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Respiration and Nutrient Release from Tree Leaf Litter MixturesAuthor(s): Kevin B. McTiernan, Philip Ineson and Paul A. CowardSource: Oikos, Vol. 78, Fasc. 3 (Apr., 1997), pp. 527-538Published by: Wiley on behalf of Nordic Society OikosStable URL: http://www.jstor.org/stable/3545614 .

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OIKOS 78: 527-538. Copenhagen 1997

Respiration and nutrient release from tree leaf litter mixtures

Kevin B. McTiernan, Philip Ineson and Paul A. Coward

McTiernan, K. B., Ineson, P. and Coward, P. A. 1997. Respiration and nutrient release from tree leaf litter mixtures. - Oikos 78: 527-538.

The effect of mixing litters on decomposition rates was investigated by incubating seven tree leaf litter types in all possible two-litter combinations under controlled laboratory conditions for 26 weeks. Inorganic N and CO2 release were monitored during the course of the incubation and final litter concentrations of N, P, Ca, Mg and K were determined. Initial Ca content provided the best correlation (r2 = 0.458 P<0.001) between total respiration of the pure and mixed units and initial litter quality. There was a very poor correlation (r2 = 0.046, P = 0.272) between total respiration and initial N content across all litters, but when alder, and mixtures with alder, were removed from the calculation the remaining litters gave a strong correlation (r2 = 0.720, P < 0.001). The majority of litter combinations showed interaction effects for CO2 release at some stage during the incubation, with eight significant positive and only one significant negative interaction for total CO2 release. All mixtures showed interaction effects for inorganic N, with release from mixtures generally occurring later than expected. Total N loss was significantly lower in four mixtures, and significantly higher in one. It is suggested that the 'mixtures effect' could be a useful management tool for modifying the timing and rate of release of N from decomposing plant residues to improve the synchrony between mineralisation and plant uptake. Increased rates of decomposition appear to have been a result of a 'sharing' of resources between the component litters of a mixture. Elemental translocation by fungal hyphae, along with diffusion, is proposed as a means by which the degradation of one litter was facilitated by the presence of another.

K. B. McTiernan, P. Ineson and P. A. Coward, Inst. of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria, UK LAI1 6JU ([email protected]).

There is an extensive literature describing the decomposition of tree litters, both in the field and laboratory, yet very few workers have considered the effect of the presence of litter from one species on the decomposition of litter from another. The vast majority of natural woodlands are mixed stands with, for example, Pe- terken's (1981) analysis of the natural tree communities of Great Britain, defining 45 out of 59 'stand types' as mixtures, with only six typically occurring as pure stands. Even in monocultures, tree litter often becomes mixed with understorey litters or falls onto the litter remains from previous years. In nature, therefore, litter rarely decomposes in isolation from other organic materials which often differ in composition and structure,

and we know little about the importance of these interactions.

It has long been recognised that certain (so called 'nurse') tree species aid the growth of other species. For example, Scots pine (Pinus sylvestris) has been found to increase the yield of Norway spruce (Picea abies) whilst experiencing no deleterious growth effects itself (Brown and Harrison 1983). Similarly, Carlyle and Malcolm (1986) reported enhanced growth of Sitka spruce (Picea sitchensis) in stands that included larch (Larix eu- rolepis). Although the mechanisms responsible have not yet been fully elucidated, Carlyle and Malcolm (1986) related this 'mixtures effect' to greater amounts of available nitrogen (N) and calcium (Ca) in the forest

Accepted 2 August 1996

Copyright ? OIKOS 1997 ISSN 0030-1299 Printed in Ireland - all rights reserved

OIKOS 78:3 (1997) 527



Table 1. Combinations of litters used and the standard nomenclature used to describe them.

Alder Alder Ash ASAL Ash Birch BIAL ASBI Birch Oak OAAL ASOA OABI Oak Pine PIAL PIAS PIBI PIOA Pine N. spruce NSAL NSAS NSBI NSOA NSPI N. spruce S. spruce SSAL SSAS SSBI SSOA SSPI SSNS S. spruce

Alder Ash Birch Oak Pine N. spruce S. spruce

floor of the mixed species stands. They also found that both mineralisation rates and treatment differences tended to be greatest for the 0-3 cm layer, suggesting that an interaction had taken place between the litters of S. spruce and larch.

