Control of Fe and Mn availability on nitrogen mineralization in subtropical paddy soils

The availability of alternative electron acceptors like Fe3+ and Mn4+ may form a bottleneck to anaerobic SOM mineralization and thereby NH4+-release in flooded paddy soils. We assessed the influence of availability of soil Fe and Mn on anaerobic N mineralization in lab incubation experiments. Collected paddy soils from Bangladesh either untreated, amended with Fe2O3, or with Mn/Al mixed oxides were anaerobically incubated. In a first 8 weeks incubation with 5 treatments from a long-term field experiment (control, N, NP, NPK and N + FYM) we found no considerable differences in evolution of soil solution Fe and Mn between the control and Fe2O3 treated soils. Whereas, the soil solution contents of Fe were lower and Mn were higher in Mn/Al mixed oxide treated soils. Similar observations were made for dissolved Fe and Mn in a second 10-week incubation experiment with four farmers field soils. Evolution of KCl-ixtractable NH4+ was not affected by Mn4+ or Fe3+ application and we therefore conclude that availability of electron acceptors was not limiting release of NH4+ in the studied soils. The large and rapid increase of exchangeable-NH4+ at the onset of the incubations provoked the question whether part of it derived from release of fixed-NH4+. A third experiment revealed, however, instead a small significant increase of the fixed-NH4+ within four weeks. In addition, the microbial biomass carbon already plateaued after two weeks. Both results suggest that released mineral N was mainly derived from biotic anaerobic N mineralization and not from defixation of NH4+. Finally, while not directly dependent on Fe and Mn application, there was a remarkable convergence in the buildup of soil exchangeable-NH4+ and soil solution Fe concentrations. This warrants further investigation and still suggests involvement of reductive Fe and Mn-oxide dissolution in NH4+-release, e.g. through release of bound organic N after reduction of these oxides. (C) 2016 Elsevier B.V. All rights reserved.