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Published online 1 January 2007
Published in Agron J 99:59-65 (2007)
DOI: 10.2134/agronj2006.0025
© 2007 American Society of Agronomy
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Soil & Crop Management

Tillage System, Preceding Crop, and Nitrogen Fertilizer in Wheat Crop

I. Soil Water Content

Rafael J. López-Bellidoa,*, Luis López-Bellidob, Jorge Benítez-Vegab and Francisco J. López-Bellidoc

a Dep. de Ciencias Agroforestales, Univ. of Huelva, Campus de La Rábida, 21819 Palos de la Frontera (Huelva), Spain
b Dep. de Ciencias y Recursos Agrícolas y Forestales, Univ. of Córdoba, Córdoba, Spain
c Dep. de Producción Vegetal y Tecnología Agraria, Univ. of Castilla-La Mancha, Spain

* Corresponding author (rafael.lopez{at}dcaf.uhu.es)

Received for publication January 31, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Under rainfed Mediterranean conditions, water economy must be based on a suitable choice of agronomic techniques. A 6-yr study was undertaken to determine the effects of tillage system, preceding crop, and N fertilizer on soil water at wheat (Triticum aestivum L.) planting and harvest in a Vertisol. Tillage treatments were no-tillage and conventional tillage. Preceding crops, in 2-yr rotations, were sunflower (Helianthus annuus L.), chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), fallow, and continuous wheat. Nitrogen fertilizer rates were 0, 50, 100, and 150 kg N ha–1 applied to wheat only. No-tillage did not provide more water at wheat planting for any of the rotations. Preceding crop effect on soil water content (SWC) at planting was as follows: fallow ≥ faba bean > wheat ≥ chickpea > sunflower. At harvest, SWC was higher in continuous wheat. Only at harvest were there differences among N fertilizer rates for SWC. Besides, measurement of SWC at harvest for sunflower, chickpea, faba bean, and fallow were performed to determine soil water storage and precipitation storage efficiency (PSE) for 2 yr. Soil water storage was higher for rotations with sunflower or fallow. Nevertheless, fallow PSE was the lowest (10%). The mean PSE was 29%. Under the conditions of this study, no-tillage is not more efficient than conventional tillage in soil water accumulation. Fallow is not a useful tool for increasing water availability.

Abbreviations: PSE, precipitation storage efficiency • SWC, soil water content


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN THE MEDITERRANEAN REGION, water availability is the major factor limiting rainfed wheat production. The greatest loss of water from the profile under Mediterranean conditions occurs through direct evaporation from the soil, drainage being negligible. Corbeels et al. (1998) report that direct evaporation can account for up to 80% of total seasonal evapotranspiration. Consequently, efforts to improve soil water storage have been based on techniques designed to curb this form of water loss, even to the extent of covering much of the soil surface with plastic, thereby increasing soil water storage and wheat yield (Fan et al., 2005). Traditionally, it was thought that soil water storage in water-deficient cropping systems could be increased by fallow. However, fallow efficiencies seldom exceed 40%, i.e., at least 60% of the precipitation received during fallow is lost to evaporation (Tanaka et al., 2005). Others techniques can be those related to soil tillage, type of crop and cultivar, planting date and density, and N fertilization (Debaeke and Aboudrare, 2004).

According to Hatfield et al. (2001), soil management practice that alters any soil component within or on the soil surface affects the processes of evaporation by modifying the available energy, the available water in the soil profile, or the exchange rate between the soil and the atmosphere. This leads to increased water storage by increased infiltration into soil as well as increased soil water losses by evaporation compared with a residue-covered surface or an undisturbed surface. Tillage, too, directly affects water storage via its effect on macropore space and accelerated evaporation. In a no-tillage system, surface residue during fallow provides physical protection for soils to control wind and water erosion, increases precipitation infiltration by protecting the soil surface from raindrop impact and subsequent crusting, and reduces evaporation by decreasing air movement immediately above the soil, changing albedo and insulating the soil surface (Horton et al., 1996; Nielsen et al., 2005; Tanaka et al., 2005). How surface residue is managed, its amount and orientation, can be a factor in minimizing evaporation and increasing soil water storage (Peterson et al., 1996; Nielsen et al., 2005). However, it should be noted that heavy-textured soils are highly drought prone in a semiarid environment, due to the large amounts of water retained at wilting point. It is characteristic for these clayey soils that significant amounts of moisture are lost even under fallow, presumably due to high evaporative fluxes from soil surface and crack wall area (Corbeels et al., 1998).

