Denitrification response to nitrate concentrations in sandy soils

The response of denitrification in soil to nitrate concentration was determined in intact cores to examine how soil C concentration and matric potential influence this relationship. In most soil cores the denitrification response to nitrate could be described by a Michaelis–Menten mathematical function. The rate of denitrification that occurred when NO3− was not limiting (Vmax) ranged from 10 to 3000 g N ha−1 d−1 and increased with increasing soil C and matric potential. In these non-stirred soils it was inferred that Vmax was attained when the concentration of NO3− in the bulk solution was sufficiently high that the demand for electron acceptors at the site of the active enzyme was met by diffusion. The Michaelis–Menten constant (Km) determined for different cores ranged from 1 to 100 mg N l−1. When Vmax values exceeded 150 g N ha−1 d−1 (which mostly occurred when water contents were relatively high) Km was more strongly correlated with Vmax than with any measured soil property. Below this value of Vmax this relationship disappeared and Km was affected most strongly by soil water variables. We conclude that in soil cores with the higher Vmax values, Km was modified by the demand for electron acceptors (other than O2) as reflected in the Vmax value, and is principally determined by O2 diffusion and C availability. However, in soil cores with lower Vmax values, restrictions on the rate of NO3− diffusion caused by low water contents were the principal controllers of Km. The implication for modeling is that rather than using a single Km value to describe the response of denitrification to NO3− concentration, a value can be estimated from predicted Vmax which varies with matric potential and C availability.