Tillage and crop rotation interactions on humic substances of a typic haplorthox from southern Brazil

Soil & Tillage Research, 26 (1993) 227-236

227

Elsevier Science Publishers B.V., Amsterdam

Tillage and crop rotation interactions on humic substances of a Typic Haplorthox from southern Brazil P.L.O. de A. Machado and M.H. Gerzabek Austrian Research Centre Seibersdorf, Department for Agriculture and Biotechnology, A-2444 Seibersdorf, Austria (Accepted 3 December 1992)

ABSTRACT The effects of soil tillage and crop rotation on humic substances of an Oxisol from southern Brazil were studied. Soils were sampled from conventional (CT) and no-tillage (NT) systems applied to soybean-wheat (S-W) and soybean-lupine-maize-wheat ( S - L - M - W ) rotations. Two additional samples from soils with secondary forest and eucalyptus were collected. Humic substances (HS) were extracted by chelating ion exchange resin and water. Grey humic acids (GHA), brown humic acids (BHA) and fulvic acids (FA) were analysed chromatographically on controlled pore glass. As expected, cultivation and tillage led to a decrease in soil organic carbon content. This diminution was somewhat lower in conventional tillage with S - L - M - W , compared with S-W rotation. The total amount of extractable HS was higher for forest soils. Cultivated soils with both CT and NT showed higher contents of GHA than BHA and FA. Crop rotation had an important influence on the quality of the organic matter as the S-W rotation with both CT and NT showed the highest GHA contents. Soils with eucalyptus exhibited the highest amount of extractable humic substances among the investigated soils.

INTRODUCTION

Oxisols are widely distributed over Brazil and many of them are acidic with low base saturation (Dematt~, 1981; Resende et al., 1988). The role of soil organic matter in alleviating nutrient and stress problems in tropical soils is well known (Morais et al., 1976; Sanchez, 1976 ), but compared with soils of temperate regions, fewer investigations exist about the composition and Correspondence to: P.L.O. de A. Machado, Austrian Research Centre Seibersdorf, Department for Agriculture and Biotechnology, A-2444 Seibersdorf, Austria.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-1987/93/$06.00

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transformations of humic substances in tropical ecosystems. The turn-over time for litter in tropical forests is generally less than 1 year whereas in temperate forests it is between 1 and 2 years (Anderson and Swift, 1983 ). Jenkinson and Ayanaba (1977) showed that residues of ryegrass added to soil decomposed four times faster in Nigeria than in England. Dystric Fluvisols from Bangladesh had low contents of high molecular weight humic substance with a low degree of polymerization of the extractable fraction (Gerzabek and U1lah, 1989). Soil tillage influences the organic matter composition. Ten years after clearing and establishing a mechanized cropping system for corn production, aromaticity of humus increased in a Vertisol from Yucat~in, Mexico (Zech et al., 1990). The State of Paran~i in southern Brazil, with only 2.3% of total land area is responsible for 25% of the national production of grains (Castro Filho et al., 1991 ). Cultivation associated with intensive tillage systems has accelerated the degradation of soil organic matter and soil erosion. Excessive tillage of an Oxisol in Brazil led to a quantitative and qualitative degradation of the soil organic matter (Roth et al., 1992 ). Some studies have demonstrated the beneficial effects of crop rotation on agroecosystems in the southern part of Brazil (Muzilli, 1981; Yamaoka, 1981; Muzilli et al., 1983; Derpsch et al., 1986; Roth et al., 1991 ). Little is known, however, about the composition of the humic substances, specially those in tropical and sub-tropical soils where different systems of crop rotation are applied. The purpose of the present study was to characterize the humic substances of an Oxisol (Typic Haplorthox) from southern Brazil and to investigate the response to different systems of soil tillage and crop rotation. For reference purposes, two additional samples from forest-Oxisols were characterized. MATERIALS AND METHODS

In 1989 soil samples were collected from a field trial at the Instituto Agron6mico do Paran~i (IAPAR) in Londrina (23°23'S and 51 ° l l ' W ) . The experiment was a tillage-crop rotation study initiated in 1981. The soil type was an Oxisol (Typic Haplorthox) with low base saturation and high clay content (79% clay, 19% silt and 2% sand). The < 2 #m soil fraction predominantly contained kaolinite and hematite (Pavan et al., 1985 ). The tillage treatments were conventional tillage (disk plough, followed by two light disk harrowings (CT) and no-tillage (Rotacaster, a rotary hoe planter (NT). Crop rotation included soybean-wheat (S-W) and soybean-lupinemaize-wheat ( S - L - M - W ) . The soil was fertilized with 40 kg K20 h a - l as KC1 and 75 kg P205, h a - 1as triple superphosphate for all crops and 30-60 kg N h a - 1 as ammonium sulphate only for maize. Tillage-rotation plots (CT S-W, CT S-L-M-W, NT S-W and NT S - L - M W) were arranged in a split-plot experimental design with four replications.

