Effects of grazing on soil respiration of Leymus ... - Springer Link

Climatic Change (2007) 82:211–223 DOI 10.1007/s10584-006-9136-0

Effects of grazing on soil respiration of Leymus chinensis steppe Bingrui Jia · Guangsheng Zhou · Fengyu Wang · Yuhui Wang · Ensheng Weng

Received: 29 November 2004 / Accepted: 18 April 2006 / Published online: 13 February 2007 C Springer Science + Business Media B.V. 2007

Abstract Soil respiration, canopy temperature, soil moisture, above and belowground biomass were observed in 2001, 2002, 2004 and 2005 at fenced and grazed typical Leymus chinensis steppes in Inner Mongolia. Based on soil respiration data obtained by the enclosed chamber method, diurnal and seasonal dynamics of soil respiration and their controlling factors were analyzed. The effects of grazing on diurnal and seasonal soil respirations were not significant. The diurnal patterns of soil respiration could be expressed as a one-humped curve and the lowest and highest values appearing from 1:00 to 3:00 and from 11:00 to 14:00, respectively. Canopy temperature had a strong influence on the diurnal variation of soil respiration. The rates of soil respiration rose to a seasonal maximum from the middle of June to the end of July and then gradually decreased. Soil moisture explained about 71.3% and 58.3% of the seasonal variation in soil respiration at fenced and grazed plots, respectively, and canopy temperature only 33.9% and 39.7%. Soil respiration rate, above and belowground biomass and soil moisture were significantly increased at the fenced plots compared to the grazed plots (P < 0.05), but the difference was not significant in canopy temperature. The mean soil respiration rates were 247.85 and 108.31 mgCO2 m−2 h−1 during the whole experiment at fenced and grazed plots, respectively. Soil respiration rate was enhanced significantly at the fenced plots, which might attribute to the increasing soil moisture and biomass. The response of soil respiration rate to grazing varied among different sites and might be related to local soil moisture status.

B. Jia · G. Zhou () · F. Wang · Y. Wang · E. Weng Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, P.R.China e-mail: [email protected] B. Jia · F. Wang Graduate Schoo1 of the Chinese Academy of Sciences, Beijing 100039, P.R.China Springer

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1 Introduction Grassland is about 44.5 × 108 ha, accounting for 30% of land surface all over the world, and stores carbon about 761 Pg C, 10.6% for the vegetation and 89.4% for the soil (Atjay et al. 1979). Soil respiration is composed mainly of microbial and root respiration. CO2 , as a greenhouse gas released from soil respiration, is a major flux in the global carbon cycle accounting for about 25% of global CO2 exchange (Bouwmann and Germon 1998) and 60–80% of total ecosystem respiration (Wofsy et al. 1993; Goulden et al. 1996; Law et al. 1999; Janssens et al. 2001). CO2 emissions from grassland soils contribute to a large portion of total CO2 flux (Raich and Schlesinger 1992). Thus, grasslands play an important role in modulating the global carbon cycle. Studies on soil respiration and related controlling factors in grassland ecosystem contribute to the understanding and evaluation of the global carbon cycle and climate change. Leymus chinensis steppe covers the largest area (30 × 104 ha) and has the highest economic value in the typical steppe zone of Inner Mongolia, primarily as grazing and mowing lands (Li et al. 1988). Due to human disturbances (mainly overgrazing), the area of degraded grassland in China has reached about 1.35 × 108 ha, accounting for one third of the available grassland area and increases with 200 × 104 ha every year (Zhou and Wang 2002). Whether grasslands operate as a sink or source for atmospheric CO2 depends on the particular land use, grazing intensity and so on (Frank 2002). However, the effects of grazing on soil CO2 effluxes are still not well understood and vary among sites. In particular, relatively few direct comparisons had been made for soil respiration rates in natural and disturbed vegetations (Raich and Schlesinger 1992). There have been several researches on how grazing influences the soil CO2 effluxes in typical grassland. Cui et al. (2000) showed that grazing decreased the release of CO2 from the soil by 77% in 1998 in Stipa grandis grasslands. Compared to non-grazing plots of 20 years in Leymus chinensis grasslands, Li et al. (2000) showed that grazing activities resulted in a little lower rate of soil CO2 effluxes from 1998 to 1999, but there was no significant difference. Other investigations, based on a shorter interval (July 18–19, 1998), showed that grazing led to a decrease in CO2 effluxes by 29% (including soil and plant dark respiration) in Leymus chinensis grasslands and an increase by 21% in Stipa grandis grasslands (Dong et al. 2000; Zhou et al. 2002). Such discrepancies between results illustrate the need for more research attention in this region. The study aims to examine differences in soil CO2 emissions between fenced and grazed Leymus chinensis steppe in Inner Mongolia. Furthermore, basal data could be served for further research on the influence of degradation and restoration on the global carbon budget in grassland ecosystems.

