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Decomposition and nutrient release from C3 and C4 plant litters in a natural grassland
Acta Oecologicu
19 (2) (1998) 115-123 IO Elsevier, Paris
Decomposition and nutrient release from C, and C, plant litters in a natural grassland Zoi Koukoura Assistant
professo7;
L.abomtq
of Range
Science (236),
Aristotle
University
of Thessaloniki,
Greece.
Received July 26 1996; revised August 25 1997; accepted August 27 1997
Abstract - The rate of litter loss, and the release of N, P, K, Ca during litter decomposition of two C, plants (Dichanfhium
ischaemum L. and L.) and one C, plant (Festuca ovina H. ) were studied for 24 months. This was achieved by placing leaf and culm litter in nylon screen bags close to the soil surface, sampling every three months to mease the changes in litter weight and chemical composition during the experimental period. Litter decomposition was faster during the first 12 months for both leaves and culms of all species. The order was D. ischaemum > E ovina > C. gryllus. The mean rate litter loss for the 12 and 24 month period was similar for leaf and culm litter. Nitrogen accumulation was observed during tbe entire experimental period for all species. Phosphorus release had occurred during the experimental period in the decreasing order: C. gryllus > D. ischaemum > E ovine. Potassium release was observed after 12 and 24 months in the decreasing order D. ischaemum > C. gryllus > E ovinu, while calcium was accumulating. The decomposition rates were similar for C, and C,.,-plants and their contribution to nutrient cycling depended on the release of the various elements during decomposition. 0 Elstkier, Paris Chrysopogon
gtyllus
1. INTRODUCTION
Litter decomposition is an important process in all terrestrial ecosystems because it controls the recycling and availability of nutrients to plants. For grasslands in particular, several studies concerning this process have appeared [ 1, 13, 19,21,29] because of the importance of nutrient recycling and humus formation in such ecosystems. The rate of litter decomposition and nutrient release varies with a number of factors, including the nature of the plant material, the available decomposer community, temperature and moisture conditions (26). Availability of nutrients in the soil can also affect decomposition rates {20]. Also, Agren and Bosatta [2] and Bosatta and Agren [IO] have shown that changes soil organic matter during decomposition are continuous functions of the substrate quality. Quality of soil organic matter or any litter substrate is defined through the feeding microbial populations, i.e. microbial production to assimilation ratio, microbial growth rate and efficiency during decomposition. Comparative studies on the physiology and competition behaviour of C, (cool season) and C, (warm season) plants have concluded that these species can occupy the same site with minimal interspecific competition [35]. However, at sites where available nitrogen is particularly low, C, plants may show an adaptive advantage over C, plants because they can utilize nitrogen more efficiently. In addition, C, plants
can often have better chances of survival because they are generally inferior food sources for herbivores and main insects compared to C, plants. In many Mediterranean type grassland ecosystems C, and C, plants coexist and therefore a better understanding of the decomposition rates and nutrient release from these plants should lead to improved management decisions relevant to the optimum utilization of such ecosystems. For example Papanastasis [28] reported that litter decomposition of C, and C, plants, which coexisted and were dominant in a Greek grassland, was affected by annual rainfall. The objective of this study was to compare decomposition rates and nutrient release from the litter of ischaemum two C, plants, namely, Dichanthium (D. is.) and Chrysopogon gryllus (C. gr.), and one C3 plant, namely, Festuca ovina (F. ov.), that coexist in temperate grasslands and were the dominant species in such an ecosystem of northern Greece. 2. MATERIALS AND METHODS
2.1. Study area The study was conducted in a natural grassland in Macedonia, northern Greece, covering and area of about 1 ha and located at a 650 m altitude. The total monthly rainfall and mean monthly temperature during the 2 year study (June 1987-June 1989) are
Z. Koukoura
116 shown in figure 1. The soil is relatively shallow, having developed on deposits of the tertiary period. The surface 20 cm soil layer was slightly acidic (pH = 5.6) and contained 1.4 % carbon and 0.14 % tatal nitrogen. Electrical conductivity of the saturation extract was 0.59 dS . m-’ and water soluble cations, namely Ca, Mg, K, and Na amounted to 4.2, 1.3, 0.5 and 0.4 meq + L-l, respectively. Major exchangeable cations included Ca, Mg and K in amounts equal to 13.2,3.4 and 1.4 meq . lOOg-‘, respectively. Available soil P was rather low at 1.7 mg . kg-’ of soil (methods of extraction and determination are given in the next section). The dominant species of vegetation were two C, perennial grasses, i.e. D. is. (8.2 % coverage), C. gr. (29.3 % coverage) and one C, perennial grass, F. ov. (32.3 % coverage).
used (3 species x 2 plant parts x 3 replications x 8 sampling dates), each bag separated from the other by a distance of about 8 cm, arranged in a complete random design. The experimental spot was covered by a plastic net, anchored to the soil surface to secure the bags in place. The experiment lasted for 24 months and 18 bags were retrieved (3 species x 2 plants parts x 3 reps), visible organisms (e.g. earthworms, insects, larvae of insects) every 3 months (at the end of every season) Extraneous mineral matter was removed and the litter residue prepared for chemical analysis. 2.3. Chemical analysis The litter residue in each bag was oven dried at 65 “C for 48 h, then weighed, ground and passed through a 0.4 mm stainless steel sieve for analysis. Total N in the litter was determined by the microKjeldahl method [7]. After dry-ashing the litter (550 “C for 4 h) [ 111, the ash was dissolved in 10 mL of dilute HCl, brought to a volume of 25 mL with distilled water and assayed for Ca and K by atomic absorption spectroscopy and for P by photospectometry using the phosphoro-molybdate method. The ashfree dry weight (a measure of the organic material in the litter) of each sample was determined by loss on ignition at 600 T for 4 h [3]. Carbon content was calculated by dividing the percentage ash-free dry weight by 1.8, as suggested by Lunt [23]. Surface soil samples were taken from the spot where the bags were placed, at the beginning and at the end of the experimental period and analysed for pH in a soil: water slurry (1: I ),
2.2. Sampling Litter from the studied species was collected on 25 May 1987, separated in two parts (leaves and culms) and dried under shade. Litter decomposition was studied by recording changes in the litter chemical characteristics with respect to time (24 months), as it was exposed to climatic and other conditions of the study area. This was achieved by loading 5 g from each plant part into 10 x 10 cm nylon screen bags (3 mm openings) and placing the bags in the study area as follows: on 1 June 1987, in an area of about 2 m*, the vegetation was cut down to a height of 5 cm, removed, and all litter bags were placed in close contact with the soil surface. A total of 144 bags was
1st year
t
Tot. month rainfall
-
Mean month temp.
