Subsoil's Coupled Carbon-Iron Redox Cycle and Its Influence on Deep Soil C Accumulation during Reforestation.

It is now well established that most terrestrial carbon (C) cycles in subsoil, and that these deep processes are severely understudied and subsequently poorly understood.  Often the subsoil C cycle periodically couples with the iron (Fe) cycle during transient oxygen limitation to redistribute Fe and create redoximorphic features (RFs).  To better understand the pedogenic consequences of subsoil coupled C-Fe redox cycling, and its influence on contemporary subsoil C accumulation during reforestation, we sampled and analyzed individual RFs from a network of two meter deep soil pits along a gentle catena (3% slope) in the Southern Piedmont. Until 1910, the entire catena was under cultivation. Since then, half the soils we excavated supported two deeply rooted pine forests while the other half remained treeless and supported grasses and forbs. 

Across this catena, within a given subsoil horizon, Fe-oxyhydroxides, percent clay, total C, and Δ14C, vary by as much as 60 mg/g, 26%, 3 mg/g, and  400‰ respectively across individual RFs and demonstrate that C-Fe cycling has a profound effect on subsoil structure, function, and heterogeneity.  RF abundance, morphology, and Fe-oxyhydroxide concentration is statistically indistinguishable under forest and grass suggesting that these RFs originated prior to the divergent land uses.  Reforestation does significantly increase total C by an average of 0.5 mg/g  (p=0.0478) and Δ14C by an average of 100‰ (p=0.0014) in RFs with low Fe-oxhydroxide concentrations.  In contrast subsoil C in RFs with high Fe-oxhydroxide concentrations are not influenced by contemporary land use.  Our findings indicate that contemporary subsoil C accumulation during reforestation is highly heterogeneous and spatially conditioned by historic C-Fe redox cycling.  We suggest that that moving beyond bulk soil sampling and analyses can improve our ability to detect and understand subsoil processes in forest ecosystems, and that historic C-Fe redox cycling may influence these processes more often than is commonly appreciated.