Effects of fertilizer on litterfall and N and P release from decomposing litter in a Pinus radiata plantation

Forest Ecology and Management, 32 (1990) 87-102

87

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E f f e c t s o f F e r t i l i z e r on L i t t e r f a l l a n d N a n d P R e l e a s e f r o m D e c o m p o s i n g L i t t e r in a P i n u s radiata Plantation C. THEODOROU and G.D. BOWEN 1

CSIRO, Division o/Soils, Private Bag No. 2, Glen Osmond, S.A. 5064 (Australia) (Accepted 2 May 1989)

ABSTRACT Theodorou, C. and Bowen, G.D., 1990. Effects of fertilizer on litterfaU and N and P release from decomposing litter in a Pinus radiata plantation. For. Ecol. Manage., 32: 87-102. Application, over a period of three years, of an N: P: K ( 806:178: 366 kg ha - 1) plus trace elements fertilizer to a 12-year-old Pinus radiata D. Don plantation growing on an orthic podzol increased litterfall from 234.6 to 377.0 g m -2, the N concentration in the litter from 0.437% to 0.540% and the P concentration from 0.043% to 0.053%. The rate of litter decomposition as indicated by loss of dry-weight in litter bags in the field was not affected by applying fertilizer to the trees from which the litter was collected; approximately 50% of dry-matter was lost after 2 years. Decomposition was greatest in the first year (30%) and then continued at a slower rate (80% in 6 years). Total N concentration increased from 0.48% to 1.07% in the litter from fertilized trees and from 0.35 to 0.96% in litter from unfertilized trees after 4 years of decomposition. The timing of leaching (0-1 month), immobilization (1-12 months) and release (12-72 months) of N in decomposing litter were similar in both treatments, but over 72 months, 2.6 mg N g - 1original litter was released from fertilized litter compared to only 1.4 mg N g-~ from unfertilized litter.

INTRODUCTION

Mineralization of nutrients from littercan contribute a substantial amount of the annual nutritional requirements of forest plantations (Miller, 1984). There is evidence that fertilization,which is widely practised in plantation forestry (Ballard, 1984), affects the rates of nutrient turnover through litter. However, it is necessary to have quantitative information on how fertilization of stands affects litterfall,decomposition of litter,and the release of nutrients in the litterlayer.There has been littlework on this topic in the Pinus radiata IPresent address: InternationalAtomic Energy Agency, Wagramerstrasse 5, P.O. Box I00, A-1400 Vienna, Austria.

0378-1127/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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C. THEODOROU AND G.D. B 0 W E N

plantations extensively grown in Australia and New Zealand, despite the wide use of fertilizers (Woods, 1981; Will, 1985). Baker et al. (1986) showed by measuring accumulated litter and litterfall that lupin growth and fertilizer, applied in large Amounts over a period of 10 years to a 14-year-old P. radiata on coastal sands, increased litterfall and its nutrient concentration over the control but did not affect the loss of organic matter or of P from litter in the litterbags over 22 months. Nitrogen was immobilized for 22 months in all treatments. To improve our understanding of the effects of fertilization on nutrient cycling in P. radiata plantations, it was necessary to quantify the extent to which fertilization increased the turnover of nutrients through litter. Objectives of this study were to determine the effects of multi-element fertilization, as commonly practised, on the amounts of N and P in litterfall, and on the rates of release of N and P from decomposing needles. MATERIALSAND METHODS

