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

Abstract

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 Feox 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 (Feox) and Mn (Mnox) ratios (p < 0.01). Mössbauer 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:Feox (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 O2 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.