Decay and vertical reallocation of organic C, and its incorporation into carbonates, in agricultural soil horizons at two different depths and rewetting frequencies

Drying–rewetting phenomena influence key processes for soil C cycling, such as organic matter (SOM) decomposition and leaching, or pedogenic carbonate formation. Soil moisture is in turn modulated by horizon position, creating different pedoclimatic conditions through depth. However, the way these conditions affect C dynamics is still unclear. We conducted an annual field mesocosm experiment in which homogenous agricultural horizons enriched with 14C-labeled wheat straw were incubated in columns in conjunction with mineral subsoil, at two depths (surface or subsurface) and under two contrasting rewetting frequencies. We performed destructive samplings along the annual period to i) assess the evolution of the organic C within the agricultural horizon, and ii) its vertical reallocation into the mineral subsoil, which was assumed to occur by means of dissolved organic C (DOC) leaching. Our results showed that a lower number of soil rewetting events prompted an initial reduction in total (bulk SOM) and labeled (fresh straw) organic C decay rates, although this effect did not extend over time. Subsurface level induced higher fresh straw-14C decomposition after one year, due to lower soil moisture limitations. Contrary to expectations, the reduction of rewetting events increased the reallocation of organic C downwards in the profile, but it prompted the contrary effect regarding upward reallocation. Further, subsurface pedoclimatic conditions selectively hampered downward reallocation of straw-14C. The amount of 14C incorporated as carbonate after the first rewetting corresponded to 1.3% of the initial litter-14C, evidencing the link between organic and inorganic C cycles. This content declined over time at subsurface but not at surface horizons. In summary, our results underline the importance of depth as a factor modulating the effects of water regime on soil C dynamics.

► Subsurface moisture conditions favored straw mineralization respect to surface on a year basis. ► Reduction of rewettings favored downward – and hindered upward – OC reallocation. ► Subsurface conditions hindered vertical reallocation of straw C respect to bulk SOC. ► Incorporation of straw C within soil carbonates was higher than its vertical reallocation.