Water economy in a rainfed Mediterranean system requires a rotation design that is appropriate in terms of both space and time. As indicated earlier, fallow traditionally formed part of semiarid rainfed systems; nowadays, however, replacement of fallow by other crops has led to cropping intensification which, paradoxically, has increased rather than reduced sustainability (Tanaka et al., 2005). Farahani et al. (1998) reported that improvements in system PSE with cropping intensification (i.e., reduced fallow frequency) were due to reducing the percentage of the system fallow time. The effect of the preceding crop on water storage may be due to its water consumption, the type and amount of residue it generates and the physical effect of its root system on the soil.

Finally, N in dryland cropping systems has a positive impact on the amount of residue returned to the soil and on belowground residue C. Rising N rates increase soil organic C and total N (Halvorson et al., 1999), and should therefore contribute to increased water storage. However, N fertilization is more closely linked with water utilization and water use efficiency.

A field study was undertaken to determine, in a wheat crop, the effects of tillage system, preceding crop and N fertilizer on soil water storage in a Vertisol under rainfed Mediterranean conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Characteristics
Field experiments were conducted in Córdoba, southern Spain (37°46' N, 4°31' W, 280 m above sea level), on a Vertisol (Typic Haploxererts) typical of the Mediterranean region, where rainfed cropping is the standard practice. Weather-related parameters for this area are as follows: average annual rainfall 584 mm (39%, October–December; 37%, January–March; 19%, April–June; and 5%, July–September); average annual evapotranspiration 1000 mm; average duration of dry period, 4 to 6 months; average annual temperature, 17.5°C; average temperature in the coldest month, 9.5°C; and average temperature in the warmest month, 27.5°C. This area is primarily grown to wheat in rotation with sunflower and, less frequently, grain legumes such as faba bean and chickpea. Fallow is also used on relatively nonfertile soils in the driest areas. Use of wheat–fallow also takes advantage of the set-aside measures under the European Union Common Agricultural Policy (CAP) reforms. Intensive conventional tillage is the most widespread practice.

Experimental Design
The long-term experiment, called "Malagón," was started in 1986, designed as a randomized complete block with a split-split plot arrangement and four blocks. Main plots were tillage system (no-tillage and conventional tillage); subplots were preceding crop, with four 2-yr rotations (wheat–sunflower, wheat–chickpea, wheat–faba bean, and wheat–bare fallow) and continuous wheat; sub-subplots were N fertilizer rate (0, 50, 100, and 150 kg N ha–1) applied to wheat. Duplicate sets of plots were established to allow all phases of the rotation to be present each year. The area of each sub-subplot was 50 m2 (10 by 5 m).

Crop Management
Specific seed drills were used in each tillage system. Weeds were controlled with glyphosate [N-(phosphomethyl)glycine] + MCPA [(4-chloro-2-methylphenoxy)acetic acid] at a rate of 0.87 + 0.60 kg a.i. ha–1 before planting. Conventional tillage treatment included moldboard plowing after crop harvest, disk harrowing at the beginning of autumn, and/or vibrating tine cultivation to prepare a proper seedbed before planting. Information on cultivar, planting, and herbicides applied during the growing season is provided in Table 1. Nitrogen fertilizer was applied to wheat plots as ammonium nitrate. At all application rates, half was applied before planting (incorporated by disk harrowing in conventional tillage plots and surface-broadcast in no-tillage plots). The remaining N was applied as top dressing at wheat tillering. Every year, wheat plots were also supplied with P fertilizer at a rate of 65 kg ha–1; this was incorporated in conventional tillage following the standard practice and banded with drilling in no-tillage plots. Soil-available K was adequate (530 mg kg–1).


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  		Table 1. Information for each crop about cultivar, planting, herbicides, and others.

