(1983) Organic Carbon, Nitrogen, and Phosphorus ...

Organic Carbon, Nitrogen, and Phosphorus Concentrations and pH in Soil Profiles as Affected by Tillage Intensity1 W. A. DiCK 2 ABSTRACT No-tillage (NT), minimum tillage (MT), and conventional tillage (CT) practices were continuously applied to a Hoy tville silty clay loam (Mollic Ochraqualf) soil (18 years) and a Wooster silt loam (Typic Fragiudalf) soil (19 years) in Ohio. The effect of the various tillage intensities on the profile (0-30 cm) distribution of organic C, N, and P concentrations and pH was investigated. Results showed that NT resulted in significantly (P < 0.05) higher organic C and N concentrations in the 0- to 15-cm soil increment of the Hoy tville soil but significantly lower concentrations in the 15- to 30-cm soil increment. For the Wooster soil, NT resulted in higher concentrations in the 0- to 7.5-cm soil increment. No significant differences were observed among tillage intensities in the 7.5- to 30-cm soil increment. Comparison of organic C concentrations in the plow layer (0-22.5 cm) of the soils at the beginning of the longterm tillage experiment and at present showed that concentrations remained constant or decreased 11 % under NT in the Hoytville and Wooster soils, respectively. Present organic C concentrations in the Hoytville 1 Contribution from the Dep. of Agronomy, Ohio Agric. Res. & Dev. Ctr., Wooster, OH 44691. Published with approval of the Director as Paper no. 72-82. Received 19 May 1982. Approved 24 Aug. 1982. * Assistant Professor, Dep. of Agronomy, The Ohio State University and The Ohio Agric. Res. & Dev. Ctr., Wooster, OH 44691.

soil were decreased 12 to 14% by long term MT or CT while a 23 to 25% decrease was observed for the Wooster soil. Organic P concentrations under NT were significantly (P < 0.05) higher in the 0- to 7.5cm increment of the Wooster soil and significantly lower in the 22.5- to 30-cm soil increment. Organic C/N, C/P, and N/P ratios were calculated and higher ratios were observed under NT than under MT or CT in the surface soil increments. Tillage intensity, however, had little effect on the ratios averaged over the entire profile (0-30 cm). Soil pH was 0.1 to 0.3 units lower (P < 0.05) under NT in all soil increments except in the 22.5- to 30-cm increment of the Wooster soil where no significant differences in pH were observed among the tillage intensities. Additional Index Words: no-till, zero tillage, minimum tillage, nutrient distribution, organic C/N/P ratios. Dick, W.A. 1983. Organic carbon, nitrogen, and phosphorus concentrations and pH in soil profiles as affected by tillage intensity. Soil Sci. Soc. Am. J. 47:102-107.

A

N INCREASING NUMBER of farmers are changing to crop production methods which are less tillage-intensive. No-tillage, defined as a crop production system

