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Decomposition of root litter and some factors regulating the process: Long-term root litter decomposition in a scots pine forest
Soil Bid. Eiochem. Vol. 16,No. 6, pp. 609-617,1984 Printedin Great Britain.All rightsreserved
0038-0717/84 $3.00+ 0.00 Copyright0 1984PergamonPressLtd
DECOMPOSITION OF ROOT LITTER AND SOME FACTORS REGULATING THE PROCESS: LONG-TERM ROOT LITTER DECOMPOSITION IN A SCOTS PINE FOREST BJ~RN BERG Section of Forest Ecology, Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala 7, Sweden (Accepted 6 March 1984)
Summary-Decomposition of root litter was studied using Scats pine roots (six diameter classes) and rhizomes from heather (three diameter classes) and cowberry (one diameter class). For Scats pine roots, root diameter was correlated with initial concentrations of N, P, S and Mg but not with organic+hemical composition. The highest nutrient concentrations were found in Scats pine roots and the lowest in heather rhizomes, with cowberry rhizomes intermediate. The highest lignin concentrations were in heather and cowberry rhizomes. In the early decomposition stages diameter and nutrient concentration correlated with mass loss rate in Scats pine roots: in a comparison Scats pine roots were degraded faster than cowberry rhizomes which, in turn, were degraded faster than heather rhizomes. Root diameter, however, may not be important in decomposition of cowberry and heather rhizomes but nutrient and lignin concentrations appear important in all three species. In the late decomposition stages only Scats pine roots could be compared and it appeared that there was a negative correlation with lignin concentration and mass loss, and no correlation with any nutrient.
INTRODUCTION The release of nutrients from litter and humus is a fundamental process in the internal biogeochemical cycle of an ecosystem. The release may take place according to different processes (Staaf and Berg, 1977; Berg and Staaf, 1981) and it appears that with the exception of leaching the release of several nutrients is proportional to litter mass loss once it has started (Staaf and Berg, 1982). With litter decomposition rate being critical for nutrient release it appears important to find factors which regulate the mass-loss rate. The decomposition of needle litter can be divided into two phases (Berg and Staaf, 1980). In the first phase decomposition rate is regulated by the concentrations of nutrients in the litter and in the second phase lignin decomposition rate is the regulating factor. This two-step regulating mechanism was based on decomposing Scats pine needles and would need to be generalized to other litter types. Most decomposition studies in forest systems have been carried out on leaf and needle litter (Swift et al., 1979) which have been incubated on the soil layers whereas little research has been done on root litter decomposition. The latter should be of interest for several reasons, one of which being that root biomass and turnover in some systems appear to be of high magnitudes (Persson, 1981) and another that formation of root litter is similar in quantity to litter formation above ground. Another reason is that the patterns for release or net uptake of nutrients may be very different when the litter is decomposing in the soil instead of on it. It was pointed out by Jenkinson (1965) that “There are formidable difficulties in following the decomposition of plant material in soil
under natural or near-natural conditions”, and for a correct quantification this is certainly true. On the other hand, a quantification is not necessarily the sole aim of a study but more often the principles of the decomposition process as such and factors regulating it. Whereas estimates on turnover of root biomass have been made, only few field investigations have been carried out on the decomposition process of root litter (Waid, 1974). The aim of my research was two-fold. One was to determine decomposition of the three dominant types of root litters in a nutrient poor Scats pine forest system and the other was to investigate regulating factors for decomposition rate. MATERIALSAND METHODS Site description
The 120- to 130-yr-old Scats pine stand studied is located at the SWECON research site, IvantjCmsheden, Central Sweden (60”49’N, 16”30’E) at an altitude of 185 m on a flat area of deep glacifluvial sand sediments. The mean annual precipitation at a nearby village is 609 mm (1931-60), and the mean annual temperature + 3.8”C. The length of the growing season is about 60 days (number of days with a daily temperature higher than 6°C; Axelsson and Brbkenhielm, 1980). The tree layer is exclusively composed of Pinus silvestris L. and has a density of 393 trees ha-’ and a height of 17-19 m. Calluna vulgaris (L.) Hull. and Vaccinium vitis-idaea L. form a well-developed bottom layer, and the bottom layer, completely covering the ground, is mainly composed of the mosses
609
610
BJ~RN BERG
Pleurozium schreberi (Brid.) Mitt. and Dicranum polysetum Sv. together with Cladonia lichens. The
most recent direct effect of forestry practices was a slight thinning in 1960. A more complete description is given by Axelsson and Brikenhielm (1980). The soil is iron podsol (Spodosol) with a weakly developed bleached (AZ) horizon (2-7 cm). The humus type is a typical mor. A loose L horizon, interwoven with living mosses and lichens, covers an F + H horizon of S-10 cm thickness. The F (AO,) and H (A& horizons are almost indistinguishable from each other. The pH range is 3.9-4.2 in the F + H horizon and 4.6-4.8 in the upper mineral soil. The parent mineral material as well as the whole soil is considered to be very poor in essential nutrients. Root collection, storing and weighing
Live roots and rhizomes (pine, cowberry and heather) were dug up from the humus- and upper mineral layers in an area less than 10 x 10 m of the 120-yr-old Scats pine stand. After drying at room temperature for a month the roots were cut into 1Ocm pieces and diameters were measured at the thinnest part of the root pieces. The pine roots were divided into the following diameter classes: 1<4<2, 2<$<3, 5
The litter-bags, made of terylene net with a mesh size of about 1 mm, measured 12 x 5 cm. An amount of l-3 root or rhizome pieces, dried at room temperature (0.61.0 g) was enclosed in each bag. The litter-bags were incubated in the F + H horizon in a measurement plot (1 x 1 m) in each of 20 blocks in a randomized block design in the stand (3 ha). The first incubation was begun in late October 1974 (Scats pine roots set 1 and heather rhizomes) and the second in October 1977 (Scats pine roots set 2 and cowberry rhizomes). Samplings were made once or twice annually during a 3-yr period. Chemical analyses were made on the initial material and on all sampled litter. On each sampling occasion one sample of each type from each of the 20 plots w-as collected. The litter-bags were transported directly to the laboratory and cleaned of ingrown material and humus particles. After drying at 85°C they were weighed individually and then pooled to one composite sample before the chemical analyses were carried out. Chemical analyses
Samples were ground (< 1 mm) in a laboratory mill. The amounts of water-soluble and ethanol-