Gustafson (1943) and Johnson (1953) showed that interactions could occur between decomposing litters, significantly influencing rates of decay. Seastedt (1984) hypothesized that nutrient release from rapidly decay- ing litter types could stimulate the decomposition of adjacent, more recalcitrant litter types. This was a view shared by Taylor et al. (1989) who attributed an accel- eration of mass loss in Populus-Alnus mixtures to a transfer of nutrients from N-rich Alnus litter to Populus litter. Conversely, inhibitory compounds such as pheno- lics and tannins (Harrison 1971, Dix 1979, Swift et al. 1979) could be expected to reduce the decomposition rates of some litter combinations. Chapman et al. (1988) found that although rates of respiration from the litter layer of Picea abies increased in mixture with Pinus sylvestris, a decrease was observed in mixtures with Alnus glutinosa and Quercus petraea. Similarly, Fyles and Fyles (1993) suggested that litters have the potential to interact, with positive or negative effects on decomposition rates.

Despite these observations, there is still no theoretical framework to quantify or model the importance of litter mixtures in nutrient cycling in ecosystems, and fundamental work is necessary to clarify the nature of these interactions. In the current work, seven tree litter types with differing initial qualities and chemistries were decomposed under controlled laboratory conditions in all possible two-litter combinations. The hypothesis was that carbon dioxide (CO2) and inorganic N release in mixtures would differ from predicted levels based on decomposition of the pure litters.

Methods

Preparation and watering of microcosms

Litter from pure stands of sessile oak (Quercus petraea Mattuschka (Liebl.)), alder (Alnus glutinosa L. Gaertn), Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L. Karsten) and Sitka spruce (Picea sitchensis (Bong.) Carr) was collected from the control plots of a tree mixtures experiment at Gisburn, U.K. (Nat. Grid

Ref. SD750585). A full site description is given in Brown (1992). In addition, common ash (Fraxinus ex- celsior L.) and silver birch (Betula pendula Roth) litters were collected from Grizedale forest, U.K. (Nat. Grid Ref. SD326915). All litters were collected immediately after leaf fall in autumn, returned to the laboratory and air-dried to constant weight. Unwanted debris was sorted from the litters, and petioles were removed. The oak, ash and alder leaves were cut into approximately 2 cm2 pieces, facilitating their use in standard microcosms (Anderson and Ineson 1982). Birch leaves (minus petioles), N. spruce and S. spruce needles were used whole, whilst Scots pine needles were cut in half.

Sub-samples of each litter were taken for determination of nutrient composition (see below) and for oven- dry weight (50'C). Aliquots, 2.0 g, of each pure litter were placed into microcosm inners (Anderson and In- eson 1982), with three replicates for each litter type. The microcosm inners were then placed in microcosms and these units constituted the pure litters.

A similar series of microcosms containing all possible combinations of two-litter mixtures was established by combining 1 g of one litter with 1 g of another. Care was taken to ensure thorough mixing to give maximum surface contact between the mixed litters. The experiment, therefore, consisted of seven pure units, together with 21 mixtures, each replicated three times. The combinations of litters used in the microcosms are shown in Table 1, along with the nomenclature used to describe them.

The litter in each microcosm was re-hydrated with 100 cm3 of distilled water, and allowed to soak for 24 h. The litter was then drained under gravity and the leachate retained for chemical analysis. Each microcosm was inoculated with 1 cm3 of a coarsely sieved suspension of a macerate of L and F1 litter in distilled water. The litter for the inoculum was freshly collected from the floor of several mixed deciduous/coniferous woodlands chosen to include the seven litters under investigation.

The microcosms were incubated at 1 70C. A ran- domised block design was employed with one replicate of each pure and mixed microcosm being placed on each of the cabinet's three shelves. After seven d the microcosms were leached with 125 cm3 of distilled water, leachates again being retained for chemical analysis. Leaching was performed by gently pouring the distilled water on to the surface of the litter, with

528 OIKOS 78:3 (1997)



draining under gravity. The leachate was re-applied twice to ensure thorough equilibration of mineralised nutrients between the litter and the leachate (Anderson and Ineson 1982). This procedure was repeated every two weeks for 26 weeks.

Respiration measurements Carbon dioxide evolution was monitored every two weeks from day 14 (i.e. half-way between leaching occasions), using infra-red gas analysis (IRGA). The microcosm inners were removed from the microcosms and placed in respiration chambers connected to the IRGA system. Evolution of CO2 was monitored over three h. The IRGA consisted of a flow-through system, capable of monitoring respiration of twelve samples automatically and continuously (Ineson and Gray 1980). The analyser was connected to a twelve-channel gas handler (Type WA161, Analytical Development Co. Ltd., UK), fitted with a sequential timer circuit which routed gas either through the analyser or to waste during a 60-min cycle. Air-flow through the analyser was maintained at 0.4 1 min-' by a diaphragm pump (ADC WA-197B pump unit) placed in line immediately before the exhaust. A full description of the IRGA system is given in Wookey et al. (1991).

