Effect of no-till cropping systems on soil organic matter in a sandy
clay loam Acrisol from Southern Brazil monitored by electron
spin resonance and nuclear magnetic resonance
CimeÂlio Bayer
a, Ladislau Martin-Neto
b,*, JoaÄo Mielniczuk
c, Carlos Alberto Ceretta
daUniversidade do Estado de Santa Catarina, C.P. 281, 88 520-000, Lages, SC, Brazil bEmbrapa-Instrumentac,aÄo AgropecuaÂria, C.P. 741, 13 560-970, SaÄo Carlos, SP, Brazil cUniversidade Federal do Rio Grande do sul, C.P. 776, 90 001-970, Porto Alegre, RS, Brazil
dUniversidade Federal de Santa Maria, 97 105-900, Santa Maria, RS, Brazil
Received 27 April 1998; received in revised form 21 October 1998; accepted 24 August 1999
Abstract
In weathered tropical and subtropical soils organic matter is crucial for soil productivity and its quantity depends heavily on soil management systems. This study evaluated the effect of no-till cropping systems on organic matter content and quality in a sandy clay loam Acrisol soil (Paleudult in US taxonomy) from Southern Brazil. Ten cropping systems with varying additions of C and N were conducted for 12 years (from 1983 to 1994). The addition of crop residues increased total organic carbon (TOC) and total nitrogen (TN) in the soil at 0±17.5 cm depth, and this increase was directly related with C and N added or recycled by the systems. The crop residues added to the soil were associated with reduced semiquinone free radical concentration, detected by electron spin resonance (ESR), in the organo-mineral aggregates <53mm and humic acid (HA)
samples, in the soil at 0±2.5 cm depth. This showed that stable organic matter originating from crop residues was less humidi®ed than the original soil organic matter. Results obtained from organo-mineral aggregates showed a higher amplitude (highest and lowest values were 5.47 and 2.091017spins gÿ1 of TOC, respectively) of semiquinone free radical
concentration than HA samples (highest and lowest values were 2.68 and 1.771017spins gÿ1
of HA, respectively). These data showed that alterations due to tillage in soil organic matter characteristics, e.g., humi®cation degree can be better identi®ed through a combination of soil physical fractionation and spectroscopic analysis. Semiquinone content in the HA samples, detected by ESR, related signi®cantly to aromaticity, as measured by nuclear magnetic resonance (NMR) of13C.
Management systems including no-till and cropping systems with high C and N additions to the soil improved its quality in Southern Brazil.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Soil organic matter; Soil tillage; Cover crops; Humic acid; ESR;13C NMR
1. Introduction
High decomposition rates of soil organic matter and associated degradation processes in cultivated *Corresponding author. Tel.: 55-16-2742477; fax:
55-16-2725958.
E-mail address: martin@cnpdia.embrapa.br (L. Martin-Neto).
soils in tropical and subtropical regions present a very disturbing problem. Sanchez and Logan (1992) showed that soil organic matter in the tropics de-composes ®ve times faster than in temperate regions. As a prime example Silva et al. (1994) detected an approximately 50% reduction in total soil organic matter content in three different soil types only three years after conventional tillage began in Northeast Brazil. This fast and signi®cant reduction was accom-panied by a severely decreased cation exchange capacity.
Soil organic matter is now viewed as the most important factor in evaluating soil management sys-tems affecting soil quality and therefore agricultural sustainability (Doran and Parkin, 1994; Sollins et al., 1996; Medeiros et al., 1996; Salinas-Garcia et al., 1997). However, the effects of tillage methods on decomposition rates of organic matter are dependent on soil type, mainly texture and mineralogy (Par®tt et al., 1997). In a clay (220 g kgÿ1
) Acrisol, Bayer (1996) determined that rate of decomposition (k) varied from 0.054 per year under conventional tillage to 0.029 per year in no-till, while for clay (680 g kgÿ1) Oxisol, also from Southern Brazil, practically no difference existed between the effect of tillage meth-ods on soil organic matter decomposition rates (k0.014 per year under conventional tillage and 0.012 per year under no-till). The reduced effect of tillage methods on decomposition rates in Oxisol with high clay, iron and aluminum oxides content (Brazil, 1973) was attributed to higher physical sta-bility of organic matter in <2 and 2±20mm soil frac-tions. This agrees with ®ndings of Guggenberg et al. (1995), that showed in one Oxisol under tropical pastures only minor alterations in organic matter associated with clay particles, compared to sand and silt particles.
