Molecular Composition and Biodegradability of Soil Organic Matter

Article pubs.acs.org/est

Molecular Composition and Biodegradability of Soil Organic Matter: A Case Study Comparing Two New England Forest Types Tsutomu Ohno,*,† Thomas B. Parr,‡ Marie−Cécile I. Gruselle,§ Ivan J. Fernandez,§ Rachel L. Sleighter,∥ and Patrick G. Hatcher∥ †

School of Food and Agriculture, University of Maine, Orono, Maine 04469-5722, United States School of Biology and Ecology, University of Maine, Orono, Maine 04469-5751, United States § School of Forest Resources, University of Maine, Orono, Maine 04469-5722, United States ∥ Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States ‡

S Supporting Information *

ABSTRACT: Soil organic matter (SOM) is involved in many important soil processes such as carbon sequestration and the solubility of plant nutrients and metals. Ultrahigh resolution mass spectrometry was used to determine the influence of forest vegetation type and soil depth on the molecular composition of the water-extractable organic matter (WEOM) fraction. Contrasting the upper 0−5 cm with the 25−50 cm B horizon depth increment, the relative abundance of lipids and carbohydrates significantly increased, whereas condensed aromatics and tannins significantly decreased for the deciduous stand WEOM. No significant abundance changes were found for the coniferous stand DOM. Kendrick mass defect analysis showed that the WEOM of the 25−50 cm B horizon was depleted in oxygen-rich and higher mass components as compared to the 0−5 cm B horizon WEOM, suggesting that higher mass WEOM components with oxygen-containing functionality show greater reactivity in abiotic and/or biotic reactions. Furthermore, using an inoculated 14-day laboratory incubation study and multivariate ordination methods, we identified the WEOM components with H:C > 1.2 and O:C > 0.5 as being correlated most strongly with biodegradability. Our findings highlight the importance of understanding soil depth differences for various forest types in the chemical composition of SOM and the processes governing SOM production and transformations to fully understand the ecological implications of changes in forest composition and function in a changing climate.

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INTRODUCTION

Studies have shown that DOM chemical properties change with soil depth. Using a fluorescence index of the ratio of emission intensities at 470 and 520 nm wavelengths (excitation wavelength fixed at 370 nm), it was reported that the DOM of a pine-dominated forest soil in Colorado became less aromatic and more microbially derived with increasing soil depth.9 Using parallel factor analysis of a fluorescence spectral array from a long-term cropping systems study site, subsoil (30−45 cm) DOM had a greater relative concentration of a component attributed to microbial decomposition products than found in surface soil (0−15 cm) DOM.10 Kaiser and Kalbitz11 have proposed a conceptual model of DOM transformational reactions with its movement down a soil profile that explains the observed trend for greater microbial DOM at soil depth. In this model, plant-derived DOM inputs initially are sorbed or precipitated onto mineral surfaces, where it undergoes microbial reprocessing and then is released by desorption or dissolution back to the dissolved state. This model accounts for the shifting from predominately plant-derived DOM near the

Sequestration of organic carbon by temperate forest soils is a vital ecosystem service that stores an estimated 195 Pg of C to an average of one meter depth.1 Studies of soil organic matter (SOM) have typically focused on the surface horizons, despite subsoils containing a greater mass of organic C.2,3 Although water-extractable organic matter (WEOM) constitutes a small fraction of SOM, it represents the most labile pool that directly participates in chemical processes, such as solubilization and complexation of metals4 and microbial activity.5 Adsorption of dissolved organic matter (DOM) to mineral surfaces, a primary mechanism for its retention in soils, is sensitive to both DOM composition and the surface chemical properties of the sorbent mineral.6 Additionally, the biodegradability of DOM has been identified as a key process determining the stabilization of DOM in soils.7 DOM is operationally defined as the organic material that is dissolved in situ and passed through a ∼0.45 μm filter, which originated from the limnology and oceanography disciplines.8 In soil studies, WEOM and DOM are often used interchangeably. However, it should be noted that WEOM does not exclusively reflect in situ materials and also includes organic matter that can readily be desorbed into the aqueous extraction solution.8 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 7229

