Contrasting fates of nitrate between organic and iron oxide-rich horizons of an acidic forest soil under oxic and suboxic conditions

Abstract

Elevated inputs of reactive nitrogen (Nr) have large and often detrimental effects on the environment and the magnitude and type of effects depend on the fate of Nr, such as water (NO3−) and air (N2O) pollution. Research has shown that soils are the primary sink of Nr in temperate forest ecosystems. Redox conditions, in conjunction with soil composition, stand to have an important but poorly constrained impact on the fate of this reactive N. In this study, we used tracer-level additions to contrast the fate of 15N–NO3- in organic (Oa) and in iron oxide-rich mineral (B) horizons under oxic and suboxic conditions. We performed stirred-jar laboratory incubations (up to 26 days) using soils from an acidic mixed-hardwood forest. We followed the fate of the tracer N into soil and solution pools (dissolved organic matter, NO3−, and NH4+), and also monitored solution concentrations of redox-sensitive species (NO2−, Fe2+, Fe3+, Mn). At the end of the incubations, ~3–7 times more tracer was recovered in soil material from the highly organic Oa horizon (57 ± 3% for oxic conditions and 73 ± 3% for suboxic) than in soil material from the organic-poor mineral B horizon (20 ± 5% for oxic, and 9 ± 2% for suboxic). For the B horizon, some 15N–NO3- remained in solution (29 ± 13%) under oxic conditions, while none remained under suboxic conditions (0.2 ± 0.02%); the main fate of 15N–NO3- went unrecovered, presumably due to gaseous losses, with more lost under suboxic (86 ± 3%) than oxic (47 ± 7%) conditions. Apparent gaseous losses were substantial in the Oa horizons as well, amounting to 35 ± 3% (oxic) and 12 ± 4% (suboxic) of tracer additions. Transformation rates of 15N–NO3- were greater under suboxic conditions for both horizon soil materials. Under both redox conditions retention was reaction rate limited in Oa horizon material and capacity limited in B horizon material. Our results indicate that in these acidic soils organic matter (OM) drives retention of nitrate in soil under both oxic and suboxic conditions, likely also accompanied by microbial denitrification; while lack of OM in iron oxide-rich mineral soils leaves nitrate available for other fates, with especially large gaseous loss in suboxic conditions. Mechanistically, it is likely that reduction of NO3− to NO2− is largely biotically driven, while subsequent reactions of NO2− are competitive between retention (nitrosation and microbial immobilization) and gasification (denitrification, chemodenitrification, self-decomposition) reactions. Both soil characteristics and redox conditions are critical determinants of NO3− fate in temperate forest ecosystems.