Decomposition and nutrient release from leaf litter of Eucalyptus globulus grown under different water and nutrient regimes

Forest Ecology and Management 171 (2002) 31–41

Decomposition and nutrient release from leaf litter of Eucalyptus globulus grown under different water and nutrient regimes C. Ribeiroa, M. Madeirab,*, M.C. Arau´joc a

Escola Superior Agra´ria de Coimbra, Bencanta, 3040 Coimbra, Portugal Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisbon, Portugal c Celulose Beira Industrial (CELBI) S.A., 3081-853 Leirosa, Portugal

b

Abstract Leaf litter from stands of Eucalyptus globulus Labill., of varying chemical composition was used to study decomposition rates and the release of nutrients (N, P, S, K, Ca, Mg). Leaf litter from control plots, which received no additional fertiliser or water (C), from fertilised plots (F), from irrigated plots (I) and from irrigated and fertilised plots (IL and ILN) was used. IL was collected during June–July and ILN during September. The C, F and I contained lower N and P than the IL and ILN, and had also a lower lignin content. Litterbags were used to study decomposition during a period of 643 days. The remaining ash free weight after 643 days of incubation ranged from 52.8% in C leaf litter to 47.9% in ILN leaf litter. The decomposition rates of leaf litter were 0.37 per year for C and 0.42 per year for ILN. Decomposition rates and weight losses were not significantly different among leaf litters, irrespective of different N and P concentrations, C/N ratios and lignin/N ratios. The concentration of N, S, and Ca in all substrates studied were higher at the end of the incubation period than at the beginning whereas the concentrations of K and Mg were lower. P concentration was lower at the end than at the beginning of the experiment, except for C leaf litter. The increase in N concentration was inversely correlated to its concentration in the original leaf litter. During the early decomposition stages (133 days), release of N, P and Ca was positively correlated to their initial concentration. Except for S and Ca, the amount of N, P, Mg and K remaining at the end of the experiment was similar to that determined after the early decomposition stages. The improvement of leaf litter quality through fertiliser application did not increase decomposition rate, but the release of N and P was enhanced. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Eucalyptus; Leaf litter; Decomposition; Nutrient dynamics

1. Introduction The decomposition of leaf litter is an important part of the nutrient cycling in forests. Amount of nutrients delivered by annual litterfall to the soil through decomposition is a great importance factor for sus* Corresponding author. E-mail address: [email protected] (M. Madeira).

tainable forest production and provides an index of forest productivity (Attiwill and Leeper, 1987). The quantification of these amounts is especially important in plantations of fast-growing trees grown as short rotation coppiced stands, e.g. Eucalyptus globulus Labill. in Portugal. For better management of such systems it is therefore important to evaluate the influence of the litter characteristics on decomposition and nutrient release.

0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 5 9 - 0

32

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

The present study was part of a larger project investigating the effects of fertilisation and water supply on the growth of Eucalyptus trees (Madeira and Pereira, 1991). E. globulus trees receiving irrigation, produced more litter than those which were not irrigated, irrespective of fertiliser supply (Madeira et al., 1995). Additionally, trees supplied with fertilisers showed higher N and P concentrations in their leaf litterfall irrespective of irrigation (Madeira et al., 1995). Such an increase of N and P concentrations, resulting in improved leaf litter quality, has generally been considered to increase rates of leaf litter decomposition, especially in the early stages of the decay (Edmonds, 1980; Melillo et al., 1982; Berg, 1986). Nevertheless, these results have not always been found. For example, Will et al. (1984), Granhall and Slapokas (1984) and Prescott (1995) did not find any increase in decay rates with an increase of N concentration in needle litter or leaf litter of several tree species which received fertilisers. The decomposition of Eucalyptus leaf litter from plots subjected to water and nutrient supply was compared to test the hypothesis that an improvement in the chemical composition of litter from fertilised plots may result in a more rapid decomposition. In order to avoid the confounding influences of chemical litter quality and microclimatic variations due to treatments, leaf litter was incubated on a site which had similar conditions of moisture and temperature. The aims of the present study were (a) to describe the influence of changes in leaf litter chemical composition due to nutrient and/or water supply on litter decomposition rates, and (b) to determine the influence of the leaf litter chemical composition on the dynamics of nutrient release. The data related to the influence of water and/or nutrients supply on the amount of leaf litterfall and its nutrient content were published elsewhere (Madeira et al., 1995).

