Ding Et Al.: Tillage Management Affecting Soil Organic Matter

ganic C (SOC). Soils of the southeastern United States of America, particularly sandy Coastal Plain soils, have Soil organic matter (SOM) is of primary importance for maintaining inherently low SOC contents (typically below 1%, Hunt soil productivity, and agricultural management practices may signifiet al., 1982). Consequently, small changes in the SOM cantly influence SOM chemical properties. However, how SOM chemical characteristics change with agricultural practices is poorly undercontent are significant to the agricultural production of stood. Therefore, in this study, we evaluated the impacts of tillage the region. An evaluation of tillage and crop residue (conventional vs. conservation) management on the structural and management practices to rebuild SOC levels has been compositional characteristics of SOM using cross-polarization magicconducted by Hunt et al. (1996). These researchers monangle-spinning (CPMAS) and total sideband suppression (TOSS) itored changes in SOC levels in numerous small tillage solid-state 13C nuclear magnetic resonance (NMR) and diffuse reflecplots and found that after 9 yr of CnT, the SOC content tance Fourier transform infrared (DRIFT) spectroscopy. We characin the top few centimeters was significantly higher than terized both physically and chemically isolated SOM fractions from a the soil under CT management. Campbell et al. (1999) Norfolk soil (fine-loamy, siliceous, thermic Typic Kandiudults) under reported that over an 11to 12-yr period, increases in long-term tillage management (20 yr). The solid-state 13C NMR results C storage in the 0to 15-cm soil depth, because of indicated that humic acid (HA) from conventional tillage (CT, 0–5 cm) was less aliphatic and more aromatic than HA from conservation adoption of no-tillage, were small (0–3 Mg ha 1 ). Most tillage (CnT). The aliphatic C content decreased with increasing depth of the differences were observed in the 0to 7.5-cm soil (0–15 cm) for both CT and CnT treatments. The reverse trend was depth, with little change in the 7.5 to 15 cm. However, true for aromatic C content. Based on reactive/recalcitrant (O/R) the short and long-term influences of disturbance on C peak ratio comparisons, HA was more reactive in the top soil (0–5 mineralization are complex and may vary depending cm) under CnT than CT. Both soil organic C (SOC) and light fraction on types of soil and plant residues (Hu et al., 1995; (LF) material were higher in the 0to 5-cm soil of CnT than CT Franzleubbers and Arshad, 1996; Alvarez et al., 1998). treatment. Our results show that long-term tillage management can The strong influence of soil management on the amount significantly change the characteristics of both physical and chemical and quality of SOM was also reported by others (Janzen fractions of SOM. et al., 1992; Ismail et al., 1994; Campbell et al., 1996). Another approach to evaluate the impact of agricultural management on SOM dynamics is to separate S organic matter strongly affects soil properties SOM into pools based on differences in decomposition such as water infiltration rate, erodibility, water rates (Wander et al., 1994; Wander and Traina, 1996a). holding capacity, nutrient cycling, and pesticide adsorpGenerally, those pools are conceptualized with one tion (Stevenson, 1994; Campbell et al., 1996; Francioso small pool having a relatively quick decomposition rate et al., 2000; Wander and Yang, 2000). It has been sug(i.e., active pool LF) and pools that are more recalcigested that proper management of SOM is the heart of trant (i.e., humus) (Stevenson, 1994). The LF is sensitive sustainable agriculture (Weil, 1992). Recent research to environmental and agricultural management factors has also recognized SOM as a central indicator of soil and can be used as a functional description of organic quality and health (Soil and Water Conservation Socimaterials (Wander and Traina, 1996a). Regardless of ety, 1995). For example, a decline in SOM (biological active or recalcitrant SOM pools, structural chemistry oxidation or erosion) significantly reduced