Illinois
Fertilizer Conference Proceedings
January 21-23, 2002
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S.A. Khan, R.L. Mulvaney, and R.G. Hoeft1
Reports that corn is sometimes nonresponsive to nitrogen (N) fertilization (e.g., Bundy and Malone, 1988; Fox et al., 1989; Roth and Fox, 1990; Meisinger et al., 1992; Brown et al., 1993; Schmitt and Randall, 1994) have stimulated recent work to identify a specific fraction of soil organic N that mineralizes readily, as a means to detect sites where N fertilization is unnecessary. After eliminating major defects in the methodology to fractionate the N in soil hydrolysates (Mulvaney and Khan, 2001), studies showed a much higher concentration of amino sugar-N for nonresponsive than for responsive soils, whereas no consistent difference was detected in their concentrations of total hydrolyzable N, hydrolyzable NH4 +-N, or amino acid-N (Mulvaney et al., 2001). In subsequent incubation experiments, nonresponsive soils produced a much larger quantity of mineral N than did responsive soils, and mineralization was accompanied by a net decrease in amino sugar-N but not in amino acid-N (Mulvaney et al., 2001).
Based on these findings, a simple soil test has been developed to estimate amino sugar-N, which avoids the need for acid hydrolysis or chemical extraction (Khan et al., 2001). This Illinois soil N test provides an unprecedented capability to detect sites where corn is unlikely to respond to N fertilization, but work by Brown (1996) indicates that nonresponsive sites can become responsive when moisture availability is limited. Such occurrences would be expected if adverse weather conditions limit mineralization of amino sugar-N.
Three specific objectives of the present project were to compare (1) hydrolyzable forms of N and amino sugar-N test values for soil samples collected from sites that were much less responsive to N fertilization in a normal growing season than when moisture was limiting; (2) production of mineral N by these samples when incubated under different moisture levels and temperatures; and (3) amino sugar-N test values before and after incubation.
The soils used (Table 1) were surface (0-30 cm) samples representing six of the 75 site-years studied by Brown (1996). The particular samples used had been collected from three different sites, each of which had been sampled in consecutive growing seasons differing in moisture supply. Soil sampling was done with a 2.5-cm (dia.) soil probe (five cores per sample) in late March or early April of 1990, 1991, or 1992, prior to planting corn. The soil cores had been frozen within 12 h after collection, and were subsequently allowed to thaw at room temperature, screened to < 2 mm while still field moist, and mixed thoroughly. A 10-g subsample of the sieved soil was extracted with 100 mL of 2 M KCl. The extracts were analyzed for (NO3- + NO2-)-N by steam distillation techniques (Mulvaney, 1996), so as to determine NO3--N in the surface 30 cm for the presidedress NO3- test (PSNT), or in the surface 60 cm for the preplant NO3- test (PPNT). The remaining soil (< 2 mm) was allowed to air-dry at room temperature, then transferred to polyethylene or paper bags for storage. Before use, each sample was ball-milled to pass through a 0.15-mm scree.
Of the chemical analyses reported in Table 1, data for pH, Bray-1 P, and exchangeable K were obtained from Brown (1996); organic C was determined by the method of Mebius (1960); and total N was determined as described by Stevens et al. (2000). The percentage response of corn to N fertilization was calculated as 100 × (optimum yield - check-plot yield)/check-plot yield, using data reported by Brown (1996) for check-plot and optimum yields. Values for optimum yield were determined by fitting N-rate and corresponding yield data to a quadratic plateau model by nonlinear regression (SAS Institute, 1993).
Soil Hydrolysates and Hydrolyzable Nitrogen
To prepare soil hydrolysates, 5-g samples of soil (three replicates) were heated (110-120°C) under reflux for 12 h in 125-mL Erlenmeyer flasks fitted with a 24/40 ground-glass joint for attachment to a 40-cm Liebig condenser, after treatment with 20 mL of 6 M HCl and two drops of octyl alcohol. The hydrolysis mixture was filtered through Whatman no. 50 filter paper under vacuum, subsequently neutralized by addition of 10 M NaOH (pH 6.5-6.8), and brought to 100 mL by addition of deionized water. Prior to use, the neutralized hydrolysates were stored under refrigeration (5°C). Using the diffusion methods described by Mulvaney and Khan (2001), analyses were performed for total hydrolyzable N, NH4+-N, (NH4+ + amino sugar)-N, and amino acid-N. Amino sugar-N was determined as the difference between (NH4+ + amino sugar)-N and NH4+-N.
