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a Division of Ecosystem Sciences, Department of Environmental Science, Policy, and Management, 151 Hilgard Hall, University of California, Berkeley, CA 94720
b Pacific Southwest Research Station, USDA Forest Service, 2400 Washington Ave., Redding, CA 96001
* Corresponding author (forsoil{at}nature.berkeley.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Researchers have noted both net losses and net gains in elemental mass as wood decomposes (e.g., Holub et al., 2001). Elemental mass changes varied between species and woody-debris component in Edmonds' (1987) study; with high C/N ratios, N was generally the least mobile element while K was the most mobile, and Mn showed some net gains. Sollins et al. (1987) documented net gains in N, P, and Mg per unit volume of large Douglas-fir logs decomposing on the ground for 80 yr. Means et al. (1992) noted both net gains and losses in Douglas-fir logs. Nitrogen and K were lost throughout the decomposition period, but K losses occurred very early. There were only slight losses or little early changes in P, Ca, and Mg, but net gains at later stages of decay, and of these elements only P showed a loss with even further decay. Busse (1994) studied decomposition of lodgepole pine (Pinus contorta Dougl.) wood and noted net losses in Ca, Mg, and K, but P remained constant. He also noted that downed logs contained <3% of the N, P, K, and Ca, and only about 8% of the Mg in the nutrient pool comprised of the logs, the O horizon and the mineral soil. In a microcosm study of mycelial cords of saprophytic basidiomycete fungi, Wells et al. (1998) demonstrated bidirectional movement of 32P, and showed that nutrients scavenged from soil are essential to the decay process of woody debris, and that nutrient relocation occurs primarily on response to demand within recently colonized resources.
The role of Mn in decomposition has largely been overlooked in past ecological literature, although the recent report of Berg (2000) highlights its role in decomposition of foliar debris. There have been more reports in the plant pathology and wood chemistry literature identifying specific roles of Mn in the degradation of phenolic substructures of the lignin component of wood (e.g., Forrester et al., 1988; Hammel, 1997; Paszczynski et al., 1985; Shortle and Shigo, 1973). Other inorganic elements, including Cu, Zn, Fe, Mo, and Ca, also may have roles in lignin degradation (Kirk and Farrell, 1987).
The decay organisms primarily responsible for the degradation of lignin are the white-rot fungi, in contrast to brown-rot fungi, which destroy the carbohydrate component and leave a residue of almost pure lignin (Preston et al., 1990; Baldock and Preston, 1995; Worrall et al., 1997). White-rot fungi can also be nonselective, resulting in little change in overall composition of wood, even though mass and structural integrity are lost. This nonselective decomposition appears to be more common in small-diameter material and in early stages of decomposition (Preston et al., 2002).
Extracellular Mn-dependent peroxidase (MnP) is formed by white-rot fungi, oxidizing Mn (II) to Mn (III), which diffuses into the wood (Bonnarme and Jeffries, 1990; Blanchette, 1984, 1991; Forrester et al., 1988; Hammel, 1997; Huynh and Crawford, 1985; Paszczynski et al., 1985). Manganese peroxidases are capable of producing hydrogen peroxide from reductants such as glutathione using oxygen as the oxidant. Enzymatically generated oxidized Mn species have an advantage over other much larger enzymes in penetrating lignocellulose complexes because the pores of the cell walls are too small for the large-molecular-weight enzymes to penetrate sound wood (Forrester et al., 1988).
Decomposition by several white-rot fungi can also result in visible black regions of accumulated MnO2 in wood (Blanchette, 1984, 1991). Oxalate has also been implicated with Mn in the process of wood decomposition. Manganese accumulations, along with oxalic acid produced by fungi or oxidative degradation of cell wall components, have been shown to degrade lignin in poplar wood by Hames et al. (1998), and spruce sawdust by Kurek and Gaudard (2000).
