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SOIL & WATER MANAGEMENT & CONSERVATION

Management Effects on Soil Carbon Dioxide Fluxes under Semiarid Mediterranean Conditions

^a Dep. de Suelo y Agua, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones, Científicas (CSIC), POB 202, 50080 Zaragoza, Spain
^b Dep. de Producció Vegetal i Ciencia Forestal, Univ. de Lleida–IRTA, Rovira Roure 191, 25198 Lleida, Spain

^* Corresponding author (jalvaro.fuentes{at}gmail.com).

	ABSTRACT

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

 
Losses of soil organic carbon (SOC) have contributed to CO₂ emissions from soils to the atmosphere and to global climate change. We hypothesized that in semiarid agroecosystems of the Mediterranean region, a shift from the traditional management system (including conventional tillage [CT] and a cereal–fallow rotation) to a more conservative system, including no-till (NT) and continuous cropping, could reduce CO₂ emissions during the cropping season. Thus, in this study, we studied the effects of tillage and cropping systems on C inputs and soil CO₂ fluxes during three cropping seasons at three different sites in the Ebro River valley (northeast Spain). Carbon inputs ranged from 650 to 6000 kg ha^–1 and seasonal average CO₂ flux ranged from 0.10 to 1.76 g CO₂ m^–2 h^–1. Differences in rainfall led to marked differences in C inputs and soil fluxes among growing seasons. Although differences among tillage treatments were weak, CO₂ fluxes under NT were always lower. Intensification of cropping systems led to an increase in C input. A move from CT to NT together with cropping intensification is suitable to increase C inputs and to reduce soil CO₂ fluxes in semiarid Mediterranean agroecosystems.

Abbreviations: AG, Agramunt site • CF, cereal–fallow rotation • CT, conventional tillage • NT, no-till • PN, Peñaflor site • PN-CC, continuous cropping system at the Peñaflor site • PN-CF, cereal–fallow rotation at the Peñaflor site • SV, Selvanera site

	INTRODUCTION

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

 
Estimates of the total soil organic carbon (SOC) of the world is close to 1500 Pg (Eswaran et al., 1993; Batjes, 1996) and about 170 Pg is contained in agricultural soils (Paustian et al., 1997). In the last two centuries, SOC losses from agricultural soils have been estimated at about 54 Pg of C (Paustian et al., 1998). These C losses have environmental and productivity effects on agroecosystems. Thus, mineralization of SOC has contributed to large CO₂ emissions from soils to the atmosphere and to global climate change (Paustian et al., 2000). In addition, the depletion of SOC has been associated with a loss of fertility and thus with a loss of agroecosystem productivity (Bauer and Black, 1994).

Soil CO₂ emissions respond mainly to a concentration gradient from locations with higher to lower CO₂ concentration. This process may be accelerated or restrained depending on soil micrometeorological conditions and management practices. Several studies have concluded that soil temperature is the main variable affecting soil CO₂ emissions and that soil water content has little or no effect (Hendrix et al., 1988; Bajracharya et al., 2000; Frank et al., 2006).

Soil management practices, especially tillage, modify soil profile properties and thus soil CO₂ emissions. Tillage, especially moldboard plowing, stimulates soil microbial activity due to greater soil aeration than conservative tillage systems such as reduced tillage or, particularly, no-till, in which soil is not altered (Angers et al., 1993). At the same time, the breakdown of soil macroaggregates under intensive tillage systems leads to an increase on soil CO₂ emissions. Six et al. (1999) observed faster soil macroaggregate turnover under moldboard plowing than no-till and, thus, a greater release of labile organic matter previously protected from soil microbes within macroaggregates.

