J For Res (2005) 10:57–60 DOI 10.1007/s10310-004-0102-2
© The Japanese Forest Society and Springer-Verlag Tokyo 2005
SHORT COMMUNICATION Wen Jie Wang · Yuan Gang Zu · Hui Mei Wang Takashi Hirano · Kentaro Takagi · Kaichiro Sasa Takayoshi Koike
Effect of collar insertion on soil respiration in a larch forest measured with a LI-6400 soil CO2 flux system
Received: January 30, 2004 / Accepted: April 27, 2004
Abstract Little information is available on the effect of root cutting by the collar pre-insertion technique on soil respiration. In this study, we found that soil respiration rates decreased with increasing depth of collar insertion in both the “with live roots intact” and “with live roots severed” treatments, but the rate of decrease was substantially higher in the former. The cutting of roots, especially fine roots, may be responsible for this result. Key words Larch (Larix gmelinii) · Soil respiration rate · Collar insertion · Root biomass
Introduction Soil respiration rate can now be made routinely with a LI-6400 soil CO2 flux system (LiCor, Lincoln, NE, USA). The soil chamber of the system is vented to equilibrate the pressure inside and outside the chamber, since pressure differences can have a strong effect on estimates of soil respiration (Nakayama 1990; Norman et al. 1992; LiCor 1997). Further improvements in soil respiration measurements can be achieved by the insertion of thin-walled polyvinyl chloride (PVC) collars prior to taking the measurements (Norman et al. 1992). These collars minimize soil surface disturbance and reduce the sudden flushing of CO2 in the soil and litter. Despite these improvements, there are still large variations in soil respiration even within a single forest, due to biotic and abiotic factors (Lloyd and Taylor W.J. Wang · Y.G. Zu · H.M. Wang Key Laboratory of Forest Plant Ecology, Northeast Forestry University, Harbin, PR China T. Hirano Graduate School of Agriculture, Hokkaido University, Sapporo, Japan K. Takagi · K. Sasa · T. Koike (*) Hokkaido University Forests, FSC, Hokkaido University, Sapporo 060-0809, Japan Tel. ⫹81-11-706-3854; Fax ⫹81-11-706-3450 e-mail:
[email protected]
1994; Fang et al. 1998; Davidson et al. 1998; Buchmann 2000; Fang and Moncrieff 2001). One of these factors is the contribution of root respiration, which can be 40%–60% of the total soil respiration rate (Bowden et al. 1993; Hanson et al. 2000). It is, therefore, critical to determine the effect of inserting the root-cutting collar on soil respiration. However, available information on the effect of the collars on soil respiration measurement is still limited (Hanson et al. 2000). Ignoring the effect of the root-cutting collar can therefore result in misleading soil respiration rates. The problem is expected to be more serious in forests with a shallow root system, such as those dominated by Larix gmelinii. These forests are widespread in central Siberia, Russia (Kajimoto et al. 2003) and northeast China (Wang et al. 2002). To examine the effect of inserting the root-cutting collar on soil respiration, a long-term CO2 flux site in a L. gmelinii forest in northeast China was selected as the study site (Wang et al. 2002). Since soil trenching is a method commonly used for estimating root respiration (Hanson et al. 2000; Lee et al. 2003), we also investigated the effect of collar insertion in such a soil trench.
Study site and methods Study site Measurements were made in a larch forest (L. gmelinii (Rupr.) Rupr.) 36 years old in 2003, located in the Laoshan Experimental Forest of the Northeast Forestry University in China (45°20⬘N, 127°34⬘E). The mean annual precipitation at Laoshan Experimental Forest was 723 mm and the annual mean air temperature was 2.8°C. The density of the larch stand was 1150 stems ha⫺1. The mean height was 18 m and mean diameter at breast height (DBH) was 17.2 cm. The soil here is characterized as a dark-brown forest soil and with about 10% soil organic matter, and pH values of 5.3–6.0. Depth of the litter layer at our study plot was about 2 cm. A more detailed description of this research site is given by Shi et al. (2001).