Most previous field studies of the decomposition of woody material have inferred age or condition of the sample material that was unidentified at the time of stand origin or treatment, and have generally involved large logs (deMontigny et al., 1993; Edmonds 1987; Lambert et al., 1980; Preston et al., 1990; Sollins et al., 1987). Other studies have been conducted under controlled laboratory conditions (Davis et al., 1994a, 1994b, 1994c; Means et al., 1992). This study differs from these earlier studies as it examines decomposition of both bark and wood of small-diameter debris, and because individually labeled samples were retrieved over a relatively long experimental period of 17 yr in the field. We report on total dry mass loss of bark and wood, changes in the C/N ratio, and changes in the concentration and mass of important inorganic elements (N, P, and Mn) under silvicultural treatments of thinning and N fertilization. Special attention is given to differential gains or losses in N, P, and Mn, and their roles in the decomposition process. This report compliments our earlier one from the same field site in which we documented changes in organic decomposition of wood and bark using solid-state 13C nuclear magnetic resonance techniques (McColl and Powers, 1998). Specific objectives of this study were to determine the short- and long-term decomposition rates, changes in the C/N ratio, and dynamics of N, P, and Mn, of bark and wood of small-diameter debris of California red fir with and without thinning and N-fertilizer treatments.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The mean annual air temperature is 9°C, with winter lows and summer highs averaging -12.4°C and 30.3°C, respectively. Total annual precipitation is about 1000 mm with nearly all falling as snow during October through March. Snow limits access to the site until June.
The surface soil is derived from recent rhyolitic pumice of 50-cm depth, overlying a buried profile derived from till of glacial volcanics. The tentative classification is frigid thapto-xerocheptic Vitrandepts, cindery over medial-skeletal.
Experimental Design and Sampling
In August 1976 (designated as Year 0), twelve 0.06-ha plots were established in a factorial combination of N fertilization and thinning treatments with three blocks per treatment. Thinning levels (no thinning vs. 3.0-m nominal spacing of residual stems) were crossed with urea fertilization at 300 kg N ha-1. Thinning was done from below resulting in an average of 1141 trees ha-1, whereas the Control averaged 16 687 trees ha-1. Felled trees were left on the ground. The four treatments are designated as: Control, Thinned, N fertilized, and Thinned + N fertilized.
Samples of stems and branches (0.5- to 5-cm diameter and 15- to 30-cm length) were cut from thinned trees, air-dried to a stable weight, individually weighed and labeled, then distributed on the forest floor, in each of the 12 treatment plots in the fall of 1976. Agricultural-grade urea (46% N) was broadcast by hand across the fertilizer plots following a light snowfall in the fall of 1976.
Collections of three woody samples were done from each plot after 3 and 17 yr.
Sample Preparation and Elemental Analyses
Samples were air-dried in the laboratory, and then bark was separated from wood of subsamples. Both portions were ground to pass a 1-mm sieve, freeze-dried and stored in a freezer in airtight containers. Just before analysis they were further ground to pass a 425 µm (40-mesh) sieve. All elemental concentrations are on an oven-dried (65°C) basis.
Total C and total N were determined on 5- to 10-mg portions wrapped in 8 by 5 mm tin capsules pressed closed into rough spheres, then concentrations were determined in an autoanalyzer with a combustion temperature of 1022°C (Carlo Erba, NA 1500, Milan, Italy). Concentrations of P and Mn were determined by digesting duplicate 200-mg subsamples in 5 mL of concentrated HNO3 at 140°C for 16 h (Zarcinas et al., 1987) then measuring concentrations on an inductively coupled plasma emission spectrophotometer with a high-dissolved-solids nebulizer (Plasma 40, Perkin-Elmer, Norwalk, CT).
Mass Changes
Total dry mass losses of wood and bark were estimated from masses of the original samples (wood and bark combined) and from subsamples of the same after decomposition, separated into wood and bark components. The proportions of wood and bark in the original samples were estimated from the mean of 20 similar fresh samples taken in 1993 (Year 17) from trees in Control plots. Results of mass losses are expressed as percentages of original mass at Year 0 on an oven-dried (65°C) basis.
Changes in elemental masses (i.e., concentration multiplied by dry mass) at Years 3 and 17 are expressed as percentages of the elemental masses at Year 0.
Statistics
Statistical analyses were made using the Statview 4.01 ANOVA program (Abacus Concepts Inc., Berkeley, CA). Treatment effects were tested by analysis of variance. If treatment effects were significant at P < 0.10, differences between treatments were determined with Fisher's protected least-significant difference (PLSD). Differences between years (i.e., differences between Years 0 and 3, and between Years 3 and 17) were determined using Student's t test.
