Caroline M. Preston

Applications of NMR to soil organic matter analysis : History and prospects

More than 30 years have passed since the first application of nuclear magnetic resonance (NMR) spectroscopy to soil organic matter (SOM). Since then, there has been an explosion of applications using 1H, 13C, 31P, and 15N NMR on both solution and solid-state samples. These have greatly enhanced our

A comparison of soil extraction procedures for 31P NMR spectroscopy

The effect of extractants on phosphorus determination by 31 P NMR spectroscopy was examined using five forest floor samples. The extractants used were: 0.25 M NaOH, 1:6 soil to Chelex in water, 1:6 soil to Chelex in 0.25 M NaOH, and a 1:1 mix of 0.5 M NaOH and O.1 M EDTA. The broadest peaks were produced by the NaOH + EDTA extraction. However, NaOH + EDTA extracts contained the highest percentage of total phosphorus and the greatest diversity of P forms. These extracts were the only ones to show peaks for polyphosphates. Metals analysis indicated that NaOH + EDTA maintained Mn in solution, which seemed to be responsible for the line broadening. The sharpest peaks, with the best separation, were produced with Chelex + NaOH, and these were improved further by increasing the pH with NaOH prior to NMR analysis. Chelex + NaOH extracted 23 to 35% of the total soil P, Chelex in water extracted 10 to 13%, NaOH alone extracted 22 to 34%, and NaOH + EDTA extracted 71 to 90%. This work suggests that, because the extractant used will affect the P forms, care must be taken when interpreting studies of P cycling in soils using 31 P NMR spectroscopy and when comparing studies using different extractants.

Litter decomposition and humus characteristics in Canadian and German spruce ecosystems: information from tannin analysis and 13C CPMAS NMR

Abstract Influences of litter and site characteristics were investigated during the decomposition of black spruce (Picea mariana (Mill.) B.S.P.) and Norway spruce (Picea abies (L.) Karst.) needle litter in litterbags in two black spruce sites in Canada (6 and 12 months) and two Norway spruce sites in Germany (6 and 10 months). Mass losses were greater for black spruce litter (mean 25.2%) than for Norway spruce (20.8%), despite lower quality of black spruce litter in terms of lower N (10.1 versus 17.1 mg g−1), higher C-to-N ratio (49.0 versus 30.3) and higher content of alkyl C (surface waxes and cutin), indicated by CPMAS 13C NMR spectroscopy. However, Norway spruce litter was higher in condensed tannins than black spruce (37.8 and 25.3 mg g−1, respectively). Tannins were lost rapidly from both species, especially in the first 6 months, with losses in 10–12 months of 75–89% of the fraction extractable in acetone/water and 40–70% of the residual fraction. Losses were greater in the German sites (mean 75.2%, 10 months, versus 68.4%, 12 months), which had earthworms present and higher temperature, precipitation and catalase activity, the latter being positively correlated with tannin loss. There was a much larger contrast in the organic layers; with the Canadian sites having lower C-to-N ratios and higher N concentrations (C-to-N, 20.3 and 29.7; N, 26.0 and 13.8 mg g−1 for Canadian and German sites, respectively). The 13C NMR spectra showed that they were poorly decomposed and unusually high in condensed tannins (consistent with chemical analysis of 28.7 and 37.6 mg g−1, Canada; and 3.5 and 5.0 mg g−1, Germany), with depletion of lignin structures. Differences in other inputs (bark, wood, roots, understorey vegetation) and in site properties (climate, decomposer community, earthworm activity) may be responsible for the considerable differences in humus properties, which would not be expected from differences in the chemical composition and short-term decomposition of needle litter. The tannin accumulation, lignin depletion and N sequestration in the black spruce sites may be related to accumulation of unavailable N and associated forest management problems in these ecosystems.

