Aggregation and C and N contents of soil organic matter fractions in a permanent raised-bed planting system in the Highlands of Central Mexico

Permanent raised bed planting with crop residue retention is a form of conservation agriculture that has been proposed as an alternative to conventional tillage for wheat production systems in the Central Highlands of Mexico. A field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batán (State of Mexico, Mexico) in 1999. The percentage of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full crop residue retention (average for maize and wheat), while the percentages free microaggregates was lower. The percentages of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds with residue retention compared to permanent raised beds with removal of the residue (average for maize and wheat), while the percentages free microaggregates and silt and clay fraction was lower. Cultivation of maize significantly reduced the large macroaggregates, while wheat reduced the silt and clay fraction (average over all systems). Cultivation of maize reduced the C and N content of the free microaggregates compared to soil cultivated with wheat, while removal of plant residue reduced the C and N content of the silt and clay fraction compared to soil where residue was retained. The C and N content of the coarse particulate organic matter (cPOM) and microaggregates within the macroaggregates was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full residue retention, while C and N content of the cPOM was significantly lower when residue was removed or partially removed compared to the soil where the residue was retained. The δ ¹³C ‰ signatures of the macroaggregates, microaggregates, the silt and clay fraction, cPOM and microaggregates within the macroaggregates were not affected by tillage or residue management when wheat was the last crop, but removal of residue reduced the δ ¹³C ‰ signatures of the macro-, microaggregates and microaggregates within the macroaggregates significantly compared to soil where the residue was retained. Retaining only 30–50% of the organic residue still improved the soil structure considerably compared to plots where it was removed completely. Permanent raised beds without residue retention, however, is a practice leading to soil degradation. Kelly Lichter and Bram Govaerts contributed equally to this publication.     

The effects of long-term fertilization on the accumulation of organic carbon in the deep soil profile of an oasis farmland

Background and aims

Deeper soils represent a poorly understood, but potentially important, sink for carbon sequestration. The objective of this study was to determine the effects of long-term fertilization on soil organic carbon (SOC), its labile fractions and aggregate-associated carbon throughout a 0–3 m soil profile.

Methods

The investigation was conducted in a field experiment started in 1990 in an oasis farmland cropped with winter wheat. The following treatments were compared with the desert from which the oasis was created: CK (no fertilizer), NPK, N2P2K, NPKR, and N2P2R2 (“2” for double fertilizer and “R” for straw residue)

Results

SOC contents increased by 14–56 % in the topsoil (0–0.2 m), but decreased by 15–22 % in the subsoil (0.2–0.6 m) under all fertilizer treatments. In the deep layer (0.6–3 m) there were significant differences between the treatments: SOC decreased by 5–9 % in treatments without straw, but increased by 4–9 % in treatments with straw. Labile fractions (particulate organic carbon and light fraction organic carbon) also showed similar trends. Both the fertilizer and CK treatments led to an increase in the amount of macro-aggregates (>0.25 mm), especially small macro-aggregates (0.25–2 mm), throughout the soil profile. SOC content was highest in the macro-aggregates, intermediate in the silt + clay fraction (<0.053 mm), and lowest in the micro-aggregates (0.25–0.053 mm). However, 44–87 % of total SOC was stored in the silt + clay fraction, especially in the deep layer (at least 80 %).

Conclusions

After 20 years of fertilizer applications, difference in SOC mainly occurred in the deep layer, and preservation of SOC in the silt + clay fraction appeared to be a prerequisite for soil-carbon sequestration. Applying inorganic fertilizer alone decreased SOC content in the silt + clay fraction in the deep layer, while the combined applications with straw resulted in higher SOC content in the silt + clay fraction in that layer, which turned out to be the main mechanism for increasing SOC content. Our study indicated that applying straw with inorganic fertilizer is the best practice for carbon sequestration, which occurred mainly in the deep soil layer.     