For evaluating tillage effects, soil organic matter is chemically fractionated, a process depending on solubility and yielding humic substances, humic and fulvic acids and humine (Stevenson, 1994). Among the components of refractory soil organic matter humic substances demonstrate the greatest amount of structural complexity. By investigating composition of different aromatic and aliphatic bio-macromolecules during humi®cation in soils, descrip-tion of the humi®cadescrip-tion process, operative during alteration of plant-derivative biomacromolecules to
humic substances, seems feasible. In recent years, non-destructive spectroscopic methods, e.g., nuclear magnetic resonance (NMR) and electron spin reso-nance (ESR), have shed some light on the structure and reactivity of humic substances. For example, from 13C NMR spectra of humic substances and to a lesser extent from organo-mineral aggregates and/or compounds of whole soil samples such as polysac-charides, lignin and alkyl-C moieties can be identi®ed (Frund et al., 1994; Preston, 1996; Kogel-Knabner, 1997).13C NMR has been extensively used to deter-mine the amount of aromatic and aliphatic groups and thus the humi®cation degree of humic substances. Generally, the higher aromaticity of humic substances indicates a more advanced stage of humi®cation (Pre-ston, 1996). This interpretation is due to the associa-tion of higher chemical recalcitrance with aromatic chemical groups in soil organic matter (Stout et al., 1985).
Another spectroscopically demonstrated molecular property of soil organic matter relating to humi®cation degree is stable free radical concentration (`spin'), as measured by ESR spectroscopy (Senesi, 1990a,b). Complex aromatic structures are believed to stabilize semiquinone free radicals in humus (Riffaldi and Schnitzer, 1972; Wikander and Norden, 1988; Senesi, 1990a,b; Stevenson, 1994). Thus, spin concentration in soil humus should increase with advancing humi-®cation (Martin-Neto et al., 1998).
More recently soil organic matter has been physi-cally fractionated by density or particle size using ultrasound for clay dispersion (Christensen, 1992). A major advantage in using a physical procedure fol-lowed by spectroscopy analysis, compared with tradi-tional chemical procedure, is the reduced possibility of alterating organic matter compounds (Baldock et al., 1992; Martin-Neto et al., 1994).
2. Materials and methods
2.1. Experimental design and treatments
The experiment was conducted at the Experimental Station of the Federal University of Rio Grande do Sul, Eldorado do Sul, State of Rio Grande do Sul, Brazil, with geographic coordinates of 308500
5200 S and 518 380
0800
W. The soil is a sandy clay loam Acrisol by the FAO legend (Paleudult by US and Dark red podzolic by Brazilian taxonomies), with 540 sand, 240 silt and 220 g kgÿ1
clay. Regional climate is humid subtropical-Cfa according to the Koeppen clas-si®cation, with annual mean temperature of 19.48C and with monthly variation from 13.9 to 24.98C. Annual rainfall is 1440 mm, varying monthly between 95.7 and 168 mm (Bergamaschi and Guadagnin, 1990).
The experimental design used randomized blocks with split-plots and three replications. The main plots (58 m) were 10 cropping systems (Table 1), and the subplots (54 m), were two N rates (0 and 120 kg haÿ1) applied to the maize crop. The present study concentrated on samples obtained from the area without N application.
When the experiment began in 1983, the soil was degraded, eroded and compacted due to intensive cultivation and erosion. Cropping systems were selected to provide from low to high amounts of crop residue and N-®xing crops. Prior to the start of the
experiment (in 1983), deep plowing was used to reduce compaction in whole experimental area. The CloverSpergula/maize, fallow/maize, and bare soil cropping systems used shallow cultivation with disc for control of weeds from 1983 to 1987. In these treatments since 1987 and in all others (Table 1), the soil was not plowed (no-till system) until sampling. Additional information on experimental procedures can be obtained in Testa et al. (1992), Teixeira et al. (1994) and Burle et al. (1997).
2.2. Soil sampling and chemical analysis
Soil samples were collected in September 1994 at depths of 0±2.5, 2.5±5.0, 5.0±7.5, 7.5±12.5 and 12.5± 17.5 cm, in an area of 0.10 by 0.50 m per plot perpendicular to maize row. The samples were air dried, ground and analyzed for total organic carbon content (TOC) by the Walkley and Black procedure, and total nitrogen (TN) by the Kjeldhal method (Page, 1982). The TOC and TN measurements were adjusted for soil density, and are reported in terms of weight/volume. The TOC and TN were estimated at soil depth of 0±17.5 cm. Total amounts of C and N added or recycled by the cropping systems were estimated from data reported by Burle et al. (1997) corresponding to C and N content in the above-ground parts of winter cover crops and maize residue plus above-ground parts of summer intercrops over a 12 year period.