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Trenching Four trench boxes were set in the larch forest after snowmelt. The boxes measured 50 ⫻ 50 cm. Trenches were prepared by making vertical cuts along the boundaries to about 50 cm below the ground surface using a steel knife and shovel. The vertical distribution of roots showed that most roots were within a depth of 40 cm from the surface at the study site (W.J. Wang, unpublished data). Roots within the trench were not removed, and were gently cut to minimize disturbance to the soil core inside the trenches. Pieces of 5mm-thick polyethylene board were inserted into the vertical cuts to inhibit root regrowth. To eliminate the effect of root debris decay on soil respiration (Hanson et al. 2000), all experiments were done 3 months after the initial trenching (Lee et al. 2003). Collars were set in both the control (with live root) and trenched box (without live root) treatments in order to assess the root-cutting effect. We use the terminology “with live roots” and “without live roots” treatments in the following text. Soil respiration measurements Soil respiration was measured with a LI-6400 and a soil CO2 flux chamber (LI-6400-09). The target value was set close to the ambient CO2 concentration (~380 µmol mol⫺1), and the ∆CO2 value was set as the factory default value (10 µmol mol⫺1) (LiCor 1997). Thin-walled polyvinyl chloride (PVC) collars were inserted at least one night (12 h) before measurement so as to avert CO2 flushing, as recommended by LiCor (1997). The collars were 10 cm in diameter and 5 or 10 cm in height. To assess the influence of the root biomass cut by the collar on soil respiration, measurements at five insertion depths (0.3, 2, 3, 5, and 8 cm) were made in soil with live root (i.e., not trenched) and at four insertion depths (0.3, 2, 5, and 8 cm) in soil without live root (i.e. within trenched boxes). The “with live roots” plots were set adjacent to the trenched boxes (about 2–5 m away). Four or five replicate measurements were made for each insertion depth. Soil temperature was determined concurrently with soil respiration with a LiCor soil temperature probe (Type E). Soil water content in the collar was measured by taking a core sample of 100 cm3 (Daiki Rika Kogyo, Tokyo, Japan) and dried in the oven at 80°C for 48 h. In both the “with live roots” and “without live roots” treatments, collars were set in four rows and four columns. Moreover, two types of insertion combinations were set in the “with live roots” treatment. One was 0.3 cm into soil, 2 cm into soil, 5 cm into soil, and 8 cm into soil, excluding litter layer. The other was 0.3 cm into litter, 0.3 cm into soil, 2 cm into soil, and 3 cm into soil. A Latin square experimental design was used so as to prevent experimental error between different rows and columns, since no insertion depth appeared twice in each row (column).
Root biomass, soil moisture and soil temperature measurements To define the relations between root biomass in the “with live roots” plots and changes in soil respiration, roots were divided into two classes: fine roots (⬍2 mm diameter) and coarse roots (ⱖ2 mm diameter). After each measurement, roots were recovered from the soil volume within the collar. Roots were rinsed carefully with tapwater in a soil sieve (0.25-mm mesh). Roots were oven-dried at 80°C for 48 h for dry mass determination. Data analysis Linear correlations between root biomass and insertion depth, between root biomass and soil respiration, were carried out using Microsoft Excel 2002. Statistical analyses for the correlation significance were performed using SPSS 10.0.
Results and discussion Assuming that the soil respiration rate using the 0.3 cm collar insertion depth as 100% in both “with live roots” and “without live roots” treatments, we found a considerable decrease in soil respiration rates with increasing collar insertion depths in the “with live roots” treatment, but a relatively small decrease in the “without live roots” treatment (Fig. 1). In particular, when the collar was inserted to depths of 5 and 8 cm, soil respiration in the “without live roots” treatments was about 40%–50% higher than that in the “with live roots” treatment.