TILLAGE AND CROP ROTATION INTERACTIONS

229

Plots were 10 m × 20 m. Detailed information about trial sites and climate is reported elsewhere (Kemper and Derpsch, 1981; Derpsch et al., 1988 ). Soil was sampled prior to summer tillage and planting in November 1989, at depth of 0-10 cm. Five cores were taken randomly from the row middles in each replication of each plot and composited. After air-drying the samples were passed through a 2.0 m m sieve, placed in plastic bags and transported to the laboratory in Seibersdorf. Soil pH measurements were made in 0.01 M CaC12 and organic carbon was determined by a wet combustion procedure (EMBRAPA, 1979). An unbuffered salt solution ( 1 N KC1) was used to displace exchangeable or extractable A1. Exchangeable cations (Ca, Mg and K) were extracted using 1 M NH4OAc, pH 7 (Camargo et al., 1986). Particle size analyses were conducted by wet sieving and the pipette method as described by Blum et al. (1989). In addition, soil samples were collected from two uncultivated sites presenting a secondary mixed hardwood forest adjacent to the tillage-crop rotation experiment and a reforestation with eucalyptus (25 °13'S and 50 ° I ' W ) . Humic substances (HS) were extracted by chelating ion exchange resin (CHELEX 100, Bio Rad, CA) and water (Danneberg and Schaffer, 1974). Compared with alkaline solvents (0.1 to 0.5N NaOH), ion exchange resin is considered to be a mild extractant (Danneberg, 1973; Kutsch, 1985 ). Gerzabek and Ullah (1989) showed that the amount of humic substances extracted by ion exchange resin varies between 13.6 and 55.6% of the organic carbon content of soils. The ratio of soil:resin:water was 10 g:25 g: 50 ml. The soil samples were shaken over-night and centrifuged for 30 min at 13 000 rpm. The supernatant was decanted off to smaller centrifuge tubes and recentrifuged at 15 000 rpm for 20 min in order to obtain clear extracts. The HSextract was diluted with water in a 100 ml glass-flask, decanted off to plastic bottles and immediately deep-frozen. This extraction procedure was repeated three additional times and the supernatant was added each time to the corresponding plastic bottle. The extracts were then freeze-dried and again deepfrozen until they were used for chromatography. The chromatography was carried out using a technique similar to that described by Gerzabek and Ullah ( 1989 ). A collumn (Chromoduls~iulen, H/Slzen, Germany) with 18 m m i.d. and 1000 m m long was filled up with controlled pore glass (CPG, Electronucleonics, Fairfield, N J) of 120-200 mesh coming size and a pore diameter of 17.7 nm. Dissolved gases were driven out using 0.02 M NaEB407and 0.05M NaC1 solutions as eluent. Calibration parameters such as salt volume (Vt) and void volume (Iio) were calculated from the CPG used and these values were also determined by chromatography of a test solution containing 0.5% blue dextran 2000 (Pharmacia, Stockholm, Sweden) and 1% benzyl alcohol. The mean standard deviation of the described chromatography is 8.2% (Danneberg, 1979). The freeze-dried samples were dissolved in the eluent and the flow rate of

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P.L.O. DE A. MACHADO AND M.H. GERZABEK

the column was maintained at 1.5 ml m i n - 1 After injection of 0.5-1.5 ml of extract and subsequent elution, the extinction was measured at 400 nm in the following eluent by a spectrophotometer. The characterization of the humic substances was made using the following parameters (a) The Ka-values of the peaks (see Figs. 1 and 2) as a measure of the molecular weight K

d

Ve-- Vo =VT-S-

Where Vt is the salt volume, Vo is the void volume, Ve is the elution volume. (b) The optical density (o.d.) per gram soil, calculated by integration of the extinction peak. (c) The humification ratio (HR) (Gerzabek and Ullah, 1989 ) was calculated as HR=O.d" g-i soil °/0Corg

where o.d. is the optical density of the total extract at 400 nm. (d) The maximum extinction of the high molecular weight fraction as a percentage of the low molecular humic substances was used as a measure of the molecular weight distribution (Gerzabek and Ullah, 1989; modified from Kononova, 1966). The higher the values of E 1/E2%, the higher is the amount of larger molecules, such as, grey humic acids E1

/E2%_Emax 1St peak× 100 Ema x 2nd peak

where E is extinction.

