Nitrous oxide emissions from denitrification and the partitioning of gaseous losses as affected by nitrate and carbon addition and soil aeration

Abstract

National inventories of N2O emissions from agricultural situations are being developed; however, the factors controlling such emissions may vary with soil and environmental conditions and management practices. This study determined the relative importance of soil aeration, as measured by water-filled pore space (WFPS), NO3- addition, C addition and C source on the amount and partitioning of gaseous N losses from denitrification from an arable soil in Atlantic Canada. Denitrification (N 2O + N2) and N2O emissions were measured on repacked soil cores using acetylene inhibition. The N2O:(N 2O + N2) ratio was frequently 0.7 or higher, indicating that most emissions occurred as N2O. N2O emissions and denitrification were negligible at a WFPS of 0.45 m3 m-3, and high at WFPS of 0.75 m3 m-3, regardless of NO 3- or C addition treatments. At a WFPS of 0.60 m 3 m-3, N2O emissions and denitrification were low and were increased by both NO3- and C addition treatments. Carbon source was investigated by amendment with glucose, red clover or barley straw. Based on the quantity of soil respiration per unit of C added in the amendment, C in the red clover and barley straw was estimated to be 48 and 28% as available as glucose C. When corrected for C availability, cumulative N2O emissions averaged 0.010, 0.011 and 0.002 mg N kg-1 soil, and cumulative denitrification averaged 0.014, 0.014 and 0.003 mg N kg-1 soil, for each 1.0 mg C kg-1 soil of available C added as glucose, red clover or barley straw, respectively. NO3- addition had no effect on denitrification, but increased N 2O emissions, especially where C availability was high. The amount of denitrification was controlled primarily by soil O2 supply, as controlled by WFPS and C availability. The N2O:(N2O + N2) ratio was generally high in cases where the supply of O 2 or NO3- was sufficient to meet the demand for terminal electron acceptors.