RICE CULTURE. Alkaline Hydrolyzable-Nitrogen Changes with Soil Depth: Implications for Calibration of Predicted Nitrogen Rates

Transcription

1 RICE CULTURE Alkaline Hydrolyzable-Nitrogen Changes with Soil Depth: Implications for Calibration of Predicted Nitrogen Rates T.L. Roberts, R.J. Norman, N.A. Slaton, and C.E. Wilson, Jr. ABSTRACT Alkaline hydrolyzable-n (AH-N) quantified by the Illinois Soil Nitrogen Test (ISNT) or Direct Steam Distillation (DSD) may index potentially mineralizable-n in the soil. Limited success has been achieved utilizing the ISNT to predict corn (Zea mays L.) yield response when soils are sampled to a depth of 12 in. (30 cm) and a strong correlation of ISNT to total N (TN) has raised concerns over the sensitivity of the method in determining potentially mineralizable-n. A study was implemented to determine the effects of site and soil depth on AH-N. Soil samples were collected from 0 to 24 in. (0 to 45 cm) in 6-in. (15-cm) depth increments and analyzed for AH-N (ISNT and DSD) and TN. Analysis of variance for AH-N showed a significant site soil depth interaction. Alkaline hydrolyzable-n ranged from 22 to 280 mg N kg soil -1 and the highest values were not always in the top 6 in. (15 cm) of the soil. Alkaline hydrolyzable-n accounted for 11% to 38% of soil TN and was variable across sites and depths. Variation in AH-N and the fraction of TN quantified as AH-N with site and soil depth indicate the importance of proper sampling depth (i.e., to the rooting depth of a given crop) for correlation and calibration of crop response using either the ISNT or DSD. Significant changes in AH-N with depth could influence crop-available N and thus the reliability of a soil test to predict crop response to N-fertilization. INTRODUCTION Plant uptake and assimilation of N is highly dependent on the mineralization of organic-n and the presence of both NH 4 + and NO 3 - in the soil solution. Correlation and 181

2 AAES Research Series 581 calibration of crop N response has always been an important focus of soil fertility research and will continue to be as agricultural production costs and economic uncertainty increase. Research has shown that subsoil nutrient levels can influence crop yield and increase the predictive capability of soil-test methods to predict crop response to nutrient additions. Subsoil K and P levels have been shown to influence the yield of both corn and soybean [Glycine max (L.) Merr.] (Mallarino and Ul-Haq, 1997). Potentially mineralizable-n in the subsoil (>6 in.) can be a significant source of plant-available N as many crops have rooting depths greater than 6 in. (15 cm). Previous studies have documented corn roots growing to depths of 24 to 36 in. (45 to 60 cm) (Laboski et al., 1998) and similar results have been found for rice (Oryza sativa L.) (Beyrouty et al., 1987). A better understanding of potentially mineralizable-n in the subsoil may clarify why at some sites the N uptake and/or yield does not correlate well when the AH-N is used at just the 0 to 6 in. (0 to 15 cm), 6 to 12 in. (15 to 30 cm), or 0 to 12 in. (0 to 30 cm) depths. The forms of organic-n and exchangeable NH 4 -N quantified by ISNT and DSD can be greatly influenced by cropping systems and soil texture. These compounds may also be adsorbed or leached depending on the soil characteristics such as ph, clay content, and OM. We hypothesized that AH-N varies with depth and is therefore not a constant fraction of TN. Thus, the following objectives were developed to determine the relationship between AH-N and soil depth: 1) determine the influence of site and soil depth on AH-N for silt loam soils; and 2) compare the fraction of TN quantified as AH-N across depths. METHODS AND MATERIALS Sixteen agricultural sites located on silt loam soils across the state of Arkansas were selected to represent a range of soil characteristics and previous crops. Sites included both agricultural experiment station and commercial production fields that were sampled in either April or May of each year prior to N fertilization. A minimum of four soil cores (1-in. diameter) were taken to form a composite sample at depth increments of 0 to 6 in. (0 to 15 cm), 6 to 12 in. (15 to 30 cm), 12 to 18 in. (30 to 45 cm), and 18 to 24 in. (45 to 60 cm), respectively, and replicated four times within a field for a total of 16 samples per site (4 depths 4 replications). The University of Arkansas Diagnostic Laboratory (Fayetteville, Ark.) analyzed soil samples for TN using an Elementar CN Variomax (Elementar Americas Inc., Mt Laurel, N.J.) according to the procedure of Nelson and Sommers (1996). Alkaline hydrolyzable-n was determined using the ISNT (Khan et al., 2001) and 10 M NaOH DSD methods (Roberts et al., 2009). All statistical analyses were carried out using JMP 7.0 (SAS Institute, Inc., Cary, N.C.). The treatment structure for all statistical analysis was a split-plot design with site representing the main-plot factor and soil depth representing the split-plot factor. Analysis of variance was conducted to determine the effects of site and soil depth on AH-N as well as the ratio of AH-N to TN and significant differences reported at the α = 0.05 level. 182

