Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

Anaerobic decomposition in wetland soils is carried out by several interacting microbial processes that influence carbon storage and greenhouse gas emissions. To understand the role of wetlands in the global carbon cycle, it is critical to understand how differences in both electron donor (i.e., organic carbon) and terminal electron acceptor (TEA) availability influence anaerobic mineralization of soil organic matter. In this study we manipulated electron donors and acceptors to examine how these factors influence total rates of carbon mineralization and the pathways of microbial respiration (e.g., sulfate reduction versus methanogenesis). Using a field-based reciprocal transplant of soils from brackish and freshwater tidal marshes, in conjunction with laboratory amendments of TEAs, we examined how rates of organic carbon mineralization changed when soils with different carbon contents were exposed to different TEAs. Total mineralization (the sum of CO₂ + CH₄ produced) on a per gram soil basis was greater in the brackish marsh soils, which had higher soil organic matter content; however, on a per gram carbon basis, mineralization was greater in the freshwater soils, suggesting that the quality of carbon inputs from the freshwater plants was higher. Overall anaerobic metabolism was higher for both soil types incubated at the brackish site where SO₄²⁻ was the dominant TEA. When soils were amended with TEAs in the laboratory, more thermodynamically favorable respiration pathways typically resulted in greater organic matter mineralization (Fe(III) respiration > SO₄²⁻ reduction > methanogenesis). These results suggest that both electron donors and acceptors play important roles in regulating anaerobic microbial mineralization of soil organic matter.     

Methane oxidation in boreal forest soils: kinetics and sensitivity to pH and ammonium

In soil incubation experiments we examined if there are differences in the kinetic parameters of atmospheric methane (CH₄) oxidation in soils of upland forests and forested peatlands. All soils showed net uptake of atmospheric CH₄. One of the upland forests included also managed (clear-cut with or without previous liming or N-fertilization) study plots. The CH₄ oxidation in the forested peat soil had a higher K_m (510 μl l⁻¹) and V_max (6.2 nmol CH₄ cm⁻³ h⁻¹) than the upland forest soils (K_m from 5 to 18 μl l⁻¹ and V_max from 0.15 to 1.7 nmol CH₄ cm⁻³ h⁻¹). The forest managements did not affect the K_m-values. At atmospheric CH₄ concentration, the upland forest soils had a higher CH₄ oxidation activity than the forested peat soil; at high CH₄ concentrations the reverse was true. Most of the soils oxidised CH₄ in the studied pH range from 3 to 7.5. The pH optimum for CH₄ oxidation varied from 4 to 7.5. Some of the soils had a pH optimum for CH₄ oxidation that was above their natural pH. The CH₄ oxidation in the upland forest soils and in the peat soil did not differ in their sensitivities to (NH₄)₂SO₄ or K₂SO₄ (used as a non-ammonium salt control). Inhibition of CH₄ oxidation by (NH₄)₂SO₄ resulted mainly from a general salt effect (osmotic stress) though NH₄⁺ did have some additional inhibitory properties. Both salts were better inhibitors of CH₄ oxidation than respiration. The differences in the CH₄ oxidation kinetics in the forested peat soil and in the upland forest soils reveal that there are differences in the physiologies of the CH₄ oxidisers in these soils.     

Methane oxidation in a temperate coniferous forest soil: effects of inorganic N

Methane oxidation rates were measured in soils obtained from a coniferous forest in northern England. The effects of depth and added K⁺ (K₂SO₄), NH₄⁺ ((NH₄)₂SO₄) and NO₃⁻ (KNO₃) on potential CH₄ oxidation were investigated in a series of laboratory incubations. The humus (H) layer soil showed much greater CH₄ oxidation rates than the other soil layers, with maximal rates of 53 and 226 ng CH₄ gdw⁻¹ h⁻¹ when incubated with initial 10 and 1000 μl CH₄ l⁻¹, respectively. Additions of the solutes K⁺, NH₄⁺ and NO₃⁻ showed differing degrees of inhibition on CH₄ oxidation, which varied with the initial CH₄ concentration, the ion added, and the ion concentration. In general, inhibition by the ions was slightly greater for incubations with an initial concentration of 1000 μl CH₄ l⁻¹ than for 10 μl CH₄ l⁻¹ under otherwise identical conditions. For K⁺ and NH₄⁺ treatments, inhibitory rates were usually less than 15%, but at high K⁺ and NH₄⁺ concentrations inhibition could reach 50%, the inhibitory effects of NH₄⁺ were consistently slightly greater than those of K⁺ at the same concentration. In marked contrast to NH₄⁺, NO₃⁻ showed a very strong inhibitory effect. Added NO₃⁻ and NO₂⁻ produced via added NO₃⁻ reduction in anaerobic ‘microsites’ are probably toxic to CH₄-oxidizing bacteria. These results, together with those from other reports, suggest that NO₃⁻ may have a greater importance in the inhibition of CH₄ oxidation in forest soils than that attributed to NH₄⁺ and needs to be investigated in a wide range of soil types from various forests.     

