Estimation of evaporation using a dual-beam surface layer scintillometer and component energy balance measurements

A dual-beam surface layer scintillometer (SLS), for the estimation of sensible heat flux density H for a path length of 101 m, was used in a mixed grassland community in the eastern seaboard of South Africa for 30 months. Measurements also included Bowen ratio (BR) and eddy covariance (EC) estimates of H. Acceptable SLS data between 0600 h and 1800 h, judged by the percent of error-free 1 kHz data exceeding 25% and an inner scale of turbulence exceeding 2 mm, showed little seasonal variation and was consistently high—between 86.7% and 94.8%. An analysis of the various Monin–Obukhov similarity theory (MOST) empirical dimensionless stability functions used for estimating H from the SLS measurements showed percent differences in H that varied from ?30% to 28% for neutral to unstable conditions, respectively and for stable continuous conditions the differences in H were within 60 W m^?2 with much larger differences for stable sporadic conditions. The good agreement in measurements of H over an extended period for the SLS, BR and EC methods demonstrates the applicability and robustness of the SLS method and the associated MOST empirical functions used for estimating H for a range of canopy heights, stability conditions and diurnal and seasonal weather conditions. Furthermore, there was no evidence for an underestimation in EC sensible heat compared to SLS and BR measurements, which implies that any lack of energy balance closure points to possible latent energy EC underestimation or due to energy fluxes not included in the shortened energy balance if the net irradiance and soil heat flux components are correct. A sensitivity analysis was used to determine the relative importance of the SLS data inputs of air temperature, atmospheric pressure, beam path length and beam height on H estimates. Worst-case errors in air temperature, atmospheric pressure, beam path length and beam height resulted in errors in H within 1.0%, 1.3%, 3.0% and 4.0%, respectively. Overall, the worst-case total percent error in SLS-estimated H is within 5.3% and the typical percent error is within 3.9%. Accounting for the error in net irradiance and soil heat flux measurements, the seasonal variation in the error in daily evaporation estimated as a residual of the energy balance is generally less than 0.2 mm (0.49 MJ m^?2) in winter when the daily evaporation was about 1 mm (2.45 MJ m^?2) and typically less than 0.4 mm (0.98 MJ m^?2) when the evaporation exceeded 4 mm (9.8 MJ m^?2). Soil heat flux density measurements can contribute significantly to the overall error.     

Multiple measurements constrain estimates of net carbon exchange by a Eucalyptus forest

Net ecosystem exchange of carbon (F_NEE) was estimated for a temperate broadleaf, evergreen eucalypt forest ecosystem at Tumbarumba in south-eastern Australia to investigate the processes controlling forest carbon sinks and their response to climate. Measurements at a range of temporal and spatial scales were used to make three different estimates of F_NEE based on: (1) the difference between fluxes of carbon input by photosynthesis and output by autotrophic plus heterotrophic respiration, (2) changes over time in the carbon pools in the above- and below-ground biomass, soil and litter, and (3) micrometeorological flux measurements that provide a continuous estimate of the net exchange. A rigorous comparison of aggregated component fluxes and the net eddy fluxes within a flux tower source area was achieved based on an inventory of the site and a detailed sampling strategy. Measurements replicated in space and time provided mean values, confidence limits and patterns of variation of carbon pools and fluxes that allowed comparisons within known limits of uncertainty. As a result of comparisons between nighttime eddy flux and chamber measurements of respiration, a revised micrometeorological method was developed for estimating nighttime carbon flux using flux tower measurements. Uncertainty in the final estimate of F_NEE was reduced through mutual constraints of each of these measurement approaches. F_NEE for the period October 2001–September 2002, with average rainfall, was an uptake of 6.7 (5.1–8.3) tC ha^?1 yr^?1 estimated from component fluxes, and 5.4 (3.0–7.5) tC ha^?1 yr^?1 estimated from the revised eddy flux method. Biomass increment was 4.5 (3.7–5.4) tC ha^?1 yr^?1 and the remaining 0.9–2.2 tC ha^?1 yr^?1 could represent a carbon sink in the soil and litter pools or lie within the confidence limits of the measured fluxes. F_NEE was reduced to ?0.1 to 2.4 tC ha^?1 yr^?1 during a period of drought and insect disturbance in October 2002–September 2003, with biomass increment being the main component reduced. The forest is a large carbon sink compared with other forest ecosystems, but this is subject to high-annual variability in response to climate variability and disturbance.     

Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season

The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ, rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO₂ pulses increased from 0.84 to 2.08 g CO₂–C m^? 2 d^? 1 with the intensity of wetting up to ca. 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH₄–C m^? 2 d^? 1 with the intensity of wetting. Wetting dry forest floor increased N₂O fluxes from 6.2 to 25.9 μg N₂O–N m^? 2 d^? 1, but there was no significant difference between all experimental wetted plots. The rainfall-induced N₂O pulses with increasing wetting intensity were opposite to that of the CO₂ pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO₂ flux and CH₄ uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q₁₀ value of the temperature response of CO₂ fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO₂ pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.     

Impact of agriculture,forest and cloud feedback on the surface energy budget in BOREAS

We explore the impact of agriculture, forest and cloud feedback on the surface energy budget using data obtained using a research aircraft, mesonet towers and model data. The forest has an order of magnitude larger roughness length, a lower albedo, a much smaller seasonal cycle in surface Bowen ratio (BR) and a weak mid-summer maximum of CO₂ uptake compared to agricultural areas, which have much smaller BR and much higher mid-summer CO₂ uptake, but a net CO₂ release and much reduced evaporation in spring and fall. Higher surface temperatures and the higher albedo over agricultural land reduce R_net near local noon in the warm season by about 50 W m⁻² in comparison with the adjacent boreal forest. The annual averaged R_net, derived from 2 years of tower data, is 14 W m⁻² less over grass sites than over forest sites. A reanalysis time-series for the BOREAS southern study area shows the coupling on daily timescales between the surface energy partition, the mean boundary layer depth, the cloud field and the long-wave and short-wave radiation fields. The albedo of the cloud field, the cloud short-wave forcing at the surface, varies over the range 0.1–0.8 with decreasing surface BR, and plays a major role in the surface energy budget. We estimate that this cloud feedback may increase albedo by 0.13 and reduce R_net by 25 W m⁻² in summer over agricultural land.     

Estimating heterotrophic and autotrophic soil respiration in a semi-natural forest of Lombardy,Italy

We studied a semi-natural forest in Northern Italy that was set aside more than 50 years ago, in order to better understand the soil carbon cycle and in particular the partitioning of soil respiration between autotrophic and heterotrophic respiration. Here we report on soil organic carbon, root density, and estimates of annual fluxes of soil CO₂ as measured with a mobile chamber system at 16 permanent collars about monthly during the course of a year. We partitioned between autotrophic and heterotrophic respiration by the indirect regression method, which enabled us to obtain the seasonal pattern of single components.The soil pool of organic carbon, with 15.8 (±4.5) kg m^?2, was very high over the entire depth of 45 cm. The annual respiration rates ranged from 0.6 to 6.9 μmol CO₂ m^?2 s^?1 with an average value of 3.4 (±2.3) μmol CO₂ m^?2 s^?1, and a cumulative flux of 1.1 kg C m^?2 yr^?1. The heterotrophic component accounted for 66% of annual CO₂ efflux. Soil temperature largely controlled the heterotrophic respiration (R² = 0.93), while the autotrophic component followed irradiation, pointing to the role of photosynthesis in modulating the annual course of soil respiration.Most studies on soil respiration partitioning indicate autotrophic root respiration as a first control of the spatial variability of the overall respiration, which originates mainly from the uppermost soil layers. Instead, in our forest the spatial variability of soil respiration was mainly linked to soil carbon, and deeper layers seemed to provide a significant contribution to soil respiration, a feature that may be typical for an undisturbed, naturally maturing ecosystem with well developed pedobiological processes and high carbon stocks.     

