In situ validation of fungal N translocation to cereal rye mulches under no-till soybean production

Background and aims: The ability of grass mulches to inhibit weed performance has been linked to their limitations on nitrogen availability to the weeds. Fungal translocation of N from the soil to the surface mulch has been confirmed in laboratories, but this mechanism has not been documented under field conditions. Methods: Experiments used ¹⁵N (NH₄)₂SO₄ , 99.7 at.%, which was uniformly injected below the soil surface at a rate of 1 mg ¹⁵ N kg⁻¹ soil. Some plots were treated with a fungicide (Captan) every 2 weeks after injection, while others were not treated. Nitrogen transfer was monitored by measuring levels in surface residue, soybean tissue, and extractable soil inorganic N pools. Results: Despite the N release from the cereal rye (Secale cereale L.) tissues ranging from 15 to 50 kg N ha⁻¹, there was a detectable increase in ¹⁵N enrichment of 10–15 % in the cereal rye tissue. Six weeks after injection, tissue from the plots not treated with fungicide contained 36 % more ¹⁵ N. Conclusions: The increased ¹⁵N enrichment in the cereal rye mulch supports laboratory observations that soil inorganic N is translocated into surface mulch via fungal mechanisms. These findings illustrate microbial-mediated sinks for nitrogen in cereal rye mulches in no-till soybean production systems.