soluble substances were determined by sonicating the milled sample three times in a sonicator bath and weighing the samples after filtration and drying. The analyses for sulfuric-acid (Klason) lignin and solid carbohydrates (xylan, mannan, galactan, rhamnan, arabinan and cellulose) in the samples were carried out according to Bethge et al. (1971). The release of water-soluble substances from whole roots was investigated on separate samples by allowing them to soak in distilled water at room temperature for 10 or 24 h. The milled samples were also analyzed for total contents of the elements C, N, P, S, Ca, Mg, Mn, K and Na. All analyses were in duplicate. Carbon was determined by the method of Nijmmik (1971). Nitrogen was determined by a semimicro Kjeldahl procedure (Nilhgard, 1972). After an acid wet oxidation in HN03 + HClO,, S was analysed turbidimetrically as BaSO,-precipitate (Blanchar et al., 1965) for P by the vanadate yellow-complex method (Jackson, 1958) and for K by a flame photometric procedure. Ca, Mg, Mn and Na were determined by atomic absorption spectrophotometry (Perkin-Elmer 403) in 1% LaCl,-solution against acid standards (Pawluk, 1967). RESULTS
AND
DISCUSSION
Comments about the substrate
For the study, live roots were dug up, dried at room temperature for a month and thus killed. This fresh root litter can, of course, be regarded as an artificial and dubious substrate. However, it appeared from samples obtained of naturally-formed root litter that it was extremely heterogeneous, representing many different stages of decomposition. From this point of view it is evident that studies with naturallyformed litter would be less meaningful. Further, we know little of the physiology of root death (H. Persson, personal communication) which in this case means knowing little about differences between the chemical composition of live roots and recently dead ones. With regard to this evident criticism we may accept the study as one where the decomposition described from the point of view of principles for decomposition and less of quantification. If applied to a practical situation like a clear-cutting the choice of substrate may be less dubious. Initial chemical composition of the roots
The chemical composition varied both with root diameter within one species and between the species. The organic chemical composition (Table 1) of the Scats pine roots was rather similar to that of Scats pine needles (cf. Berg et al., 1980) and the variation between diameter classes was small. Of the polymer carbohydrates some variation was seen only in the glucans fraction (varying between 31 and 38%) but the concentrations of sulfuric-acid lignin appeared to be fairly similar, from 21 to 22%. The heather rhizomes had lignin concentrations varying between 29 and 35%, whereas cowberry rhizomes had about 31%. For both species the concentrations were higher than for the Scats pine roots. The proportions between the carbohydrates were also different. The heather and cowberry rhizomes had very high concentrations of xylans-about 20 and
D~mposition
of root litter
611
15% respectively-as compared to less than 7% for the pine roots. The concentrations of mannans and arabinans, on the other hand, were lower than in pine roots as was also the amount of solubles. The carbon concentration was highest in heather rhizomes (52-53x) whereas cowberry rhizomes and pine roots had slightly lower concentrations (Table 2). The Scats pine roots contained the highest concentrations of N, P, S, Mg and K, followed by cowberry rhizomes, with heather rhizomes having the lowest concentrations of these components, but the latter had comparatively high concentrations of Mn and Na (Table 2). In the pine roots there were linear correlations between some of the nutrients and root diameter. Thus N varied inversely but not linearly with diameter. Also the con~ntrations of P, S, Mg and Na varied inversely and linearly with the diameter (Fig. 1, Table 3). The concentrations of the other nutrients appeared less dependent on root diameter. In their study on Scats pine needles Berg and Staaf (1980), however, found a close linear relation between the concentrations of N, S and P.
There was very little leaching ( < 1%) and although the initial decomposition rate varied with diameter (Fig. 2) the mass loss of pine root litter could be described fairly well by a first order kinetics relation for all the diameter classes (Table 4) with rate constants (k) varying between 0.195 and 0.149 yr-‘. Even so, in spite of the initial differences the accumulated mass loss after 3 yr was fairly similar. The changes in organic-chemical composition of the decomposing roots of 2-3 mm diameter revealed a pattern similar to those for needles (cf. Berg et al.,
Sire
classes
1-2 2-3
3-5
tmm) --
5-7
3 Average
root diameter
10 fmml
Fig. 1. Relationship between Scats pine root diameter (average for each diameter class), and ( x ): nitrogen concentration; (0) phosphorus concentration; (0) sulfur concentration in the six diameter classes. The size classes are indicated.
BJ~RN BERG
612
Table 2. Initial inorganic-chemical
composition of Scats pine roots and rhizomes of heather and cowberry roots of a 120-yr-old Scats pine stand Concentration
Carbon
Root-type
Nitrogen
Phosphorus
Sulphur
(mg gg’)
Potassium
Calcium
515 514 515 511 ND ND
5.1 3.4 2.9 2.6 2.5 2.6
0.69 0.73 0.61 0.60 0.50 0.44
Scats pine (Pinus siluestris) 0.87 3.05 1.6 0.82 3.82 1.8 0.66 3.17 1.9 0.62 3.11 1.9 0.48 3.15 1.8 0.35 2.19 1.8
Rhizomes l-2 mm 4 2-3 mm 4 3-5 mm $
538 526 522
3.3 3.9 2.8
0.28 0.26 0.21
Heather (Calluna vulgaris) 0.50 1.43 0.9 0.70 1.15 0.9 0.77 1.40 0.9
Rhizomes
506
5.4
0.57
Roots l-2 mm 2-3 mm 3-5 mm 5-7mm 7-9 mm 9-llmm
4 4 4 q+ 4 4
Magnesium
Manganes
Sodium
0.89 0.81 0.67 0.65 0.47 0.59
0.28 0.38 0.35 0.37 0.31 0.29
0.04 0.02 0.02 0.02 0.01 0.01
0.27 0.33 0.32
1.01 0.94 0.96
0.15 0.13 0.15
0.39
0.50
0.01
Cowberry (Vaccinium oitis-i&a) 0.44 1.51 1.4
ND = not determined.
Table 3. Linear relation between inorganic
components in Scats pine (P. siluestris) roots of different diameter classes (cf. Fig. 1)
Average
N
P
S
K
Ca
Mg
Mn
Na ..~
d N P S K Ca Mg Mn Na
cf. Fig. 1
P < 0.01
P < 0.001 cf. Fig. 1 P < 0.001
NS NS NS NS -
NS NS NS NS NS
P < 0.05
NS NS NS NS NS NS NS
P < 0.05 P
cf. Fig. 1
P < 0.05 P < 0.05 P < 0.05 NS NS
NS P < 0.05
NS NS P < 0.05 NS
NS = not significant.
1982b) (Fig. 3, Table 5) and was similar also for the other diameter classes. First a relatively fast decomposition of solubles took place possibly combined with some low leaching. Some of the cellulose and part of the arabinans were also degraded early due to their easily hydrolyzed structures whereas some other hemicelluloses like xylan, galactan and mannan did not decrease at all or only very slightly during the 3 yr. The absolute amount of the analytical ligin fraction increased initially probably due to
humification (B. Berg and 0. Theander, unpublished data). In spite of some degradation after about 1 yr, the absolute amount in this analytical fraction was still higher after 3 yr than initially. The concentration of lignin + humification products increased during the whole period of decomposition (Fig. 4) (cf. Berg et al., 1982b) and was correlated to mass loss (P < 0.01). These organic-chemical changes were followed for all the root diameter classes and appeared similar.