Several researchers have studied the role of soil tillage practices on soil CO₂ emissions and, in addition, on the soil C budget (Alvarez et al., 1995; Franzluebbers et al., 1995; Kessavalou et al., 1998). Reducing tillage intensity may lead to a decrease in SOC losses either by enhancing C inputs returned to the field (Alvarez et al., 1995) or, in contrast, by decreasing CO₂ emissions (Kessavalou et al., 1998; Curtin et al., 2000). At the same time, intensification of cropping systems may also lead to a decrease in soil C losses due to an increase in C inputs. In the semiarid regions of the Canadian prairies, the suppression of long fallowing in the rotation and the consequent switch from a cereal–fallow rotation to a continuous cereal system increased the soil C content due to greater crop residues returned to the soil (Curtin et al., 2000).

Cropping intensification may also influence soil CO₂ emissions. Jacinthe et al. (2002) observed greater soil CO₂ emissions when a greater amount of wheat (Triticum aestivum L.) residue was applied on the soil surface, probably due to a change in soil thermal properties. Curtin et al. (2000), under semiarid conditions, found greater CO₂ emissions under continuous wheat than the cropped phase of a wheat–fallow rotation.

In semiarid agroecosystems of the Ebro River valley (northeast Spain), the cereal–fallow rotation is a widespread cropping management system aimed to increase soil water content. In this area, intensive tillage with the use of moldboard plowing has also been a common traditional practice. Information about the impact of these management practices on soil CO₂ emissions in the Ebro River valley region is scarce. There are some studies comparing conventional tillage and reduced tillage in other Spanish areas with similar conditions (Sánchez et al., 2002, 2003). In these studies, however, neither the impact of no-till nor the intensification of cropping systems on soil CO₂ fluxes was evaluated. The objective of the present study was to determine the effects of tillage and cropping systems on C inputs and soil CO₂ fluxes under semiarid Mediterranean conditions. This study was performed during three consecutive cropping seasons in three long-term experiments located along the Ebro River valley.

	MATERIALS AND METHODS

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

 
Sites, Tillage, and Cropping Systems
This study was conducted during three cropping seasons, from November 2002 to June 2005, at three experimental sites located in the semiarid Ebro River valley region (northeast Spain). Sites, from higher to lower annual precipitation, were: Selvanera (475 mm), Agramunt (430 mm), and Peñaflor (390 mm). Selected site and soil characteristics are shown in Table 1 . Monthly precipitation and mean monthly air temperature recorded at the three experimental sites are presented in Table 2 .

In Agramunt (AG), the cropping system consisted of a barley–wheat rotation with four tillage treatments: CT, ST, RT, and NT. The CT treatment consisted of a moldboard plowing operation to a depth of 25 to 30 cm in October followed by a pass with a field cultivator to a depth of 15 cm. The moldboard plow consisted of three bottoms of 0.50-m width. The ST treatment consisted of subsoil tillage to a depth of 25 cm in October followed by a field cultivator to 15-cm depth. The RT treatment was implemented with one or two passes of a cultivator to 15-cm depth in October. The subsoiler and the cultivator had the same characteristics as that used in the SV site.

In Peñaflor (PN), two cropping systems were compared: a continuous barley cropping system (PN-CC) and a barley–fallow rotation (PN-CF). In the barley–fallow rotation, both phases of the rotation were represented at the field every season (PN-CF1 and PN-CF2). Three tillage systems were compared under both cropping systems: CT, RT, and NT. In the PN-CC system, the CT treatment consisted of moldboard plowing to a depth of 30 to 40 cm in November as primary tillage. The moldboard plow had the same characteristics as that used at the AG site. The RT treatment was implemented also in November by chisel plowing to a depth of 25 to 30 cm. The chisel plow consisted of five rigid shanks spaced 20 cm apart and a shank width of 5 cm. In the CT and RT plots, primary tillage was implemented every year in October, followed by a pass of a sweep cultivator to a depth of 10 to 15 cm as secondary tillage. In the PN-CF rotation, however, primary tillage was implemented in March every other year during the fallow phase of the rotation and secondary tillage in May with a cultivator pass to a depth of 15 to 20 cm. In both PN-CC and PN-CF, moldboard plowing in the CT plots was followed by a pass with a tractor-mounted scrubber consisting of a metal beam passed through the soil surface to break down large clods.

