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Soil Science Society of America Journal 64:1496-1503 (2000)
© 2000 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Crop Rotations and Nitrogen Fertilization to Manage Soil Organic Carbon Dynamics

Guillermo A. Studdert and Hernán E. Echeverría

Facultad de Ciencias Agrarias (U.N.M.P.), Estación Experimental Agropecuaria Balcarce (I.N.T.A.), Unidad Integrada Balcarce, C.C. 276, (7620) Balcarce, Buenos Aires, Argentina

gastudde{at}mdp.edu.ar


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Sustainability is influenced in many production systems by the variation of soil organic C (SOC) content and dynamics, and crop rotations. We hypothesized that arable layer SOC under conventional tillage can be managed through the amount of residue C (RC) returned to the soil as affected by tillage and fertilization. Soil organic C dynamics of a complex of Typic Argiudoll and Petrocalcic Paleudoll soils under conventional tillage between 1984 and 1995 at Balcarce, Argentina was studied for 16 crop sequences. Crops included were spring wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and corn (Zea mays L.). Eleven years of conventional tillage decreased SOC 4.1 to 8.8 g kg-1 without supplemental N and 2.8 to 7.2 g kg-1 when N fertilizer was applied. Soil organic C loss increased when soybean (1.2 Mg RC ha-1 yr-1) was present in the sequence and decreased when corn (3.0 Mg RC ha-1 yr-1) was present. The amount of RC returned by the sequences correlated with SOC in 1995 and with SOC at equilibrium , but the sequences with two summer crops (soybean, sunflower, or corn) every 3 yr showed lower SOC in 1995 (28.9–33.8 g kg-1) and at equilibrium (24.0–34.4 g kg-1) than sequences with none or one summer crop (29.7–35.0 g kg-1 either in 1995 or at equilibrium) for the same range of RC (1.4–2.6 Mg RC ha-1 yr-1). The difference between sequences in the relationship between RC and SOC were attributed to tillage timing. Under conventional tillage, arable layer SOC can be managed through the selection of the crops in the rotation and N fertilization, but the timing and intensity of tillage have to be taken into account.

Abbreviations: EU, experimental units • G1, none or only one summer crop every 3 yr • G2, two summer crops every 3 yr • RC, residue C • SOC, soil organic C • SOM, soil organic matter • WW, wheat–wheat sequence


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
CROP ROTATION is an important agroecosystem management practice to preserve and improve its sustainability. Goals of crop rotation on system productivity are known as collectively as the rotation effect. This term comprises the effect of crop combination on nutrient and water availability; pest, weed, and disease dynamics; presence of either growth inhibiting or promoting substances in the soil; and soil condition (Bullock, 1992). One of the soil components that is altered by soil management practices in the mid to long term is soil organic matter content (SOM) (Doran and Smith, 1987; Robinson et al., 1994). Soil organic matter is a key soil component because it influences soil biological, physical, and chemical properties that define soil productivity (Robinson et al., 1994) and quality (Doran and Parkin, 1994). Soil organic matter content also determines soil capacity to reorganize after the alterations produced by tillage and cropping (Kanal and Kõlli, 1996), which is a feature that characterizes sustainable agricultural systems (Addiscott, 1995). Likewise, SOM variations are associated with the amounts of CO2 emitted from or fixed in the soil and, consequently, with the greenhouse effect (Varvel, 1994). It is believed that any management practice that contributes to the maintenance or the increase of SOM is a sustainable one (Robinson et al., 1994).

Cropping decreases SOM, especially if done with conventional tillage (Lamb et al., 1985; Dalal and Mayer, 1986; Studdert et al., 1997). However, SOM loss can be reduced by including pastures in the rotation (Studdert et al., 1997) or increasing the amount of crop residues returned to the soil (Larson et al, 1978; Collins et al., 1992; Campbell and Zentner, 1993). The amount of SOM in a soil is the result of the balance between humification and mineralization rates (Campbell, 1978). Such balance could be managed by means of the amount of C returned to the soil as crop stover and roots (Larson et al., 1978; Robinson et al., 1994) and by tillage operations and fertilization (Doran and Smith, 1987; Bullock, 1992; Robinson et al., 1994). Through crop rotations it is possible to manipulate the opportunities for, the amount, and the mechanism of C return to the soil and the characteristics and temporal distribution of crop residues (Campbell, 1978). Depending on the crops selected, SOM could even be increased under conventional tillage (Varvel, 1994). However, to attenuate the universally admitted deleterious effects of conventional cropping on SOM, it would be necessary to increase the frequency of high biomass production and elevated residue C/N ratio crops in the rotation (Havlin et al., 1990; Varvel, 1994; Huggins et al., 1998), fertilize (Barber, 1979; Varvel, 1994), apply organic manure (Collins et al., 1992), irrigate (Lueking and Schepers, 1985), and/or reduce tillage intensity (Lamb et al., 1985; Havlin et al., 1990; Eghball et al., 1994).

