Managing nitrogen in cover crop systems

Dr. John Lauzon, University of Guelph (adapted from a presentation given at the workshop – Making Sen$e with Cover Crops Feb. 28, 2013)

Cover crops may be grown for many reasons including; reducing erosion, adding soil organic matter, soil structural improvement, nematode suppression, weed suppression, scavenging residual nitrogen, providing nitrogen to the next year’s crop, etc.  However not all cover crops are able to provide all of these benefits.  Nitrogen management, for instance, can vary greatly depending on the cover crop choice used.

Nitrogen Scavenging

Provided that you have selected a cover crop that is likely to establish and grows well, all cover crop species are capable of taking up residual plant available nitrogen.  Although legume species can form a symbiotic relationship with nitrogen fixing bacteria (nodules), if there is nitrogen available in the soil, they are also capable of using it.  As such, the main factor that influences nitrogen uptake from the soil is the total amount of cover crop biomass produced

Cover Crop Nitrogen Requirement

All plants, including cover crops, require nitrogen for growth.  Legume cover crops can take up soil nitrogen, but they can also get their nitrogen from fixation of atmospheric nitrogen by nitrogen fixing bacteria in root nodules.  As long as nothing affects nodule formation and function, legume cover crop growth is unlikely to be limited by nitrogen supply.  Non legume cover crops (eg. grasses, brassicas) are not capable of symbiotic nitrogen fixation, as such; their growth is limited when soil nitrogen supplies are limited.  If the main objective is to scavenge nitrogen this is not a problem, having limited growth due to low nitrogen supplies may indicate that you have accomplished this.  Having lush growth may also be an indication that nitrogen supplies to the main crop were greater than plant requirement and you may be able to save on nitrogen fertilizer without losing yield.  However, low residual plant available soil nitrogen could also result in disappointing amounts of biomass production, particularly to anyone who may want to harvest the cover crop for forage.  In these cases, there may be an economic advantage of adding nitrogen.

Nitrogen Supply from Cover Crops

The amount of nitrogen supplied to next year’s crop varies greatly depending on the cover crop type and management.  The main factors that dictate weather nitrogen will be released for the following crop, is the carbon to nitrogen ratio of the cover crop and the speed of decomposition.  For cover crops with low carbon to nitrogen ratios (less than 25:1), plant available nitrogen will be released during the early stages of decomposition, because the soil organisms eating it are getting more nitrogen than they require for growth and reproduction, so they excrete the extra as plant available nitrogen.    Figure 1, illustrates the typical pattern of change in soil organism numbers and plant available nitrogen release from low carbon to nitrogen organic materials.

Provided that soil conditions are correct for decomposition, soil organism numbers will increase when a food source (organic carbon) is added. Plant available soil nitrogen levels will also increase over time.  The amount of time required for release is dependent on how easily digestible the cover crop is, and soil factors such as temperature, moisture, pH, etc. that may influence the activity of the organisms involved.  The key here is to time the cover crop control date to best match the nitrogen release time with the nitrogen uptake pattern of the crop you intend to grow.

When high carbon to nitrogen ratio (greater than 25:1) material decomposes, the nitrogen requirement of the soil organism eating it will not be achieved.  In these cases the soil organisms can use plant available nitrogen to overcome this deficit during the early stages of decomposition.  Figure 2 illustrates the typical changes in soil organism numbers and soil plant available N levels that are typical when high carbon to nitrogen ratio organic material is added to a soil.  In this case plant available nitrogen levels decease during the early stages of decomposition.  However, due to continued soil organism respiration, carbon will leave the system as carbon dioxide leading to a surplus of soil organism nitrogen which will be released back into the soil. The nitrogen release rate however, is usually slow.   This means that, just like low carbon to nitrogen ratio materials, the key to management is matching the nitrogen release with crop requirement.

Legume cover crops like red clover that is controlled either in the fall or early spring typically have a low carbon to nitrogen ratio and will typically give nitrogen credit that is mainly dependant on the amount of cover crop biomass.  Some cover crops, like oilseed radish, will be controlled by a hard frost.   Although the carbon to nitrogen ratio is typically low enough for nitrogen release during the early stages of decomposition, the rate of release is too fast and usually any nitrogen released is lost before the following crop grows resulting in no nitrogen credit.  Still other cover crops, such as an older stand of ryegrass, can have a carbon to nitrogen ratio that results in reductions in plant available soil nitrogen during the early stages of growth.   If these cover crops are controlled too close to crop planting there can be an increase in fertilizer nitrogen requirement.  However when these cover crops are controlled earlier, for example late fall, usually there is no impact on fertilizer nitrogen requirement.

Further information on specific cover crop species can be found on the OMAF cover crop web site (http://www.omafra.gov.on.ca/english/crops/facts/cover_crops01/covercrops.htm) and the Midwest cover crop council web site (http://www.mccc.msu.edu/CCinfo/cropbycrop.html). A Cover Crop Decision Tool for Eastern Canada is found at http://decision-tool.incovercrops.ca/.