Diversity and composition of tropical soil nitrifiers across a plant diversity gradient and among land-use types

The influence of biotic interactions on soil biodiversity

Belowground communities usually support a much greater diversity of organisms than do corresponding aboveground ones, and while the factors that regulate their diversity are far less well understood, a growing number of recent studies have presented data relevant to understanding how these factors operate. This review considers how biotic factors influence community diversity within major groups of soil organisms across a broad spectrum of spatial scales, and addresses the mechanisms involved. At the most local scale, soil biodiversity may potentially be affected by interactions within trophic levels or by direct trophic interactions. Within the soil, larger bodied invertebrates can also influence diversity of smaller sized organisms by promoting dispersal and through modification of the soil habitat. At larger scales, individual plant species effects, vegetation composition, plant species diversity, mixing of plant litter types, and aboveground trophic interactions, all impact on soil biodiversity. Further, at the landscape scale, soil diversity also responds to vegetation change and succession. This review also considers how a conceptual understanding of the biotic drivers of soil biodiversity may assist our knowledge of key topics in community and ecosystem ecology, such as aboveground-belowground interactions, and the relationship between biodiversity and ecosystem functioning. It is concluded that an improved understanding of what drives the diversity of life in the soil, incorporated within appropriate conceptual frameworks, should significantly aid our understanding of the structure and functioning of terrestrial communities.

Species and rotation frequency influence soil nitrogen in simplified tropical plant communities

Among the many factors that potentially influence the rate at which nitrogen (N) becomes available to plants in terrestrial ecosystems are the identity and diversity of species composition, frequency of disturbance or stand turnover, and time. Replicated suites of investigator-designed communities afforded an opportunity to examine the effects of those factors on net N mineralization over a 12-year period. The communities consisted of large-stature perennial plants, comprising three tree species (Hyeronima alchorneoides, Cedrela odorata, and Cordia alliodora), a palm (Euterpe oleracea), and a large, perennial herb (Heliconia imbricata). Trees were grown in monoculture and in combination with the other two life-forms; tree monocultures were subjected to rotations of one or four years, or like the three-life-form systems, left uncut. The work was conducted on fertile soil in the humid lowlands of Costa Rica, a site with few abiotic constraints to plant growth. Rates of net N mineralization and nitrification were high, typically in the range of 0.2-0.8 microg x g(1) x d(-1), with net nitrification slightly higher than net mineralization, indicating preferential uptake of ammonium (NH4+) by plants and microbes. Net rates of N mineralization were about 30% lower in stands of one of the three tree species, Hyeronima, than in stands of the other two. Contrary to expectations, short-rotation management (one or four years) resulted in higher net rates of N mineralization than in uncut stands, whether the latter were composed of a single tree species or a combination of life-forms. Neither additional species richness nor replenishment of leached N augmented mineralization rates. The net rate at which N was supplied tended to be lowest in stands where demand for N was highest. Careful choice of species, coupled with low frequency of disturbance, can lead to maintenance of N within biomass and steady rates of within-system circulation, whereas pulses, whether caused by cutting and replanting or by the phenological traits of the species selected or combined, subject N supplies to leaching loss.

Cold-temperate climate: a factor for selection of ammonia oxidizers in upland soil?

Ammonia-oxidizing bacteria in various upland soils show a rather large diversity with respect to their amoA genes (coding for a subunit of the ammonium monooxygenase). It is known that the community structure of ammonia-oxidizing bacteria in upland soils is influenced by different selective factors, such as pH, gravimetric water content, fertilizer treatment, and temperature. The question, from an ecological point of view, is whether a particular ecophysiological factor, such as temperature, could select for a particular community structure of ammonia oxidizers in upland soils that would be represented by distinct clusters of the amoA gene (AmoA cluster). Studying the literature, including recent publications and our own unpublished results, we found that AmoA clusters 3a, 3b, and 9-12 apparently exhibited no preference for either subtropical/tropical soils (i.e., warm regions) or temperate cold soils. However, AmoA clusters 1 and 4 (and perhaps cluster 2) seem to occur predominantly in soils from cold-temperate regions. Here we review the evidence for a temperature effect on the global distribution of amoA genes in warm- and cold-temperate soils.