Evapotranspiration and water use efficiency in different-aged Pacific Northwest Douglas-fir stands

We used the eddy-covariance technique to measure evapotranspiration (E) and gross primary production (GPP) in a chronosequence of three coastal Douglas-fir (Pseudotsuga menziesii) stands (7, 19 and 58 years old in 2007, hereafter referred to as HDF00, HDF88 and DF49, respectively) since 1998. Here, we focus on the controls on canopy conductance (g_c), E, GPP and water use efficiency (WUE) and the effect of interannual climate variability at the intermediate-aged stand (DF49) and then analyze the effects of stand age following clearcut harvesting on these characteristics. Daytime dry-foliage Priestley–Taylor α and g_c at DF49 were 0.4–0.8 and 2–6 mm s^?1, respectively, and were linearly correlated (R² = 0.65). Low values of α and g_c at DF49 as well at the other two stands suggested stomatal limitation to transpiration. Monthly E, however, showed strong positive linear correlations to monthly net radiation (R² = 0.94), air temperature (R² = 0.77), and daytime vapour pressure deficit (R² = 0.76). During July–September, monthly E (mm) was linearly correlated to monthly mean soil water content (θ, m³ m^?3) in the 0–60 cm layer (E = 453θ ? 21, R² = 0.69), and GPP was similarly affected. Annual E and GPP of DF49 for the period 1998–2007 varied from 370 to 430 mm and from 1950 to 2390 g C m^?2, respectively. After clearcut harvesting, E dropped to about 70% of that for DF49 while ecosystem evapotranspiration was fully recovered when stand age was ～12 years. This contrasted to GPP, which varied hyperbolically with stand age. Monthly GPP showed a strong positive linear relationship with E irrespective of the stand age. While annual WUE of HDF00 and HDF88 varied with age from 0.5 to 4.1 g C m^?2 kg^?1 and from 2.8 to 4.4 g C m^?2 kg^?1, respectively, it was quite conservative at ～5.3 g C m^?2 kg^?1 for DF49. N-fertilization had little first-year response on E and WUE. This study not only provides important results for a more detailed validation of process-based models but also helps in predicting the influences of climate change and forest management on water vapour and CO₂ fluxes in Douglas-fir forests.     

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

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

Measurements of ozone deposition to a potato canopy

Potatoes are an important staple crop, grown in many parts of the world. Although ozone deposition to many vegetation types has been measured in the field, no data have been reported for potatoes. Such measurements, including the latent-heat flux, were made over a fully grown potato field in central Scotland during the summer of 2006, covering a 4-week period just after rainfall and then dry, sunny weather. The magnitude of the flux was typical of many canopies showing the expected diurnal cycles. Although the bulk-canopy stomatal conductance declined as the field dried out (～300 mmol-O₃ m^?2 s^?1 to ～70 mmol-O₃ m^?2 s^?1), the total ozone flux did not follow the same trend, indicating that non-stomatal deposition was significant. Over a dry surface non-stomatal resistance (R_ns) was 270–450 s m^?1, while over a wet surface R_ns was ～50% smaller and both decreased with increasing surface temperature and friction velocity. From the variation with relative humidity (RH) it is suggested that three processes occur on leaf surfaces: on a very dry surface ozone is removed by thermal decomposition, possibly enhanced by photolytic reactions in the daytime and so R_ns decreases as temperature increases; at 50–70% RH a thin film of liquid blocks the “dry” process and resistance increases; above 60–70% RH sufficient surface water is present for aqueous reactions to remove ozone and resistance decreases.     

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

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

Experimental drought alters rates of soil respiration and methanogenesis but not carbon exchange in soil of a temperate fen

The impact of intensified drought and rewetting on C cycling in peatlands is debated. We conducted drying/rewetting (DW) experiments with intact monoliths of a temperate fen over a period of 10 months. One treatment with original vegetation (DW-V) and one defoliated treatment (DW-D) were rewetted after an experimental drought of 50 days; another treatment was kept permanently wet (W-V). Soil water content was determined by the TDR technique, C fluxes from chamber measurements and gas profiles in the soils, and respiration from mass balancing CO₂ and CH₄ fluxes in the peat using hourly to weekly data. Zones of high root associated respiration were determined from a ¹³C labeling experiment. Autotrophic respiration contributed from 55 to 65% to an average ecosystem respiration (ER) of 92 (DW-D), 211 (DW-V), and 267 mmol m^?2 d^?1 (W-V). Photosynthesis ranged from 0 (DW-D) to 450 mmol m^?2 d^?1 (W-V), and strongly declined for about 30 days after rewetting (DW-V), while ER remained constant during the drying and rewetting event. Drying raised air-filled porosity in the soil to 2–13%, temporarily increased respiration to estimated anaerobic and aerobic rates of up to 550 and 1000 nmol cm^?3 d^?1, and delayed methane production and emission by weeks to months. Root associated respiration was concentrated in the uppermost peat layer. In spite of clear relative changes in respiration during and after drought, the impact on carbon exchange with the atmosphere was small. We attribute this finding to the importance of respiration in the uppermost and soil layer, which remained moist and aerated, and the insensitivity of autotrophic respiration to drought. We expect a similar dynamics to occur in other temperate wetland soils in which soil respiration is concentrated near the peatland surface, such as rich minerotrophic fens.     

