Emission of carbon dioxide and dynamics of inorganic N in a gradient of alkaline saline soils of the former lake Texcoco

A plan was developed to apply biosolid to soil of the former lake Texcoco to fertilize the pioneer vegetation. Because, no information exists about how differences in electrolytic conductivity (EC) might affect mineralization of biosolid and dynamics of C and N in soil, 20 soil samples forming a gradient in EC ranging from 22 to 150 dS m⁻¹ were characterized, amended with 500 mg biosolid C kg⁻¹ dry soil and incubated aerobically at 22 ± 2 °C while production of CO₂, concentrations of ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻), and NH₃ volatilization were monitored at 22 ± 2 °C for 70 days. Soil characteristics showed large variations with maximum values often >10-times larger than minimum values. The production of CO₂ in the unamended soil ranged from 25 to 159 mg CO₂-C kg⁻¹ day⁻¹ and NH₃ volatilization from 0 to 189 μg NH₃-N kg⁻¹ day⁻¹_. Application of biosolid increased production of CO₂ significantly 1.4-fold and volatilization of NH₃ 11.5-fold. The EC explained most of the variation in production of CO₂, while particle size distribution explained most of the variation in volatilization of NH₃. The concentration of NH₄⁺ in the biosolid-amended soil decreased sharply in the first 14 days, with the EC explaining most of the variation found, and remained constant thereafter with a small increase at day 70. Significant increases in the concentration of NO₃⁻ were generally found in soil with EC < 64 dS m⁻¹. The EC explained most of the variation in production of CO₂, and dynamics of NH₄⁺ and NO₃⁻ while clay positively and sand content negatively affected NH₃ volatilization. It was found that increases in EC inhibited C and N mineralization in soil of the former lake Texcoco.     

Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO₃⁻, Ca²⁺ and PO₄³⁻ as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil⁻¹ in the mineral topsoil and 3 mg g soil⁻¹ in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO₂-C g soil⁻¹ d⁻¹, respectively. We calculated high mean metabolic quotients (qCO₂) of 200 mg CO₂-C g Cmic⁻¹ d⁻¹ in the litter layer and 166 mg CO₂-C g Cmic⁻¹ d⁻¹ in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO₂-C ha⁻¹ yr⁻¹ in the tall, medium and low forest, respectively. CO₂ production was significantly increased by CaHPO₄ fertilization, but no consistent effects were caused by Ca²⁺ and NO₃⁻, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.     

Experimental nitrogen deposition alters the quantity and quality of soil dissolved organic carbon in an alpine meadow on the Qinghai-Tibetan Plateau

Dissolved organic matter (DOM) plays a central role in driving biogeochemical processes in soils, but little information is available on the relation of soil DOM dynamics to microbial activity. The effects of NO₃⁻ and NH₄⁺ deposition in grasslands on the amount and composition of soil DOM also remain largely unclear. In this study, a multi-form, low-dose N addition experiment was conducted in an alpine meadow on the Qinghai–Tibetan Plateau in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄ and KNO₃, were applied at four rates: 0, 10, 20 and 40 kg N ha⁻¹ yr⁻¹. Soil samples from surface (0–10 cm) and subsurface layers (10–20 cm) were collected in 2011. Excitation/emission matrix fluorescence spectroscopy (EEM) was used to assess the composition and stability of soil DOM. Community-level physiological profile (CLPP, basing on the BIOLOG Ecoplate technique) was measured to evaluate the relationship between soil DOC dynamics and microbial utilization of C resources. Nitrogen (N) dose rather than N form significantly increased soil DOC contents in surface layer by 23.5%–35.1%, whereas it significantly decreased soil DOC contents in subsurface layer by 10.4%–23.8%. Continuous five-year N addition significantly increased the labile components and decreased recalcitrant components of soil DOM in surface layer, while an opposite pattern was observed in subsurface layer; however, the humification indices (HIX) of soil DOM was unaltered by various N treatments. Furthermore, N addition changed the amount and biodegradability of soil DOM through stimulating microbial metabolic activity and preferentially utilizing organic acids. These results suggest that microbial metabolic processes dominate the dynamics of soil DOC, and increasing atmospheric N deposition could be adverse to the accumulation of soil organic carbon pool in the alpine meadow on the Qinghai-Tibetan Plateau.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.     