At all three experimental sites, for the NT treatment no tillage operations were done and a direct drill planter was used for sowing. In this treatment, soil was kept free of weeds by spraying a total herbicide (glyphosate [N-(phosphonomethyl)glycine]).

At all sites, tillage treatments were arranged in a randomized complete block design with three replicates in SV, PN-CC, and PN-CF and four replicates in AG. Plot size was 50 by 7 m at SV, 50 by 9 m at AG, and 33 by 10 m at PN-CC and PN-CF.

Soil Carbon Dioxide Fluxes
After sowing, soil CO₂ emissions were measured every 15 d from December 2002 to June 2005 at the PN site. At the SV and AG sites, measurements were taken once a month from December 2003 to June 2005, with the exception of the short fallow period (July–November 2004) when no measurements were made. Three measurements per plot were taken using an open-chamber system (Model CFX-1, PP Systems, Hitchin, UK) connected to an infrared gas analyzer (Model EGM-4, PP Systems). This system was based on the chamber designed by Rayment and Jarvis (1997), which was developed to ensure that atmospheric pressure fluctuations were transferred through to the soil surface. The soil CO₂ flux was calculated from the difference in CO₂ concentration between air entering and leaving the chamber. The chamber has a cylindrical diameter of 21 cm, covering a soil surface of 346 cm². Flow rate was adjusted to 900 mL min^–1. The chamber was inserted 3 cm into the soil to prevent CO₂ leaks to the atmosphere. The flux readings were taken 3 min after the chamber was inserted into the soil to avoid possible unrealistic values caused by the disturbance produced after placing the chamber into the soil (Pumpanen et al., 2004).

Daily measurements started at 1000 h and finished around 1200 h and were assumed to represent the average flux of the day (Kessavalou et al., 1998). Each plot was divided into two regions, and one measurement per region was taken each time. A whole week was used to measure the five experimental fields (one experimental field per day).

Carbon Inputs and Weather Data
The inputs of C were computed during three seasons at the PN site (2002–2003, 2003–2004, and 2004–2005) and during two seasons at SV and AG (2003–2004 and 2004–2005). The C inputs consisted of crop straw and dry root biomass at maturity.

After harvest, four soil cores (8-cm diameter by 30-cm depth) per plot (two in the row and the other two in the interrow) were collected from the top 30 cm of soil to measure the root biomass. Once at the laboratory, the soil cores were kept at 4°C until root–soil separation. Soil was washed over a 0.5-mm sieve specifically built up for this study to remove roots (Böhm, 1979). Roots separated from each soil core were transferred to an aluminum pan and weighed after oven drying for 48 h at 65°C.

Crop straw was measured before harvest. Crop plants from four 0.5-m-long rows per plot were hand harvested. The grain was removed from the plant and the straw was oven dried for 48 h at 65°C and weighed. Samples from dry straw and roots were ground and analyzed for C content.

Meteorological data were collected at the three experimental sites during the entire experimental period using automated weather stations and recorded by dataloggers (Model CR10, Campbell Scientific, Logan, UT).

Statistical analyses of data were performed using SAS (SAS Institute, 1990). Analyses of variance (ANOVA) were applied to compare tillage treatments, and differences between means were tested with Duncan's multiple range test.

	RESULTS AND DISCUSSION

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

 
Weather Conditions and Carbon Inputs
During the experimental period, total precipitation recorded was highly variable among and within growing seasons (Table 2). At PN during the 2002–2003 growing season (1 December–31 May), 244 mm of precipitation was recorded. At the same site, however, during the 2004–2005 growing season, 113 mm was recorded, with >50% of this rainfall received from April to June (Table 2). At SV and AG during the 2003–2004 growing season, 65% more precipitation was recorded than during the 2004–2005 season (Table 2). In the study area, the general rainfall pattern during the growing season was characterized by an especially wet December (i.e., at AG during December 2004, 50% of the total precipitation received during the 2004–2005 growing season was recorded) followed by a dry winter (i.e., at SV and AG from January to March 2005, 14 and 4%, respectively, of the total precipitation received during the 2004–2005 growing season was recorded) and a wet spring (i.e., at PN during April and May 2005, the 40% of the total precipitation received during the 2004–2005 growing season was recorded). Mean monthly air temperature was similar among sites and growing seasons, varying from 7.8°C during the 2003–2004 growing season at SV to 10.4°C during the 2002–2003 growing season at PN (Table 2).