I
2nd year
Figure 1. Ombmthermic diagram of the experimental area during the experimental period. (-•--)
Tot. month. rainfall. (...w..) Mean month. temper. Acta Oecologica
Deeompositiin
water soluble cations (Ca, Mg, K, Na) in the saturation extract, exchangeable cations (Ca, Mg, K, Na) by extraction with 1 N CHsCOONH,, pH 7, available P by the Olsen procedure (NaHCO, extractable) and total N by the micro-Kjeldahl method. The analytical determination of the cations was performed by atomic absorption spectroscopy and that of P calorimetrically, as previously described. Litter loss at each sampling data was expressed as a percentage of the initial biomass. Calcium, K and P content were expressed as mg . g-’ of dry litter remaining and total N, ash-free dry weight and C as a percentage of dry litter remaining. 2.4. Stastical analysis The data were analysed as a 3 x 8 x 2 (species x time x plant part) factorial experiment. As mentioned before, there were 3 reps for each species and plant part and the reported results are the mean. For comparison of means the criterion of Least Significant Difference (LSD) at P I 0.05 was used [32]. 3. RESULTS
117
from C, and C, plants titters
AND DISCUSSION
3.1. Decomposition The analysis of variance showed that litter decomposition depended on time (P < O.OOl), species (P < 0.05) and plant part (P < 0.01). There was a significant interaction among all these factors (P < 0.05). The decomposition constants (K values) for all species and plant parts are shown in table I. The total loss from the leaf litter varied from 45.9 % to 49.4 % after 12 months and reached up to 64.7 to 72.6 % after 24 months @gure 2). These losses were higher than those of the culms which varied from 32.3 % to 35.6 % after 12 months and reached up to 48.2 % to 50.3 % after 24 months. [ 121 working in a desert steppe dominated by perennial grass Stipa breviflora found a loss of 40 % in biomass after 12 months and of 40-80 % after 24-48 months. Litter decomposition rate was calculated as percentage litter loss per month (average total litter loss in each year divided by the 12). This was 4.1 % litter loss per month for leaves and 2.9 % for culms during the first 12 months and 1.9 % and 1.2 %, respectively, for next 12 months, clearly showing that leaf litter decomposed faster than that of culms and also that the decomposition was generally faster during the first twelve months. The latter could be attributed to the small seasonal changes in temperature and rainfall during the experimental period. Papanastasis [28], working in the same area, has reported that the critical factor for litter decomposition was mainly the annual rainfall. In figure I it can be seen that the mean Vol. 19 (2) ( 1998)
Table I. Decomposition
constants
(k values)
for all species
and plant
Par@. Decomposition Species
D.is.
Plant part
-constants First
(k)ag
r-’
yea
Second year 0.37
leaves
0.59
culms
0.34
0.22
c.gr.
leaves
0.51
0.31
culms
0.28
0.19
Eov.
leaves
0.56
0.35
culms
0.32
0.20
The Decomposition constants were calculated from the weight losses during the first and second year period by the formula x/x, = e”’ (Olson 1963)
monthly temperature and the total monthly rainfall for the first twelve months (first year) were 13.5 “C and 563 mm as compared to 13 ‘C and 35 1 mm for the rest twelve months (second year). MacLean and Wein [25] and Anderson [6] also found either constant or declining litter decomposition rates during a 12-24 month period Using a first order regression model, Andren and Paustian [5] found that soil temperature had little effect on the decomposition rate during the unfrozen period and when soil moisture was included the model was slightly improved. Soil moisture is directly related to the amount of rainfall which was recorded in the present study. They also found that the effects of soil moisture during warm periods were inversely related to temperature responses. Steinberger and Whitford [32], who studied litter decomposition in desert high-range lands of Israel, found that the decomposition rates were essentially zero during the dry summer when rainfall was zero. Thus, the litter dries in summer and remains too dry for the activity of micro-organisms. Our results are in agreement with these because significant differences in the litter decomposition rates of the three species were observed between the two summer seasons, the lower rate being during the second season, in which the air temperature was similar but the rainfall less than that of the first season, Significant differences (LSD = l,l> were observed among the leaf litter decay rates of the three species, which decreased in the order: D. is. > F. ov. > C. gr. after 12 months, while for D. is. the decay rate differed significantly from the other species after the 24 months period. A more detailed look at the litter loss over the first three month period (summer season) showed an initial loss of 11.3, 7.2 and 6.3 % for D. is., F. ov., C. gr. leaf litter and 7.8, 5.7 and 4.3 % for culms, respectively. During the first three months, litter loss could to be largely due to a physical loss by leaching as was reported by Bocock et al. [9].