The respiration chambers were maintained in an incubator at 15'C, the air stream supplying the chambers being humidified by bubbling through water held at the same temperature. Air came from a common supply, and the analyser was operated in differential mode, comparing the CO2 content of the sample air with that of an identical stream of air which passed through an empty chamber.

Chemical analysis of leachates Samples were stored for a short time at 20C before analysis. Ammonium-N (NH') was measured on a Technicon continuous flow autoanalyser using the salicylate-hypochlorite method, with nitroprusside as a cat- alyst (Gentry and Willis 1988). Nitrate/nitrite-N (NO-7) was also determined using a Technicon autoanalyser, operating the hydrazine/sulphanilamide method (Row- land et al. 1984).

Chemical analysis of litters Following the final leaching and respiration measurements, the litter remaining in the microcosms was removed. Several of the litter combinations had reached such an advanced stage of decomposition that resolu- tion into component litters was not possible, and only six of the combinations were chosen for further analy-

sis. These were carefully separated into their two component litters, dried to constant weight at 50'C and analysed for mineral composition. Materials from the pure litter microcosms were also analysed, together with samples of the original litters. These were ground in a freeze-mill (Spex 6700, Spex Industries Inc., USA) with three replicate H202/H2SO4 digests being performed for each litter sample.

The digest solutions were analysed for total N and phosphorus (P), potassium (K), Ca and magnesium (Mg). Total N was determined by the salicylate-nitroprusside method (Berthelot reaction) and total P by the molybdenum blue method. Potassium was determined by atomic emission, and Ca and Mg by atomic absorp- tion using an acetylene/oxygen flame, with lanthanum chloride as a releasing agent.

Statistical analysis Calculation of expected leachate and respiration rates was performed as follows:

Expected mixture AB = (Observed pure A + Observed pure B)/2

Expected values were calculated from pure litters in the same experimental block, permitting the calculation of a mean and error term. In order to calculate total values for CO2 efflux, each measurement was assumed to be the mean of the seven d preceding and following the date of analysis; the first week of the incubation being ignored. Total NH+ release was calculated by summing the leachate fluxes. Expected and observed means for total NH- and CO2 release from mixtures, as well as final chemical composition of litter residues, were analysed using a t-test procedure. Comparisons of means for total NH+ and CO2 release from pure litters were made using analysis of variance and Student Newman-Keuls tests. Litter chemical composition and CO2 concentrations were based on oven dry weights (500C).

Results

Respiration Respiration rates for the pure units (Fig. 1) tended to decrease gradually and steadily during the course of the experiment, with the exception of ash which showed a marked increase in respiration between weeks 6 and 16; CO2 release rates at week 10 being twice those at week 6. Total CO2 release for the seven pure litters (Fig. 2) followed the order: ash > birch > pine = N. spruce = alder = S. spruce > oak. Initial Ca content provided the best correlation (r2= 0.458, P < 0.001) between total

OIKOS 78:3 (1997) 529



4

3.5 T

v 2.5

02

(D 1.5

0

0.5 0 . - --------I;

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Time (weeks)

Fig. 1. Carbon dioxide release rates (mg C02-C d-l g dw-1) for the pure litters; alder (0), ash (0), birch (A), N. spruce (x), oak (*), Scots pine (0) and S. spruce (+). Data are means ? se (n = 3).

respiration of the pure and mixture units and initial litter quality (Fig. 3, Table 2), although ash and ash mixtures dominated the relationship. A scatter plot of initial N content against total C02 release (Fig. 4) showed three distinct groups, pure alder, alder mixtures, and all other litters, resulting in a very poor overall correlation (r2= 0.046, P = 0.272). However, when alder and alder mixtures were removed from the calculation, the remaining litters gave a strong positive correlation between CO2 release and initial N content (r2 = 0.720, P < 0.001). The correlation with Ca for this reduced data set was also slightly improved (r2 = 0.491, P < 0.001), when compared with the full data set, but now explained less variance than initial N content.