Site description The site was situated in S.E. South Australia (37°43'S, 140°27'E) at an elevation of about 60 m above sea level. It has an annual rainfall of about 720 mm, with a winter/spring maximum and with an average of at least 25 mm every month. The average monthly forest-floor temperature ranges from 10 ° C in winter to 20°C in summer. The aeolian podzolized sandy soil (an orthic podzol (Anonymous, 1978) Uc2.23, Northcote et al., 1975) of the plantation had a particle size distribution at 20-cm depth of 64% coarse sand, 31% fine sand, 3% clay and 0.4% silt. The total carbon content of this soil was 0.9%, total N and P were 0.033% and 0.011%, respectively, bicarbonate-extractable P was 3.9 mg kg -1, and its pH was 5.4. This study was carried out as part of an experiment established by the Woods and Forest Department of South Australia to study the responses of a 12-yearold P. radiata plantation to fertilizer application. The fertilizer trial was established as a randomized block design. Each treatment plot consisted of 48 trees at a spacing of 2.5 m (1600 trees h a - l ) . The treatment plots as well as the blocks were separated by buffer zones of untreated trees. The site was fertilized in May 1973 with N: P: K at the rate of 100: 50: 30 kg h a - 1. In October 1974 and November 1975, N: P: K at the rate of 353: 64:18 plus trace elements was broadcast.

LitterfaU collection Litterfall collection from fertilized and unfertilized trees commenced in August 1977 and continued at monthly intervals until the end of July 1981. Ten litter-collection traps, each with an area of 0.5 m 2, were placed at random within

EFFECTS OF FERTILIZER ON LITTERFALL AND N AND P RELEASE FROM P I N U S

89

two plots of each treatment. The traps consisted of a round metal frame supporting a terylene mesh bag to allow drainage. The traps were emptied monthly and the samples dried at 80 ° C for 72 h. The collections in August to November each year were sorted into male cones and needles; in other months only needles were present. The samples for each month were weighed and analysed for N and P.

Litter decomposition The needle litter for the decomposition studies was collected from December 1977 to February 1978 on hessian sheets placed in the fertilized and unfertilized plots. This litter was air-dried, mixed and placed in 20 X 20-cm terylene bags of 0.15-cm mesh (6 g litter per bag). The bags with the litter were placed on the surface of fertilized or unfertilized plots from which the litter was obtained, and pegged to the ground. Five replicate blocks in each fertilizer plot were used with a buffer zone of 50 cm between each block, with 10 bags per treatment in each block to allow for sequential sampling. The bags within each block were randomly placed on the floor with a space of 20 cm between them. The oven-dry-weight of the litter at zero time was determined by drying 10 extra samples at 80°C for 72 h. Litter bags were placed on the forest floor on 1 March 1978 and harvested sequentially after 1, 2, 3, 6, 9, 12, 15, 24, 48 and 72 months. For the first 15 months, additional bags of litter were placed on the forest floor at 3-month intervals, and collected after 3 months' exposure, each harvest coinciding with a harvest of litter from the original placement (1 March 1978). At harvest, the litter was thoroughly cleaned of any sand particles and other debris, dried, weighed, ground, and analysed for total and mineral nitrogen.

Nitrogen and phosphorus status of the forest floor The fertilized and unfertilized sites used for the measurement of litterfall and litter decomposition were sampled to determine the changes in the N and P status occurring in both the litter layer and soil with fertilization. In each of four plots in the fertilized and unfertilized sites, 5-cm-diameter cores of soil were obtained from the 0-10 cm depth. Three random cores were obtained from each plot. The litter layer, as far as it could be distinguished from the soil layer before disturbance, was separated from the soil and the three samples were bulked before analysis for total and mineral N and for total P in the litter and soil.

Analytical procedures The total N and P of litter and soil was determined by digesting in a Tecator Digestion System, and that of soil in a Kjeldahl flask. The analyses of both were carried out in an auto-analyser (McLeod, 1982). Mineral N in fresh samples of litter (1 g) and soil (10 g) was extracted by shaking with 40 ml of 2M

90

C. THEODOROU AND G.D. BOWEN

KC1 for 1 h. The total mineral N (ammonium and nitrate) was determined colorimetrically (Keay and Menagd, 1970). RESULTS

