Optimizing fen peatland water-table depth for romaine lettuce growth to reduce peat wastage under future climate warming

Forty percentage of UK peatlands have been drained for agricultural use, which has caused serious peat wastage and associated greenhouse gas emissions (carbon dioxide (CO₂) and methane (CH₄)). In this study, we evaluated potential trade-offs between water-table management practices for minimizing peat wastage and greenhouse gas emissions, while seeking to sustain romaine lettuce production: one of the most economically relevant crop in the East Anglian Fenlands. In a controlled environment experiment, we measured lettuce yield, CO_2, CH₄ fluxes and dissolved organic carbon (DOC) released from an agricultural fen soil at two temperatures (ambient and +2°C) and three water-table levels (−30 cm, −40 cm and −50 cm below the surface). We showed that increasing the water table from the currently used field level of −50 cm to −40 cm and −30 cm reduced CO₂ emissions, did not affect CH₄ fluxes, but significantly reduced yield and increased DOC leaching. Warming of 2°C increased both lettuce yield (fresh leaf biomass) and peat decomposition through the loss of carbon as CO₂ and DOC. However, there was no difference in the dry leaf biomass between the intermediate (−40 cm) and the low (−50 cm) water table, suggesting that romaine lettuce grown at this higher water level should have similar energetic value as the crop cultivated at −50 cm, representing a possible compromise to decrease peat oxidation and maintain agricultural production.     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Simulation of seasonal changes in microbial activity of maritime antarctic peat

Seasonal variations in temperature and moisture in moss peat were monitored in the field at Signy Island, Antarctica. When simulated in intact peat cores in vitro after frozen storage, these variations caused changes on O₂-uptake which closely reproduced the results for fresh samples. Respiration rate was used as a measure of aerobic decomposer activity. Supplements of sugars indicated the predominance of microbial respiration and its dependence on the availability of dissolved organic C (DOC). Low temperatures of 0° to 1°C were not rate-limiting for respiration in vivo or in vitro, and O₂-uptake was detected at ?1°C. Repeated peaks of O₂-uptake under wet conditions resulting from simulated spring freeze-thaw cycles, and a solitary peak during an autumn simulation, suggested release of DOC substrates from frost-damaged cells. Desiccation, microfaunal predation and microaerophily were thought to contribute to respiratory declines. O₂-uptake and CO₂-evolution were equivalent in peat beneath Polytrichum sampled in autumn. Peat respiration was not generally proportional to microbial biomass, but saccharolytic yeasts were dominant during the respiratory maximum in spring and correlated with O₂-uptake in a mixed culture of indigenous microflora. Yeasts grew exponentially in freezethaw cycle simulations but percolated into the peat profile in the field. The basal O₂-uptake, which may be attributable to the decomposition of redalcitrant molecules such as cellulose, was lower in simulations of spring than autumn. Although bacterial biomass increased and diversified during summer, the ratio of fungal-to-bacterial contributions to O₂-uptake in an incubated homogenate of peat sampled in autumn was 4:1.     

Soil hydrology and CO2 release of peat soils

A simple model to predict soil water components and the CO₂ release for peat soils is presented. It can be used to determine plant water uptake and the CO₂ release as a result of peat mineralization for different types of peat soils, various climate conditions, and groundwater levels. The model considers the thickness of the root zone, its hydraulic characteristics (pF, Ku), the groundwater depth and a soil‐specific function to predict the CO₂ release as a result of peat mineralization. The latter is a mathematical function considering soil temperature and soil matric potential. It is based on measurements from soil cores at varying temperatures and soil water contents using a respiricond equipment. Data was analyzed using nonlinear multiple regression analysis. As a result, CO₂ release equations were gained and incorporated into a soil water simulation model. Groundwater lysimeter measurements were used for model calibration of soil water components, CO₂ release was adapted according long‐term lysimeter data of Mundel (1976). Peat soils have a negative water balance for groundwater depth conditions up to 80—100 cm below surface. Results demonstrate the necessity of a high soil water content i.e. shallow groundwater to avoid peat mineralization and soil degradation. CO₂ losses increase with the thickness of the rooted soil zone and decreases with the degree of soil degradation. Especially the combination of deep groundwater level and high water balance deficits during the vegetation period leads to tremendous CO₂ losses.     

