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Published online 3 January 2006
Published in J Environ Qual 35:3-13 (2006)
DOI: 10.2134/jeq2005.0050
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Changes in Soil Organic Carbon Contents and Nitrous Oxide Emissions after Introduction of No-Till in Pampean Agroecosystems

Haydée S. Steinbach* and Roberto Alvarez

Facultad de Agronomía-Universidad de Buenos Aires. Av. San Martín 4453 (C1417DSE) Buenos Aires, Argentina

* Corresponding author (steinbac{at}agro.uba.ar)

Received for publication February 10, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We reviewed published results to estimate no-till effects on SOC and denitrification in the Argentine Pampas and the potential of no-till to mitigate the global warming effect. On an equivalent mass basis, 42 paired data sets were used for SOC comparisons of no-till vs. plow till (moldboard plow or disk plow), 18 paired data for comparison of no-till vs. reduced till (chisel plow or harrow disk), and 20 paired data for comparison of plow till vs. reduced till. Twenty-six denitrification data sets were used for evaluation of tillage system and fertilization effects on N2O emission. Changes in SOC under no-till were not correlated to time since initiation of experiments. Averaged over years a 2.76 Mg ha–1 SOC increase (P = 0.01) was observed in no-till systems compared with tilled systems, but no differences were detected between plow and reduced till. The SOC under tillage explained most of the SOC variation under no-till (R2 = 0.94, P = 0.01). The model had a positive intercept and predicted a relatively higher increase of SOC in areas of low organic matter level. The conversion of the whole pampean cropping area to no-till would increase SOC by 74 Tg C, about twice the annual C emissions from fossil fuel consumption of Argentina. Emissions of N2O were greater under no-till with a mean increase of 1 kg N ha–1 yr–1 in denitrification rate for humid pampean scenarios. The increased emissions of N2O might overcome the mitigation potential of no-till due to C sequestration in about 35 yr, and therefore no-till might produce global warming.

Abbreviations: NT, no-till • PT, plow till (moldboard plow or disk plow) • RT, reduced till (chisel plow or harrow disk) • SOC, soil organic carbon • {Delta}SOC, differences in soil organic carbon between tillage systems


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
CROPLAND CONVERSION from tilled systems to conservation tillage systems that minimize or eliminate tillage operations have the potential to increase soil organic carbon (SOC) and therefore mitigate global warming (Freibauer et al., 2004; Lal, 2004; West and Post, 2002). However, N2O soil emissions in no-till systems are generally higher compared with tilled systems (Mummey et al., 1998). This trace gas has a large impact on mitigation potential because 1 kg N2O–N produces the warming effect of 120 kg CO2–C (Houghton et al., 2001). Overall approaches that incorporate N2O emission estimations in the evaluation of the mitigation potential of no-till have shown that the impact of no-till may be strongly reduced (Smith et al., 2000).

The Pampas is a vast plain of around 50 Mha, which runs from 28° to 40° S lat in Argentina (Alvarez and Lavado, 1998). The relief is flat or slightly rolling and natural vegetation consist of grasslands in which graminaceous species are dominant. Mean annual rainfall ranges from 200 mm in the west to 1200 mm in the east and the mean annual temperature ranged from 14°C in the south to 23°C in the north. Agriculture is performed in the semiarid and humid portions of the region on well drained soils, mainly Mollisols formed on loess-like materials, and areas with hydromorphic soils are devoted to pasture (Hall et al., 1992). Nearly 50% of the area is devoted to agriculture with soybean [Glycine max (L.) Merr.], corn (Zea mays L.), and wheat (Triticum aestivum L.) as the main crops (Hall et al., 1992). The region is considered one of the most suitable areas for grain crop production in the world (Satorre and Slafer, 1999). Many cropland soils in the Pampas had been depleting SOC and degrading because of the sustained use of conventional tillage practices for more than a century of cultivation (Alvarez, 2001). This phenomenon led to public institutions studying tillage system effects on soils (Buschiazzo et al., 1996; Panigatti et al., 1998), and to the widespread adoption of conservation tillage systems by many farmers. Since 1990 an exponentially increased use of no-till has occurred, with around 50% of agricultural lands now cropped under this management (INDEC, 2003). This evolution may be ascribed to economics and to concern about soil erosion and fertility. Farmers usually think that SOC will increase under no-till, but the magnitude of these increases has not been well defined in the region. Possible effects of no-till on denitrification and N2O emissions are also unknown, and the climate change mitigation potential of the practice has not been assessed.

