A review of no-till systems and soil management for sustainable crop production in the subhumid and semiarid Pampas of Argentina

Soil & Tillage Research 65 (2002) 1±18

Review

A review of no-till systems and soil management for sustainable crop production in the subhumid and semiarid Pampas of Argentina MartõÂn DõÂaz-Zoritaa,b,*, Gustavo A. Duartec, John H. Groveb a

EEA INTA General Villegas, CC 153 (6230) General Villegas, Buenos Aires, Argentina Agronomy Department, N-122 Agriculture Science Center, University of Kentucky, Lexington, KY 40546-0091, USA c AACREA, Maipu 51 (6237) AmeÂrica, Buenos Aires, Argentina

b

Received 4 October 2000; received in revised form 31 August 2001; accepted 24 September 2001

Abstract The western part of the Argentine Pampas is a subhumid and semiarid region consisting of extensive plain with deep sandy and sandy-loam soils. The agricultural system includes pastures in rotation with annual grain crops and grazed crops or continuous annual row cropping. The objective of this review was to present and discuss changes in soil properties due to different soil management systems, mainly no-tillage practices, in the western part of the Argentine Pampas. The effects of tillage, crop sequences under no-till, and grazing on soil properties and crop productivity have been studied since 1990 on loamy and sandy Haplic Phaeozem (Typic Hapludolls and Entic Hapludolls) and Haplic Kastanozem (Typic Haplustolls). A database developed from the yield and soil test records of growers af®liated with Regional Consortium for Agricultural Experimentation (CREA) were also utilized in the study. The results showed that soil organic C (SOC) content depends both on soil texture and soil management. SOC decreases when the length of the row crop cycle increases and also in moldboard plow and chisel-tillage systems. Pastures and no-till row crop sequences with more years of maize (Zea mays L.) and wheat (Triticum aestivum L.), than sun¯ower (Helianthus annus L.) or soybean (Glycine max (L.) Merrill) tended to increase the SOC content in the 0±20 cm layer. Deep tillage of no-till soils with compacted layers improved maize dry matter production but, in the same experiment, yield was increased more by nitrogen fertilization than by subsoil tillage. The grazing of crop residues increases the soil bulk density only in the 0±5 cm layer of tilled soils, but did not signi®cantly change bulk density on soils under continuous no-till. Crop productivity was related to SOC content of the 0±20 cm layer of the soils. Due to the positive effect of SOC on crop yields, no-till soil management and pasture±annual row crop rotations are two practices that permit the development of sustainable production systems in the western part of the Argentine Pampas. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Soil organic matter; Crop productivity; Crop husbandry; Grazing; Sustainability; No-till; Pampas; Argentina

1. Introduction The western part of the Pampas (34±358S; 61± 638W) comprises a land area of about 4.5 million ha *

Corresponding author. Tel.: ‡54-3388-421284; fax: ‡54-3388-421284. E-mail address: [email protected] (M. DõÂaz-Zorita).

in the center of the Argentine Republic, lying within the inland Pampas (Fig. 1, Soriano et al., 1991). Sun¯ower (Helianthus annus L.), maize (Zea mays L.), soybean (Glycine max (L.) Merrill) and wheat (Triticum aestivum L.) are the principal crops of the region, and are grown in rotation with winter annual forage crops (oat (Avena sativa L.), triticale …Triticum aestivum L:Secale cereale L:†, rye (Secale

0167-1987/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 2 7 4 - 4

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Fig. 1. Location of the selected sites within the subdivisions of the Pampean region (A: rolling Pampa; B: inland ¯at Pampa; C: inland western Pampa; D: ¯ooding Pampa) showing boundaries for the subregions (


). Provinces lying partly within the area of interest are named and their boundaries shown (- - -). Inset shows location of area within South America.