Litter[aU Fertilization significantly increased the annual litterfall from 234.6 g m -2 (unfertilized plots) to 377.0 g m -2 (fertilized) (Table 1). Litterfall from fertilized trees was significantly greater than from unfertilized trees each month (Fig. 1 ) and although it varied from year to year, the mean litterfall was highest in April (43.4 g m -2 from unfertilized and 50.6 g m -2 from fertilized trees) and lowest in June (18.6 g m -2 fertilized) and August (11.4 g m -2 unfertilized). Seasonally, litterfall peaked in autumn (March to May) and was at its lowest in winter (June to August). In spring the increase of litterfall over that TABLE 1 Seasonal a n d a n n u a l litterfall (g m -2) from fertilized a n d unfertilized Pinus radiata trees over 4 years

Season and year

Unfertilizedsite Needles

Fertilized site

Male cones

Total

Needles

Male cones

Total

62.4 56.7 50.3 92.2 78.8 83.2 118.2 90.4 31.3 24.2 29.8 56.9 28.5 40.9 36.2 57.9

95.4 95.2 87.8 146.4 115.5 111.4 139.2 107.3 45.2 48.7 42.5 71.3 48.0 63.3 42.5 63.2

14.7 3.2 4.6 15.1 29.5 39.7 32.2 44.6

95.4 95.2 87.8 146.4 115.5 111.4 139.2 107.3 59.9 51.9 47.1 86.4 77.5 103.0 74.7 107.8

201.5 205.0 234.5 297.4 234.6

304.7 319.0 312.5 393.5

44.2 42.9 36.8 54.4

Seasonal Summer

Autumn

Winter

Spring

1978 1979 1980 1981 1978 1979 1980 1981 1978 1979 1980 1981 1977 1978 1979 1980

62.4 56.7 50.3 92.2 78.8 83.2 118.2 90.4 24.7 23.6 28.4 50.3 15.4 25.0 22.8 36.4

6.6 0.6 1.4 6.6 13.1 15.9 13.4 21.5

1978 1979 1980 1981 Mean

181.8 188.5 219.7 269.3

19.7 16.5 14.8 28.1

Annual 348.9 361.9 349.3 447.9 377.0

91

EFFECTS OF F E R T I L I Z E R ON L I T T E R F A L L AND N AND P RELEASE FROM P I N U S

[7 Total litterfall from fertilized trees (F) D Total littertall from unfertilized trees (UF) 80

I I

Male cones from fertilized trees Male cones from unfertilized trees

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DECEMBER JANUARY FEBRUARY MARCH

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Fig. 1. Litterfall from fertilized and unfertilized trees collected monthly over the period of 4 years after the application of fertilizer.

of the winter was due to the shedding of male cones (Table 1). This was especially so in unfertilized plots, in which needle-fall was lower in spring than in winter by some 27%. Also, there was a significant increase of 125% in male cone shedding by fertilized over that by unfertilized trees. During the 4 years of sampling, the monthly, seasonal and annual litterfall varied considerably from year to year in both treatments (Table 1, Fig. 1 ). Application of fertilizer significantly increased the concentration of N (Fig. 2a, b) and P (Fig. 3a, b) in freshly fallen litter (needles and male cones) from fertilized trees compared with unfertilized trees. The N and P concentration in freshly fallen needles was lowest in autumn (range 0.30-0.42% for N and 0.028-0.039% for P) and highest in both winter and spring (not significantly different, range 0.37-0.55% for N and 0.033%-0.079% for P (Table 2). The male cone litter from the fertilized site contained greater mean concentrations of N and P (0.693% and 0.075%, respectively) than the needle litter (0.499% and 0.048%; Table 2). The annual weighted concentration of N and P in litter of both types was 24% higher in fertilized than in unfertilized plots (Table 2 ). However, the monthly and seasonal concentration of N and P varied from year to year during the 4 years of sampling.