DICK: ORGANIC C, N, AND P CONCENTRATIONS AND PH IN SOIL PROFILES AFFECTED BY TILLAGE INTENSITY

where weed control is accomplished entirely by herbicides and tillage is limited to the opening of a small slot for seed placement, is a method rapidly being adopted by farmers throughout the U.S. This is because no-tillage reduces soil erosion and fuel use, conserves soil water, and allows row-crop production to be practiced on steeply sloping farmland. By the year 2000 an estimated 65% of the seven major crops (corn, soybeans, sorghum, wheat, oats, barley, and rye) in the U.S. will be grown by the no-tillage system and 78% by the year 2010 (Phillips et al., 1980). With no-tillage, mechanical incorporation of fertilizer within the soil plow layer is not possible and the nutrients taken up by plant roots from the subsoil and incorporated into the plant are subsequently deposited on the soil surface as plant residue. A few reports on changes in the distribution of organic matter, nutrients, and acidity (pH) in the soil profile as a result of no-tillage practices have been previously published (Van Doren et al., 1976; Blevins et al., 1977; Juo and Lal, 1979; Doran, 1980). These reports, however, dealt with studies on sites where continuous no-tillage crop production practices had been maintained for only 10 years or less and cannot describe longer-term effects of continuous no-tillage. In Ohio, experiments dealing with various tillage intensities (no-tillage, minimum tillage, and conventional tillage) were begun in 1962 and 1963 and have continued to the present. These 18- and 19-year continuous tillage experiments have allowed a much greater time for changes in organic matter, nutrients, and pH to become established. The objectives of this study were to investigate changes in organic C, N, and P concentrations and pH as affected by tillage intensity in soil profiles collected from these long-term experimental plots. MATERIALS AND METHODS Experimentation on a Hoytville silty clay loam (fine, illitic, mesic Mollic Ochraqualf) soil and a Wooster silt loam (fine, mixed, mesic Typic Fragiudalf) soil concerning the effects of tillage intensity on crop production was begun in 1963 and 1962, respectively. This corresponds to an 18-year period where the various tillage intensities have been continuously applied at the Hoytville site and a 19-year period at the Wooster site. The complete history and the soil characteristics of these sites were described by Van Doren et al. (1976). Briefly, prior to the beginning of the tillage experiment, the Hoytville site (slope, < 1%) had been maintained for 6 years in a conventional tillage corn-oats-meadow rotation and a grass meadow had been maintained for 6 years at the Wooster site (slope, 2.5 to 4.0%). Soil characteristics observed at the two sites can be attributed primarily to parent material, topography, drainage, and shrinkswell potential. The Hoytville soil has poor surface and internal drainage when wet but cracks substantially when dry. In contrast, the Wooster soil has much better surface and internal drainage and little or no shrink-swell potential. At each site nine combinations of three levels of each of two variables in a complete factorial, randomized block design with three replications were established. The tillage variable is defined as follows: 1. No-tillage (NT)—planting was accomplished directly into the proceeding years crop residue by means of a coultertype planter. 2. Minimum tillage (MT)—moldboard plow 20 to 25 cm deep with no other tillage prior to planting. 3. Conventional tillage (CT)—moldboard plow 20 to 25 cm deep and at least two other 10-cm deep secondary tillage operations prior to planting.

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The rotation variable consisted of (i) continuous corn, (ii) corn and soybeans, and (iii) corn, oats, and alfalfa meadow in a 3-year rotation. Sufficient numbers of plots were established so that each crop in each of the nine treatments appeared each year. The same tillage treatments have been maintained on each plot to the present time. Tilled treatment plots were plowed in the spring 1 to 4 weeks before planting for the Wooster soil and in the fall for the Hoytville soil. The third year (meadow) of the corn-oatsmeadow rotation received no tillage prior to planting. Common management practices were maintained for the various rotations across the tillage treatments, i.e., the total amount of N, P, K, and lime applied was the same for the three tillage treatments and in all cases was broadcast-applied. Fertilizer was broadcast in the spring prior to planting and lime was broadcast in the winter as required to maintain a pH in the Ap horizon of the continuous corn plots at or above 6.0. Herbicide materials and rates varied with years but the total amounts added to the NT plots compared to the MT or CT plots was approximately one-third greater over the experimental time period. Soil samples (2.5-cm outer diameter soil cores) from 0 to 1.25 cm, 1.25 to 2.5 cm, 2.5 to 7.5 cm at 2.5-cm increments, and from 7.5 to 30 cm at 7.5-cm increments from the NT plots and from 0 to 30 cm at 7.5-cm increments from the MT and CT plots were collected prior to tillage in the fall of 1980. Samples were obtained after first removing from the soil surface easily identified plant materials, i.e., corn stalks and leaves. The soils were air-dried and ground to pass through a 60-mesh sieve. Soil pH was determined by a glass electrode (soil-to-water ratio, 1:1), organic C by the Walkley-Black method (Allison, 1965), organic N was determined by semimicro-Kjeldahl digestion as described by Bremner (1965), and organic P was obtained by subtracting the value of inorganic P (Olsen and Dean, 1965) from the value obtained for total P (Dick and Tabatabai, 1977).