Unlocking fixed soil phosphorus upon waterlogging can be promoted by increasing soil cation exchange capacity

Phosphorus (P) adsorbed by iron (Fe) oxyhydroxides in soil can be released when the Fe(III) minerals are reductively dissolved after soil flooding. However, this release is limited in tropical soils with large Fe contents and previous studies have suggested that P sorbs or precipitates with newly formed Fe(II) minerals. This hypothesis is tested here by scavenging Fe²⁺ in flooded soils by increasing the cation exchange capacity (CEC) of soil through resin application (30 cmol_c kg^?1; Na‐form). Three soils from rice paddies with contrasting properties were incubated in aerobic and anaerobic conditions with or without resin and with or without addition of organic matter (OM) to stimulate redox reactions. Dissolved Fe was 0.1–1.1 mm in unamended anaerobic soils and decreased to less than 0.07 mm with resin addition. Anaerobic soils without resin and aerobic soils with or without resin had marginal available P concentrations (<2 mg P kg^?1; anion‐exchange membrane P). In contrast, available P increased 3‐ to 14‐fold in anaerobic soils treated with resins, reaching 16 mg P kg^?1 in combination with extra OM. Application of Ca‐forms of resin did not stimulate P availability and dissolved Ca concentrations were larger than in unamended soils. Resin addition can increase P availability, probably by a combination of reducing solution Fe²⁺ (thereby limiting the formation of Fe(II) minerals) and increasing the OM solubility and availability through reducing dissolved Ca²⁺. The soil CEC is a factor controlling the net P release in submerged soils.     

Desiccation and product accumulation constrain heterotrophic anaerobic respiration in peats of an ombrotrophic temperate bog

To gain insight into the effects of drying and rewetting events on anaerobic respiration in ombrotrophic peat soils, we investigated bacterial sulfate (SO₄) reduction and methane (CH₄) production in anaerobic incubations of intact peat microcores from 30 to 40 cm depth of Mer Bleue bog, Ontario/Canada. Concentrations of dissolved SO₄, carbon dioxide (CO₂), CH₄, acetate, and hydrogen (H₂) were recorded and net turnover rates calculated from regression. Gross rates of bacterial sulfate reduction were determined by ³⁵SO₄ tracer incubation. After incubation, the peat was dried and rewetted, with saturated peat serving as control. CO₂ production was initially rapid (up to <360 nmol cm^?3 d^?1) and slowed towards an endpoint of 2–3 mmol l^?1, which was only partly related to thresholds of Gibbs free energies of the involved processes. Acetate rapidly accumulated to levels of 600–800 μmol l^?1 and remained constant thereafter. CH₄ production (0–2.8 nmol cm^?3 d^?1) was small and delayed, even after SO₄ was depleted, by about 30–40 d. Hydrogenotrophic methanogenesis was endergonic and the process thus likely followed an acetotrophic pathway. Drying and rewetting replenished the SO₄ pool, enhanced SO₄ reduction rates and suppressed methanogenesis. The overall contribution of net SO₄ reduction and methanogenesis to the CO₂ production rate was small (0.5–22%) and only enhanced in replicates subjected to drying (35–62%). The major fraction of respiration in the incubated peat cores thus followed yet unidentified pathways.     

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing,China

Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO₂ concentrations and global warming through soil respiration. We conducted a comparative study to determine NH₄⁺ and NO₃⁻ concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO₃⁻-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH₄⁺-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg⁻¹ d⁻¹) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH₄⁺ and, NO₃⁻ concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.     