In the last two decades, agricultural soils of the southeastern Buenos Aires province in Argentina have been intensively used for conventional crop production (Darwich, 1991). Many producers have adopted conservation tillage systems in recent years, but conventional tillage (moldboard plowing) is still predominant in the area. Excessive and continuous soil cultivation is deteriorating soil properties (Echeverría and Ferrari, 1993) and erosion and nutrient deficiency problems are becoming more frequent due to SOM reductions. Since the described production mode is expected to continue, it is necessary to manage SOC dynamics through crop combination in the rotation in order to contribute to system sustainability.

We propose that SOC decrease in agricultural soils under continuous cropping and conventional tillage can be minimized through managing the amount of residues returned to the soil by means of crop selection for the rotation and fertilization. The objective of this work was to relate the change in the arable layer SOC of a complex of Typic Argiudoll and Petrocalcic Paleudoll soils during 11 yr of continuous cropping under conventional tillage, with the amount of aboveground residues restored to the soil by 16 crop sequences including wheat, soybean, sunflower, and corn.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
A long-term crop rotation experiment has been conducted since 1984 at Balcarce, Argentina (37°45' S, 58°18' W; 138 m above sea level; 870 mm mean annual rainfall; 13.7°C mean annual temperature) on a soil complex of a fine, mixed, thermic Typic Argiudoll and a fine, illitic, thermic Petrocalcic Paleudoll (petrocalcic horizon was below 0.7 m) with 2% slope (no erosion). The surface horizon of this complex has a pH of 6 (1:2.5 in water), loamy texture, 33.1 cmol kg-1 cation-exchange capacity, and 5.0 mg kg-1 Bray and Kurtz P. Prior to the establishment of the experiment, the site had been under an ungrazed predominantly grass pasture for at least 4 yr. According to previous research for the area (Studdert et al., 1997), this period assures a high SOC content that could be reduced by cropping.

The experimental design was a randomized complete block with a split-block treatment arrangement (Little and Hills, 1978) and four replicates. Spring wheat, soybean, sunflower, and corn were sown in 25 by 100 m strips during the first year of each 3-yr cycle. In the second year, the same crops were sown in 25 by 100 m strips perpendicular to those of the first year. This defined 16 crop sequences in 25 by 25 m grand plots. During the third year, the entire experiment was cropped to wheat. This 3-yr rotation cycle was repeated four times until 1995 (Table 1) .


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  		Table 1 Crop sequences evaluated in the experiment.{dagger}

 
All grand plots were split into two 12.5 by 25 m experimental units (EU). Whenever the crop on the plot was wheat, one of the EU was fertilized with urea (60 kg N ha-1 in 1984 and 1985, 90 kg N ha-1 in 1986 and 1987, and 120 kg N ha-1 thereafter) at tillering until 1987 and at sowing thereafter. Likewise, since 1990, whenever the crop on the plot was corn, the EU that had been fertilized under wheat was fertilized at planting with 120 kg N ha-1 as urea. In all cases, urea was broadcasted. This fertilization scheme determined 16 EU per block that were never N fertilized and 16 that received urea 3, 5, 7, 9, or 11 yr during the experiment (disregarding 1995, Table 1). The corresponding EU under soybean, sunflower, and corn (until 1990) were not N fertilized, but they were analyzed separately due to the possible N carryover generated by fertilizer applied to wheat and corn (since 1990) in the sequence. All crops were grown under rainfed conditions and superphosphate was banded at planting according to soil analysis.

All seedbeds were prepared by moldboard plowing, disking, and harrowing. Tillage operations for summer crops were started {approx}3 to 4 mo before planting (mid October for corn, and mid November for soybean and sunflower). For wheat, tillage operations were started immediately after the preceding crop harvest. In all cases, tillage opportunity and intensity were those required to get a weed free and not excessively fine seedbed. In the case of the row crops, chemical weed control was complemented by mechanical control throughout the growing season.