the N supply is important for their chemical and biological activities. and resulted in a deterioration of soil physical condiSpectroscopic techniques can provide useful structions, leading to crop yield reduction (Greer et al., 1996). tural information of SOM. Diffuse reflectance Fourier Therefore, it is important to maintain proper levels of transform infrared spectroscopy is considered to be one SOM to sustain soil productivity. of the most sensitive infrared techniques for humic subIntensive agricultural practices change SOM characstances analysis (Niemeyer et al., 1992; Ding et al., 2000). teristics greatly, generally a substantial loss of soil orAccording to Painter et al. (1985) and Niemeyer et al. G. Ding and B. Xing, Dep. of Plant and Soil Sciences, University Abbreviations: CnT, conservation tillage; CPMAS-NMR, cross-polarof Massachusetts, Amherst, MA 01003; J.M. Novak and P.G. Hunt, ization magic-angle-spinning nuclear magnetic resonance; CT, conUSDA-ARS-Coastal Plains Soil, Water, and Plant Research Center, ventional tillage; DRIFT, diffuse reflectance Fourier transform infraFlorence, SC 29501; D. Amarasiriwardena, School of Natural Science, red spectroscopy; HA, humic acid; LF, light fraction; O/R, reactive/ Hampshire College, Amherst, MA 01002. Received 17 Jan. 2001. recalcitrant functional group ratio calculated from peak heights of *Corresponding author ([email protected]). DRIFT spectra; SOC, soil organic C; SOM, soil organic matter; TCN, total combustible N; TOSS, total sideband suppression. Published in Soil Sci. Soc. Am. J. 66:421–429 (2002). 422 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 each soil layer was isolated using a modified density gradient (1992), this technique offers several advantages over method of Wander and Traina (1996a). The LF was collected transmission infrared spectroscopy: (i) a simple sampleby dispersion of 50 g soil of freshly sieved, field-moist soil preparation procedure; (ii) insensitivity to water associsample in a NaBr solution (density 1.5 g mL , 1:1 w/v). The ated with the sample and enhanced resolution; (iii) high mixture was shaken for 30 min and centrifuged at 7500 g resolution of the spectra because of reduction in the (8000 rpm) for 20 min. These rinses were then transferred sensitivity towards light scattering; and (iv) a more reliinto a 250-ml separatory funnel and allowed to settle overable method for quantitative estimations of functional night. After three such separations, the composite supernatant groups. Another spectroscopic technique is solid-state was filtered using a 0.45m polycarbonate membrane filter. 13C NMR spectroscopy that is probably the most useful The heavy SOM fraction which settled to the bottom of the funnel was removed. The LF materials retained on the filter tool for nondestructive characterization of SOM and its were rinsed with a 0.5 M CaCl2 and 0.5 M MgCl2 solution components (Preston, 1996; Xing and Chen, 1999; Mao followed by a final rinse in deionized water. This was done et al., 2000). Studies by Capriel (1997) and Ding et al. to avoid any remnant biological toxicity because of Na satura(2000) demonstrate that both DRIFT and 13C NMR tion of the ion-exchange sites in the LF. The weight yield of techniques are useful and suitable for examining the LF was measured and the light fraction organic C (LF-OC) effects of agricultural management on SOM. and total combustible N content were determined using a The goal of this research was to evaluate the changes LECO-CN 2000 analyzer (LECO Corp., Joseph, MI). of SOM quantity and quality under CT and CnT systems using both DRIFT and solid-state 13C NMR. SpectroExtraction, Fractionation, and Purification of HA scopic investigations of SOM changes for the Norfolk and Elemental Composition Analysis soil, located in the Southeastern Coastal Plain, have Most of extraction techniques require the organic matter never been conducted before. Furthermore, because orto be removed from soil (Stevenson, 1994). As a consequence, ganic, sustainable agricultural systems depend increasthe OC constituents would be modified to some extent. Thereingly on soil