名山河流域水稻土组分对微团聚体吸附-解吸锌的影响

通过模拟-培养试验和选择溶解法,研究了四川省名山河流域水稻土各土壤组分对原土和不同粒径微团聚体吸附解吸Zn2+能力的影响.结果表明:原土和不同粒径微团聚体对Zn2+的吸附与有机质、游离氧化铁、无定形氧化铁、阳离子交换量(CEC)含量极显著相关,原土去除各土壤组分前后对Zn2+的吸附均用Freundlich方程拟合最佳,对Zn2+的吸附能力大小顺序为:小于0.002mm0.25～2mm原土0.002～0.053mm0.053～0.25mm.与未去除土壤组分相比,去除土壤组分的原土和微团聚体对Zn2+的吸附量均有一定程度的减小,吸附量的减小以0.002mm粒径最高,0.25～0.053mm粒径最低,原土介于各微团聚体中间.去除有机质后,0.002mm、0.053～0.25mm和原土吸附Zn2+的减少量依次为39.56%±1.97%、26.68%±4.21%和36.39%±2.31%;去除游离氧化铁后,吸附Zn2+的减少量依次为30.41%±1.91%、20.14%±3.33%和28.73%±1.22%;去除无定形氧化铁后,吸附Zn2+的减少量依次为22.12%±1.27%、12.43%±2.11%和20.15%±2.62%.吸附减少量大小顺序为:去除有机质去除游离氧化铁去除无定形氧化铁,不同处理间差异极显著.去除各土壤组分后,原土和各粒径微团聚体对Zn2+的非专性吸附量显著上升,增加了Zn2+在土壤中的流动性;而专性吸附量降低,降低了原土和各粒径微团聚体对Zn2+的缓冲能力和固持能力.     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

Does soil organic carbon quality or quantity govern relative temperature sensitivity in soil aggregates?

Soil aggregates govern soil organic carbon (SOC) sequestration. But, sparse understanding about the process leads to inaccuracy in predicting potential of soil to stabilize C in warming world. We appraised effects of 43 years of fertilization on relative temperature sensitivity of SOC decomposition (Q₁₀) in soil aggregates to know whether SOC quality or quantity governs Q₁₀. Treatments were: fallow, control, 100% recommended dose of nitrogen (N), N and phosphorus (NP), N, P and potassium (NPK), and NPK + farmyard manure (FYM) (NPK + FYM). Macroaggregates, microaggregates and silt + clay (s + c) fractions were incubated for 16 weeks at 25, 35 and 45 °C, SOC quality (R₀) and Q₁₀ were computed. SOC mineralization from macro- and micro- aggregates were 34 and 28% higher than s + c across the treatments. The s + c fraction of NPK + FYM had ~ 41, 40 and 24% higher C decay rate than NPK plots at 25, 35 and 45 °C, respectively. For s + c fraction Q₁₀ increased over other aggregates. Mean Q₁₀ of s + c fraction was ~ 18.3 and 17.5% higher than macro and micro-aggregate-C, respectively. R₀ was the lowest for NPK + FYM, suggesting long-term manuring with balanced NPK significantly enhance recalcitrance of C. We observed Q₁₀ of macroaggregates and s + c fraction is controlled by C quality but C quantity governs Q₁₀ of microaggregates in Vertisol. Specifically, microaggregates of NPK + FYM were more temperature sensitive, and could be vulnerable to C loss. Hence, practices facilitating microaggregate formation should be avoided. Thus, we recommend manure application for facilitating C sequestration.

     

Soil N mineralization, nitrification and dynamic changes in abundance of ammonia-oxidizing bacteria and archaea along a 2000 year chronosequence of rice cultivation

Background and Aims

Soil mineralization, nitrification, and dynamic changes in abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) were studied to validate our hypothesis that soil mineralization and nitrification decreased along the chronosequence of rice cultivation.

Methods

Paddy soils with a 300, 700 and 2000-year cultivation history (P300, P700 and P2000) were selected to study net mineralization and nitrification processes. Dynamic abundance of AOB and AOA was estimated by quantifying their respective amoA gene copies.

Results

The net mineralization rate was higher for P300 than P700 and P2000. Potential nitrification (N _p ) and average nitrification rates (V _a ) were similar for P300 and P700 soils, but the simulated potential nitrification rate (V _p ) and nitrification rate (k₁) was 72 % and 88 % higher for P300 than P700, respectively. V _a was about 70 % lower than for P2000 than P300 and P700. AOB amoA gene copies were higher for P300 than P700 and P2000, whereas AOA abundance did not show significant differences. AOB abundance showed a positive response to NH₄ supply but AOA did not.

