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a Dep. of Agronomy, 116 ASI Building, Pennsylvania State Univ., University Park, PA 16802 USA
b Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801 USA
c Dep. of Crop Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801 USA
d USDA, 1102 S. Goodwin Ave., Urbana, IL USA
mwander{at}uiuc.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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50 g kg-1 soil, NT fields contained greater SOC, total N, and POM contents in the 0- to 5-cm depth and lower POM contents in the 5- to 15-cm depth than CT fields. In soils with sand contents higher than 50 g kg-1 soil, tillage practices did not affect the vertical distribution of SOC, total N, or POM.
Abbreviations: CR, central region • CT, conventional till • ECR, east-central region • NR, north region • NT, no-till • PMN, potentially mineralizable N • POM, particulate organic matter • SMB, soil microbial biomass • SOC, soil organic C • SOM, soil organic matter • SR, south region
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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Few studies have investigated the influence of soil texture on the relationship between tillage and SOM dynamics. Campbell et al. (1996) reported that the positive relationship between clay and SOM contents was greater in NT than in CT soils at three sites in western Canada. Paustian et al. (1997) found no general relationship between texture and the effects of tillage on SOM contents in their analysis of data from twenty-seven long-term tillage trials. At two sites where NT did not increase SOM contents relative to CT, soils were fine-textured and poorly drained and crop biomass production was decreased by NT management (Dick et al., 1986; Havlin et al., 1990). Wander et al. (1998) found NT increased SOC and POM-C contents in the 0- to 5-cm depth at the expense of SOM stored at the 5- to 17.5-cm depth in three long-term studies in Illinois. The extent of this effect varied among soils with different textures and initial C contents. No-till practices increased SOC and POM-C contents in the top 5 cm of two silt loam soils, but had no effect on POM-C or SOC concentrations at the surface of a silty clay loam; however, NT reduced total and POM-C and -N contents in the 5- to 30-cm depth.
Changes in SOM occurring within a decade of practice alteration are difficult to document because SOM includes materials that vary in their chemical, physical, and biological lability. Models of SOM dynamics have used multiple pools, differentiated by turnover rates (Paul and van Veen, 1978; Parton et al., 1987). In such models, biologically active SOM has been represented by the active and slow (Parton et al., 1987) and the microbial biomass and decomposable (Paul and van Veen, 1978) pools. Measures of biologically active SOM can be used as early indicators of change in SOM status (Gregorich et al., 1994; Powlson et al., 1987). Three measures of biologically active SOM that have been proposed as indicators of soil quality are POM, PMN, and SMB (Turco et al., 1994). Particulate organic matter is composed of sand-sized, incompletely decomposed organic materials (Cambardella and Elliot, 1992), while PMN is a measure of readily degradable organic N (Drinkwater et al., 1996), and SMB is estimated as C in the living biomass (Gregorich et al., 1994).
The objective of this study was to quantify the effects of tillage practices on the quantity and vertical distribution of biologically active SOM and total SOM in Illinois soils. Most agricultural fields in Illinois have been conventionally tilled for a half century or more. The use of conservation tillage practices such as NT within the region became common within the last decade. Accordingly, there are few long-term tillage trials in Illinois. We hypothesized that in these soils, tillage practices affect the vertical distribution but not the total quantity of biologically active SOM and that the impacts of tillage are not constant across soil textures.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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Total organic C and N and PMN were determined in all depths, and POM and SMB were determined in the 0- to 5- and 5- to 15-cm depths. The POM fraction was determined according to the method described by Gregorich and Bettany (1995). Using a Carlo Erba NA 1500 C/N analyzer (Carlo Erba, Milan, Italy), total N and organic C contents of the whole soil and POM were determined by dry combustion according to Nelson and Sommers (1982). Free carbonates were removed with sulfurous acid prior to analysis.
The PMN fraction was determined by anaerobic incubation after Waring and Bremner (1964). A field-moist soil sample (6 g) was placed in a 50-mL centrifuge tube, saturated with 10 mL of deionized water, and incubated at 40°C for 7 d. Then, 40 mL of 0.625 M K2SO4 was added to give a final concentration of 0.5 M. The tube was shaken for 1 h on a reciprocating shaker. The supernatant was filtered through Whatman no. 42 filter paper. The same procedure was used on nonincubated samples. Ammonium was determined colorimetrically (Sims et al., 1995). The PMN was determined as the NH4 recovered from the incubated soil minus NH4 recovered from the nonincubated soil.