118
Z. Koukoura
70
g
60
w 3
50
--D-
Leaves
F. ov.
---ct
Culms
F. ov.
kl .I2 -I
40 30
JJA!?ONDJFMAMJJASONDJFMAMJJAS
Sampling Figure 2. Litter loss (as percentage
of the initial
litter weight)
dates (every
for leaves
C. gr. appears to be more resistant to leaching than the other species not only during the first three months but also during the whole experimental period. 3.2. Ash-free dry weight The analysis of variance showed that the ash-free dry weight (i.e. organic matter content) of the undecomposed litter depended on time (P < O.OOl), species (P < 0.01) and plant part (P < 0.05) with a significant interaction among all these factors. Ash-free dry weight of leaves decreased to 73.0, 71.9 and 70.3 % for D. is., F. ov. and C. gr, respectively, at the end of the first 12 months and to 59.4, 57.6 and 57.0 %, respectively, at the end of the second 12 months relative to the initial levels (data not shown). The differences in the amount of ash-free dry weight reduction between the species was significant (LSD = 0.544) and decreased in the order: D. is. > F. ov. > C. gr. The greater losses in ash-free dry weight were observed during the cold and humid seasons. These losses can be attributed to the presence of water soluble carbohydrates, which are easily leached by rainfall. This is supported by the findings of Vardavakis and Koukoura [33]. The same trends were observed for the ash-free dry weight of the culms, whose original content was lower than that of the leaves. The losses among the species was decreasing in the order: D. is. > F. ov. > c. gr.
and culms
3 months) during
the experimental
period.
3.3. Nitrogen The analysis of variance showed that variation in N content yigure 3) depended on time (P < 0.001) species, (P < 0.05) and plant part (P < O.OS),with a significant interaction among all these factors (P < 0.05). Leaf N content was increased by 5.7, 19.3 and 4.2 % for D. is., C. gr and F. ov., respectively, after 12 months, relative to the initial nitrogen content. At the end of the experimental period, it was increased by 14 % for D. is. 27 % for C. gr and 11.8 % for F. ov. These figures show that N accumulation has taken place during the entire decomposition period, for all species. The difference in N accumulation among the species was significant (LSD = 0.149), decreasing in the order: C. gr. > D. is. > F. ov. Increase in N content of culms was observed only during the second year (12-24 months) and amounted to 7.8, 8.6 and 30.9 % for D. is., C. gr. and F. ov., respectively. These figures show that N immobilization has taken place during the first 12 months and accumulation in the next 12 months. Increases in litter N content, which took place during litter decomposition for 12 and 18 months, were also reported by Pastor et al. [29] working in two old fields dominated by prairie grasses in Minnesota. Fahey [16] states that increases in N content during the early stages of litter decomposition are common and presumably result from transfer by saprophytic fungi. Mineralization of litter N has tradiActa
Oecologica
119
Decomposition from C, and C, plants litters
+
Leaves
D. is.
-
Leaves
C. gr.
-c--
Leaves
F. ov.
-
Culms
D. is.
-+-
Culms
C. gr.
--o-
Culms
F. ov.
3.0
l 0.76 l O.58 0 0.55
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling Figure 3. Nitrogen
concentration
(%) of leaves
and culms
during
the experimental
tionally been related to the C:N ratio of the substrate. Lutz and Chandler [24] have shown that N mineralization should occur at C:N ratios between 2O:l and 30: 1. Above these levels microbial immobilization occurs. Edmonds [14] reported that the C:N ratio at which N mineralization occurred for needle litter of silver fir stands was 23: 1 to 35: 1 or less. In tabEe II the C:N ratios for species litter during the experimental period are given. The magnitude of these ratios and their change with time provided evidence that, for the leaf litter of all species, N mineralization had occured at the end of the experiment. This was not the case for the litter of culms except possibly for the species D. is. where C:N ratio dropped below the value of 35. Mary et al. [26] demonstrated that the availability of N in soil can control the decomposition process and strongly reduce the N immobilization potential of plant residues. Under limiting N conditions immobilization lasts longer but at a lower rate. 3.4. Phosphorus The analysis of variance showed that P concentration depended on time (p < O.OOl), species (p c 0.01) and plant part (p < 0.01) with a significant interaction among all these factors (p c 0.01). Leaf P concentration (figure 4) was 1.08, 0.85 and 0.65 mg . g-t for Vol. 19 (2) (1998)
dates (every
3 months)
period
Table II. C/N ratios experimental
for
all species
and
plant
part
during
the
period. Plant part
Species
Initial
12 months
24 months
Leaves
Culms
Leaves
Culms
Leaves
Culms 34.7
D. is.
35.5
59.2
27.3
46.2
20.2
C. gr.
30.1
76.5
21.9
65.3
17
46
F. ov.
36.2
80.1
28.2
63.6
21.1
40.6
D.is., C.gr. and Eov., respectively at the end of the first 12 months. These figures represent a reduction of 12.3, 26.7 and 8.4 % relative to the initial P concentration for the same species. After 24 months, leaf P concentration was 0.76, 0.16 and 0.72 mg . g-l, which represent a reduction of 21.6, 82 and 1.4 %, relative to the initial, for D.is., C.gr. and Eov., respectively. These figures show that P release had occurred during both periods in the decreasing order: C.gr. > D.is. > F. ov. with significant differences (LSD = 0.12) among the species. Decrease in P concentration with time was observed by Kalburtzi et al [21], studying nutrient release in an agricultural ecosystem and by Grier [ 181 in a forest ecosystem. Pomeroy [30] stated that P recy-
Z. Koukoura
120 cling influences the other nutrients and this may be exhibited in the N:P ratio. Gosz et al [17] found that the N:P ratio of plant material tended to stabilise with time, while Kalburtzi et al [21] found increase of this ratio with time. Our results showed that the N:P initial ratios of 14.4 for D.is., 17.2 for C.gr and 19.0 for F. ov. changed to 20 for D.is. and C.gr and 21.6 for F. ov. at the end of the first 12 months and to 21 and 20.8, respectively, during the next 12 months. Enright and Ogden [ 151 mentioned that a critical C:P ratio below which P is released from decomposing litter was about 650: 1, while Gosz et al. [ 171 gave a ratio of 480: 1. This ratio in this study was 5 12, 541 and 690 for D.is., C.gr and Eov., respectively, in the initial material and had changed to 434, 367 and 457.1, respectively, at the end of the experiment. These figures show that P release has occurred from the leaves of all species.
