Decomposition and nutrient release from mixtures of Gambel oak and ponderosa pine leaf litter

Forfst Ecology and Management, 47 (1992) 349-361 © 1992 -

349

Elsevier Science Publishers B.V. All rights reserved. 0378-1127/92/$05.00

Decomposition and nutrient release from mixtures of Gambel oak and. ponderosa pine leaf litter J a m e s O. K l e m m e d s o n

School of Renewable Natural Resources, University of Arizona, Tucson, Arizona 85721, U..$.A. (Accepted 15 January 1991)

ABSTRACT Klemmedson, J.O., 1992. Decomposition and nutrient relcase from mixtures of Gambel oak and ponderosa pine leaflitter.For. Ecol. Manage., 47: 349-361.

Studies were carried out to test the hypothesis that decay and release of nutrients of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) is accelerated in litter mixtures with Gambel oak (Quercus gambilii Nutt.). Litterbags containing different proportions of pine needles and oak leaves were placed in pine stands and collected over a 2-year period. At the end of the experiment mass losses from pure oak were 60% higher than those from pure pine with no evidence that the intrinsic rate of dry matter loss was affected in mixtures of litter. Litter type significantly influenced the concentration of all nutrients studied and amount of nutrients remaining for all nutrients except N and Ca. Increasing amounts of oak caused N. S, Ca and Mg to be retained more so than in 100% pine litter. Relative mobility of K and P were unaffected by litter composition. Nutrient release patterns were similar for N, S and Ca with little or no loss over the study period. Nutrient release patterns for P, Mg and K were similar with marked release (56 to 82°£ loss) over the two-year period. Possible reasons for lack of a synergistic effect between pine and oak are discussed. INTRODUCTION In the ponderosa pine forest of Arizona, Gambel oak is a common subdomin a n t u n d e r s t o r y s p e c i e s , u s u a l l y o c c u r r i n g in s m a l l , s c a t t e r e d p a t c h e s o f variable density. Where it occurs on otherwise uniform sites with ponderosa p i n e , t h e o a k h a s a f a v o r a b l e i n f l u e n c e on c o n c e n t r a t i o n a n d a v a i l a b i l i t y o f n u t r i e n t s i n t h e f o r e s t floor a n d u p p e r soil l a y e r s ( L e f e v r e a n d K l e m m e d s o n , 1980; K l e m m e d s o n , 1987; K l e m m e d s o n , 1991). T h e s e o b s e r v a t i o n s s u g g e s t t h a t i m p r o v e d f e r t i l i t y of s o i l s u n d e r p i n e - o a k s t a n d s m i g h t b e a s s o c i a t e d w i t h m o r e r a p i d d e c a y o f t h e o a k a n d a s y n e r g i s t i c effect b e t w e e n p i n e a n d o a k l e a f litters in the decay process.

~0

J.O. KLEMMEDSON

In addition tc the effects of climate, it is well-known that chemical properties of litter as well as those of the underlying soil, regulate litter decay (Van Cleve, 1974; Swif~ et al., 1979; Haynes, 1986). Differences in decomposition of organic debris have been observed under stands of different overstory (Lutz and Chandler, 1946) or understory (Ramann, 1898; Wittich, 1937; Tappeiner and Aim, 1975) compositi,vn, or when various litters were placed on soils with different hv .mus types (Bocock and Gilbert, 1957; Boccck et al., 1960). In the few studies to determine if broad-leaved species accelerate the decomposition of associated conifer needles or leaves of other species slow to decompose, the findings have been mixed (Gustafson, 1943: Thomas, 1968; Staaf, 1980a). Cthvrs who have studied decay of var~.ous species mixtures in litter bags (Day, 1982; Dw~er and Merriam, 1984) did not attempt, to investigate interaction effects of tke associated species. This paper reports research to test the hypothesis that Gambel oak leaf litter increases the rate of decay and release of r,utricnts from ponderosa pine needles when the two litter types are ir~timatcly mixed. To test this hypothesis, the litter bag method (Bocock et al., 1960) was used to measure dry matter and nutrient losses from decomposing ].ca{ iitters of variable pine-oak coml~sition over time. STUDYAREA The study was conducted about 40 km south of Flagstaff, Arizona, U.S.A. The ~i~c was about 2100 m elevation and consisted of a mosaic of small even-aged groups of ponderosa pine, averaging about 0.08 ha (0.06-0.14) in size, typical of the southwestern pine type found in Arizona and New Mexico (Cooper, 1961). The overstory is a nearly pure stand of pine, but Gambel oak occurs in the understory as small, scattered patches (~0 to 300 m 2) of variable density. The poorly-formed oak trees may reach 10-12 m height and 85 cm dbh (Hanks et al., 1983). A shrubby oak understory, origina.~;.ng from lignotubers and rhizomes (Tiedemann et al., 1987), is common. Arizona fescue (Festuca arizoldca Vasey), mountain muhly (Muhlenbergia montana (Nutt.) Hitchc.), squirreltail (Sitanion hystrix (Nutt.) J.G. Smith) and blue grama (Bouteloua gracilis (H.B.K.) Lag.) dominate grassy openings and the scant, but variable, ground cover under pine. Topography is gentle with scattered cinder cones and buttes occas ,onally inter~p~iug the rolling landscape. Soil parent material is basalt with occasional cinders in the vicinity of cinder cones. Soils are Brolliar stony clay learns; they are classified as fine, montmorillonitic Typic Argiborolls. In a typical profile, 4 to 6 cm of duff mull forest floor (Aoovcr and Lunt, 1952) overlies the ,mineral soil. From 20 to 407o of the surface is covered with stones, cobblestones and gravel. Mean annual plt~ipitation at Flagstaff is 53.0 cm; mean annual temperature is 7.4°C. January and July means are -2.90{3 and 18.9°C, respectively. A more detailed description is contained in Klemmedson (1087).