Wheat and soybean were harvested with a conventional plot combine. Sunflower and corn yields were estimated by measuring head diameter and hand harvesting (ear collection and later stationary threshing), respectively, of several 14.3-m-long crop lines per EU. The amount of C returned to the soil as aboveground residues was estimated assuming average harvest indexes of 0.45, 0.40, 0.35, and 0.45 for wheat, soybean, sunflower, and corn, respectively, since in the long term, harvest index fluctuations under rainfed conditions are more dependent on the moment when water stresses occur than on N fertilization (F. Andrade, 1997, personal communication) and a C content of 43.0%. The mean annual aboveground RC returned was calculated by averaging the estimated residue C left by each crop sequence on the ground, through the duration of the experiment, excluding 1995.

Arable layer (0–0.17 m depth) SOC was determined by the Walkley-Black procedure (Nelson and Sommers, 1982). Composite soil samples (15–20 subsamples per sample) were taken each year before crop sowing (between July and October, according to the crop to be sown), air dried, and ground to pass 0.5-mm-opening sieve. In 1984 only four samples were taken from the entire experiment area, and in 1985 and 1986 only one sample per strip of the preceding crop and one sample per block, respectively, were taken. In 1987 and 1988 soil samples were taken from each grand plot, disregarding N fertilization background. For the serial analysis throughout the experiment, SOC determined in 1984, 1985, 1986, 1987, and 1988 was assigned to each of the corresponding EU. Starting in 1989, soil samples were taken from each EU. In 1987, 1991, and 1993 only the EU that were going to be cropped to wheat were sampled. The last soil sampling was done at wheat sowing in 1995.

The model proposed by Bartholomew and Kirkham (1960) was used to describe the relationship between SOC and cropping years:

(1)
where SOCt is the value of SOC (g kg-1) at time t; SOCe is the value of SOC at equilibrium; SOC0 is the value of SOC at ; r is the exponential rate of variation (yr-1); and t is cropping year (yr). Soil organic C contents were fit to this model through nonlinear regression (Marquardt iteration method). Since N fertilization was not randomly assigned to EU, the analyses of variance to test the effect of crop sequences on SOC in 1995 and RC for unfertilized EU and for those that received N fertilizer at least once during each rotation cycle were performed separately. Duncan's multiple range test was used to compare treatment means of those two variables. Other statistical analyses (linear correlation and regression analyses) were also performed.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Eleven years of continuous conventional cropping caused a decrease in the SOC. Soil organic C contents before the last wheat sowing (1995) for the 16 crop sequences studied are shown in Fig. 1 . The difference between the initial SOC level (37.7 g kg-1) and SOC content in 1995 ranged from 2.8 to 7.2 and 4.1 to 8.8 g kg-1 for the fertilized and unfertilized EU, respectively.



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  		Fig. 1 Arable layer soil organic C after 11 yr of conventional tillage under different crop sequences (a) without N and (b) with N applied at 3, 5, 7, 9, or 11 yr during the experiment. Crop sequences are described in Table 1. C = corn, Sb = soybean, Sf = sunflower, W = wheat. The same letters within columns indicate that the sequences do not differ according to Duncan (P < 0.05)

 
For fertilized and unfertilized EU, the analysis of variance showed highly significant differences in SOC in 1995 (P < 0.01) among treatments. Although the mean separation did not show sharp differences in cropping system effects on SOC, some differences were apparent. Perhaps, the narrow range of SOC content between treatments (30.5–35.0 and 28.9–33.6 g kg-1 for fertilized and unfertilized EU, respectively) prevented clear differentiation (Fig. 1). Independent of N fertilization, the lowest SOC level corresponded with wheat–wheat (WW) and soybean–soybean sequences and SOC increased as soybean was not present or corn was present in the sequences.

Two groups of sequences can be distinguished in Fig. 1. The first one (G1) comprises those sequences that include none or only one summer crop (corn, soybean, or sunflower) every 3 yr (the first seven sequences on the left of Fig. 1, averaging 33.4 ± 1.4 and 32.0 ± 1.4 g SOC kg-1 for fertilized and unfertilized EU, respectively). The second group (G2) includes the rest of the sequences, all of which have two summer crops every 3 yr (averaging 32.6 ± 1.1 and 31.5 ± 1.3 g SOC kg-1 for fertilized and unfertilized EU, respectively).