Fig. 1. Relation between collar insertion depth and soil respiration in trenched box (“without live roots”) and control (“with live roots”) sites. The vertical bar on each column indicates the standard error of the mean. The diagonal column shows the effect of cutting coarse roots on soil respiration
59 Fig. 2. Relation between collar insertion depth and fine root biomass (left), and between collar insertion depth and total root biomass (right)
cut by collars Fig. 3. Relation between soil respiration, fine root biomass (left), and total root biomass (right) cut by collars. The bars on each data point indicate the standard error of the mean. The arrow indicates that a large coarse root (⬎5 mm) was cut in that measurement. Line 2 shows the linear relation with one data point missing, since the coarse roots were cut during measurement. Line 1 shows the linear relationship for all data
Both fine roots and total root biomass cut by the collar were linearly related to the depth of the collar insertion. The correlation was more significant for fine roots (Fig. 2). We found that the root biomass cut by collar insertion is significantly negatively correlated with soil respiration rate (P ⬍ 0.01) (Fig. 3). However, there was no difference in soil respiration rates between the 2 cm insertion depth and the 0.3 cm insertion depth, even though more roots were cut in the 2 cm treatment (Figs. 1 and 2). One plausible reason for this is the density and distribution of roots. This error caused the scatter in the correlation of Fig. 3. L. gmelinii is considered to be a typical tree species with a superficial root system (see, e.g., Kajimoto et al. 2003; Wang et al. 2002). In fact, in fertile soil sites in northern China, about 31% of the fine root biomass is found in the top 10 cm (Han and Liang 1997). Similarly, in our study sites, we also found that many fine roots were mixed with decaying litter. Moreover, the fine root density (dry mass per unit volume) in soil at 0–10 cm was 1.4, 1.6, 3.7, and 6.9 times greater than that at 10–20, 20–30, 30–40, and 40– 50 cm, respectively (data not shown). Frequency distribu-
tions reviewed by Hanson et al. (2000) showed that 40%– 60% of total soil respiration is due to root respiration. Our results show that root cut by collar insertion increased with insertion depth (Fig. 2) and this can significantly affect the subsequent soil respiration measurement. However, little attention has been given to such effect of severing roots by collar insertion on soil respiration, as different studies have reported results from collar insertions varying from 0.5 to 10.5 cm (Mariko et al. 2000; Widén and Majdi 2001; Irvine and Law 2002). Therefore, in forests with superficial root systems, root cutting by the inserted collar should be taken into account in soil respiration measurements. The importance of root respiration has been demonstrated by various methods. For instance, using a forest gapformation method by felling trees, Ohashi et al. (2000) found that root respiration accounts for more than half of the soil respiration in a cypress (Cryptomeria japonica) forest. Large variations (27%–71%) in soil respiration have been contributed to root respiration in a mixed forest using a trenched box method (Lee et al. 2003). Based on a stem-
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girdling method, more than 60% of total soil respiration was estimated to be from roots in a pine forest (Högberg et al. 2001; Nordgren et al. 2003). Although isotopic labeling methods show a lower (~40%) proportion of root respiration (Hanson et al. 2000), these studies clearly show the large contribution that roots make to soil respiration. The negative correlation between root biomass and soil respiration (Fig. 3) suggests that the reduction in root respiration following collar insertion is responsible for the sharp decrease in soil respiration in the “with live roots” plots (Fig. 1). However, the cutting of coarse roots by the collar can have a significantly different effect on soil respiration than the severing of finer roots (Fig. 3). There are two possible reasons for this. One is that the size of our collar (5 cm diameter) was too small to measure the effect of coarse root cutting. In our experience, instead of fully cutting off big coarse roots, this type of collar generally wounds the larger roots. Further studies would be needed to test this hypothesis. The second reason is that large and small roots naturally differ in activity by virtue of their size difference; it has been reported that fine roots respire more than coarse roots per unit mass. For example, even with similar nitrogen concentrations, the respiration rates of fine roots are substantially higher than those of coarse roots because of their different activities (Ryan et al. 1996; Vose and Ryan 2002). Thus, the influence on soil respiration from fine root cutting by collar insertion may be more sensitive than coarse root cutting by collar insertion. It follows that cutting of roots by collar insertion, especially fine roots, may be responsible for the decrease in the soil respiration rate in the “with live roots” plots. A large proportion of the variation in long-term soil respiration measurements can be associated with changes in soil moisture and soil temperature (Lloyd and Taylor 1994; Davidson et al. 1998; Fang and Moncrieff 2001). We examined the possible effects of these two factors on soil respiration and found little difference in both soil temperature (17.8 ⫾ 0.2° and 18.1 ⫾ 0.3°C) and soil moisture (0.18 ⫾ 0.04 and 0.17 ⫾ 0.04 m3 m⫺3) between the “without live roots” and “with live roots” treatments, respectively. Consequently, soil temperature and soil moisture correlated poorly with soil respiration rate (data not shown) and were unlikely to have contributed to differences between the two treatments (Fig. 1). From the present experiment, we conclude that the reduction in root respiration caused by collar insertion could well be responsible for the rapid decrease in soil respiration in the “with live roots” treatment. This root cutting effect by collar insertion should be taken into account in measurements of the soil respiration rate, especially in forests with superficial root systems such as those of L. gmelinii. Acknowledgments This study was sponsored in part by the Natural Science Foundation of China (No. 30300271), FFPRI-Ministry of Environment (Japan) and JSPS (MOE-10205, Basic Research Grant to T.K.). Thanks are also due to Prof. T.T. Lei for his constructive advice. W.J.W. thanks Dr. L.Y. Qu for critical reading of an earlier draft.
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