Small-scale spatial patterns in N2-fixation and nutrient availability in an arctic hummock-hollow ecosystem

Atmospheric nitrogen that is fixed by associative cyanobacteria can be released into the surrounding soil environment providing a key source of N for arctic ecosystems. Yet, little is known about nitrogen fixation by Biological Soil Crusts (BSCs) within hummock-hollow complexes that are typical of many arctic environments. In this study, we examined spatial and temporal patterns in N₂-fixation, dinitrogenase reductase (nifH) gene abundance and release of N in a low arctic hummock-hollow ecosystem. The impacts of cyanobacteria on N status in soil were evaluated by assessing soil nitrogen in relation to the cyanobacterial associations found on Hummock and Hollow BSCs. In addition, potential P limitation of N₂-fixation by cyanobacteria was assessed for Hummock and Hollow BSCs. The tops of hummocks and the bottoms of hollows were areas of high N₂-fixation, whereas minimal N₂-fixation occurred on the sides of hummock-hollow complexes. Compared with Hummock BSCs, Hollow BSCs had a higher mean growing season N₂-fixation rate, a higher mean growing season nifH abundance, a higher mean total %N and δ¹⁵N values closer to that of atmospheric N₂. Soil N status was linked to rates of N₂-fixation by BSCs indicating that these N₂-fixing associations act as important point sources of soil N in this low arctic ecosystem. Over the course of a growing season temporal variation in N₂-fixation and nifH abundance were weakly linked suggesting that N₂-fixation was carried out by complex communities of diazotrophic microorganisms and that factors such as nutrient availability may limit N₂-fixation to a greater extent than nifH abundance.     

Community composition and activities of denitrifying bacteria from adjacent agricultural soil, riparian soil, and creek sediment in Oregon, USA

We examined denitrifying bacteria from wet soils and creek sediment in an agroecosystem in Oregon, USA that received inputs of nitrogen (N) fertilizer. Our objective was to determine the variation in denitrifying community composition and activities across three adjacent habitats: a fertilized agricultural field planted to perennial ryegrass, a naturally vegetated riparian area, and creek sediment. Using C₂H₂ inhibition, denitrifying enzyme and N₂O-reductase activities were determined in short-term incubations of anaerobic slurries. A key gene in the denitrification pathway, N₂O reductase (nosZ), served as a marker for denitrifiers. Mean denitrifying enzyme activity (DEA) was similar among habitats, ranging from 0.5 to 1.8 μg N g⁻¹ dry soil h⁻¹. However, the ratio of N₂O production, without C₂H₂, to DEA was substantially higher in riparian soil (0.64±0.02; mean±standard error, n=12) than in agricultural soil (0.19±0.02) or creek sediment (0.32±0.03). Mean N₂O-reductase activity ranged from 0.5 to 3.2 μg N g⁻¹ dry soil h⁻¹, with greater activity in agricultural soil than in riparian soil. Denitrifying community composition differed significantly among habitats based on nosZ terminal-restriction fragment length polymorphisms. The creek sediment community was unique. Communities in the agricultural and riparian soil were more closely related but distinct. A number of unique nosZ genotypes were detected in creek sediment. Sequences of nosZ obtained from riparian soil were closely related to nosZ from Bradyrhizobium japonicum. Although nosZ distribution and N₂O-reductase activity differed among habitats, relationships between activity and community composition appeared uncoupled across the agroecosystem.     

Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO2 and added soil carbon

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO₂ may affect N₂-fixation in aridland ecosystems, we measured C₂H₂ reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO₂. We also measured crust and soil δ¹⁵N and total N to assess changes in N sources, and δ¹³C of crusts to determine a functional shift in crust species, with elevated CO₂. The mean rate of C₂H₂ reduction by biological soil crusts was 76.9±5.6 μmol C₂H₄ m⁻² h⁻¹. There was no significant CO₂ effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO₂. Additions of dextrose-C had a positive effect on rates of C₂H₂ reduction in soil. There was no elevated CO₂ effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C₂H₂ reduction in soils either with or without C additions were 8.5±3.6 μmol C₂H₄ m⁻² h⁻¹ and 4.8±2.1 μmol m⁻² h⁻¹, respectively. Crust and soil δ¹⁵N and δ¹³C values were not affected by CO₂ treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ¹⁵N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO₂ and concomitant changes in litter composition likely will overwhelm any physiological changes in N₂-fixation.     