How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: Stocks, molecular structure, and conversion to black carbon (charcoal)

[1] In boreal forests, fire is a frequent disturbance and converts soil organic carbon (OC) to more degradation-resistant aromatic carbon, i.e., black carbon (BC) which might act as a long-term atmospheric-carbon sink. Little is known on the effects of fires on boreal soil OC stocks and molecular composition. We studied how a surface fire affected the composition of the forest floor of Siberian Scots pine forests by comparing the bulk elemental composition, molecular structure (13C-MAS NMR), and the aromatic carbon fraction (BC and potentially interfering constituents like tannins) of unburned and burned forest floor. Fire reduced the mass of the forest floor by 60%, stocks of inorganic elements (Si, Al, Fe, K, Ca, Na, Mg, Mn) by 30–50%, and of OC, nitrogen, and sulfur by 40–50%. In contrast to typical findings from temperate forests, unburned OC consisted mainly of (di-)O-alkyl (polysaccharides) and few aromatic structures, probably due to dominant input of lichen biomass. Fire converted OC into alkyl and aromatic structures, the latter consisting of heterocyclic macromolecules and small clusters of condensed carbon. The small cluster size explained the small BC concentrations determined using a degradative molecular marker method. Fire increased BC stocks (16 g kg−1 OC) by 40% which translates into a net-conversion rate of 0.7% (0.35% of net primary production) unburned OC to BC. Here, however, BC was not a major fraction of soil OC pool in unburned or burned forest floor, either due to rapid in situ degradation or relocation.

Linking chemical reactivity and protein precipitation to structural characteristics of foliar tannins

Tannins influence ecosystem function by affecting decomposition rates, nutrient cycling, and herbivory. To determine the role of tannins in ecological processes, it is important to quantify their abundance and understand how structural properties affect reactivity. In this study, purified tannins from the foliage of nine species growing in the pygmy forest of the northern California coast were examined for chemical reactivity, protein precipitation capacity (PPC), and structural characteristics (13C NMR). Reactivity of purified tannins varied among species 1.5-fold for the Folin total phenol assay, and 7-fold and 3-fold, respectively, for the acid butanol and vanillin condensed tannin assays. There was about a 5-fold difference in PPC. Variation in chemical reactivity and PPC can be largely explained by differences in structural characteristics of the tannins determined by 13C NMR. In particular, the condensed versus hydrolyzable tannin content, as well as the hydroxylation pattern of the B-ring and stereochemistry at the C-2–C-3 position appear to influence reactivity. Due to the large differences in chemical reactivity among species, it is necessary to use a well-characterized purified tannin from the species of interest to convert assay values to concentrations. Our results suggest that structural characteristics of tannins play an important role in regulating their reactivity in ecological processes.

Linkages between phosphorus transformations and carbon decomposition in a forest soil

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution13C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state13C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution31P-NMR indicated that inorganic P (P1) increased with increasing depth, while organic-P (Po) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P1 and C fractions (r = 0.82, 0.81, −0.87, and −0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = −0.74 and −0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,31P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.

Towards a global assessment of pyrogenic carbon from vegetation fires

The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates indicating that ~50% of the PyC produced by vegetation fires potentially sequesters C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious, and therefore, PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 116-385 Tg C yr(-1) , that is ~0.2-0.6% of the annual terrestrial net primary production. According to our estimations, atmospheric emissions of soot/black C might be a smaller fraction of total PyC (<2%) than previously reported. Research on the fate of PyC in the environment has mainly focused on its degradation pathways, and its accumulation and resilience either in situ (surface soils) or in ultimate sinks (marine sediments). Off-site transport, transformation and PyC storage in intermediate pools are often overlooked, which could explain the fate of a substantial fraction of the PyC mobilized annually. We propose new research directions addressing gaps in the global PyC cycle to fully understand the importance of the products of burning in global C cycle dynamics.

Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle.