Table 1
Cropping systems used in the experiment conducted at Eldorado do Sul, Southern Brazil Cropping systemsa Winter Summer
ClSp/Ma,b Subterraneum clover (Trifolium subterraneumL.)
Spergula(Spergula arvensisL.)c
Maize (Zea maysL.) O/M Oat (Avena strigosaSchreb) Maize
OV/MVg OatVicia (Vicia sativaL.) MaizeVigna(Vigna unguiculata(L.) Walp) F/MDl Fallow MaizeDolichos (Lablab purpureos(L.) Sweet) OCl/M Oatsubterraneum clover Maize
Ma Macroptilium(Macroptilium atropurpureum(DC.) Urb.) during 8 yearsc
Maize (1988 and 1993) MC Cajanus(Cajanus cajan(L.) Milsp.) MaizeCajanus
F/Mb Fallow Maize
Di Digitaria(Digitaria decumbensStent) during 8 yearsc Maize (1988 and 1993)
BSb Fallow (bare soil) Maize (1988 and 1993) aCultivated with wheat (Triticum aestivumL.) in winter and soybeans (Glycine maxL. Merrill) in summer up to 1986.
2.3. Soil organic matter fractionation
Soil organic matter from the 0±2.5 cm layer, where the cropping systems had the highest effect on soil samples obtained from oat/maize, oatVicia/maize
Vigna, maizeCajanus and bare soil cropping systems (Burle et al., 1997), was physically and chemically fractionated.
The physical procedure used to obtain organo-mineral aggregates of different sizes was described by Martin-Neto et al. (1994). To 20 g of soil was added 70 ml of distilled water in a 100 ml snap-cap tube. This suspension was sonicated (Sonic and aterial model 300 W) at 240 W for 6 min. The fractions >150 and 53±150mm were obtained by sieving, while the fractions 20±53, 2±20 and <2mm were obtained by sedimentation in PVC tubes, assuming a mean particle density of 2.65 g cmÿ3
. To reduce sample ¯occulation during sedimentation in the PVC tubes 0.28 g of NaOH per litre of suspension was added. The sedi-mentation process was repeated for each fraction about 15±20 times until the supernatant liquid was clean. After separation of organo-mineral aggregates in the fractions of 20±53, 2±20 and <2mm, 0.77 g of CaCl2 was added per fraction to ¯occulate solid material and separate the liquid using a siphon. After this procedure, the fractions were oven dried at 6058C and ground for spectroscopic analysis.
HA extraction according to Ceretta (1995), used the classical method based on solubility characteristics (Stevenson, 1994) in samples collected in 1993 from the same cropping systems. NaOH 0.1 M was used to extract humic substances and HCl 6 M to acidify to pH 1.0, with precipitation of HA, followed by puri®cation in a solution of HClHF (5 ml concentrated HCl5 ml HF 48%990 ml distilled water). The samples were then washed with distilled water until complete removal of chloride, monitored by AgNO3 (Ceretta, 1995).
2.4. Application of spectroscopic analysis
ESR measurements were made in HA and organo-mineral aggregate samples with three replicates using a Varian ESR spectrometer, line E-109 Century in X-band (9 GHz) from the Biophysics Laboratory, Insti-tute of Physics of SaÄo Carlos, University of SaÄo Paulo, Brazil. Samples used for HA analysis weighed 13±18
and 20±50 mg for organo-mineral aggregates. Spins quanti®cation was obtained using the approximation IH2 (Poole and Farach, 1972) employing as a secondary standard a synthetic ruby, as used in Sing-er's method (Martin-Neto et al., 1994) for both HA and organo-mineral aggregates. Particularly for the organo-mineral aggregates, spectral parameters were obtained by subtracting the spectrum of the whole organo-mineral fraction from the respective spectrum of the sample treated with H2O2(resulting in oxidation of organic matter) and consisting solely of soil mineral components. Therefore subtraction eliminated mineral fraction in¯uence from the spectral para-meters of soil organic matter.
Experimental conditions for measuring ESR in organo-mineral aggregates were: magnetic ®eld (H0)3400 Gauss (G), frequency of modulation (FM)100 kHz, amplitude of modulation (AM)
2.0 G, and microwave power (P)0.2 mW. Experi-mental conditions for synthetic ruby were H0 5400 G, FM100 kHz, AM5.0 G and P2.0 mW. Absolute spin quanti®cation was made by com-parison with standard of ``Strong Pitch'' of KCl analyzed in the following conditions: H03400 G; FM100 kHz, AM0.5 G andP2 mW. For HA samples analysis conditions employed were the same as for the organo-mineral aggregates, except that the modulation amplitude was 0.5 G.