Microbial communities, biomass, and activities in soils as affected by freeze thaw cycles

Two Finnish agricultural soils (peat soil and loamy sand) were exposed to four freeze-thaw cycles (FTC), with a temperature change from −17.3±0.4 °C to +4.1±0.4 °C. Control cores from both soils were kept at constant temperature (+6.6±2.0 °C) without FTCs. Soil N₂O and CO₂ emissions were monitored during soil thawing, and the effects of FTCs on soil microbes were studied. N₂O emissions were extremely low in peat soil, possibly due to low soil water content. Loamy sand had high N₂O emission, with the highest emission after the second FTC. Soil freeze-thaw increased anaerobic respiration in both soil types during the first 3-4 FTCs, and this increase was higher in the peat soil. The microbial community structure and biomass analysed with lipid biomarkers (phospholipid fatty acids, 3- and 2- hydroxy fatty acids) were not affected by freezing-thawing cycles, nor was soil microbial biomass carbon (MIB-C). Molecular analysis of the microbial community structure with temperature gradient gel electrophoresis (TGGE) also showed no changes due the FTCs. These results show that freezing and thawing of boreal soils does not have a strong effect on microbial biomass or community structure.     

Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting

Soil compaction and soil moisture are important factors influencing denitrification and N₂O emission from fertilized soils. We analyzed the combined effects of these factors on the emission of N₂O, N₂ and CO₂ from undisturbed soil cores fertilized with (150 kg N ha⁻¹) in a laboratory experiment. The soil cores were collected from differently compacted areas in a potato field, i.e. the ridges (ρ_D=1.03 g cm⁻³), the interrow area (ρ_D=1.24 g cm⁻³), and the tractor compacted interrow area (ρ_D=1.64 g cm⁻³), and adjusted to constant soil moisture levels between 40 and 98% water-filled pore space (WFPS).High N₂O emissions were a result of denitrification and occurred at a WFPS≥70% in all compaction treatments. N₂ production occurred only at the highest soil moisture level (≥90% WFPS) but it was considerably smaller than the N₂O-N emission in most cases. There was no soil moisture effect on CO₂ emission from the differently compacted soils with the exception of the highest soil moisture level (98% WFPS) of the tractor-compacted soil in which soil respiration was significantly reduced. The maximum N₂O emission rates from all treatments occurred after rewetting of dry soil. This rewetting effect increased with the amount of water added. The results show the importance of increased carbon availability and associated respiratory O₂ consumption induced by soil drying and rewetting for the emissions of N₂O.     

Role of soil drying in nitrogen mineralization and microbial community function in semi-arid grasslands of north-west Australia

We examined effects of wetting and then progressive drying on nitrogen (N) mineralization rates and microbial community composition, biomass and activity of soils from spinifex (Triodia R. Br.) grasslands of the semi-arid Pilbara region of northern Australia. We compared soils under and between spinifex hummocks and also examined impacts of fire history on soils over a 28 d laboratory incubation. Soil water potentials were initially adjusted to −100 kPa and monitored as soils dried. We estimated N mineralization by measuring changes in amounts of nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) over time and with change in soil water potential. Microbial activity was assessed by amounts of CO₂ respired. Phospholipid fatty acid (PLFA) analyses were used to characterize shifts in microbial community composition during soil drying. Net N mineralized under hummocks was twice that of open spaces between hummocks and mineralization rates followed first-order kinetics. An initial N mineralization flush following re-wetting accounted for more than 90% of the total amount of N mineralized during the incubation. Initial microbial biomass under hummocks was twice that of open areas between hummocks, but after 28 d microbial biomass was<2 μ g⁻¹ ninhydrin N regardless of position. Respiration of CO₂ from soils under hummocks was more than double that of soils from between hummocks. N mineralization, microbial biomass and microbial activity were negligible once soils had dried to −1000 kPa. Microbial community composition was also significantly different between 0 and 28 d of the incubation but was not influenced by burning treatment or position. Regression analysis showed that soil water potential, microbial biomass N, NO₃⁻-N, % C and δ¹⁵N all explained significant proportions of the variance in microbial community composition when modelled individually. However, sequential multiple regression analysis determined only microbial biomass was significant in explaining variance of microbial community compositions. Nitrogen mineralization rates and microbial biomass did not differ between burned and unburned sites suggesting that any effects of fire are mostly short-lived. We conclude that the highly labile nature of much of soil organic N in these semi-arid grasslands provides a ready substrate for N mineralization. However, process rates are likely to be primarily limited by the amount of substrate available as well as water availability and less so by substrate quality or microbial community composition.     