RESULTS AND DISCUSSION

Table 1 shows the physico-chemical properties of the soils at the experimental site. It is worthwhile mentioning that the secondary forest adjacent to the field trial exhibited a higher organic carbon content as compared with the cultivated plots. In Parami, degradation of soil organic matter owing to cultivation and tillage has been well documented (Instituto Agronbmico do Paramt, 1979; Sidiras and Pavan, 1986; Castro Filho et al., 1991 ). The new Corgequilibrium content was 31.3% lower in conventional tillage under S-W, compared with S - L - M - W rotation. This may be due to the higher mass of crop residues in the latter case (Havlin et al., 1990; Muzilli, 1979). Compared with the secondary forest soil, conventional tillage plots showed a decrease of pH-values leading to an increase of exchangeable aluminium. This

TILLAGE AND CROP ROTATION INTERACTIONS

231

TABLE 1 Physico-chemical properties of uncultivated and cultivated Oxisols from Brazil under different tillage and crop rotation (composite samples from four replications) Soils

pH (CaC12)

Corg (%)

AI

Ca (meq per 100 g soil )

Mg

K

Sand

Silt (%)

Clay

4.87 ~ 4.26 b

3.61 5.00

0.36 1.20

0.8 5.3

2.6 2.5

0.42 0.45

2 5

19 25

79 70

4.15 b 4.77 a 4.24 b 4.96 ~

1.10 1.50 1.60 1.60

0.90 0.26 1.10 0.10

3.3 3.5 3.9 3.9

1.5 1.4 1.4 2.1

0.31 0.60 0.47 0.44

2 2 2 2

19 19 19 19

79 79 79 79

Uncultivated Secondary forest Eucalyptus

Cultivated CT l S-W 2 NTIS-W 2 CT l S - L - M - W 2 NT l S - L - M - W 2

ICT, conventional tillage; NT, no-tillage. 2S, soybean; W, wheat; L, lupine; M, maize. a'bValues followed by the same letter do not show significant differences at P = 0.01 ( n = 2 ).

effect was not observed at no-tillage plots (Table 1 ). In soils under NT the nutrients tend to accumulate in the top layer (0-10 cm ), while tilling mixes them over a greater depth, reducing relative concentrations (Derpsch et al., 1988 ). An increase in the pH of soils under no-tillage was also reported by Sidiras and Pavan (1986) for Oxisols and Alfisol from Paranfi State. Higher erosion rates, specially under conventional tillage, also lead to a decrease in nutrient contents and pH (Derpsch et al., 1986). Figures 1 and 2 show the chromatogrammes of humic substances separated by controlled pore glass. The elution curves are characterized by two peaks. The first peak at a Kd-value of approximately 0.0 is attributed to larger molecules, such as grey humic acids, the second peak at a Kd-value of approximately 0.60 represents smaller molecules (brown humic acid and predominantly fulvic acids) (Danneberg and Ullah, 1982 ). The total amount of extractable humic substances (o.d. g-1 soil; Table 2 ) was generally higher for the uncultivated soils as compared with the field plots. In contrast to the adjacent secondary forest site, the amount of grey humic acids was slightly higher than brown humic and fulvic acids in soils with NT under S - L - M - W rotation (Figs. 1 and 2). This difference seems to be more evident with CT under S-W rotation (Fig. 1 ). Compared with a forest soil, the decrease of %Cfulvicacids due to cultivation of the same soil type with SW rotation was also observed by Roth et al. (1992 ). Thus, soil cultivation presumably promotes the mineralization of the brown humic and fulvic acids and favours the increase of grey humic acids with a relatively high stability. Smaller molecules such as fulvic acids are more easily decomposed, com-

232

P.L.O. DE A. MACHADO AND M.H. GERZABEK

1.4 1.3 1.1"512 *... CT S/W

~

,.o

~ ... NT

-~ 0.9 ] 0.8 j

2

s/w

A ... CT S/L/M/W * ... NT S/L/M/W

1

e 0.41 0.3] 0.2 0.1 0.0 I ......

-0.1

'"1

.........

0.0

I"

0.1

.......

~2;'~.)(: . . . . .

I'"'

.....

0.2

I .........

0.5

I I ........

0.4

Kd

]FI'"I'"I''"~II"I

0.5

........

0.6

0.7

II .........

0.8

II '1~'

'TTTT

0.9

1.0

value

Fig. 1. Chromatogramme of humic substances extracted from four cultivated Brazilian Oxisols (composite samples from four replications). 2.4-

2.2 2.0 1.8 1.6

<>... eucalyptus

1.4

/

~

1.2

o ~ 1.0 ~ 0.8 6 0.6 0.4 0.2 0.0 I''"'"

-0.1

I .........

0.0

I ......

0.I

' ' ' 1 ' " ' ' ' " ' 1

0.2

.........

0.3

[ ' ' [ ' ' ' ' ' l l ' ' ' ' ' l ' ' ~ l ' ' H ' ' ' ' ' p

0.4

0.5

0.6

.........