Conclusions

Both N mineralization and nitrification were depressed with increased cultivation time. Archaea responded to mineralization positively rather than nitrification, which suggested that readily mineralized organic matter may play an important role in AOA.     

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO₂ and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO₂ on the contribution of fast- ( r -strategists) and slow-growing ( K -strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO₂ treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25–2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C_mic), the maximum specific growth rate (μ), growing microbial biomass (GMB) and lag-period ( t _lag) were estimated by the kinetics of CO₂ emission from bulk soil and aggregates amended with glucose and nutrients. Although C_org and C_mic were unaffected by elevated CO₂, μ values were significantly higher under elevated than ambient CO₂ for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO₂ treatments. Based on changes in μ, GMB and lag period, we conclude that elevated atmospheric CO₂ stimulated the r- selected microorganisms, especially in soil microaggregates. Such an increase in r -selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO₂ environment.     

Responses of nitrous oxide fluxes and soil nitrogen cycling to nutrient additions in montane forests along an elevation gradient in southern Ecuador

Tropical montane forests are commonly limited by N or co-limited by N and P. Projected increases in N deposition in tropical montane regions are thought to be insufficient for vegetation demand and are not therefore expected to affect soil N availability and N₂O emissions. We established a factorial N- and P-addition experiment (i.e., N, P, N + P, and control) across an elevation gradient of montane forests in Ecuador to test these hypotheses: (1) moderate rates of N and P additions are able to stimulate soil-N cycling rates and N₂O fluxes, and (2) the magnitude and timing of soil N₂O-flux responses depend on the initial nutrient status of the forest soils. Moderate rates of nutrients were added: 50 kg N ha^?1 year^?1 (in the form of urea) and 10 kg P ha^?1 year^?1 (in the form of NaH₂PO ₄ ^. 2H₂O) split in two equal applications. We tested the hypotheses by measuring changes in net rates of soil–N cycling and N₂O fluxes during the first 2 years (2008–2009) of nutrient manipulation in an old-growth premontane forest at 1,000 m, growing on a Cambisol soil with no organic layer, in an old-growth lower montane forest at 2,000 m, growing on a Cambisol soil with an organic layer, and an old-growth upper montane rainforest at 3,000 m, growing on a Histosol soil with a thick organic layer. Among the control plots, net nitrification rates were largest at the 1,000-m site whereas net nitrification was not detectable at the 2,000- and 3,000-m sites. The already large net nitrification at the 1,000-m site was not affected by nutrient additions, but net nitrification became detectable at the 2,000- and 3000-m sites after the second year of N and N + P additions. N₂O emissions increased rapidly following N and N + P additions at the 1,000-m site whereas only smaller increases occurred at the 2,000- and 3,000-m sites during the second year of N and N + P additions. Addition of P alone had no effect on net rates of soil N cycling and N₂O fluxes at any elevation. Our results showed that the initial soil N status, which may also be influenced by presence or absence of organic layer, soil moisture and temperature as encompassed by the elevation gradient, is a good indicator of how soil N cycling and N₂O fluxes may respond to future increases in nutrient additions.     

Ozone pollution influences soil carbon and nitrogen sequestration and aggregate composition in paddy soils

Background and aims

Much attention has focused on the effects of tropospheric ozone (O₃) on terrestrial ecosystems and plant growth. Since O₃ pollution is currently an issue in China and many parts of the world, understanding the effects of elevated O₃ on soil carbon (C) and nitrogen (N) sequestration is essential for efforts to predict C and N cycles in terrestrial ecosystems under predicted increases in O₃. Thus the main objective of this study was to determine whether an increases in atmospheric O₃ concentration influenced soil organic C (SOC) and N sequestration.

Methods

A free-air O₃ enrichment (O₃-FACE) experiment was started in 2007 and used continuous O₃ exposure from March to November each year during crop growth stage in a rice (Oryza sativa L.)—wheat (Triticum aestivum L.) rotation field in the Jiangsu Province, China. We investigated differences in SOC and N and soil aggregate composition in both elevated and ambient O₃ conditions.

