Tuesday, 11 July 2006 - 1:15 PM

Some Chemical and Spectroscopic Approaches to Evaluating Management Impacts on Forest Soil Organic Matter.

Caroline M. Preston, Pacific Forestry Centre, 506 West Burnside Rd., Victoria, BC V8Z1M5, Canada

Evaluating the impact of management on forest soils presents special challenges, which tend to increase depending on how far removed the situation is from management for annual agricultural crops. Forest soils tend to be spatially very heterogeneous, especially those on steep slopes, and in colder or poorly-drained areas where a thick forest floor develops over mineral soil. Soil development may be affected by tree uprooting, persistence of coarse woody debris and landslides, as well as wildfires that consume organic horizons and produce charcoal. Harvesting produces coarse residues that decompose slowly, and may be accompanied by soil disturbance, such as compaction, rutting and loss of organic matter, or by prescribed burning. Silvicultural treatments may include fertilization, thinning, and manipulation of understory vegetation. Sampling is therefore a formidable challenge, especially as sample numbers have to be severely limited for more complex types of analyses. Despite dramatic visual effects or reduction of site C, the effects of forest management on Soil Organic Matter (SOM) composition may be quite subtle and difficult to detect. Not surprisingly, there have been few studies in this area. Solution or solid-state C-13 Nuclear Magnetic Resonance (NMR) spectroscopy has long been used characterize OM, including forest floor, woody debris, foliar litter and mineral soil C. For the latter, it is usually necessary to treat samples with HF to remove paramagnetic Fe and enrich C concentration. Also, more insight may be obtained by separating physical or chemical fractions, as the total SOM C may not show detectable changes. The instrument type (e.g., field strength, probe size, spinning speed, RAMP options) also affects quality and utility of spectra. Organic forms of P are predominant in many forest sites, and can be characterized by solution P-31 NMR of extracts such as NaOH. Application of solid-state N-15 NMR is more problematic, especially at natural abundance levels. Despite the limitations imposed by sampling issues and the low sensitivity of NMR, effects of forest management on OM quality have been detected. Analysis of baseline OM properties can also be helpful in predicting site sensitivity to certain management practices, or in formulating guidelines for emulating natural disturbance or preserving biodiversity. In coastal forests of Vancouver Island, British Columbia, for example, the soil humin fraction was less decomposed in plots with N fertilization. In the same region, studies of chronosequences (recent regeneration to old-growth in age) showed little effect of a single harvest on SOM quality, as assessed by solid-state C-13 NMR. This was mainly due to the legacy of centuries of only natural disturbance prior to recent onset of logging. There was a greater influence of climate, mainly higher precipitation on west-coast vs. east-coast sites, which was also reflected in large differences in the availability and organic composition of soil P, and its potential sensitivity to increased harvesting. Harvesting is increasing in the boreal forest where many sites have thick forest floor, poor drainage, and historically have been disturbed by stand-replacing fires. Again, insights into the natural ecology and SOM characteristics can contribute to developing appropriate management regimes. For example, careful preservation of forest floor may not always be appropriate for long-term site productivity. Increasing interest in the C sink potential of managed forests requires improved understanding of the mechanisms that regulate C storage in a wide range of soils, including those with little mineral component. Radiocarbon analysis reveals C dynamics, and makes the connection to turnover times of modelled C pools, while molecular-level analysis of specific compounds provides the details associated with C transformation and stabilization. Natural abundance of C-13 and N-15 are also linked to ecosystem function and decomposition pathways. NMR provides a starting fingerprint of OM composition, but to have real usefulness for these large issues of forest management and C storage, it needs to be applied in an interdisciplinary team approach, and in conjunction with other chemical, isotopic and spectroscopic techniques.

Back to SCE Evaluating Management Impacts on Forest Soils - Oral
Back to WCSS

Back to The 18th World Congress of Soil Science (July 9-15, 2006)