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Soil Science Society of America Journal 63:1335-1341 (1999)
© 1999 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Long-Term Tillage Effects on Soil Chemical Properties and Organic Matter Fractions

I. Hussaina, K.R. Olsonb and S.A. Ebelharc

a Pindi Gheb, Pakistan
b Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801 USA
c Dep. of Crop Science, Dixon Springs Agric. Center, Simpson, IL 60550 USA

k-olson1{at}uiuc.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Change in frequency and intensity of tillage practices alters the soil properties, distribution of nutrients, and soil organic matter in the soil profile. We hypothesized that 8 yr of no-till (NT), chisel plow (CP), and moldboard plow (MP) treatments would affect chemical properties and organic matter of eroded soil. The corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation study was established in tall fescue (Festuca arundinacea L.) sod on a previously eroded, moderately well-drained, Grantsburg (Fine-silty, mixed, mesic Oxyaquic Fragiudalf) soil in southern Illinois. In the eighth year, soil pH, exchangeable Ca, and Bray P-1 were greater in NT than in CP and MP in the 0- to 5-cm soil depth. In the 0- to 5-cm soil depth, exchangeable K and Mg were greater with the CP than with the NT and MP. In the 5- to 15-cm soil depth, exchangeable Ca and Mg were greater in the MP and CP than in NT, due to mixing. Soil pH and P were greater for CP than MP and NT in the 5- to 15-cm layer. Exchangeable K in the 5- to 15-cm soil depth was greater in the MP than CP and NT. In the 0- to 5-cm soil depth, NT, CP, and MP had 38, 35, and 31% of their total C as particulate organic matter (POM), respectively. After 8 yr, CP and MP had less total organic C than NT in the 0- to 5-cm depth. In the 0- to 5-cm depth, CP and MP had less POM C than NT. The greater reduction of organic C in the POM fraction than in whole soil showed that POM was the most tillage-sensitive fraction of organic matter. After 8 yr of study, the water-stable aggregates in the 0- to 5-cm soil depth of MP and CP was reduced compared with NT. The effects of tillage treatment and associated soil erosion either resulted in different findings from tillage treatments on uneroded soil or affected the trend and magnitude of the soil property differences between treatments. For the 10-yr period prior to the establishment of the tillage experiment the site was managed as hayland. At the end of 8 yr, the NT maintained or improved nutrient retention and aggregate stability in the 0- to 5-cm layer compared with MP and CP.

Abbreviations: CEC, cation-exchange capacity • CP, chisel plow • DAP, days after planting • MP, moldboard plow • NT, no-till • POM, particulate organic matter • SOM, soil organic matter • USLE, Universal Soil Loss Equation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 
CHANGE IN FREQUENCY AND INTENSITY of tillage practices alters soil properties, distribution of nutrients, and soil organic matter in the soil profile. These changes become stable with time and could affect availability of nutrients for plant growth, crop production, and soil productivity. Long-term NT systems accumulate nutrients in the soil surface, whereas MP distributes nutrients relatively uniformly through the tillage depth. Stratification of nutrients has been observed in two long-term tillage studies under NT, whereas soil mixing promotes uniform distribution of nutrients in MP and CP (Karlen et al., 1991; Ismail et al., 1994; Mackay et al., 1987). In contrast, Karlen et al. (1994) and Franzluebbers and Hons (1996) observed differences in nutrients distributions due to tillage system.

Soil pH decreases at the soil surface in NT because of surface-applied N (Blevins et al., 1983), soil acidity (Dick, 1983), and more organic matter (Franzluebbers and Hons, 1996). Kitur et al. (1994), Ismail et al. (1994), and Karlen et al. (1991) found a higher soil pH in the surface soil of NT than MP that resulted from slow mixing of surface-applied lime.