140
days t
500 Time
Fig. 2. Accumulated forest. (x) l-2mm+;
(days)
mass loss of Scats pine root litter of different diameters in a 120-yr-old Scats pine (A)2-3mm+; (0)3-Smm+;(n) 5-7mmd1;(0)7-9mm$; (W)9-llmmd.
Decomposition
of root litter
613
I 25
I:-0815 P
Tome fyr)
Fig. 3. A generalized
graph of the degradation of the organic-chemical components of Scats pine (P. si/uesrris) root litter of 2-3 mm diameter.
Factors regulating decomposition rate in the pine root litter
In a model for decomposition of Scats pine needle litter (Berg and Staaf, 1980) it was suggested that the nutrient concentrations (N, P and S) regulated massloss rate in an early phase up to about 30-35% mass loss, at which point the lignin concentration appeared to become the regulating factor and rule the decomposition rate in a late phase. As the Scats pine roots had a chemical composition similar to that of needle litter it appeared reasonable to test this model. An additional factor to consider as an initial regulating factor for decomposition would be the root diameter. One could expect that fungal mycelia will take longer to penetrate the roots with larger diameters than thinner ones. There also was a negative linear relation between root diameter and mass loss in the first year of incubation (P < 0.05; Fig. 5, Table 6) for both sets of pine root litter. The roots had initially different concentrations of N, P, S and Mg, depending on diameter-the amounts decreasing significantly with increasing root diameter (Table 3,
40
c
1Yr
I
Root
Fig. 5. The relationship between root diameter and first year mass loss. Two sets of root litter were incubated, one set in 1974 (0) and one in 1977 (x).
Fig. 1) but fairly similar amounts of lignin. When testing for relations between concentration of nutrients and decomposition during the first 12 months of incubation significant linear relations were found for P, S and Mg (P < 0.05, Table 6). No significant relations were seen for Mn, K, Ca and Na. The relation between mass loss and initial N level followed a curved graph (Fig. 6). Also the first set of root litter incubated showed a similar relation between mass loss and initial N level. The strong linking between concentrations of N, P and S in needle litter originally reported by Berg and Staaf (1980), which made it impossible to distinguish whether any of the nutrients was more rate-regulating
1 ,x
Concentration
------x
Absolute
@ (mm1
-I
omounf
-
-.--. -1
L”
140
t
t
t
34.9
495
721
t 1085
I 500
Time
(days)
Fig. 4. Changes in concentration (solid line) and absolute amount (dashed line) of sulfuric-acid lignin in decomposing Scats pine root litter of different diameter classes. (x) I-2mm 4; (0) 2-3 mm 4; (0) 3%5mm 4; (A) 5-7mm 4; (A) 7-9mm 4; (0) 9-11 mm 4.
614
BJGRN BERG Table 4. Rate constants for decomposition of root and rhizome litters in the FH-layer of a 120-yr-old Scats pine (P. sihestris) stand. Maximum weight loss for which the first order kinetics was calculated is given Rate constant r
(VT-9
I-2mm 2-3 mm 3%5mm Z-lmm 7-9mm 9-llmm
f#J Q 4 6 I#J 4
l-2mm 4 2-3 mm r$
n
Decomposition
2 3 3 3 3 2
35 41 38 41 40 26
0.145
Cowberry rhizomes P < 0.05 0.998 3
2
25
0.106 0.080
Heather rhizomes P < 0.001 0.999 4 P < 0.01 0.992 4
3.5 3.5
32 24
of cowberry rhizomes was followed Mg
(VI)
0.195 0.153 0.153 0.166 0.172 0.149
changes for cowberry
(mg
05
for 2 yr. The rate was low, with 15% decomposition for the first year and 25% accumulated mass loss after 2 yr (Table 5). During this period the decomposition could be described by a first order kinetics with the rate constant (k) being 0.145 yr-’ (Table 4). The mass loss of the organic-chemical components followed mainly the pattern shown in Fig. 3 (Table 5). Decomposition of heather rhizome litter and organicchemical changes
The heather root litter was degraded with the lowest initial rate of the root litters investigated. For the three diameter classes studied, namely l-2, 2-3 and 3-5 mm, a tendency might be seen towards lower decomposition rate with increasing root diameter for the first year (9.0, 5.3 and 5.0% respectively). The
g-l)
07
09
3.0
I
I 0.3
I
g-‘)
09 S (mg
0.1
5.0 N (mg
I
0.5
I
Maximum mass loss (%)
Duration
Scats pine roots P < 0.05 0.920 5 P < 0.05 0.941 5 P < 0.05 0.941 5 P < 0.01 0.966 5 P < 0.01 0.990 4 NS 0.995 3
than another, was also the case in the present study. There was no relation between initial lignin concentration and mass loss in this first incubation period. For the period between 12 and 24 months there was no linear relation between root diameter and mass loss nor between any nutrient concentration and mass loss or lignin level and mass loss. After 2 yr, however, in the period between 24 and 36 months there was a significant negative relation between lignin concentration and mass loss (P < 0.001) (Table 6, Fig. 7). It thus appears that there is also a two-phase decomposition for Scats pine root litter with change in rate-determining factors. Mass loss and organic-chemical rhizome lifter
Level of significance
I 0.3
4’:) I 07
I 05 P
(mg
g-‘)
Fig. 6. The relationship between initial concentrations of nutrients in Scats pine root litter and first-year mass loss. Set 1: nitrogen (O), set 2: nitrogen (x ); phosphorus (O), sulfur (A) and magnesium (0).
;: 230
313 352 353 353 319
283
31.3 38.9 41‘6 4s. 1 42.6
28.3 36.4 35.3 39.4
q-1 9.7fO.30) 1S.o(O.87) 21.7(0.82) 25.2(1.08)
q-1 S.3(0.44) 19.8f2.93) 23.q2.62)
:z 122
0 372 960 1308
z 300
211 263
21.1 3690 37.5 38.i 38.9
q-_) 26.9(0.74) 34.q0.96) 34.8(1.63) 40,9(1.71)
3: 495 721 1085
9:
Abs. fme)
Rel. 1%)
Days incubated
~
Litter mass loss (%I
Sulfuric acid ligoin
Table 5. ~ornp~i~ion -~--
245 223 193 163
Heather (C. uufgaris) rhizome litter (2-3 mm 4) 17 3 14 I3 15 6 S 12 18
106 23 26 20
t:
1
157 156 142 122 12s
Cowberry (V. uizis-i&a) rhizome litter (t-2 mm @) 7 27 10 17 3 4 14 t 13 18 3 d 13
z 65
~~
Xylans
146 32 18 18 24
Ambinnns Abs. Imd
::
gj
Rhamnans
239 26 13 16 20
fn ethanol Abs. tm@ scats pi$e (P. PikJe$fr~) root litter ‘2-;gmm 4) 4 4 27 24 28 18 : 23 16 3 20 10
In water Abs. bs)
9
il 12 11 II
Zf
1::
z
42 47 47
Mannans Abe. (me;)
Hemicelhdoses
z 26
26
: 17
29 I1
18 14 19 13 If
gi -~-.-~
Gala&w
components in some root litters in a Scats pine forest. Standard error within parenthesis
Extractable
of organ~c~he~~ -
g 250 218
229 19s 191 174 170
191 154 162 143 138
Sum Abs. fmg)
--~-
295 289 229 206
z 269 221 232
311 265 220 229 203
gi
Cellulose ~_--
$ r. f: 8
s
P. E. s
Table 6. Linear relations between mass loss for Scats pine root litter for the time intervals O-12, 12-24 and 24-36 months and levels of nitrogen, phosphorus, sulphur, magnesium and sulphuric-acid lignin at the start of each interval as well as root diameter (cf. Figs. 3 and 4) Mass N
lOS
Diameter
P
@nmpg-‘f
”._-f”/,)
@nag-?
