Biogeochemistry of terrestrial soils as influenced by short-term flooding

Many terrestrial soils in the US Midwest are temporally flooded during the spring. The effects of short-term flooding on biogeochemical processes that occur in these soils are not fully understood and are the subject of this study. To evaluate these processes we investigated the redox-induced changes in the soil solution for three-cultivated and three-uncultivated/forest soils with different organic matter concentrations. The soils were flooded for 1, 3, 7, and 14-days under anoxic conditions in a biogeochemical reactor. Samples were analyzed for Eh; pH; NO3 (-); NH4 (+); total dissolved Mn and Fe; soluble P; dissolved organic and inorganic carbon (DOC-DIC); and evolved CO2. We found strongly contrasting responses of the terrestrial soils to flooding. Reducing conditions were established quickly in the uncultivated and more slowly in the cultivated soils. Concomitant changes in pH were higher for the uncultivated soils. The uncultivated soils showed a higher increase in the amount of NH4 (+), P, Fe, Mn than the cultivated soils over the 14-day incubation. The total amount of carbon decomposed was much greater for the uncultivated soils with approximately 900 mu g C (CO2 + DOC + DIC) decomposed per gram of soil compared to a total decomposition of 240 mu g C g (soil) (-1) for the cultivated soils indicating differences in the type of carbon decomposed. The rapid onset of reducing conditions for the uncultivated soils is attributed to a reactive carbon component that is either absent or occluded in the cultivated soils. This study demonstrates that the biogeochemically-induced changes in carbon dynamics in terrestrial soils are strongly influenced by short-term flooding and the history of soil management.