2. Materials and methods The experiments were conducted in the Furadouro area, Central Portugal (398210 N, 98240 W) at an elevation of 86 m. The climate is Mediterranean with maritime influence. At Caldas da Rainha, 12 km away from Furadouro area, the mean annual temperature is 15.2 8C; the maximum mean monthly temperature (19.3 8C) occurs in July/August, while January is the coldest month (11.2 8C) (Reis and Gonc¸alves, 1981). The total annual rainfall averages 607 mm, 75% of which falls between November and March. Drought usually extends from the end of May until the end of September. From April 1993 to March 1994, the 10-day average temperature at the litter layer surface ranged between 23.2 8C in the summer, and 4.0 8C in the winter (Jose´ Alexandre, personal communication). The decomposition study was conducted within three plots ð40 m  40 mÞ of an old E. globulus plantation established in 1965. Trees spacing was 3 m  3 m. The plots had no understory vegetation. The forest floor litter layer was 3–4 cm thick with 34.2 Mg ha1 in the L layer and 24.6 Mg ha1 in the F/H layers. The soils are derived from sands overlying sandstones (Haplic Arenosols) (FAO, 1988) (Table 1). For this study leaf litter was collected from the experimental plots of E. globulus, established in March 1986, which received a combination of fertilisation and irrigation treatments as described in detail by Pereira et al. (1989) and Madeira and Pereira (1991). Leaf litter was collected in 1991 from the following treatments: (C) control plots which received no fertiliser or irrigation except the starter fertiliser (31 kg N ha1, 26 kg P ha1, 41 kg K ha1) added at the time of planting; (F) fertilised plots which received solid fertiliser broadcasted twice per year (March and October); (I) irrigated plots which were irrigated daily

Table 1 Characteristics of the soils of the Eucalyptus stand where the experimental decomposition studies were carried out Depth (cm)

pH (H2O)

Org C (g kg1)

Kjeldhal N (g kg1)

Ca Mg K Na Al P (cmol (þ) kgl) (cmol (þ) kgl) (cmol (þ) kgl) (cmol (þ) kgl) (cmol (þ) kgl) (mg g1)

0–10 10–20 20–40 40–60

4.55 4.73 4.99 5.20

20.7 16.1 12.4 7.0

0.84 0.74 0.58 0.36

0.29 0.14 0.07 0.05

0.25 0.19 0.15 0.11

0.15 0.16 0.16 0.13

0.16 0.18 0.16 0.16

1.45 1.60 1.46 1.04

3.3 2.9 1.8 0.9

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

to maintain the soil close to field capacity (near 80%) during the potential drought period; (IL) irrigated plus liquid fertiliser plots which received irrigation as in I plus weekly addition of fertiliser in the irrigation water. The total amounts of N, P and K applied during the experimental period were, respectively, 703, 225 and 619 kg ha1 in F treatment, and 921, 321 and 876 kg ha1 in IL treatment. The leaf litter collected, for quantification purposes (Madeira et al., 1995), in C, F, I and IL plots from beginning June till middle of July 1991 was used for this study. This period coincided with the peak of leaf litterfall (especially under rainfed conditions) and with a wide range of nutrient concentrations in leaf litter due to nutrient withdrawal prior to leaffall (Madeira et al., 1995). Leaf litterfall from IL treatment collected in September (ILN) was also included in the experiment, in order to have a wider range of N concentrations. No rain events occurred during the period of leaf litter collection. The leaf litters were air dried and kept at room temperature till the experiment began. Their dry mass was determined after drying at 85 8C. Ten subsamples of each leaf litter type were collected and mixed. From the mixture five subsamples were taken and analysed for their chemical composition. The decomposition of leaf litter was determined using the nylon mesh bag technique. Litterbags were 25 cm  10 cm with a mesh size of 1 mm. Five to 6 g of leaf litter, dried at 85 8C, were enclosed in each bag which represented the weight per unit area in the range of annual litterfall in Eucalyptus plantations in Portugal (Madeira et al., 1995; Cortez, 1996). Litterbags were placed on the L horizon by pinning them to the ground. Within the experimental area 15 randomised sub-plots were chosen for placement of the litterbags. At each sub-plot, nine litterbags of each of the five experimental leaf litters were placed in late December 1992 close to each other in one row. The rows were approximately 50 cm apart. Bags were sampled every 3–4 months. At each sampling occasion one litterbag of each leaf litter type was collected from the 15 subplots. The litterbags were placed in paper bags and transported to the laboratory. Leaves were cleaned of any ingrowth material and humus particles. After drying at 85 8C they were weighed individually and then pooled for each leaf litter type to make one composite sample for analysis with two replicates.

33

Leaf litter was ground to pass a 1 mm screen before the chemical analysis was carried out. The mineral nutrients were determined after ashing (6 h at 450 8C) and dissolving ash in HCl. Ca, Mg and K were measured by atomic absorption spectrofotometry, and P by the ascorbic acid method (Watanabe and Olsen, 1965). Total nitrogen was determined using Kjeldahl digestion (Digestion System 40, Kjeltec Auto 1030 Analyser). Total S was analysed turbidimetrically as BaSO4-precipitate (Cottenie et al., 1982) after an acid wet oxidation in HNO3 þ HClO4 . Carbon was assumed to be 50% of the ash-free dry mass. The extractable amounts (water, ethanol and dicloromethane) were determined according to the methods given by TAPPI (1975, 1976). The lignin content (Klason) was determined by procedure described by Effland (1977). Decomposition rates were calculated from ash-free dry mass remaining using a single negative exponential decay model X=X0 ¼ ekt , where X/X0 is the fraction mass remaining at time t, t the time elapsed in years and k the annual decay constant (Olson, 1963). At each sampling time, the mean values for ash-free remaining dry mass in the different leaf litters were compared by using analysis of variance (ANOVA). One-way ANOVA test was applied to compare the leaf litter substrates as well as the respective decomposition rates. The Turkey’s multiple range was used to test differences between leaf litters. The level of significance used in tests was P < 0:05. Concentration and content of nutrients in the decomposing leaf litters were not subjected to statistical analysis because the chemical analysis was performed on composite samples.