 
Measurements and Calculations
The study was performed over a 6-yr period (1995–1996, 1996–1997, and 1999–2000 to 2002–2003). Soil water content was determined with two measurements per wheat plot at planting and harvest to a depth of 0.9- in 0.3-m increments, using a ThetaProbe ML 2x soil moisture sensor (AT Delta-T Devices, UK). In seasons 1997–1998 and 1998–1999 weather conditions prevented taking soil water measurements: the 1st yr, short gap in the prevailing waterlogging conditions during autumn only allowed to plant and fertilizer; the 2nd yr, owing to rainfall shortage no harvest was obtained and no soil water measurement was done. For sunflower, chickpea, faba bean, and fallow, measurement of SWC at harvest were performed from 1999–2000 to 2002–2003 to determine soil water storage and PSE. Calculations of both parameters began at harvest of the preceding crop and ended with the planting of wheat. For wheat–fallow rotation this period starts 2 yr before. Precipitation storage efficiency was calculated by dividing the increase in SWC at depths of 0 to 90 cm for the fallow period by the precipitation received during that period (Tanaka and Aase, 1987; Huang et al., 2003). With the available data, both soil water storage and PSE could be estimated only for a 2-yr period (2001–2002 and 2002–2003).

Statistical Analyses
The year was considered as a random variable, due to unpredictable weather conditions under rainfed Mediterranean conditions (Gómez and Gómez, 1984). All parameters were subjected to analysis of variance (ANOVA) using a randomized complete block design combined over years, following error term according to McIntosh (1983). Treatment means were compared using Fisher's protected least significant difference (LSD) test at P ≤ 0.05. Different correlations were also calculated. Analyses of variance were performed using Analytical Software Statistix 8.0 program (Analytical Software, 2003) to determine treatment effects.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather Conditions
Average annual rainfall in the area is 584 mm. Total rainfall in study years ranged from 256 (1998–1999) to 1007 mm (1996–1997). Rainfall was very high in 3 yr (1995–1996, 1996–1997, and 1997–1998), high in 2 yr (2000–2001 and 2001–2002), medium in 2 yr (1999–2000 and 2002–2003), and low in 1 yr (1998–1999) (Table 2).


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  		Table 2. Total and monthly rainfall for 8 yr at Córdoba (Spain).

 
In those years for which SWC data were obtained, the major findings for seasonal rainfall distribution were as follows: extremely high rainfall was recorded in the autumn of 1996–1997 (419 mm), while values for other years ranged from 183 mm (1995–1996) to 288 mm (2002–2003); winter rainfall was low in 1999–2000 (45 mm), whereas in 1995–1996 it was 467 mm; spring rainfall over the last 3 yr of the study was relatively low, ranging from 69 to 98 mm, while in the first 2 yr it was 165 mm, rising to 223 mm, in 1999–2000. Summer rainfall was insignificant in all study years (Table 2).

Soil Water Content
The total number of sources of significant variation at planting and at harvest of wheat crop, for all depths, was 30 and 26, respectively (Table 3). There were more sources of significant variation at harvest, largely due to N fertilizer treatment. This treatment was applied only to the wheat crop, and its effect on soil water content at wheat planting was very small, since by then 2 yr had elapsed since the previous application. Even so, one might have expected a greater number of significant interactions involving N, due to the considerable effect of treatment on yield (López-Bellido et al., 2007). According to Corbeels et al. (1998), this may be because at the end of the growing season there is a considerable water deficit in surface soil due to strong evapotranspirative demand; this has a marked effect on SWC, thus countering the possible effect of the N rate applied. Weather conditions (year) and preceding crop displayed the strongest effect on SWC, values being significant both at harvest and planting, for all depths studied (Table 3). Crop preceding treatment had a more marked effect at planting than at harvest, with a larger number of significant interactions for year x preceding crop, tillage system x preceding crop, and year x tillage system x preceding crop at all depths studied (Table 3). The reverse was true of fertilizer N treatment (for the reasons outlined above); the main effect was only significant at harvest—for all depths—and there were 12 significant interactions at harvest compared to four significant interactions at planting (Table 3). Tillage system was significant at depths of 0 to 30 and 0 to 90 cm at harvest, although its effect was strongly dependent on weather conditions both at planting and at harvest; the interaction year x tillage system was significant at all three depths (Table 3).


