Burning season influences the response of bird assemblages to fire in tropical savannas

Fire plays a pivotal role in structuring ecosystems and often occurs as a human-mediated disturbance for land management purposes. An important component of fire regime is the season of burn. In tropical savannas, most fire management occurs during the dry season; however, wet season burning is often used for pastoral management and may be useful for controlling introduced plant species. We used replicated, experimental fire treatments (unburnt, dry season burnt and wet season burnt), spanning two habitats (riparian and adjacent open woodland), to examine the short- (within 12 months of fire) and longer-term (within four years of fire) changes of bird assemblages in response to wet and dry season burning in tropical savannas of northern Australia. Within 12 months of fire, we observed higher abundances of birds in the burnt treatments, although some species (e.g., red-backed fairy-wren, Malurus melanocephalus) were rarely observed in burnt sites. Dry season burnt sites contained higher abundances of insectivores and granivores, while wet season burnt sites had more carnivores. Four years following burning, dry season burnt sites were characterized by lower abundances, especially of nectarivores and granivores. Dry season burnt sites also contained a different assemblage than wet season burnt sites, but few differences were observed between wet season burnt and unburnt sites. Our results confirm that differences in fire regimes can substantially alter bird assemblages, especially in riparian zones, and emphasize the importance of incorporating burning season in fire management strategies.     

Differences in soil respiration between different tropical ecosystems

We examined the relationship between soil respiration rate and environmental determinants in three types of tropical forest ecosystem—primary forest, secondary forest, and an oil palm plantation in the Pasoh Forest Reserve on the Malaysian Peninsula. In August 2000, the soil respiration rate and environmental factors (soil temperature, soil water content, soil C and N contents, biomass of fine roots, and microbes) were measured at 12–16 points in research quadrats. Soil respiration rates were 831 ± 480, 1104 ± 995, 838 ± 143, 576 ± 374, and 966 ± 578 (mean ± S.D.) mg CO₂ m⁻² h⁻¹ in the primary forest canopy and gap site, secondary forest canopy and gap site, and oil palm plantation, respectively. Although the mean soil respiration rates in the three forest ecosystems did not differ significantly, differences were evident in the environmental factors affecting the soil respiration. The major causes of spatial variation in soil respiration were fine root biomass, soil water content, and soil C content in the primary and secondary forests and oil palm plantation, respectively.     

Litter contribution to diurnal and annual soil respiration in a tropical montane cloud forest

Respiration of CO₂ from soils (R_s) is a major component of the carbon cycle of ecosystems, but understanding is still poor of both the relative contributions of different respiratory sources to R_s, and the environmental factors that drive diurnal variations in R_s. We measured total and litter-free R_s at half-hourly intervals over full 24 h periods, and thereafter twice a month for 10 months in a tropical montane cloud forest (TMCF) in Peru. Total R_s declined by about 61% during the night as a result of variations in respiration rate in the litter, which were partly correlated with the soil surface air temperature. Most of the diurnal variation of R_s in this TMCF appears to be driven by respiration in the litter layer, which contributed 37% to the total soil CO₂ efflux. Total R_s rates at this particular site would have been overestimated by 60% if derived from daytime measurements that had not been corrected for diurnal variations in R_s.     

放牧模式对内蒙古典型草原生长季土壤呼吸速率的影响

【目的】放牧改变了典型草原生产力和土壤养分循环,影响了植被和土壤微生物的生长状况,进而使草原土壤碳排放量发生变化。本研究通过分析不同放牧措施下内蒙古典型草原生长季土壤呼吸速率 (Rs) 的差异,了解不同放牧管理模式影响草原碳交换和碳平衡的主要途径。【方法】基于内蒙古典型草原全年放牧、休牧及禁牧三种放牧措施,于2014和2015年的7月和9月对Rs进行原位测定,并分析了不同放牧措施下Rs及其影响因子的差异。【结果】1) 三种放牧措施下,Rs表现为休牧样地 [CO₂ 2.00 μmol/(m2·s)] > 禁牧样地 [CO₂ 1.94 μmol/(m2·s)]> 全年放牧样地 [CO₂ 1.56 μmol/(m2·s)]。放牧对Rs的影响还存在季节效应,7月份放牧降低了Rs,而9月份放牧则提高了Rs。2) 与禁牧措施相比,放牧和休牧管理均降低了地上生物量(70.6%和47.3%)、土壤总碳含量(34.5%和32.0%)、土壤总氮含量(37.0%和34.5%),但休牧显著提高了根系生物量(37.2%)。全年放牧样地中土壤可溶性有机碳提高,但微生物磷脂脂肪酸含量下降。3) 7月份Rs主要与土壤湿度和地上生物量显著正相关,而9月份则与土壤温度和土壤PLFAs含量显著正相关。结构方程模型 (SEM) 结果显示,土壤温度 (0.905) 和湿度 (0.188) 通过影响微生物和根系的代谢环境对生长季Rs起主导作用,放牧通过降低土壤湿度和地上生物量对Rs有抑制作用 (–0.137)。【结论】全年放牧通过抑制微生物的生长降低了土壤呼吸速率,休牧通过提高根系生物量增加了土壤呼吸速率,说明放牧对内蒙古典型草原生长季土壤呼吸速率的影响途径因放牧模式的不同而不同。     