Simulating soil freeze/thaw cycles typical of winter alpine conditions: Implications for N and P availability

Seasonally snow-covered alpine soils may be subjected to freeze/thaw cycles, particularly during years having little snow and during the late winter and early spring periods. Freeze/thaw cycles can stimulate soil mineralization and could therefore be one factor regulating nitrogen (N) and phosphorus (P) availability and cycling. In this study laboratory incubation experiments using four soils having contrasting properties have been used to characterize the change in N and P forms (microbial and soluble inorganic/organic) that occur after simulated freeze/thaw cycles.Soil samples were collected from locations representing extreme examples of either direct human management (grazed meadow (site M) and extensive grazing beneath larch (site L)) or those disturbed by more natural events (recent avalanche and colonisation by alder (site A)) and from beneath the expected forest climax vegetation beneath fir (site F). Topsoil from these sites, maintained at two different water contents (20 and 30%, w/w), were exposed to either a single (SF) or four sequential (4SF) freeze/thaw cycles. Each cycle consisted of 12 h at −9 °C and 12 h at +4 °C mimicking a diurnal pattern.A SF cycle reduced microbial N for soils from sites F and A and was accompanied by a significant increase in dissolved organic nitrogen (DON) at both moisture contents. In contrast, the microbial N of soils from M and L was not affected by the freeze/thaw cycles, suggesting a particular adaptation of soil microbes to these extremes in temperature. Freeze/thaw cycles resulted in a significant increase in the net ammonification in all soils.Extractable total dissolved N (TDN) and total dissolved P (TDP) increased in all soils after a SF cycle, however, the relative importance of the different N and P forms differed. At the lower soil moisture content, NO₃⁻ concentrations remained constant or slightly decreased in all soils, except that from site M. In all other soils DON appeared to replace NO₃⁻ as the potentially mobile N source after the freeze/thaw cycles. The relative contribution of dissolved organic P to TDP after freeze/thaw remained significant, and greater than 50% in all soils.Freeze/thaw cycles, in seasonally snow covered soils, are likely to have a selective effect on the microbial biomass. Freezing and thawing resulted in a pulse of net ammonification and DON release, which represent an important influence upon N cycling in these alpine systems.     

Microbial decomposition of leached or extracted dissolved organic carbon and nitrogen from pasture soils

Microbial decomposition of extracted and leached dissolved organic carbon (DOC) and nitrogen (DON) was demonstrated from three pasture soils in laboratory incubation studies. DOC concentration in water extracts ranged between 29 and 148 mg C L^?1 and DON concentration ranged between 2 and 63 mg N L^?1. Between 17 and 61 % of the DOC in the water extracts were respired as CO₂ by microbes by day 36. DON concentrations in the extracts declined more rapidly than DOC. Within the first 21 days of incubation, the concentration of DON was near zero without any significant change in the concentration of NO₃ ^? or NH₄ ⁺, indicating that microbes had utilized the organic pool of N preferentially. Decomposition of leached DOC (ranged between 7 and 66 mg C L^?1) and DON (ranged between 6 and 11 mg N L^?1) collected from large lysimeters (with perennial pasture; 50 cm diameter?×?80 cm deep) followed a similar pattern to that observed with soil extracts. Approximately 28 to 61 % of the DOC in leachates were respired as CO₂ by day 49. The concentration of DON in the leachates declined to below 1 mg N L^?1 within 7–14 days of the incubation, consistent with the observations made with extractable DON. Our results clearly show that DOC and DON components of the dissolved organic matter in pasture soils, whether extracted or leached, are highly decomposable and bioavailable and will influence local ecosystem functions and nutrient balances in grazed pasture systems and receiving water bodies.     

Soluble organic matter and microbial biomass C and N in soils under pasture and arable management and the leaching of organic C,N and nitrate in a lysimeter study