nutrient cycling mechanisms, it is necessary fore, we used neutral pyrophosphate (Na4P2O7 ) to extract to understand the relationships between the LF, the SOM to minimize chemical modifications (Stevenson, 1994). structural and compositional changes of HA, and nutriAir-dry and sieved soil (50 g) was weighed into a 1000-ml ent retention and supply characteristics. The specific obplastic bottle, and 500 ml of 0.1 M Na4P2O7 were added. The jectives were to: (i) characterize HA structural changes; air in the bottle and solution was displaced by N gas (N2 ) and (ii) determine peak height O/R ratio for HA, which the system was shaken for 24 h at room temperature. The reflects the biological activity; and (iii) compare the light samples were extracted three times. After separation from fraction (LF) variations with soil depth under both CnT the Na4P2O7 insoluble residues by centrifuging at 1100 g (3000 rpm), the dark-colored supernatant solutions were comand CT systems. bined, acidified to pH 1 with 6 M HCl, and allowed to stand for 24 h at room temperature for the precipitation of the HA MATERIALS AND METHODS fraction. The HA was shaken for 24 h at room temperature with 0.1 M HCl/0.3 M HF solution at least for three times. Site Description and Sampling The insoluble residues (HA) was separated from the supernatant by centrifuging at 12 000 g (10 000 rpm), washed with The study was conducted using soil samples collected from deionized water until free of Cl ions, and then freeze-dried. the long-term CnT and CT research plots established in 1979 The C, H, and N contents of the isolated HAs were measured at the Clemson University Pee Dee Research and Education with a Fisons Model EA 1108 Elemental Analyzer (Mattson Center (Darlington, SC). The soil at the research site is a Instrument, Madison, WI). Norfolk loamy sand. The coordinates are 34.3 N lat. and 79.7 W long., and the elevation is 37 m above the mean sea level. Treatments were arrayed in a randomized complete block Diffuse Reflectance Fourier Transform design with split plots and five replications (Hunt et al., 1996). Infrared Analysis The CT treatment within the plots consisted of multiple disking (0–15 cm deep) and the use of field cultivators to The DRIFT spectra were collected using an Infrared Specmaintain a relatively weed free surface. Surface disking and trophotometer (Midac series M 2010, Midac Corp., Irvine, field cultivation have been completely eliminated in soil under CA) with a DRIFT accessory (Spectros Instruments, ShrewCnT plots since 1979. Because of a root-restrictive E horizon bury, MA). All HA fractions were powdered with a agate and which reforms annually in this soil (Busscher and Sojka, 1987), pestle and stored over P2O5 in a drying box. Three-milligram both tillage treatments received in-row subsoiling (30 cm solid HA samples were then mixed with KBr (total weight as deep) at planting to fracture this horizon. Additional manageto 100 mg) and reground to powder consistency. A sample ment practices for the plots such as crop rotation, fertilization, holder was filled with the mixture (powder). A microscope and pesticide application were described previously (Hunt et glass slide was used to smooth the sample surface. At the al., 1996; Novak et al., 1996). In 1999, 50 soil cores were beginning of analysis, the diffuse-reflectance cell which concollected from the top 15 cm of soil using a 2.5-cm diam. soil tained the samples was flushed with N2 for 10 min to reduce probe at random locations from one plot under CnT and one the interference from CO2-C and water molecules. The sample plot under CT treatment. The core samples were sectioned compartment was placed with anhydrous Mg(ClO4 )2 to further (5-cm increments), composited, air-dried, and sieved (2 mm). reduce atmospheric moisture. The DRIFT spectroscopy was acquired with a minimum of 100 scans collected at a resolution of 16 cm . The spectroscopy Density Gradient Separation of Light Fraction was calibrated with the background which consisted of powMaterial and Analysis dered KBr and scanned under the same environmental conditions as the sample-KBr