concentration, by 72.8, 73.9 and 63.4 % for D.is., C.gr and Eov., respectively. At the end of the experiment it varied between 0.95 and 0.72 mg/g and had been reduced by 79.7 %, 77.4 % and 74.2 % for D.is., C.gr. and Eov., respectively. It is obvious that K release had occurred during both periods. The difference in K release among the species was significant (LSD = 0.29) and decreased in the order: D.is. > C.gr > F. ov. for the first 12 months (figure 5). The greatest release was observed during the first 9 months for all studied cases (figure 5). The higher K release of the first 12 months could be attributed to more intense leaching due to high rainfall and faster decomposition rates during this period. Gosz et al. [17] working in hardwood forest at Hubbard Brook found that about 70 % of litter K was released during the first 12 months with maximum in autumn. Alexander [4] states that K is not a structural component of plant tissue and is present in plant residues as water soluble salts. This is the reason why it is so readily leached from so many different litter substrates. The leachability of this element has been also demonstrated by Attiwill [S] even for woody tree branch material. The amount of K remaining at the end of the experiment decreased in the order: D.is. > C.gr. > Eov. Potassium concentration in culms was less than that of the leaves and its changes followed the same pattern during the experiment. From our results it is obvious that K
3.5. Potassium The analysis of variance showed that K concentration depended on time (p c O.OOl), species (p < 0.01) and plant part (p < 0.01) with a significant interaction among all these factors (p < 0.05). Leaf K concentration (figure 5) varied between 1.27 and 0.73 mg/g for all species at the end of the first 12 months and had been reduced, relative to the initial
--c
Leaves
D. is.
-
Leaves
C. gr.
-
Leaves
F. ov.
-
Culms
D. is.
-t
Culms
C. gr.
-o-
Culms
F. ov.
JF
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every
3 months)
Figure 4. Phosphorus concentration (mg g-‘) of leaves and culms during the experimental
period. Aria
Oecologica
121
Decomposition from C, and C, plants litters
5.0
/r
4.5 4.0 3.5 53 aI lk
3.0 3.82
l
E 2 w B d!
r3.54
2.5
l 3.15
2.0 I-
02.80 02.30
1.5 1 .oI0.5 ,0.C I-JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every 3 months)
Figure 5. Potassium concentration (mg g-‘) of leaves and culms during the experimental
6
--C
Leaves
D. isc.
A--
Leaves
-a-
Culms
D. kc.
--t
Culms C. gr.
period.
C. gr.
-.I
:cn5 E E 4 2 -0 9
3
2
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every
3 months)
Figure 6. Calcium concentration (mg g-‘) of leaves and culms during the experimental period. Vol. 19 (2) (1998)
-
Leaves
D. is.
-
Culms
D. is.
-
Leaves
C. gr.
-
Culms
C. gr.
-o--
Leaves
F. ov.
-
Culms
F. ov.
Z. Koukoura
122 release in D.is. and C.gr. (C, species) was greater than that in F. ov. (C, species). 3.6. Calcium (mg * 8’) The analysis of variance showed that Ca concentration depended on time (P < O.OOOl), species litter (P < 0.0001) and plant part (P < 0.0001) with a significant interaction among all these factors (P < 0.05). Leaf Ca concentration (fisure 6) ranged from 3.5 to 4.9 mg . g-’ at the end of the first 12 months which corresponds to a relative increase of 31.3, 12.2 and 7.9 % for Eov., C.gr. and D.is., respectively, with respect to the initial concentration. After 24 months, Ca concentration ranged from 5.20 to 4.9 mg +g-’ , a relative increased of 85, 46 and 15 % for Eov., C.gr. and D.is., respectively. These results point to the fact that Ca accumulation had occurred during both periods, with higher values during the second. Ca accumulation followed the order: Eov. > C.gr. > D.is. with the differences among the species being statistically significant (LSD = 0.92), after the 24 months. Vogt [34], working on conifer forest ecosystem, found that the amount of Ca in leaf litter had doubled after 2 years of decomposition. Calcium concentration in culms was lower than that of the leaves and accumulation also occurred in both periods. The relative increase in concentration, after 24 month time was 52.1, 40.0 and 30.3 % in D.is., C.gr., and Eov., respectively. Significant increases (LSD = 0.92) were observed only during the summer months for all species. 4. CONCLUSIONS
The results of the decomposition experiment presented here are general in agreement with the findings of other studies concerning grasslands [6, 12, 281. Leaching and micro-organisms activity in relation to rainfall and litter chemical composition are likely to be the most important factors which determine litter loss. Litter of C.gr. appeared to be more resistant to decay than that of D.is. and Eov, while there was no separation between the decomposition rates of C, and C, plants in this study. The degree of contribution of the different species to nutrient cycling, depended on the rate of release of elements during decomposition Our results suggested that more N was accumulated in C, plants than in C,, but no N mineralisation was observed in any of the species. Potassium was the most readily released element and at a greater rate from the C, plants, while no calcium release was observed. Phosphorus release was observed for all species at the end of the experiment with higher values for C.gr.
