Carbon to nitrogen ratio and quantity of organic amendment interactively affect crop growth and soil mineral N retention

Using organic amendments to improve arable soils in the long term is a careful balancing act of applying amendments with the right carbon to nitrogen ratio (C:N ratio) at adequate quantity to avoid nitrogen (N) leaching while promoting or retaining crop growth in the short term. So far, most studies examining the relationship between C:N ratio and N mineralization and immobilization were done without plants. In this study we explored how crop biomass and N leaching change with increasing C:N ratio and quantity of organic amendments to arable soil. We conducted an open-air mesocosm experiment with organic amendments application across a range in C:N ratio (10 to 60) and quantity (10 to 50 ton ha-1) to sandy arable soil using a full-factorial design. Spring wheat was planted and grown for six months during which three rainfall events were simulated to test treatment effects on N leaching. Applying amendments with a C:N ratio of 20 and higher decreased crop biomass and increased mineral soil N, while amendments with a C:N ratio of 10 had the opposite effect. Applying larger quantities of amendments reinforced the effect of the C:N ratio on crop biomass. N leaching remained unaffected by either amendment C:N ratio or quantity or even mineral fertilizer as N leaching only occurred in the control treatment without plants. Our results suggests that growing a crop is adequate to prevent N leaching. Applying organic amendments do not pose a different risk regarding N leaching when compared to mineral fertilizer and slurry.