Carbon in the SMB was estimated using the chloroform fumigation extraction method (Vance et al., 1987). A 10-g sample of field-moist soil was placed in a 30-mL centrifuge tube, transferred to a desiccator, and fumigated with chloroform for 24 h in the dark. After chloroform removal, 30 mL of 0.5 M K2SO4 was added, the tube was shaken for 1 h on a reciprocating shaker, and the supernatant was filtered through Whatman no. 42 filter paper. Extracted C was determined with a Dohrmann Xertex DC-80 C analyzer (Dohrmann, Santa Clara, CA). The same procedure was used on a nonfumigated soil sample. Chloroform-labile C was calculated as the amount of dissolved organic C recovered from the fumigated soil minus that recovered from the nonfumigated soil. The SMB was expressed in units of chloroform-labile C.
Additional soil parameters measured were bulk density by the core method; gravimetric water content; pH at a 1:1 soil/water ratio; and texture, determined using the hydrometer method adapted from Gee and Bauder (1986). The particle size classes considered were clay (<2 µm), fine silt (2–20 µm), coarse silt (20–53 µm), and sand (53–2000 µm).
The experimental design was a split-plot in a randomized complete block. The inference space for the study is the state of Illinois. Thirty-six 16.2-ha fields were assigned to the CR, ECR, NR, or SR according to their geographic locale (Table 1). The four regions were used as blocks. Fields were treated as main plots within the blocks. Each year–field combination was considered an environment (Carmer et al., 1989), and environments were considered random within blocks. Analysis of variance was performed using PROC GLM (SAS Institute, 1994, p. 229.). Tillage, crop, region, and depth were the class variables. Each SOM fraction was measured in one of the two sampling years; accordingly, year was not a factor in the analysis. Uniformly minimum variance unbiased estimators were used to estimate field means for the PMN data, which were lognormally distributed within fields. For a lognormally distributed population, uniformly minimum variance unbiased estimators are preferable to the arithmetic mean (Parkin and Robinson, 1994). Results were considered statistically significant at P = 0.05, except where noted.
Further investigations of tillage, crop, and depth were carried out using texture as a covariate. Region was not used as a class variable in these models because texture was expected to vary with loess origin and locale (Fehrenbacher et al., 1986). Analysis of covariance was performed to assess the interaction of textural covariates (sand, coarse silt, fine silt, total silt, and clay) and tillage effects on SOM and SOM fractions. Main effects were only significant for models using sand and clay as covariates. Texture variables not found significant are not discussed further. Statistical contrasts of means were used to test three-way interactions (tillage, depth, and sand contents) (Littell et al., 1991).
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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50 kg g-1 soil, but not in sandier soils (Fig. 1c). Note that even the sandiest soils in this study were relatively fine-textured (Table 1). This sand-content effect on tillage impacts was not observed in the 5- to 15-cm (Fig. 1d) or 15- to 30-cm (data not shown) depths. The SOC contents increased linearly with clay content
. However, tillage x clay (Table 4) and tillage x clay x depth interactions were not significant (Table 5). Based on values reported for mapped units, estimated clay values were consistently greater than expected for these soils.
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Soil C/total N ratios were influenced by region and depth but not by tillage or crop (Tables 2 and 3). The C/N ratios were smaller in the SR than in the other regions and decreased with depth (data not shown). Soil C/N ratios were positively correlated with clay content
(Table 4). When clay was included in the model, a crop x tillage interaction was observed. For corn fields, soil C/N ratios were greater in CT than in NT fields (statistical contrast P < 0.10). No differences in soybean field C/N ratios were found. The R2 between C/N ratios and clay contents decreased in the following order: CT corn > NT soybean > CT soybean > NT corn.
Particulate Organic Matter
Tillage effected the vertical distribution of POM C (Fig. 2a)
but had no effect on POM-C contents in the 0- to 15-cm depth (Tables 2 and 3). The NT contained on average 33% more POM C in the 0- to 5-cm soil depth and 29% less POM C in the 5- to 15-cm depth than CT fields. The POM contents in the 15- to 30-cm depth were not evaluated. For each tillage system, POM-C contents were greater in the 0- to 5-cm layer than in the 5- to 15-cm layer (statistical contrast P < 0.001). Crop type affected POM-C contents (Table 2). Fields in soybean and corn contained on average 3.4 and 2.9 g POM-C kg-1 soil. When texture was included in the analyses, interactions between tillage and crop were noted (statistical contrast P < 0.009; Table 4). The POM-C contents decreased in the following order: CT soybean > NT soybean > NT corn > CT corn. Accordingly, the effect of crop on POM-C contents was greater in CT fields than in NT fields. The significant crop x depth interaction was due to the slightly greater stratification of POM C in soybean than in corn fields (Table 5).