December 13, 2013 April 23, 2014 June 9, 2014 June 9, 2014 dx.doi.org/10.1021/es405570c | Environ. Sci. Technol. 2014, 48, 7229−7236

Environmental Science & Technology

Article

were processed through Agilent PPL solid-phase extraction cartridges to desalt the extract for subsequent electrospray ionization FT−ICR−MS.22 FT−ICR−MS. The FT−ICR−MS analyses were conducted with a 12 T Bruker Daltonics Apex Qe FT−ICR−MS housed at the College of Sciences Major Instrumentation Cluster at Old Dominion University. The PPL processed samples (eluted in methanol) were diluted by a factor of 5 with LC−MS grade methanol and water to give a final sample composition of 50:50 (v/v) methanol:water. In order to increase the ionization efficiency, ammonium hydroxide was added immediately prior to electrospray to bring the pH up to 8. Samples were introduced by a syringe pump providing an infusion rate of 120 μL h−1 and analyzed in negative ion mode, with electrospray voltages optimized for each sample in order to maintain consistent and stable ion currents. Previous studies have shown that negative ion mode avoids the complications of the positive ion mode in which alkali metal adducts, mainly Na+, are observed along with protonated ions. Ions (in the range of 200−1200 m/z) were accumulated in a hexapole for 1.0 s before being transferred to the ICR cell. Exactly 300 transients, collected with a 4 MWord time domain, were coadded, giving about a 30 min total run time. The summed free induction decay signal was zero-filled once and Sine-Bell apodized prior to fast Fourier transformation and magnitude calculation using the Bruker Daltonics Data Analysis software. Prior to data analysis, all samples were externally calibrated with a poly(ethylene glycol) standard and internally calibrated with naturally present fatty acids and other CH2 homologous series detected within the sample. Details about the spectral postprocessing are provided in the Supporting Information. DOM Biodegradation. Biodegradability of the WEOM was determined by extracting the Oa, E, and the six B horizon soils, for both the deciduous and coniferous soils in triplicate for replication. The WEOM extractions were conducted as described above. The extracts were diluted to 20 ppm DOC for the biodegradation study.23 For each of the 20 ppm DOC extracts, 8 mL was pipetted into an individual glass culture tube. To each tube, 3 mL of nutrient solution (0.1% NH4NO3 and K2HPO4) were added to ensure adequate nutrients in the incubation units. Fresh inoculum was made by mixing organic and mineral soils from both deciduous and coniferous stands overnight with gentle shaking. After settling, 50 μL of the inoculum was added to each incubation unit. Three replicates of 20 ppm glucose were used as a reference and deionized water was used as a control. The tubes were incubated in the dark, with gentle stirring for 14 days at 25 °C in a dark incubator. After the 14 day incubation period, the solutions were filtered through 0.4 μm polycarbonate filters to remove microbial cells and any precipitates that may have formed. The DOC of the filtered solution was determined and percent biodegradability was calculated based on the DOC loss (i.e., biodegradable DOC, or BDOC). Principal Coordinate Analysis and Redundancy Analysis. Principal coordinate analysis (PCoA) and redundancy analysis (RDA) was performed in R using the capscale() function in the vegan package. This approach interprets each molecular formula observed as an individual species. Intersite similarity in molecular composition is summarized using a distance index and ordinated to reduce the dimensionality using PCoA. The principal coordinates are then related to environmental predictor variables RDA. Further details are provided in the Supporting Information.

