferred nitrogen occurs through biological fixation processes (Burns & Hardy 1987). Classic examples of nitrogen fixing systems include the symbiotic relationships between the bacteria Rhizobiumand the root systems of leguminous plants, but significant sources of nitrogen can be fixed by free living bacteria which occur on a variety of substrates and habitats including the surfaces of leaves of vascular plants. The presence of communities on the surface of leaves (known as epiphylly) is most well developed in tropical areas characterized by high rainfall and low rates of evaporation (Bentley 1978). Such communities include a variety of taxonomic groups including certain species of nitrogen-fixing bacteria (Beijerinckii, Klebsiella, and Azotobacter), and cyanobacteria (Nostoc, Scytonema, and Stigonema) as well as lichens, fungi, and bryophytes (Frieberg 1998). View examples of epiphyllous organisms from here.Though there are undoubtedly numerous interactions taking place between these epiphylls and their host leaves, the interactions centered around nitrogen fixation by cyanobacteria may have significant consequences for tropical forests characterized by low nitrogen availability.
Nitrogen-fixing organisms may serve as a buffer to nitrogen immobility and loss. The contribution of nitrogen fixing organisms has been estimated at 10 x 106 metric tons nitrogen per year for forest and woodland systems (Burns and Hardy 1975). Recent studies in a Costa Rican tropical rain forest estimated the nitrogen contribution of Scytonema (a cyanobacteria) in the phyllosphere to be as high as 1.6 + 0.8 kilograms of nitrogen per hectare per year per unit of leaf area index (Freiberg 1998).
Fixed nitrogen is released by the cyanobacteria as extracellular products but the amount of released nitrogen actually available to other plants may depend upon the proximity to the fixer (Bentley and Carpenter 1987). Further, rates of fixation are influenced by factors such as moisture, nutrient and exogenous sugar availability. The following pages will examine the relationships between the organisms comprising the phyllosphere, with particular emphasis on bryophyte-cyanobacteria-host plant interactions. Previous research focused on this important system will be reviewed.