DECOMPOSITION AND NUTRIENT RELEASE FROM LEAF LITTER

351

METHODS A two-factor litter bag decomposition study involving five litter types, seven collection times over a 2-year period, three replicates and four subsamples was designed to test the hypothesis. Each litter type had a specific composition of pine needles and oak leaves (0, 25, 50, 75, and 100% oak by mass). Pine needles and oak leaves were collected at the study site in nets at the peak of litterfall and air-dried to constant mass. No rainfall occurred between litterfall and litter collection. About 12 g of needles and/or oak leaves were weighed in the prescribed proportions noted above, placed in each of 420 prelabeled 20 x 20 cm nylon bags (0.165 n~m openings) and sewn shut. The exact mass of needles and oak leaves was recorded for each bag. In the field, bags were set out in 15 sites (i.e. five stand types, three each) that hsd been randomly selected from a pool of 30 sites (six for each of the five stand types) widely scattered across several small drainages. The five stand types were selected so the proportion of pine and oak basal area in the stand corresponded as closely as possible to the proportions of pine and oak weighed L-lto the litter bags (i.e. 0, 25, 50, 75 and 100% oak). A critical factor in design nf this stady was to assu~.'e that whether sites were occupied by pine or a mixture of pine and oak~ they were equivalent in all site factors except species composition (i.e. relative dominance of oak was not related to environmental or historical factors). The model for site selection and control of extraneous variation is found in Klemmedson (1991). Except for differences in proportion of pine and oak, the original pool of 30 sites fit within narrow ranges of climate, parent material, exposure, slope, micro-relief, and other extraneous site factors. Soil profi" ~s were similar throughout and soil texture ~fthe upper 15 cm of mineral soil did not differ significantly (p < 0.001) among the 30 sites (Klemmedson, 1991). Litter bags were placed on the forest floor at the center of each stand in a 4 x 7 one-meter grid and secured ~n place with steel pins. At seven collection times (the last at 728 d) with intervals ranging from 60 d initially up to 150 d, depending on season, four randomly selected bags were collected from each of the 15 s ~ n d s and returned to the laboratory. They were oven-dried for 48 hours at 70°C and weighed. Contents of bags with both pine and oak were separated and weighed by species, then recombincd for chemical analysis. Litter samples were analyzed for total N by micro-Kjeldald (13renmer and Mulvaney, 1982), organic C by dry combustion (Nelson and Sommers, 1982), total P using the vanado-molybdo-phosphoric yellow color method follow'-'ng dry ashing (Jackson, 1958; Chayman and Prat.t, 1961), total S by dry combustion (Tiedemann and Anderson, 1971) and cations (total Ca, Mg, K) by indue L tively coupl~d plasma (ICP) emission spectrometry (Barnes, 1977) following perchloric acid digests ~Lignin was determined by acid-detergent analysis (Van Soest, 1963).

