林业科学  2004, Vol. 40 Issue (1): 11-17   PDF    
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文章信息

Xu Xiaoniu, Wang Qin, Eiji Hirata
徐小牛, 王勤, 平田永二
Effects of Broad-leaved Trees Invasion on Organic Carbon and Nitrogen Dynamics in Pinus Luchuensis Plantation
自然侵入阔叶树对琉球松人工林碳素及氮素动态的影响
Scientia Silvae Sinicae, 2004, 40(1): 11-17.
林业科学, 2004, 40(1): 11-17.

文章历史

Received date: 2002-05-08

作者相关文章

徐小牛
王勤
平田永二

自然侵入阔叶树对琉球松人工林碳素及氮素动态的影响
徐小牛1, 王勤1, 平田永二2     
1. 安徽农业大学林学系 合肥 230036;
2. 日本琉球大学农学部 冲绳 903-0213
摘要:对日本冲绳岛北部相同土壤条件下的琉球松纯林及其混交林的土壤氮素及有机碳素、地表凋落物量、枯枝落叶量以及土壤氮素矿化速率进行了比较研究。结果表明, 琉球松纯林的地表凋落物层氮、碳平均贮量分别为133kg·hm-2和7199kg·hm-2, 混交林则分别为105kg·hm-2和6 143kg·hm-2。然而, 混交林地表 10cm矿质土层的氮、碳贮量则显著高于纯林, 氮素比纯林多493kg·hm-2, 碳素多5 554kg·hm-2。在30d的实验室培养实验中, 混交林表层土壤的氮素矿化速率高于纯林18%;而且, 混交林的落叶和土壤的碳氮比值亦明显低于松纯林。混交林土壤的年平均矿化氮素(NH4+ +NO3-)浓度高于纯林22%。与松纯林相比, 混交林通过枯枝落叶年平均氮素归还量多43.7kg·hm-2, 碳素归还量多1 655kg·hm-2。混交林具有较高的氮素归还量, 主要是混交林的针叶含氮含量较高以及大量的高含氮量的阔叶落叶所致。上述结果充分说明针阔混交导致了林分氮素循环的变化。
关键词混交林    枯枝落叶    琉球松    氮素矿化速率    有机碳素    
Effects of Broad-leaved Trees Invasion on Organic Carbon and Nitrogen Dynamics in Pinus Luchuensis Plantation
Xu Xiaoniu1, Wang Qin1, Eiji Hirata2     
1. Department of Forestry, Anhui Agricultural University Hefei 230036;
2. Faculty of Agriculture, University of the Ryukyus, Nishihara Okinawa 903-0213, Japan
Abstract: Total nitrogen (N) and organic carbon (C) in soil, forest floor and litterfall, and N mineralization rates were compared between pure and mixed Pinus luchuensis plantations on similar soils in the northern Okinawa Island, Japan. Mean accumulations of N and organic C on the forest floor were 133 kg·hm -2 and 7 199 kg·hm -2 for the pure pine stand, 105 kg·hm -2 and 6 143 kg·hm -2 for the mixed stands. However, in the surface 10 cm mineral soil, the mixed stands had an average of 493 kg·hm -2 and 5 554 kg·hm -2 more than the pure pine stand (p < 0.001). Amount of N mineralized from soil during 30-day incubation was 18 % greater in the mixed stands than in the pure stand (p < 0.01). Average seasonal soil mineralizable N (NH4++NO3-) concentration was 22% greater in the mixed stands than in the pure pine stand (p < 0.01). The mixed stands returned an average of 43.7 kg·hm -2a -1 (about 70%) and 1 655 kg·hm -2a -1 (about 39%) in litterfall more than the pure stand did. The greater litterfall N input in the mixed stands was due to higher concentration in the pine needle litter and the presence of high-N broadleaf litter as well. The results from this study suggest that conifers have different N cycling when mixed with broad-leaved trees from when in pure stand.
Key words: Mixed-species forest    Litterfall    Pinus luchuensis    Nitrogen mineralization    Organic carbon    

Soil organic matter in surface mineral soil is believed to be a major determinant of site productivity within many forest ecosystems due to its effects on soil structure, nutrients supplies (Powers et al., 1990). Likewise, the availability of soil N is the dominant limited nutrient in many forests, which has a major influence on primary productivity, species composition and species diversity in terrestrial ecosystems (Pastor et al., 1984; Wedin et al., 1990). The supply rate of N depends largely on N mineralization, which is usually regulated by both abiotic and biotic factors (Dwyer et al., 1981; Bosatta et al., 1982; Birk et al., 1986; Cornejo et al., 1994). Of the biotic factors, the supply of litterfall is the most important, because litterfall can differ widely in decomposition rate depending on the quality, quantity, and timing of inputs (McClaugherty et al., 1987; Berendse et al., 1989; Murdick et al., 1994). Thus, the nature of the vegetation can have a large effect on the N supply rate in an ecosystem.