The 13C NMR experiments on HA samples were performed using CPMAS/13C NMR in a Varian VXR-300 spectrometer, probe CP/MAS (VT), rotor 7 mm of zircon oxide, frequency of 75.4 MHz, spectral width 50 kHz, uncoupled nucleus of 1H (100 W of power), pulse of 0.5ms interval between pulses 2.0 s and rotation velocity of MAS 6.8 kHz. The number of pulses were 3600 for HA samples from the oat/maize, oatVicia/maizeVignaand maizeCajanus sys-tems and 5000 for bare soil. Contact time (200ms) was determined after evaluation, testing several times from 200 to 4000ms in one sample from maizeCajanus
system. The equipment is from CENPES/PetrobraÂs, Rio de Janeiro, Brazil. The CPMAS/13C NMR spectra are presented by Ceretta (1995).
2.5. Statistical analysis
qualitative parameters of soil organic matter was performed using mean value standard error. The rela-tion between the results of ESR in HAs and organo-mineral aggregates samples and the ESR and CPAMS/13C NMR of HAs results was evaluated by determination coef®cients (r2), with signi®cance t -tested at probability levels of 1 and 5%.
3. Results
3.1. C and N added by cropping systems
The cropping systems showed a large variation in C and N amounts in the above-ground parts of cover crops and maize residues (Table 2). During the 12 years of the experiment, additions varied between 5.6 Mg C haÿ1 and 126 kg N haÿ1 in the bare soil and 75.2 Mg C haÿ1 and 3436 kg N haÿ1 in the maize/Cajanus system, showing a very great differ-ence in crop potential for adding C to the soil via photosynthesis and N via biological ®xation or recy-cling. Legume-based systems showed much higher total N accumulation in their biomass than did non-legume crop systems, probably due to biological N ®xation. The maize yields are published in Burle et al. (1997) and the modeling of C and N trend in the soil are published elsewhere (Bayer, 1996).
3.2. TOC and TN accumulation in the soil
After 12 years, the cropping systems showed marked in¯uence on soil TOC and TN content. The TOC content in the 0±17.5 cm layer of soil varied from 29.8 to 43.7 Mg haÿ1
in the bare soil and maize/
Cajanussystems, respectively (Fig. 1a), correspond-ing to a differential accumulation rate of 1.26 Mg haÿ1 per year. Contents of TN in soils varied from 2497 in the bare soil to 3049 kg haÿ1 with utilization of
Macropitilium(Fig. 1b).
Effects of cropping systems on TOC and TN con-tents related directly to C and N amounts added and/or recycled (Fig. 1a and b). The angular coef®cient of the
Table 2
Estimation of C and N added by above-ground parts of winter cover crops and the maize residues plus the above-ground parts of summer intercrops over a period of 12 years (calculated from data of Burle et al., 1997)a
Cropping systems Carbon (Mg haÿ1) Nitrogen (kg haÿ1)
ClSp/Ma 45.8 1694
O/M 37.0 818
OV/MVg 67.3 2541 F/MDl 51.9 2202 OCl/M 52.9 1441
Ma 48.1 2125
MC 75.2 3436
F/M 15.0 378
Di 46.0 560
BS 5.6 126
aClclover, SpSpergula, Mmaize, Ooat, VVicia,
VgVigna, DlDolichos, MaMacroptillium, CCajanus, FFallow, DiDigitaria, BSbare soil.
equations, relating added amounts of C and N to their respective contents in soil, indicates that an estimated 19% of C and 28% of N present in crop residue remained in the soil. These results, mean values from cropping systems are overestimated because C and N from roots have not been included.
3.3. Soil organic matter quality
The ESR spectra of organo-mineral aggregates from the oat/maize system is given in Fig. 2. Two signals are visible in samples with sizes 53±150, 20± 53 and 2±20mm. As indicated in Fig. 2, one signal has
g2.004 and line width of around 6 G, typical of a semiquinone free radical, as detected in humic sub-stances (Senesi, 1990a) and organo-mineral aggre-gates from a Mollisol from Argentina (Martin-Neto et al., 1994). The second signal hasg2.000 and 3 G
line width and was shown to be associated with soil mineral fraction (more probably quartz) as determined by measurements in pure quartz samples. For the above 150mm samples, only that associated to quartz (g2.000 and line width 3 G) was observable. For 150±53, 53±20 and 20±2mm samples evident increase of semiquinone signal and decrease of quartz signal occurs simultaneously from higher to smaller particle size samples. For samples below <2mm, only a typical semiquinone signal (g2.004) was detected. The mineral fraction signal was excluded by subtraction in the quanti®cation process of semiquinone free radicals, as described in Section 2. For comparing chemical and physical fractionation procedures, the results of semiquinone free radical concentration determined by ESR is presented as mean values of organo-mineral aggregates <53mm, obtained by weighing TOC content in samples. For HA samples only a typical semiquinone free radical signal was detected (Senesi, 1990a; Martin-Neto et al., 1998).