All concentrations reported for organic C, N, and P are expressed on a gravimetric basis. For the statistical analyses, the concentration of the various parameters measured in the 0- to 7.5-cm soil increment for the NT plots was obtained by calculating a weighted average of the concentrations in the 0- to 1.25-, 1.25- to 2.5-, 2.5- to 5.0-, and 5.0- to 7.5-cm soil increments. Data were interpreted using analysis of variance and Duncan's Multiple Range Test. RESULTS AND DISCUSSION The main effect of tillage intensity on organic C, N, and P concentrations and on pH in soil profile samples will be the focus of this paper. No mention of rotation effects and interaction effects of tillage and rotation will be made. Data for all three tillage treatments will be presented but in most cases the results for the MT and CT treatments were similar. Organic C Among the pronounced effects caused by the different continuous long-term tillage intensities was the accumulation of organic C at the soil surface under NT. The organic C concentrations (Fig. 1) were approximately 2.5 and 2.2 times greater at the soil surface (0-1.25 cm) in the NT plots than in the MT and CT plots for the Hoytville and Wooster soils, respectively. The concentration of organic C in the NT plots rapidly decreased with increase in soil depth. The organic C concentrations calculated for the 0- to 7.5-cm soil increment in the NT plots of the Hoytville and Wooster soils were 3.01 and 1.82%, respectively, which were significantly higher (P < 0.001) than in the plowed plots (2.02 and 1.15%). The

104

SOIL SCI. SOC. AM. J., VOL. 47, 1983 ORGANIC-C (%) 1

2

3

Table 1—Effect of tillage intensity on organic C concentrations in the plow layer (0-22.5 cm) of Hoytville and Wooster soils.

4

Present organic C concentrations 7.5

Soil

concentration!

NT

MT

CT

2.02(12) 1.08(23)

2.00(14) 1.05(25)

——— % —— HOYTVILLE

15.0

SOIL

Hoytville Wooster

-. 22.5

- 3OO CL LJ

0

O en

7.5

Q

15.0

1.0

2.0

3.0

WOOSTER SOIL

22.5

30.01-

Fig. 1—Organic C concentrations in soil profiles as affected by tillage intensity. O, no-tillage; •, conventional tillage; and D, minimum tillage.

differences in the organic C concentrations in the 0- to 7.5-crri soil increment layer between the NT plots and the MT and CT plots may be due to (i) less soil-residue interaction as a result of NT, (ii) a lower rate of biological oxidation, and/or (iii) less erosion of soil high in organic matter. Soil erosion losses are greatly reduced when NT is practiced (Harold et al., 1970). Erosion, however, was not a problem at the Hoytville site under any of the tillage intensities. The reduced contact between soil and plant residues is considered the primary reason for organic matter accumulation under NT. Work reported by Brown and Dickey (1970) has shown that losses of wheat straw, left on the soil or exposed above the soil during an 18month period, ranged from 22 to 40%. Losses from 93 to 98% were observed for wheat straw buried in the soil. Below the 7.5-cm depth, the effect of soil type on organic C concentrations is evident (Fig. 1). For the Hoytville soil, organic C concentrations were significantly (P < 0.001) higher under NT than under MT or CT in the 7.5- to 15-cm increment but significantly (P < 0.001) lower in the 15- to 22.5- and the 22.5- to 30-cm soil increments. For the Wooster soil, however, comparing organic C concentrations among the three tillage treatments showed that below the 7.5-cm soil depth, concentrations were not significantly (P < 0.05) different. Several reasons for the differences between the two soils in organic C concentrations below the 7.5-cm soil depth may be postulated, one being reduced root growth in the Hoytville soil under NT. Preliminary findings reported by Van Doren et al. (1976) indicated much greater damage of the corn root system by the fungus Pythium graminicola Subr. with continuous corn under NT. Corn grown continuously in the Wooster soil did not suffer from such a disease problem when NT practices were used. A second reason may be due to the soil cracking