2 Materials and methods 2.1 Study site description The study site is located at Baiinxile Livestock Farm (lat.43◦ 55 N, long.116◦ 31 E, 1201 m above mean sea level), Xilin River Basin, Inner Mongolia. It belongs to Leymus chinensis steppe. The fenced plot has been free from grazing since it was established in early June 2001 and covered a total area of about 2.80 ha. The grazed plot was only 200 meters away from a shepherd’s habitation, characterized by sparse vegetation and bare ground due to higher frequency of free-grazing sheep throughout the year. The climate is semi-arid, Springer


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continent temperate with an annual mean temperature of −0.4 ◦ C, ranging from −21.41 ◦ C in January to 18.53 ◦ C in July. Annual precipitation is about 350.43 mm concentrating from July to September (Jiang 1985). Chestnut soil is the zonal soil type (Chen and Wang 2000). The dominant species is Leymus chinensis, accompanied with Stipa grandis, Cleistogenes squarrosa, Agropyron cristatum, Artemisia frigida, Carex korshinskyi, etc. 2.2 Soil respiration Although the static chamber method (alkali absorption method) is commonly accepted, its applicability is limited by the incapability of making short-term and continuous measurements (Li et al. 1998). In this paper, soil respiration was measured with an enclosed chamber method. The enclosed chamber was made from acrylic material. The surface area of the chamber was 50 cm × 50 cm and 15 cm in height. Two air mixture fans (12 V, 0.13 A) and a high quality precision temperature and humidity sensor (type: NKHT, Beijing Northking Electronic Technology Development Co., Ltd.) were fitted inside the chamber and connected with a Thermohygrograph (type: 3DD150, Beijing JunFang Technical Institute of Physics and Chemistry). CO2 concentration was directly measured inside the chamber by an infrared gas analyzer (type: GXH-3010D, Beijing Computer Technology & Application Institute), which was connected with the chamber in a closed configuration and calibrated regularly with CO2 standards. Soil respiration was measured with three replicates for each sampling observation. Firstly, three stainless steel frames (50 cm × 50 cm) were random inserted at 5 cm depth into the soil. Meanwhile, all green and standing-dead plants inside the frames were clipped to ground level and litter was collected. During measurements, the chamber was enclosed outside the stainless steel frame and sealed with sealing tape. Between measurements, the chamber was opened and the atmosphere was allowed to equilibrate. Each measurement took continuously 3 minutes, changes of the temperature, humidity and CO2 concentration in the closed chamber were logged every 10 seconds and used to calculate soil respiration rate. The soil respiration rates were computed from the concentration change over the measurement period. It can be calculated as follows (Dong et al. 2000): F = (m/t) · D · (V /A) = h · D · m/t where by F refers to the soil respiration rate (mg·m−2 ·h−1 ), m/t denotes linear slope of concentration change with time over the measurement period, which is thought to be effective only when the correlation coefficient (R2 ) is greater than 0.95 (Fig. 1), D is the gas density of the chamber (D = P/RT, mol m−3 , P: atmospheric pressure, T: temperature and R: air constant), V is the chamber volume, A is the surface area of the chamber, and h represents the height of the chamber. 2.3 Soil moisture and biomass The volumetric soil moisture of four layers (0–10, 10–20, 20–30 and 30–40 cm) was measured at the same time with the soil respiration measurements near the frames with the Profile Probe (Type: PR1/4, Delta-T Devices Ltd. CAMBRIDGE CB5 0EJ UK, recorded by HH2-Moisture Meter). After soil respiration measurements, 9 soil cores (3.80 cm in diameter) for root biomass determination were taken at the depth of 0–30 cm from each sampling point. Roots were Springer

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CO2 co n cen tratio n (m o l·μm o l -1 )

