鄂尔多斯高原脉冲降雨对油蒿灌丛群落土壤碳排放的影响

Precipitation is the major driver of ecosystem functions and processes in semiarid and arid regions. In such water-limited ecosystems, pulsed water inputs directly control the belowground processes through a series of soil drying and rewetting cycles. To investigate the effects of sporadic addition of water on soil CO₂ efflux, an artificial precipitation event (3 mm) was applied to a desert shrub ecosystem in the Mu Us Sand Land of the Ordos Plateau in China. Soil respiration rate increased 2.8-4.1 times immediately after adding water in the field, and then it returned to background level within 48 h. During the experiment, soil CO₂ production was between 2 047.0 and 7 383.0 mg m^-2. In the shrubland, soil respiration responses showed spatial variations, having stronger pulse effects beneath the shrubs than in the interplant spaces. The spatial variation of the soil respiration responses was closely related with the heterogeneity of soil substrate availability. Apart from precipitation, soil organic carbon and total nitrogen pool were also identified as determinants of soil CO₂ loss in desert ecosystems.     

Simulated rain addition modifies diurnal patterns and temperature sensitivities of autotrophic and heterotrophic soil respiration in an arid desert ecosystem

The timing and magnitude of rainfall events in arid and semiarid regions are expected to change dramatically in future decades, which will likely greatly affect regional carbon cycles. To understand how increases in rainfall affect the diurnal patterns and temperature sensitivities (Q₁₀) of soil respiration (R_S) and its key components (i.e. heterotrophic respiration (R_H) and autotrophic respiration (R_A)), we conducted a manipulative field experiment in a desert ecosystem of Northwest China. We simulated five different scenarios of future rain regimes (0%, 25%, 50%, 75% and 100% increase over local annual mean precipitation) each month from May to September in 2009. We measured R_S and R_H every three hours on 6 and 16 days after the rain addition, and estimated R_A by calculating the difference between R_S and R_H. We found that rain addition significantly increased the daily mean R_S and its components on the two measurement days during the growing season. However, the diurnal pattern was different between the two respiration components. Rain addition significantly increased the daily Q₁₀ value of R_H but suppressed that of R_A on Day 6. Rain addition had no influence on daily Q₁₀ value of both respiration components on Day 16 when soil moisture was lower. In addition, we observed significantly higher daily Q₁₀ of R_H than R_A under all five rain addition treatments, indicating that microbial respiration is more temperature sensitive than root respiration in a short-time scale in this desert ecosystem. Thus, partitioning soil respiration into its two components, and analyzing the differential responses of R_H and R_A to future climate changes should be considered for more accurate predictions of soil respiration and regional carbon cycle in these arid and semiarid regions.     

Promise and pitfalls of modeling grassland soil moisture in a free-air CO2 enrichment experiment (BioCON) using the SHAW model

Free-air carbon dioxide (CO₂) enrichment (FACE) experiments provide an opportunity to test models of heat and water flow under novel, controlled situations and eventually allow use of these models for hypothesis evaluation. This study assesses whether the United States Department of Agriculture SHAW (Simultaneous Heat and Water) numerical model of vertical one-dimensional soil water flow across the soil-plant-atmosphere continuum is able to adequately represent and explain the effects of increasing atmospheric CO₂ on soil moisture dynamics in temperate grasslands. Observations in a FACE experiment, the BioCON (Biodiversity, CO₂, and Nitrogen) experiment, in Minnesota, USA, were compared with results of vertical soil moisture distribution. Three scenarios represented by different plots were assessed: bare, vegetated with ambient CO₂, and similarly vegetated with high CO₂. From the simulations, the bare plot soil was generally the wettest, followed by a drier high-CO₂ vegetated plot, and the ambient CO₂ plot was the driest. The SHAW simulations adequately reproduced the expected behavior and showed that vegetation and atmospheric CO₂ concentration significantly affected soil moisture dynamics. The differences in modeled soil moisture amongst the plots were largely due to transpiration, which was low with high CO₂. However, the modeled soil moisture only modestly reproduced the observations. Thus, while SHAW is able to replicate and help broadly explain soil moisture dynamics in a FACE experiment, its application for point- and time-specific simulations of soil moisture needs further scrutiny. The typical design of a FACE experiment makes the experimental observations challenging to model with a one-dimensional distributed model. In addition, FACE instrumentation and monitoring will need improvement in order to be a useful platform for robust model testing. Only after this can we recommend that models such as SHAW are adequate for process interpretation of datasets from FACE experiments or for hypothesis testing.     