The noteworthy rainfall variability led to differences in C inputs among growing seasons (Tables 3 and 4 ). For example, at the AG site during the 2003–2004 growing season, the average of C inputs in the four tillage treatments was 4407 kg ha^–1 (Table 3). During the following growing season, however, C inputs dropped to 945 kg ha^–1 (average of the four tillage treatments) (Table 3). Compared with the small grain crop, the greater straw production of the rapeseed crop during the 2004–2005 growing season led to higher C inputs and, thus, to lower differences between the 2003–2004 and 2004–2005 growing seasons (Table 3). In PN-CC, C inputs in the 2004–2005 season were 70% lower than in the 2003–2004 season (average of the three tillage treatments) (Table 4). This difference is explained by the different rainfall amounts received during the two growing seasons (244 vs. 113 mm in 2003–2004 and 2004–2005, respectively) (Table 2). In the semiarid Ebro River valley, crop growth and yields are highly dependent on seasonal rainfall, as found in previous studies (Cantero-Martínez et al., 1995; Austin et al., 1998; Moret et al., 2007).

The intensification of the cropping systems produced greater C inputs (Table 4). The inclusion of a fallow phase in the rotation resulted in a crop every 2 yr and, consequently, in a decrease in the C inputs. During the 2002–2003 and 2003–2004 growing seasons, the total C input at PN-CC was 7120 kg ha^–1 (average of the three tillage treatments). For the same period of time in the cereal–fallow (CF) rotation, however, C inputs averaged 4451 and 3565 kg ha^–1 at PN-CF1 and PN-CF2, respectively (Table 4).

Carbon Dioxide Fluxes
Soil CO₂ fluxes under different tillage treatments and cropping systems are shown in Fig. 1 to 3. Emissions of CO₂ were generally <2 g CO₂ m^–2 h^–1, although higher values were measured during spring 2004 at the three sites (peaks of 2.7, 2.8, and 4.6 g CO₂ m^–2 h^–1 at PN-CC, AG, and SV, respectively) due to the high spring rainfall. From March to May 2004, 188, 174, and 133 mm of rainfall was collected at SV, AG, and PN-CC, respectively (Table 2). This precipitation accounted for >60% of the total precipitation received during the 2003–2004 growing season. Several studies have concluded that rainfall induces soil CO₂ fluxes (Rochette et al., 1991; Akinremi et al., 1999; Parkin and Kaspar, 2004) due to the displacement of the CO₂–rich soil atmosphere produced by water filling the soil pores followed by an increase in microbial activity due to favorable micrometeorological soil conditions for microbial decomposition (Akinremi et al., 1999; Emmerich, 2002). In our study, no relationship was obtained between soil CO₂ flux and surface soil water content (0–5 cm) (data not shown). A low effect of soil water content on soil CO₂ emissions has also been reported by other researchers (Hendrix et al., 1988; Frank et al., 2006). At the same time, high rainfall during spring stimulated crop growth, leading to higher root respiration measured by the soil chamber. Unfortunately, the surface chamber methods do not differentiate between heterotrophic-derived CO₂ and root-derived CO₂, limiting the value of these techniques for evaluation of the soil as a source or sink of atmospheric CO₂ (Kuzyakov, 2006).

View larger version (11K): [in this window] [in a new window]   Fig. 1. Soil CO2 fluxes as influenced by tillage (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till) from November 2003 to June 2005 at the Selvanera (SV) and Agramunt (AG) sites. Bars represent LSD (P < 0.05) for comparison among tillage treatments where significant differences were found.