Soluble organic N and C were extracted from soils under long-term kikuyu grass pasture, annual ryegrass pasture and annual maize production using water, 0.5 M K₂SO₄ and 2 M KCl. Quantities extracted with K₂SO₄ were more than double those extracted with water while those extracted with KCl exceeded those using K₂SO₄. Differences in soluble organic C and N between land uses were much more obvious when water rather than salt solutions were used. It was suggested that water extracts give more realistic values than salt solutions. Regardless of the extractant used, the proportion of total N present as soluble N was considerably greater than the equivalent proportion of organic C present as soluble C. While the percentage of soil organic C and total N present in the light fraction and microbial biomass was lower in the kikuyu than ryegrass and maize soils, the equivalent values for water soluble C and N were, in fact, greatest in the kikuyu soil.The leaching of organic C, N and NO₃⁻ from these soils was also measured over a 6-month period in a greenhouse lysimeter study. The soils were either left undisturbed or were disturbed (broken into clods <50 mm diameter) to simulate tillage and stimulate microbial activity. Quantities of organic C and N leached were greater from the kikuyu than other treatments and tended to be greatest from the disturbed kikuyu soil. The percentage of total soil N leached as organic N was considerably greater than that of total organic C leached as soluble C. Leaching of NO₃⁻ was greatest from the disturbed kikuyu soil and least from the undisturbed kikuyu soil. The mean percentage of total soluble N present in organic form in leachates ranged from 17 to 32% confirming the importance of this form of N to leaching losses of N from agricultural soils.     

Temperature and soil moisture effects on dissolved organic matter release from a moorland Podzol O horizon under field and controlled laboratory conditions

Organic upland soils store large amounts of humified organic matter. The mechanisms controlling the leaching of this C pool are not completely understood. To examine the effects of temperature and microbial cycling on C leaching, we incubated five unvegetated soil cores from a Podzol O horizon (from NE Scotland), over a simulated natural temperature cycle for 1 year, whilst maintaining a constant soil moisture content. Soil cores were leached with artificial rain (177 mm each, monthly) and the leachates analysed for dissolved organic carbon (DOC) and their specific C‐normalized UV absorbance determined (SUVA, 285 nm). Monthly values of respiration of the incubated soils were determined as CO₂ efflux. To examine the effects of vegetation C inputs and soil moisture, in addition to temperature, we sampled O horizon pore waters in situ and collected five additional field soil cores every month. The field cores were leached under controlled laboratory conditions. Hysteresis in the monthly amount of DOC leached from field cores resulted in greater DOC on the rising, than falling temperature phases. This hysteresis suggested that photosynthetic C stimulated greater DOC losses in early summer, whereas limitations in the availability of soil moisture in late summer suppressed microbial decomposition and DOC loss. Greater DOC concentrations of in‐situ pore waters than for any core leachates were attributed to the effects of soil drying and physico‐chemical processes in the field. Variation in the respiration rates for the incubated soils was related to temperature, and respiration provided a greater pathway of C loss (44 g C m⁻² year⁻¹) than DOC (7.2 g C m⁻² year⁻¹). Changes in SUVA over spring and summer observed in all experimental systems were related to the period of increased temperature. During this time, DOC became less aromatic, which suggests that lower molecular weight labile compounds were not completely mineralized. The ultimate DOC source appears to be the incomplete microbial decomposition of recalcitrant humified C. In warmer periods, any labile C that is not respired is leached, but in autumn either labile C production ceases, or it is sequestered in soil biomass.     

Overgrazing and soil carbon dynamics in the western Chaco of Argentina

Very little is known about the effect of overgrazing on carbon loss from soil in semi-arid savannas and woodlands of South America. Soil carbon parameters were measured in a 10,000 ha restoration project in the western Chaco of Argentina (24°43′S and 63°17′W). Three situations were compared: highly restored (HRS), moderately restored (MRS) and highly degraded (HDS). Soil and litter samples were recovered in the dry and wet seasons. SOC and CO₂–C values decreased from the HRS (7.0 kg m⁻² and 130 g m⁻²) to the HDS (1.5 kg m⁻² and 46 g m⁻²) whereas the C mineralization rate increased toward the less restored sites (0.96–2.29). Surface-litter C was similar in both sites under restoration (260 and 229 g m⁻²), being non-existent at the HDS. Leaves from woody species dominated surface-litter in the HRS, whereas grass material was predominant in the MRS. During the wet season, the SOC decreased, whereas both CO₂–C and C mineralization rate increased. The magnitude of the between-season differences was highest at the HDS (62% in SOC, 55% in CO₂, and 80% in C mineralization rate). We estimated that C loss since introduction of cattle into the forest was 58 Mg ha⁻¹, reaching a total of 2×10¹⁵ g at for the entire Chaco. These values are higher than those caused by the conversion of savannas and other ecosystems into agriculture or cultivated pastures. The amount of C fixed in the highly restored site (275 g ha⁻¹ per year) indicates that the Chaco soils have a significant potential as atmospheric carbon sinks.     