A comparison of methods for estimating fractional vegetation cover in arid regions

We compared a set of methods for estimating the fractional vegetation cover (f_c) of sparse desert vegetation over an arid region of southern Xinjiang, China. Six kinds of remote sensing inversion models (an NDVI regression, a spectral mixture analysis (SMA), a pixel dichotomy model, a three-band maximal gradient difference (TGDVI) model and two modified TGDVI models) were used to derive f_c from remote sensing data, and the results were compared with f_c values measured in the field to select an appropriate model to derive the fractional cover of sparse desert vegetation in arid regions. The NDVI regression based on field f_c and the NDVI for the sampled pixels in September 2006 showed the highest precision, while the results of 2007 showed that the NDVI regression method is inappropriate for depicting vegetation characteristics in other growing season because the empirical model highly depend on the specified in situ measurement. The SMA approaches yielded higher precision than the other models, indicating that it is applicable for analysing the coverage of sparse desert vegetation. The pixel dichotomy model can yield a high precision based on finely detailed vegetation maps. However, it requires the measurement of many parameters. The TGDVI model is simple and easy to implement, and the values that it predicted for the coverage of high-density vegetation and barren areas were close to those measured in the field, but the f_c values of sparsely vegetated areas were underestimated. The predictions of the modified TGDVI models were close to the values measured in the field, indicating that these modified models can reliably and effectively extract information on the fractional cover of sparse vegetation in an arid region. We analyzed the models’ sensitivity with respect to rainfall because the short-wavelength infrared bands used in the two TGDVI models proposed in this study are sensitive to moisture. The results showed that the modified TGDVI models’ accuracy was not affected by increasing soil moisture content caused by rain. However, the NDVI regression, SMA and TGDVI were sensitive to the change of soil moisture content. Moreover, the two modified TGDVI models yielded negative values for water sources, such as reservoirs and rivers, implying that they are effective for characterising water bodies. However, the modified TGDVI models cannot predict f_c in snow- and glacier-covered regions, producing abnormally high rather than zero values. Additionally, the predictions before and after snowfall on the top of a mountain show a linear increasing relationship, suggesting that the short-wavelength infrared band may be useful to predict snow depth.     

黑土不同土地利用方式下受水分有效性调节的土壤CO2排放的温度敏感性

The quantification of soil CO2 efflux is crucial for better understanding the interactions between driving variables and C losses from black soils in Northeast China and for assessing the function of black soil as a net source or sink of atmospheric CO2 depending upon land use.This study investigated responses of soil CO2 efflux variability to soil temperature interactions with diferent soil moisture levels under various land use types including grassland,bare land,and arable(maize,soybean,and wheat)land in the black soil zone of Northeast China.The soil CO2 effluxes with and without live roots,defined as the total CO2 efflux(FtS)and the root-free CO2 efflux(FrfS),respectively,were measured from April 2009 to May 2010 using a static closed chamber technique with gas chromatography.The seasonal soil CO2 fluxes tended to increase from the beginning of the measurements until they peaked in summer and then declined afterwards.The mean seasonal FtS ranged from 20.3±7.8 to 58.1±21.3 mg CO2-C m-2h-1 for all land use types and decreased in the order of soybean land>grassland>maize land>wheat land>bare land,while the corresponding values of FrfS were relatively lower,ranging from 20.3±7.8 to 42.3±21.3 mg CO2-C m-2h-1.The annual cumulative FtS was in the range of 107-315 g CO2-C m-2 across all land uses types.The seasonal CO2 effluxes were significantly(P<0.001)sensitive to soil temperature at 10 cm depth and were responsible for up to 62% of the CO2 efflux variability.Correspondingly,the temperature coefcient Q10 values varied from 2.1 to 4.5 for the seasonal FtS and 2.2 to 3.9 for the FrfS during the growing season.Soil temperature interacting with soil moisture accounted for a significant fraction of the CO2 flux variability for FtS (up to 61%) and FrfS (up to 67%) via a well-defined multiple regression model,indicating that temperature sensitivity of CO2 flux can be mediated by water availability,especially under water stress.     