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  		Table 3. Significant effects of year, tillage system, preceding crop, and N fertilizer on soil water content over 6 yr, and soil water storage and precipitation storage efficiency over 2 yr in a wheat crop under rainfed Mediterranean conditions.

 
No significant differences were recorded in SWC at planting as a function of tillage method in any of the profiles over the 6-yr study as a whole (Fig. 1 ). The year x tillage system interaction was not significant over the whole profile 0 to 90 cm (Table 3). Significant differences were recorded in some soil profiles in 4 yr (Table 4), but they did not provide relevant information to characterize both tillage systems in terms of water accumulation. However, Hatfield et al. (2001) reported that no-tillage has a positive effect on SWC.


Figure 1
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  		Fig. 1. Soil water content at planting and harvest in a rainfed wheat in tillage system, preceding crop, and N fertilizer treatments during 6 yr. Horizontals bars indicate LSD at P < 0.05.

 

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  		Table 4. Soil water content over 6 yr at planting and harvest of a rainfed wheat in year x tillage system interaction for significant profiles.

 
Soil water content at harvest was greater under conventional tillage over the 0- to 90-cm profile as a whole and in the 0- to 30-cm profile (Fig. 1). Year x tillage system interaction showed that over the 0- to 90-cm profile no-tillage SWC was higher than that of conventional tillage in 1995–1996 and 2001–2002, while in 1999–2000 and 2000–2001 the reverse was true. This may be accounted for by greater wheat grain yield: the higher the yield, the lower the SWC (López-Bellido et al., 2007). When SWC was higher in conventional tillage, it was significantly higher in all three profiles, whereas when SWC was higher in no-tillage, there were no differences in the surface horizon. For some reason, water retention in the surface profile was lower under the no-tillage system; this trend, though not significant, was noted in all years (Table 4). Further research into this finding might require analysis of the physical properties of the soil, and particularly of cracking, which is very common of Vertisols. Corbeels et al. (1998) suggest that due to the cracking properties of the soil, considerable evaporative losses occur up to a depth of 0.45 m.

Overall analysis disclosed that preceding crop had a strong effect on SWC at planting over the whole 0- to 90-cm profile: there were no differences between fallow and faba bean; values were significantly lower for chickpea and wheat, with no difference between the two; the lowest value was recorded for sunflower (Fig. 1). Exactly the same results were recorded for the 30- to 60-cm profile; at 60 to 90 cm, chickpea and sunflower were significantly different with the rest of preceding crop and between them; while at 0 to 30 cm only sunflower was significantly different. Norwood (2000) also observed a very dry soil profile following sunflower production. Huang et al. (2003) have shown, for various crops, that none of the preceding crops significantly affected soil water available at wheat planting. No information was provided by the year x preceding crop interaction, which is therefore not included in any table. The only noteworthy finding was the absence of significant differences in SWC in all profiles as a function of preceding crop in 1999–2000; this was due to the fact that no crops were harvested the previous year because of the drought (López-Bellido et al., 2000, 2002, 2003, 2004).

The tillage system x preceding crop interaction showed significant differences between tillage systems at planting only between the wheat–chickpea and wheat–fallow rotations (Table 5). In the wheat–chickpea rotation, SWC was higher for conventional tillage for all three profiles, a finding that ought to be attributable to lower chickpea yield under conventional tillage. However, López-Bellido et al. (2004) found no significant difference in chickpea yield between the two tillage systems in a study forming part of the same 12-yr experiment. With preceding fallow, SWC was significantly higher for no-tillage over the 0- to 90-cm profile. However, differences were too small to be of any practical interest. Under conventional tillage, only the sunflower was significantly different from the rest of preceding crops; whereas under no-tillage more preceding crops differed (Table 5).


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  		Table 5. Soil water content over 6 yr at planting of rainfed wheat in tillage system x preceding crop interaction for significant profiles.