Integrated diurnal soil respiration model during growing season of a typical temperate steppe: Effects of temperature, soil water content and biomass production

Soil respiration was measured with the enclosed chamber method in an ungrazed Leymus chinensis steppe during the growing seasons of 2001 and 2002. Soil respiration rate (R_S) was significantly influenced by air temperature (T) at the diurnal scale, and could be described by Van't Hoff's equation (R_S = R₁₀ exp(β(T − 10))). At the seasonal scale, the normalized soil respiration rate at 10 °C (R₁₀) was mainly controlled by soil water content (R² = 0.717, P < 0.001), while the sensitivity of soil respiration to temperature (Q₁₀) was partially affected by absolute growth rate (R² = 0.482, P = 0.004). Thus, soil respiration could be described as R_S = (20.015W − 84.085) (0.103AGR + 1.786)^(T−10)/10 during the growing seasons, integrating soil water content (W) and absolute growth rate (AGR) into the temperature-dependent soil respiration equation. It was validated by the observed soil respiration rates in this study (R² = 0.890, P < 0.001) and observations from near-field experiment (R² = 0.687, P = 0.011). It implied that accurately evaluating annual soil respiration should include the effects of plant biomass production and other abiotic factors besides air temperature.     

Isolation and characterisation of symbiotically effective Rhizobium resistant to arsenic and heavy metals after the toxic spill at the Aznalcóllar pyrite mine

After the toxic spill occurred at Aznalcóllar pyrite mine (Southern Spain), a wide area of croplands near the Doñana Wild Park was contaminated with 4.5 million m³ of slurries composed of acidic waters loaded with toxic metals and metalloids such as As, Sb, Zn, Pb, Cu, Co, Tl, Bi, Cd, Ag, Hg and Se. Today, 6 years after the spill, the concentration of toxic elements in these soils is still very high, in spite of the efforts to clean the zone. However, some plant species have colonised this contaminated area. Legumes possessing N₂-fixing nodules on their roots represented a significant proportion of these plants. Our objective was to use the Rhizobium-legume symbiosis as a new tool for bioremediate the affected area. We have isolated about 100 Rhizobium strains, 41 of them being resistant to high concentrations of As (300 mg l⁻¹), Cu (100 mg l⁻¹) and Pb (500 mg l⁻¹). Their phenotypes and bioaccumulation potentials have been characterised by their growth rates in media supplemented with As and heavy metals. The presence of the resistance genes in some strains has been confirmed by PCR and Southern blot hybridisation. Several Rhizobium were symbiotically effective in the contaminated soils. On the other hand, the first steps in nodule establishment seemed to be more affected by heavy metals than N₂-fixation.     

