Evolution and soil entrapment of nitrogen gases formed by denitrification in flooded soil

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Abstract
The direct field measurement of N loss by denitrification in flooded soils has been a long-standing challenge. A field experiment was conducted in puddled, flooded soil without plants to ascertain whether the directly measured evolution of (N2 + N2O)-15N from 15N-labeled nitrate applied to floodwater was an accurate measure of denitrification loss. The evolution of (N2 + N2O)-15N was determined from the 15N content of air samples collected in a chamber placed over the floodwater. The evolved (N2 + N2O)-15N increased for 2 to 3 days following addition of either 3.5, 6.9, or 10.4 kg nitrate-N-ha-1 and then decreased on succeeding days. Evolved (N2 + N2O)-15N correlated (r = 0.86) with floodwater (NO3 - + NO2 -)-N measured 48 h earlier. The total (N2 + N2O)-15N evolved over the 10 days following nitrate addition ranged from 20 to 25% of the added N. The added 15N not recovered in the soil and floodwater after 10 days ranged from 48 to 64% of the added N. This unrecovered 15N presumably represented denitrification loss, since runoff and leaching losses were negligible. The failure of evolved (N2 + N2O)-15N to quantify denitrification was attributed to entrapment of 15N gases in soil. Nitrogen gases collected from 0.04-m soil depth, but not from floodwater, remained enriched in 15N after the (N2 + N2O)-15N evolution decreased to negligible levels (less than 15 g N · ha-1-day-1). Immobilization competed with denitrification for nitrate-N. Control of algal growth with the algicide terbutryne reduced nitrate immobilization and increased denitrification from 61 to 87% of the applied 6.9 kg nitrate-N · ha-1. Application of 17 kg P · ha-1 reduced denitrification from 61 to 51%, presumably because of enhanced growth and nitrate assimilation by algae and other biota.