300 -

a

250-

200- o b 0 - cs C

150 ~~ ~~~C C

e ~~~~~~~~~~~~~~~~d 0 100 0

50

50-

Ash Birch Pine N. Alder S Oak spruce spruce

Litter type

Fig. 2. Total CO2 release (mg C02-C g dw -) for the pure litters. Data are means ?se (n = 3). Means with different letters are significantly different (P < 0.05).

260 x

240--

J 220 -x m T ~~~~~~~~X B

@~~ ~ ~~~~~~~~ X 0.458

200 {-

E 160 X

?D 160 -- < X x x 14

I X

X {~~~~~~~ 0 120

100 -

0 0.5 1 1.5 2 2.5 Initial litter Ca content (% dw)

Fig. 3. Total CO2 release (mg C02-C g dw-') as a function of initial litter Ca content (% dw) of the pure and mixed litters.

There was also a significant correlation between CO2 release and initial N content within the alder mixtures (r2 = 0.796, P < 0.05).

The time trends for CO2 efflux during the experiment are shown in Fig. 5, with each graph providing expected and observed means and associated standard errors (see Methods). Any significant deviation between the observed and expected curves represents an interaction effect (Chapman et al. 1988, Blair et al. 1990).

For total CO2 release eight mixtures showed significant (P < 0.05) positive interactions, and only one a significant (P < 0.05) negative interaction (Fig. 6). All but one of the other combinations (PIOA) showed interaction effects at some stage during the 26-week incubation (see Fig. 5), one example being OABI which showed a negative interaction until week 8, no interaction between weeks 8 and 14, and a subsequent positive interaction. Although the total CO2 efflux for several of the mixtures did not show any overall interactions there were frequently transient interactions during the course of the experiment.

Initial CO2 production from broadleaf mixtures with oak tended to be lower than expected, yet, after several samplings, this trend reversed and CO2 production became greater than expected. No such inhibition was seen for mixtures of oak with needle litters with, for example, SSOA producing significantly (P < 0.05) higher levels of CO2 from the very start of the experiment. Mixtures of oak with alder, N. spruce and S. spruce showed significantly higher total CO2 efflux than predicted, and similar results were observed for pine, when mixed with these same three litters (Fig. 6).

Inorganic nitrogen

The time course for release of NH4+ differed markedly between the pure litters, with alder and ash releasing the largest amounts (Fig. 7). Ammonium mineralisation from the alder litter was initially low, rising rapidly to

530 OIKOS 78:3 (1997)



Table 2. Initial and final Ca, Mg, K, N and P composition (% dw) of the pure litters along with certain mixtures which were separated into their component parts. Data are means ? se (n = 3). Stars indicate significance; *: P < 0.05, **: P < 0.01, P<0.001.

Litter/ Litter Initial/Final Elemental concentration (% dw) Mixture analysed

Ca Mg K N P

Alder Alder Initial 0.96 0.16 0.25 4.09 0.102 Alder Alder Final 1.24 0.16 0.06 4.69 0.096 NSAL Alder Final 1.49* 0.11*** 0.07 4.18* 0.080 SSAL Alder Final 1.27 0. 12* 0.08 4.35* 0.090 Ash Ash Initial 2.13 0.15 0.18 2.16 0.085 Ash Ash Final 2.73 0.18 0.09 3.38 0.118 Birch Birch Initial L.16 0.17 0.20 1.66 0.160 Birch Birch Final 1.45 0.18 0.10 2.42 0.161 NSBI Birch Final 1.83** 0.13** 0.09 2.37 0.140* N. spruce N. spruce Initial 1.39 0.06 0.11 0.81 0.074 N. spruce N. spruce Final 1.85 0.07 0.05 1.95 0.085 NSAL N. spruce Final 1.46*** 0.10* 0.06* 2.06 0.081 NSBI N. spruce Final 1.73 0.I1*** 0.07** 1.99 0.101** NSOA N. spruce Final 1.44** 0.10** 0.07 1.94 0.078** NSPI N. spruce Final 1.44*** 0.07 0.06** 1.87** 0.075** Oak Oak Initial 0.98 0.15 0.09 1.02 0.046 Oak Oak Final 1.16 0.13 0.06 1.37 0.054 NSOA Oak Final 1.91*** 0.11** 0.09** 1.85** 0.061 SSOA Oak Final 1.45** 0.10** 0.09* 1.70* 0.063 Pine Pine Initial 0.35 0.04 0.12 1.06 0.070 Pine Pine Final 0.52 0.05 0.07 1.15 0.055 NSPI Pine Final 0.98*** 0.05* 0.09 1.50** 0.066* S. spruce S. spruce Initial 0.79 0.04 0.14 1.21 0.082 S. spruce S. spruce Final 0.92 0.04 0.09 1.58 0.092 SSAL S. spruce Final 1.09** 0.09*** 0.08* 1.92*** 0.090 SSOA S. spruce Final 0.83* 0.06*** 0.08* 1.55 0.082**

a peak at week five, followed by a decline to a level more than twice the initial flux, at which it remained. Before week 11, NH+ was barely detectable in the leachates from ash, then mineralisation increased sharply, giving a peak at week 15 followed by a decline. In contrast, birch mineralised little NH+ until almost the end of the experiment, with an increase starting at about week 21, which was still rising at the final