cereale L.)) and pastures composed of alfalfa (Medicago sativa L.) and fescue (Festuca arundinacea L.). Crops have been grown in the subhumid and semiarid Pampas region for more than 50 years (Covas, 1989). The production systems that were historically used in this region integrated row crop cultivation and pasture grazing for beef and dairy cattle production. The area under each activity (row crops versus pasture) has varied, grazed pasture was the dominant land use until the 1990s. In the last decade, the area under grazed pasture has diminished and row crop production, mainly of summer annuals (maize, sun¯ower, and soybean), has increased. At the same time, the adoption of continuous no-till soil management is increasing in the Pampas, and is associated with the increase in row cropping (Hall et al., 1992; GarcõÂa et al., 2000). Panigatti (1998) reported that the ®rst no-till experiences in the Pampas region occurred in the 1960s at the National Institute of Agricultural Technology (INTA) research stations at Anguil (La Pampa province) and Pergamino (Buenos Aires province) with row crop production systems. In the subhumid and semiarid Pampas region, the effects of tillage, crop sequences under no-till systems, and grazing on soil properties

and crop productivity have been studied since 1990 at the General Villegas Agricultural Experimental Station of INTA and at two sites near AmeÂrica and Daireaux, all three located in the inland Pampas region within Buenos Aires province (Fig. 1, Table 1). A database has also been developed on tillage studies from the yield and soil test records of growers af®liated with Argentine Association of Regional Consortium for Agricultural Experimentation (AACREA). The objectives of this review article are to present the results of regional studies that suggest the potential evolution of soil problems that are related to crop productivity and to discuss the possible adoption of continuous no-till practices in the western part of the Argentine Pampas. A speci®c objective of the review was to evaluate factors that change soil organic C (SOC) content and the resultant implication for crop productivity. 2. Climate of the region The climate of the region is temperate with some continental characteristics. The mean annual

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Table 1 Soil pro®le characterization in three locations of the western Pampas region of Argentina (Moscatelli et al., 1980; Pazos and Moscatelli, 1998; Pazos, 1999) Location

Soil type

Horizon

Thickness (cm)

Claya (g kg 1)

Siltb (g kg 1)

Sandc (g kg 1)

General Villegas

Loamy Haplic Phaeozem (Typic Hapludoll)

A B2 B3 C

0±26 27±59 60±92 >92

252 277 212 153

262 129 224 216

486 461 564 631

AmeÂrica

Sandy Haplic Phaeozem (Entic Hapludoll)

A AC C

0±33 34±57 >57

141 134 112

133 94 106

726 772 782

Daireaux

Haplic Kastanozem (Typic Haplustoll)

A AC C

0±29 30±59 >59

131 111 106

110 91 68

759 798 826

a

<2 mm. 2±50 mm. c 50±2000 mm. b

temperature is 16.2 8C with the highest values in January …mean maximum ˆ 33:0  C† and the lowest in July …mean minimum ˆ 1:6  C†. These weather conditions allow good development and production from forage and crop species typical of temperate regions. For example, the overall mean grain yields (1993±1998) for maize, soybean, double-cropped soybean, sun¯ower and wheat are 6.0, 1.9, 1.4, 1.8 and 3.1 Mg ha 1, respectively. The average yearly dry matter production of alfalfa and fescue grazed pastures is 12 and 9 Mg ha 1, respectively. Wind is more intense and more frequent during the hottest season and, with the high temperature levels, induces the occurrence of high daily evapotranspiration rates. Mean monthly wind speeds rise from a minimum during March±May (fall season) of about 13.0 km h 1 with 24% calm to a maximum between September and February of 16.8 km h 1 with only 14% calm (Hall et al., 1992). In the latter period, wind erosion processes are more frequent on sandy textured soils or on soils with a low residue cover condition typical of summer crops grown after plow tillage. Most rainfall occurs between October and April (spring to fall seasons), and the historical yearly average is 929 mm (DõÂaz-Zorita et al., 1998). Rainfall amounts are highly variable between years, typical of subhumid and semiarid regions. Based on average rainfall amounts and the potential evapotranspiration (1092 mm per year) predicted using the FAO

Penman±Monteith equation (Allen et al., 1994), the water balance is negative (Fig. 2). The relationship between monthly evaporation and rainfall values serves as a rough guide to seasonal changes in water availability. The months when rainfall exceeds evaporation are considered wet. When rainfall is less than one-half of predicted evapotranspiration, crop growth depends on stored soil water (FAO, 1985). In the western part of the Pampas there is almost no wet period and from July to September rainfall is less than one-half of predicted evapotranspiration. The greatest negative water balance occurs between November and February and coincides with establishment and ¯owering of the main summer crops. Soil water recharge is concentrated between March and April and coincides with the beginning of fallow practices and with the seeding of pastures or winter annual species for grazing. Crop and pasture productivity is strongly related to the amount and timing of available water. For example, 40% of the variation in maximum wheat yield from farmer ®elds has been found to depend on the amount of rainfall in September and October (DõÂaz-Zorita et al., 1994; DõÂaz-Zorita and Duarte, 1998). Similar results have been found for soil water storage at sowing or during the ¯owering period and the productivity of sun¯ower (Trasmonte and DõÂaz-Zorita, 1996) or soybean (PeÂrez, 1996). Forage and grain productivity are not stable due to weather variability. In subhumid and semiarid

