One of the most significant contributions that legume cover crops make to the soil is the nitrogen (N) they contain. Legume cover crops fix atmospheric N in their plant tissues in a symbiotic or mutually beneficial relationship with rhizobium bacteria. In association with legume roots, the bacteria convert atmospheric N into a form that plants can use. As cover crop biomass decomposes, these nutrients are released for use by cash crops. Farmers should make an effort to understand this complex process because it will help them to select the proper legumes for their cropping plan, calculate when to incorporate cover crops and plant cash crops that follow, and plan fertilizer rates and schedules for those cash crops. Above all, they need to inoculate legume seed before planting with the appropriate Rhizobium species.
The N associated with cover crop biomass undergoes many processes before it is ready to be taken up for use by cash crops. The process begins with biomass N, which is the nitrogen contained in mature cover crops. From 75 to 90 percent of the nitrogen content in legume cover crops is contained in the above ground portions of the plant, with the remaining N in its roots and nodules (Shipley et al., 1992).
When legume or grass cover crops are killed and incorporated into the soil, living microorganisms in the soil go to work to decompose plant residues. The biomass nitrogen is mineralized and converted first to ammonium (NH4) and then to nitrate compounds (NO3) that plant roots can take up and use. The rate of this mineralization process depends largely on the chemical composition of the plant residues that are involved (Clement et al., 1995), and on climatic conditions.
Determining the ratio of carbon to nitrogen (C:N) in the cover crop biomass is the most common way to estimate how quickly biomass N will be mineralized and released for use by cash crops. As a general rule, cover crop residues with C:N ratios lower than 25:1 will release N quickly. In the southeastern U. S., legume cover crops, such as hairy vetch and crimson clover, killed immediately before corn planting generally have C:N ratios of 10:1 to 20:1 (Ranells and Wagger, 1997). Residues with C:N ratios greater than 25:1, such as cereal rye and wheat, decompose more slowly and their N is more slowly released.
A study conducted in 1989 reported that 75 to 80 percent of the biomass N produced by hairy vetch and crimson clover residues was released eight weeks after the cover crops were incorporated into the soil (Wagger, 1989a). This amounted to 71 to 85 pounds of N per acre. However, not all of the released N was taken up by the subsequent corn crop. The corn utilized approximately 50 percent of the N released by both residues. (This value may be con-sidered the N uptake efficiency of corn from legume residues. This value is similar to the N uptake efficiency of corn from inorganic fertilizer sources, such as ammonium nitrate.) The N not taken up by the following crops may still contribute to soil health. Living microbes in the soil may use the nitrogen to support population growth and microbial activity in the soil.
(Source – http://www.cefs.ncsu.edu/resources/organicproductionguide/covercropsfinaljan2009.pdf)Read more