Extraction and seasonal variation of NH4+ pools in different types of coastal marine sediments

Abstract

Seasonal variations in pools of porewater NH4+ and exchangeable NH4+ desorbed by 1-step (easily desorbed NH4+) and multiple KCl extractions (tightly bound NH4+) were investigated in sediments from 3 stations in Danish coastal waters. One-step KCl extractions gave higher yields in winter and lower yields in periods of increased sediment activity and temperature. The exchangeable NH4+ pool obtained by 1-step extraction was always less than the total pool removed by the multi-extraction technique, independent of sediment type. The multi-extraction technique proved the existence of sediment NH4+ pools with different exchange capacities. There was evidence to indicate that the tightly bound pool changed seasonally and entered the normal N cycle. The size and dynamic nature of this pool have not previously been recognized: maximum and minimum integrated values for Aarhus Bay, Norsminde Fjord and Randers Fjord sediments were 32, 36, 25 and 4, 9 and 6 mmol m-2, respectively. A lack of correlation between pool sizes of easily desorbed NH4+ and porewater NH4+ was not compatible with literature values of the NH4+ adsorption coefficient (K-value). K-values varied with station, depth and season (means of all values 3.7 and 11.7 by 1-step and multiple extractions, respectively), and were tn general higher than the literature values.