The estimated RC through 1995 is shown in Fig. 2 . For fertilized and unfertilized EU, the analysis of variance showed highly significant differences in RC in 1995 (P < 0.01) among treatments. Average RC returned to the soil increased if soybean was not present or corn was present in the sequences. Fertilized and unfertilized wheat in rotation returned an average of 2.1 ± 0.7 and 1.6 ± 0.5 Mg C ha-1 yr-1, respectively, and corn returned an average of 3.2 ± 0.5 and 2.8 ± 0.6 Mg C ha-1 yr-1, respectively. Soybean and sunflower returned 1.2 ± 0.5 and 2.0 ± 0.6 Mg C ha-1 yr-1, respectively, without difference due to fertilization background. The presence of soybean and corn in the sequence decreased and increased RC, respectively, and this trend was more clear if N fertilization had occurred. On the other hand, sequences of G1 restored 2.0 ± 0.4 and 1.7 ± 0.2 Mg ha-1 yr-1 in the fertilized and unfertilized EU, respectively, and the sequences in G2 restored 2.1 ± 0.4 and 1.8 ± 0.2 Mg ha-1 yr-1, respectively.



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  		Fig. 2 Mean annual aboveground residue C returned to the soil after 11 yr of conventional tillage by different crop sequences (a) without N and (b) with N applied at 3, 5, 7, 9, or 11 yr during the experiment. Crop sequences are described in Table 1. C = corn, Sb = soybean, Sf = sunflower, W = wheat. The same letters within columns indicate that the sequences do not differ according to Duncan (P < 0.05)

 
In the WW sequence, fertilized and unfertilized continuous wheat returned an average of 1.7 ± 0.6 and 1.4 ± 0.4 Mg C ha-1 yr-1, respectively (Fig. 2). Monocultured wheat was affected by Take-all [Gaeumannomyces graminis (Sacc.) Arx & D. Olivier var. tritici J. Walker] several years at random (data not shown). At Balcarce, Take-all effects occur after anthesis (A. Melegari, 1998, personal communication) just when the aboveground biomass (excluding grain) finishes its growth and grain formation starts, and, consequently, grain yield and harvest index fall. Therefore, the method employed underestimated the amount of C restored by the WW sequence, since an average normal harvest index was employed. This may have also happened, to a lesser extent, with the estimation of RC for the rest of the sequences that include wheat, since all of them comprise two continuous wheat crops in each rotation cycle (Table 1). In the latter case, the frequency of appearance and severity of Take-all were much lower than in the former (data not shown). This means that the material that is returned to the field as residue (stem, leaves, and chaff) must have been present on it after harvest, despite the low grain yield due to the disease.

The similarity of trends shown in Fig. 1 and 2 suggests a close relationship between RC and SOC level in 1995. A highly significant correlation coefficient between both variables confirms what some other authors have already reported (Larson et al., 1978; Barber, 1979; Havlin et al., 1990; Varvel, 1994). However, the correlation coefficients were higher when the relationship between RC and SOC was analyzed separately for and (Fig. 3) . Figure 3 shows that, at the same amount of RC restored, different SOC levels were reached by each sequence group. On the other hand, the equations in Fig. 3 show that the maximum SOC that each sequence group could have reached in 1995 would have been 34.7 g kg-1 (at 2.6 Mg RC ha-1 yr-1) and 34.0 g kg-1 (at 2.9 Mg RC ha-1 yr-1) for G1 and G2, respectively. The data shown in Fig. 3 suggest that (i) N fertilization, even when it has not been continuous, augmented RC (Fig. 2) and, therefore, helped maintain SOC with respect to the initial level (Fig. 1); (ii) differences observed between sequence groups indicate that residue amount is the main factor in defining SOC, but that its effect is conditioned by other factors regarding crop combination; (iii) G2 would not have reached as high a maximum as G1 could have reached, even if the amount of RC returned by G2 had been increased by almost 12%; and (iv) conventionally tilled cropping caused a decrease in SOC and, even though some sequences had had higher RC levels, it could never maintain the initial SOC levels. The latter statement disagrees with Barber (1979) and Varvel (1994), who reported that by increasing the amount of residue returned to the soil, it might be possible to increase SOC even under conventional tillage, probably because they started their studies at a low SOC level.