Effects of floating Azolla on phosphorus fluxes and recovery from former agricultural lands in wetland microcosms

The long-term fertilization results in accumulation of phosphorus especially in the top layer of the soils. Inundation of agricultural lands leads to a switch to anaerobic soil condition, causing reduction of iron and leaching of phosphate simultaneously. From the ecological and environmental perspective, high nutrients flux especially phosphorus will increase the possibility of eutrophication in aquatic system. The fern Azolla had a good potential to adsorb phosphorus, it also has distinctive nitrogen-fixing capacity. We conducted a 10-week aquarium experiment to investigate the phosphorus release capacity from two agricultural soils in the Netherlands with different Fe and P concentrations but comparable Fe/P ratios. Besides, the research questions rose to whether Azolla could use the mobilized phosphate released from the soils for growth. We also tried to find an effective indicator to estimate the actually phosphate mobilization from sediment to water layer. Results showed that the soils with high Fe and P concentrations had higher phosphate release rate compared with the soil with low Fe and P concentrations. Pore water Fe: PO₄^3? ratios were valid to identify P release to surface water, when the Fe: PO₄^3? ratios less than 8 mol mol^?1 substantial phosphorus mobilization occurred. The conclusions showed that the actual mobilization of phosphate is more important than the phosphorus retained in the sediments for the internal PO₄^3? fluxes. From 10-week experimental results, we found that Azolla can reuse the phosphate retained in soils thus removed the mobilized phosphate in a moderately low surface water nutrient loading.     

Exploration of soils developing on volcanic materials on the island of Milos,Greece

The island of Milos (Greece), part of the South Aegean volcanic arc with a typical Mediterranean climate, is covered with volcanic deposits of different ages. The objective of this study was to investigate the physicochemical and mineralogical properties of the soils developing on these volcanic deposits and their classification. Samples were taken from seven locations of soil on different parent material and of different ages. There were substantial differences in their particle size distribution, with sand ranging from 19% to 92%, silt from 3.5% to 50%, and clay from 5% to 46%. Organic matter content was low (< 2.0%). The soil pH ranged from 5.6 to 8.0. In two of the profiles, CaCO₃ equivalents of 1.4% to 24.6% were found and a calcic horizon identified. The cation exchange capacity (CEC) and specific surface area (SSA) varied between profiles ranging from 3 cmol₍₊₎ kg^− 1 to 47 cmol₍₊₎ kg^− 1 and 30 m² g^− 1 to 380 m² g^− 1, respectively. The soils exhibited high base saturation. The amounts of Al, Fe and Si extracted with ammonium oxalate (Αl_o, Fe_o and Si_o) were particularly low (< 0.1%, < 0.17%, and < 0.1%, respectively) which demonstrates the absence of amorphous clay-silicate minerals (allophane). Fe extracted with dithionite citrate bicarbonate — DCB (Fe_d) was greater than Fe_o sharing the dominance of crystalline Fe oxides. Al and Si extracted with hot 0.5 M NaOH (Al₂Ο_3NaOH and SiΟ_2NaOH) and with Τiron-C₆H₄Na₂O₈S₂, (Al₂Ο_3Τ and SiΟ_2Τ). SiΟ_2NaOH and SiΟ_2Τ were particularly high (mean values 3.4% and 4.5%, respectively), showing that amorphous silica was present. The clay fraction of the soil was dominated by the presence of 2:1 (vermiculite and smectite) and 1:1 (kaolinite) clay-silicates. Al_o+ 1/2Fe_o was low (< 0.18%), while the P-retention in most soils was less than 15%. These soils do not exhibit andic properties and hence cannot be classified as Andisols. The silica saturation index (ISS) may be used for these soils to describe a pedogenetic environment rich in Si which favours the formation of pedogenic amorphous silica. The climate is the major determinant of the evolution of these soils.     

Effects of acid sulphate on DOC release in mineral soils: the influence of SO42− retention and Al release

Dissolved organic carbon (DOC) in acid‐sensitive upland waters is dominated by allochthonous inputs from organic‐rich soils, yet inter‐site variability in soil DOC release to changes in acidity has received scant attention in spite of the reported differences between locations in surface water DOC trends over the last few decades. In a previous paper, we demonstrated that pH‐related retention of DOC in O horizon soils was influenced by acid‐base status, particularly the exchangeable Al content. In the present paper, we investigate the effect of sulphate additions (0–437 µeq l^?1) on DOC release in the mineral B horizon soils from the same locations. Dissolved organic carbon release decreased with declining pH in all soils, although the shape of the pH‐DOC relationships differed between locations, reflecting the multiple factors controlling DOC mobility. The release of DOC decreased by 32–91% in the treatment with the largest acid input (437 µeq l^?1), with the greatest decreases occurring in soils with very small % base saturation (BS, < 3%) and/or large capacity for sulphate (SO₄^2?) retention (up to 35% of added SO₄^2?). The greatest DOC release occurred in the soil with the largest initial base status (12% BS). These results support our earlier conclusions that differences in acid‐base status between soils alter the sensitivity of DOC release to similar sulphur deposition declines. However, superimposed on this is the capacity of mineral soils to sorb DOC and SO₄^2?, and more work is needed to determine the fate of sorbed DOC under conditions of increasing pH and decreasing SO₄^2?.     