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C.THEODOROUANDG.D.BOWEN

TABLE2 Seasonal and annual weighted mean concentrations (%) of nitrogen and phosphorus in litter from fertilized (F) and unfertilized (UF) Pinus radiata trees collected monthly over 4 years after fertilizer application Season and year

Nitrogen Needles

Seasonal Summer

Autumn

Winter

Spring

Phosphorus Male cones

Needles

Male cones

F

UF

F

UF

F

UF

F

UF

1978 1979 1980 1981 1978 1979 1980 1981 1978 1979 1980 1981 1977 1978 1979 1980

0.36 0.37 0.42 0.30 0.32 0.30 0.30 0.30 0.39 0.39 0.38 0.37 0.38 0.42 0.40 0.45

0.47 0.50 0.51 0.38 0.40 0.42 0.35 0.40 0.47 0.50 0.54 0.47 0.52 0.55 0.46 0.48

--------0.52 0.80 0.60 0.51 0.47 0.58 0.56 0.62

--------0.63 0.90 0.80 0.55 0.60 0.73 0.68 0.65

0.028 0.034 0.033 0.032 0.028 0.029 0.027 0.033 0.033 0.034 0.035 0.040 0.034 0.034 0.038 0.035

0.037 0.037 0.036 0.037 0.036 0.033 0.031 0.039 0.048 0.047 0.049 0.056 0.044 0.044 0.045 0.045

0.066 0.079 0.057 0.062 0.063 0.055 0.062 0.070

0.081 0.092 0.084 0.068 0.071 0.065 0.066 0.072

1978 1979 1980 1981

0.36 0.37 0.38 0.36

0.47 0.49 0.47 0.43

0.50 0.69 .0.58 0.57

0.62 0.82 0.74 0.60

0.031 0.033 0.033 0.035

0.041 0.040 0.040 0.044

0.065 0.067 0.060 0.066

0.076 0.079 0.075 0.070

Annual

F o r e a c h s e a s o n , a m o u n t s of N a n d P r e t u r n e d t o t h e f o r e s t f l o o r w e r e a p p r o x i m a t e l y t w i c e as g r e a t in f e r t i l i z e d p l o t s as i n u n f e r t i l i z e d p l o t s ( F i g s . 4 a n d 5 ) . N i t r o g e n r e t u r n t o t h e f l o o r b y l i t t e r f a l l in b o t h t h e f e r t i l i z e d a n d u n fertilized plots was similar during summer, a u t u m n a n d spring (range 1.206.38 kg N h a -1 ), b u t l o w e r d u r i n g w i n t e r ( r a n g e 0 . 9 7 - 3 . 3 5 kg N h a -1 ). T h e N r e t u r n t o t h e f l o o r in t h e m a l e c o n e s r a n g e d f r o m 0.08 t o 0.93 kg N h a - 1 in w i n t e r a n d 0.62 a n d 2.90 k g N h a - 1 i n s p r i n g (Fig. 4 ) . S u m m e r a n d a u t u m n N concentrations were greater than winter values by 60-97% in litter from unf e r t i l i z e d p l o t s , a n d b y 3 7 - 5 0 % i n l i t t e r f r o m f e r t i l i z e d p l o t s (Fig. 4 ) . W i t h o n e exception, there were no significant seasonal differences between fertilizer t r e a t m e n t s i n t h e a m o u n t o f P r e t u r n e d t o t h e p l a n t a t i o n f l o o r (Fig. 5); t h e e x c e p t i o n w a s i n f e r t i l i z e d p l o t s , w h e r e t h e s p r i n g P c o n t e n t o f l i t t e r w a s 44%

EFFECTS OF FERTILIZER ON LITTERFALL AND N AND P RELEASE FROM P I N U S

0.7

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Fig. 2. (top): Nitrogen concentration in needles litter from fertilized and unfertilized P i n u s radiata trees. Fig. 2 (bottom): Nitrogen concentration in male cones litter from fertilized and unfertilized Pinus radiata trees. Collected monthly over the period of 4 years after fertilizer application.