Mechanisms of short-term soil carbon storage in experimental grasslands

We investigated the fate of root and litter derived carbon in soil organic matter and dissolved organic matter in soil profiles, in order to explain mechanisms of short-term soil carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment between 2002 and 2004. In addition to the main experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with ¹³C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g C m^?2 and 263 g C m^?2 to 20 cm depth, while 71 g C m^?2 and 393 g C m^?2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 115 g C m^?2 and 156 g C m^?2 of soil organic carbon were derived from C4 plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of 97 g C m^?2 that was newly stored in the same soil depth, whereas with double litter this clearly exceeded the stored amount of 75 g C m^?2. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution. However, the total flow of dissolved organic carbon was not sufficient to explain the observed carbon storage in deeper soil layers, and the existence of additional carbon uptake mechanisms is discussed.     

Winter wheat carbon exchange in Thuringia,Germany

Eddy covariance measurements and estimates of biomass net primary production (NPP) in combination with soil carbon turnover modelled by the Roth-C model were used to assess the ecosystem carbon balance of an agricultural ecosystem in Thuringia, Germany, growing winter wheat in 2001. The eddy CO₂ flux measurements indicate an annual net ecosystem exchange (NEE) uptake in the range from −185 to −245 g C m⁻² per year. Main data analysis uncertainty in the annual NEE arises from night-time u¹ screening, other effects (e.g. coordinate rotation scheme) have only a small influence on the annual NEE estimate. In agricultural ecosystems the fate of the carbon removed during harvest plays a role in the net biome production (NBP) of the ecosystem, where NBP is given by net ecosystem production (NEP=−NEE) minus non-respiratory losses of the ecosystem (e.g. harvest). Taking account of the carbon removed by the wheat harvest (290 g C m⁻²), the agricultural field becomes a source of carbon with a NBP in the order of −45 to −105 g C m⁻² per year. Annual carbon balance modelled with the Roth-C model also indicated that the ecosystem was a source for carbon (NBP −25 to −55 g C m⁻² per year). Based on the modelling most of carbon respired resulted from changes in the litter and fast soil organic matter pool. Also, the crop and management history, particularly the C input to soil in the previous year, significantly affect next year’s CO₂ exchange.     

Impact of bovine urine deposition on soil microbial activity,biomass, and community structure

Nitrogen (N) from urine excreted by grazing animals can be transformed into N compounds that have detrimental effects on the environment. These include nitrate, which can cause eutrophication of waterways, and nitrous oxide, which is a greenhouse gas. Soil microbes mediate all of these N transformations, but the impact of urine on microbes and how initial soil conditions and urine chemical composition alter their responses to urine are not well understood. This study aimed to determine how soil inorganic N pools, nitrous oxide fluxes, soil microbial activity, biomass, and the community structure of bacteria containing amoA (nitrifiers), nirK, and nirS (denitrifiers) genes responded to the addition of urine over time. Bovine urine containing either a high (15.0 g K⁺ l^?1) or low salt content (10.4 g K⁺ l^?1) was added to soil cores at either low or high moisture content (hereafter termed dry and wet soil respectively; 35% or 70% water-filled pore space after the addition of urine). Changes in soil conditions, inorganic N pools, nitrous oxide fluxes, and the soil microbial community were then measured 1, 3, 8, 15, 29 and 44 days after urine addition. Urine addition increased soil ammonium concentrations by up to 2 mg g d.w.^?1, soil pH by up to 2.7 units, and electrical conductivity (EC) by 1.0 and 1.6 dS m^?1 in the low and high salt urine treatments respectively. In response, nitrate accumulation and nitrous oxide fluxes were lower in dry compared to wet urine-amended soils and slightly lower in high compared to low salt urine-amended soils. Nitrite concentrations were elevated (>3 μg g d.w.^?1) for at least 15 days after urine addition in wet urine-amended soils, but were only this high in the dry urine-amended soils for 1 day after the addition of urine. Microbial biomass was reduced by up to half in the wet urine-amended soils, but was largely unaffected in the dry urine-amended soils. Urine addition affected the community structure of ammonia-oxidising and nitrite-reducing bacteria; this response was also stronger and more persistent in wet than in dry urine-amended soils. Overall, the changes in soil conditions caused by the addition of urine interacted to influence microbial responses, indicating that the effect of urine on soil microbes is likely to be context-dependent.     