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Oecologicu
Decomposition from C, and C, plants litters [21] Kalburtzi K.L., Veresoglou D.S., Vokou D., Decomposition and nutrient release from wheat and fababean straw under field conditions, Agr. Ecos. Em. 30 (1990) 107-120. [22] Koelling M.R., Kucera C.L., Dry matter losses and mineral leaching in bluestem standing crop and litter, Ecology 46 (1965) 529-532. [23] Lunt H.A., The carbon-organic matter factor in forest soil humus, Soil Science. 32 (1931) 27-33. [24] Lutz H.J., Chandler R.F., Forest soils, John Wiley and Sons Inc., New York, 1946, pp. 514. [25] Maclean D.A., Wein R.W., Litter production and forest floor nutrient dynamics in pine and hardwood stands of New Brunswick Canada Holaretic, Ecology I (1978) I-5. [26] Mary, B. Recous, S. Darwis, D., Robin D., Interactions between decomposition of plant residues and nitrogen cycling in soil, Plant and Soil. 181 (1996) 71-82. [27] Mason C.F., Decomposition, Camelot Press, Southampton, 1977, p. 58. [28] Papanastasis V., Production of natural grasslands in relation to air temperature and precipitation in Northern Greece, Dozent
Vol. 19 (2) (1998)
123
[29]
[30] [31] [32] [33]
[34]
[35]
dissertation, Aristotle University of Thessaloniki, Greece, 1982 (In Greek). Pastor J., Stillwell M.A., Tilman D., Litter bluestem litter dynamics in Minnesota old fields, Oecologia (Berlin) 72 (1987) 327-330. Pomeroy L.R., The strategy of mineral cycling, Annu. Rev. Ecol. Syst. I (1970) 171-190. Steel R.G.D., Torrie J.H., Principles and procedures of statistics, McGraw-Hill, New York, 1980, pp. 633. Steinberger Y., Whitford W., Decomposition process in Nege’ ecosystems. Oecologia (Berlin) 75 (1988) 61-66. Vardavakis E., Koukoura Z., Quantitative studies of microflora during leaf and stem decomposition of four herb plant species, Geotech. Sci. Iss. 4 (1993) 78-90 (In Greek).. Vogt K.A., 0, consumption, CO, evolution and assimilation during microbial metabolism of starch cellulose and xylan, Thesis, New Mexico State University, Las Cruces, New Mexico, 1974. Williams J., Markley J.L., The photosynthetic pathway type of North American Shortgrass prairie species and some ecological implications, Photosynthetica 7 (1973) 262-270.
19 (2) (1998) 115-123 IO Elsevier, Paris
Decomposition and nutrient release from C, and C, plant litters in a natural grassland Zoi Koukoura Assistant
professo7;
L.abomtq
of Range
Science (236),
Aristotle
University
of Thessaloniki,
Greece.
Received July 26 1996; revised August 25 1997; accepted August 27 1997
Abstract - The rate of litter loss, and the release of N, P, K, Ca during litter decomposition of two C, plants (Dichanfhium
ischaemum L. and L.) and one C, plant (Festuca ovina H. ) were studied for 24 months. This was achieved by placing leaf and culm litter in nylon screen bags close to the soil surface, sampling every three months to mease the changes in litter weight and chemical composition during the experimental period. Litter decomposition was faster during the first 12 months for both leaves and culms of all species. The order was D. ischaemum > E ovina > C. gryllus. The mean rate litter loss for the 12 and 24 month period was similar for leaf and culm litter. Nitrogen accumulation was observed during tbe entire experimental period for all species. Phosphorus release had occurred during the experimental period in the decreasing order: C. gryllus > D. ischaemum > E ovine. Potassium release was observed after 12 and 24 months in the decreasing order D. ischaemum > C. gryllus > E ovinu, while calcium was accumulating. The decomposition rates were similar for C, and C,.,-plants and their contribution to nutrient cycling depended on the release of the various elements during decomposition. 0 Elstkier, Paris Chrysopogon
gtyllus
1. INTRODUCTION
Litter decomposition is an important process in all terrestrial ecosystems because it controls the recycling and availability of nutrients to plants. For grasslands in particular, several studies concerning this process have appeared [ 1, 13, 19,21,29] because of the importance of nutrient recycling and humus formation in such ecosystems. The rate of litter decomposition and nutrient release varies with a number of factors, including the nature of the plant material, the available decomposer community, temperature and moisture conditions (26). Availability of nutrients in the soil can also affect decomposition rates {20]. Also, Agren and Bosatta [2] and Bosatta and Agren [IO] have shown that changes soil organic matter during decomposition are continuous functions of the substrate quality. Quality of soil organic matter or any litter substrate is defined through the feeding microbial populations, i.e. microbial production to assimilation ratio, microbial growth rate and efficiency during decomposition. Comparative studies on the physiology and competition behaviour of C, (cool season) and C, (warm season) plants have concluded that these species can occupy the same site with minimal interspecific competition [35]. However, at sites where available nitrogen is particularly low, C, plants may show an adaptive advantage over C, plants because they can utilize nitrogen more efficiently. In addition, C, plants
can often have better chances of survival because they are generally inferior food sources for herbivores and main insects compared to C, plants. In many Mediterranean type grassland ecosystems C, and C, plants coexist and therefore a better understanding of the decomposition rates and nutrient release from these plants should lead to improved management decisions relevant to the optimum utilization of such ecosystems. For example Papanastasis [28] reported that litter decomposition of C, and C, plants, which coexisted and were dominant in a Greek grassland, was affected by annual rainfall. The objective of this study was to compare decomposition rates and nutrient release from the litter of ischaemum two C, plants, namely, Dichanthium (D. is.) and Chrysopogon gryllus (C. gr.), and one C3 plant, namely, Festuca ovina (F. ov.), that coexist in temperate grasslands and were the dominant species in such an ecosystem of northern Greece. 2. MATERIALS AND METHODS
2.1. Study area The study was conducted in a natural grassland in Macedonia, northern Greece, covering and area of about 1 ha and located at a 650 m altitude. The total monthly rainfall and mean monthly temperature during the 2 year study (June 1987-June 1989) are
Z. Koukoura
116 shown in figure 1. The soil is relatively shallow, having developed on deposits of the tertiary period. The surface 20 cm soil layer was slightly acidic (pH = 5.6) and contained 1.4 % carbon and 0.14 % tatal nitrogen. Electrical conductivity of the saturation extract was 0.59 dS . m-’ and water soluble cations, namely Ca, Mg, K, and Na amounted to 4.2, 1.3, 0.5 and 0.4 meq + L-l, respectively. Major exchangeable cations included Ca, Mg and K in amounts equal to 13.2,3.4 and 1.4 meq . lOOg-‘, respectively. Available soil P was rather low at 1.7 mg . kg-’ of soil (methods of extraction and determination are given in the next section). The dominant species of vegetation were two C, perennial grasses, i.e. D. is. (8.2 % coverage), C. gr. (29.3 % coverage) and one C, perennial grass, F. ov. (32.3 % coverage).