94

C. T H E O D O R O U A N D G.D. B O W E N

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Fig. 3 (top): Phosphorus concentrations in needles litter from fertilized and unfertilized Pinus radiata trees. Fig. 3 (bottom): Phosphorus concentration in male cones litter from fertilized and unfertilized Pinus radiata trees. Collected monthly over the period of 4 years after fertilizer application.

EFFECTS OF FERTILIZER ON LITTERFALL AND N AND P RELEASE FROM PINUS

95

0 Nitrogen in total litter from fertilized trees ~ Nitrogen in male cones from fertilized trees Nitrogen in total litter from unfertilized trees ~ Nitrogen in male cones from unfertilized trees (a) Total litter

6

(b) Male cones I LSDforseasonx t ] ferlJlizerinteraction P=O.01 for totallitter P=O.05

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¢--

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Fig. 4. Seasonal return of nitrogen in litterfall to the forest floor from fertilized and unfertilized Pinus radiata trees over 4 years after application of fertilizer.

o,I

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96

C. THEODOROUAND G.D.BOWEN

greater than in winter (Fig. 5). Annually, 112% more N and 102% more P was returned in the litter of fertilized plots than in the unfertilized plots. The return of N and P peaked in the spring in fertilized plots and in the autumn in unfertilized plots (Figs. 4 and 5).

Litter decomposition Dry-weight of litter in litter bags decreased over the 72-month period, and was more rapid in the first year (30-31% loss) of sampling than in the follow100 95 9O 85 8O 75 7O

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I 10

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I 25

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Time (months) from placement

Fig. 6. Percentage residual dry-weight of litter from fertilized and unfertilized Pinus radiata trees decomposing on fertilized and unfertilized sites.

Litter freshly placed at beginning of season I 20

I Litter placed 1 March 1978 (continuous decomposition) ]"

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Fig. 7. Seasonal percentage loss of dry-weight of decomposing litter from unfertilized trees placed on unfertilized site.

EFFECTS OF FERTILIZER ON LITTERFALL AND N AND P RELEASE FROM PINUS

O ....... O /,. ....... A

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97

Nitrogen concentration in litter from fertilized trees Nitrogen concentration in litter from unfertilized trees Amount of nitrogen in litter from fertilized trees Amount of nitrogen in litter from unfer~lized trees LSD for amount of nitrogen in unfertilized litter ~- P=0.05, ~- P=0.0t LSD for amount of nitrogen in fertilized litter

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Fig. 8. Nitrogen content of decomposinglitter in bags in the unfertilized plots at different times of decomposition. ing years (mean annual loss 10-18% ), with little difference between treatments (Fig. 6). Organic-matter loss from litter samples of the original placement was similar in spring, winter and a u t u m n (5-8% of t h a t at the start of the season), but significantly lower in s u m m e r (3% loss) (Fig. 7). Weight loss of litter placed on the forest floor at the beginning of each season was similar in a u t u m n and winter (14% and 15%, respectively), but lower in spring and s u m m e r (both 11% loss; Fig. 7). These losses were g r e a t e r t h a n those recorded for the same seasons in litter that had been on the forest floor 6-15 months prior to sampling (Fig. 7). In the first m o n t h of decomposition the N concentration of litter placed on the unfertilized plot dropped slightly, from 0.48% to 0.44% in litter from fertilized trees and from 0.35% to 0.33% in litter from unfertilized trees, but after this initial drop the N concentration increased to about 1.0% after 4 years in both types of litter (Fig. 8). Changes in the N concentration of decomposing litter placed on the fertilized plots were similar to those placed on the unfertilized plots. The a m o u n t of N in the litter from the fertilized trees decreased below the amount present at zero time after 15 to 24 months of decomposition, whilst t h a t of the litter from unfertilized trees decreased below the original a m o u n t after 48 months (Fig. 8). After 72 months decomposition the amount of N in the litter from both the fertilized and unfertilized trees was the same, but over this period 2.6 mg N g-1 original litter was released from fertilized litter compared to only 1.4 mg N g - 1 from unfertilized litter.