Moisture pulses,trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming,USA

Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10 cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216 h following additions of 25.4 mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH₄⁺), extractable nitrate (NO₃^?), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N₂O and CO₂ emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO₂ emissions compared to other soils but B. tectorum soils had the greatest N₂O emissions and the greatest increases in CO₂ emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8 h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.     

Long-term effects of land use and fertiliser treatments on sulphur transformations in soils from the Broadbalk experiment

Sulphur transformations were monitored in a unique set of arable, grassland and woodland soils from the Broadbalk Classical Experiment, which started in 1843. In an open incubation experiment with periodic leaching, 14–35 mg SO₄²⁻-S kg⁻¹ was mineralised in 28 weeks at 25°C, equivalent to 4.4–8.3% soil organic S. Cumulative amounts of S mineralised increased linearly during the 28 weeks, indicating constant rates of mineralisation. The rate of mineralisation was the greatest in the woodland soil (170 μg SO₄-S kg⁻¹ day⁻¹), followed by the grassland (120 μg SO₄-S kg⁻¹ day⁻¹) and the arable soil from the farmyard manure (FYM) plot (110 μg SO₄-S kg⁻¹ day⁻¹). Three soils from arable plots receiving different inorganic fertiliser treatments but no FYM had similar rates of S mineralisation (~70 μg SO₄-S kg⁻¹ day⁻¹). In an incubation experiment with ³⁵SO₄²⁻, addition of glucose greatly enhanced S immobilisation. In 132 days, the woodland and grassland soils immobilised more S than the arable soils, with or without glucose amendment. Immobilisation and mineralisation of S occurred concurrently, and both were stimulated by glucose addition. The results show that S mineralisation and immobilisation were influenced strongly by the type of land-use and long-term organic manuring, whereas annual application of sulphate-containing fertilisers for over 150 years had few effects on short-term S transformations.     

Spatial and seasonal variations of atmospheric N2O and CO2 fluxes from a subtropical mangrove swamp and their relationships with soil characteristics

Marine ecosystems are a known net source of greenhouse gases emissions but the atmospheric gas fluxes, particularly from the mangrove swamps occupying inter-tidal zones, are characterized poorly. Spatial and seasonal fluxes of nitrous oxide (N₂O) and carbon dioxide (CO₂) from soil in Mai Po mangrove swamp in Hong Kong, South China and their relationships with soil characteristics were investigated. The N₂O fluxes averaged from 32.1 to 533.7 μg m⁻² h⁻¹ and the CO₂ fluxes were between 10.6 and 1374.1 mg m⁻² h⁻¹. Both N₂O and CO₂ fluxes in this swamp showed large spatial and seasonal variations. The fluxes were higher at the landward site than the foreshore bare mudflat, and higher fluxes were recorded in warm, rather than cold, seasons. The landward site had the highest content of soil organic carbon (OC), total Kjeldahl nitrogen (TKN), nitrate (NO₃⁻–N) and total phosphorus (TP), while the bare mudflat had the highest ammonium nitrogen (NH₄⁺–N) concentration and soil denitrification potential activity. The N₂O flux was related, positively, to CO₂ flux. Soil NO₃⁻–N and TP increased N₂O flux, while soil OC and TP concentrations contributed to the CO₂ flux. The results indicated that the Mai Po mangrove swamp emitted significant amounts of greenhouse gases, and the N₂O emission was probably due to soil denitrifcation.     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

Is nitrate reduction to nitrite possible in glucose-amended alkaline saline soil under aerobic conditions?