Hot spots,hot moments,and spatio-temporal controls on soil CO2 efflux in a water-limited ecosystem

Soil CO₂ efflux is the primary source of CO₂ emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO₂ efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO₂ efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO₂ efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO₂ efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO₂ efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO₂ efflux, and during the wet season the emergence of a hot spot of soil CO₂ efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO₂ efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO₂ efflux in water-limited ecosystems.     

Factors controlling soil CO2 effluxes and the effects of rewetting on effluxes in adjacent deciduous, coniferous, and mixed forests in Korea

To better understand the factors that control forest soil CO₂ efflux and the effects of rewetting on efflux, we measured soil CO₂ efflux in adjacent deciduous, coniferous, and mixed forests in the central part of the Korean Peninsula over the course of one year. We also conducted laboratory rewetting experiments with soil collected from the three sites using three different incubation temperatures (4 °C, 10 °C, and 20 °C). Soil moisture (SM), soil organic matter (SOM), and total root mass values of the three sites were significantly different from one another; however, soil temperature (ST), observed soil CO₂ efflux and sensitivity of soil CO₂ efflux to ST (i.e., Q₁₀ = 3.7 ± 0.1) were not significantly different among the three sites. Soil temperature was a dominant control factor regulating soil CO₂ efflux during most of the year. We infer that soil CO₂ efflux was not significantly different among the sites due to similar ST and Q₁₀. Though a significant increase in soil CO₂ efflux following rewetting of dry soil was observed both in the field observations (60-170%) and laboratory incubation experiments (100-1000%), both the increased rates of soil CO₂ efflux and the magnitude of change in SM were not significantly different among the sites. The increased rates of soil CO₂ efflux following rewetting depended on the initial SM before rewetting. During drying phase after rewetting, a significant correlation between SM and soil CO₂ efflux was found, but the effect of ST on increased soil CO₂ efflux was not clear. Cumulative peak soil CO₂ efflux (11.3 ± 0.7 g CO₂ m⁻²) following rewetting in the field was not significantly different among the sites. Those evidences indicate that the observed similar rewetting effects on soil CO₂ efflux can be explained by the similar magnitude of change in SM after rewetting at the sites. We conclude that regardless of vegetation type, soil CO₂ efflux and the effect of rewetting on soil CO₂ efflux do not differ among the sites, and ST is a primary control factor for soil CO₂ efflux while SM modulates the effect of rewetting on soil CO₂ efflux. Further studies are needed to quantify and incorporate relationship of initial dryness of the soil and the frequency of the dry-wet cycle on soil CO₂ efflux into models describing carbon (C) processes in forested ecosystems.     

Effects of a biomimetic iron‐porphyrin on soil respiration and maize root morphology as by a microcosm experiment

