Loss of organic matter, elements, and organic fractions in decomposing Eucalyptus microcarpa leaf litter

J. MAHESWARAN A N D P. M. ATTIWILL' School of Botany, University of Melbourne, Parkville, Victoria, Australia, 3052

Received January 29, 1987

MAHESWARAN, J . , and ATTIWILL, P. M. 1987. Loss of organic matter, elements, and organic fractions in decomposing Ericalyptus r~zicrocarpa leaf litter. Can. J . Bot. 65: 2601 -2606.

The losses of organic matter, elements and organic fractions during the decomposition of Eucalyptus microcarpa Maiden leaf litter were measured in litterbags. The concentrations of nitrogen and phosphorus in litter increased for most of the decom- position period. At the end of 15 months the amounts of elements lost were in the order K > Na > Mg > P > N > Ca. A simple method to determine the different organic fractions according to the degree of decomposability and using a small quantity of litter (0.1 -0.5 g) was developed. The mass loss during the initial 3 months was most closely related to the amount of light organic fraction in the litter, while during the final 12 months the mass loss was most closely related to the harder organic fractions in the litter.

MAHESWARAN, J., et ATTIWILL, P. M. 1987. Loss of organic matter, elements, and organic fractions in decomposing Eucalyptus rnicrocarpa leaf litter. Can. J. Bot. 65 : 2601 -2606.

Les pertes en matikre organiques, ClCments minCraux et fractions organiques au cours de la dCcomposition de la litikre des feuilles d'Eucalyptus rnicrocarpa Maiden furent mesurees dans des sacs de litikre. Les teneurs en azote et en phosphore ont augment6 durant la plus grande partie de la pCriode de dCcomposition. Aprks 15 mois, les quantitCs d'C1Cments perdus Ctaient dans l'ordre K > Na > Mg > P > N > Ca. Une mCthode simple permettant de determiner les diffirentes fractions orga- niques selon le degrC d'altCration et n'utilisant que de petites quantitCs de litibre (0 , l -0 ,5 g) a CtC mise au point. La perte massale durant les 3 premiers mois Ctait davantage relike i la fraction organique ICgbre de la litibre, alors que durant les 12 mois suivants, elle Ctait surtout relike aux fractions organiques plus dures.

[Traduit par la revue]

Introduction Materials and methods

Several studies on the decomposition of leaf litter have been reported in eucalypt forests of Australia (e.g., Hatch 1955; O'Connell and MenagC 1983; Baker and Attiwill 1985). Most of these studies have concentrated on the loss of organic matter and mobility of inorganic nutrients during decomposition. Investigations on the status of organic constituents in decom- posing litter have received little attention.

Berg et al. (1982) studied in detail the loss of various organic fractions in the needles of Scots pine. They observed that extractive substances were lost rapidly while structural constituents such as noncellulose polysaccharides (hemicellu- loses), celluloses, and lignins decomposed slowly. Although high nutrient levels (Berg and Staaf 1980) and soluble fractions (McClaugherty et al. 1985) stimulate mass loss during the early stages of the decomposition, lignin and related sub- stances are also important in regulating long-term decomposi- tion (Fogel and Cromack 1977). High concentrations of lignin have a negative effect on decomposition rates and are more rate determining than either the C:N ratio or the nitrogen con- centration of the litter (Fogel and Cromack 1977); further- more, lignin-related substances influence the immobilization and release of nutrients (Berg and Staaf 1981).

Determination of organic fractions in litter is tedious and the large number of samples dealt with in decomposition studies poses additional problems for the researcher. In this paper, studies on the loss of organic matter, inorganic elements, and organic fractions of varying decomposability in decomposing leaf litter of Eucalyptus microcarpa have been reported. Methods used by Van Soest (1963) and other workers have been modified to allow a serial extraction on a small quantity of ground litter sample and to evaluate the different decompos- able fractions gravimetrically.

'Author to whom correspondence should be addressed.

Pnnced In Canada i Impnmi au Canada

Site description

The study was based in a 0 . I-ha plot within a mature Eucalyptrts ~~zicrocarpa Maiden forest (open dry sclerophyll type) in the One-Eye State Forest, approximately 100 km northwest of Melbourne, Victoria. The plotwas originally established to study nutrient cycling and nitrogen mineralization patterns (Adams and Attiwill 1986a, 1986b). The forest has a sparse understory, mainly of Acacia pyc- rrantha, A. acinacea, and Cassirlia arcuata. Annual litter fall is 280 g m'2, about 50% of this falling in the summer months, and the ovendry weight of the litter layer is 1250 g m-2 (Adams and Attiwill 1 9 8 6 ~ ) . Because of the small litter fall the soil surface is either bare or covered with a thin litter layer. Altitude of the forest site is 240 m. Average annual rainfall is 570 mm and annual pan evaporation is 1500 mm.

The summer of 1982 - 1983 was the driest on record in southeastem Australia. The drought continued through to the end of February 1983. Rainfall from March onward was more or less average (Fig. I), with the bulk of the rain coming in winter (rainfall for July was 130 mm).

Litterbug study

Leaf litter was collected from the plot during the peak of litter fall (October-January) by hanging litter traps at a height of about 1 m from the ground. The nets were emptied weekly, and the litter was air dried for 3 weeks. About 5 g of the airldried litter was weighed and enclosed in polyvinylchloride mesh bags (20 x 25 cm2) with 7.5-mm mesh size. Similar mesh sizes have been used in a variety of studies in Australia (e.g., Baker and Attiwill 1985) to allow invertebrate activity while minimizing particulate loss (Woods and Raison 1982). The bags were fastened at each end with tape and stapled. Separate sub- samples (n = 5) were dried at 80°C to determine moisture content.

Six locations were randomly selected along the perimeters of the plot. At each location, litterbags were layed over the litter layer and secured with nails. The bags were placed on February 2, 1983, immediately following peak litter fall. Bags (11 = 6 , except after 15 months, when n = 4) were sampled after 1, 3, 6, 9 , 12, and 15 months from the commencement of the experiment.

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CAN. J. BOT. VOL. 65. 1987

6 12 18 T IME (MONTHS1

1983 198L MONTH

FIG. 1. Percentage of initial organic matter remaining (with stan- dard deviations) and monthly rainfall during decomposition of Euca- lyptus microcarpa leaf litter. Continuous curve represents the best fit using the data for the entire 15 months for the model MI = M, . e-nl (Olson 1963). Broken curve represents the fit for the initial 3 months.

Analyses of inorganic nutrients After each collection the contents of the bags were oven-dried at

80°C for 48 h and weighed. Samples were then ground, digested (H2S0,/H20,), and analysed for nitrogen and phosphorus by auto- mated colorimetry, for potassium and sodium by flame emission photometry, and for calcium and magnesium by atomic absorption spectrophotometry.

Analyses of organic fractions The litter was serially extracted and the loss in weight after each

extraction was calculated. "Neutral detergent solution" extracts frac- tions similar to soluble carbohydrates, and "acid detergent solution" extracts fractions similar to noncellulose polysaccharides. Triethylene glycol, which selectively extracts lignin-like substances, was used next. Sulphuric acid (72%) was used after triethylene glycol to extract fractions similar to cellulose. The residue was ignited to account for the remaining cutinous substances.

Extraction with neutral detergent solution (Spalding 1979) From 0.1 to 0.5 g of oven-dried material was weighed in a 15 mL

Corex glass centrifuge tube. Ten millilitres of the neutral detergent solution (NDS) was added to this and the tube with its contents was autoclaved at 121°C for 30 min, then cooled to less than 95°C in approximately 30 min. The tubes were then centrifuged at 5600 rpm for 5 min at 5°C and resulting supernatants were removed carefully by suction. The residue was washed several times with hot distilled H20, until the supernatant was colourless. After each washing the contents were centrifuged and the supernatant was removed by suc- tion. For the final washing 95% ethyl alcohol was used instead of water to facilitate drying. The washed residues were dried at 80°C overnight and the loss in weight was calculated as the light fraction (LF) of the litter.

Extraction with acid detergent solutior~ (Spalding 1979) After treatment with NDS, the oven-dried residue was autoclaved

with 10 mL of acid detergent solution (ADS) in the same tube for 30 min at 121 'C. The contents were then treated and centrifuged as described for NDS. 'The weight loss after ovendrying was recorded as the intermediate fraction 1 (MFl) of the litter.

Extraction with activated triethylene glycol (Edwards 1973) The method described by Edwards (1973) was slightly modified

and the oven-dried residue remaining after ADS treatment was treated with triethylene glycol (Trigol) in the centrifuge tubes instead of in sintered Gooch crucibles. The residue was autoclaved at 121°C for 60 min with 10 mL of Trigol activated with HC1 to give a concentra- tion of 2% HC1. The pressure was released rapidly after autoclaving, the tubes were centrifuged, and the contents were washed with 95% ethyl alcohol to remove all traces of Trigol. Acetone was used in the final two washings. The contents were then ovendried overnight at 80°C and the loss in weight in the residue was calculated as hard frac- tion 1 (HF1).

Extraction with 72% sulphuric acid (Van Soest and Wine 1967) After treatment with Trigol, the residue was mixed with excess

72% sulphuric acid and the contents were allowed to stand in the tubes overnight. The residue along with the acid was then transferred to weighed, sintered Gooch crucibles standing in an enamel pan filled with warm water to about 1 cm. The crucibles were then washed several times with hot water and sucked dry. The residue was oven- dried at 80°C overnight and the loss in weight was recorded as inter- mediate fraction 2 (MF2).

lgnition at 500°C The crucible with the residue remaining after the 72% acid treat-

ment was heated in a "muffle furnace" at 500°C for about 4 h to ignite all remaining organic matter. The loss in weight due to ignition was measured as hard fraction 2 (HF2). The ashed residue left after ignition was assumed to be mainly silica.

Calculatiorzs The amounts of organic matter and the concentration of inorganic

elements and organic constituents remaining in each bag were calcu- lated as proportions of ash-free, oven-dried material. The decay con- stant x was estimated by fitting the model M, = Mo. e-"' (Olsen 1963), where Mo is initial mass of litter and M, is mass of litter at the end of decomposition period t years. The influence of the different organic fractions on the rate of decomposition of organic matter was analysed by simple linear regression.

Results The loss of organic matter was rapid during the early stages

of decomposition. After an initial 33% loss of organic matter during the first 3 months, the rate of decomposition decreased with only 18% lost for the next 12 months (Table 1, Fig. 1). Olson's model (1963) gave H = -0.74 f 0.05 over the entire 15 months. This fit, however, did not account for the initial rapid loss of material during the first 3 months for which H = -1.80 0.11.

The concentrations of nitrogen and phosphorus in the decom- posing litter increased progressively with time (Table 1). In contrast, after the 3rd month the concentrations of potassium, sodium, and magnesium were considerably lower than in the fresh litter.

All elements except calcium were released during the 1st month to various degrees (K > Na > Mg > P > N, Fig. 2). At the end of 3 months there was a small accumulation of nitrogen, although not in excess of the initial level. Amounts of both nitrogen and phosphorus changed little during the rest of the decomposition period and only 22 and 28%, respec- tively, of the initial amounts had been released after 15 months. About 53 % of the magnesium, 69 % of the sodium, and 92% of the potassium was lost during the 1st month and an additional 25, 15, and 2 % (respectively) were lost over the rest of the decomposition period. Calcium accumulated until the end of the 3rd month and a net release was not observed until the end of the 1st year. At the end of 15 months only 12% of

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MAHESWARAN AND ATTIWILL

TABLE 1. Percentage mass remaining of organic matter and concentrations of inorganic and organic constituents (mg g-') in Eucalyptus nlicrocarpa litter after

0 , 3, 9, and 15 months of decomposition. Standard errors are in parentheses

Months

% mass Nitrogen Phosphorus Potassium Calcium Magnesium Sodium Light fraction Intermediate

fraction 1 Intermediate

fraction 2 Hard fraction 1 Hard fraction 2

- , 120 influences the mass loss during the early stages of the decom- z 3 o position. There was an increase in HF2 during the early stages, 1 4 and the regression between HF2 and mass loss was negative.

80 During the final 12 months both MF2 and HF2 influence the t - mass loss, and despite the strong influence of LF, retard the - - u 0 rate of decomposition. $ LO - + Discussion Z 3 0 E Loss of organic matter from leaf litter of E. microcarpa 4

o L 8 12 16 (Table 1) was comparable with losses reported from other TIME I MONTHS) studies in Australia: 40% for E. marginata in 18 months

FIG. 2. Percentages of initial organic matter and elements remain- ing during decomposition of Eucalyptus tnicrocarpa leaf litter. Nitrogen, 0 ; phosphorus, 0; calcium, B, magnesium, 0; potassium, A; sodium, A ; organic matter, 0.

calcium had been released (Fig. 2). LF accounted for 57 % of the initial litter and was reduced to

44% of its initial level in 3 months (Table 1). This accounts for approximately 98% of the organic matter lost during this period. During the 1st month 29% of MF1 was lost; during the rest of the decomposition period the amount of MF1 fluctuated around a mean of 77% of its initial amount. MF2 increased with small fluctuations to 118% of its initial amount in 6 months. HF1 gradually decreased to 46% of the initial amount at the end of 9 months. The amount of HF2 in the litter was always above the initial value, except after 12 months, when the amount was below 100 % . The amount of HF2 increased by 37% in the first 3 months and the increased amount was not completely released at the end of 15 months. A 20% increase in HF1 at the end of 12 months corresponded with losses in MF2 and HF2. The reason for this increase in HF1 is not clear.

The regressions between the amounts of organic matter and the organic fractions remaining were all positive and signifi- cant when analysed with the data obtained for the entire period of decomposition (Table 2). When the data obtained for the first 3 months were analysed separately, the regression with LF was the most significant. The regression with HF2 was negative and significant. The regressions with LF, MF2, and HF2 calculated for the final 12 months were all positive and highly significant. These results suggest that the LF strongly

(Hatch 1955); 50% for E. diversicilor in 18 months (O'Connell 1981); 42% for E. paucifiora in 18 months (O'Connell and MenagC 1983); and 70 -75 % for E. obliqua in 24 months (Baker and Attiwill 1985). Olson's decay constant calculated for the entire period of decomposition did not account for the rapid initial loss and was markedly different from that calculated for the initial 3 months (Fig. 1). The mean residence time for organic matter in this forest is 5.5 years (calculated from Adams and Attiwill 1986a), which can be explained by the slower decay rates after the 3rd month.

In E. microcarpa leaf litter, no increases in the amount of nitrogen or phosphorus were observed (Fig. 2) despite increases in concentration of both these elements during the entire 15-month period of decomposition (Table 1). In most studies increases in the amount of nitrogen have been observed, at least during the early stages of decomposition. Many factors contribute to this increase (Berg and Staaf 1980). Similar increases in phosphorus are common in Australia (e.g., Attiwill 1968; O'Connell and MenagC 1983), where forest soils are generally low in phosphorus by world standards (Wild 1958). In the present study, most of the release of nitrogen and phosphorus was during the 1st month (Fig. 2). The C:N and C:P ratios of the undecomposed litter were 63: 1 and 1720: 1, respectively (Table 3). The critical C:N ratio for mineraliza- tion of nitrogen in forest soils has been reported as 20: 1 to 30: 1 (Lutz and Chandler 1946). In E. microcarpa litter, the C:N ratio was reduced to 39:l at the end of 15 months. The C:P ratio changed little over the 15 months of decomposition and was lowered to 1170: 1 at the end of 15 months. The critical C:P ratio for phosphorus mineralization was found to be 480: 1

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CAN. J . BOT. VOL. 65, 1987

TABLE 2. Coefficients of correlation ( r ) for regressions between the amount of organic matter remaining and the different organic fractions remaining of decompos- ing Eucalyptus microcarpa leaf litter for the initial 3 months, the final 12 months,

and the entire period of decomposition

Initial 3 months Final 12 months Entire period

Light fraction o,gg*** 0.77*** 0 . 9 5 ~ " Intermediate fraction 1 0.56* 0.42 0.49* Intermediate fraction 2 0.07 0.72*** 0.39* Hard fraction 1 0.68** 0.39 0,68*** Hard fraction 2 -0.78*** 0.78*** 0.33* Total intermediate fraction 0.56* 0.77*** 0.59*** Total hard fraction -0.33 0.84*** 0.60***

TABLE 3. C:N, C:P, and N:P ratios in decom- posing Eucalyptus microcap litter. Standard

errors are in parentheses

Months of decomposition C: N C:P N:P

in the Hubbard Brook Experimental Forest (Gosz et al. 1973) and 230:l in a cool temperate aspen woodland in Canada (Louisier and Parkinson 1978). O'Connell and Menage (1983) found that the C:P ratio during the decomposition of E. margi- nata litter was lowered to only 2950: 1 by the end of 18 months and there was no evidence for phosphorus mineralization. They concluded that low phosphorus concentrations and micro- bial demand for phosphorus during decomposition are impor- tant factors controlling nutrient flux in the litter layer.

In the E. microcarpa forest, critical C:N and C:P ratios were apparently not reached since nitrogen and phosphorus were not mineralized, except during the 1st month. Higher concentra- tions of nitrogen and phosphorus in the older litter indicate that there is active microbial immobilization of both nitrogen and phosphorus (Table 1). Despite increasing concentrations of nitrogen and phosphorus in the older litter, C:N and C:P ratios were greater than the critical levels.

Relatively large losses of potassium, sodium, and magne- sium during the first 3 months suggest that large proportions of these elements are associated with the LF fraction of the E. microcarpa litter. Calcium was comparatively well retained in the litter and followed similar trends to that of HF2 during decomposition. This suggests that calcium is closely associ- ated with lignin-like substances of the cell structures. In E. calophylla and E. marginata leaf litter, the release of calcium was similar to that of cellulose (O'Connell and MenagC 1983).

In the present study, HF1 decomposed rapidly compared with MF1, MF2, and HF2 (Table 1). If HF1 represents lignin- related substances, an accumulation of this fraction is expected. It is probable that large amounts of lignin were extracted with ADS or combined with HF2. During the later stages of decomposition, lighter fractions become recalcitrant so that the concentration of HF2 increases with time of decom-

position. HF2, which represents cutin-like substances, was not released until the end of 15 months and this fraction may be rate determining for the decomposition of the litter after the first 3 months. In the needles of Scots pine, noncellulose poly- saccharides were lost without initial accumulation (Berg et al. 1982). Lignins were slow to decompose, with release taking place only after 1.5-2 years. Decomposition of celluloses in Scots pine needles was rapid compared with noncellulose poly- saccharides as a group. Both cellulose and lignin showed accumulation during the early stages of decomposition.

NDS removes 98% of the nutritively available soluble con- stituents in vegetable materials (Goering and Van Soest 1970) including soluble carbohydrates, proteins, organic acids (Van Soest and Wine 1967), small amounts of noncellulose polysac- charides, and pectic substances (Bailey and Ulyatt 1970). The material remaining is mainly cell-wall constituents which depend on microbial degradation for release. ADS when used after NDS extracts acid-hydrolysable carbohydrates such as noncellulose polysaccharides and leaves a residue consisting only of cellulose, lignin, and cutinous substances. Bailey and Ulyatt (1970) found that ADS may not completely solubilize noncellulose polysaccharides and may remove some of the lignin. Goering and Van Soest (1970) described two methods of determining lignin. In both methods the samples were initially treated with ADS. In the first method the material remaining after ADS treatment was extracted with perman- ganate and the weight difference was calculated as lignin. The ash-free residue was considered as an estimate of cellulose. In the second method, the ash-free residue remaining after 72% sulphuric acid extraction for cellulose was calculated as lignin. When the second method is followed after the first, the ash- free residue is calculated as cutin. Edwards (1973) autoclaved the residue remaining after the ADS treatment with activated Trig01 to obtain lignin. Cutin, the residue resistant to all treat- ments except to ignition at 500°C, is resistant to normal micro- bial degradation (Kolattukudy 1980).

All these gravimetric methods of determinations give only an empirical evaluation of various organic constituents. Fur- thermore, these organic constituents extracted do not conform to fixed molecular structures, especially once decomposition proceeds, and continually interchange to constituents with varying decomposability. MF1 and MF2 combined together (MF) are comparable with the insoluble polymer carbo- hydrates (holocellulose) in the litter; HF1 and HF2 together (HF) are comparable with insoluble phenolic substances (largely lignin) (Berg et al. 1984). The amount of MF in E. microcarpa litter decreased by 15% during the 1st month and remained unchanged, with small fluctuations, for the

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MAHESWARAN A N D ATTIWILL 2605

0 L 8 12 16 T I M E (MONTHS)

FIG. 3. Percentages of initial organic matter, light fraction, total intermediate fraction, and total hard fraction remaining during decom- position of Eucalyptus microcarpa leaf litter. Light fraction, a; total intermediate fraction, 0; total hard fraction, H; organic matter, 0.

rest of the decomposition period (Fig. 3). The HF fraction increased by 16% during the 1st month and gradually decreased by 30% during the next 14 months.

The regressions of the amount of organic matter with the amount of M F and HF remaining were highly significant for the final 12 months (Table 2). Berg et al. (1984) showed that, although holocellulose and lignin may decompose at different rates during the early stages of litter decomposition, their decomposition rates become nearly the same after some time: This was evident in decomposing litter of E. microcarpa. After the 3rd month the percentages of initial amounts remaining of both M F and HF approached similar values and were nearly the same after 12 months. The decomposition o f lignin and holocellulose can interact in two ways. Lignin and lignin-like substances may form a resistant shield around the holocellu- losic compounds (Berg et al. 1984); secondly, isolated lignin cannot be degraded without a n available carbohydrate source (Kirk et al. 1976). Towards the later stages of decomposition, soluble carbohydrates become less available and lignin decom- position will depend more on the amount of available holocel- lulose.

The average 570 m m of annual rainfall in E. microcarpa forests of Heathcote is seasonal and most of it falls during late autumn and winter following peak litter fall in summer. Since most of the decomposition is during the first 3 months after litter fall, rainfall plays a n important role in the short-term nutrient cycling. The large LF in the undecomposed litter mainly consists of water-soluble components. This fraction accounts for about 9 8 % of the organic matter lost during the first 3 months and is important for the short-term availability of nutrients to sustain the forest. The M F and HF are associated with the loss of organic matter after the 3rd month and regulate the long-term availability of nutrients.

The use of litterbags to study the decomposition of materials enables only a small quantity of the material to be incorporated within the litterbag. Very little sample is therefore available for chemical analyses towards the later stages of decomposition. The method described in this paper for the analyses of organic fractions requires a small quantity of sample and most of the analyses are accomplished sequentially within a centrifuge tube. Furthermore, a large number of samples can be analysed in a relatively shorter period compared with conventional methods. The method also provides reliable estimates of frac- tions of varying decomposability and the dynamics of these fractions during decomposition comparable with results obtained in other studies.

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