Lignin d:)
s
Mg
(mgg-‘)
C$ 4 f$ $ 8, i$
25.4 26.9 22.4 23.6 10.1 16.1 I)+-
Period O-12 months 5.7 0.69 3.4 0.73 2.9 0.61 2.6 0.60 2.5 0.50 2.6 0.44 NS *
0.87 0.82 0.66 0.62 0.4E 0.35 t
0.89 0.81 0.67 0.65 0.47 a.59 I
22.3 21.1 21.6 21.7 20.8 22.0 NS
I-2mm d, 2-3mm I$ 3-5 mm # 5-7mm Cp 7-9mm 4 9-11rmn& tNS
12.3 10.8 9.8 6.9 17.4 11.5 -+c
Period 12-24 months 4.7 0.48 4.3 3.9 0.38 5.0 0.32 3.4 0.28 2.8 NS NS
---
0.18 0.14 O.iS o.t9 NS
37.4 36.0 35.7 34.7 34.4 33.8 NS
l-2mm 2-3 mm 3-smm 5-7mm 7-9 mm eilmm t_NS
ND 9.4 11.3 16.9 19.1 ND -tC
Period 2636 months _ 5.1 0.41 3.9 0.36 0.33 ::: 0.30 NS NS
-
111 0.17 0.13 0.12 0.14
1-2 mm 2-3mm 3%smm >7mm 7-9 mm 9-11mm C*
Cp d, # & I# r$
NS
38,l 37.5 36.0 35.4 -.. +w
NS = not significant; ND = not determined; *P < 0.05; **O.OOl< P < 0.01;***P c: 0.001.
T&e~~~~~i~e ~~~~0~~~ mtes b~~~ee~ rum from dl~e$e~~ species There was a clear difference in decomposition rate in the first year between Scats pine roots, heather roots and cowberry rhizomes of similar diameter (1-2 mf (Fig. 7). The Scats pine roots had the highest mass loss with 25.47& heather rhizomes the lowest
d~orn~osj~~~ could be described by a first-order kinetics fairly welt (Table 4). For the heather root litter with a diameter of 1-2 mm the decomposition of the organic-chemical components was followed for about 3 yr, and the changes were seen to follow mainly those shown for pine roots (Fig. 3, Table 5).
Phosphorus/Suiphur
0.3
03
0.5
(mq g”‘)
0.7 MQgnesjum
t mg g-“)
30
Pine
1
2
-
3
Nitrogen
4
5
(mg
6
7
g-Y
Fig. 7. The relations between first year mass loss and concentrations ofnutrientsfor Scats pine and heather root litter and cowberry rhizomes all of l-2 mm diameter. (x ) Nitrogen concentration; (0) phosphorus ~on~ntrat~ou; (A> sulfur ~ou~ent~at~on~ (f3) magnesium conc.entration.
D~om~sition
with 9% and cowberry rhizomes were in between with 15%. If we consider the diameter class of I-2mm only it was possible to compare nutrient composition with mass-loss rate between species. It then appears that the highest mass loss was found in the pine roots which had the highest concentrations of N, P, S and Mg. This is seen from Fig. 7 where mass toss for each species is represented as a function of nutrient concentration. Heather rhizomes with the lowest rate of mass loss were lowest in these nutrients with the exception of S, whereas cowberry rhizomes had intermediate levels. Even if there appears to be a trend there was little point in testing for statistical significance with only 3 values for decomposition. When running a multiple regression for first year mass loss as dependent on diameter, N and lignin for all the material (n = IO), it appeared that diameter was less important whereas N and lignin were the important factors (multiple r = 0.850, P < 0.01). We may conclude that the relative amount of nutrients together with initial lignin level are the dominant factors for initial mass-loss rate for root litter in this system. For the late stages we have only the observation for Scats pine root litter where lignin level determined mass loss. With regard to the amounts of lignin in heather and cowberry rhizomes, this conclusion probably is valid also for these, although it is not demonstrated in this study. It may be noted that neither diameter nor nutrient level would be important after, say 1 year’s decomposition. It has been demonstrated for Scats pine needle titter (Berg et af., 1982a) that there is a connection between litter mass loss and lignin decomposition in late stages. If so the question of what determines lignin decomposition will be important. Based on the finding by Fenn and Kirk (1981) that high concentrations of ammonium and some amino acids repress the formation of the lignolytic enzyme system, suggestions have been made (Berg er al., 1982a) that the total N concentration may exert a negative influence on lignin decomposition and consequently also on litter decomposition in late stages. This was also suggested by Herman et al. (1971). The build-up of other materials, here registered as lignin may, however, add other rate-determining factors.
of root litter
617
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Effects of initial chemical composition on decomposition of roots of three grass species. Canadian Journal of Soil Science 57, 205-215.
Jackson M. L. (1958) Soil Chemical Am&&s. Prentic~HalI, Englewood Cliffs, NJ. Jenkinson D. S. (1965) Studies on the decomposition of plant material in soil. I. Losses of carbon from ‘%Z labelled ryegrass incubated with soil in the field. Journal of Soil Science 16, 104-115. NihlgHrd B. (1972) Plant biomass, primary production and distribution of chemical elements in a beech and a planted spruce forest in south Sweden. Oikos 23, 69-81. NGmmik H. (1971) A modified procedure for determination of organic carbon in soil by wet combustion. Soil Science 111, 330-336. Pawluk S. (1967) Soil analysis by atomic absorption spectrophotometry. Atomic Absorption Newsletters 6, 53-56* Persson H. (1981) Death and replacement of fine roots in forest ecosyste&s-a neglected area of research. Swedish Coniferous Forest Project. Technical ReDort 29. l-25.
Staaf GI. and Berg B. (1477) Mobilization &f plan; nutrients in a Scats pine forest moor in central Sweden. S&a Fenniea 11, 210-217.
