Evaluating termination methods of leguminous cover crops for optimizing nitrogen synchrony

Background/Question/Methods

In agroecosystems, synchronizing availability of legume nitrogen (N) to cash crop need is essential to efficiently utilize cover crops as a fertility source. Winter annual legume cover crop termination can be achieved through a variety of means, including tillage and non-tillage methods. However, the chosen approach may affect legume decomposition rate and consequently N mineralization by impacting availability to decomposing soil microorganisms. We predict that termination method in combination with biomass nitrogen content will govern mineralization rate. The objective of this work is to evaluate a range of legume species and termination methods to determine nitrogen release rates in corn following winter annual legumes. In this study, four leguminous winter cover crop species including Austrian winter pea, hairy vetch, and balansa and crimson clovers, were terminated using four different approaches. Termination treatments included rolling-crimping, flail mowing, disking, and herbicide application. Bi-weekly inorganic soil tests and Plant Root Simulator ion resin probes were used to measure plant available NO₃^- and NH₄⁺. Cover crop biomass, total carbon, and total nitrogen were measured for each species prior to termination. 

Results/Conclusions

In both Austrian winter pea and hairy vetch, peak plant available N occurred at six weeks after kill across all termination treatments, with vetch contributing more N than pea. At six weeks, rolled vetch produced significantly greater available soil N than any other termination treatment. Biomass contributions of crimson clover were higher (7,528 kg ha^-1) than hairy vetch (6,555 kg ha^-1) and Austrian winter pea (5,813 kg ha^-1), which both contributed similar amounts of biomass to the system. Balansa clover was the lowest contributor of biomass at 4,880 kg ha^-1. Austrian winter pea and hairy vetch had the highest total nitrogen content (216 and 191 kg ha^-1, respectively), followed by crimson clover (177 kg ha^-1), and balansa clover (108 kg ha^-1). Results demonstrate that termination method is tantamount to cover crop species choice in order to maximize nitrogen contributions from annual winter leguminous cover crops.