Dry season soil conditions and soil nitrogen availability to wet season wetland rice

A pot experiment with Maahas clay soil covered three consecutive crops. After uniform growth of the first crop, the soils were subjected to different moisture conditions during the dry season. Prolonged drying before wet season flooded rice stimulated increased release of mineral nitrogen but moistening of the dry soil for a dryland crop or by occasional rain during the dry season reduced nitrogen use from the soil in the next wet season. One cycle of alternate wet and dry soil preparation for 20 days before transplanting rice improved soil nitrogen availability and plant uptake of fertilizer nitrogen.

The initial growth of rice was retarded after flooding the previously moist dryland or dried soil, but not in the continuously flooded soils.

Losses of applied nitrogen were small in continuously flooded soils and were greater in the previously moist dryland and dry treatments. Uptake of soil nitrogen, however, was much higher in the air-dried soil treatment and in the dry with alternate wet and dry preparation treatments. Total nitrogen uptake (soil+fertilizer) was also greater in those dry treatments. Uptake of soil nitrogen in the wet-season crop was roughly proportional to the amounts of ammonia measured just before transplanting.

The proportion of the uptake of immobilized fertilizer nitrogen to available soil nitrogen was constant among treatments. Release of immobilized fertilizer nitrogen was also greatly enhanced by soil drying. For 1976 wet-season crop, the availability of fertilizer nitrogen immobilized in the 1975 wet season was three times higher than that of native soil nitrogen.     

Topographic variation in heterotrophic and autotrophic soil respiration in a tropical seasonal forest in Thailand

Soil respiration is a carbon flux that is indispensable for determining carbon balance despite variations over time and space in forest ecosystems. In Kanchanaburi, western Thailand, we measured the soil respiration rates at different slope positions—ridge (plot R), upper slope (plot U), and lower slope (plot L)—on a hill in a seasonal tropical forest [mixed deciduous forest (MDF)] to determine the seasonal and spatial variations in soil respiration on the slope. The heterotrophic (organic layer and soil) and autotrophic (root) respiration was differentiated by trenching. Soil respiration rates showed clear seasonal patterns: high and low rates in rainy and dry seasons respectively, at all plots, and tended to decrease up the slope. Soil respiration rates responded significantly to soil water content in the 0–30?cm layer, but the response patterns differed between the lower slope (plot L) and the upper slope (plots R and U): a linear model could be applied to the lower slope but exponential quadratic models to the upper slope. The annual carbon dioxide (CO₂) efflux from the forest floor was also associated with the slope position and ranged from 1908?gC?m^?2?year^?1 in plot L to 1199?gC?m^?2?year^?1 in plot R. With ascending position from plot L to R, the contribution of autotrophic respiration increased from 19.4 to 36.6% of total soil respiration, while that of the organic layer decreased from 26.2 to 9.4%. Mineral soil contributed to 46.3 to 54.4% of the total soil respiration. Soil water content was the key factor in controlling the soil respiration rate and the contribution of the respiration sources. However, the variable responses of soil respiration to soil water content create a complex distribution of soil respiration at the watershed scale.     

Soil respiration in a tropical grassland

Soil respiration throughout an annual cycle was measured at three different stands in a tropical grassland situated at Kurukshetra at 29°58' N lat. and 76°51' E long. Rates of CO₂ evolution were measured by alkali absorption using 13 cm dia × 23 cm aluminium cylinders inserted 10 cm into the ground. Both movable and permanently-fixed cylinders were used. The CO₂ evolution rates for the three stands were: Stand I (dominated by Sesbania bispinosa) 49–358 mg CO₂ m^?2 h^?1; Stand II (mixed grasses) 55–378 mg CO₂m^?2 h^?1; and Stand III (dominated by Desmostachya bipinnata) 55–448 mg CO₂ m^?2 h^?1. A positive significant relation existed between rate of CO₂ evolution and soil water content (r = 0.59?0.740), and between soil respiration and temperature (r = 0.58?0.69). A statistical model developed on the basis of the relationship between CO₂ evolution rates and certain abiotic environmental factors showed 69% comparability between the calculated and observed values of soil respiration. The contribution of root and root-associated microorganisms to total soil respiration was estimated at 42% using the relationship between root biomass and CO₂ output from movable cylinders.     