Axelsson B. and BrAkenhielm S. (1980) Investigation sites of the Swedish Coniferous Forest Project-Biological and nhysiologica! features. In Srrucfure and Function of &‘c&thern- Coniferous Forests-an Ecosystem Study cf. Persson. Ed.). Ecoloaical Bulletins (Stockholm) 32.2544 Berg B. and ‘Staaf ii. (1980) D&ompositio& r&e and chemical changes of Scats pine needle litter. II. Influence of chemical composition. In Structure and Function qf Northern
Persson, 373-390.
Coniferous Forests-an Ecosystem Study (T. Ed.). Ecological Bulletins (Stockholm) 32,
Staaf H. and Berg B. (1982) Release and accumulation of plant nutrients in needle litter during decomposition. Long term decomposition of Scats pine needle litter II. C~adian
JournaZ of Botany 40, 1561-1568.
Swift M. J., Heal 0. W. and Anderson Decomoosition
in
Terrestrial
J. M. (1979) Blackwell,
Ecosystems.
London. Waid J. S. (1974) Decomposition
of roots. In BioZogy of Plant Litier DecomDosition CC. H. Dickinson and G. F. J.
Pugh, Eds), Vol.’ I, pp. London.
175-211. Academic
Press,
0038-0717/84 $3.00+ 0.00 Copyright0 1984PergamonPressLtd
DECOMPOSITION OF ROOT LITTER AND SOME FACTORS REGULATING THE PROCESS: LONG-TERM ROOT LITTER DECOMPOSITION IN A SCOTS PINE FOREST BJ~RN BERG Section of Forest Ecology, Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala 7, Sweden (Accepted 6 March 1984)
Summary-Decomposition of root litter was studied using Scats pine roots (six diameter classes) and rhizomes from heather (three diameter classes) and cowberry (one diameter class). For Scats pine roots, root diameter was correlated with initial concentrations of N, P, S and Mg but not with organic+hemical composition. The highest nutrient concentrations were found in Scats pine roots and the lowest in heather rhizomes, with cowberry rhizomes intermediate. The highest lignin concentrations were in heather and cowberry rhizomes. In the early decomposition stages diameter and nutrient concentration correlated with mass loss rate in Scats pine roots: in a comparison Scats pine roots were degraded faster than cowberry rhizomes which, in turn, were degraded faster than heather rhizomes. Root diameter, however, may not be important in decomposition of cowberry and heather rhizomes but nutrient and lignin concentrations appear important in all three species. In the late decomposition stages only Scats pine roots could be compared and it appeared that there was a negative correlation with lignin concentration and mass loss, and no correlation with any nutrient.
INTRODUCTION The release of nutrients from litter and humus is a fundamental process in the internal biogeochemical cycle of an ecosystem. The release may take place according to different processes (Staaf and Berg, 1977; Berg and Staaf, 1981) and it appears that with the exception of leaching the release of several nutrients is proportional to litter mass loss once it has started (Staaf and Berg, 1982). With litter decomposition rate being critical for nutrient release it appears important to find factors which regulate the mass-loss rate. The decomposition of needle litter can be divided into two phases (Berg and Staaf, 1980). In the first phase decomposition rate is regulated by the concentrations of nutrients in the litter and in the second phase lignin decomposition rate is the regulating factor. This two-step regulating mechanism was based on decomposing Scats pine needles and would need to be generalized to other litter types. Most decomposition studies in forest systems have been carried out on leaf and needle litter (Swift et al., 1979) which have been incubated on the soil layers whereas little research has been done on root litter decomposition. The latter should be of interest for several reasons, one of which being that root biomass and turnover in some systems appear to be of high magnitudes (Persson, 1981) and another that formation of root litter is similar in quantity to litter formation above ground. Another reason is that the patterns for release or net uptake of nutrients may be very different when the litter is decomposing in the soil instead of on it. It was pointed out by Jenkinson (1965) that “There are formidable difficulties in following the decomposition of plant material in soil
under natural or near-natural conditions”, and for a correct quantification this is certainly true. On the other hand, a quantification is not necessarily the sole aim of a study but more often the principles of the decomposition process as such and factors regulating it. Whereas estimates on turnover of root biomass have been made, only few field investigations have been carried out on the decomposition process of root litter (Waid, 1974). The aim of my research was two-fold. One was to determine decomposition of the three dominant types of root litters in a nutrient poor Scats pine forest system and the other was to investigate regulating factors for decomposition rate. MATERIALSAND METHODS Site description
The 120- to 130-yr-old Scats pine stand studied is located at the SWECON research site, IvantjCmsheden, Central Sweden (60”49’N, 16”30’E) at an altitude of 185 m on a flat area of deep glacifluvial sand sediments. The mean annual precipitation at a nearby village is 609 mm (1931-60), and the mean annual temperature + 3.8”C. The length of the growing season is about 60 days (number of days with a daily temperature higher than 6°C; Axelsson and Brbkenhielm, 1980). The tree layer is exclusively composed of Pinus silvestris L. and has a density of 393 trees ha-’ and a height of 17-19 m. Calluna vulgaris (L.) Hull. and Vaccinium vitis-idaea L. form a well-developed bottom layer, and the bottom layer, completely covering the ground, is mainly composed of the mosses
609
610
BJ~RN BERG
Pleurozium schreberi (Brid.) Mitt. and Dicranum polysetum Sv. together with Cladonia lichens. The
most recent direct effect of forestry practices was a slight thinning in 1960. A more complete description is given by Axelsson and Brikenhielm (1980). The soil is iron podsol (Spodosol) with a weakly developed bleached (AZ) horizon (2-7 cm). The humus type is a typical mor. A loose L horizon, interwoven with living mosses and lichens, covers an F + H horizon of S-10 cm thickness. The F (AO,) and H (A& horizons are almost indistinguishable from each other. The pH range is 3.9-4.2 in the F + H horizon and 4.6-4.8 in the upper mineral soil. The parent mineral material as well as the whole soil is considered to be very poor in essential nutrients. Root collection, storing and weighing
Live roots and rhizomes (pine, cowberry and heather) were dug up from the humus- and upper mineral layers in an area less than 10 x 10 m of the 120-yr-old Scats pine stand. After drying at room temperature for a month the roots were cut into 1Ocm pieces and diameters were measured at the thinnest part of the root pieces. The pine roots were divided into the following diameter classes: 1<4<2, 2<$<3, 5