Results

Elevated atmospheric O₃ (18–80 nmol mol^?1 or 50 % above the ambient) decreased the SOC and N concentration in the 0–20 cm soil layer after 5 years. Elevated O₃ significantly decreased the SOC concentration by 17 % and 5.6 % in the 0–3 cm and the 10–20 cm layers, respectively. Elevated O₃ significantly decreased the N concentration by 8.2–27.8 % in three layers at the 20 cm depth. In addition, elevated O₃ influenced the formation and transformation of soil aggregates and the distribution of SOC and N in the aggregates across soil layer classes. Elevated O₃ significantly decreased the macro-sized aggregate fraction (16.8 %) and associated C and N (0.5 g kg^?1 and 0.32 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (61 %) and associated C (1.7 g kg^?1) in the 0–3 cm layer. Elevated O₃ significantly decreased the macro-sized aggregate fraction (9.6 %) and associated C and N (1.4 g kg^?1 and 0.35 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (41.8 %) and decreased the corresponding associated N (0.14 g kg^?1) in the 3–10 cm layer. Elevated O₃ did not significantly effect the formation and transformation of aggregates in the 10–20 cm layer, yet it did significantly increase the C concentration in the macro-sized fraction (1 g kg^?1) and decrease the N concentration in the macro- and micro-sized fractions (0.24 g kg^?1 and 0.16 g kg^?1, respectively).

Conclusion

Long-term exposure to elevated atmospheric O₃ negatively affected the physical structure of the soil and impaired soil C and N sequestration.     

Soil properties associated with net nitrification following watershed conversion from Appalachian hardwoods to Norway spruce

Nitrate (NO₃-N) in soil solution and streamwater can be an important vector of nitrogen (N) loss from forested watersheds, and nitrification is associated with negative consequences of soil acidification and eutrophication of aquatic ecosystems. The purpose of this study was to identify vegetation-mediated soil properties that may control potential net nitrification dynamics and to determine if net nitrification is a function of abiotic retention or biotic inhibition. We performed a soil inoculation and incubation study and analyzed a suite of soil chemical and biological properties in soils from a 40-year-old Appalachian hardwood forest and an adjacent 37-year-old Norway spruce forest converted from Appalachian hardwoods. Our results indicate that net NO₃-N production was nine times higher in hardwood soil (mean = 183.51 mg N/kg/28 days) than in the spruce soil (mean = 18.97 mg N/kg/28 days) and differences in net NO₃-N production were attributed to differences in soil substrate quality. Soil properties that were most strongly correlated with NO₃-N production across vegetation types included total soil N, soil C:N ratio, oxalate concentration, and sulfate concentration. Establishment of a spruce monoculture in the central Appalachian hardwood ecoregion significantly altered N cycling, likely depleted soil N stores, increased soil acidity, and altered soil organic matter dynamics, thus leading to low net nitrification rates.     

Effect of tillage system on the root growth of spring wheat

Little research has examined the influence of tillage system on root growth in wheat grown on rainfed Vertisols. A 3-year field study (2003, 2004 and 2005) was carried out on a typical Vertisol (southern Spain), to determine the effects of tillage system on root growth in spring wheat (Triticum aestivum L) grown in continuous rotation with faba bean (Vicia faba L), within the framework of the long-term “Malagón” experiment started in 1986. Tillage treatments were no-tillage (NT) and conventional tillage (CT), and the experiment was designed as a randomized complete block with three replications. The following parameters were measured: above-ground biomass, grain yield, root length density (RLD), root biomass (RB) and root N content. In the topmost 10 cm of soil, higher values were found under CT than under NT for RLD in the rainiest year (20.2 km m^?3 vs. 9.6 km m^?3 respectively) and for RB (512 kg ha^?1 vs. 261 kg ha^?1 respectively) in all study years. In deeper layers, no difference was recorded between the two tillage systems. Greater wheat root development in the upper soil layer under CT may reflect the greater soil penetration resistance found in the topmost 10 cm under NT. Root separation using a sieve with a 0.5 mm mesh screen led to a marked underestimation of RLD and RB, with values up to three times higher when using a 0.2 mm mesh screen. Mean wheat root N content in the topmost 30 cm of soil accounted for over 80% of total root N content. The highest grain yield was observed under NT, since this system provided greater water storage in the soil profile in the mostly dry study years.     

Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types

Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4–5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30–50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil^?1 NH₄ ⁺or NO₃ ^?; <25 μg N g soil^?1 year^?1 net N mineralization; <8 μg N g soil^?1 year^?1 net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N–foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host–beetle systems.     

天津盐碱化沼泽湿地开垦对土壤团聚体有机与无机碳含量的影响

目前,开垦对沼泽湿地土壤有机碳的影响已有较多研究,但针对滨海盐碱化沼泽的研究较为薄弱,特别是对无机碳的影响尚不清晰,从而导致无法全面评估开垦对总碳的影响。本研究选取天津七里海盐碱化沼泽湿地和对应长期开垦(约60年)后的农田作为研究对象,采集0~15和15~30 cm两层土样,采用湿筛法得到>2、0.25~2、0.053~0.25和<0.053 mm 4个粒级水稳性团聚体。结果表明:湿地长期开垦后,表层(0~15 cm)和下层(15~30 cm)土壤大团聚体(>2 mm)比例均显著降低(-48.1%、-58.1%),微团聚体(0.053~0.25 mm)比例均显著增加(+166.1%、+70.0%);各粒级团聚体有机碳含量均显著降低(31.2%~56.8%);表层土壤(0~15 cm)中等团聚体(0.25~2 mm)和矿质颗粒组分(<0.053 mm)无机碳含量显著增加(+85.4%、+75.4%);而下层土壤(15~30 cm)各级团聚体无机碳含量均显著增加(182.3%~448.2%);表层土壤大团聚体(>2 mm)、中等团聚体(0.25~2 mm)总碳含量显著降低(-12.9%、-21.9%),而总碳含量在表层土壤微团聚体(0.053~0.25 mm)、矿质颗粒组分、下层土壤各级团聚体均无显著变化。可见,滨海盐碱化沼泽湿地开垦虽导致有机碳含量降低,但无机碳含量却具有显著反补作用,从而减缓或抑制了碳库流失。因此,在滨海盐碱化地区,今后应更加重视开垦过程中土壤无机碳动态变化及其对总碳的影响。     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Bumetanide Hyperpolarizes Madin–Darby Canine Kidney Cells and Enhances Cellular Gentamicin Uptake by Elevating Cytosolic Ca2+ Thus Facilitating Intermediate Conductance Ca2+-Activated Potassium Channels

Loop diuretics such as bumetanide and furosemide enhance aminoglycoside ototoxicity when co-administered to patients and animal models. The underlying mechanism(s) is poorly understood. We investigated the effect of these diuretics on cellular uptake of aminoglycosides, using Texas Red-tagged gentamicin (GTTR), and intracellular/whole-cell recordings of Madin–Darby canine kidney (MDCK) cells. We found that bumetanide and furosemide dose-dependently enhanced cytoplasmic GTTR fluorescence by ~60 %. This enhancement was suppressed by La³⁺, a non-selective cation channel (NSCC) blocker, and by K⁺ channel blockers Ba²⁺ and clotrimazole, but not by tetraethylammonium (TEA), 4-aminopyridine (4-AP) or glipizide, nor by Cl^? channel blockers diphenylamine-2-carboxylic acid (DPC), niflumic acid (NFA), and CFTR_inh-172. Bumetanide and furosemide hyperpolarized MDCK cells by ~14 mV, increased whole-cell I/V slope conductance; the bumetanide-induced net current I/V showed a reversal potential (V _r) ~?80 mV. Bumetanide-induced hyperpolarization and I/V change was suppressed by Ba²⁺ or clotrimazole, and absent in elevated [Ca²⁺]_i, but was not affected by apamin, 4-AP, TEA, glipizide, DPC, NFA, or CFTR_inh-172. Bumetanide and furosemide stimulated a surge of Fluo-4-indicated cytosolic Ca²⁺. Ba²⁺ and clotrimazole alone depolarized cells by ~18 mV and reduced I/V slope with a net current V _r near ?85 mV, and reduced GTTR uptake by ~20 %. La³⁺ alone hyperpolarized the cells by ~?14 mV, reduced the I/V slope with a net current V _r near ?10 mV, and inhibited GTTR uptake by ~50 %. In the presence of La³⁺, bumetanide-caused negligible change in potential or I/V. We conclude that NSCCs constitute a major cell entry pathway for cationic aminoglycosides; bumetanide enhances aminoglycoside uptake by hyperpolarizing cells that increases the cation influx driving force; and bumetanide-induced hyperpolarization is caused by elevating intracellular Ca²⁺ and thus facilitating activation of the intermediate conductance Ca²⁺-activated K⁺ channels.     