Factors controlling soil microbial respiration during the growing season in a mature larch plantation in Northern Japan

Purpose

Soil microbes contribute significantly to soil respiration (SR) in boreal forests; however, there is limited knowledge on microbial contributions from long field investigations. The objective of this study was to estimate soil microbial respiration, as well as its primary controlling factors, for a period of three consecutive years.

Materials and methods

A trenching method was used to distinguish soil microbial respiration (R _Mic) in a 55-year-old mature Japanese larch (Larix kaempferi) plantation in Northern Japan; the soil in which developed originally from volcanic soils containing pumice. We used a portable CO₂ detection system to measure the soil respiration rate during the growing season. Environmental factors, soil physiochemical characteristics, and soil microbial biomass carbon and nitrogen (MBC and MBN) were analyzed to explain the seasonal variations of SR and R _Mic.

Results and discussion

The results showed that the estimated contribution of soil microbes to SR was 78, 62, and 55% during the three successive years, respectively. Respiration attributable to decomposition of aboveground litter contributed approximately 19% to SR. The major environmental factor that affected R _Mic was soil temperature at 5 cm depth, which accounted for more than 70% of the seasonal variation in R _Mic observed. There were close relations among MBC, MBN, and soil water content, but the soil water content showed no significant relation with R _Mic.

Conclusions

The R _Mic to SR varied from 78 to 55% following 3 years of trenching treatments. Our results demonstrated the important role of soil microbes on soil respiration in this larch forest. Soil temperature was the major positive factor that influenced R _Mic, while soil water content had no significant effect. Global warming will increase the loss of C into the atmosphere by increasing the R _Mic, and could accelerate climate change.

     

Effect of forest structure on the spatial variation in soil respiration in a Bornean tropical rainforest

This study was undertaken to identify critical and practical factors explaining spatial variations in soil respiration and to estimate stand-scale soil respiration in an aseasonal tropical rainforest on Borneo Island. To this aim, we conducted soil respiration measurements at 25 points in a 40 m × 40 m subplot of a 4 ha study plot between 2002 and 2006, and examined the spatial variation in soil respiration averaged over the 4 years in relation to soil, root, and forest structural factors. In addition, we examined the spatial representativeness of soil respiration measured in the subplot using a specific scaling procedure. Consequently, we found significant positive correlation between the soil respiration and forest structural parameters such as the mean diameter at breast height (DBH), total basal area, and maximum DBH within 6 m of the measurement points. The most important factor was the mean DBH within 6 m of the measurement points, which had a significant linear relationship with soil respiration. Using the derived linear regression and an inventory dataset, we estimated the 4 ha plot-scale soil respiration. The 4 ha plot-scale estimation (6.0 μmol m⁻² s⁻¹) was nearly identical to the subplot-scale measurements (5.7 μmol m⁻² s⁻¹), which were roughly comparable to the nocturnal CO₂ fluxes calculated using the eddy covariance technique. In addition, we discuss characteristics of the stand-scale soil respiration at this site by comparing with those of other forests reported in previous literature in terms of the soil C balance. Soil respiration at our site was noticeably greater, relative to the incident litterfall amount, than soil respiration in other tropical and temperate forests probably owing to the larger total belowground C allocation by emergent trees. Overall, this study suggests the arrangement of emergent trees with larger DBH and their belowground C allocation could be primary factors controlling spatial variations in soil respiration in the tropical rainforest.     

