Soil Nitrogen Dynamics and Organic Bell Pepper Yield Are Affected By Rye–Vetch Cover Crop Residue Quality and Plastic Mulch

Cover crops can be an important source of nitrogen (N) for organic vegetable production, but N availability depends heavily on both the characteristics of the cover crop residues and crop management practices following cover crop termination.  A study was conducted over two seasons (2010 and 2011) in Michigan to investigate 1) how the proportions of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) sown in a winter cover crop mixture influence cover crop residue quantity and quality in the spring, and 2) how rye-vetch residue characteristics and the use of black plastic mulch (PM) interact to affect soil N dynamics and bell pepper (Capsicum annuum) yield the following summer.  Cover crop main plot treatments consisted of a gradient of 7 sown proportions of rye:vetch, from 100% rye (94 kg/ha) to 100% vetch (42 kg/ha), in addition to a no cover crop control.  Subplots consisted of bell pepper grown either with or without PM.  In both years, rye and vetch biomass in spring cover crop stands correlated well with fall seeding rates.  Stands with higher proportions of vetch generally contained more total N and had lower total residue C:N.  Both marketable bell pepper yields and soil inorganic N levels during the summers were positively correlated with expected N availability from the cover crop mixtures (based on total residue N concentration).  Yields and soil N levels were generally higher when PM was used, but the differences between PM and bare ground treatments were much less pronounced in 2011 than in 2010.  We suspect that differences in the magnitude and timing of summer precipitation events contributed to the discrepancy between the two years, with potentially greater N leached from bare ground treatments in 2010 than in 2011.  Our results suggest that PM can serve as an important “insurance policy” for N fertility derived from high quality cover crop residues incorporated early in the season, particularly on sandy soils. Furthermore, N availability is likely a dominant factor in the effects of both rye-vetch cover crop mixtures and PM use on bell pepper yield.  However, while higher proportions of vetch in the cover crop mixtures and the use of PM generally resulted in higher pepper yields, system performance must ultimately be based on a balanced consideration of all the services provided, their costs, and their relationship to primary production goals.