Any change in organic C contents due to tillage can affect soil cation-exchange capacity (CEC) of soil. Due to higher organic C, no-till resulted in a significant increase in CEC in the 0- to 15-cm sandy clay loam layer compared with CP and MP after 28 yr of cultivation (Mahboubi et al., 1993). After 12 yr of a tillage study, Karlen et al. (1994) found no differences in CEC due to tillage system.

Organic matter plays an important role in nutrient availability and soil aggregate stability. Soil productivity decreases when soil organic matter (SOM) declines (Bauer and Black, 1994). High residue-producing crops in combination with NT increase SOM (Havlin et al., 1990), while SOM declines with low residue-producing crops like soybean in combination with MP (Edwards et al., 1992). Crop residues have a residual effect on crop growth, organic C, and N availability (Christensen et al., 1994; Maskina et al., 1993). Accumulation and distribution of organic C in soil is affected by different tillage practices and time after initiation of tillage. Ismail et al. (1994) observed a decrease in organic C in the 0- to 30-cm silt loam layer during the first 5 yr, no change in the next 5 yr, and an increase in organic C in the last 10 yr in both NT and MP in comparison with sod plots. Organic C was higher in NT than in MP. Hunt et al. (1996), Alvarez et al. (1995), Angers and Giroux (1996), and Karlen et al. (1994) found that under NT organic C increased, compared with MP and CP, in the top 5 cm of soils with a range of soil textures, including loamy sand, silt loam, and silty clay loam.

More water-stable aggregates (>250 µm) were found in a silt loam soil after continuous wheat (Triticum aestivum L.) as compared with a wheat–fallow rotation system because of continuous addition of residue (Monreal et al., 1995). After 10 yr of NT continuous corn, Karlen et al. (1994), found that removal, maintenance, and doubling of crop residue resulted in 42, 46, and 60% wet aggregate stability, respectively.

Water-stable aggregates are divided into macro- and microaggregates, and different organic matter fractions are responsible for the formation of these aggregates. Temporary and transient binding agents are thought to be for macroaggregation (>250 µm) (Dormaar, 1983; Tisdall and Oades, 1982). Persistent binding agents are important in microaggregation (<250 µm) of soil (Tisdall and Oades, 1982).

Macroaggregates are more susceptible to physical disruption because of the labile nature of binding agents. The labile transient and temporary binding agents described by Tisdall and Oades (1982) were related to POM. Particulate organic matter is the slowly decomposable or stabilized fraction of organic matter, which is composed of root fragments in various stages of decomposition (Cambardella and Elliott, 1992). The POM was related to the slow fraction, which is physically protected and somewhat resistant to decomposition, with a turnover time of 20 to 40 yr (Parton et al., 1987).

After 20 yr, Cambardella and Elliott (1992) reported for a loam soil that total organic C as POM was 39, 18, 19, and 25% in sod, bare fallow, stubble mulch, and NT, respectively. They suggested that NT reduced POM loss caused by tillage and aeration. They found that the higher C/N ratio in the POM fraction in NT was due to less decomposition of straw on the soil surface as compared with bare fallow. The C/N ratio of the mineral-associated fraction of organic matter was not significantly different because this fraction was in a more stabilized form.

Beare et al. (1994) found on a sandy clay loam soil that after 13 yr of continuous NT and MP with grain sorghum [Sorghum bicolor (L.) Moench] and winter rye (Secale cereale L.), 36% of total organic C was POM C in both treatments. No-till had 20% more total organic C and POM C than MP after 13 yr. Organic C and POM C in sand-free aggregates at 0 to 5 cm in NT were higher than in MP surface soil samples; however, no significant differences occurred in the 5 to 15 cm layer. The C/N ratio of POM was lower in NT than in MP in the 0- to 5- and 5- to 15-cm soil layers in different size aggregates. Aggregate stability was correlated with the ratio of mineral-associated to total organic matter C and N. There was more aggregation due to humified organic matter or highly decomposed organic matter in MP. Their findings also included the loss of mineral-associated organic C as a result of tillage. After 4 yr of continuous tillage of a silt loam soil, Tissen and Stewart (1983) observed a 43% loss of organic C in the >50 µm associated macroaggregates. Particulate organic matter is important in macroaggregate formation, and it is this labile fraction that is affected by tillage. Differences in POM also depend on quality and quantity of residue, soil type, climatic conditions, and time since initiation of tillage.

