Annual average (n = 8–9 for each year) rates of (a) net ammonification and (b) net nitrification rates, and concentrations of (c) nitrogen (N_MB) and (d) carbon (C_MB) in soil microbial biomass in the O and A soil horizons at the Plešné (PL) and Čertovo (CT) plots.

<p>Vertical bars denote 95% confidence intervals.</p

Excess of Organic Carbon in Mountain Spruce Forest Soils after Bark Beetle Outbreak Altered Microbial N Transformations and Mitigated N-Saturation

<div><p>Mountain forests in National park Bohemian Forest (Czech Republic) were affected by bark beetle attack and windthrows in 2004–2008, followed by an extensive tree dieback. We evaluated changes in the biochemistry of the uppermost soil horizons with the emphasis on carbon (C) and nitrogen (N) cycling in a near-natural spruce (<i>Picea abies)</i> mountain forest after the forest dieback, and compared it with an undisturbed control plot of similar age, climate, elevation, deposition, N-saturation level, and land use history. We hypothesised that the high litter input after forest dieback at the disturbed plot and its consequent decomposition might influence the availability of C for microorganisms, and consequently, N transformations in the soil. The concentrations of dissolved organic C (DOC) and N (DON) in soil water extracts rapidly increased at the disturbed plot for 3 yeas and then continually decreased. Net ammonification exhibited a similar trend as DOC and DON, indicating elevated mineralization. Despite the high ammonium concentrations found after the forest dieback (an increase from 0.5 mmol kg^-1 to 2–3 mmol kg^-1), net nitrification was stable and low during these 3 years. After the DOC depletion and decrease in microbial biomass 5 years after the forest dieback, net nitrification started to rise, and nitrate concentrations increased from 0.2–1 mmol kg^-1 to 2–3 mmol kg^-1. Our results emphasize the key role of the availability of organic C in microbial N transformations, which probably promoted microbial heterotrophic activity at the expense of slow-growing nitrifiers.</p></div

Annual average (n = 8–9 for each year) concentrations of (a) NH₄, (b) NO₃, (c) DOC, (d) dissolved organic N (DON), (e) annual average molar DOC:TN_H2O ratios, and (f) relationship between DOC and DON concentrations in water extracts from the O and A soil horizons at the Plešné (PL) and Čertovo (CT) plots.

<p>Data on concentrations from 2008–2010 are from Kaňa et al. (2013). Vertical bars denote 95% confidence intervals. Lines in Fig 1f represent linear regressions (PL-O: y = 0.04x + 0.27, <i>R</i>^<i>2</i> = 0.70; PL-A: y = 0.04x + 0.35, <i>R</i>^<i>2</i> = 0.75; CT-O: y = 0.04x + 0.29, <i>R</i>^<i>2</i> = 0.58; CT-A: y = 0.03x + 0.38, <i>R</i>^<i>2</i> = 0.68).</p

Relationships between annual average (n = 8–9 for each year) molar DOC:NO₃ ratio in water extracts and (a) annual average net nitrification, (b) C in microbial biomass (C_MB), and (c) between C_MB and net nitrification in the O and A soil horizons at the Plešné (PL) and Čertovo (CT) plots.

<p>Lines indicate statistically significant (<i>p</i><0.01) correlations of respective parameters in soils from the PL plot.</p

Differences in soil biochemical and chemical parameters among seasons (spring, Sp; summer, S; autumn, A; and winter, W) in the O and A horizons at the Plešné (PL) and Čertovo (CT) plots from 2008–2013.

<p>Different letters in superscript indicate statistically significant differences (p<0.05) among seasonal values (a–lower value, b–higher value, n.s.–not significant); significantly differing seasons are in bold.</p

Ranges and averages (in parentheses) of soil characteristics of the O and A horizons at the study plots in the Plešné (PL) and Čertovo (CT) catchments from 2008–2013.

<p>Abbreviations: total carbon (C), total nitrogen (N), total phosphorus (P).</p><p>Ranges and averages (in parentheses) of soil characteristics of the O and A horizons at the study plots in the Plešné (PL) and Čertovo (CT) catchments from 2008–2013.</p

A simplified scheme of the main changes in C and N cycling along with the timeframes of major ecosystem changes (grey fields) at the Plešné plot, affected by the bark beetle infestation.

<p>The lines show mass weighted means of spring values of the respective variables in the O and A soil horizons. The left Y axis shows rates of net ammonification and net nitrification (μmol kg^-1d^-1), C in microbial biomass (C_MB; mmol kg^-1), and DOC concentrations in water extracts (mmol kg^-1). The right Y axis shows concentrations of NH₄ and NO₃ (mmol kg^-1) in water extracts. Values for the years 2004–2007 are based on regular annual spring samplings (late May) at the study plot (Šantrůčková, unpublished data). Similar scheme for unaffected CT plot is given in SI (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134165#pone.0134165.s004" target="_blank">S4 Fig</a>).</p

Differences in microbial community composition in different horizons in arctic soils.

<p>Principal component analysis (PCA) with relative abundances of all PFLA biomarkers. Colors indicate different horizon categories: organic topsoil (O) is dark grey, mineral topsoil (A) is light grey, mineral subsoil (B) is white, and cryoturbated material (J) is black. Symbols indicate sites: circles Cherskiy, diamonds Logata, and triangles Tazovsky. Symbols are the mean values of the coordinates for the individual categories, derived from the PCA with individual samples (n = 101). Error bars are SE. Colors of PLFA markers indicate general markers (grey), gram-positive markers (red), gram-negative markers (orange), bacterial markers (blue) and fungal markers (green).</p

Climate and Vegetation.

<p>Climate data are derived from WorldClim database including mean annual temperature (MAT), maximum temperature of the warmest month (Tmax), minimum temperature of the coldest month (Tmin) mean annual range in temperature (MART) and mean annual precipitation (MAP).</p

Properties of the microbial community.

<p>Total amount of PLFAs, fungi∶bacteria ratios and statistical results for the first three principal components derived from a PCA with relative abundances of all PLFA biomarkers. Values are mean values (± standard error) over all sites and for each horizon per site. Letters in parentheses indicate significantly different (P<0.05) groups between horizons derived from ANOVA and Tukey-HSD tests.</p