Link between paddy soil mineral nitrogen release and iron and manganese reduction examined in a rice pot growth experiment

Paddy soil indigenous N supply is often poorly related to N status and our aim was to assess its linkage to reduction of Fe3+ and Mn4+, primary terminal electron acceptors in submerged soils. Transplanted rice was grown in the greenhouse in four Bangladeshi paddy soils with distinct SOC to Fe-ox ratio (1.6 to 4.9) for 72 days firstly under continuous and then intermittent flooding. Solution buildup rates of Fe2+ and Mn2+ in the first 2-3 weeks of flooding correlated negatively with soil organic carbon (SOC) to NH4-oxalate extractable Fe (Fe-ox) and Mn (Mn-ox) ratios (p < 0.01). Mossbauer analysis detected ferrihydrite and goethite in all soils and with Mn3O4 these are the likely source minerals. An electron (e(-)) balance calculated from soil C-emission rates suggested reductive Fe and Mn dissolution to relevant e(-)-accepting processes, probably responsible for no > 50% of e(-) capture, though. Reduction of abundantly present octahedral Fe3+ in chlorites and vermiculite and their interstratified forms in these floodplain silty Inceptisols is hypothesized to also support microbial activity. Notwithstanding, a close temporal synergy existed between solution Fe and soil mineral N and their build-up rates were correlated (r: 0.77 to 0.90; p < 0.01) and with that of dissolved OC (DOC) (r: 0.84 to 0.96; p < 0.01), C emission rate (r: 0.99; p < 0.01) and SOC:Fe-ox (r: -0.71; p < 0.01). These correlations suggest Fe3+ reduction to be a relevant intermediary step in soil N mineralization, possibly through release of associated DOC, N or both. After switching to intermittent flooding dissolution of Fe, Mn and DOC were decoupled from mineral N release but since Eh remained in the Fe3+ -reduction range in three out of four soils, possibly clay-Fe3+ alternated with O-2 as e(-)-acceptor. Most importantly, in all soils N release slowed or halted after only 2 weeks of flooding but recommenced with intermittent flooding. As a next step, field experiments could verify if indigenous soil N supply also benefits from non-continuous irrigation management. Lastly, experimental proof is pending for release of clay-bound N and interlayer NH4+ following reduction of octahedral Fe3+ with consequent increased negative charge or structural destabilization, possibly an important process in floodplain paddy soils in Bangladesh.