A phylogenetic analysis of the archaeal community in the soil of the former Lake Texcoco showed that some of the clones identified were affiliated to Archeae that reduce nitrate (NO₃^?) to nitrite (NO₂^?) and NO₂^? to unknown products under aerobic conditions. Previous research suggested that this indeed might occur when an easily decomposable C-substrate is available, but little is known about the factors that control the possible processes involved. The sandy clay loam soil with pH 10 and electrolytic conductivity 56 dS m^?1 was spiked with 1000 mg glucose-C kg^?1 soil (GLUCOSE pre-treatment), 200 mg NO₃^?-N kg^?1 soil (NITRATE pre-treatment), or left unamended (CONTROL pre-treatment) and conditioned for eight days. Pre-treated soil was then added with 1000 mg glucose-C kg^?1 soil and 200 mg NO₃^?-N kg^?1 soil and amended with ammonium (NH₄⁺) (AMM treatment) and l-glutamine (GLUT treatment), acetylene (C₂H₂) (ACE treatment), oxygen (O₂) (OXI treatment), left untreated (CON treatment) or sterilized. No abiotic factors affected concentrations of NH₄⁺, NO₂^? or NO₃^?. In the CONTROL pre-treatment, concentration of NO₃^? decreased 170 mg N kg^?1 soil within 72 h, in the GLUCOSE pre-treatment with 182 mg N kg^?1 soil within 2 h and in the NITRATE pre-treatment with 272 mg N kg^?1 soil within 168 h. Mean concentration of NO₂^? was 3.2 mg N kg^?1 soil in unamended soil, 5.7 mg N kg^?1 soil in the CONTROL pre-treatment, but >20 mg kg^?1 soil in the GLUCOSE pre-treatment and ≥40 mg kg^?1 in the NITRATE pre-treatment. The application of NO₃^? and glucose increased the mean concentration of NH₄⁺ compared to the unamended soil independently of pre-treatment. It was found that microorganisms in the alkaline saline soil of the former Lake Texcoco can reduce concentrations of NO₃^? while releasing NO₂^? under aerobic conditions when an easy decomposable substrate is available without it being directly related to microbial activity and this being more outspoken when glucose or nitrate were previously added.     

Biogenic structures of two ant species Formica sanguinea and Lasius flavus altered soil C,N and P distribution in a meadow wetland of the Sanjiang Plain,China

Biogenic structures produced by soil ecosystem engineers influence the soil architecture and mediate soil functions and ecosystem services. Ant mounds in meadow wetlands are important biogenic structures with the potential of altering carbon storage and nutrient cycling in these ecosystems. In this study, we examined the soil nutrient concentrations of ant mounds and their effects on the wetland nutrient storage functions in meadow wetlands of the Sanjiang Plain, in northeastern China. The aims of this study were to investigate C, N and P variation in active ant mounds produced by Formica sanguinea Latreille and Lasius flavus Fabricius to estimate the C, N and P pools of ant mounds in comparison with control soil. The average total N (TN), total P (TP) and available P (AP) concentrations in the ant mounds of both species were higher than in the control soil. Organic carbon (C_org), DOC, NH₄⁺ and NO₃⁻ in F. sanguinea mounds were higher than in the control soil, but not for L. flavus mounds. Average concentrations of all the five types of nutrient were higher in F. sanguinea mounds than in L. flavus mounds. The variations in C_org, DOC, TN and TP concentrations in ant mounds were not significant at depths from 0 to 25 cm. NH₄⁺ and NO₃⁻ concentrations differed by soil layers for F. sanguinea mounds but not for L. flavus mounds. The C/N ratios were generally lower in the mounds than in the control soil (at 5–25 cm), but no significant differences were found for C/P ratios (except at 10–15 cm). Carbon and DOC pools were smaller, TN and AP pools were larger in ant mounds compared with the control soil, but there was no significant difference for TP pools. NH₄⁺and NO₃⁻ pools were substantially larger in F. sanguinea mounds, but smaller in L. flavus mounds, than those in the control soil. All of the five types of nutrient pools were larger in F. sanguinea than in L. flavus mounds. Ant mounds increased the spatial variability of soil nutrient pools in the wetland.     

Comparing net ecosystem exchange of carbon dioxide between an old-growth and mature forest in the upper Midwest,USA

Old-growth forests are often assumed to exhibit no net carbon assimilation over time periods of several years. This generalization has not been typically supported by the few whole-ecosystem, stand-scale eddy-covariance measurements of carbon dioxide exchange in old-growth forests. An eddy-flux tower installed in a >300-year-old hemlock–hardwood forest near the Sylvania Wilderness, Ottawa National Forest, MI, USA, observed a small annual carbon sink of CO₂ of −72 ± 36 g C m⁻² year⁻¹ in 2002 and −147 ± 42 g C m⁻² year⁻¹ in 2003. This carbon sink was much smaller than carbon sinks of −438 ± 49 g C m⁻² year⁻¹ in 2002 and −490 ± 48 g C m⁻² year⁻¹ in 2003 observed by a nearby flux tower in a 70-year-old mature hardwood forest (Willow Creek, WI). The mature forest had vegetation similar to the old-growth site prior to European settlement. Both sites had slightly larger carbon sinks in 2003, which was a drier and cooler year than 2002. However, the difference in sink strength between the two years was smaller than the uncertainty in the results arising from missing and screened data. Both sites also had significant systematic errors due to non-representative fluxes during certain micrometeorological conditions, which required careful screening. The difference in sink strength between the two sites was driven mainly by greater ER at the old-growth site (965 ± 35 g C m⁻² year⁻¹ in 2002 and 883 ± 69 g C m⁻² year⁻¹ in 2003) compared to the mature site (668 ± 21 g C m⁻² year⁻¹ in 2002 and 703 ± 17 g C m⁻² year⁻¹ in 2003). GEP was lower at the old-growth site (1037 ± 47 g C m⁻² year⁻¹ in 2002 and 1030 ± 41 g C m⁻² year⁻¹ in 2003) compared to the mature site (1106 ± 47 g C m⁻² year⁻¹ in 2002 and 1192 ± 51 g C m⁻² year⁻¹ in 2003), especially in 2003. Observations also suggested that growing season ER had greater interannual variability at the old-growth site. These results imply that old-growth forests in the region may be carbon sinks, though these sinks are smaller than mature forests, mostly likely due to greater ER.     

