重复添加植物残体后土壤微生物活性和生物量对盐度的响应特征

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes,which requires high amounts of energy;however,a single addition of plant residues can only temporarily improve energy supply to soil microbes.Therefore,a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract（EC_e） of 0.6 dS m^-1.The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve EC_e of 12.5,25.0 and 50.0 dS m^-1.The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg^-1 soil on days 0,15 and29.The soils receiving no residues were included as a control.Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg^-1 soil and higher in the non-saline soil than in the saline soils.In the saline soils,the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with3.9 g C kg^-1 at EC_e of 50 dS m^₁.Though with the same amount of C added(7.8 g C kg^-1),salinity reduced soil respiration to a lesser extent when 3.9 g C kg^-1 was added twice compared to a single addition of 7.8 g C kg^-1.After the third residue addition,the microbial biomass C concentration was significantly lower in the soils with EC_e of 25 and 50 dS m^₁ than in the non-saline soil at3.9 g C kg^-1,but only in the soil with EC_e of 50 dS m^-1 at 7.8 g C kg^-1.We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity,which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.     

Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon (C) cycling and affect components of carbon dioxide (CO₂) exchange that could lead to either net accumulation or loss of soil C. While there are many studies on the effect of water availability on biomass production and soil C stocks, much less is known about the effect of the frequency of water inputs on the components of CO₂ exchange. We grew Bermuda grass (Cynodon dactylon L.) in mesocosms under irrigation frequencies of every day (I₁ treatment, 30 d), every two days (I₂ treatment, 12 d), every three days (I₃ treatment, 30 d), and every six days (I₆ treatment, 18 d, after I₂ treatment). Rates of CO₂ exchange for estimating net ecosystem CO₂ exchange (F_N), ecosystem respiration (R_E), and soil respiration (R_S) were measured, and gross C uptake by plants (F_G) and respiration from leaves (R_L) were calculated during two periods, 1–12 and 13–30 d, of the 30-d experiment. During the first 12 d, there were no significant differences in cumulative F_N (mean ±standard deviation, 61 ±30 g C m^-2, n = 4). During the subsequent 18 d, cumulative F_N decreased with decreasing irrigation frequency and increasing cumulative soil water deficit (W), with values of 70 ±22, 60 ±16, and 18 ±12 g C m^-2 for the I₁, I₃, and I₆ treatments, respectively. There were similar decreases in F_G, R_E, and R_L with increasing W, but differences in R_S were not significant. Use of the C₄ grass growing in a C₃-derived soil enabled partitioning of R_S into its autotrophic (R_A) and heterotrophic (R_H) components using a ¹³C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest. The values of R_H and its percentage contributions to R_S (43% ±8%, 42% ±8%, and 8% ±5% for the I₁, I₃, and I₆ treatments, respectively) suggested that R_H remained unaffected across a wide range of W and then decreased under extreme W. There were no significant differences in aboveground biomass between the treatments. Nitrous oxide (N₂O) emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission, but no significant differences were found between the treatments. These findings suggest that over short periods in well-drained soil, irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses, particularly those from the microbial decomposition of soil organic matter, with no significant effect on biomass production and N₂O emission.     

Responses of net ecosystem CO2 exchange to nitrogen fertilization in experimentally manipulated grassland ecosystems

Nitrogen (N) addition enhances primary productivity of terrestrial ecosystems. However, the effects of N fertilization and/or deposition on net ecosystem CO₂ exchange (NEE) are not fully understood. The effects of N on NEE were investigated in two experimental cheatgrass ecosystems in Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs), Reno, Nevada. In this experiment, no N fertilization was added to the two EcoCELLs in the first year and two different N fertilization regimes were applied in the second year. N fertilizer was applied once to one EcoCELL (pulse fertilization, PF), and the same total amount of N in biweekly increments to the other EcoCell (gradual fertilization, GF). NEE, photosynthetically active radiation (PAR) and canopy green leaf area index (LAI) were continuously measured in the two EcoCELLs during the pretreatment and N-fertilized years. Plant N content and biomass were measured at the end of the growing season in each year. Radiation-use efficiency (RUE_CO2) was calculated as the ratio of gross ecosystem photosynthesis (GEP) to the intercepted photosynthetically active radiation (IPAR). The responses of NEE to IPAR were used to estimate the maximum ecosystem photosynthetic capacity (F_max). N fertilization stimulated canopy LAI, plant N content, F_max, RUE_CO2, NEE and biomass in both methods of N supply applications. PF led to higher LAI, F_max and NEE than GF, but both had a similar RUE_CO2 during the early growing season. GF maintained higher LAI, F_max, RUE_CO2 and NEE than PF during the late growing season. At the ecosystem level, N fertilization stimulated daily NEE directly by increasing canopy LAI, plant N content, shoot/root ratio and the maximum ecosystem photosynthetic capacity, and increased the seasonally accumulated NEE indirectly by extending the growing season. PF differed significantly from GF in its effects on NEE and RUE_CO2, possibly due to differential rates and timing of N availability. Our study suggested that these changes in the canopy RUE_CO2 and growing season under N fertilization or N deposition regimes should be considered in modeling studies of ecosystem C sequestration.     

