Temporary waterlogging alters CO₂ flux dynamics but not cumulative emissions in cultivated mineral soils

<p>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 (<span class="inline-formula">CO₂</span>) production and dissolved carbon dynamics in controlled greenhouse conditions using 32 monolithic soil columns (hereafter monoliths) (<span class="inline-formula"><i>h</i>=63 cm</span>, <span class="inline-formula"><i>d</i>=15.2 cm)</span> 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 (<i>Hordeum vulgare</i>) 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 <span class="inline-formula">∼70 <i>%</i></span> 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 (<i>Festuca arundinacea</i>) 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 <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> fluxes declined, indicating an accumulation of respired <span class="inline-formula">CO₂</span> in soil pore water. The sustained <span class="inline-formula">CO₂</span> production could not be explained solely by mobilization of Fe-associated C, as initially hypothesized. After the onset of drainage of the waterlogged monoliths, <span class="inline-formula">CO₂</span> fluxes from both soils increased more than predicted based on changes in soil moisture and temperature, likely due to the release of previously accumulated <span class="inline-formula">CO₂</span>. These post-waterlogging increases in <span class="inline-formula">CO₂</span> fluxes roughly equaled the earlier decreases during waterlogging. Thus, although off-season waterlogging strongly influenced the temporal dynamics of <span class="inline-formula">CO₂</span> fluxes, it did not alter total cumulative <span class="inline-formula">CO₂</span> emissions from the studied agricultural soils.</p>