surface to predominately microbial-derived DOM at deeper soil depths. Fluorescence-based studies of DOM are inherently limited by the lack of fine structure in their spectra. Recently, ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT−ICR−MS) has become a prevailing method for the chemical characterization of natural organic matter.12 The superior capabilities of FT− ICR−MS typically allow detection of thousands of individual DOM components. The absolute mass accuracies to five decimal places of high field FT−ICR−MS allows calculation of the elemental formulas of detected peaks based on the exact molecular masses of the constituent atoms.13 A standard approach to comparing FT−ICR−MS data is through graphical van Krevelen diagrams, which can assign each formula based on their H:C and O:C ratios into biomolecular chemical classes that typically includes proteins, lipids, carbohydrates, unsaturated hydrocarbons, lignins, condensed aromatics, and tannins.14,15 Kendrick mass defect (KMD) analysis is also a chemically meaningful method to examine the relationships between assigned formulas.16 Molecules of a specified homologous series differing only in the number of a functional group, such as a methylene or carboxyl group, have the same KMD. Forest vegetation has also been shown to influence the chemical characteristics of SOM.17,18 General circulation climate modeling forecasts (year 2100) for the northeastern U. S. predict that the prevalent spruce−fir and maple−beech− birch forest habitat types will decline and be replaced with oak−hickory and oak−pine forests.19 Our findings, coupled with climate modeling predictions of changing forest habitats, highlight the importance of understanding soil depth differences in the chemical composition of SOM and the processes governing SOM production and transformations. These insights are increasingly important to more fully understand the ecological implications of changes in forest composition and function in a changing climate. In this study, our objectives are to examine the influence of forest vegetation cover and soil depth on the molecular composition and biodegradability of the WEOM fractions from Spodosols (Typic and Lithic Haplorthods) using ultrahigh resolution FT−ICR−MS. Spodosols show high transport of WEOM due to their sandy texture, therefore we selected two profiles under deciduous and coniferous plant communities to compare OM composition of the surface horizon to that of the subsoil horizons.

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EXPERIMENTAL SECTION Field Site and Sample Analysis. The study site is part of a long-term acidification and nitrogen addition experiment using the paired watershed approach and is located at the Bear Brook Watershed in Maine (BBWM, 44°52′ N, 68°06′ W), U. S. A.20 Details are provided in the Supporting Information. Total soil C and N were determined using a LECO CN-2000 analyzer on ground samples (SPEX 8000 ball mill). Short-range ordered (SRO) mineral content was determined using an extraction of 4 h with a 0.2 ammonium oxalate solution at pH 3.21 The Al and Fe content of the ammonium oxalate extracts were measured by inductively coupled plasma-atomic emission spectroscopy. Soil WEOM fractions were extracted with hot water (3 g soil + 30 mL deionized water at 80 °C) for 16 h, centrifuged, and vacuum filtered through a 0.4 μm polycarbonate filter before the dissolved organic carbon (DOC) concentration was measured using a Shimadzu 5000 TOC Analyzer. Extracts 7230

dx.doi.org/10.1021/es405570c | Environ. Sci. Technol. 2014, 48, 7229−7236


Article

Table 1. Soil pH, Total Soil C and N, C/N Ratio, Dissolved Organic Carbon WEOM Content, and Short-Range Ordered (SRO) Al + Fe Mineral Content of the Deciduous and Coniferous Soils Collected at Depth Incrementsa total soil C g kg−1 soil

total soil N g kg−1 soil

379 20.3 31.2 19.9 28.7 26.9 33.9 30.0

13.9 0.72 1.16 0.78 1.25 1.25 1.60 1.47

soil/horizon

soil pH

Oa E B 0−5 cm B 5−10 cm B 10−15 cm B 15−20 cm B 20−25 cm B 25−50 cm

2.98 3.10 3.76 3.86 4.05 4.21 4.13 4.13

Slope

0.016b

0.195d

Oa E B 0−5 cm B 5−10 cm B 10−15 cm B 15−20 cm B 20−25 cm B 25−50 cm Slope

3.08 3.15 3.62 3.17 3.50 3.29 3.55 3.92 0.014d

454 13.5 74.9 65.4 73.6 60.8 80.5 84.7 0.466d

C/N Ratio

WEOM C g kg−1 soil

SRO Al + Fe mmol kg−1 soil

27.3 28.2 26.9 25.5 23.0 21.5 21.2 20.4

13.5 1.54 1.07 0.64 0.57 0.37 0.49 0.42

85 43 276 320 449 498 508 511

−0.268c

−0.029b

10c

26.7 18.8 22.6 22.9 23.9 24.0 24.9 26.3 0.141c

29.4 2.08 4.12 4.43 5.10 4.83 3.83 2.20 −0.067d

71 35 801 746 825 595 830 1160 10d

Deciduous

0.023d Coniferous 17.0 0.72 3.31 2.86 3.08 2.53 3.23 3.22 0.00d

a Trend in distribution of component classes in the B horizon samples evaluated by the slope term of the linear regression analysis. bSignificant at the p = 0.05 level cSignificant 0.01 level. dNot significant.