Alterations in forest detritus inputs influence soil carbon concentration and soil respiration in a Central-European deciduous forest

In a Quercetum petraeae–cerris forest in northeastern Hungary, we examined effects of litter input alterations on the quantity and quality soil carbon stocks and soil CO₂ emissions. Treatments at the Síkfőkút DIRT (Detritus Input and Removal Treatments) experimental site include adding (by doubling) of either leaf litter (DL) or wood (DW) (including branches, twigs, bark), and removing all aboveground litter (NL), all root inputs by trenching (NR), or removing all litter inputs (NI). Within 4 years we saw a significant decrease in soil carbon (C) concentrations in the upper 15 cm for root exclusion plots. Decreases in C for the litter exclusion treatments appeared later, and were smaller than declines in root exclusion plots, highlighting the role of root detritus in the formation of soil organic matter in this forest. By year 8 of the experiment, surface soil C concentrations were lower than Control plots by 32% in NI, 23% in NR and 19% in NL. Increases in soil C in litter addition treatments were less than C losses from litter exclusion treatments, with surface C increasing by 12% in DL and 6% in DW. Detritus additions and removals had significant effects on soil microclimate, with decreases in seasonal variations in soil temperature (between summer and winter) in Double Litter plots but enhanced seasonal variation in detritus exclusion plots. Carbon dioxide (CO₂) emissions were most influenced by detritus input quantity and soil organic matter concentration when soils were warm and moist. Clearly changes in detritus inputs from altered forest productivity, as well as altered litter impacts on soil microclimate, must be included in models of soil carbon fluxes and pools with expected future changes in climate.     

Rhizospheric influence on soil respiration and decomposition in a temperate Norway spruce stand

Assessments of terrestrial carbon fluxes require a thorough understanding of links between primary production, soil respiration and carbon loss through drainage. In this study, stem girdling was used to terminate autotrophic soil respiration including rhizosphere respiration and root exudation in a temperate Norway spruce stand. Rates of soil respiration and dissolved organic carbon (DOC) formation were measured in the second year after girdling, comparing an intact plant-rhizosphere continuum with an exclusive decomposer system. The molecular and isotopic composition of DOC in the soil solution was analysed with a coupled Py-GC/MS-C-IRMS system to distinguish between the carbon sources of dissolved carbon. Pyrolysis products were grouped according to their precursor origins: polysaccharides, proteins or of mixed origin (mainly derivates of lignins and proteins). When dead roots became available for decomposition, rates of heterotrophic soil respiration in girdling plots peaked at 6.5 μmol m⁻² s⁻¹, comparable to peak rates of total soil respiration (autotrophic and heterotrophic) in control plots, 6.1 μmol m⁻² s⁻¹. A significant response of soil respiration to temperature was found in control plots only, showing that an unlimiting supply of organic substrates for microbial respiration may mask any temperature effects. The enhanced decomposition in girdled plots was further supported by the isotopic composition of DOC in soil solution; all three precursor groups became isotopically enriched as the growing season progressed (polysaccharides by 2.3‰, proteins by 1.9‰, mixed origin group by 2.2‰). This indicates a trophic level shift due to incorporation of organic substrate into the microbial food chain. In the control plots’ mixed origin fraction, the isotopic composition changed over time from a signature resembling that of lignin (−28.9‰) to one similar of the protein fraction (−25.7‰). Significant temporal changes of structural DOC composition occurred in the girdling plots only. These results suggest that changes in the microbial community and in decomposition rates occurred in both girdled and control plots in the following ways: (i) increased substrate availability (dead roots) gave rise to generally enhanced performance of the decomposer community in girdled plots, (ii) root-derived exudates probably contributed to enhanced decomposition of recalcitrant lignin in the control plots and (iii) the structural composition of DOC seemed to be more a result of decomposition than of plant root exudation in all plots.     

Carbon stocks, soil respiration and microbial biomass in fire-prone tropical grassland, woodland and forest ecosystems

A thorough understanding of the role of microbes in C cycling in relation to fire is important for estimation of C emissions and for development of guidelines for sustainable management of dry ecosystems. We investigated the seasonal changes and spatial distribution of soil total, dissolved organic C (DOC) and microbial biomass C during 18 months, quantified the soil CO₂ emission in the beginning of the rainy season, and related these variables to the fire frequency in important dry vegetation types grassland, woodland and dry forest in Ethiopia. The soil C isotope ratios (δ¹³C) reflected the 15-fold decrease in the grass biomass along the vegetation gradient and the 12-fold increase in woody biomass in the opposite direction. Changes in δ¹³C down the soil profiles also suggested that in two of the grass-dominated sites woody plants were more frequent in the past. The soil C stock ranged from being 2.5 (dry forest) to 48 times (grassland) higher than the C stock in the aboveground plant biomass. The influence of fire in frequently burnt wooded grassland was evident as an unchanged or increasing total C content down the soil profile. DOC and microbial biomass measured with the fumigation-extraction method (C_mic) reflected the vertical distribution of soil organic matter (SOM). However, although SOM was stable throughout the year, seasonal fluctuations in C_mic and substrate-induced respiration (SIR) were large. In woodland and woodland-wooded grassland C_mic and SIR increased in the dry season, and gradually decreased during the following rainy season, confirming previous suggestions that microbes may play an important role in nutrient retention in the dry season. However, in dry forest and two wooded grasslands C_mic and SIR was stable throughout the rainy season, or even increased in this period, which could lead to enhanced competition with plants for nutrients. Both the range and the seasonal changes in soil microbial biomass C in dry tropical ecosystems may be wider than previously assumed. Neither SIR nor C_mic were good predictors of in situ soil respiration. The soil respiration was relatively high in infrequently burnt forest and woodland, while frequently burnt grasslands had lower rates, presumably because most C is released through dry season burning and not through decomposition in fire-prone systems. Shifts in the relative importance of the two pathways for C release from organic matter may have strong implications for C and nutrient cycling in seasonally dry tropical ecosystems.     

