Distribution of adsorbed ammonium pools in two intertidal sedimentary structures, Marennes-Oleron Bay, France

The 1-step and multiple-step KCl extraction technique was used to evaluate quantitatively the distribution of adsorbed NH4+ in the 0-5 cm strata of 2 intertidal muddy structures from Marennes-Oleron Bay, France. KCI extractio n efficiency was higher in the reconstituted sediment slurries and decrease d significantly in the presence of porewater ions, leading to underestimati ons of the size of the pool of adsorbed, easily desorbed NH4+. Various pool s of adsorbed NH4+ were displaced from the sediment after reaction with KCl in multiple steps. Pools of tightly bound NH4+ were much larger in runnels as opposed to ridges, and accounted for up to 32% (ridge) and 40% (runnel) of the NH4+ desorbed from the sediment. These pools have important implica tions for the calculation of the KD value, the dimensionless NH4+ distribut ion coefficient. Differing relationships between easily desorbed NH4+ or ti ghtly bound NH4+, on the one hand, and the sediment grain size, CaCO3 and N content, on the other, suggest multiple origins for particulate matter in the sediment.