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y=0.177x+352.19 R2 =0.987

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Fig. 1 Linear relation of CO2 concentration changes in the 3 minutes measurement period at fenced and grazed plots. P < 0.001 for both correlation coefficients

washed and then sieved. Above and belowground biomass were oven-dried at 80◦ for 24 h and weighed for dry matter. The studies were performed at the fenced and grazed Leymus chinensis steppes on June 19–September 23, 2001, June 1–September 24, 2002, September 23–25, 2004 and September 3–6, 2005. There were 20 and 7 observation dates at fenced and grazed plots, respectively. Soil respiration and soil moisture were measured once every hour with a 24-hour continuous measurement or from 6:00 to 18:00 on each observation date. 2.4 Watering treatment A quadrate (5 m × 5 m) was fully irrigated at the fenced plots on September 15, 2002. Soil respiration rate and soil moisture were measured from 6:00 to 18:00 in watered site and adjacent non-watered reference site after two days. 2.5 Statistical analysis All statistical analyses were performed by SPSS 10.0 (SPSS for Windows, Version 10.0, Chicago, Illinois). The statistical comparisons were conducted using a one-way ANOVA (Duncan’s test) for soil respiration rate, above and belowground biomass, soil moisture and canopy temperature at fenced and grazed plots. Significant differences for all statistical tests were evaluated at the level of a = 0.05. Linear and exponential regressions were performed with curve estimation.

3 Results and analyses 3.1 Diurnal dynamics of soil respiration rate at fenced and grazed plots Figure 2 showed the diurnal changes in soil respiration rate, canopy temperature and soil moisture at fenced and grazed plots on June 15–16, 2002 and August 27–28, 2002. The diurnal patterns of soil respiration were similar between fenced and grazed plots and could be expressed as one-humped curves. In general, soil respiration rates reached the maximum Springer

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Fig. 2 Diurnal dynamics of soil respiration rate (A), canopy temperature (B) and soil moisture (C) at fenced and grazed plots. Vertical bars represent the standard error of the measurement mean (n = 3) for each hour

at 11:00–14:00 and fell to the minimum at 1:00–3:00 (Fig. 2A), coinciding with the highest and lowest canopy temperatures (Fig. 2B). The diurnal dynamics of soil moisture were negligible (Fig. 2C). Thus, canopy temperature controls over the diurnal soil respiration variations. Daily average CO2 emission rates were 445.46 (June 15–16, 2002) and 78.06 (August 27–28, 2002) mg CO2 m−2 h−1 at the fenced plots, and 155.52 (June 15–16, 2002) and 62.32 (August 27–28, 2002) mg CO2 m−2 h−1 at the grazed plots (Fig. 2A). The average emission rates were found very close to those at 7:00–8:00 and 17:00–19:00, which were similar with canopy temperature (Fig. 2A and B). Thus, soil respiration rate and canopy temperature at 8:00 were approximately representative for mean daily values. 3.2 Seasonal dynamics of soil respiration rate at fenced and grazed plots The seasonal dynamics of soil respiration rate are shown in Fig. 3(A), from which we could find that the rates of soil respiration reached peak values from the middle of June to the end of July and then gradually decreased at fenced and grazed plots. The fenced pasture experienced somewhat greater seasonal variation in soil respiration rate than the grazed pasture, with a range of 1027.34 to 13479.15 mg CO2 m−2 d−1 in the fenced pasture, and 874.35 to 4724.30 mg CO2 m−2 d−1 in the grazed pasture. The maxima of above and belowground biomass were observed in middle August at the fenced plots, and in late August at the grazed plots (Fig. 3B and C), lagging behind soil Springer

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Fig. 3 Seasonal dynamics of soil respiration rate (A), above (B) and belowground biomass (C), soil moisture (D) and canopy temperature (E) during the whole experiment. Vertical bars represent the standard error of the measurement mean (n = 3) for each observation date. Different letters indicate significant differences (P < 0.05) between fenced and grazed plots

respiration rate. Biomass increased gradually and reached the maximum as a cumulative process, whereas soil respiration rate changed with environmental and biotic factors. The belowground biomass was much higher at the fenced plots than that at the grazed plots in June and July and the differences became small in August and September, similar to the change in soil respiration rate (Fig. 3A and C). Springer


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Fig. 4 Dependence of soil respiration rate on soil moisture and canopy temperature measured during the whole experiment at fenced (A) and grazed plots (B). P < 0.001 for all correlation coefficients