The distribution and abundance of soil nematodes in East African savannas

Summary Nematodes were sampled from sites under and between tree canopies in Tsavo National Park, Kenya. We tested the hypothesis that more nematodes would be present in the generally moister soil, under the canopy, with a larger biomass of green grass prevailing for many months of the year. We found that microbivorous nematodes comprised the bulk of the populations, approximately 90% of the total. Bacterivores were more numerous (approximately 3×10⁵ on average) versus fungivores (approximately 5×10⁴ on average) per m², to 10 cm depth. All four trophic groups (plant parasites and omnivore/predators in addition to the microbivore groups mentioned above) were significantly higher under acacia than baobab in a drier site, but not significantly different under the two tree species in a wetter site. Only bacterivores were significantly different with respect to distance from the tree, with higher numbers associated with the higher microbial biomasses under the tree canopies. These higher microbial biomasses were reflected in 2.5 times more potentially mineralizable N under the tree canopies at the drier site. We suggest that belowground detrital and microbivorous food webs in savannas may be similar to those in temperate grasslands. Further proof of this idea awaits more extensive research.     

中国西北棉花根系呼吸对土壤呼吸贡献率的研究

To measure the contribution of root respiration (Rr) to total soil respiration (Rt) in arid cotton fields, eighteen plots, nine for girdling and nine control, were built in an arid cotton field in the Aksu National Experimental Station of Oasis Farmland Ecosystem, Xinjiang of China. Given the difference of soil respiration between girdled plots and non-girdled control plots, the components of soil respiration, root respiration (Rr) and respiration originating from decomposition (Rd) were divided. The temperature sensitivities of R r and R d were analyzed, respectively. The results showed that the average contribution of R r to R t in arid cotton field was about 32% during the study period. The temperature-response curve of R r differed from that of Rd . The dynamic variation of R d was more related to the change of soil temperature as compared to Rr . Rr and Rd had different responses to the variation of environment, and thus new models capable of differentiating between Rr and Rd are needed for evaluating the different factors controlling these two components of soil respiration in arid cotton field.     

Response of soil respiration to precipitation during the dry season in two typical forest stands in the forest-grassland transition zone of the Loess Plateau

Forest ecosystems on the Loess Plateau are receiving increasing attention for their special importance in carbon fixation and conservation of soil and water in the region. Soil respiration was investigated in two typical forest stands of the forest-grassland transition zone in the region, an exotic black locust (Robinia pseudoacacia) plantation and an indigenous oak (Quercus liaotungensis) forest, in response to rain events (27.7 mm in May 2009 and 19 mm in May 2010) during the early summer dry season. In both ecosystems, precipitation significantly increased soil moisture, decreased soil temperature, and accelerated soil respiration. The peak values of soil respiration were 4.8 and 4.4 μmol CO₂ m⁻² s⁻¹ in the oak plot and the black locust plot, respectively. In the dry period after rainfall, the soil moisture and respiration rate gradually decreased and the soil temperature increased. Soil respiration rate in black locust stand was consistently less than that in oak stand, being consistent with the differences in C, N contents and fine root mass on the forest floor and in soil between the two stands. However, root respiration (R_r) per unit fine root mass and microbial respiration (R_m) per unit the amount of soil organic matter were higher in black locust stand than in oak stand. Respiration by root rhizosphere in black locust stand was the dominant component resulting in total respiration changes, whereas respiration by roots and soil microbes contributed equally in oak stand. Soil respiration in the black locust plantation showed higher sensitivity to precipitation than that in the oak forest.     

