Schmidt et al. 2008

Schmidt SK, Reed SC, Nemergut DR, Grandy AS, Cleveland CC, Weintraub MN, Hill AW, Costellow EK, Meyer AF, Neff JC, Martin AM. 2008. The earliest stages of ecosystem succession in high-elevation (5000 metres above sea level), recently deglaciated soils. Proceedings of the Royal Society of London, Series B 275: 2793-2802.

These authors describe the microbial community and soil parameters of a chronosequence at the foreground of a receding glacier high in the Peruvian Andes. From a combination of aerial photography and previous work at this site, a series of sites of soils of increasing ages from zero to 79 years old was established. No surface plants, even lichens, are present on any of these new soils, and soil nutrient levels (carbon, nitrogen) are very low; the only organisms present are microorganisms.

Two previous hypotheses had been proposed to explain the dynamics of very early primary succession on new soil. Organic matter has been observed to accumulate slowly in new soils; the source of this material is either aeolian deposits (i.e. wind-borne plant detritus and pollen) or in-situ fixation of CO2 and N2. These are not mutually exclusive hypotheses, but the relative contributions of each are explored in this study.

The methods used here cover an extensive list of soil parameters. Three sets of soil samples were collected: for microbiological analysis, N-fixation measurement, and all other chemical analyses. The other chemical analyses include photosynthetic pigment extraction, soil total and mineral nitrogen, pyrolysis for identifying sources of carbon compounds (i.e. microbial-autotroph, microbial-heterotroph, plant), enzyme assays for common and informative microbial enzymes, and soil stability analysis of the resistance of these new soils to erosion forces such as water runoff.

These authors focused on the cyanobacterial fraction of the microbial community in this study; some details of other components of the biota are described in an earlier paper, Nemergut et al. (2007). Cyanobacteria are autotrophs also capable of fixing atmospheric nitrogen, thus they are ideal primary colonizers of new soil as they require little more than a source of moisture and air. Analysis of the community included the use of the P-test (Martin 2002); note that as in Nemergut et al. (2007), he is one of the authors of this study. The analytical approach is very similar to that employed in the earlier study, with a comparison of discovered sequences to published sequences from around the world. In this study, cyanobacterial sequences from zero and 4-year-old soils were similar to sequences from an extremely broad sample of habitats, including Antarctic lake ice, marine subseafloor sediments, urban aerosols, forest soils, and oil-polluted soils.

The soil chronosequence showed a clear pattern of stages of primary succession at every level of analysis. The soil microbial community became both more abundant and more diverse through time, soil nutrients increased, the chemical environment included increasing amounts and diversity of complex organic molecules, key enzyme pathways became established, and soil stability increased as soils aged. N-fixation showed a peak, with increasing N-fixation activity from the zero to 4-year-old soils (by two orders of magnitude), then declining by about half in the 79-year-old soils. This mirrors and precedes a widely-observed pattern in plant primary succession, in which nitrogen-fixing plants are among the first colonizers, but decline in abundance at later stages of succession. Enzyme and organic molecule patterns were consistent with a total absence of heterotrophs in the extremely young soils, increasing occurrence of organisms capable of decomposing plant matter in the 4-year-old soils, and a molecular ecology qualitatively similar to a mature plant-associated soil in the 79-year-old soil.

The list of procedures and level of detail of analysis in this paper is impressive. Many, though certainly not all, of these techniques will be models for my own work, especially in the summer of 2010. The molecular-diversity techniques pioneered by Martin (Martin 2002, Nemergut et al. 2007, this paper) as well as the techniques of analyzing soil pigments and soil nutrients are all very interesting.