Although many studies have addressed tillage effects on SOC in the Pampas, results have not been integrated. Therefore, the impact of reducing or eliminating tillage on soil C stocks is not known. Some results are available, reporting changes in N2O emissions under no-till in comparison to plowed soils, but they are also sparse. The objective of our review was to estimate the overall global warming mitigation potential of no-till in the Argentine Pampas region through it effects on SOC balance and N2O emissions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
We compiled the results from 17 field experiments assessing tillage systems effects on SOC at 14 different sites across the Pampas (Fig. 1 , Table 1). Selection criteria were: (i) experimental designs were clearly defined and experiments were performed using machinery and practices commonly used by farmers; (ii) only tillage system varied; (iii) sampling depth was equal to or deeper than tillage depth; and (iv) SOC mass per unit area was reported or SOC concentration and soil bulk density were available so that SOC mass could be calculated to the depth of sampling (fixed depth). It was assumed that there were no effects of tillage system below sampling depth. After selection by these criteria, 53 paired data sets for comparison of no-till vs. plow till (moldboard plow or disk plow) for samplings performed at fixed depths, 20 for comparison of no-till vs. reduced till (chisel plow or harrow disk) and 30 for comparison of reduced till vs. plow till were obtained. As differences in soil bulk density between treatments lead to comparison of SOC in unequal masses of soil, results were also calculated on an equivalent mass basis (Davidson and Ackerman, 1993). By applying this correction, 42 paired data remained for comparison of no-till vs. plow till, 18 for comparison of no-till vs. reduced till, and 20 for comparison of reduced till vs. plow till.



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  		Fig. 1. Map of the Pampean Region with four main provinces delimited and location of field experiments.

 

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  		Table 1. Set of data from soil organic carbon (SOC) field experiments performed in the Pampean Region of Argentina under contrasting tillage systems. Sources of bulk density data when not informed in listed papers are shown in footnotes.

 
Mean annual temperature and rainfall at the experimental sites were calculated using unpublished data from the Servicio Meteorológico Nacional (1970–1990 period). As differences in SOC between tillage treatments could be affected by rotation and residues input to the soil (West and Post, 2002), a discrete cropping index was developed to categorize net primary productivity of different rotations as another independent variable for regression analysis. The index integrated the number of crops per year and the percentage of corn in the rotation. It was assumed that two crops per year produced double the mass of residues of one crop per year, and that corn produced twice the residue of other crops. Consequently, the cropping index varied from 1 for the wheat–wheat rotation to 2 in a double crop sequence wheat/soybean. Results from field experiments assessing tillage systems and N fertilizer rate effects on N2O emissions were also integrated (Table 2). Twenty-six data sets were obtained from sites located in the humid portion of the Pampas (ca. 900 mm rainfall) on fine-textured–rich organic matter soils. Regression methods were used to contrast denitrification rates between tillage systems and N rates.


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  		Table 2. Set of data from denitrification experiments performed in the Pampean Region of Argentina under contrasting tillage systems.

 
Using a methodology previously described (Alvarez and Lavado, 1998), an inventory of SOC contents in the upper 20 cm of soil profiles was performed for estimating possible changes of SOC of pampean soils under different tillage systems. Data used were taken from soil surveys of the main agricultural areas of the provinces of La Pampa, Buenos Aires, and Santa Fe, performed before the widespread adoption of no-till in the Pampas (INTA et al., 1980; INTA and MAGPSF, 1981, 1983; INTA, 1989). The agricultural surface of the surveyed area approached 26 Mha and was divided into 10 geographic units according to geomorphological and soil classification considerations previously defined (INTA et al., 1980; INTA, 1989). On the basis of the soil profile characteristics described in soil surveys and their corresponding area, the weighed average values of organic C was calculated for each geographical unit.

Paired t tests on groups of paired treatments were used to compare the effects of tillage on SOC or soil bulk density (P = 0.01). Simple and multiple regression analyses were used to analyze differences between stored SOC in tillage treatments or N2O emission rates. Linear (x) and curvilinear (x2) effects were tested, and also interactions between independent variables (x1 x x2, etc.) by using the common surface regression model. Forward, backward, and stepwise methods were employed for selection of variables with P = 0.05. The discrete variable, cropping index, was tested as another independent variable, along with numeric variables: texture, temperature, and rainfall. An F statistics with P = 0.01 was used to determine the significance of the regression models. Intercepts and slopes of observed vs. predicted values were compared simultaneously with 0 and 1 using the F test (P = 0.01).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
An overall significant increase over years of 4.35 and 4.15 Mg C ha–1 was observed in no-till compared with plow till and reduced till, respectively (Fig. 2a ). On the other hand, reduced till had 1.21 Mg C ha–1 greater SOC than plow till (Fig. 2a). Soil bulk densities averaged 1.29, 1.27, and 1.25 Mg m–3 in no-till, reduced till, and plow till, respectively. Consequently, no differences between reduced and plow till were observed on SOC estimated on equivalent mass basis (Fig. 2b); however, no-till had on average, 2.76 Mg ha–1 significantly greater SOC contents than either reduced or plow tilled soils. In 13 cases (22%) SOC was lower under no-till than under tillage, in 4 cases (7%) equal and in 43 cases higher (71%). Changes in SOC ({Delta}SOC = SOC no-till – SOC tilled treatments) estimated on equivalent mass were closely correlated to {Delta}SOC calculated for fixed depths (Fig. 3 ). During the initial years from implementation of no-till (ca. 3), very small changes in SOC were observed between tillage systems (equivalent mass basis). The {Delta}SOC varied between –3 and 15 Mg C ha–1 without any significant relationship with time. Because there were not enough available data of sequential sampling in each experiment for calculation of {Delta}SOC evolution rate with time, an approximation was achieved by calculating the {Delta}SOC/time ratio for each measurement (Fig. 4a , 4b). This approximation showed that {Delta}SOC/time tended to be null during the initial years after the introduction of no-till and that C was sequestrated mainly between Years 4 and 9 of the experiments. After 10 yr {Delta}SOC/time approached zero, suggesting very small changes of SOC. A mean sequestration rate of 460 kg C ha–1 yr–1 was estimated in the 4 to 9 yr period from initiation of no-till.



