The Role of Pore Structure on Nitrate Reduction in Peat Soil: A Physical Characterization of Pore Distribution and Solute Transport

Abstract

Soil type is an important factor defining terrestrial ecosystems and plays a major role for the movement of solutes and cycling of nutrients and carbon. This paper focuses on the effect of peat complex dual-porosity structure on nitrate reduction, with the main objective to show how this process is controlled by pore-scale mass transfer and exchange of nitrate between mobile and immobile pore fractions. A mesocosm experiment was conducted where input solutions of bromide (Br−) and nitrate (NO3 −) were continuously supplied downward into 40 cm depth of peat. Br− and NO3 − breakthrough curves were used to constrain transport parameters and nitrate reduction rates in the peat depth profile. The vertical distribution of potential nitrate reduction rates were compared with depth distributions of partitioning mobile-immobile pores and the exchange coefficient between the pores. The results showed that an increase of immobile pore fractions with depth increases the common interface surface area between mobile and immobile pores which constitutes to a more pronounced exchange between the two transport domains and enhances the nitrate reduction. Hence, the pore structure with mobile-immobile pore fractions and exchange rate of solutes between mobile and immobile phases play a major role in nitrate reduction in peat soils.