Subsoiling Combined With Winter Active Italian Ryegrass Can Reduce Nitrous Oxide Emissions Without Increasing Nitrate Leaching: Evidence From Lysimeter and Field Plot Trials

Abstract

Subsoiling is a common practice for improving soil structure and has recently been recognised for its potential to reduce nitrous oxide (N2O) emissions. However, its impact on nitrate (NO3−–N) leaching must also be considered if it is to be used as a mitigation strategy. This study investigated N2O emissions, NO3−–N leaching and grass yield in an Italian ryegrass pasture soil with a compacted subsoil layer using a lysimeter study. Additionally, a separate field study examined mineral nitrogen (N) concentration and the abundance of nitrifying and denitrifying genes. Both studies have four treatments, with non-subsoiling (NS), subsoiling (SS), non-subsoiling and urine (NSU), subsoiling and urine (SSU), and the urine treatments were applied at a rate of 700 kg N ha−1. The results showed that 6 months after subsoiling, soil macroporosity in all treatments remained elevated at a depth of 0–20 cm (p < 0.01) compared with all non-subsoiling treatments. This created more aerobic conditions, which suppressed N2O emissions from denitrification by 20.3% (p < 0.05) during 190 days after urine application (NSU vs. SSU). Additionally, subsoiling had no significant effect on NO3−–N leaching in the presence of winter-active Italian ryegrass. Molecular analysis of N-cycling microbial communities revealed that subsoiling had no effect on the abundance of amoA gene-carrying microorganisms involved in nitrification but reduced the abundance of nirS denitrifier genes, indicating that subsoiling primarily affected the denitrification process. These findings suggest that subsoiling can effectively reduce N2O emissions without increasing NO3−–N leaching when combined with winter-active ryegrass.