260 x

240

id 220- e0' r2 0.720 0 ? 200- x

E 180 r= 0.796

e 160

? 140 I /X? x x

2 120

100 1 I 0 1 2 3 4 5

Initial litter N content (% dw)

Fig. 4. Total CO2 release (mg C02-C g dw 1) as a function of initial litter N content (% dw) of the pure and mixed litters; alder mixtures (Li), pure alder (U), all other litters (x).

measurement. Initial NH4 release from the N. spruce was similar to alder and remained at around this level until week 15, after which time it gradually decreased. Release of NH from S. spruce was highest at the first leaching and gradually declined thereafter, whilst little NH4 was mineralised from either oak or pine during the entire incubation period (see also Fig. 8). Nitrate was not found in any leachate until week 17, and was negligible thereafter.

Total release of NH+ (Fig. 8) was not well correlated with initial litter contents of N, but generally followed the same order, with alder having the highest initial concentration of N, and releasing the greatest quantities of NH+, followed by ash. However, N. spruce mineralised N at a rate much higher than expected from initial N content (see Table 2), with mineralisation from the remaining litters being much lower and in the following order: birch > S. spruce > pine > oak. This order, with the exception of N. spruce, reflects the initial N contents of the respective litters.

The expected and observed values for total NH4 release for each litter combination are shown in Fig. 9. One mixture showed a significant (P < 0.05) positive interaction on total NH+ release, and four mixtures a significant (P < 0.05) negative interaction. As noted above for CO2, these summary data tend to be mislead-

OIKOS 78:3 (1997) 531



3.5- T ASAL 3

2.5

2

1.5

1-

0.5

0

2.5 BIAL TjASBI

1~ ~ ~~~~~/1

1 - ------

O2.5 -_>iq.s,\

OAAL ASOA OABI

1.5 ': ', 7 \ 3 :-F'

2.5 PIAL PIAS PIBI

1.5 -_

0.5

0 - ------ - ----4 - -----+--- -=------- -------

2.5 NSAL NSAS NSBI 2

1.5 tE 4 I

2.5

2.5~~ SSAL 2 25SSAS - __ __ __ __

o

0 6 10 15 20 25 0 5 10 1$ 20 25 0 5 10 15 20Q 25

Fig. 5. (part 1).

532 OLKOS 78:3 (1997)



2.5 PIOA

2

1.5

0.5 -

0 I

2.5 NSOA NSPI

2- T

1.5 -

0.5 -X

0 - I _ I _ I . I I . I I | I .

2.5 SSOA SSPI SSNS

2

1.5

0.5

0 e

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

Fig. 5. Carbon dioxide release rates (mg C02-C d- g dw-1) for the mixed units. Each graph shows expected (broken lines) and observed (solid lines) means +se (n = 3). Any deviation between the two lines indicates an interaction.

ing and examination of the time trends (Fig. 10) for NHZI release is necessary to obtain a fuller appreciation of the interactions occurring. Alder and ash mixtures were characterized by high N mineralisation, which largely matched the behaviour of the pure litters. How- ever, expected and observed values for NHt+ release rarely coincided and interaction effects, both positive and negative, occurred in every litter combination at some point during the experiment. In particular, the

220 -

200-

180- 0

160

6140-

120

100 +

W 0 mCD aWgQU X. tv

Litter combination

Fig. 6. Expected (empty bars) and observed (shaded bars) total CO2 release (mg C02-C g dw- 1) for the mixed units. Data are means + se (n = 3). Stars indicate significance; *: P < 0.05, **: P < 0.01, ***: P < 0.001.

peaks for N mineralisation tended to be delayed in mixture. Alder mixtures perhaps showed this trend most clearly, with peaks in NH+ release, corresponding

160 1200

140 1000

_120

CD 800 0

100 55

z

co 60 a

400 I-

z 40 z

-200 20

0 0

0 5 1 0 1 5 20 25

Time (weeks)

Fig. 7. Ammonium release rates (fig NH4-N d-l g dw I) for the pure litters; alder (El), ash (0), birch (A), N. spruce (x), oak (*), Scots pine (0) and S. spruce (+). Note that alder and ash (solid lines) are scaled to the right side y-axis, whereas the other litters (broken lines) are scaled to the left-side y-axis. Data are means ? se (n 3).