mixtures. Absorption spectra were The LF has been recognized to be an important soil nutrient reservoir and has been recommended as a fertility index (Wanconverted to a Kubelka-Munk function using Grams/32 software package (Galactic Corp., Salem, NH). Peak assignments der et al., 1994). In this investigation, the LF material from DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 423 Table 1. Soil organic C (SOC), total combustible N (TCN), and and intensity (by height) ratio calculation were done following light fraction (LF) in the soil under different tillage systems the methods of Niemeyer et al. (1992), and Wander and Traina (standard deviation in parentheses).† (1996b). We used ratios of labile (O-containing) and recalcitrant (C and H or N) functional groups to compare HA spectra Soil depth CnT CT with varying soil depth of different tillage treatments. cm kg m 2 SOC 0–5 2.30 (0.02)a‡ 1.22 (0.01)a 5–10 0.89 (0.01)b 1.23 (0.01)a Solid-State Carbon-13 Nuclear Magnetic 10–15 0.61 (0.01)b 0.81 (0.01)b Resonance Spectroscopy TCN 0–5 0.22 (0.01)a 0.11 (0.00)a Spectra were obtained by using the CPMAS-TOSS tech5–10 0.08 (0.01)b 0.11 (0.00)a 10–15 0.05 (0.00)b 0.06 (0.00)b niques. Xing et al. (1999), after examining several solid-state LF 0–5 1.19 (0.02)a 0.63 (0.01)a C NMR techniques including ramped CPMAS, reported that 5–10 0.17 (0.01)b 0.55 (0.01)a CPMAS-TOSS has two advantages. The first is that an ade10–15 0.10 (0.00)b 0.12 (0.00)b quate TOSS can eliminate the sidebands so that the spectrum C/N ratio of Soil shows only the true peaks for a given HA sample. The second C/N 0–5 10.4 (0.02)a 11.1 (0.01)a is that implementation of cross-polarization-TOSS can avoid 5–10 11.1 (0.02)a 11.2 (0.01)a baseline distortion from the dead time. In addition, instrument 10–15 12.2 (0.02)a 13.5 (0.03)a time required is about the same as the regular CPMAS. They g kg 1 recommended that CPMAS-TOSS be used to analyze HA LF-OC/SOC 0–5 160 (4.01)a 150 (3.48)a samples when using a 300 MHz spectrometer. In this re5–10 40 (1.55)b 130 (2.78)a 10–15 42 (1.78)b 42 (1.29)b search, HA samples were run at 75 MHz (C) in a Bruker LF-N/TCN 0–5 100 (2.45)a 90 (2.07)a MSL-300 spectrometer (Bruker, Billerica, MA) with a 7-mm 5–10 30 (1.38)b 80 (2.17)a CPMAS probe. The samples (300–350 mg) were packed in a 10–15 22 (1.29)b 22 (1.69)b 7-mm-diam. zirconia rotor with a Kel-F cap. The spinning † Aerial mass of SOC, TCN, and LF was calculated from area and soil speed was 4.5 kHz. A H 90 pulse was followed by a contact bulk density. time (tc ) of 500 s, and then a TOSS sequence was used to ‡ Means with different letters are significantly different P 0.05. remove sidebands (Schmidt-Rohr and Spiess, 1994; Xing et al., 1999). Line broadening of 30 Hz was used. The 90 -pulse length was 3.4 s and the 180 pulse was 6.4 s. The recycle management are shown in Table 1. It is clear that 20delay was 1 s with the number of scans 4096. The details yr different tillage management influenced the quantity were reported elsewhere (Xing et al., 1999). In preliminary and distribution of C, LF, and N in the soil. Twentyexperiments, we ran several samples at different contact times, year CnT treatment resulted in a significant increase in and selected 500 s because this contact time gave the best the SOC, soil-TCN, and LF in the top 0to 5-cm soil signal/noise ratio and the spectra were similar to the ones generlayer of the unfractionated soil, as compared with CT ated by direct polarization magic-angle-spinning C NMR. management. The quantities of SOC, TCN, and LF in There was no signal observed for the rotor and Kel-F cap (Mao et al., 2000), thus, no background correction was made the 0to 10-cm layer were significantly higher than those in this work. of 10to 15-cm depth under CT treatment. The SOC and TCN decreased with increasing soil depth under both tillage treatments. Statistical Analyses The quantity of LF material in the 0to 5-cm soil All data presented were the mean of at least three replicate layer in the CnT system was approximately twice high measurements, except for HA elemental composition and as that in the CT system. On the other