An initial 3-yr tillage study (Kitur et al., 1994), on an eroded soil managed as hayland for the previous 10-yr period, was extended to 8 yr with the following objectives: (i) to determine the long-term effects of NT, CP, and MP on soil chemical properties of eroded soil in a corn–soybean rotation and (ii) to determine tillage effects on the various fractions of organic matter (POM and mineral-associated fraction) of eroded soil and its effect on water-stable aggregates.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Summary and conclusions
 REFERENCES
 
Background of Study Site
A tillage experiment was started on 12 Apr. 1989 at the Dixon Springs Agricultural Research Center in Southern Illinois. The previously eroded soil was a moderately well-drained, Grantsburg silt loam (Fine-silty, mixed, mesic Oxyaquic Fragiudalf) with an average depth of 64 cm to a root-restricting fragipan. Grantsburg soils were formed in loess under forest vegetation and underlain at a depth of 2 to 5 m by siltstone. The study area has an average slope of 6%, and had been in tall fescue hayland for more than 10 yr prior to the beginning of the experiment with no application of lime or fertilizers. The tall fescue was mowed and the residue left on the plot area to protect the soil from erosion. On 12 Apr. 1989, lime (CaCO3), with 94% calcium carbonate equivalent and an effective calcium carbonate equivalent of 79%, was applied at the rate of 3.1, 4.9, and 7.3 Mg ha-1, on upper, middle, and lower rows of the plot area, respectively, to adjust the soil pH. These blocks were limed based on the differences in soil pH that existed in the fall of 1988. This difference in soil pH was probably a reflection of an increasing degree of erosion from the upper block nearer the ridge top to the lower blocks closer to the footslope. The MP, CP, and NT treatments were established on 27 Apr. 1989. From 1989 to 1994, fertilizers (P, K, and N) were applied to corn in 1989 (67, 116, and 212 kg ha-1), 1991 (67, 260, and 168 kg ha-1), and 1993 (55, 232, and 184 kg ha-1). The experimental design was two Complete Latin Squares. Each square had three rows and three columns. Each treatment was randomized six times in 18 plots with a plot size of 9 by 12 m.

Field Activities and Tillage Operations
1995 Growing Season
On 6 April, moldboard plowing and chiseling were performed to a depth of 15 cm. Before disking and planting, 50 kg ha-1 of N and 55 kg ha-1 of P in the form of diammonium phosphate (18-46-0), and 232 kg ha-1 of K in the form of KCl (0-0-60) were broadcast on all plots. Two diskings to a depth of 5 cm were applied to the CP and MP on 30 May. Corn (Pioneer 3394) was planted at a rate of 64220 seeds ha-1 on 31 May. Nitrogen in the form of anhydrous ammonia was knifed in at the rate of 168 kg N ha-1 on 26 June.

1996 Growing Season
Plowing and chiseling to a depth of 15 cm in combination with one disking to a depth of 5 cm were performed on 5 June. No fertilizer was applied during the 1996 growing season. Soybean (Pioneer 9382) was planted at a rate of 432250 plants ha-1 on 5 June. Later in the season, glyphosate [N-(phosphono-methyl) glycine] at a rate of 2.33 L ha-1 was sprayed on 2 July.

Soil Sampling
Soil samples were taken 25 d after planting (DAP) in the 1995 (before NH3 application) and 1996 growing seasons at 0 to 5, 5 to 15, and 15 to 30 cm for the determination of soil pH; CEC; Bray P-1; extractable K, Ca, and Mg; soil organic C; and water-stable aggregates. Using a 4-cm-diam. hand auger, soil cores were collected from each corner and center of 18 plots between the rows. Samples were air dried, crushed, mixed, and passed through a 2-mm sieve.