A synthetic, water‐soluble iron‐porphyrin [meso‐tetra(2,6‐dichloro‐3‐sulfonatophenyl) porphyrinate of Fe(III) chloride] has recently been proposed as a biomimetic catalyst in the process of oxidative polymerization of terrestrial humic acids, to increase their conformational stability and thus contribute to a reduction of soil CO₂ release into the atmosphere. This study was aimed at investigating changes in selected soil chemical properties, CO₂ efflux, and maize root morpho‐topology after the addition of iron‐porphyrin as a microcosm‐style experiment, located in a greenhouse. The addition of mature compost was also included as an experimental factor in order to reveal synergistic effects in regard to freshly added organic materials. Iron‐porphyrin determined a negligible effect on soil organic budget in both unplanted and planted microcosms. Conversely, the biomimetic catalyst was found to have significant and contrasting effects on soil respiration, apparently reflecting different iron porphyrin–plant–compost interactions. Consequently, iron‐porphyrin significantly reduced CO₂ efflux from the bare (unplanted) soil, which was, conversely, stimulated in maize‐planted microcosms. Additionally, combined iron‐porphyrin and compost addition synergistically acted in increasing soil respiration in planted microcosms. Moreover, root biomass was increased with the addition of iron‐porphyrin, and a further effect on maize root morphology was noted when used in combination with compost; notably the length of coarse and fine roots increased. We hypothesized that the efficacy of iron‐porphyrin in reducing CO₂ efflux from soil may be mediated by morphological changes in the plant‐root system.     

CO2 efflux from deciduous forest litter and soil in response to simulated acid rain treatment

Using both field and laboratory measurements of CO₂ evolution as an index of decomposer activity, forest microcosms were used to evaluate the impact of simulated acidic precipitation on decomposition. The following pH treatments: 5.7, 4.5, 4.0, and 3.5 annual average were applied for a 30 mo period. No statistically significant effect of treatment on decomposition could be found in the field measurements. When the microcosm was partitioned into 01 and 02 litter, mineral soil (A and B horizons), and roots within the mineral soil horizons for laboratory determination of CO₂ efflux, only the 02 litter exhibited a statistically significant decrease as a function of treatment. The data collected do not allow a complete evaluation of the potential impact of this decrease. However, efflux of CO₂ from the 02 layer was small compared to the other layers, and this may account for the failure to detect a significant response in the field measurements. Although the field data did not exhibit a significant response, there is sufficient question concerning the 02 response to warrant additional investigation, especially since many plants derive a major portion of their nutritional requirements directly from the 02 litter layer.     

Forest responses to CO2 enrichment and climate warming

Two of the major uncertainties in forecasting future terrestrial sources and sinks of CO₂ are the CO₂-enhanced growth response of forests and soil warming effects on net CO₂ efflux from forests. Carbon dioxide enrichment of tree seedlings over time periods less than 1 yr has generally resulted in enhanced rates of photosynthesis, decreased respiration, and increased growth, with minor increases in leaf area and small changes in C allocation. Exposure of woody species to elevated CO₂ over several years has shown that high rates of photosynthesis may be sustained, but net C accumulation may not necessarily increase if CO₂ release from soil respiration increases. The impact of the 25% rise in atmospheric CO₂ with industrialization has been examined in tree ring chronologies from a range of species and locations. In contrast to the seedling tree results, there is no convincing evidence for CO₂-enhanced stem growth of mature trees during the last several decades. However, if mature trees show a preferential root growth response to CO₂ enrichment, the gain in root mass for an oak-hickory forest in eastern Tennessee is estimated to be only 9% over the last 40 years. Root data bases are inadequate for detecting such an effect. A very small shift in ecosystem nutrients from soil to vegetation could support CO₂-enhanced growth. Climate warming and the accompanying increase in mean soil temperature could have a greater effect than CO₂ enrichment on terrestrial sources and sinks of CO₂. Soil respiration and N mineralization have been shown to increase with soil temperature. If plant growth increases with increased N availability, and more C is fixed in growth than is released by soil respiration, then a negative feedback on climate warming will occur. If warming results in a net increase in CO₂ efflux from forests, then a positive feedback will follow. A 2 to 4°C increase in soil temperature could increase CO₂ efflux from soil by 15 to 32% in eastern deciduous forests. Quantifying C budget responses of forests to future global change scenarios will be speculative until mature tree responses to CO₂ enrichment and the effects of temperature on terrestrial sources and sinks of CO₂ can be determined.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F₁₀) of 2.6 μmol m⁻² s⁻¹ and the Q₁₀ of 3.7. No evidence of displacement of CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