The litter-bags, made of terylene net with a mesh size of about 1 mm, measured 12 x 5 cm. An amount of l-3 root or rhizome pieces, dried at room temperature (0.61.0 g) was enclosed in each bag. The litter-bags were incubated in the F + H horizon in a measurement plot (1 x 1 m) in each of 20 blocks in a randomized block design in the stand (3 ha). The first incubation was begun in late October 1974 (Scats pine roots set 1 and heather rhizomes) and the second in October 1977 (Scats pine roots set 2 and cowberry rhizomes). Samplings were made once or twice annually during a 3-yr period. Chemical analyses were made on the initial material and on all sampled litter. On each sampling occasion one sample of each type from each of the 20 plots w-as collected. The litter-bags were transported directly to the laboratory and cleaned of ingrown material and humus particles. After drying at 85°C they were weighed individually and then pooled to one composite sample before the chemical analyses were carried out. Chemical analyses
Samples were ground (< 1 mm) in a laboratory mill. The amounts of water-soluble and ethanol-
soluble substances were determined by sonicating the milled sample three times in a sonicator bath and weighing the samples after filtration and drying. The analyses for sulfuric-acid (Klason) lignin and solid carbohydrates (xylan, mannan, galactan, rhamnan, arabinan and cellulose) in the samples were carried out according to Bethge et al. (1971). The release of water-soluble substances from whole roots was investigated on separate samples by allowing them to soak in distilled water at room temperature for 10 or 24 h. The milled samples were also analyzed for total contents of the elements C, N, P, S, Ca, Mg, Mn, K and Na. All analyses were in duplicate. Carbon was determined by the method of Nijmmik (1971). Nitrogen was determined by a semimicro Kjeldahl procedure (Nilhgard, 1972). After an acid wet oxidation in HN03 + HClO,, S was analysed turbidimetrically as BaSO,-precipitate (Blanchar et al., 1965) for P by the vanadate yellow-complex method (Jackson, 1958) and for K by a flame photometric procedure. Ca, Mg, Mn and Na were determined by atomic absorption spectrophotometry (Perkin-Elmer 403) in 1% LaCl,-solution against acid standards (Pawluk, 1967). RESULTS
AND
DISCUSSION
Comments about the substrate
For the study, live roots were dug up, dried at room temperature for a month and thus killed. This fresh root litter can, of course, be regarded as an artificial and dubious substrate. However, it appeared from samples obtained of naturally-formed root litter that it was extremely heterogeneous, representing many different stages of decomposition. From this point of view it is evident that studies with naturallyformed litter would be less meaningful. Further, we know little of the physiology of root death (H. Persson, personal communication) which in this case means knowing little about differences between the chemical composition of live roots and recently dead ones. With regard to this evident criticism we may accept the study as one where the decomposition described from the point of view of principles for decomposition and less of quantification. If applied to a practical situation like a clear-cutting the choice of substrate may be less dubious. Initial chemical composition of the roots
The chemical composition varied both with root diameter within one species and between the species. The organic chemical composition (Table 1) of the Scats pine roots was rather similar to that of Scats pine needles (cf. Berg et al., 1980) and the variation between diameter classes was small. Of the polymer carbohydrates some variation was seen only in the glucans fraction (varying between 31 and 38%) but the concentrations of sulfuric-acid lignin appeared to be fairly similar, from 21 to 22%. The heather rhizomes had lignin concentrations varying between 29 and 35%, whereas cowberry rhizomes had about 31%. For both species the concentrations were higher than for the Scats pine roots. The proportions between the carbohydrates were also different. The heather and cowberry rhizomes had very high concentrations of xylans-about 20 and
D~mposition
of root litter
611
15% respectively-as compared to less than 7% for the pine roots. The concentrations of mannans and arabinans, on the other hand, were lower than in pine roots as was also the amount of solubles. The carbon concentration was highest in heather rhizomes (52-53x) whereas cowberry rhizomes and pine roots had slightly lower concentrations (Table 2). The Scats pine roots contained the highest concentrations of N, P, S, Mg and K, followed by cowberry rhizomes, with heather rhizomes having the lowest concentrations of these components, but the latter had comparatively high concentrations of Mn and Na (Table 2). In the pine roots there were linear correlations between some of the nutrients and root diameter. Thus N varied inversely but not linearly with diameter. Also the con~ntrations of P, S, Mg and Na varied inversely and linearly with the diameter (Fig. 1, Table 3). The concentrations of the other nutrients appeared less dependent on root diameter. In their study on Scats pine needles Berg and Staaf (1980), however, found a close linear relation between the concentrations of N, S and P.
There was very little leaching ( < 1%) and although the initial decomposition rate varied with diameter (Fig. 2) the mass loss of pine root litter could be described fairly well by a first order kinetics relation for all the diameter classes (Table 4) with rate constants (k) varying between 0.195 and 0.149 yr-‘. Even so, in spite of the initial differences the accumulated mass loss after 3 yr was fairly similar. The changes in organic-chemical composition of the decomposing roots of 2-3 mm diameter revealed a pattern similar to those for needles (cf. Berg et al.,
Sire
classes
1-2 2-3
3-5
tmm) --
5-7
3 Average
root diameter
10 fmml
Fig. 1. Relationship between Scats pine root diameter (average for each diameter class), and ( x ): nitrogen concentration; (0) phosphorus concentration; (0) sulfur concentration in the six diameter classes. The size classes are indicated.
BJ~RN BERG
612
Table 2. Initial inorganic-chemical
composition of Scats pine roots and rhizomes of heather and cowberry roots of a 120-yr-old Scats pine stand Concentration
Carbon
Root-type
Nitrogen
Phosphorus
Sulphur
(mg gg’)
Potassium
Calcium
515 514 515 511 ND ND
5.1 3.4 2.9 2.6 2.5 2.6
0.69 0.73 0.61 0.60 0.50 0.44
Scats pine (Pinus siluestris) 0.87 3.05 1.6 0.82 3.82 1.8 0.66 3.17 1.9 0.62 3.11 1.9 0.48 3.15 1.8 0.35 2.19 1.8
Rhizomes l-2 mm 4 2-3 mm 4 3-5 mm $
538 526 522
3.3 3.9 2.8
0.28 0.26 0.21
Heather (Calluna vulgaris) 0.50 1.43 0.9 0.70 1.15 0.9 0.77 1.40 0.9
Rhizomes
506
5.4
0.57
Roots l-2 mm 2-3 mm 3-5 mm 5-7mm 7-9 mm 9-llmm
4 4 4 q+ 4 4
Magnesium
Manganes
Sodium
0.89 0.81 0.67 0.65 0.47 0.59
0.28 0.38 0.35 0.37 0.31 0.29
0.04 0.02 0.02 0.02 0.01 0.01
0.27 0.33 0.32
1.01 0.94 0.96
0.15 0.13 0.15
0.39
0.50
0.01
Cowberry (Vaccinium oitis-i&a) 0.44 1.51 1.4
ND = not determined.
Table 3. Linear relation between inorganic
components in Scats pine (P. siluestris) roots of different diameter classes (cf. Fig. 1)
Average
N
P
S
K
Ca
Mg
Mn
Na ..~
d N P S K Ca Mg Mn Na
cf. Fig. 1
P < 0.01
P < 0.001 cf. Fig. 1 P < 0.001
NS NS NS NS -
NS NS NS NS NS
P < 0.05
NS NS NS NS NS NS NS
P < 0.05 P
cf. Fig. 1
P < 0.05 P < 0.05 P < 0.05 NS NS
NS P < 0.05
NS NS P < 0.05 NS
NS = not significant.