Carbon, nitrogen and temperature controls on microbial activity in soils from an Antarctic dry valley

The Antarctic dry valleys are characterized by extremely low temperatures, dry conditions and lack of conspicuous terrestrial autotrophs, but the soils contain organic C, emit CO₂ and support communities of heterotrophic soil organisms. We have examined the role of modern lacustrine detritus as a driver of soil respiration in the Garwood Valley, Antarctica, by characterizing the composition and mineralization of both lacustrine detritus and soil organic matter, and relating these properties to soil respiration and the abiotic controls on soil respiration. Laboratory mineralization of organic C in soils from different, geomorphically defined, landscape elements at 10 °C was comparable with decomposition of lacustrine detritus (mean residence times between 115 and 345 d for the detritus and 410 and 1670 d for soil organic matter). The chemical composition of the detritus (C-to-N ratio=9:1-12:1 and low alkyl-C-to-O-alkyl-C ratio in solid-state ¹³C nuclear magnetic resonance spectroscopy) indicated that it was a labile, high quality resource for micro-organisms. Initial (0-6 d at 10 °C) respiratory responses to glucose, glycine and NH₄Cl addition were positive in all the soils tested, indicating both C and N limitations on soil respiration. However, over the longer term (up to 48 d at 10 °C) differential responses occurred. Glucose addition led to net C mineralization in most of the soils. In the lake shore soils, which contained accumulated lacustrine organic matter, glucose led to substantial priming of the decomposition of the indigenous organic matter, indicating a C or energetic limitation to mineralization in that soil. By contrast, over 48 d, glycine addition led to no net C mineralization in all soils except stream edge and lake shore soils, indicating either substantial assimilation of the added C (and N), or no detectable utilization of the glycine. The Q₁₀ values for basal respiration over the −0.5-20 °C temperature range were between 1.4 and 3.3 for the different soils, increasing to between 3.4 and 6.9 for glucose-induced respiration, and showed a temperature dependence with Q₁₀ increasing with declining temperature. Taken together, our results strongly support contemporaneous lacustrine detritus, blown from the lake shore, as an important driver of soil respiration in the Antarctic dry valley soils.     

Accumulation of nitrous oxide and depletion of oxygen in seasonally frozen soils in northern Japan - Snow cover manipulation experiments

It has been suggested that soil-thawing and snow-melting are critical triggers for vigorous emissions of nitrous oxide (N₂O) from soils in cold regions. However, because soil freezing is affected by air temperature and snow cover, accurate predictions that estimate subsequent emissions of this important greenhouse gas are difficult to make. In this study, we measured in situ soil gas N₂O and oxygen (O₂) concentrations at two experimental sites in northern Japan over the period of a year, from November 2008 to October 2009, to clarify the factors stimulating N₂O production in soil at low temperatures. The sites were N-fertilized bare arable lands with different soil frost depths and snowmelt rates, according to the snow cover management imposed. Winter-to-spring net N₂O fluxes, ranging from −0.10 to 1.95 kg N₂O-N ha⁻¹, were positively correlated with the annual maximum soil frost depth (ranging from 0.03 to 0.41 m; r = 0.951***). In the plots with deeper maximum soil frost, winter-to-spring N₂O fluxes represented 58% to 85% of the annual values. Soil N₂O production was stimulated when the soil frost depth was greater than 0.15 m or the daily mean soil temperature at 0.05-m depth was below −2.0 °C. In the soil with the greatest frost depth, soil gas N₂O concentrations at the depth of 0.10 m peaked at 46 ppm when soil gas O₂ concentrations fell down to 0.12 m³ m⁻³ under soil temperature below 0.0 °C. Snowmelt acceleration had no stimulating effect on N₂O production in the soil during the winter-to-spring period.     

Plant growth promoting potential of psychrotolerant Dyadobacter sp. for pulses and finger millet and impact of inoculation on soil chemical properties and diazotrophic abundance

