Is KCl a reliable extractant of¹⁵NH ⁺₄ added to coastal marine sediments?

Abstract

The release of NH ⁺₄ and¹⁵N-labelled NH ⁺₄ by one-step KCl extraction was assessed in different types of coastal marine sediments. KCl was efficient to extract NH ⁺₄ from sandy sediments and less efficient in silt sediments, where an extended extraction period was required for obtaining a maximum NH ⁺₄ yield. Extraction at 0 or 20 °C had only a little effect on the efficiency of KCl. KCl gave always non complete recovery of¹⁵NH ⁺₄ in silt sediments. However, the added label could be fully recovered by addition of 80 μmol·cm⁻³ exogenous NH ⁺₄ prior to KCl, or when NaCl or ASW replaced KCl.¹⁵NH ⁺₄ was added to non-biological silt sediment, which was incubated at 0 °C up to 16 hours, to see the effect of physical processes on the partition of¹⁵NH₄ among porewater (29–49%) exchangeable (9–30%) and non-extractable, organic bound pools (24–42%). Total¹⁵N recovery was approximately 100%. KCl failed to remove¹⁵NH₄ which entered to unknown, bound pools in sediment. Only shortly after addition of¹⁵N (0.1 h), the extraction period resulted in significantly different¹⁵N recoveries (P < 0.05) in KCl extractable NH ⁺₄ , 17% versus 9% of label was recovered after 1 min or 60 min extraction of sediment, respectively. Two hours of incubation time were required for complete equilibrium of¹⁵NH ⁺₄ among porewater, exchangeable and organic bound pools. Sediments (silt) to which¹⁵NH ⁺₄ has been added in order to measure NH ⁺₄ turn-over and KCl is used as extractant, should be incubated for at least 2 hours, before taking a zero-time sample.