Effect of warming on the temperature dependence of soil respiration rate in arctic,temperate and tropical soils

We examined the response of the temperature coefficient (Q₁₀) for soil respiration rate to changes in environmental temperature through a laboratory incubation experiment. Soil samples were collected from three climatic areas: arctic (Svalbard, Norway), temperate (Tsukuba, Japan) and tropical (Pasoh, Malaysia). The arctic and temperate soils were incubated at 8 °C (control), 12 °C (4 °C warming) and 16 °C (8 °C warming) for 17 days. The tropical soil was incubated at 16 °C (8 °C cooling), 24 °C (control) and 32 °C (8 °C warming). Before and after the incubation experiment, the temperature dependence of soil microbial respiration was measured using an open-airflow method with IRGA by changing the temperature in a water bath. The initial Q₁₀ before the incubation experiment was larger in the soils from higher latitudes: 3.4 in the arctic soil, 2.9 in the temperate soil, and 2.1 in the tropical soil. The response of the microbial respiration rate to change in temperature differed among the three soil types. The temperature dependence of respiration rate in the arctic soil did not change in response to warming by 4 and 8 °C with a Q₁₀ of about 3. On the other hand, the Q₁₀ in the temperate soil decreased with increasing incubation temperature: from 2.8 in soils incubated at 8 °C to 2.5 at 12 °C and 2.0 at 16 °C. In the tropical soil, the Q₁₀ was not changed even by the 8 °C warming with a value of 2.1, whereas the Q₁₀ was increased from 2.1 to 2.7 by the 8 °C cooling. These results suggest that the response of microbial respiration to climatic warming may differ between soils from different latitudes.     

水土保持措施对板栗林土壤呼吸的影响

为阐明板栗林土壤呼吸对水土保持措施的响应,采用IRGA法,对不同类型板栗林的土壤呼吸从2009年3月至2010年9月开展为期1年半的定位观测。结果表明:1)采取水土保持措施后,样地的土壤水分状况得到一定程度的改善,尤其随着采取水土保持措施年限的延长,其对土壤水分时空分布影响更为显著。2)4个板栗林样地的土壤呼吸速率均呈明显的单峰曲线变化,水土保持措施对土壤呼吸的季节动态无明显影响。3)样地Ⅰ和Ⅲ的参考呼吸R10分别为1.718和1.595μmol/(m2.s);而采取水土保持措施后,样地Ⅱ和Ⅳ的R10均表现为一定程度的降低,分别为1.092和1.324μmol/(m2.s)。样地Ⅰ和Ⅲ的土壤呼吸的温度敏感性指数Q10分别为1.927和1.899;采取水土保持措施后,样地Ⅱ和Ⅳ的Q10均表现为略微增加。采取水土保持措施后,土壤温度和土壤湿度对土壤呼吸速率的影响有一定程度的增强。研究结果可为把水土保持措施作为土壤严重侵蚀地区一种潜在的固碳减排模式提供基础数据。     

Contributions of root respiration to total soil respiration before and after frost in Populus euphratica forests

Temporal changes in soil CO₂‐efflux rate was measured by a canopy‐gap method in a Populus euphratica forest located at the both sides of Tarim River banks (W China). Soil CO₂‐efflux rates in situ were correlated with key soil biotic (e.g., fungal, bacterial, and actinomycetes populations) and abiotic (e.g., soil moisture, temperature, pH, organic C) variables. Two kinds of measurement plots were selected: one under the crown of a living Populus euphratica tree and the other under a dead standing Populus euphratica tree. Diurnal variations in soil respiration in these plots were measured both before and after the occurrence of the first frost. Soil respiration of the dead standing Populus euphratica (Rd) was assumed to be a measure of heterotrophic respiration rate (Rh), and root respiration rate (Rr) was estimated as the difference between soil respiration under living (Rl) minus soil respiration under dead standing Populus euphratica. Daily variation of Rr contribution to the total soil respiration in Populus euphratica forests were analyzed before and after the frost. The contribution of root respiration to total soil respiration before and after frost varied from 22% to 45% (mean 30%) and from 38% to 50% (mean 45%), respectively. In addition, Rh was significantly correlated with soil temperature both before and after frost. In contrast, Rr was not significantly correlated with soil temperature. Change in Q₁₀ of Rr was different from that of Rh from before the frost to after the frost. Variation of Q₁₀ of Rr from before the frost to after the frost was larger than that of Q₁₀ of Rh. Thus, the results indicate that different soil respiration models are needed for Rr and Rh because different factors control the two components of soil respiration.