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Fig. 2. Mean water balance at the General Villegas Agricultural Experimental Station of INTA (DõÂaz-Zorita, 1996). Et0 denotes evapotranspiration according to the FAO Penman±Monteith method.

environments, the amount of water available for growth has an important in¯uence on crop productivity on non-irrigated lands (Read and Warder, 1974; Tessier et al., 1990), either because of the quantity and distribution of rainfall or the capacity of soil to store water. Availability of water is closely linked to soil properties such as SOC content and texture (Nichols, 1984; McDaniel and Munn, 1985; Hutson, 1986; Rawls et al., 1991). The way to diminish plant production instability due to water shortages is to adjust soil and crop management practices with the objective of increasing water storage capacity. System manipulation to maximize rainfall storage in the root zone not only increases the productivity of dryland agricultural systems but also protects the soils from erosion threats (Gimenez et al., 1997). 3. Soils and crop productivity in the region The most frequently cropped soils in the region are Mollisols, developed from eolian sediments of the

Pleistocene era, with dominantly udic and thermic waterand temperature regimes, respectively(Moscatelli et al., 1980). Based on the World Reference Base for Soil Resources (WRB) classi®cation system, these soils mostly correspond to the groups of Phaeozem and Kastanozem (ISSS±ISRIC±FAO, 1988; Pazos and Moscatelli, 1998; Pazos, 1999). The variability in soil texture was originally caused by differences in the composition of the parent material, but strong erosive processes, especially at the beginning of the agricultural period, may have accentuated the original differences (Covas and Glave, 1988). The predominant landscape is comprised of ¯at to gently rolling continental dunes (Hall et al., 1992). Abrupti-luvic Phaeozem (Thapto-Argic Hapludolls) are the predominant soil types in the east and center of the western part of the Pampas. These soils, due to differences in the thickness of the topsoil, exhibit non-uniform crop productivity, reduce the choice of ®eld crops and are grouped largely in classes III±V in

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Fig. 3. Relationship between wheat grain yield and SOC at the 0±20 cm soil depth in 134 farmer ®elds of the semiarid Pampas (DõÂaz-Zorita et al., 1999). If TOC < 42:0 Mg ha 1, yield ˆ 74:4 ‡ 70:0  TOC and if TOC > 42:0 Mg ha 1, yield ˆ 2938:6 …r2 ˆ 0:477; P < 0:01†.

the land capability classi®cation system (USDA, 1961). The presence of a ®ne textured subsurface horizon impedes normal root development and soil water movement. Consequently, it diminishes the volume of soil available to the crop for water and nutrient uptake and sometimes leads to salinization or sodi®cation processes. In these soils, the aerial dry matter and yield of wheat, maize and other crops is diminished when the topsoil thickness diminishes (DõÂaz-Zorita, 1996). Other researchers, e.g. Frye et al. (1982) in Kentucky, USA, also found that crop productivity was positively related to top soil thickness. Perennial pastures have low productivity in the presence of this eda®c limitation (DõÂaz-Zorita et al., 1993). Animal grazing systems are found widely in this area and almost no annual cropping for grain is done. In the rest of the western Pampas the soils are classi®ed as Typic Hapludolls, Entic Hapludolls or Typic Haplustolls (Haplic Phaeozem or Haplic Kastanozem, Pazos and Moscatelli, 1998), have moderate limitations for crop production and are found mostly in land capability class II (USDA, 1961). These soils are deep sandy to sandy-loam textured, well-drained, with low to medium SOC contents and low water storage capacity. Top SOC content has been found to

be the property that is best related to the yield of the principal crops of the region (e.g. wheat, Fig. 3, DõÂazZorita et al., 1999). As a consequence of erosion processes, a loss of 1.0 Mg ha 1 of SOC from the 0±20 cm surface layer of soils with SOC contents below 42 Mg ha 1 is associated with a reduction in wheat grain yield of approximately 40 kg ha 1 (DõÂazZorita et al., 1999). SOC plays a key role in soil productivity in this region through its positive effects on soil aggregation and the storage and supply of water and nutrients. Increased SOC reduces soil compactability (Thomas et al., 1996; Quiroga et al., 1998a; DõÂaz-Zorita and Grosso, 2000). Consequently, C production (crop yields, harvest index, etc.) and conservation (crop residue management, tillage, etc.) are the main objectives of sustainable agricultural systems for this region of Argentina. 4. Factors that change SOC content The quantity of ®ne mineral particles (clay and silt) is one of the intrinsic factors that control the SOC level in soils of the region (Buschiazzo et al., 1991; Alvarez and Lavado, 1998). An increasing presence of clay and