Effect of elevated carbon dioxide and water stress on gas exchange and water use efficiency in corn

CO₂ has been predicted to increase in the future, and thus leading to possible changes in precipitation patterns. The objectives of this study were to investigate water use and canopy level photosynthesis of corn plants, and to quantify water use efficiency in corn plants under two different CO₂ levels combined with four different water stress levels. Corn plants were planted in sunlit plant growth chambers and a day/night temperature of (28/18 °C) was applied. From 21 days after emergence (DAE), the eight treatments including two levels of carbon dioxide concentrations (400 and 800 μmol mol⁻¹) and four levels of water stress (well-watered control, “mild”, “moderate”, and “severe” water stress) treatments at each CO₂ level were imposed. Height, number of leaves, leaf lengths, and growth stages of corn plants were monitored from nine plants twice a week. Corn plants were separately collected, dried, and analyzed for the biomass accumulation at 21 and 60 DAE. Soil water contents were monitored by a time domain reflectometry (TDR) system (15 probes per chamber). The “breaking points” (changes from high to low rates of soil water uptake) were observed in the bottom of soil depth for the water stressed conditions, and the “breaking points” under ambient CO₂ appeared 6-9 days earlier than under elevated CO₂. Although approximately 20-49% less water was applied for the elevated CO₂ treatments than for ambient CO₂ from 21 DAE, higher soil water contents were recorded under elevated CO₂ than under ambient CO₂. However, corn growth variables such as height, leaf area, and biomass accumulation were not significantly different in CO₂ or water stressed treatments. This result may be explained by considering that significant differences in canopy level gross photosynthesis among the water stress treatments was observed only toward the end of the experiment. The higher soil water contents observed under elevated CO₂ resulted mainly from less water use than under ambient CO_2. WUE (above ground biomass per water use since 21 DAE) at the final harvest was consistently higher and varied with a smaller range under elevated CO₂ than under ambient CO₂. This study suggests that less water will be required for corn under high-CO₂ environment in the future than at present.     

Uncoupling of microbial CO2 production and release in frozen soil and its implications for field studies of arctic C cycling

Knowledge of seasonal trends and controls of soil CO₂ emissions to the atmosphere is important for simulating atmospheric CO₂ concentrations and for understanding and predicting the global carbon cycle. This is particularly the case for high arctic soils subject to extreme fluctuating environmental conditions. Based on field measurements of soil CO₂ efflux, temperature, water content, pore gas composition in soil and frozen cores as well as detailed temperature experiments performed in the laboratory, we evaluated seasonal controls of CO₂ effluxes from a well-drained tundra heath site in NE-Greenland. During the growing season, near-surface temperatures correlated well with observed CO₂ effluxes (r²>0.9). However, during intensive thawing of near-surface layers we observed up to 1.5-fold higher effluxes than expected due to temperature alone. These high rates were consistent with high CO₂ concentrations in frozen soil (>10% CO₂) and suggested a spring burst event during soil thawing and a corresponding trapping of produced CO₂ during winter. Laboratory experiments revealed that microbial soil respiration continued down to a least −18 °C and that up to 80% of the produced CO₂ was trapped in soil at temperatures between 0 and −9 °C. The trapping of CO₂ in frozen soil was positively correlated with soil moisture (r²=0.85) and led to an abrupt change of the temperature sensitivity (Q₁₀) observed for soil CO₂ release at 0 °C with Q₁₀ values below 0 °C being up to 100-fold higher than above 0 °C. The results of sub-zero CO₂ production allowed us to predict the microbial soil respiration throughout the year and to evaluate to what extent burst events during thawing can be explained by the release of CO₂ being produced and trapped during winter. Taking only the upper 20 cm of the soil into account, winter soil respiration accounted for about 40% of the annual soil respiration. At least 14% of the winter CO₂ production was trapped during the winter 2000-2001 and observed to be released upon thawing. Thus, the site-specific winter soil respiration is an important part of the annual C cycle and CO₂ trapping should be accounted for in future field and modelling studies of soil respiration dynamics in arctic ecosystems. In conclusion, we have discovered a soil moisture dependent uncoupling of CO₂ production and release in frozen soils with important implications for future field studies of Arctic C cycling.     