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RESULTS |
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Variation in total dry mass losses in our study was determined largely by the moisture regime, especially as the summers are warm and dry. Even after 17 yr, we observed some very small branches that were physically intact and with much bark attached. These particular branches were kept relatively dry, suspended in litter above the forest floor, especially in openings and thinned areas throughout the dry summers. Where contact with soil occurred, decomposition was accelerated. Similar observations were made by Busse (1994), Erickson et al. (1985), and Edmonds et al. (1986), who documented decomposition of conifer debris in northwestern forests. In contrast, Baldock and Preston (1995) noted generally more rapid decomposition of small woody debris compared with large, but their Canadian studies were not in a Mediterranean climate like ours where summer drought retards decomposition (Larsen et al., 1981; Powers and Edmonds, 1992).
After 3 yr there were significant (P < 0.10) decreases in total dry mass losses of bark because of N fertilization. Wood dry mass loss increased in all treatments compared with Controls. Nitrogen is a controlling element of wood decomposition by white-rot fungi, with both stimulation and retardation effects having been noted (Blanchette, 1991; Boyle, 1998; Fog, 1988). Results of controlled laboratory experiments suggest that low N levels stimulate lignin degradation by some white-rot fungi, whereas high N stimulates polysaccharide degradation (Blanchette, 1991; Merrill and Cowling, 1966; Levi and Cowling, 1969). As pointed out by Kirk and Farrell (1987), N-limited conditions are natural for white-rot fungi because concentrations of N in wood are typically low. However, there are also conflicting reports of accelerated lignin decomposition because of additions of N in laboratory experiments (e.g., Yoshihara et al., 1987). Fog (1988) made a detailed review of effects of N on organic matter decomposition. He concluded that the negative effect of N is mainly found for organic matter with high C/N ratios (>50/1) such as straw and wood, whereas the positive effect of N is typical for organic matter with low C/N ratios. The negative effect could be explained in part by N blocking production of certain enzymes, at least in Basidiomycetes, thus enhancing breakdown of the most available cellulose and accumulating more recalcitrant lignocelluloses (Fog, 1988).
After 17 yr there were no significant differences in total dry mass losses because of treatments of both bark and wood (Tables 1 and 3). However, we noted differences in C fractions in selected samples from the study site using C13 NMR techniques (McColl and Powers, 1998). We assumed that the C spectra reflected the distribution of the C types quantitatively. Results suggested a one-third decrease of carbohydrate in wood of the Control treatment over 17 yr, but no changes in the Thinned + N fertilized treatment. Controls also contained higher proportions of aliphatic-C in their wood and a lower proportion of O-alkyl C than Thinned + N fertilized samples.
In addition to the possible retardation of decomposition by added N, discussed above, we attributed these differences to microclimates (McColl and Powers, 1998). In warmer, drier, thinned plots (where mean annual temperature of the forest floor was 20.3°C, and annual throughfall was 383 mm) compared with unthinned Controls (where mean annual temperature of the forest floor was 13.6°C, and annual throughfall was 392 mm). Moisture is an obvious major factor affecting wood decay fungi (Boddy, 1983), and low temperatures and high humidity appear to favor delignification of wood by fungi (Blanchette, 1991).
C/N Ratio
Changes in C/N ratios were similar to those for N concentrations because changes in N concentrations were greater than those for C. For bark, there were no treatment effects either at Year 3 or 17 (Table 1). For wood, thinning treatments reduced the C/N ratio at Year 3, but by Year 17 these effects had disappeared (Table 3). For both bark (Table 2) and wood (Table 4), there were no differences in the C/N means between Years 0 and 3, but differences were significant between Years 3 and 17. This underscores the value of long-term decomposition studies.
There has been discussion about the efficacy of the C/N ratio as an indicator of N mineralization in woody debris, and whether there is a single critical C/N at which N is released and becomes available for plant use (Busse, 1994; Edmonds, 1987; Fog, 1988; Herman et al., 1977). Means et al. (1992) found that lignin content was a better predictor of N release from Douglas-fir logs than was the C/N ratio.