Laboratory Soil Chemical Analysis Methods
Soil pH was determined using 1:1 soil/water ratio. Extractable soil P was measured using the Bray P-1 method (Kundsen and Beegle, 1988) and the standard soil scoop method described by Peck (1988). Exchangeable Ca, Mg, and K were extracted by 1 M NH4OAc (pH = 7.0). Calcium, Mg, and K were determined on a atomic absorption spectrophotometer (Lanyon and Heald, 1982; Kundsen et al., 1982). Cationexchange capacity was measured using NH+4 saturation (Chapman, 1965) and diffusion of NH3 (Mulvaney, 1996). Particle-size distribution of the <2-mm fraction was determined by the pipette and sand sieving method (Soil Survey Staff, 1984).

Particulate Organic Matter and Aggregate Stability
Particulate organic matter was determined (Cambardella and Elliott, 1992) on 10 g of soil (<2 mm) dispersed in 30 mL of 5 g L-1 sodium hexametaphosphate by shaking on a reciprocal shaker for 18 h. The dispersed material was sieved through the 53-µm sieve and the sieve was rinsed several times in a beaker. The suspended fraction was dried at 50°C in a forced air oven. Dried samples were ground and analyzed for organic C and N. This fraction was identified as mineral-associated organic C and N. Particulate organic matter was obtained by subtracting the mineral-associated fraction from whole soil organic C and N values. Organic C and N for whole soil and mineral samples were determined by using a LECO CNS-2000 Analyzer (Leco Corp., St. Joseph, MI).

Soil aggregate stability was determined by wetting a 25-g soil sample on a 250-µm screen for 10 min and subsequent wet sieving for 10 min (Kemper and Rosenau, 1986).

Plant Residue and Soil Erosion
The percentage of surface residue was determined for each plot by using the line-transect method (Hill et al., 1989) after planting. The soil loss rates were determined using the Universal Soil Loss Equation (USLE) (Walker and Pope, 1983).

Statistical Analysis
Statistical analyses were performed using the Statistical Analysis System (SAS Inst., 1995). Analysis of variance and least square means for all parameters were calculated by General Linear Model (GLM) procedure using Latin Square Design with year as a factor. The least square means were obtained from tillage x year interactions.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Summary and conclusions
 REFERENCES
 
Soil pH
Tillage effects on soil chemical properties at 25 DAP during 1995 and 1996 are presented in Tables 1 and 2 . Soil pH was higher in NT than in MP and CP during both years in the 0- to 5-cm soil layer. These differences were only significant (P = 0.05) during 1995. In the 5- to 15- and 15- to 30-cm soil layers, tillage effect on soil pH was not significant in both years. At 15 to 30 cm, pH was lower with all tillage systems in comparison with the 0- to 5- and 5- to 15-cm soil layers. The higher soil pH in NT than in MP in the 0- to 5-cm layer was a result of slower reaction of lime in NT compared with MP, probably due to the lack of mixing (Karlen et al., 1991; Ismail et al., 1994). Nitrogen was applied to corn in 1989, 1991, 1993, and 1995. However, lowering of soil pH in NT, as previously reported by Blevins et al. (1983) and Dick (1983), as a result of nitrification of N fertilizer, mineralization of N in plant residues, and accumulation in the soil surface was not observed in our study. The majority of the N fertilizer was applied in deep bands between corn rows using knife-application equipment, although some was surface-applied before corn planting. Higher soil pH in the 0- to 5-cm soil layer with NT was a result of the presence of the higher amount of exchangeable Ca compared with CP and MP during both years. Lower soil pH values in the 5- to 15-cm soil layer during 1996 (Table 2) compared with 1995 (Table 1) could be attributed to crop uptake, leaching of exchangeable bases, and the effect of N fertilizer applied during corn production years.


