Abundances and potential activities of nitrogen cycling microbial communities along a chronosequence of a glacier forefield

Abstract

Glacier forefields are ideal ecosystems to study the development of nutrient cycles as well as single turnover processes during soil development. In this study, we examined the ecology of the microbial nitrogen (N) cycle in bulk soil samples from a chronosequence of the Damma glacier, Switzerland. Major processes of the N cycle were reconstructed on the genetic as well as the potential enzyme activity level at sites of the chronosequence that have been ice-free for 10, 50, 70, 120 and 2000 years. In our study, we focused on N fixation, mineralization (chitinolysis and proteolysis), nitrification and denitrification. Our results suggest that mineralization, mainly the decomposition of deposited organic material, was the main driver for N turnover in initial soils, that is, ice-free for 10 years. Transient soils being ice-free for 50 and 70 years were characterized by a high abundance of N fixing microorganisms. In developed soils, ice-free for 120 and 2000 years, significant rates of nitrification and denitrification were measured. Surprisingly, copy numbers of the respective functional genes encoding the corresponding enzymes were already high in the initial phase of soil development. This clearly indicates that the genetic potential is not the driver for certain functional traits in the initial phase of soil formation but rather a well-balanced expression of the respective genes coding for selected functions.

Figure 1

Site map illustrating the Damma glacier forefield. Circles mark the sampling locations. Five circles representing the soil samples of the same developmental stage are labeled with the corresponding sample code: 10, 50, 70, 120 and 2000a.

Figure 2

Potential (a) and relative enzyme activities (b) for nitrogen fixation, mineralization, nitrification and denitrification are displayed (n=5, error bars represent standard error of means). Only significant differences as revealed by one way ANOVA on ranks (P<0.05) are indicated by different letters. n.d. indicates that all five replicates of potential N fixation were below the detection limit of 0.2 pmol N h⁻¹ g⁻¹ soil.

Figure 3

Gene copy numbers of functional genes involved in major steps of the nitrogen cycle (nifH, aprA, chiA, amoA AOB, amoA AOA, nirK, nirS and nosZ) are displayed related to gram soil (a) and nanogram DNA (b) (n=5, error bars represent standard error of means). Only significant differences as revealed by one way ANOVA on ranks (P<0.05) are indicated by different letters.

Figure 4

Correlations of the gene copy numbers and the corresponding potential enzyme activities are displayed for nitrogen fixation (a), mineralization (b, c), nitrification (d, e) and denitrification (f, g and h). The graphs plot log-transformed copy numbers vs log-transformed enzyme activities. In the top left corner of each plot the parameters of the regression analysis, including P-values of t-tests, are provided.