Figure 1. Dissolved organic carbon concentration of the WEOM (a) and total soil C (b) content of deciduous and coniferous forest type soils as a function of short-range ordered mineral content. Deciduous data are the filled circles and coniferous data are the open triangles.

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RESULTS AND DISCUSSION Soil Chemical Characterization. The basic chemical properties of the soils are shown in Table 1. Total soil C data shows characteristic trends in forested Spodosols with a Crich surface Oa horizon, a C-depleted eluviated E horizon, and a C-accumulating B horizon. We focus our discussion throughout this study on the illuvial B horizon soils, where SOM is accumulating. The coniferous B horizon soils were more acidic with a range of pH 3.2−3.9, compared to pH 3.8−4.2 for the deciduous B horizons. The increasing pH with greater soil depth of the B horizon was significant for the deciduous soils. There was a strong vegetation effect, with total C and N contents in the coniferous B horizons being more than twice the deciduous B horizons (Table 1). Under both forest types, total C declined between 0 and 5 cm and 5−10 cm B horizons

and then generally increased with greater soil depth. The slope coefficient for total C was positive for both soil types but was not statistically significant. Total N for the deciduous horizons decreased with depth, then increased to a concentration greater than in the 0−5 cm B horizon, resulting in a significantly lower C/N ratio with greater soil depth (Table 1). Total N for the coniferous soils generally declined with soil depth, resulting in a significantly higher C/N ratio with depth. DOC concentration of the B horizon WEOM declined significantly with greater soil depth in the deciduous stand, whereas no clear depth trend was observed in the coniferous stand (Table 1). SRO minerals have been shown to adsorb DOM in soils, presumably through ligand exchange reactions between the −OH functional group of DOM and mineral surface −OH functional groups.24 The significant negative 7231



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Table 2. Number Average Assigned Molecular Formula, m/z, and the Distribution of van Krevelen Chemical Classes from the FT−ICR−MS Analysis of the WEOM Components for Deciduous and Coniferous Soils Collected at Depth Incrementsa soil/horizon

average formula

m/z

% lipid

Oa E B 0−5 cm B 5−10 cm B 10−15 cm B 15−20 cm B 20−25 cm B 25−50 cm Slopeb

C22.7H30.3O8.96N1.62S0.31P0.007 C24.2H29.7O8.75N1.50S0.39P0.008 C23.9H29.5O8.99N1.51S0.41P0.006 C23.5H31.1O8.77N1.55S0.46P0.008 C23.3H31.5O8.78N1.50S0.46P0.009 C23.5H32.8O8.52N1.45S0.47P0.016 C23.3H32.0O8.70N1.52S0.46P0.011 C23.3H32.3O8.53N1.47S0.47P0.011 H: 0.102,d O: −0.016c

478.3 493.2 493.3 489.3 486.9 485.4 486.2 483.8 −0.333d

6.3 11.5 10.0 12.5 13.1 16.3 14.3 15.6 0.209c

Oa E B 0−5 cm B 5−10 cm B 10−15 cm B 15−20 cm B 20−25 cm B 25−50 cm Slopeb

C22.7H29.4O9.41N1.46S0.27P0.008 C23.6H29.1O9.21N1.27S0.24P0.009 C22.8H29.8O8.86N1.62S0.40P0.006 C23.0H29.7O8.96N1.59S0.39P0.003 C22.8H29.1O9.16N1.49S0.39P0.006 C23.1H29.3O9.22N1.53S0.40P0.004 C23.1H30.3O9.09N1.53S0.41P0.006 C23.3H31.3O8.84N1.64S0.44P0.008 C: 0.018c