Semiquinone free radical concentration in the organo-mineral aggregates in the 0±2.5 cm of soil layer was affected by cropping systems (Fig. 3). The number of spins gÿ1 TOC varied from 5.471017 in bare soil to 2.091017 in the mai-zeCajanus system. Samples in soils under oat/ maize and oatVicia/maizeVigna systems showed intermediate values, 3.571017 and
3.821017spins gÿ1
TOC respectively.
Fig. 2. ESR spectra of organo-mineral aggregates (>150, 53±150, 20±53, 2±20 and <2mm) from an Acrisol soil from Southern Brazil
under no-tillage with oat/maize cropping system. Two signals can be observed, one with g2.004, typical of semiquinone free radical in soil organic matter, and other withg2.000, associated to quartz in sand particles.
Fig. 3. Relationship between semiquinone free radical concentra-tions in HA samples (spins gÿ1 of HAs) and organo-mineral
aggregates (spins gÿ1of TOC) in an Acrisol soil from Southern
Generally the semiquinone free radical concentra-tion, indicating degree of humi®cation of soil organic matter (Martin-Neto et al., 1998), decreased with increased residue addition to the soil by cropping systems.
In the HA samples, semiquinone free radicals var-ied from 2.681017spins gÿ1 HA in bare soil to 1.771017spins gÿ1 HA in the maizeCajanus
systems (Fig. 3). Compared to organo-mineral aggre-gates, free radical variation amplitude values in HA samples were low.
Semiquinone free radical concentration in organo-mineral aggregates and HAs were closely related (r20.93) (Fig. 3). Linear regression adjustment of values obtained for organo-mineral aggregates and HA samples showed a variation of 3.541017 spins gÿ1
TOC in organo-mineral aggregates com-pared to 11017spins gÿ1
HA in the humic acid. Quanti®cation of semiquinone free radicals by ESR and aromaticity by CPAMS/13C NMR for HA samples showed a close relationship (r20.94) (Fig. 4).
4. Discussion
The cropping systems under no-till during 12 years signi®cantly increased total C and N in the soil to 0± 17.5 cm depth (Fig. 1a and b). The C and N accumu-lated in the soil in this experiment carried out in a
subtropical, warm and humid region equals, or even surpasses that reported by Reicosky et al. (1995) in temperate region. According to Bayer and Mielniczuk (1997a,b), soil tillage determines if C and N added by crop residue accumulate in the soil. These authors observed that after 9 years under conventional tillage, in the same soil as that used in this experiment, even with high crop residue addition no signi®cant soil C and N accumulated, due to high rates of organic matter decomposition.
Distribution of C and N in the soil pro®le follows the natural pattern with higher concentration in sur-face layers. After the ®rst 3 years of this experiment, C and N increases were small and restricted to 0±2.5 cm layer, but by the 5th year, this effect reached the 0± 7.5 cm layer (Testa et al., 1992; Teixeira et al., 1994). In the 9th and 11th years, C and N contents were also increased in the 0±12.5 cm (Pavinato, 1993) and 0± 17.5 cm layers (Burle et al., 1997), respectively.
Soil organic matter increase (under high residue addition and no-till) produced other important soil improvements. Studies on the same experiment have shown improvements in structural stability (Silva and Mielniczuk, 1997a,b); water retention and lowering of maximum daily soil temperature (Bragagnolo and Mielniczuk, 1990); CEC and other chemical charac-teristics (Testa et al., 1992; Pavinato, 1993; Burle et al., 1997); microbial biomass and activity (Cattelan and Vidor, 1990a,b); N availability (Teixeira et al., 1994); and an increase in the maize yield (Burle et al., 1997). In addition to these agricultural bene®ts, the possibility of a reduced CO2 emission owing to C retention in the soil under no-till system and high crop residue addition has highly signi®cant implications for the greenhouse effect. The 13.9 Mg haÿ1
of soil C difference in 12 years between bare soil and the maize/
Cajanus represents an approximate 50 Mg of CO2 reduction per hectare cropped.