Many studies showed that near-surface soil moisture was the most important influence on soil respiration rate among all depths, however, no standardized sample depth was defined, e.g. 0–5, 0–10 or 0–20 cm (Keith et al. 1997; Chen et al. 1999; Li et al. 2000). This paper presented soil moisture at the depth of 0–10 cm. A parallel pattern between soil moisture (Fig. 3D) and soil respiration rate (Fig. 3A) was observed at fenced and grazed plots. In particular, the dramatic rise in soil respiration rate at the fenced plots took place on July 22 and August 21, 2001, associating with anomalous increase of soil moisture. Also, canopy temperature (Fig. 3E) had a similar variation with soil respiration rate (Fig. 3A), but not clear like soil moisture. During the whole period, soil moisture accounted for 71.3% and 58.3% of the seasonal variation in soil respiration at fenced and grazed plots, respectively, and canopy temperature was only 33.9% and 39.7% (Fig. 4). These results indicated that the seasonal dynamic of soil respiration was controlled mainly by soil moisture in the semi-arid grassland, and canopy temperature was secondary. 3.3 Comparisons of soil co2 emissions between fenced and grazed plots To test if grazing induced significant differences in soil respiration rate and in its relevant controlling factors, the corresponding data at fenced and grazed plots were compared with one-way ANOVA (Fig. 3). There were significant differences in soil respiration rate between fenced and grazed plots (P < 0.05). From Table 1 we could find that the mean soil respiration rates were 247.85 and 108.31 mgCO2 m−2 h−1 during the whole experiment at fenced and grazed plots, respectively. Compared with the previous studies in the same region (Table 1), the results were similar to that of Chen et al. (1999) and were within the range of that from Li et al. (2000) at fenced sites, and similar to Chen et al. (2003) but lower than that of Li et al. (2000) at grazed sites. Compared to the world’s major temperate grasslands (55.31– 347.42 mg CO2 m−2 h−1 , Raich and Schlesinger 1992), the values were also within the range. Springer

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197.19 ± 29.44 101.04

108.31 ± 7.17

206.77 ± 28.01

247.85 ± 12.12

Leymus chinensis During the whole experiment

5/20–10/20/1999 6/5–10/15/2001

5/20–10/20/1998

256.28 ± 35.06

267.70 ± 32.21

Leymus chinensis

Leymus chinensis

6/25–9/25/1997

253.15 ± 42.21

Stipa grandis

Period of observation (month/day/year)

Fenced plots

Dominant vegetation

Grazed plots

Alkali absorption method Enclosed chamber method

Alkali absorption method Alkali absorption method

Method

The present paper

Chen et al. (2003)

Li et al. (2000)

Chen et al. (1999)

Source

Table 1 Comparisons of the mean soil respiration rates (±S.E.) in typical temperate grassland in Inner Mongolia (mg CO2 m−2 h−1 )

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The above and belowground biomass were significantly lower in the heavily grazed area as compared to the ungrazed area during four years, especially the aboveground biomass was reduced by 51–88% (Fig. 3B and C). Soil moisture decreased significantly by grazing, but the differences were not distinct in canopy temperature (Fig. 3D and E). The sparse vegetation and bare ground in the grazed pasture might accelerate soil surface evaporation and lead to decreasing soil moisture, but did not seem to do so in canopy temperature.

4 Discussions Many researches indicated that a temperature-water interaction term explained seasonal variation in CO2 efflux if water was limited in arid and semi-arid regions (Wildung et al. 1975; Mathes and Schriefer 1985; Janssens et al. 2001). The authors found that the temperature effect was manifested only when there was sufficient soil moisture to permit significant root and microbial CO2 production. There was a positive correlation between soil respiration rate and net primary productivity (Raich and Schlesinger 1992; Raich and Potter 1995) or gross primary productivity (Janssens et al. 2001). Soil respiration was significantly higher at the fenced plots when compared to the grazed plots, which was probably attributed to increasing soil moisture and biomass. Soil respiration originated mainly from root and microbial activity, which both were strongly influenced by available soil moisture (Howard and Howard 1993). Li et al. (2000) showed that above and belowground biomass were significantly decreased by grazing, but soil moisture was only slightly influenced, which might result in the similar soil respiration rate between fenced and grazed plots (Table 1). Soil moisture was similar between grazed plots in this study and degraded Leymus chinensis steppe (precipitation was only 60 mm during experiment) studied by Chen et al. (2003), which was far lower than that in the study site of Li et al. (2000). Thus, soil respiration rates measured in degraded Leymus chinensis steppe in this study and Chen et al. (2003) were similar, lower than those of Li et al. (2000). These results suggested that soil moisture was a major limiting factor influencing soil respiration rates in this region. In the present study, enclosure enhanced soil moisture (Fig. 3D), which might facilitate soil CO2 emission. From the watering treatment in Fig. 5 we could find that approximately a 4-fold of respiration-enhancement was achieved with a 1-fold of waterenhancement. Although canopy temperature was low (18.47 ◦ C on average), soil moisture would still greatly stimulate soil respiration rate, which provided the direct evidence about the role of soil moisture in enhancing soil CO2 emission. The observed increase in soil respiration rate might result from an enhancement of above and belowground biomass and subsequent litter deposition, which could supply more substrates available for decomposition. Thus, in the semi-arid grassland with a very thin humus layer, grazing could cause a C limitation for the soil microbes. Root respiration was a primary contributor to the total soil respiration. Li et al. (2002) indicated that the contribution of root respiration to total soil respiration was estimated to be about 27% on average, ranging from 14% to 39% in the growing season in Leymus chinensis steppe. The increase in root must contribute to release more CO2 (Berntson and Bazzaz 1996) and exudation (an important carbon and energy source for rhizosphere soil microorganisms). In addition, the effects of grazing on root varied in different growth phases, which might also indirectly affect soil respiration rate. Relevant researches from other countries showed that the soil respiration was increased in degraded grasslands during restoration (Tongway and Ludwig 1996). Grazing reduced the Springer

