[M-IS21_28PM2] Biogeochemistry

[MIS21-23] Soil nitrite transformation along a forest slope and controlling factors

*Megumi KUROIWA¹, Kazuo ISOBE¹, Hiroyu KATO¹, Sho MURABAYASHI¹, Yuka KANEKO¹, Tomoki ODA¹, Nobuhito OHTE¹, Shigeto OTSUKA¹, Keishi SENOO¹ (1.Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Keywords:Forest soil, Nitrite, 15N tracer, Dissolved organic nitrogen, Nitrification

We conducted a tracer study to clarify the spatial heterogeneity of nitrite (NO₂^-) dynamics in forest soils. Because of its reactive nature, NO₂^- does not usually accumulate in forest soils. This low concentration and experimental difficulties of accurate quantification have hampered quantitative detailed analyses of gross NO₂^- production and consumption in terrestrial environments. However, NO₂^- is an intermediate in many N transformation processes including nitrification and denitrification. Furthermore NO₂^- can also be reduced to gaseous N and react with organic matter not only biologically but also chemically. Thus NO₂^- dynamics may control whole N retention/emission characteristics in forest soils. We added ¹⁵NO₂^- to mineral top soils derived from a slope of a Japanese cedar forest. Primary properties of soils such as concentration of inorganic N, pH and water content differed geographically; N concentration, pH and water content are lower in the upper soils. NO₂^- production and consumption rates gradually increased from upper slope to lower slope. Quite short mean residence time of NO₂^- implies that NO₂^- consumed very rapidly anywhere in slope. The dominant pathway of NO₂^- consumption change geographically. It is suggested that the conversion to DON and gaseous N is more important in upper soils. On the other hand, conversion to NO₃^- (nitrification) is dominant in lower soils.At this presentation, we focus on geographical difference of NO₂^- dynamics and their regulation by environmental factors.