Soil Nitrogen Test
The procedure described by Khan et al. (2001) was employed to estimate (NH4+ + amino sugar)-N.
Six-g samples of air-dried soil (< 0.15 mm), representing two normal and two dry seasons (12 samples from each of site-years 1, 3, 4, and 6), were weighed onto Whatman QM-A quartz filter material in the cup of a 5.5-cm (dia.) polypropylene Büchner funnel and leached under vacuum with two 30-mL aliquots of 0.01 M CaCl2, while taking care to prevent loss of soil in the leachate. Soil moisture content was adjusted to 30 or 60 percent of water-holding capacity (WHC), and triplicate cups containing the moistened soil samples were then placed in an incubator maintained at 85-90 percent relative humidity, and either 20 or 30°C. After 1, 2, 4, or 8 wk, 12 samples representing each site-year were transferred to 125-mL polyethylene bottles, and mineral N was extracted by shaking the soil sample with 60 mL of 2 M KCl for 2 h on a reciprocal shaker and filtering the resulting suspension through Whatman no. 42 filter paper under vacuum. The bottle was rinsed twice with 10 mL of deionized water from a wash bottle, so as to ensure complete recovery of the incubated soil sample. Soil extracts were analyzed by accelerated diffusion methods (Khan et al., 1997) to determine NH4+-N and (NO3- + NO2-)-N.
To monitor changes in soil-test N during the 8-wk incubation period, extracted soil samples were dried for at least 2 d at 25°C under a flow of forced air from a fan. One g of each sample was then transferred to a 1-pint (473-mL) wide-mouth Mason jar, and the soil test of Khan et al. (2001) was performed to estimate amino sugar-N.
At biweekly intervals, samples incubated for 4 or 8 wk were leached under vacuum with two 30-mL aliquots of 0.01 M CaCl2. Following leaching, the soil moisture content was readjusted to 30 or 60 percent of the soil's waterholding capacity, and incubation was resumed at 20 or 30°C. The leachates were analyzed for NH4+-N and (NO3- + NO2-)-N as described previously for soil extracts.
To serve as a zero-time control, triplicate soil samples (6 g) representing each site-year were leached and then extracted for determination of inorganic N, prior to soil N testing.
All analytical determinations reported were performed in triplicate. Mean values for replicate incubations were compared on the basis of a least significant difference (LSD) at the 0.001 probability level.
Estimation of plant-available N is complicated enormously by the dynamic nature of soil N, owing largely to the effects of temperature and moisture supply on N-cycle processes. In the Illinois soil N test developed by Khan et al. (2001), (NH4+ + amino sugar)-N is estimated as an indicator of potentially mineralizable soil N, so as to detect sites where corn does not respond to N fertilization. This technique was developed using soil samples collected from sites that received normal rainfall during the growing season, but subsequent experience has shown that a yield response sometimes occurs with sites having a high test level, particularly in cases involving a dry and/or cool spring.
The present study was designed to evaluate the hypothesis that soil testing for amino sugar-N may not correctly predict a lack of Nfertilizer response if adverse weather conditions limit mineralization. The soils used had been collected by Brown (1996) from three farms prior to consecutive growing seasons that differed substantially in rainfall. At each farm, field plot location varied somewhat between the two seasons, which accounts for the difference in soil series between site-years 1 and 2, while no such difference occurred at the other two sites (Table 1).
According to field history information collected by Brown (1996) and summarized by Table 1, none of the sites had been manured for at least three years prior to the growing season studied, while soil-test goals for cash-grain cropping in Illinois were usually exceeded with respect to pH (6.0), Bray-1 P (40-50 lb acre-1), and exchangeable K (260-300 lb acre-1). The only exceptions involved low pH values for site-years 5 and 6, and a low K-test level for site-year 5. Yet these occurrences are probably of no significance, because they coincided with the highest check-plot yield (site-year 5) or the highest yield with N fertilization (site-year 6).
Further examination of Table 1 reveals that corn was much less responsive to N fertilization during a normal growing season (site-years 1, 3, and 5) than when moisture was limiting (siteyears 2, 4, and 6). In the former case, the percentage response at the optimal N rate ranged from 0 to 17 percent, as opposed to a range from 28 to 234 percent under dry conditions. The highest percentage response, obtained for site-year 4, can be attributed to an exceptionally low check-plot yield.