hand, soil under solid-state C NMR data because of the high cost and low CT management at the 5to 10-cm layer had signifiavailability of the instrument. However, preliminary solidcantly higher LF than soil under CnT. The dependency state NMR experiment with one HA sample indicated minimal variations, which was consistent with the result of an extensive of tillage and depth on LF distribution was confirmed NMR study in our lab (Mao et al., 2000). The HA elemental by the two-way ANOVA which showed that there was composition and NMR measurements were performed on a significant tillage and depth interaction (P 0.01) composite samples. when the quantity of LF was compared between tillages. The fraction of total SOC and total combustible N (TCN) Additionally, regression analyses between the quantity pools in the unfractionated soil and in LF was compared using of LF material and the quantity of SOC showed a signifia two-way Analyses of Variance (ANOVA). Also, the O/R cant linear relationship (r 2 between 0.89 and 0.97, P ratios generated from DRIFT peak heights were examined 0.01) (data not shown). This relationship indicates that between tillages and by depth using the ANOVA. Different tillage treatments and soil depths were the experimental fac89 to 97% of the variation in quantity of LF material tors and the interactions between tillage and depth were examisolated from soil under both tillages can be accounted ined. SigmaStat software (SPSS Corp., Richmond, CA) was for by the SOC content. used for each test at a 0.05 level of significance. The isolated LF material accounted for between 4 and 16% of the total SOC and 2 to 10% of soil TCN from the Norfolk soil (Table 1). Only at the 5to 10RESULTS cm soil depth was there a significant difference of LFYields of Light Fraction Material and Elemental OC and LF-N percentages between tillages. Regression Composition of Humic Acid analyses confirmed a significant relationship (r 2 between 0.79 to 0.95) between the LF-OC vs. the SOC The total quantities of SOC, TCN, and LF found in soils calculated from bulk density under CnT and CT content and the LF-TCN vs. the soil-TCN content. The 424 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 Table 2. Elemental composition of humic acids on an ash-free generic chemical characteristics of the components presbasis and atomic H/C and C/N ratios. ent in the samples. Unsubstituted aliphatic C is indicated Sample Depth C H N H/C C/N by signals in the 0to 50-ppm region. Carbons in proteinaceous materials (amino acids, peptides, and proteins) cm g kg 1 have resonances between 40 and 60 ppm, and C in carboCnT 0–5 527 41 39 0.94 15.7 CnT 5–10 479 36 31 0.92 18.1 hydrates gives signals between 60 to 108 ppm. Signals CnT 10–15 516 35 31 0.81 19.4 between 108 to 162 ppm are because of aromatic C, CT 0–5 546 40 37 0.88 17.2 while those near 155 ppm arise from phenolic C, indicatCT 5–10 548 40 35 0.88 18.3 ing the presence of Oand N-substituted aromatic CT 10–15 538 40 31 0.89 20.3 groups (e.g., phenolic OH and aromatic NH2 ). The strong signals between 170 and 180 ppm come from C soil C/N ratio for both tillages increased slightly with in carboxyl groups, with possibly some overlapping from soil depth, but not significantly (Table 1). The elemental compositions of the HAs from both phenolic, amide, and ester carbons (Stevenson, 1994; CnT and CT systems are displayed in Table 2. ExaminaMao et al., 2000). tion of the data showed that the HAs from two tillage It was difficult to directly compare the HA spectra of systems were similar to each other. The C content of different treatments because visual comparison showed HA under CnT was slightly lower in the middle layer no major differences in terms of presence or absence (5–10 cm) than that of other two layers. The N content of specific peaks. However, we can obtain detailed inforwas higher in the top soil than that of deeper layers for mation from these spectra by peak area integration. both tillages. The HA atomic C/N ratio increased with The relative content of major C-types, calculated by soil depth for both tillages, similar to the soil C/N ratio integrating