Assessment of Two Chemical Extraction Methods as Indices of Available Nitrogen

Abstract

A soil test for estimating mineralizable N in soil is needed to improve fertilizer use efficiency. Our objectives were to: (i) test the effectiveness of two chemical indices in estimating potentially mineralizable N (N0) in soils, and (ii) determine if these indices could be used to differentiate the impact of cultural practices on the N‐supplying capacity of soils. We collected samples from the 0‐ to 0.15‐m depth of 42 soils representing all agroecological regions in Saskatchewan, Canada. We determined (i) N mineralized (Nmin), N0, and the rate constant (k) by aerobic incubation at 35°C for 24 wk, and (ii) the NH4‐N extracted by (a) 2 M KCl heated at 100°C for 4 h, and (b) a steam‐distilled phosphate‐borate solution buffered at pH 11.2. The association between N0 and the hot 2 M KCl extracted NH4‐N was close (r2 = 0.78, significant at P < 0.001); the relationship between N0 and the phosphate‐borate NH4‐N was slightly less precise (r2 = 0.73, P < 0.001). When NH4‐N extracted with cold 2 M KCl was subtracted from the NH4‐N extracted with hot extractants, the association with Nmin was weaker (r2 ≤ 0.50). The value of k for the 42 soils was generally constant (avg. 0.067 ± Sxt 0.05 = 0.040 wk−1). In a second experiment, we compared the two chemical extractants vs. the N mineralized in a 24‐wk incubation regarding their effectiveness in differentiating the effects of eight 37‐yr crop rotation treatments on N‐supplying capacity. These three indices performed effectively in this regard. For example, the 2 M KCl NH4‐N was closely associated with N mineralized in 24 wk (r2 = 0.92), as was phosphate‐borate NH4‐N (r2 = 0.88; both significant at P < 0.001). We concluded that the chemical extractants may provide a quick test for assessing N‐supplying capacity of the soil.

Comparison of Chemical Methods of Assessing Potentially Available Organic Nitrogen from Organic Residues Applied to a Sandy Soil

“…In fact, the present study also shows us that method 4 was more effective in predicting apparent net N mineralization during aerobic incubation (Table 3). This was true when the initial N min present was not subtracted from the total N value determined and had been already observed by other authors (Campbell et al 1997;Jalil et al 1996;Rodrigues 2000). Gianello and Bremner (1986) had also considered this method as the most suited for predicting N mineralization from SOM, when compared to biological methods.…”

“…Moreover, they claim that phosphate -borate solution is a powerful SOM extractant, considering that this solution extracts labile organic N from minerals' surface as well as soluble N org . Other authors have also evaluated the present method, concluding that the amounts of N min correlated with N mineralized during the aerobic incubation of 63 soils during 24 weeks (r 2 ¼ 0.77) and with canola uptake during a greenhouse experiment (Ila'ava and Waring 1992; Jalil et al 1996). However, others have found this method to be only fairly correlated to results obtained with maize or potato crops (Hong, Fox, and Piekielek 1990;Rodrigues 2000) or several different crops grown in 123 different soils (Xu et al, 1996).…”

“…Every half hour, the mixtures were shaken manually, and after the 4 h, they were contrifuged and analyzed for mineral N content (Jalil et al 1996).…”

“…In fact, this study shows that this procedure and procedure 3 generated similar results (Table 3) (P , 0.05). Jalil et al (1996) incubated 42 different soils, 65% of which contained more than 500 g kg 21 sand, and concluded that raising extraction time to 4 h but maintaining the temperature of the soil led to higher amounts of N extracted, as well as higher correlation with N 0 (r 2 ¼ 0.65). Their results are not consistent with the ones obtained in the present study, for the correlation with method 3 and N 0 are worse than those of method 2 (Table 3).…”

Optical sensors have potential for determining nitrogen fertilizer topdressing requirements of canola in Saskatchewan

“…However, traditional composite soil NO 3 -N tests do not account for either within-field spatial variability (Raun et al 1998) or potential mineralization of organic N during the growing season. The potential for various fractions of organic N to be used as indicators of potentially mineralizable N (Jalil et al 1996;Khan et al 2001) is limited by the fact that mineralization is affected by variables such as soil temperature and moisture (Akinremi et al 1999) and soil wetting and drying cycles (Mikha et al 2005). While grid-or landscape-directed soil sampling techniques characterize spatial soil N fertility (Franzen et al 1998), the labour requirements and costs of such procedures can be prohibitive.…”