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Potentially Mineralizable Nitrogen
No-till soils contained 54% more PMN in the 0- to 5-cm depth than CT soils. In both tillage systems, the concentration of PMN was highly stratified, with most labile N located in the 0- to 5-cm depth. Tillage did not affect PMN contents in the 5- to 15- or 15- to 30-cm depths, or on PMN contents overall (0–30 cm) (Table 2, Fig. 2b). The PMN contents were greater (statistical contrast P < 0.03) in fields planted to corn than in fields planted to soybean. Overall (0–30 cm), PMN concentrations averaged 10.7 mg NH4–N kg-1 soil in corn fields and 6.6 mg NH4–N kg-1 soil in soybean fields.
At all depths, PMN had a negative linear response
to sand content and there was a tillage x sand interaction (at P < 0.10) (Table 4). No-till soils had greater PMN contents than CT soils in soils with low sand content; however, tillage had no effect on PMN contents in sandier soils. While the tillage x sand x depth interaction was not statistically significant (Table 5), the effect of texture and tillage on PMN was most expressed in the 0- to 5 cm depth (data not shown).
As was the case for POM, the PMN crop x sand x depth interaction was significant (Table 5). The negative relationship between PMN and sand contents was strongest in soybean fields in the 0- to 5- and 5- to 15-cm depths (corn 0–5 cm:
; corn 5–15 cm,
; soybean 0–5 cm:
; soybean 5–15 cm,
). Corn fields contained greater PMN contents than soybean fields (data not shown). The tillage x crop and tillage x crop x clay interactions were significant at P < 0.10 (Table 4), with the effect of crop slightly greater in NT than in CT fields.
Soil Microbial Biomass
Tillage did not affect SMB contents at either depth (Fig. 2d, Table 2). The SMB was greater in the 0- to 5-cm depth than in the 5- to 15-cm depth. Soybean fields contained 72.5 mg chloroform-labile C kg-1 soil, while corn fields contained 66.1 mg chloroform-labile C kg-1 soil in the 0- to 15-cm depth. The crop x sand interaction was significant at P = 0.05 (Table 4). This was due to a small positive influence of sand on SMB contents in fields planted to corn and a small negative influence of sand in fields planted to soybean. The SMB was greater in soybean fields than in corn fields, most likely because of differences in residue input from the previous year's crop. A clay x depth interaction (at P = 0.10) was due to a slight positive linear response of SMB to clay content in the 5- to 15-cm depth that was not present at the 0- to 5-cm depth (0–5 cm:
; 5–15 cm:
).
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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Our results reflect the relatively recent adoption of conservation practices. Although changes in soil characteristics continue to occur for decades after conversion to NT (Dick et al., 1991), the most dramatic changes occur within the first decade of practice adoption (Dick, 1983).
Our findings generally indicate that NT affected the vertical distribution but not the overall quantity of SOC, total N, POM, or PMN. Also, NT increased the concentration of SOC and biologically active organic matter (POM and PMN) in the upper 5 cm of soil and decreased SOC and POM in the 5- to 15-cm layer. Trends in total N, PMN, and SMB (P < 0.10) also suggest that biologically active and total SOM fractions were relatively depleted below the surface in NT soils. These results agree with the general findings of Wander et al. (1998) from a decade-old trial at three Illinois sites where NT increased POM in surface soils (0–5 cm) at the expense of POM stored at depth (5–17.5 cm). These on-farm findings indicate that continuous NT for several years (5) will redistribute C within the profile without necessarily increasing SOC storage (Dick et al., 1991; Karlen and Cambardella, 1996).
In this study, trends in POM were more sensitive indicators of tillage-based depth effects than trends in SMB or PMN. Our failure to find significant main factor effects on the SMB might be due to the large spatial (Winter and Beese, 1995), temporal (Kaiser and Heinemeyer, 1993), and methodological (Jenkinson, 1976) variability associated with the SMB measure. The PMN was most responsive to crop type and probably the associated fertility practices. Shifts in POM were the most sensitive indicators of tillage effects on the vertical distribution of SOM.