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

Summary Net mineralization of N and net nitrification in field-moist clay soils (Evesham-Kingston series) from arable and grassland sites were measured in laboratory incubation experiments at 4, 10 and 20°C. Three depth fractions to 30 cm were used. Nitrate accumulated at all temperatures except when the soil was very dry (=0.13 cm³ cm^–3). Exchangeable NH₄-ions declined during the first 24 h and thereafter remained low. Net mineralization and net nitrification approximated to zero-order reactions after 24 h, with Q₁₀ values generally <1.6. The effect of temperature on both processes was linear although some results conformed to an Arrhenius-type relationship. The dependence of net mineralization and net nitrification in the field soil on soil temperature (10 cm depth) and moisture (0–15, 15–25, 25–35 cm depths) was modelled using the laboratory incubation data. An annual net mineralization of 350 kg N ha^–1 and net nitrification of 346 kg N ha^–1 were predicted between September 1980 and August 1981. The model probably overstressed the effect of soil moisture relative to soil temperature.     

Conservation agriculture, increased organic carbon in the top-soil macro-aggregates and reduced soil CO2 emissions

Background and aims

Conservation agriculture, the combination of minimal soil movement (zero or reduced tillage), crop residue retention and crop rotation, might have the potential to increase soil organic C content and reduce emissions of CO₂.

Methods

Three management factors were analyzed: (1) tillage (zero tillage (ZT) or conventional tillage (CT)), (2) crop rotation (wheat monoculture (W), maize monoculture (M) and maize-wheat rotation (R)), and (3) residue management (with (+r), or without (?r) crop residues). Samples were taken from the 0–5 and 5–10?cm soil layers and separated in micro-aggregates (< 0.25?mm), small macro-aggregates (0.25 to 1?mm) and large macro-aggregates (1 to 8?mm). The carbon content of each aggregate fraction was determined.

Results

Zero tillage combined with crop rotation and crop residues retention resulted in a higher proportion of macro-aggregates. In the 0–5?cm layer, plots with a crop rotation and monoculture of maize and wheat in ZT+r had the greatest proportion of large stable macro-aggregates (40%) and highest mean weighted diameter (MWD) (1.7?mm). The plots with CT had the largest proportion of micro-aggregates (27%). In the 5–10?cm layer, plots with residue retention in both CT and ZT (maize 1?mm and wheat 1.5?mm) or with monoculture of wheat in plots under ZT without residues (1.4?mm) had the greatest MWD. The 0–10?cm soil layer had a greater proportion of small macroaggregates compared to large macro-aggregates and micro-aggregates. In the 0–10?cm layer of soil with residues retention and maize or wheat, the greatest C content was found in the small and large macro-aggregates. The small macro-aggregates contributed most C to the organic C of the sample. For soil cultivated with maize, the CT treatments had significantly higher CO₂ emissions than the ZT treatments. For soil cultivated with wheat, CTR-r had significantly higher CO₂ emissions than all other treatments.

Conclusion

Reduction in soil disturbance combined with residue retention increased the C retained in the small and large macro-aggregates of the top soil due to greater aggregate stability and reduced the emissions of CO₂ compared with conventional tillage without residues retention and maize monoculture (a cultivation system normally used in the central highlands of Mexico).     