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Fig. 4. SOC content evolution (0±20 cm layer) in a typical production system from the western part of the Argentine Pampas. Each site-year is an average of three to six farmer ®elds. Bars are standard error of the means (DõÂaz-Zorita, 1997).

silt particles in the soil is associated with an increasing level of SOC. The use of the soil, whether under row crop rotations or pastures, due to the occurrence of degradation processes, reduces the SOC level per unit of clay and silt compared with the relationship observed under native conditions. There are losses in SOC content during row cropping with moldboard or disk tillage practices and gains in SOC with increasing years under pastures in the row crop±pasture cycle (Fig. 4). The inclusion of legumes and cattle grazing in crop sequences on Entic Haplustolls (sandy Haplic Phaeozem) from the semiarid Pampas region have positive effects on SOC and N contents (Miglierina et al., 2000). In this region, physical degradation of the soil (hydraulic properties, compaction, etc.) with tillage depends on the original soil properties, and soils with coarse textures tend to degrade faster than those with ®ne textures (Puricelli, 1985). Composite pastures (alfalfa and perennial grasses) provide grazing for animal production and are the main management system that leads to recovery of soil productivity (Puricelli, 1985; Studdert et al., 1997). This process is a consequence of biological ®xation

of nitrogen by the legume and regeneration of good soil aggregation by the grass root system. The persistence of these species under grazing, and consequently their capacity to improve soil productivity, depends on several environmental and management factors. According to DõÂaz-Zorita and Davies (1995), SOC content and aggregate stability increased as the time under pasture increased, especially in the third year after pasture establishment. The relative rate of organic C recovery was related to soil type. It was greater in Typic Hapludolls (loamy Haplic Phaeozem) than Entic Hapludolls (sandy Haplic Phaeozem), probably due to differences in the frequency of days with favorable moisture conditions for biological activity. Also, within an area of homogeneous climate, an increase in ®ne particle content caused an increase in SOC concentration probably because of the protective effects of clay on organic compounds (Buschiazzo et al., 1991). Biological activity was improved with greater SOC levels and this activity enhanced the development of porosity, reduced the bulk density and increased soil aggregate stability (Osterheld and LeoÂn, 1993). SOC degradation is associated mostly with annual row crop production systems using tillage. When soil

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Fig. 5. SOC content in the top layer (0±30 cm layer) of a Typic Hapludoll (loamy Haplic Phaeozem) from a permanent fescue pasture (PP) subjected to 6 years of a maize±soybean crop rotation under no-till (NT), chisel-till (CT) or moldboard-till (MT) soil management (DõÂazZorita, 1999).

physical disturbance is intensive (number of tillage trips during the year, quantity of harvest residues mixed with the soil, etc.), SOC losses increase and promote further changes in the physical, chemical and biological properties of the soils. In a long-term tillage study at the General Villegas Agricultural Experimental Station (Drabble, Buenos Aires, Argentina), SOC in the 0±30 cm layer of a Typic Hapludoll (loamy Haplic Phaeozem) under continuous row cropping for 6 years, diminished 19% with moldboard plowing, 7% with chisel plowing and 0.4% with no-till (DõÂazZorita, 1999, Fig. 5). Similar results have been reported in similar studies from other sites in the region (Kruger, 1996a; Quiroga et al., 1996; Buschiazzo et al., 1999) and the world (e.g. Cornish and Pratley, 1991; Crovetto, 1998; Lal et al., 1998). In agreement with these observations, results from almost 50 farmer ®elds near AmeÂrica (Buenos Aires, Argentina) show that the SOC content of the 0±20 cm soil layer during the rotational cycle with annual crops is lower when tillage operations are performed than where continuous no-till is practiced (Fig. 6). In the

