Markku Koskinen

Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO2) production and dissolved carbon dynamics in controlled greenhouse conditions using 32 monolithic soil columns (hereafter monoliths) (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. The 1.5 year study comprised three growth cycles with alternating growing and off-seasons. Spring barley (Hordeum vulgare) was grown in all monoliths during the growing seasons. In turn, during all three off-seasons, half of the monoliths were subjected to waterlogging lasting seven weeks, while in the other half soil moisture was maintained at ∼70 % field capacity. Within these water treatment groups (waterlogged and control), the monoliths were further divided into two plant treatment groups: in half of the monoliths, an overwintering cover crop (Festuca arundinacea) was grown, while in the other half soil was left bare for the off-seasons. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths and CO2 fluxes were measured at the surface. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO2 fluxes declined, indicating an accumulation of respired CO2 in soil pore water. The sustained CO2 production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the onset of drainage of the waterlogged monoliths, CO2 fluxes from both soils increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO2. These post-waterlogging increases in CO2 fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO2 fluxes, it did not alter total cumulative CO2 emissions from the studied agricultural soils.

Mineral soil forest floors often act as net methane (CH4) sinks. The contribution of the different components of the forest floor, such as soil, shrubs, mosses and their roots and the roots of trees, to the sink and the factors affecting their contribution are not well known. Predicting the CH4 flux dynamics of forests requires understanding the component fluxes and drivers, such as microbial population, soil moisture and temperature, and composition and coverage of the forest floor vegetation.The CH4 exchange of the forest floor at the SMEAR II experimental forest in central Finland (Hari & Kulmala, 2005) was monitored using manual and automated chambers for a total of more than 10 years (2006-2016 manually, 2021-mid 2023 and mid 2025 onwards automatically). In addition, a manipulation experiment was conducted using manual chambers where either tree, shrub or mycorrhizal roots were excluded by trenching. Also, the effect of above ground vegetation (shrubs, mosses) on CH4 flux dynamics was studied. The humus and soil layers next to the automated chambers were inspected for presence of methanotrophs.We found that the forest floor is a persistent sink for CH4 through the year, CH4 being consumed even during winter on all measurement plots. The general trend in the long-term measurements was towards a larger sink during growing season. Increasing soil temperature increased the sink during the growing season, while soil moisture decreased it. During growing season, a diurnal pattern was observed where higher CH4 consumption occurred during night time.In the trenching experiment the exclusion of tree, shrub or mycorrhizal roots did not affect soil CH4 uptake, however, the cutting all above ground vegetation increase CH4 uptake compared to presence of normal vegetation (shrubs and mosses). Based on the probe-targeted metagenomic sequencing, methanotrophic bacteria within the organic and mineral soil layers consisted mainly of alphaproteobacterial high-affinity oxidizers, including taxa potentially adapted to oxygen-limited conditions.Hari, P., & Kulmala, M. (2005). Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II). Boreal Environment Research, 10(5), 315-322.

Agricultural soils are the largest anthropogenic source of nitrous oxide (N₂O), a potent greenhouse gas and an ozone-depleting substance. We investigated the seasonal and diurnal variability of N₂O fluxes and their controlling factors in an agricultural ecosystem at the SMEAR-Agri Viikki site (Helsinki, Finland) over three years. The field was cultivated with timothy (Phleum pratense) in 2022 and was renewed in spring 2023 with barley (Hordeum vulgare) undersown with red clover (Trifolium pratense) and grasses; in 2024 the site was managed for silage production. N₂O emissions in 2022 showed no consistent seasonal pattern but a high early-summer emission peak, whereas 2023 and 2024 were characterised by multiple emission events with smaller magnitudes. Minimum fluxes occurred in autumn 2022 (0.003 µg m⁻² s⁻¹), winter 2023 (0.008 µg m⁻² s⁻¹) and summer 2024 (0.006 µg m⁻² s⁻¹). The highest fluxes were observed in 2022 summer (0.089 µg m⁻² s⁻¹) and spring (0.088 µg m⁻² s⁻¹), while peak emissions in 2023 (0.028 µg m⁻² s⁻¹) and 2024 (0.032 µg m⁻² s⁻¹) occurred during autumn. Our results highlight strong interannual variability in both the timing and magnitude of N₂O emissions, likely linked to changes in crop N utilization and management, soil conditions and meteorological drivers. Seasonal variations in N₂O emissions during spring, summer and autumn were primarily driven by soil and meteorological factors, including air and soil temperature, soil moisture, water-filled pore space, electrical conductivity and redox potential. During winter, however, N₂O fluxes showed little association with these variables, suggesting a shift in controlling processes under cold conditions. Overall, the findings reveal substantial seasonal and interannual complexity in N₂O dynamics and underscore the importance of integrating soil conditions, management practices and seasonal context when assessing and mitigating N₂O emissions from managed agricultural systems.Keywords: Nitrous oxide flux, greenhouse gas, seasonal variability, agricultural ecosystems, soil moisture, temperature.