Contrasting effects of bamboo leaf and its biochar on soil CO2 efflux and labile organic carbon in an intensively managed Chinese chestnut plantation

The effects and associated mechanisms of the application of organic residues or their derived biochar on the dynamics of soil organic C and soil CO₂ efflux in planted soils are poorly understood. This paper investigated the impact of bamboo leaf and the derived biochar applications on soil CO₂ efflux and labile organic C in an intensively managed Chinese chestnut plantation in a 12-month field study. The treatments studied included Control, application of bamboo leaf (Leaf), and application of biochar (Biochar). The Leaf treatment increased (P?2 efflux and concentrations of water-soluble organic C (WSOC) and microbial biomass C (MBC). The Biochar treatment increased soil CO₂ efflux and WSOC and MBC only in the first month after application, but such effects diminished thereafter. The annual cumulative soil CO₂ emission was increased by 16 % by the Leaf treatment as compared to the Control, but there was no difference between the Biochar and Control treatments. The soil organic C (SOC) storage was increased by biochar addition but not by bamboo leaf addition. An exponential relationship between soil temperature and soil CO₂ efflux was observed regardless of the treatment. Soil CO₂ efflux was correlated to soil WSOC (P?Q ₁₀) of soil CO₂ efflux was ranked as Leaf?>?Biochar?>?Control. In comparison with the application of fresh bamboo leaf, pyrolyzed bamboo leaf (biochar) application decreased CO₂ effluxes and increased C sequestration in the soil.     

The Effect of Contrasting Moistening Regimes on CO<Subscript>2</Subscript> Emission from the Gray Forest Soil under a Grass Vegetation and Bare Fallow

The effect of contrasting moisture regimes on the CO₂ emission from the gray forest soils (Haplic Luvisols (Loamic, Cutanic, Humic)) under a grass vegetation and bare fallow was studied in a field simulation experiment in June–September, 2015 (Moscow region). Two short soil droughts (53 and 34 days) and a long one (94 days) were simulated on plots isolated from precipitation. A variant with regular irrigation, where the soil moisture was maintained 60–70% of their water holding capacity, was used as a control. Over the whole observation period, the CO₂ emissions from the soils studied decreased by a factor of 1.8 compared to the control only in the variant with the grass vegetation under prolonged drought. During the first hours after irrigation of the dry plots, the soil respiration intensified due to the “Birch effect”. The magnitude of this effect was 84–104% in the soils under the grass vegetation and 114–133% in the fallow areas. Owing to this phenomenon, the total CO₂ emission from the soils subjected to two short droughts was equal to the CO₂ flux under regular moistening for the grass plots and exceeded it by almost 1.3 times for the fallow plots as compared to the control. However, the share of extra CO₂ flux induced by moistening of the dry soils did not exceed 8–10% of the total CO₂ emission over the whole observation period.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

Lysimeter study with a cambic Arenosol exposed to artificial acid rain: I. Concentrations of ions in leachate

The effects of artificial acid rain on soil leachate composition were studied in a lysimeter experiment. Cambic Arenosol (Typic Udipsamment) in monolith lysimeters was treated for 6 1/2 yr with 125 mm yr^?1 artificial rain in addition to natural precipitation. Artificial acid rain was produced from groundwater with H₂SO₄ added. pH levels of 6.1, 4 and 3 were used. Increasing content of H₂SO₄ in the artificial rain increased the concentration of Ca²⁺ and Mg²⁺ in the leachate significantly. The pH of the leachate was slightly reduced only by the most acidic treatment (pH 3). The H^+? retention was not accompanied by a proportionate increase in the Al ion concentration. A slight increase in the Al ion concentration was only observed in the leachate from the pH 3-treated lysimeter. We conclud that cation exchange and/or weathering were the main buffer mechanisms in the soil. The study supports conclusions from other acidification studies, that acidic precipitation is likely to increase the leaching of Ca²⁺ and Mg²⁺ from soils.     

