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NMR - an essential tool for research in forest ecology.

The use of NMR is advancing the understanding of the life cycle of the forest.

Compared to its impact on forest products research, nuclear magnetic resonance (NMR) spectroscopy has seen relatively little utilization on the forest ecology, side. This is unfortunate, because whatever one's perspective on the forest, from the chemist's point of view, it is a place for carbon chemistry on a grand scale. Some of the highest levels of biomass production and C storage are found in BC's coastal rainforest. What grows must die and decay, in the process replenishing the soil and releasing C[O.sub.2] into the atmosphere. Living biomass and its decomposition stages present the analytical chemist with materials that are complex, heterogeneous, largely insoluble and often mixed with mineral matter. Molecular-level analyses are complex and may only identify, a small portion of the carbon. At the Pacific Forestry Centre, NMR has been used to characterize virtually every carbon pool in the forest - foliage, litterfall, woody debris, forest floor, soil organic matter, and dissolved organic carbon. The technique of 13C NMR with Cross-Polarization and Magic-Angle Spinning (CPMAS NMR) is ideal for solids, and solution 13C NMR is used for extracts such as tannins from plant material and humic fractions from mineral soil.

How Wood Rots

BC's coastal forests are characterized by massive amounts of coarse woody debris, often over 1 m in diameter. These logs can decay for centuries, finally becoming dark reddish-brown, fragmented, and collapsing under their own weight. The ovoid masses become covered with forest floor and eventually form buried woody horizons. Previous ecological studies had been largely restricted to changes in nutrient content, density and moisture, with little information and much speculation on changes in carbon chemistry. Solid-state 13C CPMAS NMR shows that the initial stages of decay, probably dominated by white-rot fungi, proceed with little change in the proportions of carbohydrate and lignin. In the later stages, there is usually a shift to dominance by brown-rot fungi and a relative increase in the rate of loss of carbohydrate. The final reddish-brown masses are almost entirely guaiacyl lignin, only slightly altered by some oxidation and loss of the 3-carbon side chain [1,2].

Figure 1 shows an example for Douglas-fir; the relatively fresh wood (decay class 1) is dominated by the signal at 73 ppm for C2, C3 and C5 of cellulose. The spectrum of decay class 5 is typical of guaiacyl lignin, with methoxyl at 57 ppm, the broad side-chain signal at 74 ppm, and phenolic C at 148 ppm. This type of information is essential in understanding the role of coarse woody debris in biodiversity (habitat, seedling sites), soil formation, nutrient cycling and carbon budgets.

When a Leaf Falls

Understanding of the decay process for foliar litter has been clouded by misunderstanding about its organic composition. Decay models have often been hypothesized on a two-compartment model of "lignin" pins "cellulose" (the "lignin" in fact being the acid-insoluble residue from proximate analysis). However, 13C CPMAS NMR shows that foliar litter is actually rather low in lignin (5-10%), and may contain similar or higher proportions of two other biopolymers, cutin and condensed tannins (CT) [3]. The latter (also found in bark) may play an important role in carbon and nutrient cycling, as they bind with proteins and inhibit decomposition and N release. Both solution and solid-state 13C NMR have been essential in studying the role of CT in forest ecosystems. We can detect the diagnostic features of CT in foliage and forest floor (double phenolic peak at 145 and 155 ppm, broad peak at 105 ppm in dipolar-dephased spectra, Figure 2), and also rise solution and solid-state 13C NMR to characterize purified tannin extracts. Information from the ecological studies is now also proving useful in characterizing woody residues from logging and sawmilling, including material reclaimed from old landfills [4].

Into The Soil

Carbon levels drop as we proceed down through the forest floor and organic horizons and into mineral soil. Size or density fractionation, sometimes with HF treatment, can produce fractions with higher C content suitable for 13C CPMAS NMR, or soluble fractions (humic and fulvic acid) can be extracted with NaOH [5]. These provide information on the nature of the original input (eg., roots, decomposed wood, charcoal) and the patterns of decomposition. NaOH extracts are also useful for characterizing soil P. A study in progress has shown higher proportions of organic P forms (monoesters, diesters, pyrophosphate, phosphonate) from mineral soils on the cooler, wetter west coast of Vancouver Island, and higher proportions of orthophosphate P on the drier, warmer east side (Figure 3). Such information is useful in understanding the factors limiting P availability, and the sensitivity of the sites to disturbance.