2.3 1.4

2.31 (0)§

1.25(11)

t Initial organic C concentrations for the long-term tillage experimental sites were estimated from organic matter values published by Van Doren et al. (1976) using the conventional "Van Bemmelem Factor" of 1.724. This factor is based on the assumption that soil organic matter contains 58%C(seeAllison, 1965). t NT, no-tillage; MT, minimum tillage; and CT, conventional tillage. § Figure in parentheses is the percentage decrease in organic matter calculated from the equation (1 -B/A)100 where A equals the initial and B the present organic C concentrations.

that occurs in the Hoytville soil when dry. This allows aeration to occur below the soil surface so that oxidation of organic matter can proceed. When tillage (plowing) is performed, the amount of organic matter available for oxidation remains relatively constant throughout the soil profile. Without tillage, however, only small amounts of organic matter oxidized below the soil surface as a result of cracking are ever replaced. These results suggest that in some soils, i.e., where substantial cracking can occur, oxidation of organic C can continue to proceed in the subsurface layers even without mechanical tillage. In a soil where cracking does not occur, a less oxidative environment exists under NT than under CT (Doran, 1980). Therefore the organic matter originally present below the surface of the Wooster soil under NT would not be readily oxidized and changes in organic matter concentrations would occur primarily at or near the soil surface. A concern of the agricultural community has been the decline in soil organic matter due to intensive conventional cropping practices and soil erosion. Present organic C concentrations in the plow layer of the MT and CT plots and in a soil increment of similar thickness in the NT plots were compared with organic C concentrations found in the plow layer at the beginning of the tillage experiment (Table 1). The present organic C concentrations presented in Table 1 represent average concentrations for a 0- to 22.5-cm soil plow layer and were obtained by summation and averaging values for the individual soil increments. The results show that at both sites the present organic C concentrations, after long-term tillage intensities had been maintained, were substantially higher in the NT plots than in the MT and CT plots. For the Hoytville site, where a corn-oats-meadow rotation under conventional tillage had been practiced prior to the initiation of the long-term tillage experiment, the organic C concentration after 18 years of continuous NT was essentially unchanged. A decrease of 12 and 14% was observed for the MT and CT treatments, respectively. The organic C concentration at the Wooster site decreased from that observed in the grass meadow at the beginning of the tillage experiment but the decrease was less for NT (11%) than for MT (23%) and CT (25%). A similar observation was reported by Lal (1974) when he found organic C concentrations for tropical soils decreased less when NT practices were used than when the soils were plowed. Table 1 also shows that MT practices maintained slightly higher organic C concentrations than CT practices.

DICK: ORGANIC C, N, AND P CONCENTRATIONS AND PH IN SOIL PROFILES AFFECTED BY TILLAGE INTENSITY

0.1

ORGANIC-N (%) 0.2 0.3 0.4

0.01

0.5

r.5

ORGANIC-P (%)

0.02

0.03

ao4

105

aos

7.5 HOYTVILLE SOIL HOYTVILLE SOIL

15.0

15.0

" 22.5

•22.5

3ao

0.1

0.2

0.3

Q_

30.0 0.01

Ld Q

0.02

0.03

7.5

7.5

WOOSTER WOOSTER SOIL

15.0

SOIL

15.0

22.5

22.5

30.01-

30.0L

Fig. 2—Organic N concentrations in soil profiles as affected by tillage intensity. O, no-tillage; •, conventional tillage; and D, minimum tillage.

Fig. 3—Organic P concentrations in soil profiles as affected by tillage intensity. O, no-tillage; •, conventional tillage, and D, minimum tillage.