The objectives of this study are to determine the amount and variation of organic C and total N in the forest floor and soil for different stand types and to examine the effect of broad-leaved trees invasion on soil and litterfall nitrogen dynamics in Pinus luchuensis plantation.

1 Methods 1.1 Study site

The study site is located in Yona Experimental Forest of the University of the Ryukyus, northern part of Okinawa Island, Southwest Japan. The latitude and longitude of the site are 26°45′30″N and 128°05′E, respectively. The climate is warm and humid, with a mean annual air temperature of 21.6 ℃ and a mean annual precipitation of 2 660 mm. The mean minimum air temperature is 10.8 ℃ in January (Yona Experimental Forest).

A total of 16 hm2 P. luchuensis plantations was established there in the 1950s. Most of those plantations were invaded by volunteer broad-leaved trees, especially by Schima wallichii after repeatedly severe disturbance of typhoons which caused about 10%~20% of the individuals to breakdown and to uproot in the 1970s. The forest structure differed with the disturbance regime and topographical feature. Additionally, those stands had not received any management treatments such as logging or thinning since 1980s. Three 20 m×20 m sampling plots were set up, of which one was in the pure pine stand (P-C) and two in the pine-broadleaved tree mixture (P-A and P-B). The general description of the structures for the sampling stands and their environment is given in Tab. 1. A multi-story structure was formed in the mixed stand, within which pine trees occupied the upper canopy (15~20 m high), and sub-dominant, S. wallichii occupied the sub-canopy layer (8~15 m high). The soil on the site is yellow soil (Forest Soil Division, GESF, 1976) developed from tertiary sandstone (Yamamori, 1994).

Tab.1 Structural characteristics of the sampling stands showing species with DBH over 3.0 cm
1.2 Soil and forest floor sampling

After plot installation, the forest floor and mineral soil were, respectively, sampled in February, June and October 1997, from six 1 m×1 m quadrats randomly located within each plot. L, and F + H layers were lifted from each quadrat. The composition of L layer was sorted into leaf, small wood (including branches and barks less than 2.0 cm in diameter), and sexual organs (including flower, fruit and cone). Their wet weights were measured in the field. Litter from the six quadrats was thoroughly mixed to give one litter composite per plot; and partial samples were extracted for dry weight determination and chemical analysis. Mineral soil to 30 cm was extracted in each quadrat with a cylindrical sampler with an inner diameter of 10 cm. Soil samples were oven-dried at 70℃ to a constant weight and ground to pass through a 2 mm sieve prior to analysis.

1.3 Litterfall collection

Litterfall was collected in the pure pine and pine-broadleaved tree plots. Five litter traps with an opening of 1.2 m in diameter were placed in each plot. Litterfall was collected monthly from August 1996 to July 1999. Litter was sorted by tissue type (needle, leaf, small wood, sexual organ, and miscellaneous), dried at 70℃ to a constant mass, then weighed. Subsamples from each plot were ground for chemical analysis.

1.4 Mineralization experiment

To determine if differences in soil N mineralization among plots were related to species composition or litter quality, we conducted a 30-day laboratory incubation study, which involved three plots, one soil layer, and five laboratory replications. In July 1997, mineral soil was resampled at six random points for each plot. Mineral soil was extracted at each point with a cylindrical sampler for 0~10 cm layer, and delivered to the laboratory soon. The fresh soil samples from six sampling points were passed through a 2 mm sieve, and thoroughly mixed. Ten 20 g subsamples were taken from each of the composite samples. Two subsample were for moisture determination, three for initial mineralizable N determination, and five for 30 d of incubation at 25℃. The soil samples for incubation were placed in a 100 mL polyethylene bottle. The mouth of the bottle was sealed with gas-permeable plastic. Before incubation, the soil moisture of each subsample was adjusted to field capacity using deionized, distilled water. We chose a soil moisture of field capacity and a soil temperature of 25℃ to simulate optimal soil environmental conditions.