used (3 species x 2 plant parts x 3 replications x 8 sampling dates), each bag separated from the other by a distance of about 8 cm, arranged in a complete random design. The experimental spot was covered by a plastic net, anchored to the soil surface to secure the bags in place. The experiment lasted for 24 months and 18 bags were retrieved (3 species x 2 plants parts x 3 reps), visible organisms (e.g. earthworms, insects, larvae of insects) every 3 months (at the end of every season) Extraneous mineral matter was removed and the litter residue prepared for chemical analysis. 2.3. Chemical analysis The litter residue in each bag was oven dried at 65 “C for 48 h, then weighed, ground and passed through a 0.4 mm stainless steel sieve for analysis. Total N in the litter was determined by the microKjeldahl method [7]. After dry-ashing the litter (550 “C for 4 h) [ 111, the ash was dissolved in 10 mL of dilute HCl, brought to a volume of 25 mL with distilled water and assayed for Ca and K by atomic absorption spectroscopy and for P by photospectometry using the phosphoro-molybdate method. The ashfree dry weight (a measure of the organic material in the litter) of each sample was determined by loss on ignition at 600 T for 4 h [3]. Carbon content was calculated by dividing the percentage ash-free dry weight by 1.8, as suggested by Lunt [23]. Surface soil samples were taken from the spot where the bags were placed, at the beginning and at the end of the experimental period and analysed for pH in a soil: water slurry (1: I ),
2.2. Sampling Litter from the studied species was collected on 25 May 1987, separated in two parts (leaves and culms) and dried under shade. Litter decomposition was studied by recording changes in the litter chemical characteristics with respect to time (24 months), as it was exposed to climatic and other conditions of the study area. This was achieved by loading 5 g from each plant part into 10 x 10 cm nylon screen bags (3 mm openings) and placing the bags in the study area as follows: on 1 June 1987, in an area of about 2 m*, the vegetation was cut down to a height of 5 cm, removed, and all litter bags were placed in close contact with the soil surface. A total of 144 bags was
1st year
t
Tot. month rainfall
-
Mean month temp.
I
2nd year
Figure 1. Ombmthermic diagram of the experimental area during the experimental period. (-•--)
Tot. month. rainfall. (...w..) Mean month. temper. Acta Oecologica
Deeompositiin
water soluble cations (Ca, Mg, K, Na) in the saturation extract, exchangeable cations (Ca, Mg, K, Na) by extraction with 1 N CHsCOONH,, pH 7, available P by the Olsen procedure (NaHCO, extractable) and total N by the micro-Kjeldahl method. The analytical determination of the cations was performed by atomic absorption spectroscopy and that of P calorimetrically, as previously described. Litter loss at each sampling data was expressed as a percentage of the initial biomass. Calcium, K and P content were expressed as mg . g-’ of dry litter remaining and total N, ash-free dry weight and C as a percentage of dry litter remaining. 2.4. Stastical analysis The data were analysed as a 3 x 8 x 2 (species x time x plant part) factorial experiment. As mentioned before, there were 3 reps for each species and plant part and the reported results are the mean. For comparison of means the criterion of Least Significant Difference (LSD) at P I 0.05 was used [32]. 3. RESULTS
117
from C, and C, plants titters
AND DISCUSSION
3.1. Decomposition The analysis of variance showed that litter decomposition depended on time (P < O.OOl), species (P < 0.05) and plant part (P < 0.01). There was a significant interaction among all these factors (P < 0.05). The decomposition constants (K values) for all species and plant parts are shown in table I. The total loss from the leaf litter varied from 45.9 % to 49.4 % after 12 months and reached up to 64.7 to 72.6 % after 24 months @gure 2). These losses were higher than those of the culms which varied from 32.3 % to 35.6 % after 12 months and reached up to 48.2 % to 50.3 % after 24 months. [ 121 working in a desert steppe dominated by perennial grass Stipa breviflora found a loss of 40 % in biomass after 12 months and of 40-80 % after 24-48 months. Litter decomposition rate was calculated as percentage litter loss per month (average total litter loss in each year divided by the 12). This was 4.1 % litter loss per month for leaves and 2.9 % for culms during the first 12 months and 1.9 % and 1.2 %, respectively, for next 12 months, clearly showing that leaf litter decomposed faster than that of culms and also that the decomposition was generally faster during the first twelve months. The latter could be attributed to the small seasonal changes in temperature and rainfall during the experimental period. Papanastasis [28], working in the same area, has reported that the critical factor for litter decomposition was mainly the annual rainfall. In figure I it can be seen that the mean Vol. 19 (2) ( 1998)
Table I. Decomposition
constants
(k values)
for all species
and plant
Par@. Decomposition Species
D.is.
Plant part
-constants First
(k)ag
r-’
yea
Second year 0.37
leaves
0.59
culms
0.34
0.22
c.gr.
leaves
0.51
0.31
culms
0.28
0.19
Eov.
leaves
0.56
0.35
culms
0.32
0.20
The Decomposition constants were calculated from the weight losses during the first and second year period by the formula x/x, = e”’ (Olson 1963)
monthly temperature and the total monthly rainfall for the first twelve months (first year) were 13.5 “C and 563 mm as compared to 13 ‘C and 35 1 mm for the rest twelve months (second year). MacLean and Wein [25] and Anderson [6] also found either constant or declining litter decomposition rates during a 12-24 month period Using a first order regression model, Andren and Paustian [5] found that soil temperature had little effect on the decomposition rate during the unfrozen period and when soil moisture was included the model was slightly improved. Soil moisture is directly related to the amount of rainfall which was recorded in the present study. They also found that the effects of soil moisture during warm periods were inversely related to temperature responses. Steinberger and Whitford [32], who studied litter decomposition in desert high-range lands of Israel, found that the decomposition rates were essentially zero during the dry summer when rainfall was zero. Thus, the litter dries in summer and remains too dry for the activity of micro-organisms. Our results are in agreement with these because significant differences in the litter decomposition rates of the three species were observed between the two summer seasons, the lower rate being during the second season, in which the air temperature was similar but the rainfall less than that of the first season, Significant differences (LSD = l,l> were observed among the leaf litter decay rates of the three species, which decreased in the order: D. is. > F. ov. > C. gr. after 12 months, while for D. is. the decay rate differed significantly from the other species after the 24 months period. A more detailed look at the litter loss over the first three month period (summer season) showed an initial loss of 11.3, 7.2 and 6.3 % for D. is., F. ov., C. gr. leaf litter and 7.8, 5.7 and 4.3 % for culms, respectively. During the first three months, litter loss could to be largely due to a physical loss by leaching as was reported by Bocock et al. [9].