3. Results The concentration of extractables (water, ethanol and dicloromethane) was similar for all leaf litters (Table 2). The I, IL and especially ILN leaf litters had higher content of lignin than the others. The C, F and I leaf litter substrates showed similar concentration of N and S which were lower than those in IL and ILN leaf litter. The leaf litter from fertilised plots, F, IL and ILN, had higher P concentrations than the unfertilised ones. It is evident that the ILN leaf litter substrate had the highest levels of N, P and S. Concentrations of Mg

34

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

Table 2 Concentration (mg g1) of organic matter (OM), mineral elements, extractables in water (EW), ethanol (EE) and dicloromethane (ED) and lignin (LI) (Klason) in the Eucalyptus leaf litter substrates used in the decomposition experimenta Litter type

OM

N

Ca

Mg

K

P

S

Extractables EW

C F I IL ILN

938 932 940 947 943

5.38 5.43 5.23 8.05 10.71

a a a b c

16.5 19.2 16.3 13.4 14.4

a a ab b b

1.50 1.60 1.50 1.70 1.50

a a a a a

3.80 3.70 3.90 3.50 3.60

a a a a a

0.23 0.40 0.35 0.48 0.78

a bc c b d

0.57 0.59 0.57 0.68 0.82

a ab a b c

121 111 110 106 114

LI

EE a a a a a

126 148 159 159 140

C/N

LI/N

87 86 90 59 44

38 38 42 27 24

ED a a a a a

186 186 171 171 150

a a ab ab b

206 208 217 220 254

a a ba bc c

a a a b c

a a a b b

a The litter substrates were from: (C) control plots; (F) plots receiving solid fertiliser supply; (I) irrigated plots; (IL) plots receiving both irrigation and liquid fertiliser. The C, F, I and IL substrates were collected during peak litterfall (beginning June to middle of July). The ILN substrate was collected from IL plots during September. Values in the same column with different letters are statistically different (Turkey’s test, P < 0:05).

and K were similar in all litter samples. The IL and ILN leaf litters had the lowest Ca concentration of all used in this study. The pattern of weight loss was similar for all leaf litters, and showed a fast weight loss (20–25% of initial weight) during the early (133 days) decomposition stages (Fig. 1). That period was followed by negligible weight loss in the summer months. The weight loss determined during the early decomposition stages was about 40–50% of that for the whole

experiment period. The C leaf litter showed the highest values for residual weight during the experimental period, with occasional significant differences to the others, e.g. IL and I leaf litters at 239 and 425 days after litter placement (Table 3). The residual dry weight at the end of the experiment (643 days after field placement) was highest (52.8%) for the C leaf litter and lowest (47.9%) for the ILN leaf litter, while F, I and IL experimental leaf litters showed intermediate values (50.3, 48.8 and 48.8%, respectively)

Fig. 1. Relative weight of Eucalyptus litter remaining (ash-free basis) during the experimental period. The leaf fitter substrates were from: (C) control plots; (F) plots which received solid fertiliser supply; (I) irrigated plots; (IL) plots receiving irrigation and liquid fertilisers. Values are means ðn ¼ 15Þ and bars are 1 S.D. WT: winter; SP: spring; SM: summer; AT: autumn.

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

35

Table 3 Weight remaining (average and standard deviation) after 239, 425 and 643 days, and decomposition rates after 317 and 643 days of decomposition of leaf litter substratesa Litter type

Weight remaining (% of initial) 239 days

425 days

Decomposition rate 643 days

317 days

643 days

k (per year) C F I IL ILN

78.2 74.6 72.1 73.9 76.1

(3.7) a (2.5) ab (2.6) b (3. 1) b (3.6) a

60.3 58.7 55.3 55.6 56.4

(5.4) (3.0) (5.5) (2.6) (3.5)

a a b b a

52.8 50.3 48.8 48.8 47.9

(4.8) (5.6) (7.2) (7.3) (3.8)

a a a a a

0.42 0.45 0.49 0.50 0.47

(0.05) (0.06) (0.09) (0.07) (0.07)

a a a a a

Adj. R2

k (per year)

0.94 0.94 0.95 0.94 0.94

0.37 0.38 0.39 0.41 0.42

(0.06) (0.06) (0.07) (0.07) (0.04)

Adj. R2 a a a a a

0.94 0.94 0.94 0.93 0.96

a Abbreviations as in Table 2 for leaf litter substrates. Values in the same column with different letters are statistically different (Turkey’s test, P < 0:05).