352

J.o. KLEMMEDSON

TABLE1 F r a t i o s a n d l e a s t s i g n i f i c a n t difference (%) a t p < 0.05 (LSD) from a n a l y s i s of v a r i a n c e for p e r c e n t a g e d r y m a t t e r a n d n u t r i e n t s r e m a i n i n g a s a f u n c t i o n of litter type, site w i t h i n litter t y p e and e l a p s e d t i m e of decay Source of variation DF

Dry matter

N

P

S

Ca

Mg

K

F Ratios Litter ~ype (L)

4

Site wt~ L ($9, error a

1O

Elapsed time (T) LxT T x S wf~ L, error b

6 24 60

69.94 **t 0.54ns

362.50** 20.82** 3.90"* 2.28"*

12.65"*

9.04**

2.62ns

267.02** 27.44** 4.61"* 2.65 *°

2.43ns 1.63ns

267.50** 493.95** 3.27** 9.72**

6.86"*

5.23 "°

LSD 2 Litter type

3.66

Elapsed time

3.59

8.59

9.19

10.52

-

10.74

9.42

6.45

8.38

-

8.94

6.75

t *Significant a t p < 0.05; **significant a t p < 0.01; ns = nonsignificant. Steel a n d Torrie (1960, p. 236).

Percentages of initial mass of dry matter or nutrient remaining were calculated as a ratio of final mass to initial mass x 100. A linear model for a two-factor experiment with repeated collections over time (Winer, 1971) was employed in data analysis. The model was: X~k = U + Li +

(S/L)~ +

T k + ( T L ) i k + eijk

where X~k is a random observation representing any cell of the L x T matrix, Li is the ith litter type, T is the kth elapsed time, S / L is t h e j t h site within litter type and e~k is experimental error. Form of the analysis of variance is shown in Table 1. Separation of metals for differences significant at p < 0.05 was performed by LSD. RESULTS AND DISCUSSION

Dry matter The highly significant {p < 0.01) effect of litter type and elapsed time on percentage dry matter remaining (Table 1) is clearly evident in Fig. 1. Dry matter loss displayed characteristic decay patterns (Lousier and Parkinson, 1976; Bunnell and Tait, 1974); they appear to approximate a negative ex-

DECOMPOSITIONANDNUTRIENTRELEASEFROMLEAFLITTER ponential. When plotted together, graphs for the five litter types form a family of curves with increasing loss as percentage oak increased. Decomposition constants (i.e. k; Olson, 1963) for litters with increasing amount of oak, beginning with no oak, were 0.154, 0.193, 0.262, 0.279 and 0.321. During the first 60 days, bags with 100% oak lost twice as much mass as those with 100% pine needles. This approaches the difference in decay rate Tappeiner and Aim (1975) observed between red pine (Pinus resinosa Ait.) and paper birch (Betula papyrifera Marsh.). Explanation of the significant L x Tinteraction is obscure; only the lack of change in dry matter of 100% oak litter between 511 and 600 d would seem to explain this (Fig. 1). By separating contents of litter bags after collection, it was possible to compute weight loss for beth pine and oak components of the mixed litters. When percentage of initial mass remaining is plotted as a function of elapsed time (Fig. 2), these data convincingly demonstrate t h a t pine and oak components in each of the five treatments decayed at a rate characteristic of the ~pecies, regardless of the amount of the other species present. Clearly, there was no synergistic effect between pine and oak even though an exchange of nutrients between species can be expected (Gilbert and Bocock, 1960; Anderson, 1973; Staaf, 1980a). These findings agree with those of Thomas (1968) and Staaf (1980a) who found, respectively, t h a t addition of dogwood (Comus florida L.) leaves to loblolly pine (Pinus taeda L.) leaf litter, and of raspberry (Rubus idaeus L.) leaves to beech (Fagus sylvatica L.) leaf litter, had little or no effect on decay rate. In the latter case, this occurred despite significant accumulation of N and P in beech leaves from raspberry leaves and the underlying soil. Moreover, there was no evidence (Fig. 2) of any inhibitory effect of oak leaf tannins as noted by Harrison (1971). In the pH range of forest

I001

~

9o 8o

7o ~

6o

~ Q

5o 40

o_____o ~ o~o 25

a--O A--A 1 ~ 200 400 600 ELAPSEDTIME (DAYS)

800

Fig. 1. Percentagedry matter remainingas a functionofelapsed time ofdecompositionfor litters with varyingproportionsof pine needlesand oak leaves.

354

J.O. KLEMMEI~ON

~

I00~

~

"° 70

0~ ~ 5o 4O 0

.0o

40o

2O0

.0o

ELAPSED TIME (DAYS)

Fig. 2. Percentage dry matter remaining o~separate pine and oak components for each of the five litter substrates as a function of elapsed time.