Impact of high cadmium and nickel soil concentration on selected physiological parameters of Arundo donax L.

The purpose of this study was to investigate the effects of high cadmium and nickel soil concentrations on selected physiological parameters of Arundo donax L. A 2-year pot experiment was held in the field and the pots were irrigated with aqueous solutions of Cd and Ni in concentrations of 5, 50 and 100 ppm, against the control (tap water). At the end of the cultivation periods the pots soil was divided into three equal zones and total and NH₄OAc extractable Cd and Ni concentrations were determined. The top zone exhibited the highest metal content. Cadmium and nickel total concentrations at the end of the experiment were up to 973.8 mg kg⁻¹ and 2543.3 mg kg⁻¹ respectively, while NH₄OAc extractable Cd was up to 291.7 mg kg⁻¹ and Ni up to 510.3 mg kg⁻¹. Stomatal conductance ranged between 0.3 and 0.8 mol CO₂ m⁻² s⁻¹, intercellular CO₂ concentration ranged between 212.9 and 243.0 ppm CO₂, stomatal resistance between 0.6 and 1.3 s cm⁻¹, chlorophyll content (SPAD values) between 46.3 and 57.0 and chlorophyll fluorescence (F_v/F_m) ranged between 0.8 and 0.9. All studied physiological parameters did not show statistically significant differences among control and heavy metal treated plants for both years; therefore, high soil cadmium and nickel concentration did not inhibit stomatal opening and did not affect the function of the photosynthetic machine of A. donax plants.     

Effect of freezing on soil nitrogen mineralization under different plant communities in a semi-arid area during a non-growing season

The influences of winter climate on terrestrial ecosystem processes have been the subject of growing attention, which is necessary to make the predictions about ecological responses to global warming in the future. However, little information can be found about the impacts of a large range of soil temperature fluctuation (e.g. −10 to 5 °C) over winter on the soil nitrogen (N) dynamics in the field. In the present study, we employed an intact soil core in situ incubation technique, and measured soil N mineralization and nitrification rates under three plant communities, i.e. a grassland, a shrub and a plantation, during the non-growing season (October 2004–April 2005) in Inner Mongolia, China. Our results demonstrate the significant effects of different plant communities on soil net N mineralization and the great temporal variations of soil N dynamics during the incubation period. The mean soil net N mineralization rates were 0.93, 0.77 and −1.28 mg N m⁻² d⁻¹, respectively, in the grassland, shrub and plantation. The mean soil NH₄⁺-N in the three plant communities declined by 40%, but the mean soil NO₃⁻-N increased by 190% by the end of the incubation compared with their initial concentrations at the beginning of incubation. The differences in plant communities significantly affected their soil N mineralization rates, accumulations and turnover rates, which followed the order: grassland > shrub > plantation. During the winter time, the studied soils experienced the three phases consisting of mild freezing (−7 to −2 °C soil), deep freezing (approximately −10 °C soil) and freeze–thaw (−2 to 5 °C soil). The results suggest that temporal variations of soil N mineralization are positively affected by the soil temperature and the soil nitrification is dominant in the N transformation process during the non-growing season. Our study indicates that the soil N mineralization over winter can make a substantial contribution to the mineral N pool that plants are able to utilize in the upcoming spring, but may also pose a great risk of mineral N leaching loss if great rainfalls occur during spring and early summer.     