1.5 Chemical analysis and data treatment

Soil pH was determined in a 1:2 soil/water mixture by glass electrode. Total organic carbon and total nitrogen in forest floor and soil samples were determined by dry combustion with a C-N Corder (Yanaco, MT-500, Kyoto, Japan). The mineralizable N (NH4++NO3-) in initial and incubated subsamples were extracted by adding 100 mL of 2mol·L-1 KCl, shaking for 2 h and extracting for an additional 22 h. The extract was filtered and analyzed for NH4+ and NO3- concentrations by steam distillation (Hidaka, 1997). Net mineralization was calculated as the difference between (NH4++NO3-) concentrations of the incubated and initial soil samples.

In order to ascertain whether C and N dynamics differed significantly between the pure and mixed pine stands, t-test was used, and significance was accepted at p=0.05.

2 Results 2.1 Forest floor organic carbon and nitrogen

Forest floor accumulation averaged 14.2 Mg·hm-2 in the pure pine stand (P-C) with an average of 7.2 Mg C·hm-2 and 133 kg N·hm-2 (Tab. 2). Forest floor in the mixed stands (P-A and P-B) was about 11.9 Mg·hm-2. The accumulations of organic C and total N in the forest floor were significantly lower under the mixed stands than the pure pine stand (p < 0.001). Differences within stand for different sampling times were also found for organic C and N in forest floor (p < 0.05). Generally, higher accumulation occurred in October. It was due to the effect of typhoons which caused 3.0~4.8 Mg·hm-2 of litterfall in August 1997 (Xu et al., 2002).

Tab.2 Organic carbon and nitrogen accumulation in forest floor
2.2 Litterfall organic carbon and nitrogen dynamics

Needle litterfall N concentration for P. luchuensis pine ranged from 6.7 g·kg-1 in pure stand (P-C) to 7.9 g·kg-1 in the mixed one (P-B), and for the subdominant, S. wallichii, ranged from 11.6 g·kg-1 in the P-A to 12.1 g·kg-1 in the P-B (Tab. 3). N concentration of leaf litterfall differed significantly (p < 0.001) between the two main species, P. luchuensis and S. wallichii; however, no differences were found for the same species between stand types and between sampling years. Again, N concentration of miscellaneous material was significantly greater (p < 0.05) in the P-B than in P-A and P-C (Tab. 3), which could be attributed to the different composition between the pure and mixed pine stands.

Tab.3 Mean nitrogen concentration with standard error in parentheses in litterfall

The annual N input by litterfall averaged 58.2 kg N·hm-2 in the pure pine stand (P-C), which was significantly lower (p < 0.01) than that in the mixed stands (104 kg N·hm-2 for P-A and 100 kg N·hm-2 for P-B; Tab. 4). Within either the pure or the mixed stand, N content was significantly lower in the third year than the first two years, but not significantly different between the first and second years (p > 0.143).

Tab.4 Weight, nitrogen and organic carbon input by litterfall

The annual C input by litterfall averaged 6 243 kg·hm-2, 5 490 kg·hm-2 and 4 211 kg·hm-2 for P-A, P-B, and P-C, respectively (Tab. 4). And the mixed stands were significantly greater than the pure pine stand (p < 0.05).

2.3 Soil nitrogen dynamics

Seasonal patterns for soil moisture and NO3- and NH4+ concentrations are shown in Fig. 1. Except for P-A in October, soil moisture generally did not differ significantly (p > 0.05) among stand types. Soil NO3- and NH4+ concentrations differed significantly (p < 0.05) among stand types. Mean NH4+ and NO3- concentrations (mg·kg-1) in the top 10 cm mineral soil were, respectively, 5.4 mg·kg-1 and 3.9 mg·kg-1 for the pure pine stand (P-C), 5.9 mg·kg-1 and 4.4 mg·kg-1 for P-A, and 6.8 mg·kg-1 and 5.6 mg·kg-1 for P-B.

Fig.1 Seasonal percentage of soil moisture(A), and NH4+(B)and NO3-(C) concentrations in the top 10 cm of mineral soil for the sampling plots under Pinus liukiuensis plantations.Vertical bar is one standard error.

N mineralization rate of the top 10 cm mineral soil during 30-day incubation under the mixed stands was significantly higher than under the pure pine stand(p < 0.01). Significant differences were apparent among the sampling plots (Fig. 2).