TABLE 2 Initial concentration of nutrients and lignin in pine needles and oak leaves. All differences between pine and oak were significant at p ~ 0.001 Litter type Nutrient C

N

P

S

487 446

4.0 9.7

0.4 1.8

0.5 0.8

Ca

Mg

K

Lignin

1.3 3.5

1.3 4.6

169 132

g kg-l Pine Oak

3.7 8.3

floor a n d s u r f a c e soils of this s t u d y a r e a (close to p H 7.0) o a k t a n n i n s a r e m u c h less i n h i b i t o r y t h a n a t p H 4.0 ( B a s a r a b a a n d S t a r k e y , 1966).

Nutrients Concentration The higher quality of litters with oak leaves can be sccn by comparing initial con,~ntration of nutrient elements and Iignin in pine needles a n d oak leaves (Table 2). Concentration differences ranged from 1.8-fold for S to 4.2~fold for P for the d e m e n t s , while lignin w a s 28% higher in pine; in every case ditYeronccs were highly significant (p < 0.001). In m o s t studies relating litter decay to litter quality, going back ~othe work of Melin (1928) and Fogel a n d Cromack (1977), N a n d l i g 1 ~ ,

DECOMPOSITION AND NUTRIENT RELEASE FROM LEAF LITTER

I

o

.~-

8o[

,

60

140

°i

o

,

,

{S

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/

*

/

40

,

130.

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~

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.

~e.~.

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I

7ot

*"

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*

/.

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o,

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~-~°~--o" "

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s~°~o~ , 250

355

, 560

~-

0

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~ i 750

~:'-::o

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-'.--=I,-~ •

.o--o__6 • oo, o--o

o

0--0

25

~.~*-~=~

'°1 ~ ' ~ s ~ . ~ ~ O" 0

" 250

" 5oo

7'0 5

ELAPSED TIME ( cloy= )

Fig. 3. Relative concentration of nutrients for p i n e - o a k l i t t e r mixtures as a function of elapsed

time of decay. or combinations thereof, have received the most attention as expressions of litter quality (Mcentemeyer, 1978; Melillo et al., 1982; Berg et al., 1984). Concentrations of all elements were significantly (p • 0.01) influenced by litter type, elapsed time and the L x T interaction (ANOVA not shown). Litter types differed in concentration for all nutrients from time zero. Trends in the absolute concentration with time (not shown here) were a function of litter type and mobility of nutrients. For less mobile nutrients (hi, S, Ca), concentration did not change significantly for litters dominated by pine. For mobile nutrients, and for litters dominated by oak, significant changes usually occurred within 200 d. For nutrients that increased in concentration with elapsed time (IN, S and Ca), the increase in relative concentration was roughly proportional to amount of oak in the litter mixture (Fig. 3). For elements that declined in concentration with time (P, Mg and K), the effect of oak was not as clear-cut, especially in the case of K (Fig. 3). Nutrient loss In the analysis of variance (Table 1), litter type (L) significantly influenced percentage nutrients remaining for P, S and Mg at thep< 0.01 level, K at the

356

J.O.KLEMMEDSON

p < 0.05 level and was nonsignificant for N and Ca. Elapsed time (T) significantly influenced N, P, S, Mg and K remaining at t h e p < 0.01 level and Ca at the p < 0.05 level. The L x ~/'interaction significantly influenced all nutrients, but Ca (p < 0.05) at the p < 0.01 level. Two basic patterns of nutrient loss can be distinguished (Fig. 4). Nitrogen, S and Ca characterize a pattern of relative stability in nutrients throughout the decay period; but of these only S showed a significant toss (about 10%). Nitrogen displayed the most consistent pattern among treatments, losing N initially, then gaining N, then losing to a level of near constancy for the last 420 d. Although this pattern is common for N (Staaf and Berg, 1982), the magnitude of the accumulation phase is usually much greater (Anderson, 1973; Gosz eta]., 1973) than observed here. This pattern, sometimes without the leaching phase, is also common for S (Staaf, 1980b; Staaf and Berg, 1982). The more characteristic nutrient release patt:~rn for Ca involves gains (Lousier and Parkinson, 1978; ~taaf, 1980b) or losses ~Thomas, 1969; Gosz et al., 1973) up to 5 0 % of initial amounts. The second general pattern of nutrient release, characterized by P, Mg and K, was one of marked release throughout the d~:cay period (Fig. 4). As expected, loss of K was the greatest, amounting to 82% of original K; losses of P (65%) and Mg (56%) were somewhat smaller. Although 100% oak leaf substrates tended to accumulate Mg during the first 200 d, the overall pattern for P, K and Mg was rapid removal until day 511. Re!ease of these nutrients then essentially cev~ed, the curves leveled off and curves for Mg and K, representing different pine-oak combinations converged (Fig. 4). Convergence amovg treatments was especially noteworthy for K. This pattern of loss is characteristic of

,oo -~om

120

.,.. .

t,o_ o ' - ~ ± o J

%

~

\oiO

~

~

9

8c~

o

o

o.