Organic N Organic N concentrations (Fig. 2) closely followed the pattern observed for organic C at the two sites studied. Concentrations at the soil surface (0-1.25 cm) in the NT plots were approximately twice as great as in the MT and CT plots. No significant differences were observed among tillage treatments in organic N concentrations below the 7.5-cm soil depth for the Wooster soil. The Hoytville soil had significantly (P < 0.01) higher concentrations of organic N in the NT plots in the 7.5- to 15-cm increment but significantly (P < 0.001) lower concentrations in the 15- to 22.5- and 22.5- to 30-cm soil increments. Higher rates of N fertilizer have generally been required to achieve the same level of yield for NT as for CT during the first years of NT cropping (Thomas et al., 1973; Kang and Yunusa, 1977). This was attributed to greater immobilization of N fertilizer by the soil microorganisms during the decomposition of fresh plant residues of high C/N ratio. This effect is diminished in subsequent cropping years and results by Juo and Lal (1979) showed that grain yields of maize were consistently higher under NT than under plowed treatments after 6 years of cropping at the same N rate. In the present study equal amounts of N were applied to each tillage treatment. The total amount of N in the 0 to 30-cm soil profile, however, was significantly (P < 0.05) greater under NT than where the plots had been plowed.

tillage treatments only in the 22.5- to 30-cm increment where the organic P concentration in the NT plots was lower than in the MT or CT plots. For the Wooster soil, a significantly (P < 0.05) greater organic P concentration was observed under NT compared to MT and CT in the 0- to 7.5-cm increment. Figure 3 indicates that unlike organic C and N concentrations, the greatest organic P concentrations under NT did not occur at the soil surface but in the soil increments between 2.5 to 15.0 cm in depth. The greater organic P concentrations in the 2.5to 15.0-cm soil increments observed under continuous long-term NT may be due to the movement of organic P compounds into the soil profile. Pinck et al. (1941) have shown that organic P compounds are more mobile in the soil than inorganic P.

Organic P Organic P concentrations in the Hoytville and Wooster soil profile samples as affected by tillage intensity are shown in Fig. 3. For the Hoytville soil profile, significant (P < 0.05) differences were observed among the three

Organic C, N, and P Relationships The effect of tillage intensity on organic C/N, C/P, and N/P relationships is shown in Table 2. The average C/N ratios in the two soils were very similar among the three tillage intensities. The highest C/N ratios observed were at the soil surface (0-1.25 cm) of the NT plots and as the soil depth increased, the C/N ratio decreased. The relatively low C/N ratios indicate that soil organic matter has undergone humification and favors N mineralization. Since large amounts of organic N accumulated at the surface of the NT plots compared to the MT and CT plots, mineralization may contribute an important part of the available N to plants during the growing season. However, the higher C/N ratios at the surface (0to 1.25- and 1.25- to 2.5-cm increments) under NT compared to MT or CT also indicate that even though substantial humification had taken place, the organic matter in the NT plots is still less humified and more carbonenriched compared to the plots where tillage has been applied.

106

SOIL SCI. SOC. AM. J., VOL. 47, 1983 Table 2—Organic C, N, and P relationships in Hoytville and Wooster soils as affected by tillage intensity. Ratio under tillage intensity specified! Organic C/organic N