(Table 3). The values, however, were not significantly different. The single exponential model (Olson, 1963) showed a good fit for all leaf litters (Table 3). The decomposition rate after 317 days of decomposition, was lowest for C leaf litter (0.42 per year) and highest for IL leaf litter (0.50 per year), but they were not significantly different. At the end of the experimental period, C leaf litter gave the lowest value (0.37 per year) and ILN leaf litter the highest (0.42 per year). F, I and IL leaf litters showed intermediate values (0.38, 0.39 and 0.41 per year, respectively). The k values at the end of the experiment were also not significantly different among litters (Table 3). During the first 133 days of decomposition, the N concentration of the leaf litter with the highest N content decreased markedly whereas the leaf litters with lower N (C, I and F) showed no change or an increase. After 133 days there was an increase in the N concentration in all leaf litters. The increase was more pronounced in the C, F and I leaf litters than in IL and ILN leaf litters (Fig. 2a). N concentrations increased from 5.6–5.8 to 10.4–11.0 mg g1 in C, F, and I litters, and from 8.5 and 11.3 mg g1 to 12.3 and 13.8 mg g1 in IL and ILN litters, respectively. This increase was negatively correlated with the initial N concentration in the leaf litters (r ¼ 0:98; P ¼ 0:002). The concentration of P in the leaf litters decreased during the first 133 days of decomposition, except for litter from C (Fig. 2b). The decrease was highest in ILN leaf litter and lowest in I and F leaf litters, which were also low in initial P concentration. After that period the P concentration increased in all the substrates.

However, except for C leaf litter, the P concentration in decomposing substrates was lower by the end of the experiment than the initial values, especially in the ILN leaf litter. The S concentration in litter substrates showed negligible changes during the initial 239 days of decomposition (Fig. 2c). After that period a substantial increase in S concentration (0.56–0.64 mg g1) was observed in all leaf litters, and the ILN leaf litter showed the highest concentration values along the experiment period. A different trend was observed for the concentration of Ca which increased during the first 315 days of decomposition with a slight decrease afterwards, reaching higher values overall than in the initial substrates (Fig. 2d). Concentrations of K and Mg in leaf litters declined strongly during the first 133 days of decomposition (Fig. 2e and f). This decrease was particularly marked for K. Concentrations of both Mg and K however started to increase again at the end of the experimental period. Net amount of N and P was released from the leaf litters during first 133 days of incubation (Fig. 3a and b), which did not follow the pattern of weight loss. The amount of N released ranged from about 11% (C leaf litter) to about 40% (in ILN leaf litter) of total N content of leaf litter. The amount of P released was higher and ranged from 23 to 66% in the leaf litter of C and ILN, respectively. After the early losses of N and P, no further losses of N and P were observed. Amounts of N and P released during the early decomposition stage were positively correlated with the corresponding initial contents (r ¼ 0:98 and 0.90

36

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

Fig. 2. Mean concentrations (ash-free basis) of (a) nitrogen, (b) phosphorous, (c) sulphur, (d) calcium, (e) magnesium and (f) potassium in Eucalyptus leaf litter during experimental period. Abbreviations as in Fig. 1.

for N and P, respectively; P < 0:05). Such a correlation was also observed for the amounts of N and P released during the whole experimental period (r ¼ 0:98, P < 0:01 for N; r ¼ 0:85, P < 0:05 for P). During the early decomposition stages, S was released from decomposing leaf litters in amounts (22.0–28.5% of total) similar to that of weight loss (Fig. 3c). The amount of S increased in all leaf litters, during the remaining period of decomposition. At the

end of the experiment, the absolute amount of S was higher (83.6–97.7% of total) than the amount measured at the end of the first stages of decomposition (71.5–78.0%) (Figs. 2 and 3). Such an increase of S was negatively correlated with its initial concentration (r ¼ 0:97; P < 0:05) of leaf litters. As for N and P, net release of S at the end of the experimental period was positively correlated with its initial concentration in leaf litters (r ¼ 0:99; P < 0:01).

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

37

Fig. 3. Changes in absolute amounts of (a) nitrogen, (b) phosphorous, (c) sulphur, (d) calcium, (e) magnesium and (f) potassium in Eucalyptus leaf litter during experimental period. Abbreviations as in Fig. 1.

The net release of Ca occurred after the 239 days of decomposition and at the end of study was similar (31.0–40.6%) among litter substrates (Fig. 3d). The initial loss of Ca (2.2–18.5%) was much smaller than for other nutrient elements. This loss, however, was positively correlated with the initial Ca concentration in litter substrates (r ¼ 0:89; P < 0:05).

The Mg release was of similar magnitude in various substrates (Fig. 3e). The Mg loss occurred mostly during the first 133 days of decomposition (37.9– 53.0%). At the end of experiment net release of Mg was 48.0–57.2% of initial amounts. Potassium was released much more rapidly than either Ca or Mg. Loss of K (Fig. 3f) was similar in all studied litters, and

38

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

accounted for 85.0–88.6% during the first 133 days of decomposition.

