DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION Relationships Between Soil Nitrogen Availability Indices, Yield, and Nitrogen Accumulation of Wheat

ney, 1982), have been proposed. Stanford and Smith (1972) developed a biologically based, long-term incuThe success of variable rate N fertilizer application rests on our bation method whereby potentially mineralizable N can ability to predict the contribution of soil N to growing crops. We assessed relationships between soil N availability indices (SNAIs), be estimated using one-pool (Stanford and Smith, 1972), yield, and total N accumulation of wheat (Triticum aestivum L.) two-pool (Molina et al., 1983), or incremental models grown in a typical glacial till landscape in Saskatchewan, Canada. (Ellert and Bettany, 1988). Incubation methods are Soil samples were collected at 3-m intervals along a 300-m transect time-consuming by nature, and thus more recent recomprised of low (LCFS) and high catchment footslopes (HCFS), search has focused on the development of more rapid and low (LCSH) and high catchment shoulders (HCSH). Total soil chemical extraction methods, such as the use of hot KCl, N and C, organic C, mineral N, depth of A horizon, spring soil moisfor estimating potentially available N (e.g., Gianello and ture, grain yield, and total plant N were measured. Soil N availability Bremner, 1986; Smith and Li, 1993; Jalil et al., 1996; indices used in this study included: (i) cumulative N released during a 2-wk aerobic incubation (NMIN ); (ii) potentially mineralizable N Curtin and Wen, 1999). Anion-exchange membranes estimated using a 16-wk aerobic incubation (N0 ); (iii) NO3 sorbed on also have been used to estimate soil N availability (Qian anion-exchange membranes (NO3AEM ); (iv) N extracted with hot KCl and Schoenau, 1995; Ziadi et al., 1999). (NKCl ); and (v) N hydrolyzed with hot KCl (NHYDR ). Although all Because many of the methods used to estimate N SNAIs were significantly correlated to yield and, with the exception availability measure, in part, the release of N from some of N0, total plant N when analyzed across the transect, typically 40% component of the soil organic matter pool, various indiof the yield variability was explained. Forward stepwise regression ces have been found to be closely related to total soil revealed that most SNAIs failed to explain more variability in crop organic matter levels (Keeney, 1982; Wang et al., 2001). N accumulation than did basic soil properties or relative elevation. Although these results do not invalidate the use of SNAIs for soil The quantity and quality of soil organic matter is known testing purposes, it is clear that SNAIs must be combined with addito vary within fields, with the most pronounced variabiltional information about field scale variability for predicting fertilizer ity usually occurring on hummocky or rolling terrain N requirements. Without this information, grid sampling as a means (Gregorich and Anderson, 1985; Verity and Anderson, of assessing N requirements remains ill-advised for glacial till semi1990). This variability has been attributed to loss and arid landscapes. redistribution of topsoil (Gregorich and Anderson, 1985; Pennock and de Jong, 1990; McConkey et al., 1997) and moisture (Verity and Anderson, 1990) from B N is often the most limiting nutrient for upper to lower slope positions. It follows that labile plant growth, there has been considerable interest fractions of soil organic matter, and thus potentially over the years in estimating plant-available soil N to available N, are likely to vary according to topography. predict fertilizer N requirements (Bremner, 1965; KeeThe goal of variable rate fertilizer N application is to ney, 1982). Although a large proportion of the total soil correctly predict the ability of a soil to supply N and N remains physically and chemically protected from thereby adjust fertilizer N application rates to reflect the microbial degradation in the stable soil organic matter variability in the N requirements of the growing crop. pool, and thus is unavailable for plant uptake (Jenkinson Interest in identifying suitable techniques for estimatet al., 1987), more labile fractions of soil organic matter, ing soil N availability for precision farming applications which are generally much smaller, remain an important led us to the objective of this study; to assess the relationsource of N (Jenkinson and Parry, 1989). ships between several SNAIs and the yield and total N Over the years, numerous SNAIs, based on either accumulation of unfertilized wheat grown on a glacial biological or chemical principles (Bremner, 1965; KeeF. Walley and T. Yates, Dep. of Soil Science, 51 Campus Drive, Abbreviations: CV, coefficient of variation; HCFS, high catchment Univ. of Saskatchewan, Saskatoon, SK, Canada, S7N 5A8; J.-W. van footslopes; HCSH, high catchment shoulders; LCFS, low catchment Groenigen, Dep. of Water and Environment, Alterra, P.O. Box footslopes; LCSH, low catchment shoulders; SNAI, soil N availability 47, 6700 AA Wageningen, the Netherlands; C. van Kessel, Dep. of index; NMIN, cumulative N released during a 2-wk aerobic incubation; Agronomy and Range Science, Univ. of California-Davis, Davis, CA N0, potentially mineralizable N estimated using a 16-wk aerobic incu95616. Received 24 July 2001.*Corresponding author: (walley@sask. bation; NO3AEM, NO3 sorbed on anion-exchange membranes; NKCl, N usask.ca). extracted with hot 2 M KCl; NHYDR, N hydrolyzed with hot 2 M KCl; SOC, soil organic C. Published in Soil Sci. Soc. Am. J. 66:1549–1561 (2002).