3 B.R. Wells Rice Research Studies 2009 RESULTS AND DISCUSSION The interaction between site and soil depth was significantly different for alkaline hydrolyzable soil-n determined by ISNT and DSD suggesting that location (i.e. soil series, climate, previous crop, parent material, etc.) influences the rate and direction of change in AH-N with depth. Although there was no consistent relationship between AH-N and depth across all sites, the ISNT and DSD quantified significantly greater concentrations of N in the 0 to 6 in. (0 to 15 cm) depth increment compared to the 6 to 12 in. (15 to 30 cm) increment at all sites except 2 and 11 (ISNT only) (Fig. 1). Alkaline hydrolyzable-n quantified by the ISNT or DSD below 12 in. (30 cm) varied and increased, decreased, or remained constant compared with the concentrations quantified in the 6 to 12 in. (15 to 30 cm) increment within each site. For some soils, the AH-N concentrations quantified by the ISNT (sites 7 and 8) and DSD (sites 2 and 6) in the 0 to 6 in. (0 to 15 cm) depth were similar to concentrations in soil depths below 12 in. (30 cm). Alkaline hydrolyzable-n concentrations at depths >12 in. (30 cm) cannot be accurately predicted based solely on the AH-N in the 0 to 6 in. (0 to 15 cm) depth of the soil. When an individual soil depth was considered, AH-N was highly variable from site to site indicating that it may be influenced by long-term crop rotations, soil manipulation (i.e., land leveling), tillage and naturally inherent variations in N cycling. If soil depths >6 in. (15 cm) influence crop response to N fertilizer, then a soil profile similar to site 2 may help to explain why crops grown on some soils do not respond to additions of N. For example, the 6 to 12 in. (15 to 30 cm) depth and 12 to 18 in. (30 to 45 cm) depth of soil at site 2 had a significantly higher quantity of AH-N and the 18 to 24 in. (45 to 60 cm) depth was not significantly different from the 0 to 6 in. (0 to 15 cm) depth. Mulvaney et al. (2006) highlighted the importance of subsoil fertility on ISNT calibration, but unfortunately potentially available N from soil below 12 in. (30 cm) has been largely or completely ignored. Crop rooting depth should be a primary factor when determining sampling depths for correlation and calibration of AH-N for yield and crop response to N fertilizer. The percentage of TN quantified as AH-N was highly variable and ranged from 8% to 38% across all sites and depths (Fig. 2). Analysis of variance for the percentage of TN quantified as AH-N showed a significant site by soil depth interaction for both the ISNT and DSD. Differences in the percentage of TN quantified as AH-N were proportional to the differences in recovery by the ISNT and DSD methods within a site. The percentage of TN quantified as AH-N was greatest for site 2 at depths below 6 in. (15 cm) (Fig. 2) and may be accounted for in the high levels of exchangeable NH 4 -N, but the N quantified by DSD represented as much as 38% of the TN in the 6 to 12 in. (15 to 30 cm) depth. The percentage of TN quantified by DSD for sites 9 and 10 were almost twice the percentage for the ISNT method at soil depths below 6 in. (15 cm). Concentrations of AH-N quantified by DSD were greatest in soil at the sites with the both the highest (site 2) and lowest (site 10) TN concentrations and in several cases represented almost one-third of the TN for that depth. In one-half of the sites, the percentage of TN quantified by ISNT was similar in trend and magnitude to that 183

