Carbon sequestration in forest soils: effects of soil type, atmospheric CO2 enrichment, and N deposition

Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO₂ and to increased N deposition is not completely understood. We studied the effects of CO₂ concentrations at 370 and 570 μmol CO₂ mol^?1 air and increased N deposition (7 against 0.7 g N m^?2 year^?1) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open‐top chambers containing an acidic loam and a calcareous sand. The added CO₂ was depleted in ¹³C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m^?2) exceeded that in the calcareous sand (340 ± 40 g C m^?2) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m^?2 in the acidic loam and for 2800 ± 170 g C m^?2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico‐chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m^?2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO₂ enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO₂ concentrations increased C contents by 50–170 g C m^?2. Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO₂ concentrations had no significant impact on soil C concentrations. Up to 40% of the ‘new’ carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO₂‐derived ‘new’ C was incorporated into the labile and easily mineralizable pool in the soil.     

Quality and potential biodegradability of soil organic matter preserved in permafrost of Siberian tussock tundra

Soil organic matter (SOM) quality and biodegradability the permafrost underlying Siberian wet tussock tundra (Kolyma river basin, northeast Siberia) were analyzed and compared to the characteristics of the contemporary active layer. For this purpose, three permafrost affected soil cores (down to 3 m depth) and seven active layer soil cores (down to 0.3 m depth) were sampled. The samples were divided into particular layers, which were analyzed separately. SOM stability was assessed using a simple chemical fractionation (sequential extraction by cold and hot water, and hot acid). SOM biodegradability and soil mineralization potentials were tested in short-term laboratory incubations. The active layer contained 24 kg C m⁻² and 70 kg C m⁻² was preserved in 3 m of permafrost. The chemical quality and biodegradability of permafrost SOM were very similar to that of the active layer mineral horizon, and independent from depth. The only exceptions were (1) higher solubility of permafrost SOM in water, indicating its higher mobility and potential leakage after permafrost thawing and (2) higher nutrient (N, P) concentrations available to a dense permafrost microbial community, which could support decomposition of more complex substrates under suitable temperature conditions after thawing. The mineralization potential of the upper 1 m deep permafrost, which could melt by 2100 according to permafrost degradation models, was 6.7 g C m⁻² d⁻¹ (optimum conditions of 20 °C, field water capacity), which is comparable to that of the contemporary active layer of 0.5 m depth (7.5 g C m⁻² d⁻¹). Under field conditions, SOM mineralization rate would reasonably be significantly lower due to prevailing anoxia (high water table) and diffusion constraints in the deep and flooded soil profile.We conclude from our results that the permafrost (1) cryopreserves a high SOM amount, which is distributed to considerable depths, being of similar chemical quality and biodegradability to that of the active layer mineral horizon SOM, and (2) contains a dense living microbial community, which is able to decompose the present SOM rapidly without any obvious chemical limitation under suitable conditions.     

Annual soil CO2 effluxes in the High Arctic: The role of snow thickness and vegetation type

Little work has been done to quantify annual soil CO₂ effluxes in the High Arctic region because of the difficulty in taking winter measurements. Since the effects of climate change are expected to be higher in Arctic than in temperate ecosystems, it is important that summer measurements are extended to cover the entire year. This study evaluates the quantity and quality of soil organic C (SOC) and seasonal controls of soil CO₂ effluxes in three soils under three dominating types of vegetation (Dryas, Cassiope, and Salix) at Svalbard. Measurements included soil CO₂ effluxes in the field and the laboratory, temperature, water content, and snow thickness. About 90% of the variation in soil respiration throughout 1 year was due to near-surface soil temperatures which ranged from −12 to +12 °C. Total annual soil CO₂ effluxes varied from 103 g C m⁻² at soils under Cassiope, 152 g C m⁻² under Dryas sites, and 176 g C m⁻² under Salix, with 20%, 14%, and 30%, respectively, being released during a 6-month winter period. The sensitivity of soil respiration with respect to soil temperature was the same year round and differences in winter CO₂ effluxes at the three vegetation types were mainly related to subsurface soil temperatures controlled by snow depth. The quantity and quality of soil organic matter varied under the different vegetation types. Soils under Salix had the largest and most labile pool of SOC and were characterized by a long period of snow cover. In contrast, soils under Cassiope were more nutrient-poor, more acidic and held the smallest amount of total and labile SOC, whereas soils under Dryas remained snow-free most of the winter and therefore had the coldest winter conditions. Thus, winter soil respiration rates under Dryas and Cassiope were significantly lower than those under Salix; under Dryas this was mainly due to snow depth, under Cassiope this was a combination of snow depth and poor litter quality. It is concluded that winter respiration is highly variable across Arctic landscapes and depends on the spatial distribution of snow, which acts as a direct control on soil temperatures and indirect on vegetation types and thereby, the amount and quality of soil organic matter, which serve as additional important drivers of soil respiration.     

