Mari Pihlatie

Nitrous oxide emissions following application of residues and fertiliser under zero and conventional tillage

Emissions of N2O were measured following combined applications of inorganic N fertiliser and crop residues to a silt loam soil in S.E. England, UK. Effects of cultivation technique and residue application on N2O emissions were examined over 2 years. N2O emissions were increased in the presence of residues and were further increased where NH4NO3 fertiliser (200 kg N ha−1) was applied. Large fluxes of N2O were measured from the zero till treatments after residue and fertiliser application, with 2.5 kg N2O-N ha−1 measured over the first 23 days after application of fertiliser in combination with rye (Secale cereale) residues under zero tillage. CO2 emissions were larger in the zero till than in the conventional till treatments. A significant tillage/residue interaction was found. Highest emissions were measured from the conventionally tilled bean (Vicia faba) (1.0 kg N2O-N ha−1 emitted over 65 days) and zero tilled rye (3.5 kg N2O-N ha−1 over 65 days) treatments. This was attributed to rapid release of N following incorporation of bean residues in the conventionally tilled treatments, and availability of readily degradable C from the rye in the presence of anaerobic conditions under the mulch in the zero tilled treatments. Measurement of 15N-N2O emission following application of 15N-labelled fertiliser to microplots indicated that surface mulching of residues in zero till treatments resulted in a greater proportion of fertiliser N being lost as N2O than with incorporation of residues. Combined applications of 15N fertiliser and bean residues resulted in higher or lower emissions, depending on cultivation technique, when compared with the sum of N2O from single applications. Such interactions have important implications for mitigation of N2O from agricultural soils.

Annual cycle of methane emission from a boreal fen measured by the eddy covariance technique

The northern wetlands are one of the major sources of methane into the atmosphere. We measured annual methane emission from a boreal minerotrophic fen, Siikaneva, by the eddy covariance method. The average wintertime emissions were below 1 mg m-2 h-1, and the summertime emissions about 3.5 mg m-2 h-1. The water table depth did have any clear effect on methane emissions. During most of the year the emission depended on the temperature of peat below the water table. However, during the high and late summer the emission was independent on peat temperature as well. No diurnal cycle of methane flux was found. The total annual emission from the Siikaneva site was 12.6 g m-2. The emissions of the snow free period contributed 91% to the annual emission. The emission pulse during the snow melting period was clearly detectable but of minor importance adding only less than 3% to the annual emission. Over 20% of the carbon assimilated during the year as carbon dioxide was emitted as methane. Thus methane emission is an important component of the carbon balance of the Siikaneva fen. This indicates need of taking methane into account when studying carbon balances of northern fen ecosystems.

Spatial variation in plant community functions regulates carbon gas dynamics in a boreal fen ecosystem

The aim of this study was to asses how the variability in carbon gas exchange at the plant community scale affected the C gas exchange estimates at the ecosystem scale in a fen that was homogeneous in a micrometeorological sense, that is, had an even surface topography and plant cover. CO2 and CH4 exchange was measured at the plant community scale with chambers and at the ecosystem scale with the eddy covariance (EC) technique. Community-scale measurements were upscaled to the ecosystem scale by weighting the community-specific estimates by the area of the community. All communities were net CO2 sinks and CH4 sources during the growing season, but net ecosystem production (NEP) and CH4 emissions ranged from 21 to 190 g CO2-C m-2 and from 4.3 to 13 g CH4-C m-2, respectively, between the communities. The seasonal estimates of NEP and CH4, upscaled to the 200 m radius from the EC tower, were 82 and 7.9 g CH4-C m-2, which agreed well with the EC measurements. As the communities differed markedly in their C gas dynamics, their proportions controlled the ecosystem scale estimates. Successful upscaling required detailed knowledge on the proportions and leaf area of the communities.

Gas concentration driven fluxes of nitrous oxide and carbon dioxide in boreal forest soil

Nitrous oxide (N2O) and carbon dioxide (CO2) fluxes were measured in a boreal forest during two growing seasons with soil gradient and chamber methods. N2O fluxes obtained by these two techniques varied from small emission to small uptake. N2O fluxes were of the same order of magnitude, however, the fluxes measured by the soil gradient method were higher and more variable than the fluxes measured with chambers. The highest soil gradient N2O fluxes were measured in the late summer and the lowest in the autumn and spring. In the autumn, litter fall induced a peak in N2O concentration in the organic O-horizon, whereas in the spring N2O was consumed in the O-horizon. Overall, the uppermost soil layer was responsible for most of the N2O production and consumption. Soil gradient and chamber methods agreed well with CO2 fluxes. Due to the very small N2O fluxes and the sensitivity of the flux to small concentration difference between the soil and the ambient air, the flux calculations from the O-horizon to the atmosphere were considered unreliable. N2O fluxes calculated between the soil A- and O-horizons agreed relatively well with the chamber measurements.