The ESR data shows lower semiquinone free radical concentration in the organo-mineral aggregates and HA samples under cropping systems returning higher crop residues to the soil (Fig. 3), suggesting a less advanced stage of humi®cation in freshly added organic matter. In cropping systems which add residue to the soil surface an increase occurs in the ``pool'' of easily decomposable organic materials utilized by microorganisms in obtaining energy for biosynthesis. In cropping systems with very high additions of
Fig. 4. Relationship between aromaticity degree (%), determined by 13C NMR, and semiquinone free radical concentration (spins gÿ1 of HAs), determined by ESR, in HA samples of 0±
organic materials, C substance quantity can exceed microbial metabolization capacity. Thus crop residues added to the soil are only partially decom-posed by microorganisms, forming less humidi®ed organic matter with lower semiquinone free radical concentration. Probably other factors also affect this process, e.g., crop residues composition particu-larly those generating aliphatic compounds and decomposition time. Overall, the important signi®-cance for the environment of understanding carbon dynamics in the soil justi®es additional research (Sollins et al., 1996).
Our data also show a different amplitude of varia-tion of semiquinone free radical concentravaria-tion in organo-mineral aggregates and HA samples (Fig. 3). The most probable hypothesis accounting for this result is that physically fractionated samples are composed by humic substances at different stages of humi®cation. Thus less humidi®ed, more aliphatic humic substances probably contributed little to semi-quinone free radical formation. In the organo-mineral aggregates <53mm, differences in semiquinone level may be greater because they strongly depend on differences in total humi®ed organic matter molecules added to the soil. However, in HA macro-molecule preparation, the chemical procedure exclude substances with low humi®cation degree (as fulvic acid and eventually non-humic substances) resulting in similar compounds for different cropping systems. There was a higher amplitude of variation of semi-quinone concentration in the organo-mineral aggre-gates from soil under different cropping systems than in the HA samples chemically extracted from soil under the same systems. This suggests higher sensi-tivity of physical fractionation than that of traditional chemical procedures, to detect qualitative differences in soil organic matter. However, coherence exists between the data obtained with organo-mineral aggre-gates and HA samples (Fig. 3), con®rming the validity of chemical procedures for qualitative studies on soil organic matter.
The combined use of physical fractionation and spectroscopic analysis, particularly ESR and NMR, depends on mineral content of samples, mainly Fe3
content. For many Brazilian soils this creates an impediment that must be overcome. For example, Bayer (1996) working with an Oxisol having Fe2O3 content between 133 and 274 g kgÿ1
obtained a ESR
spectrum which was useless for performing qualitative analysis of organo-mineral aggregates. In this case chemical procedures present the best alternative for organic matter analysis. Research should be done on reducing interference by Fe3
, using ditionite, HF, HFHCl, magnetic separation and density separa-tion (Oades et al., 1987; Arshad et al., 1988; Baldock et al., 1992; Preston and Newman, 1992; Preston et al., 1994; Skjemstad et al., 1994).
Finally, correlation between semiquinone free radi-cal content, detected by ESR, and aromaticity degree, detected by CPAMS/13C NMR (Fig. 4), indicate that semiquinone free radicals are good indicators of humic substances aromaticity, encouraging ESR use in qualitative studies of organic matter. Recently, Martin-Neto et al. (1998) demonstrated the feasibility of using semiquinone content to indicate humi®cation degree of HAs in a climosequence area with very different rates of annual precipitation.
5. Conclusions
Total organic C and N contents increased linearly with addition of crop residues in no-till cropping systems in a sandy clay loam Acrisol from Southern Brazil. In these management systems, soil functions as an atmospheric CO2sink, thus augmenting no-tillage system bene®ts to agriculture and the environment. This double advantage for no-till systems is of special importance to tropical and subtropical areas.
Semiquinone free radical concentrations in organo-mineral aggregates and HA samples were shown to have decreased with increase of crop residues to the soil after 12 years, a fact explained by the addition of organic matter at a stage of humi®cation lower than that of native humic substances in the soil. Values observed in organo-mineral aggregates showed a higher variation amplitude of semiquinone free radical concentrations due to cropping systems than did HA samples, suggesting higher sensitivity of physical fractionation compared to chemical procedures for qualitative change detection in soil organic matter. Both sets of data, obtained with organo-mineral aggre-gates and HA samples, showed the same tendency.
con®rming the usefulness of semiquinone level as a qualitative indicator of humic substances.
Acknowledgements
Thanks are expressed to Professor Otaciro Rangel Nascimento of the University of SaÄo Paulo, Institute of Physics of SaÄo Carlos, Laboratory of Biophysics for permitting the use of a Varian ESR spectrometer. This work was supported in part by EMBRAPA and CNPq (fellowships to authors C.B, L.M-N and J.M.).
References
Arshad, M.A., Ripmeester, J.A., Schnitzer, M., 1988. Attempts to improve solid state13C NMR spectrum of whole mineral soils.
Can. J. Soil Sci. 68, 593±602.