Are enchytraeid worms (Oligochaeta) sensitive indicators of ammonia-N impacts on an ombrotrophic bog?

Enchytraeid worms (Oligochaeta) are the dominant mesofauna in wet acidic habitats. They have key roles in biogeochemical cycling, and can be used as biological indicators. Here we report the response of these worms to in situ ammonia-N (NH₃-N) deposition on an ombrotrophic bog. Three years of NH₃-N fumigation from an automated release system has created a gradient of NH₃-N concentrations downwind of the release pipe ranging from 83 μg m⁻³ (near source) to 4.5 μg m⁻³ NH₃-N (60 m from release pipe); the ambient NH₃-N concentration is 0.56 μg m⁻³ NH₃-N. Peat pH and mineral N content have increased near the ammonia release pipe. We hypothesised that enhanced N deposition at the site would have improved litter quality and thus, enchytraeid distribution would be increased along the transect compared to ambient. However, neither litter quality nor enchytraeid abundance and diversity were affected by NH₃-N despite increases in peat pH and mineral N. This suggests that three years of ammonia fumigation was not enough time for plant matter exposed to ammonia to become incorporated into the peat litter layer. Enchytraeids appear not to be sensitive indicators of NH₃ fumigation because there was no effect below-ground of peat chemistry on litter quality.     

Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Relationship between rate of carbon dioxide evolution,microbial biomass carbon,and amount of dissolved organic carbon as affected by temperature and water content of a forest and an arable soil

Abstract

Using an Ochrept soil of a forest at climax stage or of an arable site at Kita‐Ibaraki, a city in central Japan, the rates of carbon dioxide (CO₂)‐carbon (C) evolution, the amounts of microbial biomass carbon (MBC) and the amounts of dissolved organic carbon (DOC) were measured in a laboratory with special reference to the incubation temperature and the soil water content. The rates of CO₂‐C evolution increased exponentially with increase in the incubation temperature in the range of 4–40°C. The temperature coefficients (Q₁₀) were 2.0 for the forest and 1.9 for the arable soil. The amounts of MBC were almost constant of 980 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 340 μg g^‐1 soil in the incubation temperature up to 31 °C for the arable soil. The amounts of DOC in soil solutions were almost constant at 3.1 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 3.8 μg g^‐1 soil in the incubation temperature up to 31°C for the arable soil. The rates of CO₂‐C evolution and the amounts of DOC increased with increase in soil water content (% of soil dry weight) up to 91% for the forest or up to 26% for the arable soil. However, the rates of CO₂‐C evolution and the amounts of DOC were almost constant within soil water content in the range of 91–160% or 26–53%, respectively. The amounts of MBC of the forest or arable soil were almost constant over a wide range of soil water content in the range of 41–220% or 8–73%, respectively. The rates of CO₂‐C evolution of both the forest and the arable soils were highly correlated with the amounts of DOC, but not with the amounts of MBC, under laboratory conditions in the case that the amounts of DOC were changed by various treatments. The regression equation,     

Carbon and nitrogen mineralization after maize harvest between and within maize rows: a microcosm study using 13C natural abundance