Emissions of nitrous oxide from boreal agricultural clay and loamy sand soils

Long-term studies of greenhouse gas fluxes from agricultural soils in different climate regions are needed to improve the existing calculation models used in greenhouse gas inventories. The aim of this study was to obtain more information on nitrous oxide (N2O) emissions from agricultural mineral soils in the boreal region. N2O emissions were studied during 2000–2002 on two soil types in Finland, a loamy sand and a clay with plots of grass, barley and fallow. N2O fluxes were measured with static chambers throughout the year. Other parameters measured were water filled pore space (WFPS), soil mineral nitrogen concentration, soil porosity, soil temperature and depth of soil frost. The annual fluxes from the clay soil ranged from 3.7 to 7.8 kg N ha–1 and those from sandy loam from 1.5 to 7.5 kg N ha–1. On average 60% of the annual fluxes occurred outside the growing season, from October to April. Increasing the number of freeze-thaw events was found to increase the fluxes during winter and during the thawing period in spring. The results suggest that N2O fluxes from these boreal mineral soils do not vary much as a function of applied fertiliser N and could probably be better estimated from soil physical properties, including soil porosity.

Pinus sylvestris as a missing source of nitrous oxide and methane in boreal forest

Boreal forests comprise 73% of the world’s coniferous forests. Based on forest floor measurements, they have been considered a significant natural sink of methane (CH4) and a natural source of nitrous oxide (N2O), both of which are important greenhouse gases. However, the role of trees, especially conifers, in ecosystem N2O and CH4 exchange is only poorly understood. We show for the first time that mature Scots pine (Pinus sylvestris L.) trees consistently emit N2O and CH4 from both stems and shoots. The shoot fluxes of N2O and CH4 exceeded the stem flux rates by 16 and 41 times, respectively. Moreover, higher stem N2O and CH4 fluxes were observed from wet than from dry areas of the forest. The N2O release from boreal pine forests may thus be underestimated and the uptake of CH4 may be overestimated when ecosystem flux calculations are based solely on forest floor measurements. The contribution of pine trees to the N2O and CH4 exchange of the boreal pine forest seems to increase considerably under high soil water content, thus highlighting the urgent need to include tree-emissions in greenhouse gas emission inventories.

Neglecting diurnal variations leads to uncertainties in terrestrial nitrous oxide emissions

Nitrous oxide (N2O) is an important greenhouse gas produced in soil and aquatic ecosystems. Its warming potential is 296 times higher than that of CO2. Most N2O emission measurements made so far are limited in temporal and spatial resolution causing uncertainties in the global N2O budget. Recent advances in laser spectroscopic techniques provide an excellent tool for area-integrated, direct and continuous field measurements of N2O fluxes using the eddy covariance method. By employing this technique on an agricultural site with four laser-based analysers, we show here that N2O exchange exhibits contrasting diurnal behaviour depending upon soil nitrogen availability. When soil N was high due to fertilizer application, N2O emissions were higher during daytime than during the night. However, when soil N became limited, emissions were higher during the night than during the day. These reverse diurnal patterns supported by isotopic analyses may indicate a dominant role of plants on microbial processes associated with N2O exchange. This study highlights the potential of new technologies in improving estimates of global N2O sources.

Temporal Variation of Ecosystem Scale Methane Emission From a Boreal Fen in Relation to Temperature, Water Table Position, and Carbon Dioxide Fluxes

We have analyzed decade-long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable-slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process-based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10-year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space-to-time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales. (Less)

Above- and belowground fluxes of methane from boreal dwarf shrubs and Pinus sylvestris seedlings

AimsThe contribution of boreal forest plants to the methane (CH4) cycle is still uncertain. We studied the above and belowground CH4 fluxes of common boreal plants, and assessed the possible contribution of CH4 producing and oxidizing microbes (methanogens and methanotrophs, respectively) to the fluxes.MethodsWe measured the CH4 fluxes and the amounts of methanogens and methanotrophs in the above- and belowground parts of Vaccinium myrtillus, Vaccinium vitis-idaea, Calluna vulgaris and Pinus sylvestris seedlings and in non-planted soil in a microcosm experiment.ResultsThe shoots of C. vulgaris and P. sylvestris showed on average emissions of CH4, while the shoots of the Vaccinium species indicated small CH4 uptake. All the root-soil-compartments consumed CH4, however, the non-rooted soils showed on average small CH4 emission. We found methanotrophs from all the rooted and non-rooted soils. Methanogens were not detected in the plant or soil materials.ConclusionsThe presence of plant roots seem to increase the amount of methanotrophs and thus CH4 uptake in the soil. The CH4 emissions from the shoots of C. vulgaris and P. sylvestris demonstrate that the plants have an important contribution to the CH4 exchange dynamics in the plant-soil systems.

Processes in Living Structures

Cells are the basic functional units in forest ecosystems. Plants have strong cell wall, formed by cellulose and lignin. Cell membrane isolates the cell from its surroundings, starch acts as storage and enzymes enable synthesis of new compounds. Membrane pumps allow penetration of cell membrane and pigments capture of light energy. We call enzymes, membrane pumps and pigments as functional substances. The biochemical regulation system changes the concentrations and activities of the functional substances: In summer, metabolism is very active, but in winter, vegetation is dormant and tolerates low temperatures. The action of the biochemical regulation system generates emergent regularities in the functional substances, called the state of the functional substances. The effect of environmental factors on metabolism is built in the complex chain of enzymes, membrane pumps and pigments, acting in each metabolic task. The process-specific state of functional substances and the environmental factors determine the rate of each metabolic process. Microbes have dominating role in the soil. Together with soil fauna, microbes break down macromolecules with extracellular enzymes to small molecules that can penetrate the microbial cell membrane through membrane pumps. The microbial metabolism utilises the small carbon-rich molecules for the energy needs, growth and synthesis of the extracellular enzymes.