Denitrification in surface soils of a riparian forest: effects of water, nitrate and sucrose additions

We studied the effects of adding water, nitrate and sucrose on denitrification in a riparian forest that receives nitrate in drainage from cornfields. We used large (3-6 m2) flow-through chambers to sample N2O emissions from surface soils. N2 emissions were estimated by comparing N2O efflux in chambers with or without acetylene addition. Water and solutions of nitrate or sucrose were sprinkled on the soil within the chambers and subsequent changes in N2O efflux were observed continuously for several days. In 23 of 25 cases the soil beneath the chambers released N2O at 0.19-17 micrograms N2O-N m-2 h-1 prior to experimental treatments. In two chambers, there was consistent uptake of N2O from the atmosphere at 2.2 and 1.0 micrograms N2O-N m-2 h-1 prior to treatments. Release N2O was sometimes stimulated by adding water, usually stimulated by adding sucrose and always stimulated by adding nitrate. Stimulation by either nitrate and sucrose separately may reflect the heterogeneity of soil conditions that limit denitrification. After additions of nitrate or sucrose, N2O efflux rates usually peaked at more than 10 times pre-treatment rates, but effects of adding water were less pronounced. Responses to treatments began within minutes or hours and usually did not persist for more than a day. The rapid response suggests that denitrifiers possess a stock of enzymes ready to exploit rapidly changing conditions in the soil. Acetylene added to the air in the chambers penetrated at least 40 cm into the soil within 10 h, reaching concentrations of 3-8 ml l-1 in the soil gas. Adding acetylene alone had no consistent effect on N2O flux. However, adding acetylene and nitrate together resulted in greater stimulation of N2O efflux than adding nitrate alone, presumbably because acetylene inhibites N2O reduction to N2. The lack of effect of acetylene alone suggests that inhibition of N2O reduction may not be effective in situ unless nitrate is also added. Based on the combined effects of acetylene and nitrate, we infer that 8.2-90% of the N2O produced after adding nitrate without acetylene was converted to N2. The amount of N2O-N released after adding nitrate and acetylene was only 0.19-5.5% of the nitrate-N added.