98

C. THEODOROU AND G.D. BOWEN

Effect of fertilization on the nitrogen and phosphorus status of litter and soil The amounts of mineral N present in the litter decomposing in bags at different seasons from June 1978 to June 1979 was highest in spring, ranging from 215 to 246 mg N kg -1 litter, and lowest in winter, ranging from 32 to 40 mg N k g - 1 litter. In summer it ranged from 52 to 70 mg N kg-1 litter and in autumn from 34 to 48 mg N kg-1 litter. There was no significant difference in mineral N content between litter from fertilized and unfertilized trees. Mineral N content in spring was significantly higher than in the other seasons, which did not differ significantly. Three years after fertilization the concentrations of total N and P in resident forest-floor litter were 7900 + 1792/~g g-1 and 250+58/~g g-~, respectively, with the unfertilized treatment and 10 600 + 900 jug g- ~ and 410 + 102 gg g-1, respectively, with the fertilized treatment. Mineral N content was 8 + 1.0 #g g-1 litter in the absence of fertilizer application and 13.2 + 3.9 gg g-1 where fertilizer had been applied. Soil total N and P concentrations were 330 + 92/~g g-~ and 110+33 gg g-l, respectively, with the unfertilized treatment and 750 + 218 #g g-~ and 290_+ 78 #g g-l, respectively, with the fertilized treatment. Soil mineral N content was about 3 times higher with the fertilized (8.0 _+1.7 gg g- 1) than with the unfertilized treatment (2.8 + 0.6 #g g - 1). DISCUSSION Fertilization increased the monthly and annual litterfall significantly, but did not affect the rate of litter decomposition (per unit original mass). Thus, accumulation of litter would be expected to be higher in a fertilized stand than in an unfertilized site. This is consistent with the finding of Florence and Lamb (1974) that for the first 20 years the rate of litter accumulation in P. radiata stands depends on the site productivity, such that litterfall and litter buildup in high-site-quality stands was much greater than in low-site-quality stands. However, beyond 20 years, when annual rates of litterfall at high-quality sites varied little, Florence and Lamb (1974) considered that accumulation of soil organic matter (i.e., the net result of litterfall and decomposition) is probably related to differences in litter decomposition rates rather than to site productivity variation. The rate of decomposition recorded in these studies may be lower than that in the plantation floor because of the restriction the mesh size of the litterbag may impose on the size of decomposing fungi (St. John, 1980), or of the decomposer/comminuting soil fauna (Edwards and Heath, 1963; Will, 1967) entering the bag. In addition, litterbag studies, although they give a useful indication of the rapidity of nutrient cycling from fresh litter, may not reflect nutrient cycling rates from total floor litter (Vogt et al., 1983). Litter decomposition rate was similar at both the fertilized and unfertilized sites. The loss of organic matter after 1- and 2-year decomposition (30% and