480.4 484.1 480.2 482.8 482.2 487.3 486.2 488.7 0.330d

5.1 8.4 9.7 9.8 8.5 8.6 10.3 12.0 0.075e

% protein

% lignin

Deciduous 13.8 65.0 8.5 60.5 8.9 65.2 10.6 64.4 11.2 65.2 11.8 64.2 11.1 65.5 11.1 64.8 0.075e 0.002e Coniferous 13.6 63.8 9.7 62.3 11.4 65.0 11.3 64.3 11.2 64.3 10.9 64.4 11.7 65.4 11.7 65.9 0.014e 0.045e

% carbohydrate

% unsaturated

% condensed aromatic

% tannin

2.7 0.7 0.7 0.6 0.9 1.0 1.3 1.2 0.027d

2.9 3.5 1.7 1.3 0.7 0.8 0.7 0.9 −0.033e

5.2 11.8 8.8 6.0 4.4 2.7 3.2 3.0 −0.223c

4.2 3.4 4.7 4.6 4.4 3.2 3.8 3.4 −0.058c

3.3 0.4 1.2 1.1 1.0 1.1 0.9 1.1 −0.006e

1.5 2.2 1.9 1.4 1.4 1.3 1.2 1.7 −0.010e

7.8 11.7 5.2 6.3 6.7 7.0 5.0 3.1 −0.081e

4.8 5.3 5.5 5.9 7.0 6.7 5.6 4.5 −0.035e

a

Trend in distribution of component classes in the B horizon samples evaluated by the slope term of the linear regression analysis. bOnly significant slopes for elements in formula are noted. cSignificant at the p = 0.05 level. dSignificant at the p = 0.01 level. eNot significant.

possibly due to beech bark disease that has affected the BBWM study site,26 may also be plausible causes for higher aliphatic contents in the deciduous than in coniferous B horizons. For instance, suberin, an aliphatic polyester and major component of roots, has been found to accumulate in soils upon root death.27 Other root inputs to soils, such as root exudates, may also increase long-term soil C sequestration28 by adsorption onto mineral surfaces We further analyzed the soil depth effects on WEOM composition by directly contrasting the 0−5 cm B horizon against the 25−50 cm B horizon for both forest types using a Venn diagram analysis in combination with van Krevelen diagrams, in order to identify formulas unique to the 0−5 cm and to the 25−50 cm B horizon soils for the deciduous and coniferous forest types (Figure 2). The unique formulas in the 0−5 cm depth represent those SOM components that are removed through biotic and abiotic soil processes during transport down the soil profile, and the unique ones in the 25− 50 cm depth are produced in the subsoil, presumably through microbial processing. The unique WEOM components in the deciduous 25−50 cm depth soil have shifted out of the condensed aromatic space to more coverage of the aliphatic (higher H/C values) region (Figure 2b). Visual inspection of the shift in the van Krevelen diagram for the coniferous soil contrasts indicates less shifting of the space occupied into the aliphatic space (H/C > 1.5) (Figure 2c and d). The distribution of relative contents of the van Krevelen diagram classifications shown in Figure 2 are given in Table 3. The increase in lipid, protein, and carbohydrate content in the subsoil was much greater for the deciduous WEOM as compared to the coniferous WEOM. The condensed aromatic WEOM abundance was much lower in both deciduous and coniferous soils, which is likely to be due to adsorption of aromatic components to the SRO minerals present in the soil profile (Table 3). The greater upward shifting trajectory of the van Krevelen space of the deciduous subsoil provides supporting evidence that deciduous SOM reflects a greater degree of microbial