An outline of soil nematode succession on abandoned fields in South Bohemia

Secondary succession of nematodes was studied in 1–48-year-old abandoned fields on cambisols in South Bohemia, Czech Republic, and compared with cultivated field and sub-climax oak forests. Bacterivores were the predominant group in the cultivated field whereas in forests root-fungal feeders (mainly Filenchus) were almost as abundant as bacterivores. The total abundance of nematodes in the cultivated field averaged 868 × 10³ ind m⁻². During the first three years of succession the abundance practically did not change (775 × 10³ ind m⁻²), the fauna was still similar to that in cultivated field but the biomass increased mainly due to Aporcelaimellus. Then the abundance increased up to 3731 × 10³ ind m⁻² in 7–8-year-old abandoned fields, plant parasites (Helicotylenchus) dominated and the fungal-based decomposition channel was activated. Later the abundance stabilised at between 1086 and 1478 × 10³ ind m⁻² in 13–25-year-old successional meadow stages with high population densities of omnivores and predators. The total abundance of nematodes was low in the 12–13-year-old willow shrub stage (594 × 10³ ind m⁻²), increased in the 35–48-year-old birch shrub stage (1760 × 10³ ind m⁻²) and the nematode fauna developed towards a forest community. The diversity and maturity of nematode communities generally increased with the age of abandoned fields but the highest values were in meadow stages (81–113 species, 57–68 genera, MI 2.73–3.30). The development of meadow arrested succession towards forests or diverted succession towards a waterlogged ecosystem. The succession of nematodes was influenced by the method of field abandonment (bare soil vs. legume cover, mowing) that affected the formation of either a shrub or meadow stage, and by the soil water status. The composition of the nematode fauna indicated that the soil food web could recover faster from agricultural disturbance under successive meadows than under shrubs.     

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

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

Endogeic and anecic earthworm abundance in six Midwestern cropping systems

Endogeic and juvenile anecic earthworm abundance was measured in soil samples and anecic populations were studied by counting midden numbers at the sites of two long-term cropping systems trials in South-central Wisconsin. The three grain and three forage systems at each site were designed to reflect a range of Midwestern USA production strategies. The primary objectives of this work were to determine if the abundance of endogeic or anecic earthworms varied among cropping systems or crop phases within a cropping system and were there specific management practices that impacted endogeic or anecic earthworm numbers. The earthworms present in the surface soil were: Aporrectodea tuberculata (Eisen), A. caliginosa (Savigny), A. trapezoides (Dugés); and juvenile Lumbricus terrestris (L.). True endogeic abundance was greatest in rotationally grazed pasture [188 m^?2 at Arlington (ARL) and 299 m^?2 at Elkhorn (ELK)], and smallest in conventional continuous corn (27 m^?2 at ARL and 32 m^?2 at ELK). The only type of anecic earthworm found was L. terrestris L. There was an average of 1.2 middens per adult anecic earthworm and the population of anecics was greatest in the no-till cash grain system (28 middens m^?2 at ARL, 18 m^?2 at ELK) and smallest in the conventional continuous corn system (3 middens m^?2 at ARL, 1 m^?2 at ELK). Earthworm numbers in individual crop phases within a cropping system were too variable from year-to-year to recommend using a single phase to characterize a whole cropping system. Indices for five management factors (tillage, manure inputs, solid stand, pesticide use, and crop diversity) were examined, and manure use and tillage were the most important impacting earthworm numbers across the range of cropping systems. Manure use was the most important management factor affecting endogeic earthworm numbers; but no-tillage was the most important for the juvenile and adult anecic groups and had a significantly positive influence on endogeic earthworm counts as well. The pesticides used, which were among the most commonly applied pesticides in the Midwestern USA, and increasing crop diversity did not have a significant effect on either the endogeic or anecic earthworm groups in this study. Consequently, designing cropping systems that reduce tillage and include manure with less regard to omitting pesticides or increasing crop diversity should enhance earthworm populations and probably improve sustainability.     