View larger version (11K): [in this window] [in a new window]   Fig. 2. Soil CO2 fluxes as influenced by tillage (CT, conventional tillage; RT, reduced tillage; NT, no-till) from November 2002 to June 2005 in the continuous cropping system at the Peñaflor site (PN-CC). Bars represent LSD (P < 0.05) for comparison among tillage treatments where significant differences were found.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Soil CO2 fluxes as influenced by tillage (CT, conventional tillage; RT, reduced tillage; NT, no-till) from November 2002 to June 2005 in the cereal–fallow rotation at the Peñaflor site (PN-CF1 and PN-CF2). Bars represent LSD (P < 0.05) for comparison among tillage treatments where significant differences were found.

View this table: [in this window] [in a new window]   Table 5. Effect of tillage treatment and cropping system on mean soil CO2 fluxes during different cropping seasons at the three experimental sites.

Intensification of cropping systems led to an increase in soil CO₂ fluxes (Table 5). During the 2003–2004 cropping season in PN-CC, the seasonal mean CO₂ flux, as an average of the three tillage treatments, was 1.36 g CO₂ m^–2 h^–1. The same average calculated in the cropped phase of PN-CF1 was 1.06 g CO₂ m^–2 h^–1. In the same experimental plots, López et al. (2005) observed an 80 to 90% decrease in the crop residue cover during the fallow phase of the barley–fallow rotation. Consequently, the amount of residues on the soil surface during the crop phase of the CF rotation was low. In the continuous cropping (CC) system, a substantial fraction of residues from the previous crop was not decomposed at sowing. As a result, lower soil CO₂ was emitted during the crop phase of the CF rotation than with the CC system. During the 2004–2005 cropping season, however, the suppression of the fallow phase from the rotation did not lead to greater soil CO₂ flux compared with CF (Table 5). The low rainfall registered during the 2004–2005 season (113 mm from December 2004 to June 2005), probably led to a limitation in the activity of soil microbes and, thus, to less difference in soil CO₂ between cropping systems. In the CF rotation, greater soil CO₂ flux in the cropped phase than in the fallow phase was attributed to the absence of root respiration during fallow. It is known that fallow leads to more favorable moisture conditions for microbial decomposition (Grant, 1997; Paustian et al., 2000). Under our conditions, however, fallowing is not an efficient practice to increase soil water storage in the study area (Moret et al., 2006).

	CONCLUSIONS

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

 
In semiarid Mediterranean agroecosystems, crop residue production has a strong dependence on seasonal precipitation. In these areas, precipitation is low and highly variable from season to season. Results from this study indicate that rainfall variability influenced the amount of C input returned to the soil in each growing season. On the other hand, C input was less affected by tillage. In dry seasons, however, NT led to slightly greater crop biomass and greater C inputs into the soil compared with CT, due probably to the greater soil water storage. Likewise, soil CO₂ fluxes showed greater differences among cropping seasons than among tillage treatments. As observed for C inputs, the greatest soil CO₂ fluxes were measured during cropping seasons with high rainfall. Despite the fact that differences among tillage treatments were low in general, NT always showed the lowest CO₂ fluxes.

Long fallowing in the cereal–fallow rotation led to a decrease in C returned to the soil. At the same time, this cropping system reduced measured soil CO₂ fluxes compared with continuous cropping, mainly because of the absence of root respiration during the fallow phase.

Our results suggest that a move from a CT system to a NT system together with a suppression of the long fallow period from the rotation are suitable to increase C inputs to the soil and, at the same time, to reduce soil CO₂ fluxes in semiarid Mediterranean agroecosystems.

	ACKNOWLEDGMENTS

 
The field and laboratory assistance of Sofía Alcrudo, María Josefa Salvador, Ricardo Gracia, Silvia Martí, and Carlos Cortés is gratefully acknowledged. This research was supported by the Comisión Interministerial de Ciencia y Tecnología of Spain (Grants AGL2001-2238-CO2-01 and AGL 2004-07763-C02-02) and the European Union (FEDER funds). J. Álvaro-Fuentes was awarded an FPI fellowship by the Spanish Ministry of Science and Education.

	NOTES

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

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication September 5, 2006.

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