Western Indian Himalaya is very rich in biodiversity. Being a cold climatic region, it possesses various psychrotolerant and psychrophilic microorganisms. Psychrotolerant bacterium Dyadobacter sp. was isolated from this region and studied for its plant growth promoting potential against four legumes and finger millet. This bacterium was able to grow at nitrogen (N) deficient medium at both 10°C and 28°C and gave positive nifH amplification that confirms the psychrotolerant and diazotrophic nature of this bacterium. Pot trial-based study showed that this bacterium was able to promote plant growth by fixing atmospheric nitrogen (N₂) and making it available to plants. Agronomical parameters, leaf nitrate reductase activity, and total chlorophyll content were recorded at 30, 45, 60, and 90 days after sowing and found to be increased over their respective controls. The 16S rDNA and nifH genes were quantified by q-PCR to study the dynamics of total bacterial and diazotrophic abundance due to inoculation of Dyadobacter sp. in soil. Soil chemical properties related to soil fertility were also studied at different time intervals after sowing. We found positive correlation among soil pH, soil nifH gene abundance, soil nitrate concentration, and plant leaf nitrate reductase activity. PCR-DGGE was performed to study persistence of Dyadobacter sp. in soil after inoculation, which showed good persistence of plant growth promoting rhizobacteria (PGPR). Hence, it is concluded that Dyadobacter sp. has potential to promote plant growth by fixing atmospheric N₂ and making it available to plant. Further, psychrotolerant nature of this bacterium can be exploited to enhance plant growth in cold climate agriculture due to its ability to fix atmospheric N₂ at low temperature.     

Seasonal changes in the spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils in Indonesia

We evaluated the spatial structures of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N₂O and CO₂ fluxes in the wetter season (1.85 mg N m⁻² d⁻¹ and 4.29 g C m⁻² d⁻¹, respectively) were significantly higher than those in the drier season (0.55 mg N m⁻² d⁻¹ and 2.73 g C m⁻² d⁻¹, respectively) and averaged CH₄ uptake rates in the drier season (−0.62 mg C m⁻² d⁻¹) were higher than those in the wetter season (−0.24 mg C m⁻² d⁻¹). These values of N₂O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO₂ and CH₄ fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N₂O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N₂O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N₂O fluxes and they may have depended on litter amount distribution. CO₂ fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO₂ fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation.     

Nitrous oxide flux from soil amino acid mineralization

Nitrous oxide (N₂O) is a greenhouse gas produced during microbial transformation of soil N that has been implicated in global climate warming. Nitrous oxide efflux from N fertilized soils has been modeled using NO₃⁻ content with a limited success, but predicting N₂O production in non-fertilized soils has proven to be much more complex. The present study investigates the contribution of soil amino acid (AA) mineralization to N₂O flux from semi-arid soils. In laboratory incubations (−34 kPa moisture potential), soil mineralization of eleven AAs (100 μg AA-N g⁻¹ soil) promoted a wide range in the production of N₂O (156.0±79.3 ng N₂O-N g⁻¹ soil) during 12 d incubations. Comparison of the δ¹³C content (‰) of the individual AAs and the δ¹³C signature of the respired AA-CO₂-C determined that, with the exception of TYR, all of the AAs were completely mineralized during incubations, allowing for the calculation of a N₂O-N conversion rate from each AA. Next, soils from three different semi-arid vegetation ecosystems with a wide range in total N content were incubated and monitored for CO₂ and N₂O efflux. A model utilizing CO₂ respired from the three soils as a measure of organic matter C mineralization, a preincubation soil AA composition of each soil, and the N₂O-N conversion rate from the AA incubations effectively predicted the range of N₂O production by all three soils. Nitrous oxide flux did not correspond to factors shown to influence anaerobic denitrification, including soil NO₃⁻ contents, soil moisture, oxygen consumption, and CO₂ respiration, suggesting that nitrification and aerobic nitrifier denitrification could be contributing to N₂O production in these soils. Results indicate that quantification of AA mineralization may be useful for predicting N₂O production in soils.     

Fertilization practices affect biological nitrogen fixation by modulating diazotrophic communities in an acidic soil in southern China

Biological nitrogen (N) fixation (BNF) driven by diazotrophs is an important pathway for N input in agricultural ecosystems.However,free-living BNF and its associated diazotrophic communities under different fertilization practices in acidic soils are poorly studied.Here,we conducted a long-term(29 years) fertilization experiment to explore how fertilization affected free-living BNF via changing biotic and abiotic variables.The fertilization treatments included an unfertilized control (CK),chemi...     

