Utilizing 15N To Understand Rye Mulch Effects On Soybean/Weed Competition.

Cover crop mulches have proven effective at reducing weed emergence through both physical and chemical mechanisms.  They also influence N dynamics either with enhanced N availability in the case of legumes, or via immobilization in the case of grasses.  The objectives of these studies were to establish the relative importance of N immobilization as a mechanism of weed control and determine the longevity of the impact in soybeans no-till planted into a rye (Secale cereale L) mulch.  In the first study, ¹⁵N as (NH₄)₂SO₄ (99.7 at.) was injected below the soil surface in situ into 0.58 m² microcosms to monitor the N-transfer between the rye surface residue, soybean plants, and extractable soil inorganic N pools.  To determine the relative impact of fungal mediated transfer of N from the soil to surface residues, some plots received weekly doses of fungicide meant to eliminate fungal translocation. Fifteen percent of the isotope moved into the surface residue within eight weeks.  Fungicide applications had little impact on upward movement.  In the second study, six rye residue rates (0, 2000, 5000, 8000, 12000, and 15000 kg ha-1) were monitored every two weeks for ¹⁵N movement.  Surprisingly, the two highest rates decomposed much faster than lower rates and leading to a net N release over an eight week period.  Moderate rates of rye, however, caused significant immobilization and contributed to redroot pigweed (Amaranthus retroflexus L.) control by limiting soil N concentrations.