by Matthew Brown and Julie Grossman
This fall, graduate student Matthew Brown and Dr. Julie Grossman in the Soil Science Department, began the first phase of research looking at ways to better utilize legume cover crops as on-farm sources of nitrogen and carbon. This project is supported by funds from a Southern SARE grant. One of the major components of the study is to better understand how different methods of terminating winter cover crops affect the timing of nitrogen availability and the ability to match the subsequent crop’s needs. They will look at four different legume cover crops and four kill strategies to determine which work best for a particular crop and soil management regime. Species to be evaluated as part of this project are Austrian winter pea (Pisum arvense), Balansa clover (Trifolium michelianumssp balansae), ‘Auburn Sunrise’ variety of crimson clover (Trifolium incarnatum), ‘Auburn Early Cover’ variety of hairy vetch (Vicia villosa). The choice of these four varieties is the outcome of recent work evaluating 14 legume cultivars at three sites across North Carolina. The termination methods we chose are based on common and novel practices used by a wide variety of farms that utilize cover crops in their rotation plan: flail mowing, herbicide, rolling, and disking.
Microbes and fungi in the soil are responsible for the break down and release of nutrients from cover crop residue, a process called mineralization. Timing of nitrogen mineralization depends on the quality of the plant residue added and on the termination method used to kill the cover crop. With greater incorporation into the soil and breaking apart of plant material, there is increased soil-to-residue contact, likely stimulating the bacteria and fungi to break down the material more rapidly. We think that this, in turn, means that the process of decomposing plant proteins into ammonium, then nitrite, and finally plant-available nitrate will occur at a more rapid pace. Increasing soil nitrogen in this way, though beneficial for soil biota, might not necessarily deliver nitrogen at the right time to the cash crop. Rapid mineralization of the cover crop residue can be counter to the intentions of the farmer because nitrogen is not available when the crop needs it most and then it is at risk to environmental loss.
It is becoming clear in cover crop management that choosing reliable cultivars is only one major component of a larger crop rotation plan. In a study recently completed by the Grossman lab, nitrogen contributed from cover crop biomass alone ranged from 100-200 lbs ac-1 from vetch and 50-150 lbs ac-1 from clover, with a similar yield from Austrian winter pea. Finding a termination method that is compatible with crop nutrient needs can increase the use of cover crops and the benefits they provide. Used properly, leguminous cover crops can provide a terrific source of cheap nitrogen that can be incorporated into a sound nutrient management plan.