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  		Table 1 Soil pH and extractable Ca, Mg, K, and P (Bray P-1) at three soil depths as affected by different tillage systems during the 1995 growing season

 

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  		Table 2 Soil pH and extractable Ca, Mg, K, and P (Bray P-1) at three soil depths as affected by different tillage systems during the 1996 growing season

 
Exchangeable Cations
During 1995 and 1996, exchangeable Ca was significantly (P = 0.05) greater with NT than with CP and MP in the 0- to 5-cm layer (Tables 1 and 2). In the 5- to 15-cm layer, exchangeable Ca and Mg differences were not significant. Lack of tillage and concentration of crop residues at the soil surface in NT contributed to its higher concentration of exchangeable Ca in the 0- to 5-cm layer. Mixing of the soil by plowing and disking resulted in a trend toward more exchangeable Ca and Mg in MP and CP at 5 to 15 cm. A greater amount of exchangeable Mg at 15 to 30 cm in comparison with the 0- to 5- and 5- to 15-cm layers in all tillage treatments might be due to weathering and leaching of lime materials.

Exchangeable K was significantly greater (P = 0.05) in CP than in MP and NT in the 0- to 5-cm layer (Tables 1 and 2) during 1995. In the 5- to 15-cm layer, MP and CP had significantly (P = 0.05) higher soil K values than NT because of mixing by the plowing and disking. Differences in K due to tillage were not significant at 15 to 30 cm. Potassium levels were greater in all tillage systems in 1995 (Table 1) than in 1996 (Table 2) because K was applied in 1995 but not in 1996. From 1989 to 1992, K accumulated in the 0- to 5-cm layer of NT because of the absence of tillage, which was consistent with previous observations by other researchers (Ismail et al., 1994; Karlen et al., 1991). Less K in the 0- to 15-cm layer of NT compared with CP in 1995 might be the result of significantly greater (P = 0.05) exchangeable Ca, which could have promoted more K fixation in NT. In addition, the higher mobility of K with water (Jones and Hinesly, 1986), higher moisture in NT after application of K, and presence of continuous pores could have lead to leaching of exchangeable K.

Soil Phosphorus (Bray P-1)
Soil P was higher in NT and CP than in MP in the 0- to 5-cm layer in both 1995 and 1996 (Tables 1 and 2). The CP had significantly (P = 0.05) higher soil P than NT and MP in 1995 (Table 1). The NT and CP had higher soil P than MP in 1996 (Table 2). Tillage had no significant effect on soil P in the 5- to 15-cm and 15- to 30-cm soil layers. A higher concentration of soil P in the 0- to 5-cm soil layer could be due to the higher soil organic C in NT and CP compared with the MP. The reduction in soil loss by erosion in the 0- to 5-cm depth of NT (Table 3) and an increase in organic P due to residue on the soil surface also contributed to the higher amount of soil P (Bray P-1). Absence of mixing of P fertilizer in NT may have reduced the fixation of soil P because of less contact of phosphatic fertilizer with soil colloids, which increased its availability (Schomberg et al., 1994). In the 5- to 15-cm soil layer, the soil P was lower in all tillage treatments, which could be explained by lower organic C in this layer and fixation by exchangeable Ca.


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  		Table 3 Effect of tillage treatments on residue at planting and estimated soil loss

 
Surface Residue and Estimated Soil Loss
Table 3 includes the 1995 and 1996 surface residue and estimated soil loss. The NT maintained significantly more residue on the soil surface than the CP and MP during both years (Table 3). Crop residue on the soil surface was higher with corn than with soybean as the previous crop because of higher residue production from corn and a lower rate of decomposition of corn residue. On Grantsburg soil with 5 to 9% slopes, the estimated average annual soil loss for the corn and soybean rotation, using USLE (Walker and Pope, 1983), was 7.9, 21.1, and 29.5 Mg ha-1 with NT, CP, and MP, respectively (average of yearly values reported in Table 3). The greater amounts of crop residue (Table 3) under NT protected the soil from water erosion and kept soil loss below the tolerance level of 8.4 Mg ha-1 yr-1, whereas with less residue on the MP and CP soil surface, rill erosion was observed.