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  		Fig. 3 Arable layer soil organic C after 11 yr of conventional tillage as a function of mean annual residue C returned by different crop sequences arranged in two groups: G1, none or only one summer crop every 3 yr; G2, two summer crops every 3 yr. With N = received N fertilizer 3, 5, 7, 9, or 11 yr during the experiment (Table1); without N = never received N fertilizer

 
Considering that the initial SOC level was the same for all the sequences and that they presented different levels of SOC after 11 yr of conventional cropping, the dynamics of SOC variation must also have been different. Table 2 shows the parameters estimated for Eq. [1], and Fig. 4 shows the resulting curves. The general shape of this curves has already been shown by several authors when describing the variation of SOC as a function of time under conventional cropping (Campbell, 1978; Lamb et al., 1985; Dalal and Mayer, 1986; Studdert et al., 1997). Dalal and Mayer (1986) reported different curve shapes according to the type of soils and the kind of natural vegetation present before cropping, whereas Lamb et al. (1985) showed different curve shapes in relation to tillage systems. In this study, curve shapes differ among sequences and between N fertilization status.


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  		Table 2 Results of fitting Bartholomew and Kirkham (1960) model to soil organic (SOC) as a function of cropping years for different crop sequences

 


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  		Fig. 4 Change in soil organic C as a function of time under conventional cropping for different crop sequences. Curves result from fitting a model proposed by Bartholomew and Kirkham (1960). Crop sequence description and model parameters can be seen in Tables 1 and 2, respectively. With N = received N fertilizer 3, 5, 7, 9, or 11 yr during the experiment (Table1); without N = never received N fertilizer

 
Table 2 shows that, in most cases and excluding WW, one or two soybean crops every 3 yr in the sequence reduced SOCe and r (29.6 ± 3.0 vs. 33.0 ± 1.5 g kg-1 and 0.171 ± 0.083 vs. 0.327 ± 0.262 yr-1 when soybean was present or not, respectively). Figure 4 shows that whenever a soybean crop is present in the sequence, the corresponding curves are below the rest, with the exception of Fig. 4a and 4b where curves representing SOC variation under WW were below the rest of the curves. Likewise, when one (Fig. 4a–4d) or two (Fig. 4e and 4f) corn crops were included every 3 yr in the sequence, resulting curves are above the rest. On the other hand, when N was applied, SOCe and r are higher than when no N was applied (32.5 ± 2.7 and 30.2 ± 2.5 g kg-1, and 0.352 ± 0.273 and 0.182 ± 0.065 yr-1, respectively). When a soil is put under cropping, its SOC tends to a new equilibrium in relation to the conditions of cropping (Barber, 1979; Robinson et al., 1994). Estimated parameters of Eq. [1] allowed us to characterize how the sequences tended to the equilibrium if management conditions were kept constant. Higher SOCe values indicate a reduction in the total SOC decrease expected to occur, whereas higher r values mean that such decline occurs in a shorter period. The latter can be seen in half-life data in Table 2. Half life is lower when there were no soybean crops in the sequence (2.8 ± 1.3 yr), when at least one corn crop was present (3.1 ± 1.3 yr), and when N fertilizer was applied (3.3 ± 2.8 yr) than when at least one soybean crop was included in the sequence, no corn crop was included, and N fertilizer was not applied (5.1 ± 2.7, 4.4 ± 2.8, and 4.3 ± 1.8 yr, respectively). These results agree with those reported by others with respect to the effect of soybean and corn crops in the sequence (Barber, 1979; Havlin et al, 1990; Varvel, 1994; Eghball et al., 1994; Huggins et al., 1998) and N fertilization (Barber, 1979; Varvel, 1994) on SOC level.