0.7

I ' ' ' ' '~''

0.8

'1'''

0.9

"

rrrr[

-

1.0

Kd va]ue Fig. 2. Chromatogramme of humic substances extracted from two uncultivated Brazilian Oxisols (composite samples from four replications).

pared with high molecular weight fractions like grey humic acids (Gerzabek et al., 1991). Soybean-wheat rotation under CT showed higher contents o f high molec-

TILLAGEAND CROP ROTATIONINTERACTIONS

233

TABLE2 Some characteristics of humic substances of uncultivated and cultivated Oxisols from Brazil under different tillage (CT, conv. till.; NT, no-till. ) and crop rotation (S, soybean; W, wheat; L, lupine; M, maize). Composite samples from four replications Soils

Uncultivated Secondary forest Eucalyptus Cultivated CT S-W NT S-W CT S - L - M - W NT S - L - M - W

o.d.~ per g soil

HR 2

E 1/ E2% 3

8.23 16.74

2.28 3.35

39.6 120.2

3.16 2.84 2.06 3.47

2.87 1.89 1.28 2.17

781.9 201.2 256.2 134.6

1Optical density of total extract at 400 nm. 2Humification ratio. 3Measure of the molecular weight distribution.

ular weight humic substances as compared with the S - L - M - W plots. This tendency was less evident for no-tillage systems. As indicated by the molecular weight distribution (E 1/E2%; Table 2 ), it seems that soil tillage also played a role on the quality of the organic matter. Conventional tillage under both S - L - M - W and S-W rotations was characterized by lower contents of low molecular weight humic substances (brown humic acid and predominantly fulvic acids ). Above all, the latter rotation showed a high impact of soil tillage on larger molecules as increasing their amount (E 1/E2%; Table 2 ). Fulvic acids are considered to be the most reactive molecules in the soil because of their high content of functional groups (mainly COOH-groups) (Schnitzer and Khan, 1978 ). Humic acids, despite their lower reactivity, play an important role in the amelioration of physical properties of Oxisols by increasing the aggregate stability (Roth et al., 1992 ). Both cultivated and forest soils had similar types of peaks, but the distribution of higher and smaller molecules during the elution procedure was somewhat different. The amount of organic molecules between the Kd-values of 0.05 and 0.5 was slightly higher for forest soils. This is similar to the observations of Gerzabek et al. (1989) for some forest sites in Austria. Therefore, the subsequent degradation of the organic matter through cultivation under both conventional and no-tillage systems was, above all, reflected by the low content of organic molecules (mainly brown humic acids) between the Kdvalues of 0.05 and 0.5. Uncultivated soils under eucalyptus showed the highest peaks for both larger and smaller organic molecules at a Kd-value of 0.015 and 0.57, respectively

234

P.L.O.DE A. MACHADOAND M.H.GERZABEK

(Fig. 2). Moreover, both peaks were of practically the same size. However, in contrast to the secondary forest soil, fulvic- and brown humic acid contents were somewhat lower than the contents of grey humic acids. This observation is also well identified through the molecular weight distribution (E 1/E2%, Table 2). Compared with soils under virgin forest, Della Bruna et al. ( 1991 ) reported a lower biological activity in an Ultisol under Eucalyptus plantation. These authors concluded that the low microbial activity could be due to the high C: N ratio and antibacterian substances present in the Eucalyptus litter. CONCLUSION

The chromatogrammes of the soil humic substances were characterized by two peaks indicating the clear separation of larger molecules (predominantly grey humic acid), which eluted at a Ko-value of 0.0 and smaller molecules (predominantly fulvic acids ), which eluted at Kd = 0.6. Soil cultivation under both conventional and no-tillage systems resulted in a decrease of Corg-Contents and in higher amounts of grey humic acids as compared with the adjacent forest site. Soybean-wheat rotation showed the highest contents of larger molecules within the cultivated soils. Under both S-W and S - L - M - W rotations, conventional tillage with more intensive soil inversion through ploughing led to a greater decline in the contents of fulvic acids. It was suggested that the differences in the organic matter turnover and humic substances characteristics, between the two forest sites, were due to lower microbial activity and therefore less mineralization in the eucalyptus-forest soil as compared with the secondary forest adjacent to the field-experiment site. Since the data presented here were based on composite soil samples from four replications in the field, it is noteworthy to remark that the results must be interpreted with caution. ACKNOWLEDGEMENTS

We would like to thank G.B. de Medeiros for allowing us to collect samples from the field trial at IAPAR. We greatly acknowledge Dr. M.A. Pavan, I. Bognola and Dr. C.H. Roth for providing soil samples and P. Herger for assistance with computing and graphic art. The senior author wishes to thank the Federal Chancellery of Austria for awarding him a doctorate scholarship.

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