 
No correlation was found between the amount of residue left by the preceding crop and SWC at planting (data not shown) under no-tillage. Land coverage by faba bean, chickpea and sunflower residue cannot be compared with that of wheat in terms of amount and distribution over the soil, so perhaps these preceding crops failed to have the expected effect. The absence of correlation for wheat monoculture may be ascribed to the small amount of residue left. Further studies are required to ascertain whether, as a function of N fertilizer rates, wheat residues affect SWC at planting of faba bean, chickpea and sunflower crops. Farahani et al. (1998) indicated that residue is not the key component but just one component of the system. Due to marked evaporative flux during the Mediterranean summer, covering land with straw may not be sufficient to increase soil water storage.

Only at harvest were there significant differences among N fertilizer rates for SWC (Fig. 1). At 0 to 30 and 60 to 90 cm, SWC was only significantly higher than the rest only at a fertilizer rate of 0 kg N ha–1, due to the lower yield obtained at that rate (López-Bellido et al., 2007). Significant differences were recorded between all rates at 30 to 60 cm, except that there was no significant difference between 50 and 100 kg N ha–1 (Fig. 1). This highlights the importance of the 30- to 60-cm horizon for crop soil water consumption as the N fertilizer rate increases. Over the whole profile (0–90 cm), the difference between rates was: 0 > 50 ≥ 100 ≥ 150 (Fig. 1). Corbeels et al. (1998) found no significant differences in SWC at wheat planting between crops treated with N fertilizer and untreated crops.

The year x N fertilizer interaction was significant for 30- to 60- and 0- to 90-cm profiles at planting and harvest (Table 3); however, it has not been showed for not provide more information. Tillage system x preceding crop interaction was significant for 0- to 30-, 30- to 60-, and 0- to 90-cm profiles at harvest (Table 3), and likewise it was not showed due to its irrelevance.

Soil Water Storage and Precipitation Storage Efficiency
Nine of the 15 sources of variation for soil water storage were significant; most involved preceding crop and its interactions (Table 3). Precipitation storage efficiency was only significant for four sources of variation (Table 3). In the 2 yr in which soil water storage was analyzed results differed considerably, rising from 26 mm in 2001–2002, to 130 mm in 2002–2003. This difference was reflected in PSE, which climbed from 17% in the 1st yr to 42% in the 2nd yr. There was no significant difference in either year between SWC at wheat planting over the 0- to 90-cm profile and SWC at harvest of the preceding crop. The marked year-on-year difference between years—although the mean value of SWC at harvest of the preceding crop was greater in 2001–2002 (281 mm) than in 2002–2003 (252 mm)—was due to higher rainfall before planting in 2002–2003 (259 mm) compared to 2001–2002 (124 mm).

Neither soil water storage nor PSE were affected by tillage system (Table 6). Corbeels et al. (1998) have suggested that reducing tillage and maintaining surface residues reduce precipitation runoff and increase infiltration, thereby increasing PSE. Similarly, Peterson et al. (1996) have shown that no-tillage improves the storage of precipitation in the soil profile compared with conventional tillage. Norwood (1999) reports that PSE increases as tillage is reduced during the summer fallow period before wheat planting.


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  		Table 6. Soil water storage and precipitation storage efficiency in rainfed wheat in tillage system, preceding crop, and N fertilizer treatments over 2 yr.

 
Preceding crop and year x preceding crop interaction were significant for both soil water storage and PSE (Table 3). Additionally, soil water storage was significant for the interactions tillage system x preceding crop and year x tillage system x preceding crop (Table 3). Sunflower and fallow stored more water than the other preceding crops, with no significant difference between them. There were no significant differences between wheat, chickpea, and faba bean (Table 6). The sunflower rotation clearly stored the most water, since the sunflower extracts more water and thus the land is much more receptive to the heavy rains characteristic of the Mediterranean climate; this is borne out by the highest PSE value (46%). The wheat–fallow rotation also stored more water than the rest, although it required 18 months to do so; the PSE value was the lowest of all (10%). There were no significant differences in PSE among the remaining preceding crops (Table 6). In the Mediterranean summer, evaporative demand is so strong that it robs the soil of most of the rain falling during the fallow period. Farahani et al. (1998) report that the amount of precipitation stored in the soil profile during fallow is extremely low, ranging from a minimum of 11% to a maximum of 27%. Corbeels et al. (1998) emphasized that under fallow an important discharge occurs during the growing season. They showed that profile recharge under wheat was very similar to that under fallow. According to Norwood (1999), in addition to differences in previous crop water use, soil water content at wheat planting can also be affected by differences in tillage and crop residue effects on PSE. Year x preceding crop interaction showed no differences in soil water storage in 2001–2002, whereas in 2002–2003 the preceding sunflower crop stored most water, followed by fallow and finally by the other crops, among which there were no significant differences (Table 7). In 2001–2002 the highest PSE was recorded for the wheat–faba bean rotation, and the lowest for wheat–fallow. In 2002–2003, wheat–sunflower showed the highest PSE and wheat–fallow the lowest. Only the wheat–fallow rotation stored more water under the no-tillage system, although this result is based on only 2 yr, and further research is required to obtain more reliable statistics. Meanwhile, there was no significant different between the two tillage systems for the other rotations (Table 7). Yule (1984) suggested that PSE by fallow might depend on the occurrence of cracks. Therefore, the use of crop residues should be beneficial both in preventing excessive soil cracking and in reducing evaporative losses. Tanaka and Aase (1987) showed that fallow PSE increased from under 25% to around 40% as tillage intensity decreased from moldboard plow to no-till.