Cation-Exchange Capacity
After 8 yr of tillage, no significant (P = 0.05) differences between treatments were observed for CEC in the 0- the 5-, 5- to 15-, and 15- to 30-cm soil layers (Table 4) . No-till had a slightly greater CEC in the 0- to 5-cm layer than under the CP and MP systems in 1996. This slightly higher CEC in the NT soil could be attributed to its greater organic C content. There was a significant correlation (r2 = 0.52) between CEC and organic C in the 0- to 5-cm layer of NT. No significant correlation of CEC and organic C was observed in CP and MP in the eighth year. Cation-exchange capacity was not significantly greater in MP and CP as compared with the NT in the 5- to 15- and 15- to 30-cm layers in 1996. Cation-exchange capacity tended to be higher in the 15- to 30-cm layer than in the 5- to 15-cm layer in all tillage treatments. This was attributed to 5% more clay in the 15- to 30-cm layer than in the 0- to 5- and 5- to 15-cm layers (Table 4). After 8 yr of tillage, CEC was better correlated with organic C (r2 = 0.40) than clay in the 0- to 5-cm layer, whereas CEC was better correlated with clay contents than organic C in the 5- to 15-cm (r2 = 0.84) and 15- to 30-cm (r2 = 0.72) layers in all tillage systems.


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  		Table 4 Effect of different tillage systems on soil cation-exchange capacity (CEC) and clay at different depth with time at Dixon Springs

 
Cation-exchange capacity did not change significantly (P = 0.05) within a tillage system with time because of the absence of any significant (P = 0.05) increase in clay contents and organic C across years in all tillage systems. No significant differences were observed between the first year (Table 4) and eighth year CEC in the 0- to 5- and 15- to 30-cm layers. Cation-exchange capacity was significantly (P = 0.05) lower at the eighth year than the first year in 5- to 15-cm layer of all tillage systems.

Soil Texture
After 8 yr of tillage, clay contents were not significantly (P = 0.05) affected by tillage in the 0- to 5-, 5- to 15-, and 15- to 30-cm soil layers (Table 4); however, CP and MP had a nonsignificantly higher clay content than the NT system in all three soil layers. The reason for the slightly higher clay contents under CP and MP could be the translocation of subsoil material from the underlying Bt horizon, which has higher clay contents, by tillage equipment into the Ap horizon. Clay contents were lower in all tillage systems in the 0- to 5-cm layer than in the 5- to 15- and 15- to 30-cm layers. While comparing tillage effects over time, the MP and CP had 1.5 and 2% higher clay contents after 7 yr in the 5- to 15-cm soil layer as a result of the plowing and mixing of subsoil into the topsoil. In the 15- to 30-cm layer, the increase in clay contents (3%) may be the result of soil erosion (Table 3) reducing the Ap horizon thickness and more of the Bt horizon material being within 30 cm of the current soil surface.

Soil Organic Matter and Aggregate Stability
After 8 yr of tillage, total organic C (Table 5) in the whole soil, POM, and the mineral-associated fraction were significantly (P = 0.05) different in all tillage systems in the order: NT > CP > MP in the 0- to 5-cm soil layer (Table 5). The NT, CP, and MP systems had 38, 35, and 31% of their total C as POM C, respectively. The NT had higher organic C contents in all organic matter fractions in the surface soil layer; this may have been due to the reduced decomposition rate, as a consequence of less soil residue contact, lower aeration, and lower soil temperature (Hussain, 1997) in NT. Another reason for the higher organic C contents could be higher fungal growth under the NT, without buried crop residue, (Table 3) compared with CP and MP. The requirements of maintenance energy are lower for fungi than bacteria, which resulted in the transformation of more organic matter into the humified organic fraction in the NT treatment (Alvarez et al., 1995). The higher organic C content in the NT system was also a result of the lower soil erosion (Table 3) as a consequence of more residue on the soil surface. Organic C contents in the whole soil decreased 17 and 30% in the CP and MP compared with the NT, while the POM C contents decreased 22 and 43% in the CP and MP compared with the NT in the 0- to 5-cm soil layer.