Baldock, J.A., Oades, J.M., Waters, A.G., Peng, X., Vassalo, A.M., Wilson, M.A., 1992. Aspects of the chemical structure of soil organic materials as revealed by solid-state13C NMR
spectro-scopy. Biogeochemistry 16, 1±42.
Bayer, C., 1996. Dinamica da mateÂria orgaÃnica em sistemas de manejo de solos. Tese de Doutorado em CieÃncia do Solo, PPG-Agronomia, UFRGS, Porto Alegre, 241 pp.
Bayer, C., Mielniczuk, J., 1997a. CaracterõÂsticas quõÂmicas do solo afetadas por meÂtodos de preparo e sistemas de cultura. R. bras. Ci. Solo 21, 105±112.
Bayer, C., Mielniczuk, J., 1997b. NitrogeÃnio total de um solo submetido a diferentes meÂtodos de preparo e sistemas de cultura. R. bras. Ci. Solo 21, 235±239.
Bergamaschi, H., Guadagnin, M.R., 1990. Agroclima da Estac,aÄo Experimental AgronoÃmica. Departamento de Plantas Forra-geiras e Agrometeorologia da UFRGS, Porto Alegre, 96 pp. Bragagnolo, N., Mielniczuk, J., 1990. Cobertura do solo por
resõÂduos de oito sequeÃncias de culturas e seu relacionamento com a temperatura e umidade do solo, germinac,aÄo e crescimento inicial do milho. R. bras. Ci. Solo 14, 91±98. Brazil, 1973. MinisteÂrio da Agricultura. Departamento Nacional de
Pesquisa AgropecuaÂria. DivisaÄo de Pesquisa PedoloÂgica. Levantamento de reconhecimento dos solos do Rio Grande do Sul. Recife, 431 pp. (Boletim TeÂcnico, 30).
Burle, M.L., Mielniczuk, J., Focchi, S., 1997. Effect of cropping systems on soil chemical characteristics, with emphasis on soil acidi®cation. Plant and Soil 190, 309±316.
Cattelan, A., Vidor, C., 1990a. Sistemas de culturas e a populac,aÄo microbiana do solo. R. bras. Ci. Solo 14, 125±132.
Cattelan, A., Vidor, C., 1990b. Flutuac,oÄes na biomassa, atividade e populac,aÄo microbiana do solo, em func,aÄo de variac,oÄes ambientais. R. bras. Ci. Solo 14, 133±142.
Ceretta, C.A., 1995. Fracionamento de N orgaÃnico, substaÃncias huÂmicas e caracterizac,aÄo de aÂcidos huÂmicos de solo em sistemas de cultura sob plantio direto. Tese de Doutorado em CieÃncia do Solo, PPG-Agronomia, UFRGS, Porto Alegre, 127 pp.
Christensen, B.T., 1992. Physical fractionation of soil and organic matter in primary particle size and density separates. Adv. Soil Sci. 20, 2±90.
Doran, J.W., Parkin, T.B., 1994. De®ning and assessing soil quality. In: Doran, J.W., Coleman, D.C., Bezdicek, D.F., Stewart, B.A. (Eds.), De®ning Soil Quality for a Sustainable Environment. SSSA Spec. Publ. 35 SSSA, Madison, WI, pp. 3±21. Frund, R., Guggenberger, G., Haider, K., Knicker, H.,
Kogel-Knabner, I., Ludeman, H-D., Luster, J., Zech, W., Spiteller, M., 1994. Recent advances in the spectroscopic characterization of soil humic substances and their ecological relevance. P¯anze-nernahr. Bodenk. 157, 175±186.
Guggenberg, G., Zech, W., Thomas, R.J., 1995. Lignin and carbohydrate alteration in particle-size separates of an oxisol under tropical pastures following native savana. Soil Biol. Biochem. 27, 1629±1638.
Kogel-Knabner, I., 1997.13C and15N NMR spectroscopy as a tool in soil organic matter studies. Geoderma 80, 243±270. Martin-Neto, L., Andriulo, A.E., Tragheta, D.G., 1994. Effects of
cultivation on ESR spectrum of organic matter from soil size fractions of a mollisol. Soil Sci. 157, 365±372.
Martin-Neto, L., Rosell, R., Sposito, G., 1998. Correlation of spectro-scopic indicators of humi®cation with mean annual rainfall along a temperate grassland climosequence. Geoderma 81, 305±311. Medeiros, J.C., Serrano, R.E., Martos, J.L.H., GiroÂn, V.S., 1996. Effect of various soil tillage systems on structure development in a Haploxeralf of central Spain. Soil Technol. 11, 197±204. Oades, J.M., Vassalo, A.M., Waters, A.G., Wilson, M.A., 1987.