118
Z. Koukoura
70
g
60
w 3
50
--D-
Leaves
F. ov.
---ct
Culms
F. ov.
kl .I2 -I
40 30
JJA!?ONDJFMAMJJASONDJFMAMJJAS
Sampling Figure 2. Litter loss (as percentage
of the initial
litter weight)
dates (every
for leaves
C. gr. appears to be more resistant to leaching than the other species not only during the first three months but also during the whole experimental period. 3.2. Ash-free dry weight The analysis of variance showed that the ash-free dry weight (i.e. organic matter content) of the undecomposed litter depended on time (P < O.OOl), species (P < 0.01) and plant part (P < 0.05) with a significant interaction among all these factors. Ash-free dry weight of leaves decreased to 73.0, 71.9 and 70.3 % for D. is., F. ov. and C. gr, respectively, at the end of the first 12 months and to 59.4, 57.6 and 57.0 %, respectively, at the end of the second 12 months relative to the initial levels (data not shown). The differences in the amount of ash-free dry weight reduction between the species was significant (LSD = 0.544) and decreased in the order: D. is. > F. ov. > C. gr. The greater losses in ash-free dry weight were observed during the cold and humid seasons. These losses can be attributed to the presence of water soluble carbohydrates, which are easily leached by rainfall. This is supported by the findings of Vardavakis and Koukoura [33]. The same trends were observed for the ash-free dry weight of the culms, whose original content was lower than that of the leaves. The losses among the species was decreasing in the order: D. is. > F. ov. > c. gr.
and culms
3 months) during
the experimental
period.
3.3. Nitrogen The analysis of variance showed that variation in N content yigure 3) depended on time (P < 0.001) species, (P < 0.05) and plant part (P < O.OS),with a significant interaction among all these factors (P < 0.05). Leaf N content was increased by 5.7, 19.3 and 4.2 % for D. is., C. gr and F. ov., respectively, after 12 months, relative to the initial nitrogen content. At the end of the experimental period, it was increased by 14 % for D. is. 27 % for C. gr and 11.8 % for F. ov. These figures show that N accumulation has taken place during the entire decomposition period, for all species. The difference in N accumulation among the species was significant (LSD = 0.149), decreasing in the order: C. gr. > D. is. > F. ov. Increase in N content of culms was observed only during the second year (12-24 months) and amounted to 7.8, 8.6 and 30.9 % for D. is., C. gr. and F. ov., respectively. These figures show that N immobilization has taken place during the first 12 months and accumulation in the next 12 months. Increases in litter N content, which took place during litter decomposition for 12 and 18 months, were also reported by Pastor et al. [29] working in two old fields dominated by prairie grasses in Minnesota. Fahey [16] states that increases in N content during the early stages of litter decomposition are common and presumably result from transfer by saprophytic fungi. Mineralization of litter N has tradiActa
Oecologica
119
Decomposition from C, and C, plants litters
+
Leaves
D. is.
-
Leaves
C. gr.
-c--
Leaves
F. ov.
-
Culms
D. is.
-+-
Culms
C. gr.
--o-
Culms
F. ov.
3.0
l 0.76 l O.58 0 0.55
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling Figure 3. Nitrogen
concentration
(%) of leaves
and culms
during
the experimental
tionally been related to the C:N ratio of the substrate. Lutz and Chandler [24] have shown that N mineralization should occur at C:N ratios between 2O:l and 30: 1. Above these levels microbial immobilization occurs. Edmonds [14] reported that the C:N ratio at which N mineralization occurred for needle litter of silver fir stands was 23: 1 to 35: 1 or less. In tabEe II the C:N ratios for species litter during the experimental period are given. The magnitude of these ratios and their change with time provided evidence that, for the leaf litter of all species, N mineralization had occured at the end of the experiment. This was not the case for the litter of culms except possibly for the species D. is. where C:N ratio dropped below the value of 35. Mary et al. [26] demonstrated that the availability of N in soil can control the decomposition process and strongly reduce the N immobilization potential of plant residues. Under limiting N conditions immobilization lasts longer but at a lower rate. 3.4. Phosphorus The analysis of variance showed that P concentration depended on time (p < O.OOl), species (p c 0.01) and plant part (p < 0.01) with a significant interaction among all these factors (p c 0.01). Leaf P concentration (figure 4) was 1.08, 0.85 and 0.65 mg . g-t for Vol. 19 (2) (1998)
dates (every
3 months)
period
Table II. C/N ratios experimental
for
all species
and
plant
part
during
the
period. Plant part
Species
Initial
12 months
24 months
Leaves
Culms
Leaves
Culms
Leaves
Culms 34.7
D. is.
35.5
59.2
27.3
46.2
20.2
C. gr.
30.1
76.5
21.9
65.3
17
46
F. ov.
36.2
80.1
28.2
63.6
21.1
40.6
D.is., C.gr. and Eov., respectively at the end of the first 12 months. These figures represent a reduction of 12.3, 26.7 and 8.4 % relative to the initial P concentration for the same species. After 24 months, leaf P concentration was 0.76, 0.16 and 0.72 mg . g-l, which represent a reduction of 21.6, 82 and 1.4 %, relative to the initial, for D.is., C.gr. and Eov., respectively. These figures show that P release had occurred during both periods in the decreasing order: C.gr. > D.is. > F. ov. with significant differences (LSD = 0.12) among the species. Decrease in P concentration with time was observed by Kalburtzi et al [21], studying nutrient release in an agricultural ecosystem and by Grier [ 181 in a forest ecosystem. Pomeroy [30] stated that P recy-
Z. Koukoura
120 cling influences the other nutrients and this may be exhibited in the N:P ratio. Gosz et al [17] found that the N:P ratio of plant material tended to stabilise with time, while Kalburtzi et al [21] found increase of this ratio with time. Our results showed that the N:P initial ratios of 14.4 for D.is., 17.2 for C.gr and 19.0 for F. ov. changed to 20 for D.is. and C.gr and 21.6 for F. ov. at the end of the first 12 months and to 21 and 20.8, respectively, during the next 12 months. Enright and Ogden [ 151 mentioned that a critical C:P ratio below which P is released from decomposing litter was about 650: 1, while Gosz et al. [ 171 gave a ratio of 480: 1. This ratio in this study was 5 12, 541 and 690 for D.is., C.gr and Eov., respectively, in the initial material and had changed to 434, 367 and 457.1, respectively, at the end of the experiment. These figures show that P release has occurred from the leaves of all species.
