Abstract

Food and fiber production depends heavily on fertilizer N-inputs to sustain high yields. Various biological processes in soils and plants have generally restricted grain crops from obtaining N use efficiencies in excess of 70%. Nitrogen that could not be accounted for in N balance studies has often been assumed to have been lost to the atmosphere by denitrification or leached from the system. We evaluated wheat (Triticum aestivum L.) grain yield and soil profile inorganic N (ammonium N + nitrate N) accumulation in four long-term Oklahoma experiments where fertilizer N was applied annually (at a wide range of N rates) for more than 23 yr. Results showed that soil profile (0–8 ft) inorganic N accumulation did not increase until N rates exceeded that required for maximum yield. We suggest that the soil-plant system is able to buffer against soil accumulation of inorganic N. Major buffering mechanisms include increased plant protein, increased plant N volatilization, and denitrification in soil when N rates exceed those required for maximum yield. Annual N fertilization rates that increased soil profile inorganic N accumulation exceeded those required for maximum yields by more than 20 Ib N/acre in all experiments. This suggests the presence of a fertilization “safety zone” with regard to fertilizer N recommendations. Applying more fertilizer N than that required for maximum grain yield did not immediately pose a risk to groundwater quality. The same processes that prevent 100% use of fertilizer N by crops also prevent effective soil accumulation of inorganic N and risk of subsequent leaching to groundwater.

Research Question

United States consumption of fertilizer N has exceeded 10 million tons annually for the past decade. During this same period, the relationship of fertilizer N use and groundwater nitrate contamination has become increasingly important in the research community. Researchers have long acknowledged that crops seldom use more than 50% of applied fertilizer N. The many fates of fertilizer N have also been well documented and accepted in the scientific community. What happens to the fertilizer N that is not used by crops? Is it lost to the atmosphere or does it leach into the groundwater as nitrate?

Literature Summary

Established N processes that affect the presence and concentration of inorganic N (ammonium N and nitrate N) in soils were examined with regard to the influence of fertilizer N use. Several of the processes that result in loss of N from the soil-plant system (plant N volatilization, increased plant N removal, denitrification and leaching) may be more active when N is applied at high rates. Recent research results suggested that potential leaching losses did not take place in four continuous winter wheat experiments until N had been applied at rates in excess ofthat needed for maximum grain yield.

Study Description

The relationship of applied N fertilizer with grain yield and soil profile inorganic N accumulation was evaluated in long-term (> 23 yr) continuous winter wheat experiments. Fertilizer rates ranged from zero to greater than two times the rate required for maximum grain yield. Accumulation of ammonium and nitrate N was determined by taking three soil cores (0–8 fi) from each plot where N had been applied annually at fixed rates, and in plots where no N had been applied over this 23-plus yr period. Soil profile inorganic N accumulation did not begin until after grain yields peaked (Fig. 1). This suggests that the processes responsible for low fertilizer N use efficiency in crops are the same processes that buffer against inorganic N accumulation in the subsoil. The safety zone illustrated in Fig. 1 represents the amount of excess N that can be applied (beyond that required for maximum yield) without increasing soil profile inorganic N accumulation.

Fig. 1

Effect of annually applied fertilizer N on wheat grain yield and soil profile (0-8 ft) inorganic N accumulation following 23 yr of continuous production, Lahoma, OK.

 

Applied Question

How can we improve fertilizer N use efficiency?

Fertilizer N use efficiency is highest when fertilizer rates are low. By using smaller inputs of fertilizer N in crop production, less N is lost from the system and N use efficiency is higher.

What are the consequences of applying less fertilizer N in crop production?

The most obvious consequence is decreased crop production. Additionally, protein contents will decrease in the harvested grain or forage. Natural processes, like denitrification and N volatilization from plants, are shown to be active even when fertilizer N is not applied, hence crop N use efficiency (for nonfertilizer, soil available N) would still be less than 100%.

What are the implications of this N-buffering concept for crop production and the environment?

Several important factors emerge from this concept or philosophy.

1. The natural processes responsible for this buffer are operative across the entire range of fertilizer N rates commonly used. Thus, it is unlikely that fertilizer N use efficiency will ever approach 100% for a particular cropping system.

2. Since some of the same processes (e.g., volatile losses and immobilization) that contribute to inefficient crop use of fertilizer N also reduce or prevent accumulation of subsoil inorganic N when excess fertilizer N is applied, improving fertilizer N use efficiency could lead to increased risk of nitrate N movement to groundwater.

3. There is not a single fertilizer N rate that is optimum for crop yield. Maximum yield may be obtained over a range (± 10 lb or more N/acre) of fertilizer rates with little or no change in the risk of nitrate contamination of groundwater, especially in semi-arid dryland production systems. Our inability to accurately predict yield potential does not seriously limit our ability to recommend enough fertilizer N to support maximum yield without effecting risk to the environment.

4. The processes responsible for soil-plant buffering of inorganic N are not unique to semi-arid dryland production systems. Irrigated crop production and production in humid regions will also be characterized by a degree of soil-plant buffering against inorganic N accumulation or movement to the subsoil. Consequently, since movement to the subsoil must precede movement into groundwater, this natural buffering will delay groundwater contamination.

5. Soil testing, especially subsoil analysis for nitrate N, should be an important input for managing fertilizer N. Subsoil nitrate N levels substantially higher than surface nitrate N levels may be an indication of past excess N input and a signal that future inputs of N should be decreased.

Copyright © 1995. . Copyright © 1995 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 5585 Guilford Rd., Madison, WI 53711 USA