The effects of NT on SOM storage may be associated with biomass production, which is usually correlated with crop yield. Yield may be reduced by conservation tillage in fine-textured soils in humid regions where soils are cold in spring (Griffith et al., 1988; Karlen, 1990). Models describing the effects of tillage on SOM storage have assumed that yield is reduced under NT (Donigan et al., 1995; Kern and Johnson, 1993). In our study, anecdotal information about crop yields provided by farmers cooperating in the project suggest there was no substantial yield difference between CT and NT. Based on eight replicated trials, Angers et al. (1997) reported that in eastern Canada, crop production and residue inputs were equal in CT and NT and that NT generally did not contain more SOC or N than CT. They also reported that SOM concentrations were greater in the surface (10 cm) of NT than CT and that the reverse was true in the lower plow depth (20–40 cm). Residue placement and a cool climate that would slow decay rates and minimize the effects of fall tillage were thought to minimize tillage practice effects on total SOM contents. Residue burial can increase the SOC contents of the lower plow layer in CT compared with NT (Ishmail et al., 1994; Haynes and Beare, 1996). Other authors have noted the influence of climate on the relationship between tillage practices and SOM contents. Decomposition rates, which are generally increased by disruption of soil aggregates and increased aeration, may not always be greater in CT. Franzluebbers and Arshad (1996) did not observe greater total SOM, SMB, and PMN contents in NT than in CT treatments at three sites in a semiarid climate in western Canada. They supposed that greater soil water conservation under NT may have resulted in greater decomposition of SOM and that the cold climate minimized the effects of CT on decomposition rates.
Few studies have investigated the effects of texture on tillage impacts on SOM. Campbell et al. (1996) investigated the effects of texture in an 11-yr comparison of NT and CT at three sites in Western Canada. They found soil C storage in the 0- to 15-cm depth of NT exceeded that in CT by 0, 1.6, and 3.9 Mg ha-1 in a sandy loam, a silt loam, and a clay soil, respectively. In our study, the textural range of samples was comparatively narrow and the soils, classified mostly as silty clay loams, contained almost no coarse fragments. We explored the relationship between soil size separates (sand, fine silt, coarse silt, and clay), and tillage effects on SOC and found that tillage effects on SOM were influenced by soil texture. Sand content was the most statistically effective textural covariate. There was no correlation between sand and clay contents. Although clay contents were positively correlated with SOC contents at all soil depths, clay did not influence the effects of tillage on SOC sequestration or stratification. Sand content [1 - (silt + clay)] is the functional equivalent of the silt plus clay variable used in the CENTURY model (Parton et al., 1987). Their finding that decay of labile SOM decreases as silt plus clay contents increase is consistent with our observations. We found that the use of NT in soils with low sand content increased total and biologically active SOM, in particular POM, in the top 5 cm. However, in soils with < 50 g sand kg-1 soil, use of NT decreased POM C in the 5- to 15-cm depth. Tillage had little effect on SOM in sandier soils. Similar but less pronounced effects were found for PMN contents.
The influence of sand content on SOM stratification by depth in CT and NT may be indirect and is probably physical in nature. Hassink (1996) asserted that the stabilization of applied residues in soils is a function of the unsaturated protective capacity of the soil rather than the quantity of fine particles per se. Soils with low sand content may have a larger unsaturated protective capacity. If so, then in our study, POM increased where residues and/or root biomass were concentrated and soil C saturation capacity was unfilled. Storage porosity [1 - (bulk density/2.65) - (% macropores)] and SOC were positively related (P < 0.0001). Tillage may have had little effect on biologically active SOM in soils with high sand content because of limited unsaturated protective capacity. Hassink and Whitmore (1997) asserted that the rate at which SOM becomes stabilized by physical protection is proportional to the concentration of free organic matter and the fraction of pore space that could contain SOM. The greater capacity of low sand content soils to sequester C added as residues may have been a function of porosity rather than clay content (Wander et al., 1998). The influence of sand content within the narrow range of soil textures considered in this work is not well understood.
The influence of tillage on soil porosity and O2 supply has been explored by Topp et al. (1997), who hypothesized that rapid biological activity in the surface of NT may exhaust and limit O2 diffusion to subsurface soils, especially in fine-textured soils where SOM is highly stratified. They determined that the upper rooting zone of fall-tilled soils contained more O2 during spring than soils that were under NT. This may explain why in the soils with low sand content under NT, POM was concentrated in the 0- to 5-cm depth and depleted in the 5- to 15-cm depth. The distribution of POM may reveal a texturally dependent distribution of root-derived material.
Alternatively, SOM decomposition rates may increase with sand content. Comparatively slow decay rates in the soils with low sand content may have allowed accumulation of biologically active SOM to occur where residue placement, and possibly root growth, was concentrated in the surface of NT and subsurface of CT soils.
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Conclusion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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No-tillage of poorly drained, fine-textured soils that are not erosion prone may lead to SOM stratification without increasing SOM sequestration. Tillage effects were expressed in the finest-textured soils, where NT had more POM in the surface 5 cm and less in the next 10 cm than CT fields. In sandier soils, the use of NT did not increase stratification or SOM or POM contents. The rapid shifts in POM distribution that were the result of NT adoption identified changes in the quantity, allocation, and conservation of young, biologically active SOM.Kaiser Heinemeyer 1993
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ACKNOWLEDGMENTS |
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Received for publication December 31, 1997.
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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