The sequestration of carbon in soil is not completely understood, and quantitative information about the rates of soil organic carbon (SOC) turnover could improve understanding. We analyzed the effects of the uneven distribution of crop residues after harvest of silage maize on C and N losses (CO₂‐C, dissolved organic carbon (DOC) and nitrogen (DON), and NO₃^–) from a Haplic Phaeozem and on the occurrence of priming effects induced by the decomposition of accumulated maize residues. Soil columns were taken from a continuous maize (since 1961) field after harvest i) between maize stalk rows (M_bare), ii) within the maize rows including a standing maize stalk (M_stalk), and iii) from a continuous rye (since 1878) field after tillage (rye stalk and roots were mixed into the Ap horizon). The soil columns were incubated for 230 days at 8 °C with an irrigation rate of 2 mm 10^–2 M CaCl₂ per day. Natural ¹³C abundance was used to distinguish between maize‐derived C (in SOC and maize residues) and older C originating from former C₃ vegetation. The uneven distribution of maize residues resulted in a considerably increased heterotrophic activity within the maize rows as compared with soil between seed rows. Cumulative CO₂ production was 53.1 g CO₂‐C m^–2 for M_stalk and 23.3 g CO₂‐C m^–2 for M_bare. The contribution of maize‐derived C to the total CO₂ emission was 83 % (M_stalk) and 67 % (M_bare). Calculated as difference between CO₂‐C release from M_stalk and M_bare, 19 % of the maize residues (roots and stalk) in M_stalk were mineralized during the incubation period. There was no or only a marginal effect of the accumulation of maize residues in M_stalk on leaching of DOC, DON, and NO₃^–. Total DOC and DON leaching amounted to 2.5 g C m^–2 and 0.16 g N m^–2 for M_stalk and to 2.1 g C m^–2 and 0.12 g N m^–2 for M_bare. The contribution of maize‐derived C to DOC leaching was about 25 % for M_stalk and M_bare. Nitrate leaching amounted to 3.9 g NO₃^–‐N m^–2 for M_stalk and to 3.5 g NO₃^–‐N m^–2 for M_bare. There was no priming effect induced by the decomposition of fresh maize residues with respect to CO₂ or DOC production from indigenous soil organic carbon derived from C₃ vegetation.     

Soil chemistry versus environmental controls on production of CH4 and CO2 in northern peatlands

Rates of organic carbon mineralization (to CO₂ and CH₄) vary widely in peat soil. We transplanted four peat soils with different chemical composition into six sites with different environmental conditions to help resolve the debate about control of organic carbon mineralization by resource availability (e.g. carbon and nutrient chemistry) versus environmental conditions (e.g. temperature, moisture, pH). The four peat soils were derived from Sphagnum (bog moss). Two transplant sites were in mid‐boreal Alberta, Canada, two were in low‐boreal Ontario, Canada, and two were in the temperate United States. After 3 years in the field, CH₄ production varied significantly as a function of peat type, transplant site, and the type–site interaction. All four peat soils had very small rates of CH₄ production (< 20 nmol g^?1 day^?1) after transplant into two sites, presumably caused by acid site conditions (pH < 4.0). One peat soil had small CH₄ production rates regardless of transplant site. A canonical discriminant analysis revealed that large rates of CH₄ production (4000 nmol g^?1 day^?1) correlated with large holocellulose content, a large concentration of p‐hydroxyl phenolic compounds in the Klason lignin, and small concentrations of N, Ca and Mn in peat. Significant variation in rates of CO₂ production correlated positively with holocellulose content and negatively with N concentrations, regardless of transplant site. The temperature response for CO₂ production varied as a function of climate, being greater for peat formed in a cold climate, but did not apply to transplanted peat. Although we succeeded in elucidating some aspects of peat chemistry controlling production of CH₄ and CO₂ in Sphagnum‐derived peat soils, we also revealed idiosyncratic combinations of peat chemistry and site conditions that will complicate forecasting rates of peat carbon mineralization into the future.     

Long‐term carbon loss and CO2‐C release of drained peatland soils in northeast Germany