Methane fluxes from three ecosystems in tropical peatland of Sarawak, Malaysia

Methane fluxes were measured monthly over a year from tropical peatland of Sarawak, Malaysia using a closed-chamber technique. The CH₄ fluxes in forest ecosystem ranged from −4.53 to 8.40 μg C m⁻² h⁻¹, in the oil palm ecosystem from −32.78 to 4.17 μg C m⁻² h⁻¹ and in the sago ecosystem from −7.44 to 102.06 μg C m⁻² h⁻¹. A regression tree approach showed that CH₄ fluxes in each ecosystem were related to different underlying environmental factors. They were relative humidity for forest and water table for both sago and oil palm ecosystems. On an annual basis, both forest and sago were CH₄ source with an emission of 18.34 mg C m⁻² yr⁻¹ for forest and 180 mg C m⁻² yr⁻¹ for sago. Only oil palm ecosystem was a CH₄ sink with an uptake rate of −15.14 mg C m⁻² yr⁻¹. These results suggest that different dominant underlying environmental factors among the studied ecosystems affected the exchange of CH₄ between tropical peatland and the atmosphere.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Long-term effects of metal-containing farmyard manure and sewage sludge on soil organic matter in a fluvisol

Our aim was to establish the long-term effects of repeated applications after 20 y of organic amendments (farmyard manure at 10 t ha⁻¹ y⁻¹, and urban sewage sludge at two different rates, 10 t ha⁻¹ y⁻¹ and 100 t ha⁻¹ every 2 y) on the quality of a sandy and poorly buffered soil (Fluvisol, pH 6). Chemical characteristics and biodegradability of the labile organic matter, which is mainly derived from microbial biomass and biodegradation products of organic residues, were chosen as indicators for soil quality. The organic C content had reached a maximal value (30.6 g C kg⁻¹ in the 100 t sludge-treated soil), i.e. about 2.5 times that in the control. Six years after the last application, the organic C content and the microbial biomass content remained higher in sludge-treated soils than in the control. In contrast, the proportion of labile organic matter was significantly lower in sludge-treated soils than in manure-treated and control soils. The labile organic matter of sludge extracts appeared less humified than that of manure-treated and control soils.     

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

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

CO2 emission in an intensively cultivated loam as affected by long-term application of organic manure and nitrogen fertilizer

A long-term field experiment was conducted to examine the influence of mineral fertilizer and organic manure on the equilibrium dynamics of soil organic C in an intensively cultivated fluvo-aquic soil in the Fengqiu State Key Agro-Ecological Experimental Station (Fengqiu county, Henan province, China) since September 1989. Soil CO₂ flux was measured during the maize and wheat growing seasons in 2002-2003 and 2004 to evaluate the response of soil respiration to additions and/or alterations in mineral fertilizer, organic manure and various environmental factors. The study included seven treatments: organic manure (OM), half-organic manure plus half-fertilizer N (NOM), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (PK) and control (CK). Organic C in soil and the soil heavy fraction (organo-mineral complex) was increased from 4.47 to 8.61 mg C g⁻¹ and from 3.32 to 5.68 mg C g⁻¹, respectively, after the 13 yr application of organic manure. In contrast, organic C and the soil heavy fraction increased in NPK soil to only 5.41 and 4.38 mg C g⁻¹, respectively. In the CK treatment, these parameters actually decreased from the initial C concentrations (4.47 and 3.32 mg C g⁻¹) to 3.77 and 3.11 mg C g⁻¹, respectively. Therefore, organic manure efficiently elevated soil organic C. However, only 66% of the increased soil organic C was combined with clay minerals in the OM treatment. Cumulative soil CO₂ emissions from inter-row soil in the OM and NPK treatments were 228 and 188 g C m⁻² during the 2002 maize growing season, 132 and 123 g C m⁻² during the 2002/2003 wheat growing season, and 401 and 346 g C m⁻² yr⁻¹ in 2002-2003, respectively. However, during the 2004 maize growing season, cumulative soil CO₂ emissions were as high as 617 and 556 g C m⁻², respectively, due to the contribution of rhizosphere respiration. The addition of organic manure contributed to a 16% increase in soil CO₂ emission in 2002-2003 (compared to NPK), where only 27%, 36% and 24% of applied organic C was released as CO₂ during the 2002 and 2004 maize growing seasons and in 2002-2003, respectively. During the 2002/2003 wheat growing season, soil CO₂ flux was significantly affected by soil temperature below 20 °C, but by soil moisture (WFPS) during the 2004 maize growing season at soil temperatures above 18 °C. Optimum soil WFPS for soil CO₂ flux was approximately 70%. When WFPS was below 50%, it no longer had a significant impact on soil CO₂ flux during the 2002 maize growing season. This study indicates the application of organic manure composted with wheat straw may be a preferred strategy for increasing soil organic C and sequestering C in soil.     