澳大利亚盐渍土中自然草的盐分积累状况及其生理学特性

Salinity is a major soil contamination problem in Australia. To explore salinity remediation, we evaluated the concentrations of sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca) in roots and shoots and in the supporting soil of the naturally occurring grasses, Cynodon dactylon and Thinopyrum ponticum, at two salt-affected sites, Gumble and Cundumbul in central-western New South Wales, Australia. The physiological parameters of the two grass species, including net photosynthetic rate (Pn), stomatal conductance (gs), and intercellular CO2 concentration (Ci), were investigated using one mature leaf from C. dactylon and T. ponticum populations. Increasing salinity levels in the topsoil had a significant influence on Ci and gs, whereas no significant effect occurred on Pn in C. dactylon and T. ponticum. The Pn values in C. dactylon and T. ponticum were greater at Cundumbul than at Gumble. The greater Mg concentration facilitated greater Pn in C. dactylon and T. ponticum populations at Cundumbul than Gumble. With increasing salinity levels in the soil, Na accumulation increased in C. dactylon and T. ponticum. The ratio between K and Na was > 1 in roots and shoots of both populations irrespective of the sites. Bioaccumulation factor (BF) and translocation factor (TF) results revealed that K and Na translocations were significantly higher in T. ponticum than in C. dactylon, whereas Ca and Mg translocations were significantly higher in C. dactylon than in T. ponticum. Accumulation of Na, K, Mg, and Ca ions was higher in T. ponticum than in C. dactylon; therefore, we suggest that T. ponticum as a greater salt accumulator than C. dactylon could be used for revegetation and phytoremediation of the salt-affected soils.     

大气CO2浓度升高对黄花蒿生长和氮磷钾养分吸收的影响

Annual wormwood (Artemisia annua L.) is the only viable source of artemisinin, an antimalarial drug. There is a pressing need to optimize production per cultivated area of this important medicinal plant; however, the effect of increasing atmospheric carbon dioxide (CO₂) concentration on its growth is still unclear. Therefore, a pot experiment was conducted in a free-air CO₂ enrichment (FACE) facility in Yangzhou City, China. Two A. annua varieties, one wild and one cultivated, were grown under ambient (374 μmol mol^-1) and elevated (577 μmol mol^-1) CO₂ levels to determine the dry matter accumulation and macronutrient uptake of aerial parts. The results showed that stem and leaf yields of both A. annua varieties increased significantly under elevated CO₂ due to the enhanced photosynthesis rate. Although nitrogen (N), phosphorus (P), and potassium (K) concentrations in leaves and stems of both varieties decreased under elevated CO₂, total shoot N, P, and K uptake of the two varieties were enhanced and the ratios among the concentrations of these nutrients (N:P, N:K, and P:K) were not affected by elevated CO₂. Overall, our results provided the evidence that elevated CO₂ increased biomass and shoot macronutrient uptake of two A. annua varieties.     

Two-dimensional simulation of airflow and carbon dioxide transport over a forested mountain: Part II. Carbon dioxide budget analysis and advection effects

