A Response Surface Study to Explore Nitrogen Immobilization to Rye Surface Residue in a Mulched Cropping Systems.

Weed management in organic grains production offers many unique challenges not associated with conventional grain production, mainly due to the lack of highly effective herbicide management systems.  In organic cropping systems, a balance between weed suppression and soil conservation is paramount.  An emerging technology that utilizes roller-crimped mulches has the potential to minimize soil degradation while enhancing weed suppression.  Cover crop mulches suppress weeds through physical and chemical mechanisms.  Current research suggests that a third nutritive N-immobilization mechanism may play an important role in reducing weed/crop interference.  The objective of this study was to establish the optimal rye residue mulch rates that maximize N immobilization, and determine the N deficit response to the subsequent escaped pigweeds.  Six rye (Secale cereale ‘Wheeler’) residue rates (0, 2000, 5000, 8000, 12000, and 15000 kg ha^-1) were applied as dry matter in 0.58 m² microcosm that had the light stable isotope ¹⁵N as (NH₄)₂SO₄ (99.7 at. %) injected below the soil surface in situ at 2 ppm ¹⁵N in order to monitor N-transfer between the rye surface residue, extractable soil inorganic N pools, pigweed (Amaranthus Retroflexus L.), and soybean  (Glycene max L. ‘NC Roy’) tissues,  throughout the season in a mulched soybean system.   Non-constant rye decomposition was observed between the five different rye surface mulch treatments where increased decomposition was observed in the for the 8000 and 12,000 kg ha^-1 rye residue rates, and at Goldsboro.  Despite rye N loss observed for all rye residue rates an increase of fungal transferred ¹⁵N was observed in the rye surface mulches.  Extractable soil inorganic N declined at Beltsville and Goldsboro, 6 and 4 kg N ha^-1 respectively.  Consequently, pigweeds 10 cm from the soybean row experiences intense competition for available N where the ‘Rye Roots Only’ treatment negatively influenced pigweed N assimilation equivalently to 8000 kg ha^-1.  These findings support the nutritive weed suppressive mechanisms of high biomass rye cover crop mulches, especially at rates below 8000 kg ha^-1 during the critical weed free period of soybeans, thereby enhancing the weed suppression of the system.