Table 2 also shows that G1 sequences result in higher SOCe and r than G2 sequences (32.6 ± 1.7 and 30.4 ± 3.1 g kg-1, and 0.402 ± 0.261 and 0.162 ± 0.063 yr-1, respectively). Figure 5 shows the relationship between SOCe and the amount of RC returned by each sequence until 1995. Similarly to what was shown in Fig. 3, at the same level of RC, G1 sequences tended to a higher SOCe than G2 sequences. This difference is larger when RCs are lower either due to sequences including poor residue-return crops or to lack of fertilization (Fig. 2, Table 2). Likewise, there were highly significant correlations between annual RC return through 1995 and parameter and sequences. It is worth pointing out that quadratic equations that explain the relationship between RC and either SOC or SOCe for G1 sequences in Fig. 3 and 5, respectively, are almost the same. This means that G1 sequences almost reached SOCe in 11 yr of conventional cropping. On the other hand, while high-RC-returning G2 sequences approached equilibrium in the same period (SOC for these sequences in Fig. 1 is almost the same as SOCe in Table 2), SOC levels of lower-RC-returning G2 sequences would continue falling for a longer time (SOC for these sequences in Fig. 1 is higher than SOCe in Table 2) if management conditions were kept constant. Since overall explored range is almost the same for G1 and G2, differences between them shown in Fig. 3 and 5, respectively, could be attributed to the way tillage operations were done.



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  		Fig. 5 Arable layer soil organic C at equilibrium according to the model proposed by Bartholomew and Kirkham (1960) as a function of mean annual residue C returned by different crop sequences arranged in two groups: G1, none or only one summer crop every 3 yr; G2, two summer crops every 3 yr. With N = received N fertilizer 3, 5, 7, 9, or 11 yr during the experiment (Table1); without N = never received N fertilizer

 
Conventional tillage affects C dynamics in the soil through its effect on crop residue decomposition and on soil aeration and through provoking the exposure of SOC fractions protected within the aggregates (Doran and Smith, 1987; Eghball et al., 1994). Tillage opportunity and frequency alter those effects since they also are related to temperature and soil water content. Tillage operations for summer crops, either for seedbed preparation or weed control, were done mainly during spring and early summer, when temperatures were higher. Sequences in G2 comprised eight summer crops out of 11 crops through the experiment. This could be associated with faster residue decomposition and greater SOC mineralization and explains what differences shown in Fig. 3 and 5. Similarly, this relationship between tillage opportunity and SOC dynamics can also explain the particular results observed under the WW sequence.

As stated before, it should be assumed that RC returned by WW was underestimated. However, the discussed relationships of SOC in 1995, SOCe (Fig. 3 and 5, respectively), and r with RC appear to apply also to WW (Fig. 2, Table 2). Since in reality RC was higher than estimated, the only explanation for those relationships could be how tillage was done. Tillage operations for seedbed preparation for wheat were started as soon as the preceding crop was harvested. This cropping strategy when wheat was sown after wheat means that the most aggressive tillage operations had to be done during the summer and early fall, which could have provoked more intense SOC mineralization, despite the supposedly higher RC restored.

It has been demonstrated that SOC dynamics could be managed through the selection of crops to be included in the rotation. Higher frequency of low aboveground biomass–producing crops, such as soybean, produce greater decreases of SOC. On the contrary, the amount of residues to be returned to the soil may be increased through the inclusion in the rotation of high above- ground biomass–producing crops and through fertilization, thus attenuating SOC decrease. However, the decision about which crops to include in the rotation according to the amount of residues produced has to be coupled with awareness of the tillage intensity and opportunity associated with each of them. The relationship between the amount of residues restored to the soil and SOC dynamics under conventional tillage may be greatly altered by the way and the time tillage operations are done. The high SOC content characteristic of the soils of the area under study could be preserved through a careful choice of the crops to include in the rotation and complementing this with other management practices such as alternation of cropping and pasture periods to restore SOC levels (Studdert et al., 1997) and use of conservation tillage to diminish the SOC decrease (Lamb et al., 1985; Havlin et al., 1990; Eghball et al., 1994).


   ACKNOWLEDGMENTS
 
This study was made possible thanks to the long-term crop rotation experiment at Balcarce, Argentina conducted since 1984 in the Unidad Integrada Balcarce (UIB) with financial support of the Instituto Nacional de Tecnología Agropecuaria. We want to express our gratitude to the scientists who have participated and have been in charge of the conduct of the experiment during different periods, to the field personnel who have helped with most of the field work, and to the personnel of the Soil, Plant, and Water Analysis Laboratory of the UIB who ran a great part of the soil analyses. We also want to express our appreciation to the Agencia Española de Cooperación Internacional that funded the stay of the senior author in Spain, where most of this paper was written, and for the Project PICT 97 08-00089 that funded its publication.

Received for publication August 17, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome