DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS Fertilizer Nitrogen Recovery in Long-Term Continuous Winter Wheat

et al., 1982; Webster et al., 1986). Nitrogen balance research in agricultural production systems continues Fertilizer N recovery in crop production systems seldom exceeds to focus on NO3–N leaching (Jokela and Randall, 1989; 50%. Two long-term winter wheat (Triticum aestivum L.) experiments were selected for use of 15N to evaluate fertilizer recovery in Olson and Swallow, 1984). However, little evidence exthe grain, straw, and soil. Each experiment included variable nitrogen ists to document leaching losses from nonpoint sources rates that had been incorporated preplant each year for .20 yr. In of fertilizer N in many crop production systems (West1988, ammonium nitrate enriched with 11.888 atom % 15N erman and Kurtz, 1972; Varvel and Peterson, 1990). (NH4NO3) was applied to microplots within the main plots of these Recent work has focused attention on gaseous N loss long-term experiments. Conventional ammonium nitrate (0.366% 15N) from plants. Ammonia losses from plants have been was added to the microplots the following 2 yr to allow measurement measured in wheat (Harper et al., 1987; Parton et al., of residual effects of the enriched fertilizer. Three years after the 15N 1988) and corn (Francis et al., 1993). In experiments microplots were established, soil cores (0.025 m in diam.) were taken conducted by Sharpe et al. (1988), it was estimated that to a depth of 1.20 m and partitioned into 0 to 0.15, 0.15 to 0.30, 0.30 21% of the applied N was lost from the wheat plant and to 0.45, 0.45 to 0.60, 0.60 to 0.90, and 0.90 to 1.20 m. From all microplots, percentage 15N recovered in the grain and straw at harvest and in the soil as volatilized NH3 in an experiment that reported soil was determined and expressed as atom % excess corrected for no NO3–N leaching. Without exception, pre-1980s 15N background abundance. Total fertilizer N recovery (15N removed in balance work, which attributed unaccounted N loss to the grain and straw plus that remaining in the soil) decreased with leaching and denitrification, did not consider plant N increasing N application at both locations. Results from this study loss as a potential sink. Because of this, past 15N balance combined with previously published findings from these locations studies that documented crop removal, leaching, and suggest that fertilizer N recovery was greater where there was: (i) no fertilizer N that remained in the soil can now be used evidence of priming (increased net mineralization of organic N pools to estimate combined losses from denitrification and when low rates of fertilizer N are applied); and (ii) reduced soil–plant plant volatilization. buffering (N that can be applied in excess of the amount needed for Recent work by Johnson and Raun (1995) proposed maximum yield without resulting in increased soil profile inorganic N accumulation). a soil–plant buffering concept that was defined as the ability of the soil–plant system to limit the amount of inorganic N accumulation in the rooting profile when N fertilization rates exceed that required for maximum R of fertilizer N in crop production sysgrain yields. The researchers indicated that decreased tems has been investigated for years, however, N soil–plant buffering will likely increase the risk for balance studies seldom account for .50% of the fertilNO3–N leaching. Their work noted that a consequence izer N applied (Rasmussen and Rohde, 1991). Early of the buffering mechanisms (increased gaseous plant work by MacVicar et al. (1950) reported fertilizer N N loss, increased grain protein, increased denitrification, recovery levels ,47% when evaluating sudangrass [Sorincreased immobilization, and increased ammonia volaghum bicolor (L.) Moench] and oat (Avena sativa L.). tilization from soils when N fertilizer rates exceed that Their work, which employed 15N labeled fertilizer, conrequired for maximum yield) was poor N-use efficiency sidered denitrification to be a significant sink for unacin crop production (often ,50%). However, they also counted N. Similar studies with sudangrass by Carter noted that the buffering mechanisms are desirable, conet al. (1967) attributed unrecovered 15N to gaseous loss sidering that losses and removal through these sinks since potential leaching losses were accounted for. More decrease the potential for NO3–N leaching (Johnson recent N balance studies have attributed sizable losses and Raun, 1995). of fertilizer N to denitrification (Owens, 1960; ChichesThe objectives of this experiment were (i) to deterter and Smith, 1978; Olson, 1980; Fredrickson et al., mine the efficiency of N fertilizer applications in contin1982). Unaccounted fertilizer N has also been attributed uous winter wheat; and (ii) to determine the residual to immobilization in surface soil horizons (Fredrickson effect of 15N fertilizer applications on winter wheat in experiments where the same N rates have been applied for .20 yr. Dep. of Plant and Soil Sciences, 044 North Ag. Hall, Oklahoma State Univ., Stillwater, OK 74078. Contribution from the Okla. Agric. Exp. Stn. Received 16 Jan. 1998. *Corresponding author (wrr@agr. MATERIALS AND METHODS okstate.edu). Two continuing long-term winter wheat fertility experiments were selected for additional study. Both experiments Published in Soil Sci. Soc. Am. J. 63:645–650 (1999).