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Fig. 5 Comparisons of soil moisture and soil respiration rate between watering plots and control plots. Vertical bars represent the standard error of the measurement mean (n = 3) for each hour

annual amount of soil respiration in a tallgrass prairie by 17.5% (Bremer et al. 1998). In the eastern Amazon Basin, annual emissions were decreased by 33% in the degraded pasture compared to those in the active pasture which was improved by scientific managements and reasonable grazing (Davidson et al. 2000). In tropical pasture in Panama, the reductions in respiration rates (including soil and plant dark respiration) due to grazing were roughly 41% (Wilsey et al. 2002). However, some researches showed that grazing could accelerate soil CO2 effluxes in grasslands (Risser et al. 1981). Stark et al. (2002) showed that grazing significantly enhanced soil respiration in the suboceanic tundra heaths but not in the subcontinental tundra heaths. Why is the response of soil CO2 effluxes to grazing so different? Kucera and Kirkham (1971) suggested that soil CO2 effluxes declined as soil moisture fell below the permanent wilting point of soil microorganisms or exceeded field capacity. As pointed out by Moncrieff and Fang (1999), the soil respiration rates were inhibited when the soil moisture was less than 15% and more than 35%. Linn and Doran (1984) explained this phenomenon mechanistically for microbial respiration. They thought low water conditions limited soluble organic-C substrates in water films and high water conditions limited O2 diffusion through pore spaces. Hence, soil CO2 release can be limited either by extremes of saturation or water deficits (Kucera and Kirkham 1971; Edwards 1975; Raich and Schlesinger 1992; Howard and Howard 1993; Davidson et al. 2000; Lee et al. 2002). This implies that the positive or negative response of soil respiration rate to grazing might be related to local soil moisture status. In other words, reductions of aboveground biomass (Fig. 3B) and subsequent litter accumulation by severe grazing facilitated the increase of evaporation, which at last aggravated the drought stress in dry sites, but played a well-drained role in very wet sites and an enhancement of microbial and root respiration may occur at intermediate water content. Springer


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Drought-induced declines in root respiration were much more apparent in warm soils than that in cool soils (Bryla et al. 2001), which might partially explain variable reductions induced by grazing among different regions. Besides environmental characteristics, the differences in climate, plant community and grazing management practices among reported studies were possible reasons, which will require further research. In addition, the reductions of soil respiration due to grazing could be compensated with animal CO2 emission. However, this holistic point of view was often not taken into account.

5 Conclusions The mean soil respiration rates were 247.85 and 108.31 mg CO2 m−2 h−1 during the whole experiment at fenced and grazed Leymus chinensis steppe, respectively. The diurnal patterns of soil respiration was a one-humped curve and similar with canopy temperature. The diurnal dynamics of soil moisture were negligible. However, soil moisture was the main influencing factor in the seasonal variation of soil respiration in the semi-arid grassland, accounting for 71.3% and 58.3% at fenced and grazed plots, respectively. Canopy temperature was secondary, accounting for only 33.9%–39.7%. There were significant differences in soil respiration rate, above and belowground biomass and soil moisture between fenced and grazed plots (P < 0.05), and the difference was not significant in canopy temperature. Soil respiration was significantly higher at the fenced plots when compared to the grazed plots, which was probably attributed to the increasing soil moisture and biomass. The extent and direction of grazing impacts on soil respiration varied among different sites, which might be related to local soil moisture status. The results obtained in this study could be useful within the investigation of global carbon cycle, and also helpful within the frame of ‘grassland in Asia’. Acknowledgements This work was jointly supported by NKBRSF (No.G2006CB400502), National Natural Science Foundation of China (No. 40231018), and the Chinese Academy of Sciences (Nos. KZCX1-SW-01-12, KSCX2-SW-133).

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