relationship of WEOM concentration with SRO minerals in both stands is likely the result of increasing WEOM sorption to SRO minerals (Figure 1a). Total soil C was linearly related (p = 0.001) to SRO minerals (Figure 1b). The higher concentration of SRO minerals in coniferous soils is due to more intensive podsolization as a result of higher organic matter inputs and acidity from conifer litter. Ultrahigh Resolution Mass Spectrometry. The average molecular properties of the WEOM are shown in Table 2. There were differences in the B horizon WEOM chemical compositions of the deciduous and coniferous stands. With increasing soil depth, the average assigned H content increased and O content decreased significantly for the deciduous stand B horizons. In contrast, for the coniferous stand B horizons, assigned C content was greater with increasing soil depth (Table 2). The WEOM m/z decreased significantly with greater depth for the deciduous soil B horizons, whereas it increased significantly for the coniferous B horizons. The van Krevelen diagram chemical classifications show that, in both soil types, the lignin-type DOM components are dominant, accounting for about 65% of the assigned peaks (Table 2). Lipids ranged from 5 to 15% and proteins from 9 to 14% in relative abundance. There was no significant soil depth effect in the relative content of any of the classifications for the coniferous B horizon WEOM. For the deciduous soil WEOM, relative protein and carbohydrate contents increased significantly with greater soil depth, whereas condensed aromatic and tannin contents decreased significantly with soil depth (Table 2). Organic matter molecules in the aliphatic region (H/C > 1.5) of the van Krevelen are likely to be microbially derived.25 These soil depth compositional changes for the deciduous soil WEOM are consistent with the supposition that soils at depth are enriched with microbially derived C and that C cycling is greater in soils under deciduous species. However, other factors may contribute the change in WEOM composition observed for the deciduous stand soils. Higher root necromass in the B horizon and higher root mortality in the deciduous soil, 7232



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Figure 2. van Krevelen diagrams of unique WEOM components identified by Venn diagram analysis of surface (0−5 cm) and subsoil mineral (25− 50 cm) soil horizons. (a) Deciduous forest type, 0−5 cm B horizon soil WEOM, (b) deciduous forest type, 25−50 cm B horizon soil WEOM, (c) coniferous forest type, 0−5 cm B horizon WEOM, (d) coniferous forest type, 25−50 cm B horizon WEOM.

Table 3. Percent Relative Distribution of van Krevelen Diagram Chemical Classes for the Assigned Formulas of WEOM Components Unique to the 0−5 cm Surface and the 25−50 cm Subsurface B Horizon Soils soil/set

% lipid

% protein

% lignin

0−5 cm 25−50 cm

6.4 28.1

4.6 11.9

56.7 51.0

0−5 cm 25−50 cm

5.2 17.0

5.6 8.4

57.4 63.6

% carbohydrate Deciduous 1.6 4.1 Coniferous 4.2 2.8

% unsaturated

% condensed aromatic

% tannin

4.7 3.0

19.7 0.7

6.1 1.2

4.4 2.7

15.5 3.2

7.7 2.3

carboxylate functional groups and is a signature of DOM adsorption processes.29 This suggests that adsorption is an important process controlling the chemical composition of WEOM with depth in these deciduous soils. Close inspection also reveals that above KMD values of about 0.4, the depleted components at a given KMD value begins with the less aliphatic (i.e., more aromatic) members of the homologous series. Unlike the deciduous stand soils, the KMD analysis of the soil depth contrast for the coniferous stand soils reveals little difference between the two horizons, indicating lack of depthrelated adsorption or production (Figure 3b). This may be a result of greater organic matter coating of the SRO mineral surfaces due to the higher soil C content in the coniferous stand (Table 1).

processing than coniferous SOM, which had much smaller trajectory shifts in the van Krevelen space (Figure 2, Table 3). Abiotic processes, such as cotransport with water from the surface to the subsoil, may also contribute to the presence of the aliphatic compounds at lower soil horizons. Kendrick mass defect (KMD) analysis is a complementary graphical approach to FT−ICR−MS data.16 For the deciduous upper and lower B horizon contrast, the WEOM components that are depleted in the subsoil have KMD > 0.65 and KM > 630, suggesting that aromatic or oxygen-rich WEOM formulas are depleted in the subsoil, in contrast to the 0−5 cm soil (Figure 3a). A study with DOM derived from a variety of sources has shown that the depletion of the higher molecular weight components is most likely through oxygen-containing 7233



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Figure 3. Kendrick mass defect analysis (using CH2 group) of surface (0−5 cm) and subsoil mineral (25−50 cm) soil horizons. (a) Deciduous forest type, 0−5 cm B horizon soil and 25−50 cm B horizon soil WEOM, (b) coniferous forest type, 0−5 cm B horizon and 25−50 cm B horizon WEOM.