Elemental Concentration
There were no treatment effects on elemental concentrations in bark at Year 3 or 17, with one exception (Table 1). The exception was at Year 17 for P concentration where Thinning without N fertilization decreased the P concentration (Table 1), probably because of greater leaching in the more open thinned plots (crowns had closed much more on plots receiving N fertilization). Similarly, for wood there was only one exception, that being at Year 3 where the N fertilized treatment had the lowest N concentration. Differences were not great, but they were a consistent result that we cannot explain.
Comparisons between years for overall means of elemental concentrations revealed the same pattern for bark (Table 2) and wood (Table 4). Concentrations of N and Mn did not change significantly between Years 0 and 3, but increased significantly between Years 3 and 17, whereas P concentrations significantly decreased between Years 0 and 3 but then stabilized through Year 17.
Other studies have revealed both increases and decreases in concentrations of elements as woody debris decomposes (Busse 1994; Edmonds 1987; Means et al., 1992; Ostrofsky et al., 1997; Sollins et al., 1987; Wells et al., 1998). Ostrofsky et al. (1997) noted that temporal trends in wood cation concentration varied among the decay fungi tested in a controlled study with red spruce (Picea rubens Sarg.) blocks incubated for periods up to 8 mo. In western Washington, Edmonds (1987) was one of the first researchers to conduct detailed field studies of nutrient dynamics during decomposition of twigs, cones, and small branches. Edmond's studies lasted 2 to 5 yr, depending on the species. His findings are comparable with our 3-yr elemental data, but his results may not indicate elemental dynamics over longer periods. Also, his branch samples were discs with bark intact, 6 to 10 cm in diameter and 4 cm thick, thus exposing relatively more wood surface than would occur naturally, possibly leading to artificially high decomposition rates.
Of the elements that we examined, all showed declining mass except for Mn, which increased in bark (Table 2) and was stable in wood (Table 4). Sollins et al. (1987) noted that Mn had not been previously measured in studies of log decay. They documented high concentrations (up to 250 µg. g-1) in large logs of three conifer tree species, but at the time they were unable to give a specific metabolic role for Mn that would explain its very high concentrations.
The terms "immobilization" and "release" used by many researchers to describe elemental dynamics in decomposing debris could be misleading, as there is movement of elements both in and out of the decomposing material which results in changes in the concentrations and absolute amounts with time of decomposition. The terms "net gain" and "net loss" are probably more-appropriate terms to use. As noted by Edmonds (1987), a better picture of the dynamics of the nutrients regime is obtained by examining changes in absolute masses of elements over time rather than just elemental concentrations.
Elemental Mass
Changes in elemental mass in bark and wood were not concomitant with weight losses either across treatments (Table 1 and 3) or years (Table 2 and 4). Considering treatment effects on bark, there were no significant (P < 0.10) differences in elemental masses at Year 3 or Year 17 (Table 1). Although small, there were net gains in N mass in the treatments with fertilizer N at Year 3, which may be because of residue of the N fertilizer on the bark. For wood at Year 3, Thinning treatments reduced N-mass loss, and the Thinning + N fertilized treatment reduced P mass loss, but there were no significant effects of the treatments on wood at Year 17 (Table 3).
Comparisons between years show significant differences in mass changes for both branch components and for individual elements (Tables 2 and 4). For bark, loss in total dry mass and N mass were similar over the 17-yr period. Regardless of period, losses averaged between 3 and 4% annually. Phosphorus and Mn content of bark followed a different mobility pattern with most change occurring in the first 3 yr. Average annual mass loss of P was 13% yr-1 between Years 0 and 3, but only 3.9% yr-1 between Years 0 and 17. An opposite pattern emerged for Mn, which accumulated at a rate of 6% yr-1 for the first 3 yr, but averaged a much lesser rate thereafter (Table 2). Mass changes in wood were different. Total dry mass declined steadily for the 17 yr (Table 4), but N declined at an average rate of 7.3% for the first 3 yr, and only 1.8% for the full 17 yr. Phosphorus and Mn followed similar patterns in wood as in bark, with the greatest changes occurring in the first 3 yr. The more noteworthy results, however, are the net gains in Mn mass for both bark and wood (Tables 1– 4). Such net gains could come from the soil or forest floor. Presumably, Mn is transported from the surrounding soil and organic matter to the decaying woody debris by white-rot fungi decomposing lignin.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication May 24, 2002.
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