We apply a high-resolution atmospheric model to assess the influence of mesoscale advection of CO₂ on the estimation of net ecosystem exchange (NEE) using eddy-covariance CO₂ flux measurements at a Fluxnet-Canada forest site located on sloping terrain on Vancouver Island, Canada. The numerical simulation is performed for fair-weather conditions over an idealized two-dimensional mountain bounded by water. The model is enhanced to include a CO₂ budget with a treatment of canopy photosynthesis and soil respiration.The simulation captures the transport of CO₂ by nocturnal drainage flows and weak land breezes. The resulting vertical profiles and time evolution of CO₂ concentration show a significant variation near the ground, associated with stability changes in the atmospheric boundary layer. The simulated vertical CO₂ gradients are found to be large around sunset and sunrise. The decrease of CO₂ concentration over land after midnight and the CO₂ accumulation over the neighboring water surface indicate CO₂ advection.A CO₂ budget analysis of the numerical-model output shows that the mean horizontal and vertical advection have significant fluctuations and opposite signs during daytime, with the net result that they largely counteract each other. At night, mean advection results in the underestimation by 20% of the nocturnal respiration. The estimated NEE at night is dominated by sub-grid-scale vertical flux in this simulation. Further evaluation using 3D simulations with higher resolution is needed to see if our results hold where vertical fluxes are much better resolved.     

Influence of Selenium on Cadmium Toxicity in Cucumber (Cucumis sativus cv. 4200) at an Early Growth Stage in a Hydroponic System

An experiment was established to assess the ability of selenium (Se) to reduce cadmium (Cd) toxicity when tomato was grown hydroponically. A factorial experiment was arranged in a completely randomized design with six replicates in cucumber (Cucumis sativus cv. 4200). The Se was applied at four levels [0 mg L^–1 (Se0), 2 mg L^–1 Se (Se1), 4 mg L^–1 Se (Se2), and 6 mg L^–1 Se (Se3)], whereas Cd was applied at three levels [0 µM Cd (Cd0), 5 µM Cd (Cd1), and 7 µM Cd (Cd2)]. The Se improved the dry weight of roots even when plants were exposed to Cd. Treatment Se1 improved the dry weight of shoots in Cd1 and Cd2. Treatments Se1 and Se2 improved photosynthesis in Cd1. Treatment Se1 significantly improved stomatal conductance in Cd2 at all levels of Se relative to Cd2. The greatest Cd concentration in leaves was observed in Cd2 × Se0 and while Se concentration in solution increased in response to Se1, Se2, and Se3. The greatest Se level reduced Cd uptake the most. Growth and photosynthetic attributes can be negatively affected by Cd, but Se has the ability to buffer, or improve, several attributes.     

水分和CO2对玉米光合性能及水分利用率的影响

利用环境生长室探讨不同CO₂浓度和土壤水分亏缺处理下玉米植株生物量、气孔形态与分布特征、叶片气体交换参数、叶绿素荧光参数等生长及生理指标的变化规律。以‘郑单958’ 玉米品种为试材,利用环境生长室设置2个CO₂浓度和4个土壤水分梯度对玉米进行CO₂浓度和水分处理。结果表明：1）不同程度土壤水分亏缺均显著降低玉米地上生物量（P<0.05）,但CO₂浓度升高增加了轻度水分亏缺条件下玉米地上生物量（P<0.01）和总生物量（P<0.01）。2）大气CO₂浓度升高导致轻度和中度水分亏缺条件下玉米的净光合速率（P_n）分别提高15.8%（P<0.05）和25.7%（P=0.001）,而CO₂浓度升高却降低了玉米叶片蒸腾速率（P<0.001）和气孔导度（P<0.001）,最终导致玉米瞬时水分利用效率均显著提高（P<0.001）。3）不同水分处理对玉米叶片气孔密度和单个气孔形态特征均造成显著影响（P<0.01）。因此,大气CO₂浓度升高可以增加轻度水分亏缺条件下玉米叶片氮含量、叶片非结构性碳水化合物含量和光合电子传递速率,从而提高玉米植株的生物量累积以及叶片碳同化能力和水分利用效率。研究结果将为深入理解气候变化背景下玉米对大气CO₂浓度升高和土壤水分亏缺的生理生态响应机制提供科学依据。     

Effects of Folsomia candida (Collembola) on the microbial biomass in a grassland soil

Summary The effects of the presence of Folsomia candida on substrate-induced respiration, CO₂-C evolution, bacterial count and NH ₄ ⁺ -N were investigated in a grassland soil. Differences in these parameters, with the exception of NH ₄ ⁺ , were correlated with the age of the collembolan Folsomia candida. In the presence of juvenile animals total CO₂-C evolution was enhanced, but substrate-induced respiration and the bacterial count were unchanged. In fumigated soil with imagos, substrate-induced respiration and the number of bacteria were increased, but total CO₂-C evolution was unaltered. Different food selection strategies between adults and juvenile animals may explain the results.