Carbon dioxide, nitrous oxide and methane dynamics in boreal organic agricultural soils with different soil characteristics

The annual carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) dynamics were measured with static chambers on two organic agricultural soils with different soil characteristics. Site 1 had a peat layer of 30 cm, with an organic matter (OM) content of 74% in the top 20 cm. Site 2 had a peat layer of 70 cm but an OM content of only 40% in the top 20 cm. On both sites there were plots under barley and grass and also plots where the vegetation was removed. All soils were net sources of CO₂ and N₂O, but they consumed atmospheric CH₄. Soils under barley had higher net CO₂ emissions (830 g CO₂-C m⁻² yr⁻¹) and N₂O emissions (848 mg N₂O-N m⁻² yr⁻¹) than those under grass (395 g CO₂-C m⁻³ yr⁻¹ and 275 mg N₂O-N m⁻² yr⁻¹). Bare soils had the highest N₂O emissions, mean 2350 mg N₂O-N m⁻² yr⁻¹. The mean CH₄ uptake rate from vegetated soils was 100 mg CH₄-C m⁻³ yr⁻¹ and from bare soils 55 mg CH₄-C m⁻² yr⁻¹. The net CO₂ emissions were higher from Site 2, which had a high peat bulk density and a low OM content derived from the addition of mineral soil to the peat during the cultivation history of that site. Despite the differences in soil characteristics, the mean N₂O emissions were similar from vegetated peat soils from both sites. However, bare soils from Site 2 with mineral soil addition had N₂O emissions of 2-9 times greater than those from Site 1. Site 1 consumed atmospheric CH₄ at a higher rate than Site 2 with additional mineral soil. N₂O emissions during winter were an important component of the N₂O budget even though they varied greatly, ranging from 2 to 99% (mean 26%) of the annual emission.     

Rice root-derived carbon input and its effect on decomposition of old soil carbon pool under elevated CO2

Rice (Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l⁻¹ CO₂ (elevated CO₂) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO₂ in one growing season, and (2) the effect of the newly input C on decomposition of the more recalcitrant native soil organic C. The initial δ¹³C value of the experimental soil was −25.8‰, which was 6‰ less depleted in ¹³C than the plants grown under elevated CO₂. Significant changes in δ¹³C of the soil organic C were detected after one growing season. The amount of new soil C input was estimated to be 0.9 t ha⁻¹ (or 2.1%) at 30 kg N ha⁻¹ and 1.8 t ha⁻¹ (4.1%) at 90 kg N ha⁻¹. Changes in soil δ¹³C suggested that the surface 5 cm of soil received more C input from plants than soils below. Laboratory incubation (25 °C) of soils from different horizons indicated that increased availability of the labile plant-derived C in the soil reduced decomposition of the native soil organic C. Provided the retardant effect of the new C on old soil organic C holds in the field in the longer-term, paddy soils will likely sequester more C from the atmosphere if more plant C enters the soil under elevated atmospheric CO₂.     

Methane oxidation in temperate soils: effects of inorganic N

Additions of inorganic nitrogen (N) to an oak soil with significant potential for methane (CH₄) oxidation resulted in differential reduction in CH₄ oxidation capacity depending on N species added. Nitrate, rather than nitrite or ammonium, proved to be the strongest inhibitor of CH₄ oxidation in oak soil. Both high (CH₄ at 10 μl l⁻¹) and low (CH₄ at 5 ml l⁻¹) affinity CH₄ oxidation in oak soil was completely inhibited at a nitrate concentration similar to that present in an alder soil from the same experimental site. The alder soil showed no capacity for low affinity CH₄ oxidation. A ‘low nitrate’ forest soil (oak) showed high affinity, low capacity CH₄ oxidation upto around 1 ml l⁻¹ CH₄, above which both high and low affinity CH₄ oxidation became apparent following a lag phase, indicating either an induced high affinity uptake mechanism or the existence of distinct low affinity and high affinity methanotroph populations. High affinity CH₄ oxidation became saturated at CH₄ concentrations >500 μl l⁻¹, while low affinity CH₄ oxidation became saturated at ∼30 ml l⁻¹ CH₄. In a ‘high nitrate’ forest soil (alder), CH₄ oxidation appeared to be due to high affinity CH₄ oxidation only and became undetectable at CH₄ concentrations >5 ml l⁻¹.     