same area, and also based on almost 50 farmer ®elds, no signi®cant difference in SOC due to tillage was observed during the pasture cycle of the rotations used on the soils in this region (Fig. 6). The high SOC content in the top layer of no-till soils and reduced soil disturbance promote greater populations of earthworms adapted to soils with abundant organic residues (Radke and Berry, 1993; Falco et al., 1995). No-till cropping promotes greater presence of Bradyrhizobium japonicum (Sagardoy, personal communication) and improved soybean nodulation from inoculants (DõÂaz-Zorita and FernaÂndez Canigia, 1999). These differences were thought due to better water storage conditions at the surface of no-till soils. Alves et al. (1999) compared the N cycle in no-till and till systems in tropical soils from Brazil concluding that biological N ®xation is signi®cantly favored in notill systems because of native soil N immobilization. FernaÂndez Canigia et al. (2000a,b) studied the effects of tillage practices and crop sequences under no-till on microbial diversity in Typic Hapludolls (loamy Haplic Phaeozem). Preliminary results from this research

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Fig. 6. SOC content evolution (0±20 cm layer) in continuous no-till (NT) and chisel-till (CT) production systems from the western part of the Argentine Pampas. Each site-year is an average of 3±10 farmer ®elds. Bars are standard error of the means (Duarte, unpublished data).

show that microbial diversity was more affected by changes in crop productivity and the quality of crop residues than by tillage. Increasing no-till soybean, maize or sun¯ower productivity caused microbial diversity to increase. No signi®cant relationships between soil properties and biodiversity were observed. In a maize±soybean rotation, bacterial diversity and the number of substrates used by soil bacteria tended to be greater after long-term (9 years) than short-term (2 years) use of no-till practices (FernaÂndez Canigia et al., 2000a). Soil C storage in soils from temperate regions under no-till practices has been positively related to the amount and quality of residues produced and returned to the soil (Paustian et al., 1997). Greater differences in SOC accumulation have been described after the beginning of no-till practices, reaching a steady state or period with minimal further change after 5±10 years (Staley et al., 1988; Dick et al., 1991). In Daireaux (Buenos Aires, Argentina), SOC quantities in the 0± 30 cm layer were evaluated for four crop sequences after 4 years of no-till applied to a previously tilled soil. Carbon accumulation was increased with no-till crop production relative to the initial condition, when

no grazing was done. As the proportion of maize in the sequence increased, SOC accumulation tended to be greater (Fig. 7, DõÂaz-Zorita and Grove, 1999b). As a consequence of losses in SOC after tillage, several changes in soil physical properties (increased susceptibility to compaction, loss of aggregate stability, and reduced geometric mean aggregate size) were found (Table 2). Soil structure maintenance in crop sequences that include maize and wheat and the loss of structure when soybean or sun¯ower are grown have been described for rotations under conventional tillage management (Arrigo et al., 1993). DõÂaz-Zorita and Grosso (2000) and AragoÂn et al. (2000) found that increased SOC reduced the compactability of soils of the Pampas, independent of texture. Similar results have been reported for soil from the US (Wagner et al., 1994; Thomas et al., 1996) and the UK (Ball et al., 2000). The loss of SOC in cultivated soils of the semiarid Pampas region makes them more susceptible to compaction, with adverse effects on soil hydraulic conductivity and plant root extension (Quiroga et al., 1999). The conservation of original conditions (SOC, pore size distribution and aggregation) in an agricultural

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Fig. 7. Effect of different crop sequences on the SOC content (0±20 cm layer) of an Entic Haplustoll (sandy Haplic Phaeozem) in the western part of the Argentine Pampas under continuous no-till management for 4 years. This experiment was established after 4 years of a maize± sun¯ower rotation under chisel-tillage practices (initial condition). W: wheat; Sy: soybean; M: maize; S: sun¯ower; Og: oats grazed directly. Bars are standard errors of the means (DõÂaz-Zorita and Grove, 1999a).

soil can be achieved with row crop±pasture rotations or with no-till row cropping systems. In the ®rst system, the inclusion of tillage practices during the row crop component of the cycle necessitates the presence of pastures in the production system and limits the duration of row cropping. Results from almost 50 farmer ®elds near AmeÂrica (Buenos Aires, Argentina) showed that when tillage is practiced frequently, the inclusion of pastures in the crop sequence

favors increased SOC levels. Pilatti et al. (1988), in the southern part of Santa Fe province, and Galantini and Rosell (1997), in the semiarid Pampas, reported similar results. In no-till farmer ®elds, no signi®cant differences in SOC were observed when comparing annual crop and pasture components of rotational cycles (Fig. 6). When SOC is already high, little or no increase in C storage can be expected in pasturelands (Schanabel et al., 2001).