Dinitrogen and N2O emissions in arable soils: Effect of tillage, N source and soil moisture

A laboratory investigation was performed to compare the fluxes of dinitrogen (N₂), N₂O and carbon dioxide (CO₂) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with ¹⁵N-labeled fertilizers (100 mg N kg⁻¹ soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N₂O and CO₂ fluxes were measured by a gas chromatography (GC) system while total N₂ and N₂O losses and their ¹⁵N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N₂ and N₂O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N₂+N₂O, N₂O and CO₂ from the NT soils than from the CT soils. Compared with the addition of nitrate (NO₃⁻), the addition of ammonium (NH₄⁺) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH₄⁺ on the N₂ and/or N₂O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N₂O emission under the lower moisture condition. Total and fertilizer-induced emissions of N₂ and/or N₂O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO₂ fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N₂O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO₃⁻ form of N fertilizer would reduce potential N₂O emissions from arable soils.     

Performance of para-nitrophenyl phosphate and 4-methylumbelliferyl phosphate as substrate analogues for phosphomonoesterase in soils with different organic matter content

Phosphomonoesterase (PMEase) activity plays a key role in nutrient cycling and is a potential indicator of soil condition and ecosystem stress. We compared para-nitrophenyl phosphate (pNPP) and 4-methylumbelliferyl phosphate (MUP) as substrate analogues for PMEase in 7 natural ecosystem soils and 8 agricultural top soils with contrasting C contents (8.0-414 g kg⁻¹ C) and pH (3.0-7.5). PMEase activities obtained with pNPP (0.05-5 μmol g⁻¹ h⁻¹) were significantly less than activities obtained with MUP (0.9-13 μmol g⁻¹ h⁻¹), especially in soils with a high organic matter content (>130 g kg⁻¹). Only PMEase activities assayed with MUP correlated significantly with total C and total N (r=0.7, P<0.01 all), and pH (r=−0.71, P<0.01). PMEase activities obtained with the two substrate analogues were correlated when expressed on a C-content basis (r=0.8, P<0.001), but not when expressed on an oven-dry soil weight basis. This indicated that interference by organic matter is related to the quantity rather than to the quality of organic matter. Overall, assaying with MUP was more sensitive compared to assaying with pNPP, particularly in the case of high organic and acid soils.     

Changes in soil free-living diazotrophic community and co-occurrence patterns along desert wetland degradation gradient in the Mu Us Desert, northern China

Climate change and human activity have led to the degradation of desert wetlands. Free-living diazotrophs are vital for soil nitrogen input. However, a comprehensive understanding of how soil free-living diazotrophic communities and their co-occurrence patterns respond to desert wetland degradation is lacking. Here, quantitative polymerase chain reaction (qPCR), amplicon sequencing targeting nitrogenase gene (nifH), and network analysis were used to investigate the abundance, diversity, community composition, and co-occurrence patterns of soil free-living diazotrophs along the wetland degradation gradient, i.e., non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and severely degraded (SD), in the southeastern Mu Us Desert, northern China. The abundance and Shannon, Simpson, Chao 1, and ACE indexes decreased (P < 0.05) by 14.6%, 20.7%, 2.1%, 46.5%, and 45.0%, respectively, in SD wetland, whereas no significant difference (P > 0.05) was observed between ND and LD wetlands. The relative abundance of Proteobacteria generally decreased (by 53.5%–19.7%) across the different degradation levels, while the relative abundance of Cyanobacteria increased (by 6.2%–40.1%) from ND to MD levels. The abundance, diversity, and community composition of diazotrophs were most strongly related to soil organic carbon, followed by total nitrogen, moisture, and pH. The least number of network nodes and edges and the lowest density were observed for MD and SD wetlands, indicating that the complexity of free-living diazotrophic networks was reduced by continued degeneration. Overall, severe desert wetland degradation affected the abundance, diversity, and network complexity of soil free-living diazotrophs more negatively than light degradation. This degradation promoted the growth of autotrophic diazotrophs and inhibited the growth of heterotrophic diazotrophs. These changes were mostly related to the loss of soil organic carbon.     