Soil testing for NO3-, either before (PPNT) or after (PSNT) planting, has been recommended in several states for identifying sites where N fertilization will be ineffective in producing a yield response (Bundy and Meisinger, 1994). Both the PPNT and PSNT were performed by Brown (1996), and the data thereby obtained for the six soils studied in our work are included in Table 1. Assuming a critical value of 16 mg NO3--N kg-1 for the PPNT (Schmitt and Randall, 1994) and 21 mg NO3--N kg-1 for the PSNT (Fox et al., 1989; Bundy and Andraski, 1993), neither test identified any of the site-years studied as being nonresponsive. This would indeed be expected when soil moisture is limiting, owing to reduced mineralization. Yet even under normal growing conditions, NO3- testing was completely ineffective in detecting nonresponsive sites. The latter finding is consistent with previous work by Mulvaney et al. (2001) and Khan et al. (2001), and can be attributed to the transient nature of mineral N in soils as affected by numerous N-cycle processes.
As an alternative to NO3- testing, a simple soil N test has been developed by Khan et al. (2001), in which (NH4+ + amino sugar)-N is estimated from the NH3 liberated upon heating a 1-g soil sample for 5 h with 2 M NaOH. In their work, 25 Illinois soils were classified correctly as being responsive or nonresponsive to N fertilization, on the basis of a critical test value of 225-235 mg N kg-1. When the same test was applied to the six soils studied in our work, the results (Table 2) indicated two soils (siteyears 1 and 2) as slightly responsive, and the remaining four as nonresponsive, with little difference between seasons of normal and deficient rainfall. Comparison of Table 1 and Table 2 shows the test results to be reasonably consistent with the fact that very little, if any, Nfertilizer response was observed during a normal growing season (site-years 1, 3, and 5), whereas the soil test was considerably less reliable for predicting yield response when rainfall was limiting (site-years 2, 4, and 6).
The Illinois soil N test originated by Khan et al. (2001) has its origin in the finding that nonresponsive sites are detectable by determination of amino sugar-N in soil hydrolysates (Mulvaney et al., 2001). When N-distribution analyses were performed on the soils studied in our work, concentrations of total hydrolyzable N, hydrolyzable NH4+-N, and amino acid-N decreased in the order: site-years 3 and 4 > siteyears 5 and 6 > site-years 1 and 2 (Table 2). This order is consistent with the soil N test data, but is not entirely consistent with concentrations of amino sugar-N, which were higher for site-years 1 and 2 than for site-years 5 and 6. The latter discrepancy is likely attributable to partial liberation of amino sugar-N as NH4+ during acid hydrolysis (Camargo et al., 1997), since values for hydrolyzable (NH4+ + amino sugar)-N followed essentially the same trend observed for soil test values.
The data in Table 2 suggest little, if any, difference between normal and dry growing seasons in the soil content of potentially mineralizable N, and thus support the concept that the N-fertilizer responses observed for siteyears 2, 4, and 6 were due, at least in part, to the effect of a limited moisture supply on mineralization. To check this possibility, a comparison was made of mineral N production by soil samples from growing seasons with adequate (site-years 1 and 3) or deficient (siteyears 4 and 6) moisture supply, during incubations at each of two temperatures and two soil moisture contents. The results (Figure 1) leave little doubt that all four of the soils studied had substantial capacity to generate mineral N, including both of those that were responsive to N fertilization during a dry growing season. As expected, mineralization was enhanced considerably, and in all cases, by an increase in temperature and/or moisture content.
According to Figure 1, mineral N production was more limited for site-year 1 than for site-year 3, 4, or 6. This is also evident from Table 3, which summarizes the recovery of NH4+-N and (NO3- + NO2-)-N following the entire 8-wk incubation period. Yet Table 1 shows that site-year 1 was completely nonresponsive to N fertilization, which can no doubt be attributed to weather conditions that were highly favorable for mineralization. Moreover, the data in Figure 1 and Table 3 suggest that, if such conditions had existed at the two most responsive site-years (4 and 6), the N-fertilizer responses that occurred may well have been eliminated.
Comparison of Table 2 and Table 3 reveals that production of mineral N by the incubated soil samples followed the same trend obtained when these soils were tested prior to incubation. Further evidence that the Illinois soil N test provides a reliable estimate of potentially mineralizable N is presented by Table 4, which reports the net change observed when soil-test N was measured before and after incubation for 8 wk. As would be expected for any mineralizable form of soil N, a decrease always occurred upon incubation. The magnitude of this decrease was usually greater for 30ºC than for 20ºC, but was not directly related to the quantitative production of mineral N, presumably owing to the inherent complexities associated with N-cycle processes.