OIKOS 78:3 (1997) 533



3.5

a

3 T b

0)2.5- z

0) -2 B

1.5-

I_ C z

0.5 - d

8 d

Alder Ash N. Birch S. Pine Oak spruce spruce

Litter type

Fig. 8. Total NH release (mg NH4-N g dw-1) for the pure litters. Data are means + se (n = 3). Means with different letters are significantly different (P < 0.05).

to the peak for pure alder, being observed two to four weeks later than expected. The magnitude of these peaks also varied, often being lower than expected, except for ASAL and NSAL. Following this displaced peak the alder mixtures continued to release greater quantities of NH+ than expected until the end of the incubation, with the exception of ASAL.

Mixtures with N. spruce tended to pass through a 'lag' phase where release was much lower than expected, but then increased sharply, reversing the initial interaction (Fig. 10). This was the case for NSBI, NSOA and NSPI, whereas SSNS appeared to have only reached this transition by the end of the experiment. Similar to N. spruce, NH+ mineralisation from birch mixtures tended to lag behind expected values, subse-

3

2.5 E

*D B

Litter combination

Fig. 9. Expected (empty bars) and observed (shaded bars) total NH4 release (mg NH4-N g dw- I)for the mixed units. Data are means ? se (n = 3). Stars indicate significance; *: P < 0.05, **: P<0.01, ***: P< 0.00.

quently rising to give positive interactions. At the end of the incubation period OABI still appeared to be in the 'lag' phase, whereas PIBI was apparently in transition. The marked increase in NH+ release noted to- wards the end of the incubation for pure birch was also apparent in the mixtures.

Digestion analyses

Pure litters The chemical compositions of the litters before and after 26 weeks of decomposition are presented in Table 2. Calcium concentrations in all pure litters increased during the experiment, with pine showing the largest proportional increase and S. spruce the smallest. Mag- nesium concentrations decreased slightly in S. spruce and oak, whilst increasing slightly in the other litters. Potassium decreased in concentration in all pure litters, with alder losing the highest proportion and oak the lowest. In contrast, N concentrations increased in all litters, although only to a limited extent for alder and pine. N. spruce had by far the largest relative N accumulation, more than doubling its initial concentration, whilst ash had the second highest accumulation.

Mixtures Mixed litters showed very different changes in nutrient concentrations to the pure litters (see Table 2). To summarise; the chemical compositions of two litters decomposed in mixture were much more alike than were the equivalent pure residues. For example, the concentrations of N in the litter residues of S. spruce and alder at the end of the experiment were 1.6 and 4.7%, whereas the SSAL mixture resulted in 1.9 and 4.4%, respectively. The nutrient contents of the two litter residues from mixtures only diverged in NSOA and NSPI for K+, and SSOA for Ca2+.

Discussion

The initial Ca content of the pure and mixed litters correlated well with total respiration, apparently indi- cating that across a wide range of litter types Ca is an important resource quality attribute. This is in agreement with the relationship described by Heal et al. (1978), to explain the first year mass loss of a number of litter types. However, this may have been due to a correlation between N and Ca, and closer examination of the mixtures contributing to this high correlation showed that without ash the relationship was non-significant.

The N content of a litter is generally considered to be a good indicator of initial decomposition rates (En- riquez et al. 1993, Cotrufo et al. 1995) and, overall, our results confirmed this. However, alder did not fit this

534 OIKOS 78:3 (1997)



600 ASAL T 500 it

400 300

200 100

400 BIAL 600 ASBI

350 - 'TIT 500-

250 1 4005 , 1 200 3 300 ?