N2O emissions from humid tropical agricultural soils: effects of soil moisture,texture and nitrogen availability

We studied soil moisture dynamics and nitrous oxide (N₂O) fluxes from agricultural soils in the humid tropics of Costa Rica. Using a split-plot design on two soils (clay, loam) we compared two crop types (annual, perennial) each unfertilized and fertilized. Both soils are of andic origin. Their properties include relatively low bulk density and high organic matter content, water retention capacity, and hydraulic conductivity. The top 2–3 cm of the soils consists of distinct small aggregates (dia. <0.5 cm). We measured a strong gradient of bulk density and moisture within the top 7 cm of the clay soil. Using automated sampling and analysis systems we measured N₂O emissions at 4.6 h intervals, meteorological variables, soil moisture, and temperature at 0.5 h intervals. Mean daily soil moisture content at 5 cm depth ranged from 46% water filled pore space (WFPS) on clay in April 1995 to near saturation on loam during a wet period in February 1996. On both soils the aggregated surface layer always remained unsaturated. Soils emitted N₂O throughout the year. Mean N₂O fluxes were 1.04±0.72 ng N₂O-N cm⁻² h⁻¹ (mean±standard deviation) from unfertilized loam under annual crops compared to 3.54±4.31 ng N₂O-N cm⁻² h⁻¹ from the fertilized plot (351 days measurement). Fertilization dominated the temporal variation of N₂O emissions. Generally fluxes peaked shortly after fertilization and were increased for up to 6 weeks (‘post fertilization flux’). Emissions continued at a lower rate (‘background flux’) after fertilization effects faded. Mean post-fertilization fluxes were 6.3±6.5 ng N₂O-N cm⁻² h⁻¹ while the background flux rate was 2.2±1.8 ng N₂O-N cm⁻² h⁻¹. Soil moisture dynamics affected N₂O emissions. Post fertilization fluxes were highest from wet soils; fluxes from relatively dry soils increased only after rain events. N₂O emissions were weakly affected by soil moisture during phases of low N availability. Statistical modeling confirmed N availability and soil moisture as the major controls on N₂O flux. Our data suggest that small-scale differences in soil structure and moisture content cause very different biogeochemical environments within the top 7 cm of soils, which is important for net N₂O fluxes from soils.     

Field margins and management affect settlement and spread of an introduced dew-worm (Lumbricus terrestris L.) population

To study the feasibility of earthworm introduction for increasing the macroporosity and permeability of arable heavy clay, deep-burrowing earthworms (Lumbricus terrestris L.) were inoculated into a tile drained experimental field in Jokioinen, S-W Finland in autumn 1996. Inoculation with the Earthworm Inoculation Unit technique was at the up-slope end of the field, in the field margins under permanent grass, and inside the four 0.46 ha plots of the field. The experiment was monitored on three occasions. In 1998 the L. terrestris population had persisted in low numbers only in field and plot margins. By 2003, when the field had been under set-aside grass for three years, density had grown in the margins and L. terrestris were also found inside the field at a very low density. The third monitoring was in autumn 2009, after a further four years as set-aside and a subsequent division of the field into no-till and ploughing management, and looked at the effects of management (margins, no-till, ploughing), distance from the inoculation and sub-drainage on L. terrestris abundance. The abundance displayed a clear gradient over the field, declining from 14 ind. and 18 g m^?2 at 5–9 m from inoculation, to 1 ind. and 2 g m^?2 at 56–60 m distance. Margins had the highest abundances (16 ind. and 32 g m^?2), followed by no-till (4 ind. and 4 g m^?2) and ploughing (1 ind. and 1 g m^?2). Abundances were significantly higher above the tiles than between them (P < 0.05). The results demonstrate the importance of no-till and sub-drain line habitats as settlement supports for the inoculated population. Field margins proved to be decisive for inoculation success, by providing bridgeheads for population establishment and later by acting as source areas for the colonisation of the field. This finding highlights the general importance of field margins in the dispersal ecology of earthworms in arable landscapes.     