Wildfires release substantial quantities of carbon (C) into the atmosphere but they also convert part of the burnt biomass into pyrogenic organic matter (PyOM). This is richer in C and, overall, more resistant to environmental degradation than the original biomass, and, therefore, PyOM production is an efficient mechanism for C sequestration. The magnitude of this C sink, however, remains poorly quantified, and current production estimates, which suggest that ∽1-5% of the C affected by fire is converted to PyOM, are based on incomplete inventories. Here, we quantify, for the first time, the complete range of PyOM components found in-situ immediately after a typical boreal forest fire. We utilized an experimental high-intensity crown fire in a jack pine forest (Pinus banksiana) and carried out a detailed pre- and postfire inventory and quantification of all fuel components, and the PyOM (i.e., all visually charred, blackened materials) produced in each of them. Our results show that, overall, 27.6% of the C affected by fire was retained in PyOM (4.8 ± 0.8 t C ha−1), rather than emitted to the atmosphere (12.6 ± 4.5 t C ha−1). The conversion rates varied substantially between fuel components. For down wood and bark, over half of the C affected was converted to PyOM, whereas for forest floor it was only one quarter, and less than a tenth for needles. If the overall conversion rate found here were applicable to boreal wildfire in general, it would translate into a PyOM production of ∽100 Tg C yr−1 by wildfire in the global boreal regions, more than five times the amount estimated previously. Our findings suggest that PyOM production from boreal wildfires, and potentially also from other fire-prone ecosystems, may have been underestimated and that its quantitative importance as a C sink warrants its inclusion in the global C budget estimates.

Litter Quality and Its Potential Effect on Decay Rates of Materials from Canadian Forests

Decomposition is influenced by a wide array of factors including macroclimate, microclimate, soil biota, soil nutrients, substrate piece size and substrate quality. To separate the influence of some of these factors a 10-year study, the Canadian Intersite Decomposition Experiment, was established in 1992 to measure the decay of 11 standard litter types on a range of forest types at 21 sites across Canada. As part of the study we analysed the initial elemental contents (N, P, S, K, Ca, Mg) and carbon (C) fractions (extractables, cellulose, hemicellulose, lignin) by13C NMR and wet chemical proximate analysis in a total of 37 primarily foliar litter types representative of the range of species found at the different CIDET sites. Litter types especially non-conifer species varied greatly in their qualities. Principal component analyses showed that the litter types could be distinguished by the elemental macronutrient contents through the ratio of N+P+K:S, by proximate chemical analyses through the ratio of water soluble:acid fractions, and by NMR through the ratio of O-alkyl:alkyl C. Litter quality data was used in three simple models of litter decay to predict how the mass loss of the different litter types could vary. Two models using a linear or single exponential decay equation and litter lignin and N content predicted a 2–5 fold difference in total mass loss for the different litter types. A third model using a summed exponential decay equation for three chemical fractions and a ligno-cellulose index predicted that for all but one litter type, variation in mass loss between types would be less than a 20%.

Characterization of organic matter in a forest soil of coastal British Columbia by NMR and pyrolysis-field ionization mass spectrometry

Organic matter in the soil profile under a young Douglas-fir stand in coastal British Columbia was characterized by examining intact samples of fresh litterfall and organic horizons (LF, H), and fractions (floatables, humic acid [HA], fulvic acid [FA], humin [HU]) from the three mineral horizons (Ae, Bm, BC). Some 30–40% of the carbon in the mineral horizons was found in poorly-decomposed plant material floatable in water, a fraction whose characteristics changed little with depth, and which contained over 1% Fe. The proportion of soil C in HA plus FA was approximately 8%, but the ratio of C in FA/HA increased with depth. Solid-state 13C NMR spectra of litterfall, LH and H samples showed effects of decomposition, in particular a decrease in 0-alkyl C from litterfall to LH to H, and degradation of resolution from LF to H. For the mineral soil fractions, both floatables and de-ashed HU (‘HUd’ prepared by HCl/HF treatment) indicated high levels of the original plant biopolymers, including a large alkyl component. Solution 13C spectra of the HAs from mineral horizons showed little difference with depth, except that peaks due to lignin were more pronounced for the Bm HA. The NMR spectra of FAs were high in 0-alkyl and carboxyl C. Pyrolysis-field ionization mass spectrometry confirmed and extended the results from NMR and chemical analyses, in particular demonstrating the accumulation of suberin in some fractions and the leaching and decomposition of lignin components with increasing depth in the mineral horizons. The general features of the HA, FA and HUd from this forest soil, and the effects of decomposition and pedogenesis were similar to those widely found for agricultural and forest soils. However, the accumulation of suberin, and the leaching and decomposition of lignin are particularly associated with forest soils. The low proportion of soil C in HA and FA, and the high proportion in poorly decomposed, iron-rich plant fragments suggest that decomposition is somewhat limited at this site, which is classified as having low fertility. The high accumulations of alkyl C from suberin may also indicate, or contribute to inhibition of decomposition.