New Directions

The seeds of many tree seeds require specific conditions of chilling, moisture and darkness ("stratification") for germination. Most of this research is empirical, and tests for moisture and readiness to germinate are both slow and destructive. We used bench-top pulse NMR with the Bruker MiniSpec for rapid, nondestructive moisture determination in seeds [6], and MAS NMR to obtain high-resolution 13C and 1H spectra of seeds about to germinate [7]. There is great potential for NMR studies of seed germination and seedling cold-hardening, and the possibility of developing some rapid tests for the nursery industry.

These and other projects would not have been possible without the help and support of many collaborators over the years, who assisted through times of no instrument, and the solution-only Bruker WM 250. Work is currently being carried out on a Bruker MSL 300 with solution and solid-state capability, on loan from National Defence. It is still, I believe, the only modern NMR worldwide that is dedicated to research in forest ecology. Future plans include quantitative CPMAS studies of foliage, including amounts of specific biopolymers, use of phenolic polymers as models for humic acids, and application of CPMAS 15N NMR to soil N.

References

1. Preston, C.M., Sollins, P. and Sayer, B.G., 'Changes in Organic Components for Fallen Logs in Old-growth Douglasfir Forests Monitored by 13C Nuclear Magnetic Resonance Spectroscopy', Can. L For. Res., 20:1382-1391, 1990.

2. Preston, C.M., Trofymow, J.A., Niu, J. and Fyfe, C.A., '13C CPMAS NMR Spectroscopy and Chemical Analysis of Coarse Woody Debris in Coastal Forests of Vancouver Island', For. Ecol. Manage., submitted, 1998.

3. Preston, C.M., Trofymow, J.A. Sayer, B.G. and Niu. J., '13C CPMAS NMR Investigation of the Proximate Analysis Fractions Used to Assess Litter Quality in Decomposition Studies', Can. J. Bot., 75:1601-1613, 1997.

4. Preston, C.M., Forrester, P. and Sauder, E.A., '13C CPMAS NMR Analysis of the Fines of Log Sortyard Residues' in Making a Business from Biomass (R.P Overend and E. Chornet, Eds.), Elsevier Science Ltd, pp. 757-763, 1997.

5. Preston, C.M., 'Applications of NMR to Soil Organic Matter Analysis: History and Prospects', Soil Sci., 161:144-166, 1996. 6. Nygren, M. and Preston, C., 'Determination of Seed Moisture Content in Pinus Contorta (L.) by Low Resolution Pulsed NMR', in Dormancy and Barriers to Germination, D.G.W. Edwards (Ed.), Forestry Canada, Pacific Forestry Centre, Victoria, BC, pp. 97-102, 1993.

7. Sayer, B.G. and Preston, C.M., 'A Carbon-13 Magic Angle Spinning Nuclear Magnetic Resonance Study of the Germination of Conifer Seeds', Seed Sci. Tech., 24:321-329, 1996.

Caroline Preston, FCIC, completed her PhD (1975) in carbohydrate NMR with Laurie Hall, FCIC at the University of British Columbia. After moving to Ottawa, ON she completed a PDF with Environment Canada in Raman spectroscopy of phosphates, and in 1978 she joined Agriculture Canada to work on nitrogen and organic matter in soil. In 1986, she moved to the Pacific Forestry Centre in Victoria, BC where her research interests include applications of NMR to organic matter, and 15N tracer techniques. She can be reached at cpreston@pfc.forestry.ca.
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Title Annotation:nuclear magnetic resonance spectroscopy
Author:Preston, Caroline M.
Publication:Canadian Chemical News
Date:Jan 1, 1998
Words:1307
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