Table 2 Changes in aggregate geometric mean diameter (CGMD) (wet sieving), maximum bulk density (MAX.BD) (Proctor method) and aggregate size distribution in the top layer (0±15 cm depth) of a Typic Hapludoll (loamy Haplic Phaeozem) under three tillage systems for 6 years (DõÂazZorita, 1999)a System

No-till Chisel-till Moldboard-till a

CGMDb (mm)

0.69 a 0.68 a 1.13 b

MAX.BDc (Mg m 3)

1.45 a 1.48 a 1.54 b

Aggregates (%) <2 mm

>8 mm

7.2 a 22.2 b 30.8 b

82.6 b 60.3 a 49.9 a

Different letters within a column indicate a signi®cant difference between tillage treatments (Tukey, P < 0:05). De Leenheer and De Boodt (1958). c ASTM (1985). b

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5. Other changes in soil properties under continuous no-till systems Compaction of the topsoil under no-till systems has been described in several studies in the Pampas region (Andriulo and Rosell, 1988; Kruger, 1996b; Buschiazzo et al., 1999; Ferreras et al., 2000). Results from Pergamino (Buenos Aires, Argentina) and Anguil (La Pampa, Argentina) show that soil physical status at the beginning of no-till systems is a critical factor in the productivity of no-till systems in the Pampas (Ferrari, 1997; Quiroga et al., 1998b). Pedogenic causes have been attributed to the origin of low macroporosity in ®ne loamy soils of the Pampas region (Taboada et al., 1998). Mechanical impedance to root growth has been shown to limit root elongation and is related to reductions in plant shoot growth and grain yield. A compacted soil layer, because it has high strength and low porosity, con®nes crop roots to layers above and reduces the volume of soil that can be exploited by the plant for nutrients and water (Hammel, 1994). Compaction also reduces plant growth and yields by affecting water in®ltration, aeration and plant diseases. Several studies have pointed out that the roots of some species can penetrate compact layers, creating easily accessible pathways for the roots of succeeding crops, and leaving biopores which may increase water movement and gaseous diffusion (Unger and Kaspar, 1994). Shallow compaction may not represent a signi®cant problem in no-till soils where there is an accumulation of organic C at the surface causing a decline in maximum bulk density values (Thomas et al., 1996). Soil strength, low soil moisture and lower soil N availability are factors that can contribute to reduced vigor in no-till crops. Tillage pans can occur in many coarse textured soils and these must be disturbed by a form of deep tillage (e.g. paraplowing) so that yield can be maximized. Results from a study of the effects of subsoiling practices performed on a Typic Hapludoll (loamy Haplic Phaeozem) under three tillage systems (no-till, chisel-till and moldboard-till) showed that the poor initial vegetative growth of no-till maize crops was improved by deep-tillage using a paraplow to a depth of 40 cm (DõÂaz-Zorita, 2000). The effect of this practice did not modify the grain yields of the maize crops that were improved by the N fertilization treatment (DõÂaz-Zorita, 2000). Bulk density values

varied between tillage systems with greater values for no-till. And although the soil bulk density values would not be considered limiting for normal crop growth (Vepraskas, 1994), differences in crop yield were related to differences in the bulk density of the 0±15 cm layer at seeding and also to the N fertilization treatments (Fig. 8). The residual effect of the deep tillage on soil penetration resistance lasted no longer than one crop cycle after its implementation. Cone penetrometer resistance values were also greater for the surface layer (0±10 cm) of no-till soils than for tilled soils. This part of the soil is directly in¯uenced by the presence of residue cover and remains moist longer with no-till than when shallow tillage is performed. The hardness of these soils diminishes when the water content increases, so the period with potentially limiting soil impedance is less in no-till soils than in tilled soils (Quiroga et al., 1998b). Traditional production systems in the western part of Argentine Pampas consider crop residues a part of the animal feeding regime during winter or as a way to maintain animals while conserving pastures during frozen periods. Grazing crop residues diminishes the ®xed C input in these systems and consequently limits organic matter accumulation at the soil surface (DõÂazZorita and Grove, 1999a). Trampling induced compaction is another consequence of crop residue grazing and in continuous no-till systems this damage cannot be ameliorated by tillage. Relatively little literature deals directly with the impact of animal production systems on soil properties under contrasting tillage practices (e.g. no-till versus till). Thus, the effect of intensive animal trampling during winter after maize and soybean harvest on soil properties was studied in a Typic Hapludoll (loamy Haplic Phaeozem) under no-till, chisel-till and moldboard-till systems (DõÂaz-Zorita, unpublished data). It was observed that the pressure to the soil induced by animal trampling deforms and compacts more at the surface (0±5 cm) of till than no-till systems (Fig. 9). No signi®cant differences in bulk density values were observed in the 5±15 cm layer. Greater changes in the bulk density values of tilled, rather than no-till, soils were also reported when grazing over southern Brazil soils (Silva et al., 2000). Duarte and DõÂaz-Zorita (1999) compared the penetration resistance pro®les of four farmer ®elds near AmeÂrica (Buenos Aires, Argentina) after 4 years of direct

