Nitrate leaching: modifying the loss from mineralized organic matter

The impact on nitrate leaching of agronomic practices designed to immobiliz e nitrogen in autumn and winter was investigated over 4 years. Experimental treatments (reducing tillage depth, incorporating harvest residues, reduci ng fertilizer N by growing unfertilized grass or by spring-sown rather than autumn-sown crops) were compared with a control treatment in which autumn crops were sown after burning harvest residues, mouldboard ploughing and se edbed preparation. Winter cover cropping was also compared with winter fall owing. In the first year, incorporation of harvest residues or reducing til lage depth significantly decreased nitrate leaching compared with the contr ol. Unfertilized grass did not affect leaching in the first winter but sign ificantly decreased it in years 2 and 3. When winter cover crops were grown , nitrate leaching was never less than that under an autumn-sown cereal, an d in the subsequent year leaching could be significantly greater. Winter fa llowing caused the most nitrate leaching over the year. In the winter follo wing a spring-sown crop, leaching under an autumn-sown crop greatly increas ed. Summed over 4 years, most leaching occurred with the winter fallow-spri ng cropping treatment; it was 18% mon than where a winter cover crop preced ed the springs crop. Reducing tillage depth or incorporating harvest residu es did not significantly decrease leaching. Unfertilized grass ley followed by an autumn-sown cereal in the fourth year was the only treatment that si gnificantly reduced leaching loss compared with the control. Incorporating harvest residues resulted in a balance between annual N inputs and outputs. All other treatments required substantial net annual N mineralization to b alance annual inputs and outputs.