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99

46%, respectively) were comparable to those obtained for P. radiata by Will (1967) in New Zealand (32% and 51%, respectively) and Baker and Attiwill (1985) in Australia (28% and 50%, respectively). The higher rate of weight loss in the first year (Fig. 6) was probably due to the leaching of water-soluble materials and to the rapid breakdown of easily decomposable substances. This initial rapid loss of weight also occurred with litter freshly placed on the floor at the beginning of each season (Fig. 7). The continual decomposition over the seasons indicates a wide range of temperatures and moistures at which this can occur. The forest-floor temperatures for winter, spring and autumn in this region typically range from 9.6°C to 16.9 °C (R.H. Ruiter, CSIRO, personal communication, 1982), yet the decomposition rates for these months were similar and were significantly higher than those for summer. Although forest-floor temperatures in summer (17.6-19.6 ° C) are typically higher than in other seasons, rainfall and hence litter moisture is low, indicating moisture to be an important factor in determining decomposition at this site. The inhibiting effect of low litter moisture on decomposition of P. radiata litter was observed by Woods and Raison (1983) and Baker and Attiwill (1985). However, comparison of the rates of decomposition of the litter placed at different times on the forest floor (Fig. 7) indicated that moisture had little effect on the initial rate of decomposition. This is in agreement with the findings of Swift et al. (1979) that, apart from the climate and edaphic conditions, the physical and chemical qualities of the substrate can determine the rate of decomposition by in~uencing the composition of the decomposer community and their rate of activity. Fertilization increased the concentration of N and P in the litter and in the amounts of N and P returned to the forest floor. Miller et al. {1976) also reported increased N concentration in litterfall after a 6-year period of fertilization with this element. Increased levels of N in the litterfall in our studies were related to elevated concentrations of total N in the litter and soil at the fertilized site. Thus, the application of fertilizer increased the N capital of the site through uptake and recycling rather than through residual fertilizer, since it could be expected that all the applied N not taken up by the trees would have leached out of the analysed layer (Theodorou, 1986). The concentration of N in the confined litter increased with decomposition, as reported in other studies (Berg and SSderstrSm, 1979; Baker and Attiwill, 1985; Baker et al., 1986). Such increases, in part, reflect higher rates of mineralization of carbon than of N. However, increased concentrations of N in decomposing litter have also been attributed to the importation of N from outside sources by mycelium of decomposer fungi (Berg and SSderstrSm, 1979) or by asymbiotic N fixation (Baker and Attiwill, 1984). The total N content of the litter, after an initial decrease in the first m o n t h of decomposition (probably due to leaching out of N), remained the same or increased for 15 and 48 months in fertilizer and non-fertilizer litter, respec-

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A N D G.D. B O W E N

tively.After this time a decline in the amount of N indicated net mineralization. Over the 72 months, more N per g original litterwas mineralized from the fertilizedthan from the unfertilizedlitter. The C: N ratio at the time of net mineralization in the decomposing litter was relativelyhigh (unfertilized52, fertilized59) compared to decomposing cereal straw, which showed net immobilization at a C:N ratio of 55 (Bartholomew, 1965). The C : N ratio serves only as a general guide to the mineralization-immobilization potentialof a given ecosystem, and deviations from this general guide can be expected with differentsubstrates (Ladd, 1987). Although the decomposing litterwas not sampled frequently enough to draw definite conclusions as to the exact time that net N mineralization started in the two types of litter,N contents of decomposing litter (Fig. 8) point to the possibilitythat net mineralization started earlierin litterfrom fertilizedtrees than from unfertilizedtrees,probably because of higher N content. Generally, immobilization continues until a criticalN concentration, which satisfiesthe requirements of the decomposer population, isreached (Berg and Staaf, 1981 ). The criticalN concentration varies widely and is affected by both organic chemical composition and environmental conditions (Gosz, 1984). The critical N concentration for the mineralization of P. radiata litterwas found by Baker and Attiwill (1985) to be greater than 1.0%, whereas in these studies it was 0.79% for litterfrom fertilizedtrees and 0.96% for litterfrom unfertilized trees. In litterfrom fertilizedtrees, the criticalN concentration was reached earlierthan in that from unfertilizedtrees.Increased N in litterresultingfrom increased amounts of N available to the tree decreases the polyphenol and organic-acid content of litterand thus facilitatesN mineralization (Vitousek et al.,1982; Gosz, 1984). The combination of these two factorsprobably caused earlier mineralization of N in the litterfrom the fertilizedtrees than in that from unfertilized trees. These studies clearly indicated that applying a fertilizer containing N: P: K plus trace elements to a P. radiata plantation resulted in increased litterfall. Also, since decomposition was not accelerated, such increased litterfall probably leads to increased accumulation of litter on the forest floor and hence to increased levels of N and P for recycling.

ACKNOWLEDGEMENTS

W e are gratefulto Miss Michelle Borrett for technical assistance and to Mr. T.A. Beech for the chemical analyses. Some financialassistance came from a fund provided by a number of Australian forestryorganizations.

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REFERENCES

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