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  		Table 5 Effect of 8 yr of different tillage systems on organic C, water-stable aggregates, total N, and C/N ratio in surface (0–5 cm) soil at Dixon Springs

 
Trends for total N in the 0- to 5-cm soil layer for all three organic matter fractions were the same as observed for organic C, except that the differences in POM N between the NT and the CP were not significant (P = 0.05) (Table 5). The NT, CP, and MP had 27, 24, and 21% of their total N as POM N in the surface soil, and 8 yr of tillage resulted in the 24 and 51% loss of POM N in the CP and MP compared with the NT.

Eight years of tillage resulted in a loss of 41 and 53% of macroaggregates (>250 µm) in CP and MP compared with the NT in the 0- to 5-cm layer (Table 5). The mineral fraction organic matter with a smaller C/N ratio contains more humified organic C and is more stable than the POM fraction. Higher organic C, lack of tillage, and slower rate of residue decomposition on the soil surface promoted the aggregation in NT.

In the 5- to 15-cm soil (Table 6) , the differences due to tillage in organic C and total N for whole soil, POM, and mineral fraction were not statistically significant at the P = 0.05 level. The POM C/total C ratio was 26, 27 and 30% in NT, CP, and MP, respectively. Particulate organic matter N was 20, 19, and 20% of total N in the 5- to 15-cm soil depth in NT, CP, and MP, respectively. Even though the MP had 10 and 7% higher POM C than NT and CP in subsurface soil and the POM C/total C ratio was also greater in the 5- to 15-cm subsoil layer of MP compared with the NT, the aggregate stability was still significantly greater in the NT. A higher amount of organic C and the wider C/N ratio in the POM fraction with the MP compared with CP and NT in subsurface soils could be attributed to the incorporated residue and root fragments by moldboard plowing. In the surface and subsurface soil, a greater C/N ratio (Tables 5 and 6) of the mineral fraction in NT compared with CP and MP might be due to the presence of some labile organic C in the mineral fraction SOM (Beare et al., 1994).


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  		Table 6 Effect of 8 yr of different tillage systems on organic C, water-stable aggregates, total N, and C/N ratio in subsurface (5–15 cm) soil at Dixon Springs

 

   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
REFERENCES
 
After 8 yr of NT, CP, and MP on a previously eroded Grantsburg silt loam, the NT reduced the soil erosion by maintaining higher surface residue and reduced surface disturbance. Tillage did not affect CEC; however, the NT had higher exchangeable Ca, Mg, and Bray P-1 in the surface soil than the MP. Exchangeable K was lower in the NT than the CP in the 0- to 5-cm layer, probably because of more leaching.

Eight years of tillage resulted in a significant reduction in water-stable aggregates in both the 0- to 5- and 5- to 15-cm layers of MP and CP. The NT maintained higher level of organic C content and macroaggregation than CP and MP with time. Soil organic C was higher in the POM and mineral-associated fraction of soil organic matter in the NT because of a lower decomposition rate. Tillage reduced a significant proportion of the wider C/N ratio and tillage sensitive POM in CP and MP, which contributed to the lower aggregate stability of soils under the CP and MP than the NT. Loss of macroaggregates and organic C could make the tilled soil more vulnerable to water erosion because of a reduction in its slaking resistance.SAS Institute 1995


   ACKNOWLEDGMENTS
 
This work was funded as part of Hatch Project no. 15-359 and in cooperation with North-central Regional Project no. 174. Published with the approval of the Director, Illinois Agricultural Experiment Station, Urbana, IL.

Received for publication April 30, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Summary and conclusions
 REFERENCES
 




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