Influence of earthworm activity on aggregate-associated carbon and nitrogen dynamics differs with agroecosystem management

Earthworms are known to be important regulators of soil structure and soil organic matter (SOM) dynamics, however, quantifying their influence on carbon (C) and nitrogen (N) stabilization in agroecosystems remains a pertinent task. We manipulated population densities of the earthworm Aporrectodea rosea in three maize-tomato cropping systems [conventional (i.e., mineral fertilizer), organic (i.e., composted manure and legume cover crop), and an intermediate low-input system (i.e., alternating years of legume cover crop and mineral fertilizer)] to examine their influence on C and N incorporation into soil aggregates. Two treatments, no-earthworm versus the addition of five A. rosea adults, were established in paired microcosms using electro-shocking. A ¹³C and ¹⁵N labeled cover crop was incorporated into the soil of the organic and low-input systems, while ¹⁵N mineral fertilizer was applied in the conventional system. Soil samples were collected during the growing season and wet-sieved to obtain three aggregate size classes: macroaggregates (>250 μm), microaggregates (53-250 μm) and silt and clay fraction (<53 μm). Macroaggregates were further separated into coarse particulate organic matter (cPOM), microaggregates and the silt and clay fraction. Total C, ¹³C, total N and ¹⁵N were measured for all fractions and the bulk soil. Significant earthworm influences were restricted to the low-input and conventional systems on the final sampling date. In the low-input system, earthworms increased the incorporation of new C into microaggregates within macroaggregates by 35% (2.8 g m⁻² increase; P=0.03), compared to the no-earthworm treatment. Within this same cropping system, earthworms increased new N in the cPOM and the silt and clay fractions within macroaggregates, by 49% (0.21 g m⁻²; P<0.01) and 38% (0.19 g m⁻²; P=0.02), respectively. In the conventional system, earthworms appeared to decrease the incorporation of new N into free microaggregates and macroaggregates by 49% (1.38 g m⁻²; P=0.04) and 41% (0.51 g m⁻²; P=0.057), respectively. These results indicate that earthworms can play an important role in C and N dynamics and that agroecosystem management greatly influences the magnitude and direction of their effect.     