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Fig. 8. Effect of soil bulk density (BD) in the 0±15 cm layer and N fertilization (100 kg N ha 1) on maize yield on a Typic Hapludoll (loamy Haplic Phaeozem) from the western part of the Argentine Pampas (N ˆ 0 in treatments without N fertilization and N ˆ 1 in treatments with N fertilization) (DõÂaz-Zorita, 2000, permission from Elsevier).

Fig. 9. Effect of grazing maize residues on the bulk density of a Typic Hapludoll (loamy Haplic Phaeozem) under three continuous tillage systems (CT: chisel-till; MT: moldboard-till; NT: no-till). Bars are standard errors of the means (DõÂaz-Zorita, unpublished data).

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soil to resist compaction were attributed to differences in structural stability between tillage systems (Table 2). Well-structured soils can maintain a stable pore system longer than soils formed from weak structural units (Kay, 1990). These observations suggest that crop residues could be grazed, without inducing signi®cant increases in soil compaction, if row cropping after the pasture portion of the rotation cycle is done without tillage (no-till). 6. No-till and crop productivity

Fig. 10. Soil penetration resistance in two farmer alfalfa±fescue pastures established either with chisel-till (CT) or no-till (NT) grazing. Bars are standard errors of the means (Duarte and DõÂaz-Zorita, 1999).

grazing of pastures established either with or without tillage. Greater soil penetration resistance values were found after grazing pastures that were planted with tillage (Fig. 10). The differences in the ability of the

As a result of long-term tillage trials at the General Villegas Agricultural Experimental Station of INTA and data from grower ®elds, it has been found that average crop yields with use of no-till systems in the western part of the Argentine Pampas are similar to those observed with other tillage systems (Figs. 11 and 12). Buschiazzo et al. (1999) summarized the effects of tillage practices on crop yield at six experimental sites located in the subhumid and semiarid Pampas region, concluding that average yields of soybean, wheat, and sorghum (Sorghum bicolor L.) were generally greater with conservation tillage (no-till and reduced-till) than with conventional tillage (moldboard-till). Maize and

Fig. 11. Sun¯ower, maize, soybean, and wheat yields under chisel-till (CT) or no-till (NT) in the ``Zona Oeste'' CREA groups from the western part of the Pampas, Argentina. Average of 200 grower ®elds for 1993±1997. DCS: double-cropped soybean. Bars are standard errors of the means (Database of ``Zona Oeste'' AACREA Groups).

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Fig. 12. Effect of tillage practices on annual maize and soybean grain yield on a Typic Hapludoll (loamy Haplic Phaeozem) under three continuous tillage systems (CT: chisel-till; MT: moldboard-till; NT: no-till). Data averaged over nine production seasons. Bars are standard errors of the means (DõÂaz-Zorita, unpublished data).