150 - 200 1 100 50 A - 100

0 I I I 0

350 OAAL 600 T ASOA 80 - QABI 300 j 500 70 250 400 60

400~ ~ T4 400- NSAL B00- NSAS 200 - NSB5

210 0 200 | 30 + - 20 I 50 ,L100 10c

0 I I H- 0 0

350 PIAL 600- PIAS 80 PIBI 300 500 - 70

250 IT 4 00 6 0 +tp

50 - 250 400 60

200 ' 3~~~~~~~~00 40-

100 5 10 [ 200 5 1 20 2 0 50 10100 (p, l10 0 i 0

-F--7t 05

400 NSAL 600 NSAS 200 NSBI 35050 300 } 150

250 ~~~~~~~400 150~~~~~~~~~~~~~~~~~0 200 3~~~~~~~00 150 200

~~~~-T50 100

1000i 00 I I0

350 SSAL 600 SSAS 100 5581B 300 {500 $8 26'0- 400

25 ~~~~~~~~~~~~~~~~60 300 150 40

100 ~~~~~~~200 100~~~~~~~~~~~~~~~~2 50 100- 20 --

0 I I 0 -0

0 5 10 1520 25 0 5 10 1520 25 0 5 10 15 20 25

Fig. 1 0. (part 1).



25 - PIOA

20 -

15 -

10

5

0

60 NSOA 60 NSPI

35- - SSOA 40 SSPI 100 SSNS

25 ~~~~~~~30 20 25 -60

15 ~~~~~~~~~~~~20- 15 - ~~~~~15 --40 10 10 20 5 5 0 - - - 0 I I 0

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

Fig. 10. Ammonium release rates (gig NH4-N d-1 g dw-1) for the mixed units. Each graph shows expected (broken lines) and observed (solid lines) means +se (n = 3). Any deviation between the two lines indicates an interaction.

general pattern (see Figs 4 and 5), although there was a significant correlation between CO2 release and initial N content within alder mixtures. Alder had the highest initial N content (Table 2) and, whilst appearing to have been partly responsible for positive interaction effects (see Fig. 6), this high concentration did not result in high levels of respiration in the pure alder units. This suggests that either some other factor was limiting (e.g. Ca, see Table 2) or that the high levels of N were inhibitory to microbial activity. Reduced decomposition rates under high N availability have been reported (Fog 1988) especially for later stages of decomposition when lignin degradation becomes a significant proportion of overall decomposition (Berg and Ekbohm 1991). If Ca had been limiting then one would expect that combinations of alder with litters relatively rich in Ca (e.g. ash) would result in enhanced decomposition and, consequently, increased CO2 output. There was no clear evidence for such an effect (Fig. 5), possibly because Ca is an integral part of the cell wall (as calcium pectate) and is, therefore, not easily moved or translocated.

Since eight combinations showed a significant (P < 0.05) positive interaction for total CO2 production whilst only one gave a significant (P< 0.05) negative interaction (Fig. 6), it is probable that the mixing of litters is more likely to have a positive rather than negative influence on decomposition rate, negating the

suggestion of Fyles and Fyles (1993) that positive and negative effects are equally likely. The lack of interaction for CO2 output in PIOA confirmed the observation of Klemmedson (1992) that a mixture of Pinus ponderosa with Quercus gambilii showed no decomposition interactions.

It is useful to compare our results with those of Chapman et al. (1988) since four of the litters used here (alder, N. spruce, oak and Scots pine) were collected from pure stands used in their study. Chapman et al. (1988) observed higher than expected respiration in the forest floor of NSPI mixtures and lower than expected respiration for NSOA and NSAL. Only the results from the NSPI mixture are in agreement with the present work. We found no interaction effects for NSAL, and NSOA showed significantly higher respiration than expected. Faunal interactions in the study of Chapman et al. (1988), may explain these conflicting results since lower than expected numbers of enchy- traeid worms and nematodes were found in the NSOA stands (fauna were excluded from our microcosms). However, the differences in faunal numbers observed were only of the order of 10%, with no significant differences for other components of the soil fauna, and it seems unlikely that such a small difference in enchy- traeid and nematode numbers could reverse a positive respiration interaction to a negative one.

536 OIKOS 78:3 (1997)



These contrasting findings may also be due to the respiration measurements of Chapman et al. (1988) being performed eight to nine months after litterfall. In the present study, using fresh litters and representing the initial stages of decomposition, CO2 efflux was much greater in the NSOA mixture than in either of the pure litters and far in excess of that anticipated. This suggests increased initial biological activity in the mixture; the oak litter in the NSOA microcosms was visibly more decomposed than pure oak, showing large areas of bleaching not found in any of the pure oak units. Thus, the reduced respiration values found by Chap- man et al. (1988) may have been due to the forest floor litter of the mixed stands being at a more advanced stage of decomposition than the litter in the corresponding pure stands.