Peatlands are common in many parts of the world. Draining and other changes in the use of peatlands increase atmospheric CO₂ concentration. If we are to make reliable quantitative predictions of that effect, we need good information on the CO₂ emission rates from peatlands. The present study uses two different methods for predicting CO₂‐C release of peatland soils: (i) a 40‐year field investigation of balancing organic carbon stocks and (ii) short‐term CO₂‐C release rates from laboratory experiments. To estimate long‐term losses of peat, and its resulting C input to the atmosphere, we combined highly detailed maps of surface topography and its changes, and the organic C contents and bulk densities of a drained peatland from different years. Short‐term CO₂‐C release rates were measured in the laboratory by incubating soil samples from several soil horizons at various temperatures and soil moistures. We then derived nonlinear CO₂‐C production functions, which we incorporated into a numerical simulation model (HYDRUS). Using HYDRUS, we calculated daily soil water components and CO₂‐release for (i) real‐climate data from 1950 to 2003 and (ii) a climate scenario extending to 2050, including an increase in temperature of 2°C and 20% less rainfall during the summer half year, i.e. from April to September inclusive. From our field measurements, we found a mean annual decrease of 0.7 cm in the thickness of the peat. Large losses (> 1.5 cm year^?1) occurred only during periods when groundwater levels were low (i.e. a deep water‐table). The annual CO₂‐C release results in a mean loss from the peat of about 700 g CO₂‐C m^?2, mostly as a direct contribution to the atmosphere. Both methods produced very similar results. The model scenarios demonstrated that CO₂‐C loss is mainly controlled by the groundwater (i.e. water‐table) depth, which controls subsurface aeration. A local climate scenario estimated a c. 5% increase of CO₂‐C losses within the next 50 years.     

Plant roots are more important than temperature in modulating carbon release in a limed acidic soil

Lime is a common amendment to overcome soil acidity in agricultural production systems. However, plant root effects on lime and soil carbon (C) dynamics in acidic soils under varied temperature remain largely unknown. We monitored root effects of soybean on the fate of lime applied to an acidic soil at 20 and 30°C in growth chambers. Soil respired CO₂ was continuously trapped in columns without and with plants until the final stage of vegetative growth. Lime‐derived CO₂ was separated from total respired CO₂ based on δ¹³C measurements in CO₂. Leaching was induced at early and late vegetative growth stages, and the leachates were analysed for dissolved organic (DOC) and inorganic C (DIC) concentrations. Soil respiration significantly increased with lime addition at both temperatures (p < 0.001). The presence of soybean doubled the recovery of lime‐derived CO₂‐C at 20°C at the early growth stage; however, by the end of the experiment, the contribution of lime‐derived CO₂‐C to soil respiration was negligible in all treatments, indicating that the contribution of lime to soil respiration was shortlived. In contrast, DIC and DOC concentrations in leachates remained elevated with liming and were greater in the presence of soybean. We observed no main temperature effects and no interactive effects of temperature and soybean presence on lime‐derived CO₂‐C, DIC and DOC. These results highlight the role of plant‐modulated processes in CO₂ release and C leaching from lime in acidic soils, whereas an increase in temperature may be less important. Temperature and plant roots alter the rate of key processes controlling C dynamics in a limed acidic soil. Lime‐derived CO₂‐C, DIC and DOC increased more in the presence of plants than with increased temperature. Root effects are more important than temperature for inorganic and organic carbon dynamics in limed acidic soils.     

Carbon flux from decomposing 13C-labeled transgenic and nontransgenic parental rice straw in paddy soil

Purpose

Genetic modification of Bt rice may affect straw decomposition and soil carbon pool under flood conditions. This study aims to assess the effects of cry gene transformation in rice on the residue decomposition and fate of C from residues under flooded conditions.

Materials and methods

A decomposition experiment was set up using ¹³C-enriched rice straws from transgenic and nontransgenic Bt rice to evaluate the soil C dynamics and CH₄ or CO₂ emission rates in the root and non-root zones. The concentrations and stable carbon isotope compositions of the soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), CH₄, and CO₂ of the root and non-root zones were determined from 7 to 110 days after rice straw incorporation.

Results and discussion

Rice straw incorporation into soil significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates. The percentage of ¹³C-SOC remaining in the root zone was significantly lower than that in the non-root zone with rice straw decomposition. The DOC and MBC concentrations significantly increased in both the root and non-root zones between 0 and 80 days after rice straw incorporation. However, no significant differences were found after Bts (Bt rice straw added into soil) and Cks (nontransgenic Bt rice straw added into soil) incorporation in the root and non-root zones. This result may be attributed to the priming effects of sufficient oxygen and nutrients on straw degradation in the root zone.