the spectral profile according to standard changes. The HA H/C ratio of CnT plot slightly declined chemical shift ranges (Xing et al., 1999) is shown in Fig. with depth while this ratio was almost constant for 2. The most noticeable feature was at 60 to 96 ppm CT soil. region (Fig. 1 and Fig. 2B), i.e., carbohydrate-C (aliphatic C bonded to OH groups, ether oxygens, or ocSolid-State Carbon-13 Nuclear Magnetic curring in saturated five or six-membered rings bonded Resonance Spectroscopy of Humic Acid to oxygens). This C content for CnT in the top soil (0–5 cm) was 23.9%, and was 18.3% for CT (Fig. 3B). The The HA 13C CPMAS-TOSS NMR spectra of both CT and CnT are shown in Fig. 1. The HA spectra revealed difference between the two treatments can be attributed Fig. 1. Cross-polarization magic angle-spinning total sideband suppression 13C NMR spectra of humic acids in a Norfolk soil under different tillages: (1A) conservation tillage treatment (CnT1, 0–5 cm; CnT2, 5–10 cm; and CnT3, 10–15 cm); (1B) conventional tillage treatment (CT1, 0–5 cm; CT2, 5–10 cm; and CT3, 10–15 cm). DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 425 to the accumulation of carbohydrate materials from Another interesting feature was revealed by the 108 to 162 ppm of NMR spectra and their integration results fresh residue input in the top soil of CnT treatment. The reverse trend was true in the 10to 15-cm layer, (Fig. 1A,B, and Fig. 2C). The two most pronounced peaks in this region were recorded at 131 ppm (ring which showed carbohydrate-C content was higher in CT than that of CnT system. There was not much difference carbons in which the ring is not substituted by strong electron donors such as O and N) and at 155 ppm (phein the 5to 10-cm layer between both tillages. The lowest carbohydrate-C for both tillage managements occurred nols and aromatic amines). Aromatic-C (31.7%) in the 0to 5-cm layer under CT was higher than that (28.1%) at 10to 15-cm soil layer. The carbohydrate-C decreased with soil depth for CnT. But for CT management, the of CnT treatment (Fig. 2C). Similarly, HA aromatic-C content in 5 to 10 cm of CT system was greater than carbohydrate-C content was almost the same in the first two layers. that of CnT plot. However, the aromatic-C content was almost the same in the 10 to 15 cm for both tillages, The total aliphatic C (0–108 ppm) of HA for CT treatment (Fig. 2A) decreased from 52.5% in the top even though aromatic-C in both treatments increased with soil depth. The aromaticity (expressed in terms of soil (0–5 cm) to 40.1% at the depth of 10 to 15 cm. Similarly, the aliphatic C of HA for CnT treatment aromatic-C as a percentage of the aliphatic-C aromatic-C, according to Hatcher et al., 1981) increased decreased from 58.8% in the top soil (0–5 cm) to 40.8% at the depth of 10 to 15 cm. Furthermore, when comfrom 32.3% in the top soil of CnT to 50.7% at the 10to 15-cm layer, and from 31.7 to 50.8% for CT treatparing the total aliphatic-C (0–108 ppm) and carbohydrate-C (60–96 ppm) of HA between the two treatments ment (Fig. 2D). Carboxyl groups were relatively enriched in CT treatment (data not shown). The value of (Fig. 2A and 2B), it was evident that both aliphatic-C and carbohydrate-C were higher in the top (0–5 cm) carboxyl-C increased with soil depth for both treatments, which was consistent with the report by Stearman soil of CnT than CT. The HA alkyl-C content (0–50 ppm, data not shown) at the 10to 15-cm layer was et al. (1989). The chemical shift of carbonyl-C was distinct only for a few HA samples (e.g., CnT2 and CT3) higher in CnT than CT. Fig. 2. Solid-state 13C NMR data under different tillage systems: (A) aliphatic-C (0–108 ppm); (B) carbohydrate-C (60–96 ppm); (C) aromatic-C (108–162 ppm); and (D) aromaticity (108–145 ppm)/(0–162 ppm). 426 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 Fig. 3. Diffuse reflectance Fourier transform infrared spectra of humic acids in a Norfolk soil under different tillage treatments: (3A) conservation tillage treatment (CnT1, 0–5 cm; CnT2, 5–10 cm; and CnT3, 10–15 cm); (3B) conventional tillage treatment (CT1, 0–5 cm; CT2, 5–10 cm;