An appropriate time-window for measuring soil CO2 efflux: a case study on a Black soil in north-east China

Abstract

Soil CO₂ efflux rate is influenced by soil temperature which varies with time within a day. In order to determine a measuring time-window which can represent the daily average soil CO₂ efflux rate from a Black soil in north-east China, soil CO₂ efflux rates from no-tillage (NT) and mouldboard plough tillage (MP) plots were measured at a 2-h interval over 48 h four times in the growing season of 2008. Results showed that during the course of measurements, NT soil had a higher soil CO₂ efflux rate than MP soil. Daily average soil CO₂ efflux rate was matched relatively well with the CO₂ efflux rate occurring between 09:00 h and 13:00 h, and between 19:00 h and 23:00 h. Our results indicate that the soil CO₂ efflux rate measured between 09:00 and 11:00 h represents the daily average soil CO₂ efflux rate during sunny days. When the measurements were conducted outside this time window, a procedure to adjust the CO₂ efflux rates measured between 07:00 and 21:00 h (outside of the optimum time-window) to estimate daily average soil CO₂ efflux rate is described.     

Lysimeter Study on the Effects of Different Rain Qualities on Element Fluxes from Shallow Mountain Soils in Southern Norway

This study focuses on fluxes of elements from, and changes in the soil properties of shallow organic material rich soil as a result of changes in precipitation acidity. Intact soil columns including natural vegetation from two areas (one exposed to acidic precipitation and one unpolluted) were used in a lysimeter experiment. The lysimeters were watered with simulated normal rain (pH 5.3) or simulated acidic rain (pH 4.3) for four years. Sulphuric acid and ammonium nitrate were used to regulate the quality of the simulated rain. Significantly more SO₄ ^2? was leached from lysimeters receiving acid rain. Rain acidity had no significant effect on NO₃ ^? leaching. Significantly more Mg²⁺ was leached from lysimeters receiving acid rain, but this only applied for the soils from the unpolluted area. Four years of treatment did not cause any significant effect on the soil acidity and the amounts of base cations in the soil. The more acidic rain did, however, cause a significant lower cation exchange capacity. For the soils from the polluted area the acid precipitation did cause a lowering of the exchangeable K⁺ in the upper 5 cm of the soil. Different quality of the soil organic material indicated by different vegetation types appeared to cause significant differences in the amount of components leached from the soil, but did not cause any difference in response to the different rain qualities.     

Pore‐space CO2 dynamics in a deep,well‐aerated soil

Most studies implicitly consider soil carbon dioxide (CO₂) efflux as the instantaneous soil respiration and thereby neglect possible changes in the amount of CO₂ stored in the soil pore‐space. We measured the CO₂ concentration profile of a well‐aerated soil continuously to evaluate the dynamics of the stored CO₂ and to analyse the influence of environmental factors. For 25% of the observation period, changes in the amount of stored CO₂ accounted for more than 15% of the soil‐CO₂ efflux. The following factors were identified to interfere with steady‐state CO₂ storage: (i) the fluctuating groundwater table altered the volume of the vadose zone, causing viscous airflow in air‐filled soil pores, (ii) atmospheric turbulence caused pressure‐pumping at the soil–atmosphere interface and (iii) intense rain greatly reduced the diffusivity of the uppermost soil layer. The friction velocity above the canopy was strongly correlated with fluctuations in the differential pressure between soil air and atmosphere, but no static pressure gradient could be detected because of the permeable nature of the soil. Unexpected short‐term declines in the soil CO₂ concentration were observed during intense rainfall events. These declines were explained by the intensified CO₂ saturation deficit of the infiltrating rainwater caused by the carbonate chemistry of the soil solution.     

Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration (R_s), soil temperature (T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R_s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO₂ efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO₂ efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m⁻² s⁻¹ in the forest, abandoned field and olive grove, respectively. R_s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0-15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R_s was controlled by soil moisture. Exponential and linear models adequately described R_s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R_s in the forest and olive grove. However, in the abandoned field, a single variable - either soil temperature (growing season) or rewetting index (dry season) - was sufficient to explain between 51 and 63% of the soil CO₂ efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO₂ efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.