Conclusions

Bt gene insertion did not affect the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates in both the root and non-root zones. However, rice straw incorporation and root exudation significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates.     

High Carbon Dioxide Evasion from an Alpine Peatland Lake: The Central Role of Terrestrial Dissolved Organic Carbon Input

We measured carbon dioxide (CO₂) fluxes across air?Cwater interface with floating chambers in Lake Medo (a small, shallow lake in peatland) on the eastern Tibetan Plateau in the warm season of 2009. During the study period, mean CO₂ fluxes was 488.63?±?1,036.17?mg?CO₂?m^?2?h^?1. The flux rate was high compared to those of lakes in other regions, and represented a ??hotspot?? of CO₂ evasion. Temporal variation of CO₂ flux was significant, with the peak value in the beginning and lowest point in the end of warm season. High concentration of dissolved organic carbon (DOC) in lake water (WDOC) was found to highly correlated to CO₂ flux (R?=?0.47, P?<?0.01, n?=?54). Besides, fluorescence index of WDOC showed its terrestrial origin character. In accordance with lakes in northern and boreal regions, terrestrial DOC concentration in water column was the most important regulator of CO₂ flux from this lake. We suggest that large area of peatlands in catchments support high concentration of DOC in this lake, and consequently high CO₂ evasion.     

CH4 production potential in a paddy soil exposed to atmospheric CO2 enrichment

Abstract

An anaerobic incubation experiment was conducted to investigate methane (CH₄) production potential in soil samples collected from a paddy field after exposure to free-air CO₂ enrichment (FACE). The FACE experiment with two CO₂ levels, ambient and ambient + 200 p.p.m.v CO₂ during the rice growing season, was conducted at Shizukuishi, Iwate Prefecture, Japan. The soil was a wet Andosol. Soil samples were taken from the surface (0–1 cm) and the sub-surface (1–10 cm) soil layers 2 months after rice harvest. Sub-samples of the fresh soils were put into glass bottles and submerged under N₂ gas headspace during the incubation. The results showed that, prior to incubation, the contents of total C and dissolved organic C (DOC) were significantly greater in FACE soil than ambient soil. During the incubation, CH₄ production potential was approximately 2–4-fold higher in FACE soil than ambient soil and approximately 500–1,000-fold greater in surface soil than sub-surface soil. In general, the FACE soil contained more DOC than ambient soil, particularly in the surface soil layer. These findings suggest that FACE treatment exerted long-term positive effects on CH₄ production and increased organic C content in this paddy soil, particularly in the surface soil layer.     

Carbon dynamics in Appalachian peatlands of West Virginia and Western Maryland

Abundant production of organic matter that decomposes slowly under anaerobic conditions can result in substantial accumulation of soil organic matter in wetlands. Tedious means for estimating production and decomposition of plant material, especially roots, hampers our understanding of organic matter dynamics in such systems. In this paper, I describe a study that amended typical estimates for both production and decomposition of organic matter by measuring net flux of carbon dioxide (CO₂) over the peat surface within a conifer swamp, a sedge-dominated marsh, and a bog in the Appalachian Mountain region of West Virginia and western Maryland, USA. The sites are relatively productive, with net primary production (NPP) of 30 to 82.5 mol C m^?2 yr^?1, but peat deposits are shallow with an average depth of about 1 m. In summer, all three sites showed net CO₂ flux from the atmosphere to the peat during the daytime (?20.0 to ?30.5 mmol m^?2 d^?1), supported by net photosynthesis, which was less than net CO₂ flux from the peat into the atmosphere at nighttime (39.2 to 84.5 mmol m^?2 d^?1), supported by ecosystem respiration. The imbalance between these estimates suggests a net loss of carbon (C) from these ecosystems. The positive net CO₂ flux seems to be so high because organic matter decomposition occurs throughout the peat deposit — and as a result concentrations of dissolved inorganic carbon (DIC) in peat pore waters reached 4,000 Μmol L^?1 by late November, and concentrations of dissolved organic carbon (DOC) in peat pore waters reached 12,000 Μmol L^?1. Comparing different approaches revealed several features of organic matter dynamics: (i) peat accretion in the top 30 cm of the peat deposit results in a C accumulation rate of about 15 mmol m^?2 d^?1; however, (ii) the entire peat deposit has a negative C balance losing about 20 mmol m^?2 d^?1.     