Microbial activity in pig slurry-amended soils under semiarid conditions

Soil amendment with manures from intensive animal industries is nowadays a common practice that may favorably or adversely affect several soil properties, including soil microbial activity. In this work, the effect of consecutive annual additions of pig slurry (PS) at rates of 30, 60, 90, 120 and 150 m³ ha⁻¹ y⁻¹ over a 4-year period on soil chemical properties and microbial activity was investigated and compared to that of an inorganic fertilization and a control (without amendment). Field plot experiment conducted under a continuous barley monoculture and semiarid conditions were used. Eight months after the fourth yearly PS and mineral fertilizer application (i.e. soon after the fourth barley harvest), surface soil samples (Ap horizon, 0-15 cm depth) from control and amended soils were collected and analysed for pH, electrical conductivity (EC), contents of total organic C, total N, available P and K, microbial biomass C, basal respiration and different enzymatic activities. The control soil had a slightly acidic pH (6.0), a small EC (0.07 dS m⁻¹), adequate levels of total N (1.2 g kg⁻¹) and available K (483 mg kg⁻¹) for barley growth, and small contents of total organic C (13.2 g kg⁻¹) and available P (52 mg kg⁻¹). With respect to the control and mineral fertilized soils, the PS-amended soils had greater pH values (around neutrality or slightly alkaline), electrical conductivities (still low) and contents of available P and K, and slightly larger total N contents. A significant decrease of total organic C was observed in soils amended at high slurry rate (12.3 g kg⁻¹). Compared with the control and mineral treatments, which produced almost similar results, the PS-amended soils were characterized by a higher microbial biomass C content (from 311 to 442 g kg⁻¹), microbial biomass C/total organic C ratio (from 2.3 to 3.6%) and dehydrogenase (from 35 to 173 μg INTF g⁻¹), catalase (from 5 to 24 μmol O₂ g⁻¹ min⁻¹), BAA-protease (from 0.7 to 1.9 μmol  g⁻¹ h⁻¹) and β-glucosidase (from 117 to 269 μmol PNP g⁻¹ h⁻¹) activities, similar basal respirations (from 48 to 77 μg C-CO₂ g⁻¹ d⁻¹) and urease activities (from 1.5 to 2.2 μmol  g⁻¹ h⁻¹), and smaller metabolic quotients (from 6.4 to 7.7 ng C-CO₂ μg⁻¹ biomass C h⁻¹) and phosphatese activities (from 374 to 159 μmol PNP g⁻¹ h⁻¹). For example, statistical analysis of experimental data showed that, with the exception of metabolic quotient and total organic C content, these effects generally increased with increasing cumulative amount of PS. In conclusion, cumulative PS application to soil over time under semiarid conditions may produce not only beneficial effects but also adverse effects on soil properties, such us the partial mineralization of soil organic C through extended microbial oxidation. Thus, PS should not be considered as a mature organic amendment and should be treated appropriately before it is applied to soil, so as to enhance its potential as a soil organic fertilizer.     

Measuring microbial uptake of nitrogen in nutrient-amended sandy soils—A mass-balance based approach

Soil microbial biomass N is commonly determined through fumigation-extraction (FE), and a conversion factor (K_EN) is necessary to convert extractable N to actual soil biomass N. Estimation of K_EN has been constrained by various uncertainties including potential microbial immobilisation. We developed a mass-balance approach to quantify changes in microbial N storage during nutrient-amended incubation, in which microbial uptake is determined as the residual in a ‘mass-balance’ based on soil-water N before and after amended incubation. The approach was applied to three sandy soils of southwestern Australia, to determine microbial N immobilisation during 5-day incubation in response to supply of 2.323 mg C g⁻¹, 100 μg N g⁻¹ and 20 μg P g⁻¹. The net N immobilisation was estimated to be 95-114 μg N g⁻¹ in the three soils, equivalent to 82.7-85.1% of soil-water N following the amendment. Such estimation for microbial uptake does not depend on fumigation and K_EN conversion, but for comparison purposes we estimated ‘nominal’ K_EN values (0.11-0.14) for the three soils, which were comparable to previously reported K_EN from soils receiving C and N amendment. The accuracy of our approach depends on the mass-balance equation and the integrated measurement errors of the multiple N pools, and was assessed practically through recoveries of added-N when microbial uptake can be minimised. Near-satisfactory recoveries were achieved under such conditions. Our mass-balance approach provides information not only about changes in the microbial biomass nitrogen storage, but also major N-pools and their fluxes in regulating soil N concentrations under substrate and nutrient amended conditions.     

Subtropical plantations are large carbon sinks: Evidence from two monoculture plantations in South China

Quantifying the net carbon (C) storage of forest plantations is required to assess their potential to offset fossil fuel emissions. In this study, a biometric approach was used to estimate net ecosystem productivity (NEP) for two monoculture plantations in South China: Acacia crassicarpa and Eucalyptus urophylla. This approach was based on stand-level net primary productivity (NPP, based on direct biometric inventory) and heterotrophic respiration (R_h). In comparisons of R_h determination based on trenching vs. tree girdling, both trenching and tree girdling changed soil temperature and soil moisture relative to undisturbed control plots, and we assess the effects of corrections for disturbances of soil moisture and soil moisture on the estimation of soil CO₂ efflux partitioning. Soil microbial biomass and dissolved organic carbon were significantly lower in trenched plots than in tree girdled plots for both plantations. Annual soil CO₂ flux in trenched plots (R_h-t) was significantly lower than in tree-girdled plots (R_h-g) in both plantations. The estimates of R_h-t and R_h-g, expressed as a percentage of total soil respiration, were 58 ± 4% and 74 ± 6%, respectively, for A. crassicarpa, and 64 ± 3% and 78 ± 5%, respectively, for E. urophylla. By the end of experiment, the difference in soil CO₂ efflux between the trenched plots and tree-girdled plots had become small for both plantations. Annual R_h (mean of the annual R_h-t and R_h-g) and net primary production (NPP) were 470 ± 25 and 800 ± 118 g C m⁻² yr⁻¹, respectively, for A. crassicarpa, and 420 ± 35 and 2380 ± 187 g C m⁻² yr⁻², respectively, for E. urophylla. The two plantations in the developmental stage were large carbon sinks: NEP was 330 ± 76 C m⁻² yr⁻¹ for A. crassicarpa and 1960 ± 178 g C m⁻² yr⁻¹ for E. urophylla.     