2oo

,oo

60o

8oo

~

~o~ ~

G=;,;o'

~

~

800

ELAPSED TIME( doys )

Fig. 4. Percentageofinitial nutrient mass remaining as a functionofelapsedtime ofdecomposition and six nutrients.

for five fitter treatments

DECOMPOSITION AND NUTRIENT RELEASE FROM LEAF LITTER

357

highly mobile nutrients subject to leaching (Gosz et al., 1973; Lousier and Parkinson, 1078). The amounts of nutrients associated with the minimum values obtained presumably represent nutrients immobilized by microbial biomass (Swift et al., 1979; Staaf and Berg, 1982). Relative nutrient loss Because dry matter was lost at a fairly steady rate over the two year study period, it serves well as a base for calculating nutrient/dry matter ratios (Fig. 5) for comparing relative loss of nutrients. Litter composition affected the relative mobility of nutrients and the position of nutrients in the mobility series. Thus, N, S, Ca and Mg exhibited net immobilization (significant a t p < 0.01) with increasing amounts of oak in the litter mixtures c9mpared to pure pine litter. Relative losses of K and P were unaffected by litter composition. In 100% pure pine litter, the mobility series was K > P ffiMg > N ffi S ffi Ca whereas in 100% oak litter the series was K > P > Mg > S = N > Ca (Fig. 5). These mobility series are generally consistent compared with the consensus of a large number of studies reported by Lousier and Parkinson (1978). Further insight into the effect of oak on N retention in litter is reve.qled when relative N concentration (NjNo, where No and Nt are initial and fmal N concentrations, respectively) is regressed against C/N ratio for each o f t h e five litter type~ over elapsed time from 0 to 728 d (Fig. 6). Bccanse C varied less than 10% among litter types and over time (data not shown), No essentially controls the lo~,er position of each regression line along the X-axis. Because No is the same for Y-values of all points (collection times) along any one ofthe five regression lines, l"~tdetermines the slope of each line. Slopes of the regressions steepen and N ret,~ntion increases with decreasing C/N ratio (i.e. increasing proportion of oak) and increasing rate of decomposition (k) of the litter sub-

2.o 1.6

1.21 -

-

t

i

c

0.8

,z,

P~ ~

_

0.4

0.0

25

_ _ =

50

~K

75

lOO

OAK ~N UTTER.(~)

Fig. 5. The ratio of percentage nutrient remaining to percentage dry matter remaining after 728 d a s a function of percentage oak in litter substrates (n = 12).

358

J.O.KLEMMEDSON

strates (see above for k values). Thus, relative nutrient concentration at 728 d increased from 125% for pure pine litter to about 160% for litter types with 50% or more oak leaves. This agrees with findings of Berg and Staaf (1981) and Bosatta and Staaf (1982); the relationship is common to many decomposing fitters (Becock, 1963; Anderson, 1973; Klemmedson and Blaser, 1988). Reasons for linearity of the regressions in Fig. 6 are not clear. 180 Oak 728 d - e ' ~

o ~

~ 160

°•

o

0

.,oo

o

~ 120 100 •

0

, 20

.

,

•

,

40

•

60 C

]

, 80

N

•

, 1 O0

,

i 120

,

i 140

RATIO

Fig. 6. Relativenitrogen concentration(%) as a functionof C/N ratio for five litter treatments. Each regressionline represents a data set for time zero(lowestpoint on lines) and each of seven collectiontimes (n = 12). Although after 728 d, the C/N ratio of oak leaves was 28, a value at or near the level where net N release might be expected (Swift et al., 1979; Staaf, 1980b), the data in Fig. 4 do not indicate t h a t net N release was occurring. Following the rational of Parnas (1975), net N release would not be expected to occur until the C/N ratio became constant and clearly t h a t condition did not prevail by 728 d, even in the pure oak leaf litter (Fig. 6). One is tempted to reason t h a t a synergistic effect in the mixed litters was limited by N availability, t h a t upon reaching the critical C/N ratio in oak leaves, N would be released (Swift et al., 1979; Ja.'~sson and Persson, 1982) and possibly used by microorganisms inhabiting pine needles. However, other evidence indicates t h a t the importance of N diminisi~.Cs after the initial phase of decay and t h a t lignin content (Berg et al., 1982) or the holocellulose-lignin ratio (Berg et al., 1984, McClaugherty and Berg, 1987) become rate determining in later stages of decay of many litters. This is consistent with earlier suggestions of Van Cleve (1974) and Fogel and Cromack (1977).