Micro-topographic patterns unravel controls of soil water and temperature on soil respiration in three Siberian tundra systems

Spatial and temporal patterns of soil respiration rates and controlling factors were investigated in three wet arctic tundra systems. In situ summer season carbon dioxide fluxes were measured across a range of micro-topographic positions in tussock tundra, wet sedge tundra, and low-centre polygonal tundra, at two different latitudes on the Taimyr Peninsular, central Siberia. Measurements were carried out by means of a multi-channel gas exchange system operating in continuous-flow mode.Measured soil respiration rates ranged from 0.1 g CO₂-C m^?2 d^?1 to 3.9 g CO₂-C m^?2 d^?1 and rate differences between neighbouring sites in the micro-topography (microsites) were larger than those observed between different tundra systems. Statistical analysis identified position of the water table and soil temperature at shallow depths to be common controls of soil respiration rates across all microsites, with each of these two factors explaining high proportions of the observed variations.Modelling of the response of soil respiration to soil temperature and water table for individual microsites revealed systematic differences in the response to the controlling factors between wet and drier microsites. Wet microsites – with a water table position close to the soil surface during most of the summer – showed large soil respiration rate changes with fluctuations of the water table compared to drier microsites. Wet microsites also showed consistently higher temperature sensitivity and a steeper increase of temperature sensitivity with decreasing temperatures than drier sites. Overall, Q₁₀ values ranged from 1.2 to 3.4. The concept of substrate availability for determining temperature sensitivity is applied to reconcile these systematic differences. The results highlight that soil respiration rates in wet tundra are foremost controlled by water table and only secondarily by soil temperature. Wet sites have a larger potential for changes in soil respiration rates under changing environmental conditions, compared to drier sites.It is concluded that understanding and forecasting gaseous carbon losses from arctic tundra soils and its implication for ecosystem-scale CO₂ fluxes and soil organic matter dynamics require good knowledge about temporal and spatial patterns of soil water conditions. The water status of tundra soils can serve as a control on the temperature sensitivity of soil respiration.     

Soil–atmosphere greenhouse gas exchange in a cool,temperate Eucalyptus delegatensis forest in south-eastern Australia

Forests are the largest C sink (vegetation and soil) in the terrestrial biosphere and may additionally provide an important soil methane (CH₄) sink, whilst producing little nitrous oxide (N₂O) when nutrients are tightly cycled. In this study, we determine the magnitude and spatial variation of soil–atmosphere N₂O, CH₄ and CO₂ exchange in a Eucalyptus delegatensis forest in New South Wales, Australia, and investigate how the magnitude of the fluxes depends on the presence of N₂-fixing tree species (Acacia dealbata), the proximity of creeks, and changing environmental conditions. Soil trace gas exchange was measured along replicated transects and in forest plots with and without presence of A. dealbata using static manual chambers and an automated trace gas measurement system for 2 weeks next to an eddy covariance tower measuring net ecosystem CO₂ exchange. CH₄ was taken up by the forest soil (?51.8 μg CH₄-C m^?2 h^?1) and was significantly correlated with relative saturation (S_r) of the soil. The soil within creek lines was a net CH₄ source (up to 33.5 μg CH₄-C m^?2 h^?1), whereas the wider forest soil was a CH₄ sink regardless of distance from the creek line. Soil N₂O emissions were small (<3.3 μg N₂O-N m^?2 h^?1) throughout the 2-week period, despite major rain and snowfall. Soil N₂O emissions only correlated with soil and air temperature. The presence of A. dealbata in the understorey had no influence on the magnitude of CH₄ uptake, N₂O emission or soil N parameters. N₂O production increased with increasing soil moisture (up to 50% S_r) in laboratory incubations and gross nitrification was negative or negligible as measured through ¹⁵N isotope pool dilution.The small N₂O emissions are probably due to the limited capacity for nitrification in this late successional forest soil with C:N ratios >20. Soil–atmosphere exchange of CO₂ was several orders of magnitude greater (88.8 mg CO₂-C m^?2 h^?1) than CH₄ and N₂O, and represented 43% of total ecosystem respiration. The forest was a net greenhouse gas sink (126.22 kg CO₂-equivalents ha^?1 d^?1) during the 2-week measurement period, of which soil CH₄ uptake contributed only 0.3% and N₂O emissions offset only 0.3%.