Turnover of low molecular weight dissolved organic C (DOC) and microbial C exhibit different temperature sensitivities in Arctic tundra soils

Polar ecosystems are currently experiencing some of the fastest rates of climate warming. An increase in soil temperature in High Arctic regions may stimulate soil permafrost melting and microbial activity, thereby accelerating losses of greenhouse gases. It is therefore important to understand the factors regulating the rates of C turnover in polar soils. Consequently, our aims were to: (1) assess the concentration of low molecular weight (MW) dissolved organic carbon (DOC) in soil, (2) to investigate the temperature-dependent turnover of specific low MW compounds, and (3) to analyse the influence of substrate concentration on C cycling. Microbial mineralisation of labile low MW DOC in two High Arctic tundra soils was investigated using soil solutions spiked with either ¹⁴C-labelled glucose or amino acids. Spiked solutions were added to the top- and sub-soil from two ecosystem types (lichen and Carex dominated tundra), maintained at three temperatures (4–20 °C), and their microbial mineralisation kinetics monitored. ¹⁴CO₂ evolution from the tundra soils in response to ¹⁴C-glucose and -amino acid addition could best be described by a double first order exponential kinetic equation with rate constants k₁ and k₂. Both forms of DOC had a short half-life (t_1/2) in the pool of microbial respiratory substrate (t_1/2 = 1.07 ± 0.10 h for glucose and 1.63 ± 0.14 h for amino acids; exponential coefficient k₁ = 0.93 ± 0.07 and 0.64 ± 0.06 h^?1 respectively) whilst the second phase of mineralisation, assumed to be C that had entered the microbial biomass, was much slower (average k₂ = 1.30 × 10^?3 ± 0.49 × 10^?4 h^?1). Temperature had little effect on the rate of mineralisation of ¹⁴C used directly as respiratory substrate. In contrast, the turnover rate of the ¹⁴C immobilized in the microbial biomass prior to mineralisation was temperature sensitive (k₂ values of 0.99 × 10^?3 h^?1 and 1.66 × 10^?3 h^?1 at 4 and 20 °C respectively). Concentration-dependent glucose and amino acid mineralisation kinetics of glucose and amino acids (0–10 mM) were best described using Michaelis–Menten kinetics; there was a low affinity for both C substrates by the microbial community (K_m = 4.07 ± 0.41 mM, V_max = 0.027 ± 0.005 mmol kg^?1 h^?1). In conclusion, our results suggest that in these C limiting environments the flux of labile, low MW DOC through the soil solution is extremely rapid and relatively insensitive to temperature. In contrast, the turnover of C incorporated into higher molecular weight microbial C pools appears to show greater temperature sensitivity.     

Effects of tanneries wastewater on chemical and biological soil characteristics

Effluents from leather processing, a major industry that produces up to 64320 t wastewater year⁻¹ in the town of León (Guanajuato, Mexico), are normally discharged to the river Turbio without treatment. This water is downstream used to irrigate agricultural land. Tannery wastewater contains valuable nutrients, but also contaminants, such as salts and chromium (Cr), that might affect soil processes and crop production. We investigated how almost 25 years of irrigation of agricultural land with water from the river Turbio affected soil characteristics, dynamics of carbon (C) and nitrogen (N), and microbial biomass C. Soil sampled from three adjacent fields irrigated with tannery effluent (soil A), in the vicinity irrigated with well water (soil B), and at a distance of 10 km from the irrigation canals (soil C), was characterized while dynamics of C and N were measured in an aerobic incubation experiment. Irrigation with water from the river Turbio for over 25 years had significantly increased the electrolytic conductivity from 0.64 to 2.29 dS m⁻¹, organic C and total N content two-times, total concentration of Cr four-times, copper (Cu) two-times and sodium (Na) six-times in the clayey soils (P < 0.05). Microbial biomass was two-times larger in soil A than in soil C, while the activity of proteases and hydrolases releasing ninhydrin positive compounds and organic C appeared not to be affected. The concentrations of ammonium (NH₄⁺) and nitrate (NO₃⁻) were not significantly different between the soils. The concentration of nitrite (NO₂⁻) was approximately twice larger in soil A than in soil C (P < 0.05). Although there appeared to be no adverse impact on soil characteristics and microbial biomass, oxidation of NO₂⁻ was inhibited indicating that the biological functioning of the soil might be affected. The increase in heavy metals in soil was limited, but continued irrigation with water from the river Turbio might increase sodicity and salinity that could deteriorate soil and pose a threat to future crop production.