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  		Table 7. Soil water storage and precipitation storage efficiency over 2 yr in a rainfed wheat in year x preceding crop, tillage system x preceding crop, and preceding crop x N fertilizer interactions.

 
The lower N rates (0 and 50 kg N ha–1) stored more water, and stored it more efficiently, than the higher rates (Table 6). Although differences were significant, their magnitude was not considered of great practical importance. Differences could be attributed to higher water consumption by preceding crop as a result of yield increase in response to the residual N left by wheat (López-Bellido et al., 2002, 2003, 2004). Preceding crop x N fertilizer interaction showed for continuous wheat and the wheat–sunflower rotation that there was a clear difference between the lower and higher rates; whereas for the other rotations differences were not recorded (Table 7). Within each N rate, rotations with sunflower and fallow tended to store more water than the others.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This 6-yr study showed that water storage dynamics in a Vertisol under rainfed Mediterranean conditions is heavily influenced by the strong evaporative demand in summer, which largely cancels out the effect of any measures previously taken to improve water storage. The variation in SWC between planting and harvest is very slight, suggesting that rainfall during the growing season is either used by the crop or lost through evaporation in the spring, but not stored. Nontillage does not increase water availability at planting in wheat monoculture or in 2-yr rotations with faba bean, fallow, chickpea or sunflower.

In the treatments studied, crop rotation would appear to be a much more influential factor in water economy. Faba bean as preceding crop provides the same amount of water as fallow, as well as producing a protein-rich harvest. The amount of water is lower for wheat monoculture and rotation with chickpea, and considerably lower for rotation with sunflower, since sunflower extracts the small amount of water available in the soil in summer. Fallow would appear not to be an efficient means of storing water, and thus is not to be recommended. Soil water content at wheat harvest does not vary as a function of preceding crop, despite differences in yield (López-Bellido et al., 2007), probably because during the latter half of spring marked loss through direct evaporation from the soil probably equalizes SWC. The effect of preceding crop on SWC is more evident under no-tillage system, perhaps because the influence of previous crop on soil physical properties does not undergo the modifications implicit in conventional tillage.

The effect of N fertilizer on SWC is only observed at wheat harvest, leaving more water at zero N fertilizer, over the whole profile, due to lower yield (López-Bellido et al., 2007).


   ACKNOWLEDGMENTS
 
We thank following for their excellent assistance in the laboratory and field work: Joaquín Muñoz, José Muñoz, and Auxiliadora López-Bellido. Our thanks are also expressed to ABECERA for providing the land and allowing us to use their field facilities. Special thanks are extended to INIA assisting with financial resources to conduct this long-term field experiment. This study was funded by the Spain's Plan Nacional I+D (Projects AGF95-0553, AGF97-0498, and AGL2000-0460).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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R. J. Lopez-Bellido, L. Lopez-Bellido, J. Benitez-Vega, and F. J. Lopez-Bellido
Tillage System, Preceding Crop, and Nitrogen Fertilizer in Wheat Crop: II. Water Utilization
Agron. J., January 1, 2007; 99(1): 66 - 72.
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