Spring 1993 (v5n3)


Alfalfa and the nitrogen cycle in the Corn Belt.

Peterson, T.A. and M.P. Russelle

J Soil and water Conservation 46(3):229-235. 1991

This article provides the following estimates regarding alfalfa's contribution to the cycling of nitrogen in agricultural systems.

  1. Alfalfa forage is about 17.5 percent crude protein, 2.8 percent nitrogen.
  2. Roughly 50 percent of the nitrogen in the herbage is derived from N2 fixation in the seeding year and 80 percent is derived from fixation in succeeding years.
  3. Published estimates of annual N2 fixation by alfalfa range from 70 kilograms per hectare (63 lb/acre) for seedling stands to 400 kilograms per hectare (356 lb/ acre) in mature stands.
  4. Alfalfa roots have the capacity to absorb water and nutrients at depths of 11 meters. This characteristic makes alfalfa an efficient interceptor of residual soil nitrogen.
  5. The total nitrogen content of soil increases during alfalfa growth, possibly through excretion of symbiotically fixed nitrogen, or sloughing off of old root cells. Recent estimates agree that about 56 kilograms per hectare (50 lb/acre) are added per year.
  6. There is little nitrate leaching loss from a stand of developing alfalfa. However, nitrate leaching losses following destruction of an alfalfa stand can be significant under certain soil, climatic, and irrigated conditions.

 

Discussion

Alfalfa acts as a buffer in the nitrogen cycle by adjusting the amount of nitrogen fixed from the atmosphere to complement soil nitrogen available from other sources. Alfalfa may also be able to intercept and remove nitrate-nitrogen from great depths in the soil and prevent it from being leached into groundwater. Because of its high nitrogen content, care must be exercised when rotating a stand of alfalfa to other crops.

The authors recommend practices that prevent excessive or poorly-timed nitrogen mineralization and minimize potential leaching of nitrogen during noncropped periods. For example, the common practice of fall tilling alfalfa stands when rotating to a nonlegume should probably be discouraged; instead, growers should plowdown early spring regrowth. For a 3-year old alfalfa stand, this could contribute 380 kilograms nitrogen per hectare (339 lb/acre) to the available soil nitrogen pool over the next five years. In addition to minimizing nitrate-nitrogen leaching losses, this practice would permit alfalfa to use water in the fall and early spring, water that might otherwise be available for leaching.

In summary: "Producers and policymakers should be aware of the potentially negative effects of alfalfa and of ways to minimize chances for groundwater contamination when rotating to another crop. Most important, it is crucial that producers recognize the nitrogen supplied by legumes and manure, and reduce fertilizer nitrogen application rates for succeeding crops accordingly."

(RTN.1 29)

Contributed by David Chaney


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