Effect of temperature on the mineralization of C and N of fresh and mature compost in sandy material

Information about the mineralization rate of compost at various temperatures is a precondition to optimize mineral N fertilization and to minimize N losses in compost‐amended soils. Objectives were to quantify the influence of the temperature on the mineralization rate and leaching of dissolved organic carbon (DOC) and nitrogen (DON), NO₃^—, and NH₄⁺ from a fresh (C : N = 15.4) and a mature (C : N = 9.2) organic household waste compost. Compost samples were mixed with quartz sand to ensure aerobic conditions, incubated at 5, 10, 15, 20, and 25°C and irrigated weekly for 112 days. For the fresh compost, cumulative CO₂ evolution after 112 days ranged from 36% of the initial C content at 5°C to 54% at 25°C. The CO₂ evolution was only small in the experiments with mature compost (1 to 6% of the initial C content). The data were described satisfactorily by a combined first‐order (fresh compost) or a first‐order kinetic model (mature compost). For the fresh compost, cumulative DOC production was negatively related to the temperature, probably due to leaching of some of the partly metabolized easily degradable fractions at lower temperatures. The production ratios of DOC : CO₂‐C decreased with increasing temperature from 0.094 at 5°C to 0.038 at 25°C for the fresh and from 1.55 at 5°C to 0.26 at 25°C for the mature compost. In the experiments with fresh compost, net release of NO₃^— occurred after a time lag which depended on the temperature. Cumulative net release of NO₃^— after 112 days ranged from 1.8% of the initial N content at 5°C to 14.3% at 25°C. Approximately 10% of the initial N content of the mature compost was released as NO₃^— after 14 days at all temperatures. The DOC : DON ratios in the experiments using fresh compost ranged from 11.5 to 15.7 and no temperature dependency was observed. For the mature compost, DOC : DON ratios were slightly smaller (7.4 to 8.9). The DON : (NH₄⁺ + NO₃^—) ratio decreased with increasing temperature from 0.91 at 5°C to 0.19 at 25°C for the fresh compost and from 0.21 at 5°C to 0.12 at 25°C for the mature compost. The results of the dynamics of C and N mineralization of fresh and mature compost can be used to assess the appropriate application (timing and amount) of compost to soils.     

Application of wood ash accelerates soil respiration and tree growth on drained peatland

Forested peatlands contain large pools of terrestrial carbon. As well as drainage, forest management such as fertilizer application can affect these pools. We studied the effect of wood ash (application rates 0, 5 and 15 t ha^?1) on the heterotrophic soil respiration (CO₂ efflux), cellulose decomposition, soil nutrients, biomass production and amount of C accumulated in a tree stand on a pine‐dominated drained mire in central Finland. The ash was spread 13 years before the respiration measurements. The annual CO₂ efflux was statistically modelled using soil temperature as the driving variable. Wood ash application increased the amounts of mineral nutrients of peat substantially and increased soil pH in the uppermost 10 cm layer by 1.5–2 pH units. In the surface peat, the decomposition rate of cellulose in the ash plots was roughly double that in control plots. Annual CO₂ efflux was least on the unfertilized site, 238 g CO₂‐C m^?2 year^?1. The use of wood ash nearly doubled CO₂ efflux to 420–475 g CO₂‐Cm^?2 year^?1 on plots fertilized with 5–15 t ha^?1 of ash, respectively. Furthermore, ash treatments resulted also in increased stand growth, and during the measurement year, the growing stand on ash plots accumulated carbon 11–12 times faster than the control plot. The difference between peat C emission and amount of C sequestered by trees on the ash plots was 43–58 g C m^?2, while on the control plot it was 204 g C m^?2. Our conclusion is that adding wood ash as a fertilizer increases more C sequestration in the tree stand than C efflux from the peat.     

