ORGANIC MATTER AND REDUCIBLE IRON CONTROL OF AMMONIUM PRODUCTION IN SUBMERGED SOILS

Abstract

Ammonium production is the key process for nitrogen (N) nutrition of wetland rice because N mineralization stops at ammonium production in submerged soils. Earlier results emphasized the importance of organic matter in controlling the release of ammonium in submerged soils. We studied the relationships between mineralizable N and ammonium in soil solution and reducible iron and organic C in 15 diverse West African soils. Mineralizable N released under anaerobic incubation at 30°C for 2 weeks was significantly correlated to organic C (r=0.79) and iron extracted by EDTA (EDTA-Fe) (r=0.86) or ammonium oxalate (Amox-Fe) (r=0.75). Ammonium in soil solution at 2 weeks after flooding also was correlated to organic C (r=0.71), EDTA-Fe (r=0.60) or Amox-Fe (r=0.60) although the correlation coefficients were lower than those obtained with mineralizable N. Multiple regression analysis of mineralizable N and ammonium in solution with organic C and reducible Fe showed that mineralizable N and ammonium in solution can be predicted from organic C and reducible iron status of the soils. The prediction of mineralizable N was better than the release of ammonium in soil solution. Our results underscore the importance of organic matter and reducible iron in controlling the production of ammonium in wetland rice soils.

Acknowledgments