Controls over pathways of carbon efflux from soils along climate and black spruce productivity gradients in interior Alaska

Small changes in C cycling in boreal forests can change the sign of their C balance, so it is important to gain an understanding of the factors controlling small exports like water-soluble organic carbon (WSOC) fluxes from the soils in these systems. To examine this, we estimated WSOC fluxes based on measured concentrations along four replicate gradients in upland black spruce (Picea mariana [Mill.] BSP) productivity and soil temperature in interior Alaska and compared them to concurrent rates of soil CO₂ efflux. Concentrations of WSOC in organic and mineral horizons ranged from 4.9 to 22.7 g C m⁻² and from 1.4 to 8.4 g C m⁻², respectively. Annual WSOC fluxes (4.5-12.0 g C m⁻² y⁻¹) increased with annual soil CO₂ effluxes (365-739 g C m⁻² y⁻¹) across all sites (R²=0.55, p=0.02), with higher fluxes occurring in warmer, more productive stands. Although annual WSOC flux was relatively small compared to total soil CO₂ efflux across all sites (<3%), its relative contribution was highest in warmer, more productive stands which harbored less soil organic carbon. The proportions of relatively bioavailable organic fractions (hydrophilic organic matter and low molecular weight acids) were highest in WSOC in colder, low-productivity stands whereas the more degraded products of microbial activity (fulvic acids) were highest in warmer, more productive stands. These data suggest that WSOC mineralization may be a mechanism for increased soil C loss if the climate warms and therefore should be accounted for in order to accurately determine the sensitivity of boreal soil organic C balance to climate change.     

Unusually high levels of bio-available phosphate in the soils of Ogasawara Islands, Japan: Putative influence of seabirds

Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P₂O₅ kg⁻¹, average: 0.04 g P₂O₅ kg⁻¹ as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P₂O₅ kg⁻¹, average: 2.93 g P₂O₅ kg⁻¹, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm⁻², n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm⁻², n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils.     

Zinc toxicity on N2O reduction declines with time in laboratory spiked soils and is undetectable in field contaminated soils

Denitrification assays in soils spiked with zinc salt have shown inhibition of the N₂O reduction resulting in increased soil N₂O fluxes with increasing soil Zn concentration. It is unclear if the same is true for environmentally contaminated soils. Net production of N₂O and N₂ was monitored during anaerobic incubations (25 °C, He atmosphere) of soils freshly spiked with ZnCl₂ and of corresponding soils that were gradually enriched with metals (mainly Zn) in the field by previous sludge amendments or by corrosion of galvanized structures. Total denitrification activity (i.e. the sum of N₂O+N₂ production rate) was not inhibited by freshly added Zn salts up to 1600 mg Zn kg⁻¹, whereas N₂O reduction decreased by 50% (EC₅₀) at total Zn concentrations of 231 mg Zn kg⁻¹ (ZEV soil) and 368 mg Zn kg⁻¹ (TM soil). In contrast, N₂O reduction was not reduced by soil Zn in any of the field contaminated soils, even at total soil Zn or soil solution Zn concentrations exceeding more than 5 times corresponding EC₅₀'s of the freshly spiked soil. The absence of adverse effects in the field contaminated soils was unrelated to soil NO₃ or organic matter concentration. Ageing (2-8 weeks) and soil leaching after spiking reduced the toxicity of Zn on N₂O reduction, either expressed as total Zn or soil solution Zn, suggesting adaptation reactions. However, no full recovery after spiking was identified at the largest incubation period in one soil. In addition, the denitrification assay performed with sewage sludge showed elevated N₂O release in Zn contaminated sludges (>6000 mg Zn kg⁻¹ dry matter) whereas this was not observed in low Zn sludge (<1000 mg Zn kg⁻¹ dry matter) suggesting limits to adaptation reactions in the sludge particles. It is concluded that the use of soils spiked with Zn salts overestimates effects on N₂O reduction. Field data on N₂O fluxes in sludge amended soils are required to identify if metals indeed promote N₂O emissions in sludge amended soils.