Fig.2 Net N mineralization of the top 10 cm mineral soil in 30-day incubation for the sampling plots in luchu pine plantation.Net N mineralization rates with same letters do not differ significantly (p>0.05) among the sampling plots.Vertical bar is one standard error.

Total N concentrations for the fine soil (≤2 mm particles) in 0~10 cm under the mixed stands (P-A and P-B) were significantly higher (p < 0.01) than that under the pure pine stand (P-C; Tab. 5). For 10~30 cm soil layer, Total N concentrations showed little or no stand type difference. Like soil N, soil organic C in the mixed stands was higher than that in the pure pine stand, but the differences were only in the top 10 cm soil layer (Tab. 5).

Tab.5 Organic carbon, nitrogen, pH and bulk density of the mineral soil
3 Discussion

In this study, the forest floor N content was greater in the pure pine stand than in the mixed stands (Tab. 2). However, in the top 10 cm of mineral soil, the mixed stands had an average of 493 kg N·hm-2 more than the pure stand did (Tab. 5). Because of no pretreatment measurements, we cannot rule out the possibility of inherent site differences. However, the relative uniformity of environment and soil type within the study area suggests that differences are probably related to the volunteer broad-leaved trees, which can either accelerate N turnover in the forest floor (Takahashi, 2000) or cause difference in N distribution rather than in total N within the system (Alban, 1982). The present study demonstrated that the significant effect of broad-leaved trees on N accumulation was limited to the top 10 cm mineral soil layer (Tab. 5). This is in agreement upon the result from Lefevre and Klemmedson (1980).

Again, in pine-broadleaved tree mixture, the net rate of soil N mineralization averaged 18 % greater than that in the pure pine stand (Fig. 2). A higher mineral N concentration of surface mineral soil was also present in pine-broadleaf mixture (Fig. 1B and 1C). These results are interpreted as the influence of volunteer broad-leaved trees dominated by S. wallichii, on the accumulation and cycling of N. In the mixed stands, the concentration of total N in broadleaf litter was about 62% greater than in the pine needle litter (Tab. 3), and broadleaf litter reached an average of 39 % of the annual total leaf litter. Thus, the annual N input in litterfall in the mixed stands was 1.7 times that in the pure pine stand. The greater annual litterfall N input and relatively lower C:N ratio in the mixed stands may be correlated to the higher net N mineralization in forest floor and mineral soil (Pastor et al., 1984; McNulty et al., 1991; Reich et al., 1997; Prescott et al., 2000). Some other studies have demonstrated a vegetation effect on soil N mineralization (Wedin et al., 1990; Gower et al., 1992; Compton et al., 1998; Hirai, 1999). We do not have adequate pretreatment data to conclusively demonstrate that soil N was similar within each sampling stand, but several indirect lines of evidence suggest differences were not apparent. The results from this study showed the importance of stand type (or species composition) differences in determination of organic matter quality in affecting N cycling, particularly the difference between conifer and broadleaf, and between-species interaction (Klemmedson, 1987; Perry et al., 1987). Our previous research has showed that leaf litter of S. wallichii decomposed much faster than the pine needle (Xu et al., 2000). After two years' decomposition, leaves of S. wallichii disappeared completely, while the pine needles lost only 40% of total N and 55% of organic C. Thus, net N release may be strongly affected by species for they affect on litter quality. More rapid turnover of total N and organic C in forest floor and mineral soil is expected in the mixed stands because of the presence of the relatively favorable quality of broadleaf litter.

Annual N input in litterfall in this study ranged from 58.2 kg N·hm-2 in the pure pine stand to 103.7 kg N·hm-2 in the mixed stand (P-A) over 3 years, which were much higher than warm temperate coniferous forests (Vogt et al., 1986; Tsutsumi, 1987). High annual N input in litterfall for the mixed stand may be contributed to the different structural characteristics from the pure pine stand. A multi-story structure was formed in the pine-broadleaved mixture, which is good to generate beneficial microclimate, and to increase stand productivity (Xu etal., 1997). Thus, the mixed stand appears high productivity (Xu et al., 2002), which contributes to the greater annual rates of litterfall and related N input.

Acknowledgement

We thank Dr. N. Yamamori, emeritus professor of the University of the Ryukyus for his advice and invaluable comments. We also thank the staff of Yona Experimental Forest, University of the Ryukyus, for their support to field and laboratory works. This study was made possible by partial support from the Japanese Ministry of Education, Science, Sports, and Culture grant to X.N. Xu.

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