4 AAES Research Series 581 quantified by DSD, but in the other eight sites (9 to 16) the fractional AH-N recovery was highly variable with no distinct trend. The percent recovery of TN as AH-N by the ISNT in the top 6 in. (15 cm) of the soil profile averaged 13.4%, which is similar to the percentage reported by Laboski et al. (2008). Comparison of AH-N fractions below 6 in. (15 cm) cannot be made as previous studies have not reported TN at 6 to 12 in. (15 to 30 cm) nor has soil sampled at depths greater than 12 in. (30 cm) been analyzed for AH-N. It is important to note that within a profile, the amount of TN quantified as AH-N is often inversely proportional to TN. In soil depths, such as the top 6 in. (15 cm), where TN is high the resulting ratios are often low. For most sites, except where soybean was the previous crop, the ratio of AH-N to TN is numerically greater at depths below 6 in. (15 cm) where TN decreases (Fig. 2). These results suggest that the N in the top 6 in. (15 cm) of the soil is primarily found in organic-n forms that are not readily available for mineralization. High rates of biological activity and humification in the topsoil result in N conversion to more recalcitrant forms, which do not readily mineralize. Uptake and utilization of organic-n at depths >6 to 12 in. appear to be highly feasible, especially in areas of active rooting. Alkaline hydrolyzable-n as measured by the ISNT or DSD methods are significantly influenced by the interaction of site and soil depth. The ISNT and DSD have been investigated due to their use as plant-available N indices and ability to predict crop response to N fertilizer. These data identify the potential importance of N found at depths greater than 12 in. (30 cm) for crop growth. Alkaline hydrolyzable-n has been proposed as a predictor of potentially mineralizable-n and based on these findings can be significantly impacted by the location and the depth of soil being analyzed. SIGNIFICANCE OF FINDINGS With the concurrent development of N-ST*R, a soil-based N test for rice production in Arkansas, the importance of soil sampling depth on the success of a particular soil testing method has been highlighted. Traditionally rice has been thought of as a shallow rooted crop, but previous work has shown that rice roots can grow and access nutrients at depths much greater than 6 in. (15 cm). Correlation and calibration of crop response to potentially mineralizable-n or AH-N must be accomplished at the same soil depth as the crop s rooting depth in order to correctly evaluate the method s predictive ability. Changes in the fraction of AH-N with soil depth that are in contrast with changes in TN may suggest the need for depths to be weighted differently based on their relative magnitude of potentially mineralizable-n. Previous crops may also influence the ratio of AH-N to TN and should be taken into consideration when comparing different crop rotations as the available-n may change even though TN may not. The changes in AH-N with depth and changes in the fraction of AH-N to TN explain the need for 0 to 18 in. (0 to 45 cm) sample depths for N-ST*R on silt loam soils, and help to strengthen N-ST*R s predictive ability ensuring that the correct N fertilizer rates will be recommended for rice. 184

5 B.R. Wells Rice Research Studies 2009 ACKNOWLEDGMENTS This research was supported by the Arkansas Rice Research and Promotion Board and the U.S. Rice Foundation. LITERATURE CITED Beyrouty, C.A., B.R. Wells, R.J. Norman, J.N. Marvel, and J.A. Pillow, Jr Characterization of rice roots using a minirhizotron technique. p In: H.M. Taylor (ed.). Minirhizotron observation tubes: Methods and applications for measuring rhizosphere dynamics. ASA Spec. Publ. 50. ASA, Madison, Wis. Khan, S.A., R.L. Mulvaney, and R.G. Hoeft A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Sci. Soc. Am. J. 65: Laboski, C.A.M., R.H. Dowdy, R.R. Allmaras, and J.A. Lamb Soil strength and water content influences on corn root distribution in a sandy soil. Plant and Soil. 203: Laboski, C.A.M., J.E. Sawyer, D.T. Walters, L.G. Bundy, R.G. Hoeft, G.W. Randall, and T.W. Andraski Evaluation of the Illinois soil nitrogen test in the North Central region of the United States. Agron. J. 100: Mallarino, A.P. and M. Ul-Haq Topsoil and subsoil potassium as affected by long-term potassium fertilization of corn-soybean rotations. Commun. Soil Sci. Plant Anal. 28(17/18): Mulvaney, R.L., S.A. Khan, and T.R. Ellsworth Need for a soil-based approach in managing nitrogen fertilizers for profitable corn production. Soil Sci. Soc. Am. J. 70: Nelson, D.W. and L.E. Sommers Total carbon, organic carbon, and organic matter. pp In: D.L. Sparks et al. (eds.). Methods of soil analysis. Part 3. SSSA Book Ser. 5. SSSA, Madison, Wis. Roberts, T.L., R.J. Norman, N.A. Slaton, C.E. Wilson Jr., W.J. Ross, and J.T. Bushong Direct steam distillation as an alternative to the Illinois soil nitrogen test. Soil Sci. Soc. Am. J. 73:

6 AAES Research Series 581 Soil Depth (cm) Alkaline Hydrolyzable-N Fig. 1. Alkaline hydrolyzable-n (AH-N) determined Direct Steam Distillation (DSD) as influenced by soil depth for 186

7 B.R. Wells Rice Research Studies 2009 (mg N kg soil-1) by the Illinois Soil Nitrogen Test (ISNT) and each site. Error bars represent one standard deviation. 187

8 AAES Research Series 581 Soil Depth (cm) Total N Fig. 2. Percentage of total soil N (TN) as influenced by soil depth 188

9 B.R. Wells Rice Research Studies 2009 (mg N kg soil-1) for each site. Error bars represent one standard deviation. 189