DECOMPOSITIONANDNUTRIENTRELEASEFROMLEAFLITTER

359

ACKNOWLEDGEMENTS R e s e a r c h r e p o r t e d h e r e w a s suppo.,~ced b y t h e M c I n t i r e - S t e n n i s C o o p e r a t i v e Forestry Research Program in cooperation with the Arizona Agric. Exp. Sta., J o u r n a l A r t i c l e No. 7145. T h a n k s a r e d u e t o J n s t i n e M c N e i l for l a b o r a t o r y a n a l y s i s a n d g r a p h i c s , R o b e r t K u e h l for s t a t i s t i c a l c o n s u l t a t i o n a n d A . R. Tiedemann for comments on an earlier draft of the manuscript.

REFERENCES

Anderson, J.M., 1973. The breakdown and decomposition of sweet chestnut (Castanea sativa Mill.) and beech (Fagus sylvatica L.) leaf litter in two deciduous woodland soils H. Changes in the carbon, hydrogen, nitrogen and polyphenol content. Oecologia (BerL), 12: 275-288. Barnes, R.M., 1977. Review ofthe applications of the inductively coupled plasm& In: R.M. Barnes (Editor) Applications of inductively coupled plasmas to emission spectreecopy. The Franklin Inst. Press, Philadelphia, FA, pp. 3-49. Basaraba, J. and Starkey, R.L., 1966. Effect of plant tannins on decomposition of organic substances. Soil Sci., 101:17-23. Berg, B., Ekbehm, G. and McClaugherty, C., 1984. Lignin and holecellulose relations during long-term decomposition of some tbrest litters. Long-term decomposition in a Scots pine forest. IV. Can. J. Bet., 62: 2540-2550. Berg, B., Hannus, I~, Popoff, T. and Theander, O., 1982. Changes in organic chemical components of needle litter during decomposition. Long-term decomposition in a Scots pine forest. L Can. J. Bet., 60:1310-1319. Berg, B. and H. Staaf. 1981. Leaching, accumulation and release of nitrogen in decomposing forest litter.'. In: Clark, F.E. and T. Rosswall (Editors) Terrestrial Nitrogen Cycles. Ecol. BulL (Stockholm), 33:163-178. Boceck, K.L. and Gilbert, O., 1957. The disappearance of leaf litter ,~nder different woodland conditions. Plant Soil, 9:179-185. Becock, K.L., Gilbert, O., Capstick, C.K., Twinn, D.C., Waid, J.S. and Woodman, M.J., 1960. Changes in leaf litter when placed on the surface of soils with contrasting humus type~ I. Losses in dry weight of oak and ash leaf litter. J. Soil Sci., 11:1-9. Becock, K.L., 1963. Changes in the amount of nitrogen in decomposing leaf litter of sessile oak (Quercuspetraea).J. Ecol., 51: 556-566. Bosatts, E. and Stsaf, H., 1982. The control of nitrogen turnover in forest litter. Oikos, 39: 143-151. Bremner, J.M. and Mulvaney, C.S., 1982. Nitrogen - - Total. In: A.L. Page (Editor), Methods of Soil Analysis, Part 2. Chemicr~ and Microbiological Properties. 2nd Edition, Agronomy 9: 595-624. BunneU, F.L. and Tait, D.E.N., 1974. Mathematical simulation models of decomposition prosesses. In: A.J. Holding, O.W. Heal, S.F. Maclcan Jr. and P.F. Flanagan (Editors), Soil Organisms and Decomposition in Tundra. Tundra Biome Steering Committee, Stockholm, pp. 207-225. Chapman, H.D. and Pratt, P.F., 1961. Methods of analysis for soils, plants and water. University of California. 309 pp. Cooper, C.S., 1961. Pattern in ponderosa pine forests. Ecology, 42: 492-499. Day, P.P.Jr., 1982. Litter decomposition rates in a seasonally flooded Great Dismal Swamp. Ecology, 63: 670-678.

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