A mass-balance approach to measuring microbial uptake and pools of phosphorus in nutrient-amended soils

Soil microbial biomass P is usually determined through fumigation-extraction (FE), in which partially extractable P from lysed biomass is converted to biomass P using a conversion factor (K_p). Estimation of K_p has been usually based on cultured microorganisms, which may not adequately represent the soil microbial community in either nutrient-poor or in altered carbon and nutrient conditions following fertilisation. We report an alternative approach in which changes in microbial P storage are determined as the residual in a mass balance of extractable P before and after incubation. This approach was applied in three low-fertility sandy soils of southwestern Australia, to determine microbial P immobilisation during 5-day incubations in response to the amendment by 2.323 mg C g⁻¹, 100 μg N g⁻¹ and 20 μg P g⁻¹. The net P immobilisation during the amended incubations determined to be 18.1, 14.1 and 16.3 μg P g⁻¹ in the three soils, accounting for 70.6-90.5% of P added through amendment. Such estimates do not rely on fumigation and K_p values, but for comparison with the FE method we estimated ‘nominal’ K_p values to be 0.20-0.31 for the soils under the amended conditions. Our results showed that microbial P immobilisation was a dominant process regulating P concentration in soil water following the CNP amendment. The mass-balance approach provides information not only about changes in the microbial P compartment, but also about other major P-pools and their fluxes in regulating soil-water P concentrations under substrate- and nutrient-amended conditions.     

Assessing the uncertainty of estimated annual totals of net ecosystem productivity: A practical approach applied to a mid latitude temperate pine forest

Values for annual NEP of micrometeorological tower sites are usually published without an estimate of associated uncertainties. Few authors quantify total uncertainty of annual NEP. Moreover, different methods to assess total uncertainty are applied, usually addressing only one aspect of the uncertainty. This paper presents a robust and easy to apply method to quantify uncertainty of annual totals of Net Ecosystem Productivity (NEP), related to multiple factors involved therein. The method was applied to NEP observations for a Scots pine forest (Loobos) in the Netherlands. Total uncertainty of annual NEP for the Loobos site was on average ±32 g C m⁻² a⁻¹ (±8% of NEP), which is a quarter of the standard deviation of annual NEP (127 g C m⁻² a⁻¹).     

Soil organic C stabilization and thresholds in C saturation

When building soil organic matter (SOM) contents in agricultural production systems, stabilization of both pre-existing as well as added C is important. A laboratory mineralization experiment was conducted over 374 days to evaluate the effect of pre-existing SOM on soil C mineralization after addition of organic matter (OM) using sugar cane. The SOM gradient used here stretched from 21 to 106 g C kg⁻¹ soil and was a result of different periods of continuous cultivation of 5, 20, 35 and 105 years in comparison to a forest soil. The rate of organic C mineralization was found to be dependent on the status of pre-existing soil organic C (SOC). Highly degraded soil which had been under continuous cultivation for 35 years and more showed the highest rate of C mineralization per unit SOC (117.9 mg C g⁻¹ C) while forest soil had the lowest amount of C mineralized per unit SOC (73.5 mg C g⁻¹ C). Forest soil had the highest amount of increased C mineralization as a result of organic matter (OM) additions (8.0 mg C g⁻¹ soil) followed by the highly degraded soil that had been under cultivation for 105 years (5.5 mg C g⁻¹ soil). Additional mineralized C as a function of time after forest conversion declined progressively within the first 20 years of continuous soil use. Soil which had been under continuous cultivation for 20 years had the lowest amount of additional mineralized C (4.0 mg C g⁻¹ soil). SOM stabilization efficiency in the studied soils appears to be highest with intermediate cultivation history of about 20 years. These soils that have been recently converted to cultivation also appear to have a greater ability to stabilize added OM than the most degraded soils investigated in this study. It is thus advisable to provide intervention strategies to reverse SOM decline for farming communities at an intermediate stage before the soils are highly depleted of SOC.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm⁻³ (free POM_<1.6), occluded particulate organic matter with densities <1.6 g cm⁻³ (occluded POM_<1.6) and 1.6-2.0 g cm⁻³ (occluded POM_1.6-2.0) and mineral-associated SOM (>2.0 g cm⁻³) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the ¹³C/¹²C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM_<1.6>occluded POM_<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.     

Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils

Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH₄¹⁵NO₃ (6.2 g N m⁻², 25 atom % excess ¹⁵N) was applied, and recovery of ¹⁵N in the pool taken as indicative of occurrence of DNRA. Up to 5% of applied ¹⁵N was recovered in the pool 2 d after addition of N, glucose (44.6 g C m⁻²) and l-cysteine (7.7 g m⁻², 0.9 g N m⁻², 2.3 g C m⁻²). concentrations were positively correlated with soil pH, ratio, bulk density, sand content and concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source.     

Humus accumulation and microbial activities in calcari-epigleyic fluvisols under grassland and forest diked in for 30 years

The accumulation and transformation of organic matter during soil development is rarely investigated although such processes are relevant when discussing about carbon sequestration in soil. Here, we investigated soils under grassland and forest close to the North Sea that began its genesis under terrestrial conditions 30 years ago after dikes were closed. Organic C contents of up to 99 mg g⁻¹ soil were found until 6 cm soil depth. The humus consisted mainly of the fraction lighter than 1.6 g cm⁻³ which refers to poorly degraded organic carbon. High microbial respiratory activity was determined with values between 1.57 and 1.17 μg CO₂-C g⁻¹ soil h⁻¹ at 22 °C and 40 to 70% water-holding capacity for the grassland and forest topsoils, respectively. The microbial C to organic C ratio showed values up to 20 mg C_mic g⁻¹ C_org. Although up to 2.69 kg C m⁻² were estimated to be sequestered during 30 years, the microbial indicators showed intensive colonisation and high transformation rates under both forest and grassland which were higher than those determined in agricultural and forest topsoils in Northern Germany.     

Carbon exchange in a freshwater marsh in the Sanjiang Plain, northeastern China

Northern wetlands are critically important to global change because of their role in modulating atmospheric concentrations of greenhouse gases, especially CO₂ and CH₄. At present, continuous observations for CO₂ and CH₄ fluxes from northern wetlands in Asia are still very limited. In this paper, two growing season measurements for CO₂ flux by eddy covariance technique and CH₄ flux by static chamber technique were conducted in 2004 and 2005, at a permanently inundated marsh in the Sanjiang Plain, northeastern China. The seasonal variations of CO₂ exchange and CH₄ flux and the environmental controls on them were investigated. During the growing seasons, large variations in net ecosystem CO₂ exchange (NEE) and gross ecosystem productivity (GEP) were observed with the range of −4.0 to 2.2 (where negative exchange is a gain of carbon from the atmosphere) and 0-7.6 g C m⁻² d⁻¹, respectively. Ecosystem respiration (RE) displayed relatively smooth seasonal pattern with the range of 0.8-4.2 g C m⁻² d⁻¹. More than 70% of the total GEP was consumed by respiration, which resulted in a net CO₂ uptake of 143 ± 9.8 and 100 ± 9.2 g C m⁻² for the marsh over the growing seasons of 2004 and 2005, respectively. A significant portion of the accumulated NEE-C was lost by CH₄ emission during the growing seasons, indicating the great potential of CH₄ emission from the inundated marsh. Air temperature and leaf area index jointly affected the seasonal variation of GEP and the seasonal dynamic of RE was mainly controlled by soil temperature and leaf area index. Soil temperature also exerted the dominant influence over variation of CH₄ flux while no significant relationship was found between CH₄ emission and water table level. The close relationships between carbon fluxes and temperature can provide insights into the response of marsh carbon exchange to a changing climate. Future long term flux measurements over the freshwater marsh ecosystems are undoubtedly necessary.