Dissolved Organic Matter Biodegradation. DOM biodegradability is an important property in the stabilization of soil organic matter.7,30 For both forest types, biodegradation of the B horizon WEOM increased with soil depth (Figure 4).

Figure 4. Percent biodegradability of WEOM extracted from the Oa, E, and all B horizon soils from both deciduous and coniferous forest type soils.

Figure 5. van Krevelen diagram of WEOM components from both forest types and all B horizon soil depths, colored according to rank correlations to biodegradability. The color scale correlates to rank coefficients from −1 (blue) to +1 (red).

This is likely due to the greater aliphatic (H/C > 1.5) content at the lower soil depths (Table 2). Similar depth trends for both forested and cropland soils have been observed.30 Similar to the approach used by Singer,31 we used a two-stage ordination combining PCoA and RDA to reduce high dimensional mass spectral data into a few key compositional axes, which was then utilized to establish the relationship among the individual peaks and biodegradability with Spearman rank correlation (Figure 5). Clearly, the region bound by H:C > 1.2 and O:C > 0.5 are predominantly positively correlated with biodegradability, as indicated by the abundance of red data points in this area. This is the same region that was found to be most bioavailable for ice-locked glacier DOM,31 suggesting that DOM from diverse sources share common chemical and biological properties. Histograms of the distribution of the rank correlation coefficients for each of the van Krevelen diagram classification groups are shown in Supporting Information, Figure S1.

Carbohydrates, proteins, and tannins have greater bioavailable components, whereas the lipids have greater unavailable components. To further characterize the properties of the components with high positive (>+0.6) and negative ( +0.6) and Negative Rank Correlation (< −0.6) with Respect to Biodegradation parameter

positive rank correlated components

negative rank correlated components

438 20.3 30.4 2.96 80

482a 26.0a 37.0a 0.90a 19b

7.47 0.09 0.002 1.52 0.38 7.61 0.37

6.81a 0.34a 0.004c 1.43a 0.26a 8.93a 0.34a

m/z number of C number of H number of N % of components with N number of O number of S number of P H:C O:C DBE DBE/C a

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potential reactivity. The effect of soil depth on WEOM composition was more pronounced in the deciduous stand 0−5 and 25−50 cm contrast, with an increased relative aliphatic content in the subsoil and the concomitant decrease in aromatic abundance in the surface. Forest composition not only influences subsoil WEOM chemical composition and the potential for C stabilization but also influences the chemical composition of WEOM present in surface waters draining landscapes of varied forest compositions. Hydrologic pathways, such as shallow versus deep baseflow, will also influence stream WEOM composition and biodegradability. The results from this study provide strong evidence that WEOM biodegradability is directly linked to its chemical composition. Future changes in forest composition due to factors like climate change or shifts in hydrologic patterns, such as the climate-induced intensification of precipitation events, is likely to affect soil processes by altering the quantity and chemical composition of soil organic matter, increasing microbial mineralization of soil C, as well as hydrologic flow paths. Understanding these dynamic interactions on forested landscapes will be increasingly important for effective management of ecosystem services in the decades ahead.

ASSOCIATED CONTENT

S Supporting Information *

Expanded experimental section and a supporting figure referenced in the text is provided. A database of the assigned formulas from the mass spectra of all the samples from the study is also provided as a separate text file. This material is available free of charge via the Internet at http://pubs.acs.org

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ACKNOWLEDGMENTS

This project was supported by NSF DEB-1056692. It has also been supported by Hatch funds provided by the Maine Agricultural and Forest Experiment Station. This is MAFES Journal no. 3373.

p < 0.05. bNA = not applicable. cNS = not significant.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 207-581-2975. Fax: 207581-2999. Notes

The authors declare no competing financial interest. 7235



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Molecular Composition and Biodegradability of Soil Organic Matter

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