Soluble organic nitrogen in temperate forest soils

Very few studies have been related to soluble organic nitrogen (SON) in forest soils. However, this nitrogen pool could be a sensitive indicator to evaluate the soil nitrogen status. The current study was conducted in temperate forests of Thuringia, Germany, where soils had SON (extracted in 0.5 M K₂SO₄) varying from 0.3 to 2.2% of total N, which was about one-third of the soil microbial biomass N by CFE. SON in study soils were positively correlated to microbial biomass N and soil total N. Multiple regression analysis also showed that mineral N negatively affected SON pool. The dynamics of the SON was significantly affected by mineralization and immobilization. During the 2 months of aerobic incubation, the SON were significantly correlated with net N mineralization and microbial biomass N. SON extracted by two different salt solution (i.e. 1 M KCl and 0.5 M K₂SO₄₎ were highly correlated. In mineral soil, SON concentrations extracted by 1 M KCl and 0.5 M K₂SO₄ solutions were similar. In contrast, in organic soil layer the amount of KCl-extractable SON was about 1.2-1.4 times higher than the K₂SO₄-extractable SON. Further studies such as the differences of organic N form and pool size between SON and dissolved organic N (DON) are recommended.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Basal organic phosphorus mineralization in soils under different farming systems

Soil organic P (P_o) mineralization plays an important role in soil P cycling. Quantitative information on the release of available inorganic P (P_i) by this process is difficult to obtain because any mineralized P_i gets rapidly sorbed. We applied a new approach to quantify basal soil P_o mineralization, based on ³³PO₄ isotopic dilution during 10 days of incubation, in soils differing in microbiological activity. The soils originated from a 20 years old field experiment, including a conventional system receiving exclusively mineral fertilizers (MIN), a bio-organic (ORG) and bio-dynamic (DYN) system. Indicators of soil microbiological activity, such as size and activity of the soil microbial biomass and phosphatase activity, were highest in DYN and lowest in MIN. In order to assess P_o hydrolysis driven by phosphatase in sterile soils, a set of soil samples was γ-irradiated. Basal P_o mineralization rates in non-irradiated samples were between 1.4 and 2.5 mg P kg⁻¹ day⁻¹ and decreased in the order DYN>ORG≥MIN. This is an amount lower, approximately equivalent to, or higher than water soluble P_i of MIN, ORG and DYN soils, respectively, but in every soil was less than 10% of the amount of P isotopically exchangeable during one day. This shows that physico-chemical processes are more important than basal mineralization in releasing plant available P_i. Organic P mineralization rates were higher, and differences between soils were more pronounced in γ-irradiated than in non-irradiated soils, with mineralization rates ranging from 2.2 to 4.6 mg P kg⁻¹ day⁻¹. These rates of hydrolysis, however, cannot be compared to those in non-sterile soils as they are affected by the release of cellular compounds, e.g. easily mineralizable P_o, derived from microbial cells killed by γ-irradiation.     

CH4 oxidation and N2O emissions at varied soil water-filled pore spaces and headspace CH4 concentrations

Emission of N₂O and CH₄ oxidation rates were measured from soils of contrasting (30-75%) water-filled pore space (WFPS). Oxidation rates of ¹³C-CH₄ were determined after application of 10 μl ¹³C-CH₄ l⁻¹ (10 at. % excess ¹³C) to soil headspace and comparisons made with estimates from changes in net CH₄ emission in these treatments and under ambient CH₄ where no ¹³C-CH₄ had been applied. We found a significant effect of soil WFPS on ¹³C-CH₄ oxidation rates and evidence for oxidation of 2.2 μg ¹³C-CH₄ d⁻¹ occurring in the 75% WFPS soil, which may have been either aerobic oxidation occurring in aerobic microsites in this soil or anaerobic CH₄ oxidation. The lowest ¹³C-CH₄ oxidation rate was measured in the 30% WFPS soil and was attributed to inhibition of methanotroph activity in this dry soil. However, oxidation was lowest in the wetter soils when estimated from changes in concentration of ¹²⁺¹³C-CH₄. Thus, both methanogenesis and CH₄ oxidation may have been occurring simultaneously in these wet soils, indicating the advantage of using a stable isotope approach to determine oxidation rates. Application of ¹³C-CH₄ at 10 μl ¹³C-CH₄ l⁻¹ resulted in more rapid oxidation than under ambient CH₄ conditions, suggesting CH₄ oxidation in this soil was substrate limited, particularly in the wetter soils. Application of and (80 mg N kg soil⁻¹; 9.9 at.% excess ¹⁵N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N₂O emissions. The highest N₂O emission (119 μg ¹⁴⁺¹⁵N-N₂O kg soil⁻¹ over 72 h) was measured from the 75% WFPS soil and was mostly produced during denitrification (18.1 μg ¹⁵N-N₂O kg soil⁻¹; 90% of ¹⁵N-N₂O from this treatment). Strong negative correlations between ¹⁴⁺¹⁵N-N₂O emissions, denitrified ¹⁵N-N₂O emissions and ¹³C-CH₄ concentrations (r=−0.93 to −0.95, N₂O; r=−0.87 to −0.95, denitrified ¹⁵N-N₂O; P<0.05) suggest a close relationship between CH₄ oxidation and denitrification in our soil, the nature of which requires further investigation.     