Soil increment, cm

Soil

MT

CT

NT

MT

CT

NT

MT

CT

7.5 1.25 2.50 5.0

10.7 11.4 11.1 10.2

10.2

10.5

55.0

61.2

5.9

10.1 9.7 9.5 6.9 9.2

9.9

9.2

10.8 9.2 6.7 9.3

49.5 47.3 32.3 52.1

55.0 52.3 32.9 48.8

61.8 51.7 31.6 51.6

7.4 10.4 8.9 7.4 5.6 5.1 5.0 4.6 5.5

5.4

- 7.5 -15.0 -22.5 -30.0

79.1 119.0 99.5 75.1 66.0

5.6 5.3 4.9 5.3

5.7 5.6 4.8 5.5

9.1

9.1

53.4

55.0

6.0

9.1 9.1 8.7 9.0

52.8 53.7 47.4 51.8

53.1 53.4 48.6 52.5

8.2 11.0 9.5 7.7 6.5 5.9 5.6 5.6 6.3

5.8

8.9 8.9 8.4 8.8

79.4 121.0 96.0 71.2 59.3 54.9 49.0 36.1 54.9

5.9 6.0 5.7 5.9

5.8 5.9 5.6 5.8

0 0 1.252.50-

Hoytville

5.0 7.5 15.0 22.5

Average Wooster

Organic N/organic P

Organic C/organic P

NT

0 - 7.5 0 - 1.25 1.25- 2.50 2.5 - 5.0 5.0 - 7.5 7.5 -15.0 15.0 -22.5 22.5 -30.0

9.7 11.0 10.1 9.2 9.1 9.4

Average

8.8

8.8 7.3

9.8 6.7

t NT, no-tillage; MT, minimum tillage; and CT, conventional tillage.

The effect of tillage intensities on the C/P and N/P ratios was also minimal except in the 0- to 7.5-cm soil increment where NT ratios were higher than those observed for MT and CT. The C/P ratios at the soil surface (0-1.25 cm) of the NT plots, 121 and 119 for the Hoytville and Wooster soils, respectively, are much higher than normally found in soils but the amount of organic P relative to organic C is still sufficient for mineralization to occur. A C/P ratio of 200 or less is generally required for net mineralization of P (Tisdale and Nelson, 1975). The N/P ratio is believed to be closely tied to mineralization and immobilization reactions with a high N/P ratio resulting in increased N mineralization and a low PH 6.5

7.0

7.5

7.0

7.5

7.5 HOYTVILLE SOIL

15.0

£ 22.5

o

30.O 6.5

^/

7.5 WOOSTER

N/P ratio in increased P mineralization (Barrow, 1960). Inspection of results in Table 2 would suggest, therefore, that at the soil surface of the NT plots, where the highest N/P ratios were observed, more N would be mineralized relative to P than in the MT or CT plots where lower N/P ratios were observed.

pH The NT treatment led to significantly (P < 0.01) decreased pH levels compared to MT and CT throughout the soil profile (0-30 cm) for the Hoytville soil and in the 0- to 22.5-cm increments for the Wooster soil (Fig. 4). The decrease in pH was not only observed deeper into the profile for the Hoytville soil but the amount of decrease was also greater. The pH in the 22.5- to 30-cm increment of the Hoytville soil for the CT compared to the MT treatment was also found to be significantly higher (P > 0.05). The observation that the surface soil becomes more acidic under NT than under CT has been previously reported (Blevins et al., 1977; Moschler et al., 1973). Acidification is primarily due to nitrification of surface-applied N fertilizer. Blevins et al. (1977) found that surface pH decreased with increasing nitrogen application rates after 5 years of continuous corn. Surface-applied lime has been shown to be effective in neutralizing soil acidity under NT (Moschler et al., 1973) because it creates contact directly with the soil layer where most of the acidity is produced. Soil acidity produced deeper in the soil profile, however, cannot be as effectively neutralized under NT compared to MT or CT where mixing of the soil and lime occurs.

SOIL

15.0

22.5

300 L-

Fig. 4—pH values in soil profiles as affected by tillage intensity. O, notillage; •, conventional tillage; and D, minimum tillage.

CONCLUSIONS The distribution of organic C, N, and P and pH in soil profiles (0-30 cm) was changed as a result of various tillage intensities being continuously applied for an 18to 19-year period. Organic C and N accumulated at the soil surface under NT compared to MT or CT. Concentrations of organic P were increased under NT to a lesser extent than that observed for organic C and N and the

FIXEN & LUDWICK: P & K FERTILIZATION OF IRRIGATED ALFALFA ON CALCAREOUS SOILS: I

highest concentrations observed were in the 2.5- to 15cm soil increments and not at the soil surface. Soil pH was decreased throughout the soil profile under NT. Soil type influenced the changes in pH values and organic C, N, and P concentrations in soil profiles where long-term NT practices had been maintained. ACKNOWLEDGMENTS Appreciation is expressed to Drs. G.B. Triplet! and D.M. Van Doren for their foresight in establishing the long-term tillage intensity experiment. Thanks are given to Victor Martin, Jon Page, and Cindy Mask for technical support.

107