The 21 combinations resulted in four significantly (P < 0.05) lower, and one significantly (P < 0.05) higher, values for total NH+ release. However, total release was not an effective way of presenting the observed results for NH+ release since all mixtures showed interactions during the 26-week incubation, and the time course for N release is of importance. In nearly all cases the observed trend was for increased initial retention of N followed by increased release, when compared to expected values (Fig. 10). Klemmedson (1992) and Fyles and Fyles (1993) observed greater retention of N in their mixtures, although in neither case did the experiment last sufficiently long for net mineralisation to take place. In contrast, Blair et al. (1990) observed higher than expected initial N mineralisation in mixtures of Cornusflorida with Acer rubrum as well as C. florida with A. rubrum and Quercus prints. The lack of an obvious interaction on N dynamics for the other mixture in their study (A. rubrum with Q. prints) appears to be an exception.

The 'synchrony' theme of the Tropical Soil Biology and Fertility Programme (TSBF) has identified the need to synchronise the release of nutrients from above- ground inputs and roots with plant growth demands (Swift 1987), with the suggestion that lower initial mineralisation of N followed by increased mobilisation may lead to greater synchrony. Myers et al. (1994) stated in their review that, despite being an important component of the synchrony strategy, there was incon- clusive evidence to support the concept of using residue quality (i.e. by mixing resources such as litters) to achieve synchrony. However, in our study, net N mineralisation was retarded (Fig. 10), providing evidence that using organic matter inputs composed of a mixture of plant litters may enable the release of N to be controlled, giving the plants opportunity to utilise more of the N released. Another characteristic of the N interactions found in the present work was that maximum N releases tended to be lower in mixtures (Fig. 10) than expected. Peaks in N release potentially repre- sent times of maximum loss of N from the system

because plants may be unable to utilise all of the available N before it is leached away from the roots (Swift 1987). This phenomenon is particularly important in agroforestry, but may also be relevant in understanding nutrient cycling in natural ecosystems. We suggest that management of litter mixtures could lead to improved synchrony between mineralisation of N and plant uptake.

Six of the eight significant positive interactions on total CO2 efflux were mixtures with Scots pine or oak. Since these litters had the lowest initial concentrations of P, as well as being very poor in N (see Table 2), it seems likely that a 'sharing' of resources between the component litters took place, resulting in the increased rates of decomposition observed. However, such trans- fers did not always result in a positive interaction on respiration and several other studies have noted interaction effects on N dynamics with no apparent effect on decomposition rates (Staaf 1980, Blair et al. 1990, Klemmedson 1992).

Chapman et al. (1988) hypothesised that the differential concentrations of nutrients and rates of decomposition of mixed litters allows translocation of nutrients between litters and hence more rapid utilisation of substrates. We suggest that this translocation was carried out by fungi, which we observed to have formed hyphal bridges between the component litters of a mixture. This was supported by the destructive analyses carried out on several litter combinations at the end of the experiment (see Table 2), which demonstrated that the chemical compositions of two litters decomposed in mixture were much more alike than equivalent pure residues. This phenomenon appears to be governed by nutrient gradients, with movement being from high to low concentration and consequently, diffusion via a water film cannot be discounted as one means of nutrient transfer. The hypothesis that fungi were mainly responsible for the movement of nutrients observed in the current study is supported by studies which have demonstrated translocation of nutrients by sapro- trophic fungi (e.g. Watkinson 1984). Similarly, in a microcosm experiment, Staaf (1980) observed that N from a protein source placed below a 1-cm thick A02 layer was later found in the AO, litter. We suggest that translocation of nutrients from litter, humus and soil is likely to be a general phenomenon.

It is clear that a fuller understanding of the mechanisms governing litter decomposition in mixture is necessary because of the widespread occurrence of mixtures in nature and the potential for management. The current investigation included the largest number of litter combinations ever investigated in one study, yet highlighted the fact that more work is needed on inter- and intra-species interactions to develop a theoretical framework to describe the mechanisms under- pinning the 'mixtures effect' and consequences for nutrient cycling. Specific areas for further research in-

GIKOS 78:3 (1997) 537



elude the importance of ratios of litters occurring in a mixture, and comparisons of interactions in field litter bags and laboratory microcosm systems.

Acknowledgements - The authors would like to thank Sunder- land Local Education Authority and the European Commu- nity for financing aspects of this work. The assistance of the staff of the Chemistry, Library and Computer sections at ITE Merlewood is gratefully acknowledged. In memory of Carol Anne Raine (1967-1989) - rest in peace.

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