4. Discussion Several studies have indicated that the chemical and biochemical quality of litter affects dry-weight changes during decomposition (Edmonds, 1980; Melillo et al., 1982; Berg, 1986; Harmon et al., 1990; Vitousek et al., 1994). Besides differences in N, P and S concentrations, the various leaf litters studied showed significant differences in C/N ratio and lignin/N ratio. The C/N ratio varied from 44 in ILN leaf litter to 90 in I leaf litter. The lignin/N ratio (Table 2) varied from 24 in ILN to 42 in I litter. Despite the differences in the quality indices, the studied substrates showed similar weight loss rates. The initial lignin/N ratio of the substrate has been considered as an important factor controlling the earlier phases of decomposition (Melillo et al., 1982; Harmon et al., 1990). Harmon et al. (1990) reported differences in decomposition rates when such an index was below 20. The lignin/N values of leaf litters in this study (24–42) covered the range which did not presumably affect their decomposition rates. The C/N ratio has been reported as a good predictor of leaf litter decomposition especially when the litter substrates have a low lignin content or show a wide range of lignin content (Taylor et al., 1989). The range of initial N levels in the leaf litters in this study (5.2– 10.8 mg g1) was lower than that reported by Taylor et al. (1989), and had relatively high lignin contents without showing a wide range of them. Therefore the leaf litters did not show characteristics which had allowed a marked effect of C/N ratio on decomposition rates. Our results do not agree with the model suggested by Berg (1986), who proposed that the level of nutrients (N, P and S) in the litter may control its decomposition rate in the early phase (Berg, 2000). Berg et al. (1987) and Cotrufo et al. (1995) reported that litter mass loss was positively correlated with N concentration in litter only during the first year of decomposition. This relationship was not observed in our experiment. In fact, the leaf litter from I treatment, with the lowest N concentration, showed higher weight loss during the early decomposition phase than

the leaf litter from C treatment which had similar N concentration. The ILN leaf litter with the highest N concentration among leaf litters also showed low decomposition rates (Table 3). Our results agree well with those reported by Will et al. (1984), Berg and Tamm (1991), Prescott (1995) and Granhall and Slapokas (1984) where no significant increase in decomposition rates was observed when high N in leaf litter were achieved by fertiliser application. According to Berg and Tamm (1991) the early decomposition phase can also be related to the content of water soluble substrates. The studied leaf litters had similar and relatively low amount of water soluble substances, which may be another factor for the similar pattern of their weight loss. Harmon et al. (1990) stated that the loss of readily leachable fraction during the early decomposition phase, may decrease the substrate quality for subsequent decomposition processes by removing the soluble labile carbon. However during the early decomposition stages, i.e. first 133 days of incubation, N and P were released in higher proportion from leaf litter with higher initial concentration of these elements. This resulted in a considerable narrowing of the range of nutrient contents of leaf litters studied. Such a pattern may approximate the substrate behaviour during the remaining period of decomposition (Fig. 2) and may be a factor in reducing differences in the decomposition of the studied leaf litter types. Our results agree well with those reported by Prescott et al. (1993). In the absence of any difference in vegetation and climatic conditions, decomposition rates were similar for the studied substrates irrespective of the N and P concentration, C/N ratios and lignin/N ratios. The range of N and P concentrations in the experimental leaf litters was similar to that found in leaf litter of E. globulus plantations in Portugal (Madeira et al., 1995; Cortez, 1996). Additionally, the decomposition rates determined in the present study are similar to those determined in other studies with E. globulus leaf litters whose N concentrations varied from 5.3 to 11.1 mg g1 (Madeira, 1986; Serralheiro and Madeira, 1991; Cortez, 1996; Jones et al., 1999), which had the same range in concentrations as in the experimental leaf litters. Therefore, it is unlikely that any increase in N and P concentrations of leaf litter through fertiliser application will increase