Stoichiometric constraints on the microbial processing of carbon with soil depth along a riparian hillslope

Soil organic matter (SOM) content is a key indicator of riparian soil functioning and in the provision of ecosystem services such as water retention, flood alleviation, pollutant attenuation and carbon (C) sequestration for climate change mitigation. Here, we studied the importance of microbial biomass and nutrient availability in regulating SOM turnover rates. C stabilisation in soil is expected to vary both vertically, down the soil profile and laterally across the riparian zone. In this study, we evaluated the influence of five factors on C mineralisation (C_min): (i) substrate quantity, (ii) substrate quality, (iii) nutrient (C, N and P) stoichiometry, (iv) soil microbial activity with proximity to the river (2 to 75 m) and (v) as a function of soil depth (0–3 m). Substrate quality, quantity and nutrient stoichiometry were evaluated using high and low molecular weight ¹⁴C-labelled dissolved organic (DOC) along with different nutrient additions. Differences in soil microbial activity with proximity to the river and soil depth were assessed by comparing initial (immediate) C_min rates and cumulative C mineralised at the end of the incubation period. Overall, microbial biomass C (MBC), organic matter (OM) and soil moisture content (MC) proved to be the major factors controlling rates of C_min at depth. Differences in the immediate and medium-term response (42 days) of C_min suggested that microbial growth increased and carbon use efficiency (CUE) decreased down the soil profile. Inorganic N and/or P availability had little or no effect on C_min suggesting that microbial community growth and activity is predominantly C limited. Similarly, proximity to the watercourse also had relatively little effect on C_min. This work challenges current theories suggesting that areas adjacent to watercourse process C differently from upslope areas. In contrast, our results suggest that substrate quality and microbial biomass are more important in regulating C processing rates rather than proximity to a river.     

The influence of temperature and water table position on carbon dioxide and methane emissions from laboratory columns of peatland soils

Laboratory columns (80 cm long, 10 cm diameter) of peat were constructed from samples collected from a subarctic fen, a temperate bog and a temperate swamp. Temperature and water table position were manipulated to establish their influence on emissions of CO₂ and CH₄ from the columns. A factorial design experiment revealed significant (P < 0.05) differences in emission of these gases related to peat type, temperature and water table position, as well as an interaction between temperature and water table. Emissions of CO₂ and CH₄ at 23°C were an average of 2.4 and 6.6 times larger, respectively, than those at 10°C. Compared to emissions when the columns were saturated, water table at a depth of 40 cm increased CO₂ fluxes by an average of 4.3 times and decreased CH₄ emissions by an average of 5.0 times. There were significant temporal variations in gas emissions during the 6-week experiment, presumably related to variations in microbial populations and substrate availability. Using columns with static water table depths of 0, 10, 20, 40 and 60 cm, CO₂ emissions showed a positive, linear relation with depth, whereas CH₄ emissions revealed a negative, logarithmic relation with depth. Lowering and then raising the water table from the peat surface to a depth of 50 cm revealed weak evidence of hysteresis in CO₂ emissions between the falling and rising water table limbs. Hysteresis (falling > rising limb) was very pronounced for CH₄ emissions, attributed to a release of CH₄ stored in porewater and a lag in the development of anaerobic conditions and methanogenesis on the rising limb. Decreases in atmospheric pressure were correlated with abnormally large emissions of CO₂ and CH₄ on the falling limb. Peat slurries incubated in flasks revealed few differences between the three peat types in the rates of CO₂ production under aerobic and anaerobic conditions. There were, however, major differences between peat types in the rates of CH₄ consumption under aerobic incubation conditions and CH₄ production under anaerobic conditions (bog > fen > swamp), which explain the differences in response of the peat types in the column experiment.