青藏高原不同草地利用方式对土壤粒径分形特征的影响

研究青藏高原草地土壤粒径结构分形特征,为该地区土壤质量评价和生态恢复提供科学依据。以青藏高原4种高寒草地(放牧、围栏禁牧、围栏禁牧+补植、未干扰)为对象,采用分形理论,研究不同利用方式对高寒草地土壤颗粒组成及分形特征的影响,明确土壤粒径分形特征的影响因素。结果表明：与放牧和围栏禁牧+补植相比,围栏禁牧草地中黏粒和粉粒体积分数分别增加了60%—91.1%、43.5%—80.1%,禁牧能够促进土壤砂粒向黏粒和粉粒转变。不同草地利用方式对分形维数有显著影响,单重分形维数D值依次为放牧草地<围栏禁牧+补植草地<未干扰草地=围栏禁牧草地,多重分形维数,包括信息维数D₁、信息维数/容量维数比值D₁/D₀和关联维数D₂依次为放牧草地<围栏禁牧+补植草地<围栏禁牧草地<未干扰草地。单重分形维数D与土壤黏粒、粉粒呈极显著正相关(P<0.01);砂粒、黏粒、粉粒、有机碳和全氮是多重分形维数的限制因素。信息维数D₁、信息维数/容量维数比值D₁...     

Impacts of urea N addition on soil microbial community in a semi-arid temperate steppe in northern China

Nitrogen (N) addition has been well documented to decrease plant biodiversity across various terrestrial ecosystems. However, such generalizations about the impacts of N addition on soil microbial communities are lacking. This study was conducted to examine the impacts of N addition (urea-N fertilizer) on soil microbial communities in a semi-arid temperate steppe in northern China. Soil microbial biomass carbon (C), biomass N (MBN), net N mineralization and nitrification, and bacterial and fungal community level physiological profiles (CLPP) along an N addition gradient (0–64 g N m^?2 year^?1) were measured. Three years of N addition caused gradual or step increases in soil NH₄-N, NO₃-N, net N mineralization and nitrification in the early growing season. The reductions in microbial biomass under high N addition levels (32 and 64 g N m^?2 year^?1) are partly attributed to the deleterious effects of soil pH. An N optimum between 16 and 32 g N m^?2 year^?1 in microbial biomass and functional diversity exists in the temperate steppe in northern China. Similar N loading thresholds may also occur in other ecosystems, which help to interpret the contrasting observations of microbial responses to N addition.     

Soil organic matter dynamics in density and particle-size fractions following destruction of tropical rainforest and the subsequent establishment of Imperata grassland in Indonesian Borneo using stable carbon isotopes

Background and aims

Large portions of the deforested areas in Southeast Asia have been ultimately replaced by the invasive grass Imperata cylindrica, but the dynamics of soil organic matter (SOM) during such land transitions are poorly understood. This study presents SOM dynamics in density and particle-size fractions following rainforest destruction and the subsequent establishment and persistence of Imperata grassland.

Methods

We examined soil C stock and natural ¹³C abundance in these fractions to depths of 100 cm. We predicted future soil C storage and evaluated C turnover rates in these fractions using a simple exponential model. Because soil texture strongly affects soil C storage, two chronosequences of soils differing in soil texture were compared (n?=?1 in each chronosequence).

Results

The clay-associated SOM increased in all soil layers (0–100 cm) along the forest-to-grassland chronosequence, whereas light-fraction SOM in the surface soil layer (0–5 cm) decreased.

Conclusions

In the surface layer, all SOM fractions exhibited rapid replacement of forest-derived C to grassland-derived C, indicating fast turnover. Meanwhile, δ¹³C values of the light fraction in the surface layer indicated that forest-derived charcoal and/or occluded low-density organic matter constituted unexpectedly large proportions of the light fraction. Mathematical modelling (0–50 cm) showed that grassland-derived C in the clay and silt fractions in all soil layers increased almost linearly for at least 50 years after grassland establishment. In the meantime, the forest-derived C stock in the clay fraction constituted 82 % of the total stable C pool at 0–50-cm depths even under steady-state conditions (t = ∞), indicating that residue of forest-derived SOM associated with clay largely contributed to preserving the soil C pool. Comparing soils with different soil textures, clay and silt particles in coarse-textured soil exhibited a substantially higher degree of organo-mineral interactions per unit volume of clay or silt compared to fine-textured soils.