Very little difference was observed in Illinois N-test values for soil samples collected from sites where corn was nonresponsive to N fertilization in a good growing season but responsive when moisture was limiting. The range in N response observed for such sites was attributed to variability in mineralization, based on incubation studies showing little difference between responsive and nonresponsive siteyears in production of mineral N under the same conditions, and a concurrent decrease in N-test values. To identify nonresponsive sites by the Illinois soil N test, normal growing conditions must be assumed.
Appreciation is expressed to H.M. Brown, L.C. Gonzini, and J.J. Warren for providing the Nresponse data and soil samples used in our work, and to K.D. Smith for collecting soil test data for pH, Bray-1 P, exchangeable K, and NO3-.
Table 1. Information about soils and sampling sites
Table 2. Concentrations of hydrolyzable and soil-test N for soils studied
1 S.A. Khan is a Research Specialist in Agriculture and R.L. Mulvaney is a Professor, Dept. of Natural Resources and Environmental Sciences, Univ. of Illinois. R.G. Hoeft is a Professor, Dept. of Crop Sciences, Univ. of Illinois.
Brown, H.M. 1996. Evaluation of nitrogen availability indices. Ph.D. thesis, University of Illinois, Urbana-Champaign, IL. Brown, H.M., R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. In: 1993 Illinois Fertilizer Conference Proceedings (R.G. Hoeft, ed.). pp. 43-49.
Bundy, L.G., and T.W. Andraski. 1993. Soil and plant nitrogen availability tests for corn following alfalfa. Journal of Production Agriculture, 6:200-206.
Bundy, L.G., and E.S. Malone. 1988. Effect of residual profile nitrate on corn response to applied nitrogen. Soil Science Society of America Journal, 52:1377-1383.
Bundy, L.G., and J.J. Meisinger. 1994. Nitrogen availability indices. In: Methods of Soil Analysis. Part 2. Biological Methods (R.W. Weaver et al., ed.). SSSA Book Ser. 5. Soil Science Society of America, Madison, WI. pp. 951-984.
Camargo, F. A. de O., C. Gianello, and C. Vidor. 1997. Comparative study of five hydrolytic methods in the determination of soil organic nitrogen compounds. Communications in Soil Science and Plant Analysis, 28:1303-1309.
Fox, R.H., G.W. Roth, K.V. Iversen, and W.P. Piekielek. 1989. Soil and tissue nitrate tests compared for predicting soil nitrogen availability to corn. Agronomy Journal, 81:971-974.
Khan, S.A., R.L. Mulvaney, and R.G. Hoeft. 2001. A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Science Society of America Journal, 65:1751-1760.
Khan, S.A., R.L. Mulvaney, and C.S. Mulvaney. 1997. Accelerated diffusion methods for inorganic-nitrogen analysis of soil extracts and water. Soil Science Society of America Journal, 61:936-942.
Mebius, L.J. 1960. A rapid method for the determination of organic carbon in soil. Analytica Chimica Acta, 22:120-121.
Meisinger, J.J., V.A. Bandel, J.S. Angle, B.E. O'Keefe, and C.M. Reynolds. 1992. Presidedress soil nitrate test evaluation in Maryland. Soil Science Society of America Journal, 56:1527-1532.
Mulvaney, R.L. 1996. Nitrogen - Inorganic forms. In: Methods of Soil Analysis. Part 3. Chemical Methods (D.L. Sparks et al., ed.). SSSA Book Ser. 5. Soil Science Society of America, Madison, WI. pp. 1123-1184.
Mulvaney, R.L., and S.A. Khan. 2001. Diffusion methods to determine different forms of nitrogen in soil hydrolysates. Soil Science Society of America Journal, 65:1284-1292.
Mulvaney, R.L, S.A. Khan, R.G. Hoeft, and H.M. Brown. 2001. A soil organic nitrogen fraction that reduces the need for nitrogen fertilization. Soil Science Society of America Journal, 65:1164-1172.
Roth, G.W., and R.H. Fox. 1990. Soil nitrate accumulations following nitrogen-fertilized corn in Pennsylvania. Journal of Environmental Quality, 19:243-248.
SAS Institute. 1993. SAS User's Guide: Statistics. SAS Institute, Cary, NC.
Schmitt, M.A., and G.W. Randall. 1994. Developing a soil nitrogen test for improved recommendations for corn. Journal of Production Agriculture, 7:328-334.
Stevens, W.B., S.A. Khan, R.L. Mulvaney, and R.G. Hoeft. 2000. Improved diffusion methods for nitrogen and 15nitrogen analysis of Kjeldahl digests. Journal of the AOAC International, 83:1039-1046.