Emission of CO2, CH4 and N2O from lakeshore soils in an Antarctic dry valley

We measured soil profile concentrations and emission of CO₂, CH₄ and N₂O from soils along a lakeshore in Garwood Valley, Antarctica, to assess the extent and biogeochemical significance of biogenic gas emission to C and N cycling processes. Simultaneous emission of all three gases from the same site indicated that aerobic and anaerobic processes occurred in different layers or different parts of each soil profile. The day and location of high gas concentrations in the soil profile corresponded to those having high gas emission, but the pattern of concentration with depth in the soil profile was not consistent across sites. That the highest gas concentrations were not always in the deepest soil layer suggests either limited production or gas diffusion in the deeper layers. Emission of CO₂ was as high as 47 μmol m⁻² min⁻¹ and was strongly related to soil temperature. Soil respiration differed significantly according to location on the lakeshore, suggesting that factors other than environmental variables, such as the amount and availability of O₂ and nutrients, play an important role in C mineralization processes in these soils. High surface emission (maximum: 15 μmol m⁻² min⁻¹) and profile gas concentration (maximum: 5780 μL L⁻¹) of CH₄ were at levels comparable to those in resource-rich temperate ecosystems, indicating an active indigenous population of methanogenic organisms. Emission of N₂O was low and highly variable, but the presence of this gas and NO₃ in some of the soils suggest that denitrification and nitrification occur there. No significant relationships between N₂O emission and environmental variables were found. It appears that considerable C and N turnover occurs in the lakeshore soils, and accurate accounting will require measurements of aerobic and anaerobic mineralization. The production and emission of biogenic gases confirm the importance of these soils as hotspots of biological activity in the dry valleys and probable reservoirs of biological diversity.     

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.     

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.     

Micro-scale CO2 and CH4 dynamics in a peat soil during a water fluctuation and sulfate pulse

We investigated the response of CO₂ and CH₄ production to a water table fluctuation and a SO₄²⁻ pulse in a bog mesocosm. Net gas production rates in the mesocosm were calculated from concentration data by diffusive mass-balances. Incubation experiments were used to quantify the effect of SO₄²⁻ addition and the distribution of potential CO₂ and CH₄ production rates. Flooding of unsaturated peat resulted in rapid depletion of O₂ and complex patterns of net CH₄, CO₂, and H₂S production. Methane production began locally and without a time lag at rates of 3-4 nmol cm⁻³ d⁻¹ deeper in the peat. Similar rates were determined after a time lag of 10-60 days in the surface layers, whereas rates at lower depths declined. Net CO₂ production was largest immediately after the water table position was altered (100-300 nmol cm⁻³ d⁻¹) and declined to −50-50 nmol cm⁻³ d⁻¹ after a few weeks. SO₄²⁻ addition (500 mM) significantly increased potential CH₄ production rates in the surface layer from an average of 132-201 nmol cm⁻³ d⁻¹ and reduced it below from an average of 418-256 nmol cm⁻³ d⁻¹. Our results suggest that deeper in the peat (40-70 cm) under in situ conditions, methanogenic populations are less impaired by unsaturated conditions than in the surface layers, and that at these depths after flooding the substrate availability for CH₄ and DIC production is significantly enhanced. They also suggest that methanogenic and SO₄²⁻-reducing activity were non-competitive in the surface layer, which might explain contradictory findings from field studies.