Reija Kronberg

Waterlogging of aerobic soils can induce microbially catalysed reductive dissolution of iron oxides and the subsequent mobilisation of associated carbon. Additionally, the resulting shift in microbial metabolism and community structure may eventually affect the accrual of microbially derived carbon into soils. It remains, however, unknown whether temporary anaerobic conditions significantly influence soil carbon cycling in cultivated boreal mineral soils, which are increasingly prone to off‐season waterlogging due to climate change. We investigated the effects of off‐season waterlogging on carbon–mineral associations and microbial carbon immobilisation and community structure in a 1.5‐year greenhouse experiment conducted with intact soil profiles (monoliths) from two agricultural fields in southern Finland. The experiment comprised three cycles of alternating warm growing seasons and cold off‐seasons. Spring barley ( Hordeum vulgare ) was cultivated in all monoliths for the growing seasons, while Tall Fescue ( Festuca Arundinacea ) was under‐sown into half of them as an overwintering cover crop. During the off‐seasons, half of the monoliths were subjected to waterlogging for 7 weeks. We found that waterlogging induced shifts in microbial communities favouring anaerobic taxa capable of iron reduction. Despite this, repeated waterlogging had no significant impact on the iron‐associated or the total mineral‐associated carbon, nor the microbial residue carbon content in either soil. However, waterlogging reduced belowground carbon inputs by limiting cover crop root growth in the subsoil. Overall, our findings suggest that off‐season waterlogging of cultivated mineral soils in a cold humid climate does not compromise their ability to stabilise carbon in mineral‐associated forms.

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.

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.

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.

Soil minerals play an essential role in organic matter (OM) stabilization by reducing its bioavailability thereby slowing its turnover time, and contributing to long-term carbon (C) storage (Lavallee et al. 2019). In humid soils, iron (Fe) and aluminum (Al) oxides are particularly significant contributors to C stabilization (Rasmussen et al. 2018, Salonen et al. 2024). However, Fe oxides are sensitive to changes in soil oxidation-reduction (redox) state. Accordingly, temporary waterlogging leading to anaerobic conditions may result in reductive Fe dissolution, potentially compromising the ability of soils to stabilize OM (Huang et al. 2021). Increased OM availability could thus lead to increased soil CO₂ fluxes, although soil water saturation is traditionally thought to suppress C mineralization (Huang et al. 2021, Moyano et al. 2013. So far, it remains unknown whether the dissolution of Fe-bound OM during temporary anaerobic conditions significantly impacts the C cycle in boreal mineral soils, which are increasingly susceptible to off-season waterlogging due to climate change. We studied the impact of off-season waterlogging on coupled Fe and C dynamics in two distinct arable mineral soils in a 1.5-year long soil monolith experiment. The experiment consisted of three alternating growing and off-seasons, and it was conducted under controlled greenhouse conditions. In total 32 soil monoliths (d 15.2 cm, h 63 cm) were collected from two fields with silty clay and sandy loam texture in southern Finland. During the growing seasons, barley (Hordeum vulgare) was grown in all monoliths while Tall Fescue (Festuca arundinacea) was under sown into half of the monoliths as an overwintering cover crop. After barley harvest, half of the monoliths underwent an excessive irrigation treatment, inducing water saturation for approximately 50 days, while the remaining monoliths served as controls, maintaining soil moisture at 50% water filled pore space. Soil moisture, temperature and redox potential were measured continuously at three soil depths (10, 30 & 50 cm). During the off-seasons, porewater samples were collected from the saturation treatment at the same depths for analysis of dissolved C, nitrogen (N) and Fe. CO₂ and CH₄ fluxes were measured with a dark manual chamber. After the experiment, the monoliths were destructively sampled for determination of Fe-associated OC (Fe-OC), mineral associated OC and the total C and N contents in the entire soil profile (along with other analyses not detailed here). Waterlogging induced reducing conditions in both soils within one to two weeks which was accompanied by reductive Fe dissolution, especially in silty clay soil. However, the porewater Fe concentrations remained low (max 10 µmol l^-1) (Kronberg et al. 2024) and accordingly, repeated waterlogging resulted only a slight and statistically insignificant reduction in soil Fe-OC content in silty clay and no changes in sandy loam soil. No clear differences between the water treatments were found in the total mineral associated OC content in neither soil. Waterlogging altered CO₂ flux dynamics but had no effect on cumulative emissions. No significant CH₄ production was observed (Kronberg et al. in revision). In conclusion, our preliminary results suggest that off-season waterlogging at soil temperature +4–12 °C may have minimal impact on the ability of soil minerals to stabilize C in cultivated boreal mineral soils. However, increasing the length or the frequency of waterlogging events could alter this outcome. Additionally, the long-term effects remain to be studied as the combined changes in soil C inputs, soil structure and chemistry of soil constituents could potentially alter soil C sequestration.