concentration, by 72.8, 73.9 and 63.4 % for D.is., C.gr and Eov., respectively. At the end of the experiment it varied between 0.95 and 0.72 mg/g and had been reduced by 79.7 %, 77.4 % and 74.2 % for D.is., C.gr. and Eov., respectively. It is obvious that K release had occurred during both periods. The difference in K release among the species was significant (LSD = 0.29) and decreased in the order: D.is. > C.gr > F. ov. for the first 12 months (figure 5). The greatest release was observed during the first 9 months for all studied cases (figure 5). The higher K release of the first 12 months could be attributed to more intense leaching due to high rainfall and faster decomposition rates during this period. Gosz et al. [17] working in hardwood forest at Hubbard Brook found that about 70 % of litter K was released during the first 12 months with maximum in autumn. Alexander [4] states that K is not a structural component of plant tissue and is present in plant residues as water soluble salts. This is the reason why it is so readily leached from so many different litter substrates. The leachability of this element has been also demonstrated by Attiwill [S] even for woody tree branch material. The amount of K remaining at the end of the experiment decreased in the order: D.is. > C.gr. > Eov. Potassium concentration in culms was less than that of the leaves and its changes followed the same pattern during the experiment. From our results it is obvious that K
3.5. Potassium The analysis of variance showed that K concentration depended on time (p c O.OOl), species (p < 0.01) and plant part (p < 0.01) with a significant interaction among all these factors (p < 0.05). Leaf K concentration (figure 5) varied between 1.27 and 0.73 mg/g for all species at the end of the first 12 months and had been reduced, relative to the initial
--c
Leaves
D. is.
-
Leaves
C. gr.
-
Leaves
F. ov.
-
Culms
D. is.
-t
Culms
C. gr.
-o-
Culms
F. ov.
JF
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every
3 months)
Figure 4. Phosphorus concentration (mg g-‘) of leaves and culms during the experimental
period. Aria
Oecologica
121
Decomposition from C, and C, plants litters
5.0
/r
4.5 4.0 3.5 53 aI lk
3.0 3.82
l
E 2 w B d!
r3.54
2.5
l 3.15
2.0 I-
02.80 02.30
1.5 1 .oI0.5 ,0.C I-JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every 3 months)
Figure 5. Potassium concentration (mg g-‘) of leaves and culms during the experimental
6
--C
Leaves
D. isc.
A--
Leaves
-a-
Culms
D. kc.
--t
Culms C. gr.
period.
C. gr.
-.I
:cn5 E E 4 2 -0 9
3
2
JJASONDJFMAMJJASONDJFMAMJJAS
Sampling
dates (every
3 months)
Figure 6. Calcium concentration (mg g-‘) of leaves and culms during the experimental period. Vol. 19 (2) (1998)
-
Leaves
D. is.
-
Culms
D. is.
-
Leaves
C. gr.
-
Culms
C. gr.
-o--
Leaves
F. ov.
-
Culms
F. ov.
Z. Koukoura
122 release in D.is. and C.gr. (C, species) was greater than that in F. ov. (C, species). 3.6. Calcium (mg * 8’) The analysis of variance showed that Ca concentration depended on time (P < O.OOOl), species litter (P < 0.0001) and plant part (P < 0.0001) with a significant interaction among all these factors (P < 0.05). Leaf Ca concentration (fisure 6) ranged from 3.5 to 4.9 mg . g-’ at the end of the first 12 months which corresponds to a relative increase of 31.3, 12.2 and 7.9 % for Eov., C.gr. and D.is., respectively, with respect to the initial concentration. After 24 months, Ca concentration ranged from 5.20 to 4.9 mg +g-’ , a relative increased of 85, 46 and 15 % for Eov., C.gr. and D.is., respectively. These results point to the fact that Ca accumulation had occurred during both periods, with higher values during the second. Ca accumulation followed the order: Eov. > C.gr. > D.is. with the differences among the species being statistically significant (LSD = 0.92), after the 24 months. Vogt [34], working on conifer forest ecosystem, found that the amount of Ca in leaf litter had doubled after 2 years of decomposition. Calcium concentration in culms was lower than that of the leaves and accumulation also occurred in both periods. The relative increase in concentration, after 24 month time was 52.1, 40.0 and 30.3 % in D.is., C.gr., and Eov., respectively. Significant increases (LSD = 0.92) were observed only during the summer months for all species. 4. CONCLUSIONS
The results of the decomposition experiment presented here are general in agreement with the findings of other studies concerning grasslands [6, 12, 281. Leaching and micro-organisms activity in relation to rainfall and litter chemical composition are likely to be the most important factors which determine litter loss. Litter of C.gr. appeared to be more resistant to decay than that of D.is. and Eov, while there was no separation between the decomposition rates of C, and C, plants in this study. The degree of contribution of the different species to nutrient cycling, depended on the rate of release of elements during decomposition Our results suggested that more N was accumulated in C, plants than in C,, but no N mineralisation was observed in any of the species. Potassium was the most readily released element and at a greater rate from the C, plants, while no calcium release was observed. Phosphorus release was observed for all species at the end of the experiment with higher values for C.gr.
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