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

decomposition rates of leaf litter for E. globulus plantations in Portugal. The fast weight loss observed for the leaf litters of this study during the early decomposition stages (i.e. first 133 days of decomposition) is similar to that reported by Cortez (1996), for E. globulus leaf litter, and by Granhall and Slapokas (1984), Berg (1986), Harmon et al. (1990), Berg and Tamm (1991) and Slapokas and Granhall (1991) for several species. Such a pattern was also observed in a lysimetric study with E. globulus leaf litter (Madeira, 1986), in which the concentration of carbon in the leachates was high especially during the first 3–4 weeks of decomposition and decreased steadily during 3–4 months before reaching almost constant values. The rapid release of K during the early decomposition stages and the slow leaching observed for Mg have also been reported for other tree species (Lousier and Parkinson, 1978; Granhall and Slapokas, 1984; Blair, 1988a). The low proportion of K remaining early in the incubation (Fig. 3f) is consistent with the high mobility of K due to lack of incorporation of this element into organic structures (Marschner, 1995). The slower release of Ca as compared to K and Mg was also reported by O’Connell (1988) and Cortez (1996) for leaf litter of E. diversicolor and E. globulus, respectively, and by Lousier and Parkinson (1978), Granhall and Slapokas (1984) and Blair (1988a) for other leaf litter types. The release of Ca during the early decomposition stages depended on its initial concentration in leaf litters as found by Granhall and Slapokas (1984) for other forest species. Low mobility showed by Ca during the first half of the experiment period (Fig. 3d) indicates that its release is more dependent on biotic activity than on leaching. This is consistent with the importance of Ca as a structural component in cell walls of a leaf. Our data on N release dynamics are consistent with other studies that have demonstrated an initial loss of N during the early stages of decomposition (Granhall and Slapokas, 1984; Slapokas and Granhall, 1991; Cortez, 1996). Additionally our data indicated N was lost from leaf litters even when they had low N concentration. In spite of differences in initial N concentration, the leaf litters of this study showed negligible variation in N content after the early decomposition stages, which is in agreement with their N concentration levels. This increment was then

39

positively correlated with weight mass loss (r ¼ 0:96; P < 0:05). As N loss did not equal weight loss, C/N ratio of studied samples decreased till the end of the experiment. Very little N is released from an organic substrate till the C/N ratio has been lowered to a critical value of 20–35 (Lousier and Parkinson, 1978; Edmonds, 1980; Whitmore and Handayanto, 1997). By the end of the experiment the C/N ratio in leaf litters still varied from 36 (ILN leaf litter) to 48 (C leaf litter), and thus the critical conditions for N release may not have been achieved in all of them. Compared to N, patterns of P release appear to have varied more widely across the range of leaf litters studied. The fast loss of P observed during the early decomposition stages has also been reported for several leaf litters (Lousier and Parkinson, 1978; Slapokas and Granhall, 1991; Polglase et al., 1992; Prescott et al., 1993; Cortina and Vallejo, 1994; Cortez, 1996). Polglase et al. (1992) reported that the initial P release, by both direct leaching and through microbial biomass, increased with the inorganic P content in litters, and this content increased with application of fertilisers. This could be the reason for the high proportion of P release from IL and ILN leaf litters. Similar to N, the leaf litters of this study showed negligible variation in P content after the early decomposition stages, and showed an increase in P concentration. These changes in the P content of the leaf litter with decomposition suggested that the C/P ratio exhibited by leaf litters at the end of the experiment (872–1234) was distinctly higher than those commonly reported as critical (230– 480) for P mineralisation. The fast release of S observed for the leaf litters studied is not uncommon. Blair (1988b), O’Connell (1988), and Cortina and Vallejo (1994) have reported initial decrease in S content in field incubations. After the first 133 days of incubation, there was an increase in S concentration (2.0–2.1 times) of leaf litters, which was stronger than that observed for N (1.4–1.7) and P (1.3–1.6) (Fig. 2). Moreover, after the first 133 days of incubation S content in the studied leaf litters increased 12.1–19.2% till the end of the experiment. This suggests that the increase in S concentration was related to C loss (weight loss) and immobilisation. Immobilisation of S in litter samples is also corroborated by the decrease of the N/S ratio from 12.5 to 8.8 during the second half of the experiment period.

40

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41

Our data indicate that nutrient mobility varied in relation to leaf litter type as suggested by O’Connell (1988) for other substrates. At the end of the experiment, the overall relative mobility of the nutrients examined was K > Mg > Ca > P > S > N for C leaf litter, and K > P > Mg > N > Ca > S for ILN leaf litter. Differences are mostly based on P and N dynamics in decomposing leaf litter, i.e. the higher the N and P concentrations more rapidly they were released. We may note that nutrient mobility also varied during the experiment period. In Eucalyptus plantations, the flux of nutrients from recently fallen litter to the soil will depend on amount of litterfall and litter characteristics. The annual weights of C leaf litter and IL leaf litter were 2.50 and 3.91 Mg ha1, respectively (average of 2 years) (Madeira et al., 1995). Considering the pattern of nutrient release presented in this study, the amounts of N, P, S, Ca, Mg and K released during the initial stages of decomposition of litter from the C treatment should be 2.21, 0.13, 0.36, 2.81, 1.42 and 8.14 kg ha1, respectively. The amount released from IL leaf litter should be 7.81 (N), 0.90 (P), 0.76 (S), 1.15 (Ca), 2.84 (Mg) and 11.63 (K) kg ha1. If a period of 643 days, i.e. the same of the present study, is considered, the net nutrient release should reach, in the same order as above, 0.55, 0.09, 0.07, 14.48, 1.80 and 8.52 kg1 for C leaf litter, and 9.29, 1.03, 0.18, 21.27, 3.80 and 12.26 kg1 for IL leaf litter. The differences in the release of K, Mg and Ca may be related to amount of leaf litterfall. Besides the differences in the amount of leaf litterfall, differences observed for N, P, and S (at lower extent) are also influenced by the characteristics of leaf litters. Moreover the importance of the nutrient concentration in E. globulus fallen litter, especially if we consider that most of the release of N, P and S took place in a short period of time, should be considered. Decomposition of leaf litter of improved quality implies a higher flux of nutrients to the soil where they may subsequently become available for uptake by E. globulus plantations.