Characterization of organic matter in particle size and density fractions from a red-brown earth by solid-state13C NMR. Aust. J. Soil Res. 25, 71±82.
Page, A.L., 1982. Methods of Soil Analysis. American Society of Agronomy, Madison, WI, 1159 pp.
Par®tt, R.L., Theng, B.K.G., Whitton, J.S., Shepherd, T.G., 1997. Effects of clay minerals and land use on organic matter pools. Geoderma 75, 1±12.
Pavinato, A., 1993. Teores de carbono e nitrogeÃnio do solo e produtividade de milho afetados por sistemas de cultura. Dissertac,aÄo de Mestrado em CieÃncia do Solo, PPG-Agronomia, UFRGS, Porto Alegre, 122 pp.
Poole, C.P., Jr., Farach, H.A., 1972. The Theory of Magnetic Resonance. Wiley, Somerset, NJ, 353 pp.
Preston, C.M., 1996. Applications of NMR to soil organic matter analysis: history and prospects. Soil Sci. 161, 144±166. Preston, C.M., Newman, R.H., 1992. Demonstration of spatial
heterogeneity in the organic matter of de-ashed humin samples by solid-state CPMAS/13C NMR. Can. J. Soil Sci. 72, 13±19.
Preston, C.M., Newman, R.H., Rother, P., 1994. Using CPMAS/13C NMR to assess effects of cultivation on the organic matter of particle size fractions in a grassland soil. Soil Sci. 157, 26±35. Reicosky, D.C., Kemper, W.D., Langdale, G.W., Douglas, C.L., Rasmunssen, P.E., 1995. Soil organic matter changes resulting from tillage and biomass production. J. Soil Water Conserv. 50, 253±261.
Salinas-Garcia, J.R., Hons, F.M., Matocha, J.E., 1997. Long-term effects of tillage and fertilization on soil organic matter dynamics. Soil Sci. Soc. Am. J. 61, 152±159.
Sanchez, P.A., Logan, T.J., 1992. Myths and science about the chemistry and fertility of soils in the tropics. In: Lal, R., Sanchez, P.A. (Eds.), Myths and Science of Soil of the Tropics. SSSA Spec. Publ. 29 SSSA, Madison, WI, pp. 35±46. Senesi, N., 1990a. Application of electron spin resonance (ESR)
spectroscopy in soil chemistry. Adv. Soil Sci. 14, 77±130. Senesi, N., 1990b. Molecular and quantitative aspects of the
chemistry of fulvic acid and its interaction with metal ions and organic chemicals. Part I. The electron spin resonance approach. Anal. Chim. Acta 232, 51±75.
Silva, J.E., Lemainski, J., Resck, D.V.S., 1994. Perdas de mateÂria orgaÃnica e suas relac,oÄes com a capacidade de troca catioÃnica em solos da regiaÄo de cerrados do Oeste Baiano. R. bras. Ci. Solo 18, 541±547.
Silva, I.F., Mielniczuk, J., 1997a. Ac,aÄo do sistema radicular de plantas na formac,aÄo e estabilizac,aÄo de agregados do solo. R. bras. Ci. Solo 21, 113±117.
Silva, I.F., Mielniczuk, J., 1997b. Avaliac,aÄo do estado de agregac,aÄo do solo afetado pelo uso agricola. R. bras. Ci. Solo 21, 313± 319.
Skjemstad, J.O., Clarke, P., Taylor, J.A., Oades, J.M., 1994. The removal of magnetic materials from surface soils. A solid state CPMAS/13C NMR study. Aust. J. Soil Res. 32, 1215±
1229.
Sollins, P., Homann, P., Caldwell, B.A., 1996. Stabilization and destabilization of soil organic matter: mechanisms and controls. Geoderma 74, 65±105.
Stevenson, F.J., 1994. Humus Chemistry Ð Genesis, Composition, Reactions. Wiley, New York, 496 pp.
Stout, J.D., Goh, K.M., Rafter, T.A., 1985. Chemistry and turnover of naturally occurring resistant organic compounds in soil. In: Paul, E.A., Ladd, J.N. (Eds.), Soil Biochemistry, vol. 5. Marcel Dekker, New York, pp. 1±73.
Teixeira, L.A.J., Testa, V.M., Mielniczuk, J., 1994. NitrogeÃnio do solo, nutric,aÄo e rendimento de milho afetados por sistemas de cultura. R. bras. Ci. Solo 18, 207±214.
Testa, V.M., Teixeira, L.A.J., Mielniczuk, J., 1992. Caracteristicas quõÂmicas de um PodzoÂlico Vermelho-escuro afetadas por sistemas de culturas. R. bras. Ci. Solo 16, 107±114.