1982b) (Fig. 3, Table 5) and was similar also for the other diameter classes. First a relatively fast decomposition of solubles took place possibly combined with some low leaching. Some of the cellulose and part of the arabinans were also degraded early due to their easily hydrolyzed structures whereas some other hemicelluloses like xylan, galactan and mannan did not decrease at all or only very slightly during the 3 yr. The absolute amount of the analytical ligin fraction increased initially probably due to
humification (B. Berg and 0. Theander, unpublished data). In spite of some degradation after about 1 yr, the absolute amount in this analytical fraction was still higher after 3 yr than initially. The concentration of lignin + humification products increased during the whole period of decomposition (Fig. 4) (cf. Berg et al., 1982b) and was correlated to mass loss (P < 0.01). These organic-chemical changes were followed for all the root diameter classes and appeared similar.
140
days t
500 Time
Fig. 2. Accumulated forest. (x) l-2mm+;
(days)
mass loss of Scats pine root litter of different diameters in a 120-yr-old Scats pine (A)2-3mm+; (0)3-Smm+;(n) 5-7mmd1;(0)7-9mm$; (W)9-llmmd.
Decomposition
of root litter
613
I 25
I:-0815 P
Tome fyr)
Fig. 3. A generalized
graph of the degradation of the organic-chemical components of Scats pine (P. si/uesrris) root litter of 2-3 mm diameter.
Factors regulating decomposition rate in the pine root litter
In a model for decomposition of Scats pine needle litter (Berg and Staaf, 1980) it was suggested that the nutrient concentrations (N, P and S) regulated massloss rate in an early phase up to about 30-35% mass loss, at which point the lignin concentration appeared to become the regulating factor and rule the decomposition rate in a late phase. As the Scats pine roots had a chemical composition similar to that of needle litter it appeared reasonable to test this model. An additional factor to consider as an initial regulating factor for decomposition would be the root diameter. One could expect that fungal mycelia will take longer to penetrate the roots with larger diameters than thinner ones. There also was a negative linear relation between root diameter and mass loss in the first year of incubation (P < 0.05; Fig. 5, Table 6) for both sets of pine root litter. The roots had initially different concentrations of N, P, S and Mg, depending on diameter-the amounts decreasing significantly with increasing root diameter (Table 3,
40
c
1Yr
I
Root
Fig. 5. The relationship between root diameter and first year mass loss. Two sets of root litter were incubated, one set in 1974 (0) and one in 1977 (x).
Fig. 1) but fairly similar amounts of lignin. When testing for relations between concentration of nutrients and decomposition during the first 12 months of incubation significant linear relations were found for P, S and Mg (P < 0.05, Table 6). No significant relations were seen for Mn, K, Ca and Na. The relation between mass loss and initial N level followed a curved graph (Fig. 6). Also the first set of root litter incubated showed a similar relation between mass loss and initial N level. The strong linking between concentrations of N, P and S in needle litter originally reported by Berg and Staaf (1980), which made it impossible to distinguish whether any of the nutrients was more rate-regulating
1 ,x
Concentration
------x
Absolute
@ (mm1
-I
omounf
-
-.--. -1
L”
140
t
t
t
34.9
495
721
t 1085
I 500
Time
(days)
Fig. 4. Changes in concentration (solid line) and absolute amount (dashed line) of sulfuric-acid lignin in decomposing Scats pine root litter of different diameter classes. (x) I-2mm 4; (0) 2-3 mm 4; (0) 3%5mm 4; (A) 5-7mm 4; (A) 7-9mm 4; (0) 9-11 mm 4.
614
BJGRN BERG Table 4. Rate constants for decomposition of root and rhizome litters in the FH-layer of a 120-yr-old Scats pine (P. sihestris) stand. Maximum weight loss for which the first order kinetics was calculated is given Rate constant r
(VT-9
I-2mm 2-3 mm 3%5mm Z-lmm 7-9mm 9-llmm
f#J Q 4 6 I#J 4
l-2mm 4 2-3 mm r$
n
Decomposition
2 3 3 3 3 2
35 41 38 41 40 26
0.145
Cowberry rhizomes P < 0.05 0.998 3
2
25
0.106 0.080
Heather rhizomes P < 0.001 0.999 4 P < 0.01 0.992 4
3.5 3.5
32 24
of cowberry rhizomes was followed Mg
(VI)
0.195 0.153 0.153 0.166 0.172 0.149
changes for cowberry
(mg
05
for 2 yr. The rate was low, with 15% decomposition for the first year and 25% accumulated mass loss after 2 yr (Table 5). During this period the decomposition could be described by a first order kinetics with the rate constant (k) being 0.145 yr-’ (Table 4). The mass loss of the organic-chemical components followed mainly the pattern shown in Fig. 3 (Table 5). Decomposition of heather rhizome litter and organicchemical changes
The heather root litter was degraded with the lowest initial rate of the root litters investigated. For the three diameter classes studied, namely l-2, 2-3 and 3-5 mm, a tendency might be seen towards lower decomposition rate with increasing root diameter for the first year (9.0, 5.3 and 5.0% respectively). The
g-l)
07
09
3.0
I
I 0.3
I
g-‘)
09 S (mg
0.1
5.0 N (mg
I
0.5
I
Maximum mass loss (%)
Duration
Scats pine roots P < 0.05 0.920 5 P < 0.05 0.941 5 P < 0.05 0.941 5 P < 0.01 0.966 5 P < 0.01 0.990 4 NS 0.995 3
than another, was also the case in the present study. There was no relation between initial lignin concentration and mass loss in this first incubation period. For the period between 12 and 24 months there was no linear relation between root diameter and mass loss nor between any nutrient concentration and mass loss or lignin level and mass loss. After 2 yr, however, in the period between 24 and 36 months there was a significant negative relation between lignin concentration and mass loss (P < 0.001) (Table 6, Fig. 7). It thus appears that there is also a two-phase decomposition for Scats pine root litter with change in rate-determining factors. Mass loss and organic-chemical rhizome lifter
Level of significance
I 0.3
4’:) I 07
I 05 P
(mg
g-‘)
Fig. 6. The relationship between initial concentrations of nutrients in Scats pine root litter and first-year mass loss. Set 1: nitrogen (O), set 2: nitrogen (x ); phosphorus (O), sulfur (A) and magnesium (0).
;: 230
313 352 353 353 319
283
31.3 38.9 41‘6 4s. 1 42.6
28.3 36.4 35.3 39.4
q-1 9.7fO.30) 1S.o(O.87) 21.7(0.82) 25.2(1.08)
q-1 S.3(0.44) 19.8f2.93) 23.q2.62)
:z 122
0 372 960 1308
z 300
211 263
21.1 3690 37.5 38.i 38.9
q-_) 26.9(0.74) 34.q0.96) 34.8(1.63) 40,9(1.71)
3: 495 721 1085
9:
Abs. fme)
Rel. 1%)
Days incubated
~
Litter mass loss (%I
Sulfuric acid ligoin
Table 5. ~ornp~i~ion -~--
245 223 193 163
Heather (C. uufgaris) rhizome litter (2-3 mm 4) 17 3 14 I3 15 6 S 12 18
106 23 26 20
t:
1
157 156 142 122 12s
Cowberry (V. uizis-i&a) rhizome litter (t-2 mm @) 7 27 10 17 3 4 14 t 13 18 3 d 13
z 65
~~
Xylans
146 32 18 18 24
Ambinnns Abs. Imd
::
gj
Rhamnans
239 26 13 16 20
fn ethanol Abs. tm@ scats pi$e (P. PikJe$fr~) root litter ‘2-;gmm 4) 4 4 27 24 28 18 : 23 16 3 20 10
In water Abs. bs)
9
il 12 11 II
Zf
1::
z
42 47 47
Mannans Abe. (me;)
Hemicelhdoses
z 26
26
: 17
29 I1
18 14 19 13 If
gi -~-.-~
Gala&w
components in some root litters in a Scats pine forest. Standard error within parenthesis
Extractable
of organ~c~he~~ -
g 250 218
229 19s 191 174 170
191 154 162 143 138
Sum Abs. fmg)
--~-
295 289 229 206
z 269 221 232
311 265 220 229 203
gi
Cellulose ~_--
$ r. f: 8
s
P. E. s
Table 6. Linear relations between mass loss for Scats pine root litter for the time intervals O-12, 12-24 and 24-36 months and levels of nitrogen, phosphorus, sulphur, magnesium and sulphuric-acid lignin at the start of each interval as well as root diameter (cf. Figs. 3 and 4) Mass N
lOS
Diameter
P
@nmpg-‘f
”._-f”/,)
@nag-?
Lignin d:)
s
Mg
(mgg-‘)
C$ 4 f$ $ 8, i$
25.4 26.9 22.4 23.6 10.1 16.1 I)+-
Period O-12 months 5.7 0.69 3.4 0.73 2.9 0.61 2.6 0.60 2.5 0.50 2.6 0.44 NS *
0.87 0.82 0.66 0.62 0.4E 0.35 t
0.89 0.81 0.67 0.65 0.47 a.59 I
22.3 21.1 21.6 21.7 20.8 22.0 NS
I-2mm d, 2-3mm I$ 3-5 mm # 5-7mm Cp 7-9mm 4 9-11rmn& tNS
12.3 10.8 9.8 6.9 17.4 11.5 -+c
Period 12-24 months 4.7 0.48 4.3 3.9 0.38 5.0 0.32 3.4 0.28 2.8 NS NS
---
0.18 0.14 O.iS o.t9 NS
37.4 36.0 35.7 34.7 34.4 33.8 NS
l-2mm 2-3 mm 3-smm 5-7mm 7-9 mm eilmm t_NS
ND 9.4 11.3 16.9 19.1 ND -tC
Period 2636 months _ 5.1 0.41 3.9 0.36 0.33 ::: 0.30 NS NS
-
111 0.17 0.13 0.12 0.14
1-2 mm 2-3mm 3%smm >7mm 7-9 mm 9-11mm C*
Cp d, # & I# r$
NS
38,l 37.5 36.0 35.4 -.. +w
NS = not significant; ND = not determined; *P < 0.05; **O.OOl< P < 0.01;***P c: 0.001.
T&e~~~~~i~e ~~~~0~~~ mtes b~~~ee~ rum from dl~e$e~~ species There was a clear difference in decomposition rate in the first year between Scats pine roots, heather roots and cowberry rhizomes of similar diameter (1-2 mf (Fig. 7). The Scats pine roots had the highest mass loss with 25.47& heather rhizomes the lowest
d~orn~osj~~~ could be described by a first-order kinetics fairly welt (Table 4). For the heather root litter with a diameter of 1-2 mm the decomposition of the organic-chemical components was followed for about 3 yr, and the changes were seen to follow mainly those shown for pine roots (Fig. 3, Table 5).
Phosphorus/Suiphur
0.3
03
0.5
(mq g”‘)
0.7 MQgnesjum
t mg g-“)
30
Pine
1
2
-
3
Nitrogen
4
5
(mg
6
7
g-Y
Fig. 7. The relations between first year mass loss and concentrations ofnutrientsfor Scats pine and heather root litter and cowberry rhizomes all of l-2 mm diameter. (x ) Nitrogen concentration; (0) phosphorus ~on~ntrat~ou; (A> sulfur ~ou~ent~at~on~ (f3) magnesium conc.entration.
D~om~sition
with 9% and cowberry rhizomes were in between with 15%. If we consider the diameter class of I-2mm only it was possible to compare nutrient composition with mass-loss rate between species. It then appears that the highest mass loss was found in the pine roots which had the highest concentrations of N, P, S and Mg. This is seen from Fig. 7 where mass toss for each species is represented as a function of nutrient concentration. Heather rhizomes with the lowest rate of mass loss were lowest in these nutrients with the exception of S, whereas cowberry rhizomes had intermediate levels. Even if there appears to be a trend there was little point in testing for statistical significance with only 3 values for decomposition. When running a multiple regression for first year mass loss as dependent on diameter, N and lignin for all the material (n = IO), it appeared that diameter was less important whereas N and lignin were the important factors (multiple r = 0.850, P < 0.01). We may conclude that the relative amount of nutrients together with initial lignin level are the dominant factors for initial mass-loss rate for root litter in this system. For the late stages we have only the observation for Scats pine root litter where lignin level determined mass loss. With regard to the amounts of lignin in heather and cowberry rhizomes, this conclusion probably is valid also for these, although it is not demonstrated in this study. It may be noted that neither diameter nor nutrient level would be important after, say 1 year’s decomposition. It has been demonstrated for Scats pine needle titter (Berg et af., 1982a) that there is a connection between litter mass loss and lignin decomposition in late stages. If so the question of what determines lignin decomposition will be important. Based on the finding by Fenn and Kirk (1981) that high concentrations of ammonium and some amino acids repress the formation of the lignolytic enzyme system, suggestions have been made (Berg er al., 1982a) that the total N concentration may exert a negative influence on lignin decomposition and consequently also on litter decomposition in late stages. This was also suggested by Herman et al. (1971). The build-up of other materials, here registered as lignin may, however, add other rate-determining factors.
of root litter
617
Berg B. and Staaf H. (1981) Leaching, accumulation and release of nitrogen in decomposing forest litter. In Terrestrial ~jtrogen Cycles (F. E. Clark and T. Rosswall, Eds). Ecological Btdletius (Stockholm) 33, 163-l 78.
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