Nitrogen transformations with special reference to gaseous N losses from zero-tilled soils of Saskatchewan, Canada

Abstract

This paper summarises the work on N transformations under zero-till (ZT) in comparison to the conventional-till (CT) farming system in chernozemic soils of Saskatchewan, Canada. Cumulative gaseous N losses (N 2 O + N 2) measured with acetylene inhibition-soil core technique ranged from 1 to 7 kg ha −1 year −1 N for CT and from 12 to 16 kg ha −1 year −1 N for ZT fields. In both CT and ZT, gaseous N losse were 2–5 times higher for a wheat-fallow than a continuous-wheat rotation. The denser surface soil and consistently higher moisture content of ZT fields were identified as the main reasons for higher rates of denitrification. The potential denitrification rates were markedly higher in ZT and the population of denitrifiers was up to six times higher than in the CT fields. Crop residues doubled the gaseous N losses. Temperature above 5°C did not alter denitrification rates nor did a wide range of mineral N. The contribution of lower soil horizons towards gaseous N losses was negligible. Mole fraction of N 2O [N 2O/(N 2O + N 2)] showed a reverse relationship with soil moisture and varied from 28 to 98% in the total gaseous N products. About 35% of autumn-applied 15N-labelled fertilizer N was lost via denitrification and 7–20% became immobilized by the following spring. Leaching was not responsible for the lower efficiency of fertilizer N. Due to adequate N fertilization and good straw mulch conditions, yield, N uptake and protein content of wheat were highest in the ZT system. The ZT system was also efficient in conserving more moisture from over-winter snowfall and rains.