Freeze–thaw cycles (FTCs) are recognized as important regulators of nitrous oxide (N₂O) emissions in temperate and boreal soils, yet the underlying drivers and processes during FTC remain poorly understood. We set up two controlled incubation experiments to investigate the effect of freezing and thawing on N2O emissions, emission drivers and N2O forming processes. Soil samples were collected during spring freeze-thaw period from a biodiversity cover crop plot trial in Helsinki, southern Finland. In experiment 1, we studied the effects of soil depth and cover crops (CC) on N₂O emissions, soluble nutrients (N, P, C) and active N cycling genes (RNA) during consecutive freezing and thawing phases over a 3-week period. In experiment 2, we studied N2O emissions and their isotopologue ratios in a dynamic automated FTC experiment with alternating freezing (–4 °C) and thawing (+4 °C) cycles. To support the controlled experiments, we conducted twice weekly N2O flux measurements in the field during the spring freezing-thawing period.Throughout the experiment 1, we found significantly greater N2O production at the surface (0 – 2 cm) compared to the subsurface (9 – 11 cm) soil depth. To support this, soil nitrate, ammonium, total dissolved N, dissolved organic C and soluble reactive P concentrations where higher in the topsoil compared to the subsurface. In experiment 2, N2O emissions occurred during thawing periods but were stimulated by freezing. Based on isotopologue ratios, the N2O originated predominantly from denitrification. Field N2O flux data support the laboratory results showing higher N2O emissions during freeze-thaw, and smaller during warm periods, and that cover crop treatments potentially lead to higher N2O emissions during soil freeze-thaw. Overall, the findings demonstrate the episodic nature of freeze-thaw related N2O emissions governed by substrate dynamics in soil that support conditions suitable for “hot moments”.

Nordic agricultural production faces multiple challenges in a warming climate. Although predictions indicate an overall increase in annual mean precipitation in northern Europe, increasing temperatures may lead to earlier snowmelt and drying of soil during springtime, as well as longer growing season and higher evapotranspiration increasing the risk of summer droughts. The use of cover crops in agriculture is one of the climate-smart practices that have multiple benefits, such as increasing SOC, reducing N losses, and increasing biodiversity. Still, the question whether cover crops and their diversity increase resilience against climate extremes such as drought, and how the combined effects of cover crops, their diversity and drought affect greenhouse gas (GHG) emissions from soil remain largely unknown. We studied the effect of cover crop diversity and drought on soil and crop C and N dynamics and GHG (CO2, N2O) emissions in a biodiversity cropland experiment with or without shelters that remove 50% of incoming precipitation for two years. GHG emissions were measured with the manual dark chamber method twice a week during growing season and once a week during off-season. Soil temperature and water content were measured continuously, and the soil was sampled for mineral N and total C and N analysis seasonally.The preliminary results showed that reduced rainfall did not affect N2O emissions significantly during the growing season in either year. During off-season, reduced rainfall led to elevated N2O emissions irrespective of cover crop diversity treatments. However, the effect was absent in the second year, indicating that factors other than drought were driving the N2O production. Contrary to N2O, drought did not affect CO2 emissions during off-season in either year. Overall, during both years off-season N2O emissions dominated the annual N2O balance in all diversity treatments, highlighting the importance of including off-season measurements to the annual N2O balance estimation.

Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO₂) and methane (CH₄) production, and dissolved carbon dynamics in controlled greenhouse conditions using 32 soil profiles (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. During the 1.5-year study comprising three growth cycles, spring barley (Hordeum vulgare) was grown during the growing seasons. During the off-seasons, half of the monoliths were subjected to waterlogging episodes of seven weeks, while in the control monoliths soil moisture was maintained below field capacity. Additionally, overwintering cover crop (Festuca arundinacea) was grown in half of the monoliths. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths, and CO₂ and CH₄ fluxes were measured at the surface. Temporary waterlogging did not induce CH₄ production in either soil. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO₂ fluxes declined, indicating an accumulation of respired CO₂ in soil pore water. This sustained CO₂ production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the initiation of drainage, CO₂ fluxes from both soils and plant treatments increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO₂. These post-waterlogging increases in CO₂ fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO₂ fluxes, it did not alter total cumulative CO₂ emissions from the studied agricultural soils.

Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO₂) and methane (CH₄) production, and dissolved carbon dynamics in controlled greenhouse conditions using 32 soil profiles (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. During the 1.5-year study comprising three growth cycles, spring barley (Hordeum vulgare) was grown during the growing seasons. During the off-seasons, half of the monoliths were subjected to waterlogging episodes of seven weeks, while in the control monoliths soil moisture was maintained below field capacity. Additionally, overwintering cover crop (Festuca arundinacea) was grown in half of the monoliths. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths, and CO₂ and CH₄ fluxes were measured at the surface. Temporary waterlogging did not induce CH₄ production in either soil. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO₂ fluxes declined, indicating an accumulation of respired CO₂ in soil pore water. This sustained CO₂ production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the initiation of drainage, CO₂ fluxes from both soils and plant treatments increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO₂. These post-waterlogging increases in CO₂ fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO₂ fluxes, it did not alter total cumulative CO₂ emissions from the studied agricultural soils.

Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO₂) and methane (CH₄) production, and dissolved carbon dynamics in controlled greenhouse conditions using 32 soil profiles (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. During the 1.5-year study comprising three growth cycles, spring barley (Hordeum vulgare) was grown during the growing seasons. During the off-seasons, half of the monoliths were subjected to waterlogging episodes of seven weeks, while in the control monoliths soil moisture was maintained below field capacity. Additionally, overwintering cover crop (Festuca arundinacea) was grown in half of the monoliths. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths, and CO₂ and CH₄ fluxes were measured at the surface. Temporary waterlogging did not induce CH₄ production in either soil. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO₂ fluxes declined, indicating an accumulation of respired CO₂ in soil pore water. This sustained CO₂ production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the initiation of drainage, CO₂ fluxes from both soils and plant treatments increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO₂. These post-waterlogging increases in CO₂ fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO₂ fluxes, it did not alter total cumulative CO₂ emissions from the studied agricultural soils.

Increasingly variable rainfall patterns expose soils to more frequent waterlogging in humid climates. Yet, the effects of waterlogging on soil organic matter decomposition in mineral soils remain uncertain. We studied the impact of off-season waterlogging on carbon dioxide (CO₂) and methane (CH₄) production, and dissolved carbon dynamics in controlled greenhouse conditions using 32 soil profiles (h=63 cm, d=15.2 cm) sampled from two agricultural fields (silty clay, sandy loam) in southern Finland. During the 1.5-year study comprising three growth cycles, spring barley (Hordeum vulgare) was grown during the growing seasons. During the off-seasons, half of the monoliths were subjected to waterlogging episodes of seven weeks, while in the control monoliths soil moisture was maintained below field capacity. Additionally, overwintering cover crop (Festuca arundinacea) was grown in half of the monoliths. Soil temperature and moisture were continuously monitored, dissolved organic (DOC) and inorganic carbon (DIC) concentrations in pore water were analyzed at three depths, and CO₂ and CH₄ fluxes were measured at the surface. Temporary waterlogging did not induce CH₄ production in either soil. Contrary to our hypothesis, waterlogging did not increase soil DOC content. Instead, on-going microbial/rhizospheric activity promoted an increase in DIC content while CO₂ fluxes declined, indicating an accumulation of respired CO₂ in soil pore water. This sustained CO₂ production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the initiation of drainage, CO₂ fluxes from both soils and plant treatments increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated CO₂. These post-waterlogging increases in CO₂ fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of CO₂ fluxes, it did not alter total cumulative CO₂ emissions from the studied agricultural soils.

Reduction–oxidation (redox) reactions are ubiquitous in nature and are responsible for the energy acquisition of all organisms. Redox reactions are electron transfer reactions that necessarily involve two participants: one being oxidised (electron donor) and one being reduced (electron acceptor). The availability of terminal electron acceptors (TEAs) is a major determinant of the extent to which carbon in organic matter can be oxidised in an ecosystem. This is most important under waterlogged conditions, such as in peatlands, where the diffusion of O ₂ , the most effective common TEA, into soil is blocked by water. Under these conditions, alternative TEAs can be used by microbiota to continue organic matter oxidation. Decomposition processes in soil can be characterised by its redox state, i.e. which TEA is responsible for organic matter oxidation at a given time. This can, in principle, be measured as a voltage between the soil solution and a known reference electrode, known as the redox potential. Current soil ecosystem models do not depict the use of alternative TEAs well. This limits their applicability for predicting soil carbon loss under different drainage regimes and, thus, their usefulness for assessing the best management practices for soil carbon preservation and water course protection. The most common determinant of the mode of decomposition presently used in ecosystem models is the water table level (WTL), which relies on the assumption that the redox state of a peatland ecosystem responds predictably to changes in the WTL. We conducted a 2-year redox monitoring experiment in a boreal mesotrophic peatland under three drainage regimes: undrained, short-term drainage, and long-term drainage. In addition, an ombrotrophic plot that had undergone long-term drainage was monitored. Snapshot assessments of the activity of three major metabolic enzymes – arginine deaminase, protease, and urease – were also undertaken at the mesotrophic plots as an indicator of differences in microbial activity between drainage regimes. We found that the WTL was a poor temporal predictor of redox potential but that (1) the position of major transition zones between oxic and anoxic states and (2) enzymatic activities within the peat profile were somewhat determined by the dominant WTL depth. In the undrained plots especially, redox potential values reflecting oxic or suboxic conditions were often found below the WTL, whereas anoxia was present above the WTL at the drained plots. Preceding redox potential was found to affect the activities of protease and urease but not arginine in all plots.