sun¯ower yields were lower with conservation tillage, probably because of crop N de®ciencies in conservation tillage systems. No-till double-cropped soybean yield are generally higher and more stable between seasons (Ferrari, 1997). Several researchers, e.g. Blevins et al. (1971) and Lindstrom et al. (1974), showed that the presence of soil residue cover in no-till soils favors soil water conservation via reduced evaporation, with improved crop yields. In Daireaux (Buenos Aires, Argentina), when no-till was initiated after 4 years of tillage, lower ®rst-year corn yields were observed for no-till than for chisel-till (Fig. 13). But, in the following seasons, no signi®cant differences in sun¯ower or maize yields were observed between tillage practices and after 4 years, corn yields were greater for no-till than for chisel-till (Fig. 13). Murdock et al. (2001) and DõÂaz-Zorita et al. (unpublished data) observed that 2 years without tillage was too short a time period for recovery of the better structure found in continuously no-till soils in Kentucky, USA. These authors also observed that tillage disruption by chisel plowing of otherwise continuous no-till soils often reduced yields of summer crops (maize and double-cropped soybean), especially in seasons with fewer rainfall events. Similar results

have been reported elsewhere, indicating that the bene®ts of no-till management, in terms of soil structure related processes, require no-till continuity for periods of time longer than 2 years (Voorhees and Lindstrom, 1984; Bergh et al., 2000; Karunatilake et al., 2000; McGarry et al., 2000). No-till adoption is increasing in the western Pampas (Fig. 14), as well as in the rest of the Pampas region, where the total area under no-till represents more than 30% of the total area cropped annually (GarcõÂa et al., 2000). A short-term change in crop productivity is not the main factor explaining the increasing adoption of no-till practices. In general, no-till adoption in the subhumid and semiarid Pampas is related to soil water conservation that allows adequate planting dates for maximal yields. Yeates et al. (1996) observed that notill increased the potential number of sowing dates the in semiarid-tropics of northern Australia. Other bene®ts additional to SOC conservation and related improvement in soil properties as determined by a wide range of studies in the US and other countries are a reduction in labor costs, better timing of crop harvest and pasture grazing, and reduced fuel and machinery requirements (Phillips, 1984). The no-till system reduces the energy input into maize and soybean

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Fig. 13. Sun¯ower and maize yield evolution with time under chisel-till (CT) or no-till (NT) on an Entic Hapludoll (sandy Haplic Phaeozem) from the western part of the Argentine Pampas. Bars are standard errors of the means (DõÂaz-Zorita and Grove, 1999b).

production by 7 and 18%, respectively, when compared to the moldboard plowing followed by disking (Phillips et al., 1980). Differences in production costs also support the increment of no-till. GarcõÂa et al. (2000) reported

that direct costs for establishing maize, soybean, and wheat in the Pampas region with no-till are lower than or similar to conventional till. The adoption of notill allowed growers to adjust their management system and acquire potential ®nancial bene®ts relative to

Fig. 14. Evolution of the adoption of no-till practices for row crops by the ``Zona Oeste'' CREA groups from the western part of the Pampas, Argentina (Database of ``Zona Oeste'' AACREA Groups).

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management practices with tillage. Similar behavior has been described in other countries, e.g. Australia, New Zealand and the US (Frengley, 1983; Thomas, 1985; Cornish and Pratley, 1991). No-till adoption varies between crop species depending on the availability of weed and fertilizer management information. For example, abundant information is available for no-till maize and soybean production but is lacking for sun¯ower. In the case of wheat, low no-till adoption is related to lower economic bene®ts with this crop and the usually lower N fertilizer rates and consequent production costs when planted with tillage. Adoption of no-till systems in the western Pampas region favors the inclusion of double crop sequences (e.g. wheat/soybean) mostly in ®elds under continuous grain cropping rather than in those ®elds with grain crop±pasture rotations. The integration of annual grain crops and livestock cycles brings bene®ts to both production components (e.g. N ®xation during the pasture cycle) and also some con¯icts (e.g. choice of crop sequences and timing for livestock operations). The ¯exibility in the integration of crops and livestock varies, depending on land capability, personal, ®nancial, and economic factors, and tradition (Mann, 1991; Tow and Schultz, 1991). 7. Conclusions The success and sustainability of agricultural production systems in the subhumid and semiarid Pampas of Argentina depends on the production and conservation of SOC. In this region, the proportion of the area under annual crops is increasing and, because no-till systems are the best alternative for producing and conserving SOC, it is recommended their implementation after the elimination of soil compaction limitations and when crop's N requirements are met. Beef or dairy cattle production on permanent pastures should be included in rotation with tilled row crop systems because pastures restore SOC and soil productivity. On the other hand, the inclusion of pastures in rotation with annual crops under no-till systems is more of an economic, than a soil management decision because both pastures and no-till row cropping maintain the productivity of the soils in the subhumid and semiarid Pampas of Argentina.

15

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