5. Conclusions The increase of N, P and S concentrations in leaf litter of E. globulus through fertiliser application did

not cause significant in weight loss or decomposition rate. However the amount of N, P, and S released during the early decomposition stages in a proportion depended on their initial concentration in leaf litters. In spite of differences in P and N concentrations in the leaf litter, the leaf litters of this study showed negligible variations of N and P contents in the remaining litter after the early decomposition stages. The recycled amount of N, P (and S to some extent) depends on both differences in the amount of litterfall and differences in their nutrient releasing pattern based on the composition of leaf litters.

Acknowledgements The research work was supported by grants from Junta Nacional de lnvestigac¸a˜ o Cientı´fica (JNICT), through the research project PMCT/AGR/593/90. The Celulose Beira Industrial (CELBI), S.A. contributed with field work facilities and technical assistance. The field work was carried out at CELBI’s field station at Furadouro, and the laboratory work was conducted at Instituto Superior de Agronomia. The authors thank Dr. P. Khanna for help in improving the manuscript. References Attiwill, P.M., Leeper, G.W., 1987. Forest Soils and Nutrient Cycles. Melbourne University Press, Melbourne. Berg, B., 1986. Nutrient release from litter and humus in coniferous forest soils—a mini review. Scand. J. For. Res. 1, 359–369. Berg, B., 2000. Litter decomposition and organic matter turnover in northern forest soils. For. Ecol. Manage. 133, 13–22. Berg, B., Tamm, C.O., 1991. Decomposition and nutrient dynamics of litter in long-term optimum nutrition experiments. Scand. J. For. Res. 6, 305–321. Berg, B., Staaf, H., Wessen, B., 1987. Decomposition and nutrient in needle litter from nitrogen-fertilized Scots pine (Pinus sylvestris) stands. Scand. J. For. Res. 2, 399–415. Blair, J.M., 1988a. Nutrient release from decomposing foliar litter of three tree species with special reference to calcium, magnesium and potassium dynamics. Plant and Soil 110, 49–55. Blair, J.M., 1988b. Nitrogen, sulphur and phosphorus dynamics in decomposing deciduous leaf litter in the southern Appalachians. Soil Biol. Biochem. 20, 697–701. Cortez, N., 1996. Compartimentos e Ciclos de Nutrientes em Plantac¸o˜ es de Eucalyptus globulus Labill. ssp. globulus e Pinus pinaster Aiton. Dissertac¸a˜ o de Doutoramento. Universidade Te´ cnica de Lisboa, Lisbon.

C. Ribeiro et al. / Forest Ecology and Management 171 (2002) 31–41 Cortina, J., Vallejo, V.R., 1994. Effects of clearfelling on forest floor accumulation and litter decomposition in a radiata pine plantation. For. Ecol. Manage. 70, 299–310. Cotrufo, M.F., Ineson, P., Roberts, J.D., 1995. Decomposition of birch leaf litters with varying C-to-N ratios (short communication). Soil Biol. Biochem. 27 (9), 1219–1221. Cottenie, A.L., Kiekens, G., Veighe, G., Camerlync, K., 1982. Chemical analysis of plant and soils. Laboratory of Analytical Agrochemistry, State University of Ghent, Ghent. Edmonds, R.L., 1980. Litter decomposition and nutrient release in Douglas-fir, red alder, western hemlock and Pacific silver fir ecosystems in western Washington. Can. J. For. Res. 10, 327–337. Effland, M.J., 1977. Modified procedure to determine acidinsoluble lignin in wood and pulp. Techn. Assoc. Pulp Paper Ind. 60 (10), 143–144. FAO, 1988. FAO/UNESCO soil map of the world, revised legend, with corrections. World Resources Report 60. FAO, Rome (Reprinted as Technical Paper 20, ISRIC, Wageningen, 1994). Granhall, U., Slapokas, T., 1984. Leaf litter decomposition in energy forestry. First year nutrient release and weight loss in relation to the chemical composition of different litter types. In: Pertu, K. (Ed.), Ecology and Management of Forest Biomass Production Systems. Swedish University of Agricultural Science Report, vol. 15. Department of Ecology and Environmental Research, pp. 131–153. Harmon, M.E., Baker, G.A., Spycher, G., Greene, S.E., 1990. Leaf litter decomposition in the Picea/Tsuga forests of Olympic National Park, Washington, USA. For. Ecol. Manage. 31, 55–66. Jones, H., Madeira, M., Herraez, L., Dighton, J., Fabia˜ o, A., Gonzales-Rio, F., Fernandez-Marcos, M., Gomez, C., Tome´ , M., Feith, H., Magalha˜ es, M.C., Howson, G., 1999. The effect of organic-matter management on the productivity of Eucalyptus globulus stands in Spain and Portugal: tree growth and harvest residue decomposition in relation to site and treatment. For. Ecol. Manage. 122, 73–86. Lousier, J.D., Parkinson, D., 1978. Chemical element dynamics in decomposing leaf litter. Can. J. Bot. 56, 2795–2812. Madeira, M., 1986. Influeˆ ncia dos Povoamentos de Eucalipto (Eucalyptus globulus Labill.) no Solo, Comparativamente aos Povoamentos de Sobreiro (Quercus suber L.) e de Pinheiro (Pinus pinaster Ait.). Doctor Thesis. Instituto Superior de Agronomia, Lisbon. Madeira, M., Pereira, J.S., 1991. Productivity, nutrient immobilisation and soil chemical properties in Eucalyptus globulus plantation under different irrigation and fertilisation regimes. Water Air Soil Pollut. 54, 621–634. Madeira, M., Arau´ jo, M.C., Pereira, J.S., 1995. Effects of water and nutrient supply on amount and on nutrient concentration of litterfall and forest floor litter in Eucalyptus globulus plantations. Plant and Soil 168–169, 287–295. Marschner, A., 1995. Mineral Nutrition of Higher Plants. Academic Press, London. Melillo, J.M., Aber, J.D., Muratore, J.F., 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology 63, 621–626.

41

O’Connell, A.M., 1988. Nutrient dynamics in decomposing litter in karri (Eucalyptus diversicolor) forests of south-western Australia. J. Ecol. 76, 1186–1203. Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology 44, 322–331. Pereira, J.S., Linder, S., Arau´ jo, M.C., Pereira, H., Ericsson, T., Borralho, N., Leal, L.C., 1989. Optimisation of biomass production in Eucalyptus globulus plantations—a case study. In: Pereira, J.S., Landsberg, J.J. (Eds.), Biomass Production by Fast-Growing Trees. Kluwer Academic Publishers, Dordrecht, pp. 101–121. Polglase, P.J., Jokela, E.J., Comerford, N.B., 1992. Nitrogen and phosphorous release from decomposing needles of southern pine plantations. Soil Sci. Soc. Am. J. 563, 914–920. Prescott, C.E., 1995. Does nitrogen availability control rates of litter decomposition in forest. Plant and Soil 168–169, 83–89. Prescott, C.E., Taylor, B.R., Parsons, W.F.J., Durall, D.M., Parkinson, D., 1993. Nutrient release from decomposing litter in Rocky Mountain coniferous forests—influence of nutrient availability. Can. J. For. Res. 23, 1576–1586. Reis, R.M., Gonc¸alves, M.Z., 1981. Caracterizac¸a˜ o Clima´ tica da Regia˜ o Agrı´cola do Ribatejo e Oeste. O Clima de Portugal, Fasc, vol. XXXII. Instituto Nacional de Meteorologia e Geofı´sica, Lisboa. Serralheiro, F., Madeira, M., 1991. Acari colonization of Quercus suber and Eucalyptus globulus litter. In: Dusbcisek, F., Bukva, V. (Eds.), Modern Acarology. Academia/SPB Academic Publishing, Prague/Hague, pp. 353–358. Slapokas, T., Granhall, U., 1991. Decomposition of litter in fertilized short-rotation forests on a low-humified peat bog. For. Ecol. Manage. 41, 143–165. TAPPI, 1975. Water solubles in wood and pulp. Technical Association of the Pulp and Paper Industry T207. TAPPI, 1976. Alcohol–benzene and dichloromethane solubles in wood and pulp. Technical Association of the Pulp and Paper Industry T204. Taylor, B.R., Parkinson, D., Parsons, W.F.J., 1989. Nitrogen and lignin content as predictors of litter decay rates: a microcosm test. Ecology 70, 97–104. Vitousek, P.M., Turner, D.R., Parton, W.J., Sanford, R.L., 1994. Litter decomposition on the Mauna Loa environmental matrix, Hawaii. 1. Patterns, mechanisms, and models. Ecology 75 (2), 418–429. Watanabe, F.S., Olsen, S.R., 1965. Test of an ascorbic acid method for determining phosphorous in water and NaHCO3 extracts from soil. Soil Sci. Soc. Am. Proc. 291, 677–678. Whitmore, A.P., Handayanto, E., 1997. Simulation the mineralisation of N crop residues in relation to residue quality. In: Cadish, G., Giller, K.E. (Eds.), Driven by Nature. Plant Litter Quality and Decomposition. CAB International, Wallingford, pp. 337– 348. Will, G.M., Hodgkin, P.D., Madgwick, H.A.L., 1984. Nutrient losses from litter bags containing Pinus radiata litter influences of thinning, clearfelling and urea fertilizer. NZ J. For. Sci. 13, 291–304.