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  		Fig. 2. Differences in soil organic C between tillage systems ({Delta}SOC) from experiments conducted in the Argentine Pampa Region related to experiment duration. (a) {Delta}SOC calculated for fixed depth, (b) {Delta}SOC calculated on an equivalent mass basis. {Delta}SOC = NT – PT (solid circles) denotes SOC differences between no-till and plow till. {Delta}SOC = NT – RT (open circles) denotes SOC differences between no-till and reduced till (chisel and disk till).

 


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  		Fig. 3. Relationship between changes in SOC ({Delta}SOC = SOC no-till – SOC tilled treatments) estimated on equivalent mass basis and changes in SOC ({Delta}SOC) C estimated for fixed depths, from experiments conducted in the Argentine Pampa Region. {Delta}SOC NT – PT (solid circles) denotes SOC differences between no-till and plow till. {Delta}SOC NT – RT (open circles) denotes SOC differences between no-till and reduced till (chisel and disk till).

 


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  		Fig. 4. Annualized soil organic C differences related to experiments duration from experiments conducted in the Argentine Pampa Region. (a) {Delta}SOC calculated for fixed depth, (b) {Delta}SOC calculated on an equivalent mass basis. {Delta}SOC NT – PT (solid circles) denotes SOC differences between no-till and plow till. {Delta}SOC NT – RT (open circles) denotes SOC differences between no-till and reduced till (chisel and disk till).

 
Soil organic C under no-till could be very well predicted using SOC under tilled treatments as an independent variable, both for fixed depth (Fig. 5a ) or on an equivalent mass basis (Fig. 5b). In both regression models intercepts were significantly greater than zero, indicating that relative increases of SOC were higher in low SOC soils from semiarid environments than in rich ones located in humid regions. The regression model in Fig. 5b (equivalent mass basis) estimates increases of around 15% in SOC under no-till for soils with 20 Mg C ha–1 (0–20 cm), and around 5% in soils with 80 Mg C ha–1 (Fig. 6 ). Average SOC levels of pampean soils for some of the main cropping areas decrease from 72 Mg C ha–1 in the humid east to 24 Mg C ha–1 in the semiarid west (Fig. 7a ). Soil organic C increases in no-till compared with reduced and plow till ranged from 3% in the east to 13% in the west (Fig. 7b). Total amount of C that may be sequestered by full adoption of no-till in the Pampas was estimated to be 74 Tg C. This amount is about twice the country's estimated 40 Tg C yr–1 (CIA World Factbook, 2004) annual C emissions related to fossil fuel consumption.



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  		Fig. 5. Relationships of soil organic C (SOC) between no-till and plow or reduced till, from experiments conducted in the Argentine Pampa Region, calculated for fixed depth (a) and on an equivalent mass basis (b). PT (solid circles) denotes plow till, and RT (open circles) denotes reduced till (chisel and disk till).

 


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  		Fig. 6. Relative change in soil organic C (SOC) of no-till to SOC in plow till from experiments conducted in the Argentine Pampa Region. Data were estimated using equations fitted in Fig. 5. NT, no-till; PT, plow till.

 


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  		Fig. 7. (a) Map of the Argentinean Pampean Region with areas delimited by different soil organic C contents in the 0- to 20-cm depth (Mg ha–1) calculated from soil survey performed before no-till introduction. (b) Potential relative change in soil organic C by adoption of no-till (%) calculated on an equivalent mass basis using equations from Fig. 5.

 
Although few experiments assessed tillage effects on denitrification as the only source of variation, a good relationship between N2O emissions in plow till and no-till was found (Fig. 8a ). The regression had an intercept not different from zero and shown that emissions of N2O were fivefold greater under no-till than in plowed soils. These differences were reported during the initial stages of growth of wheat and corn, when soil nitrate contents were high, disappearing further into the growing cycle. Greater denitrification rates were also reported when N fertilizer was applied to the crops. Regressions fitted to results from different fertilization experiments had similar slopes and a significant general fit could be performed to all results but those with low R2 (Fig. 8b). On average, 0.022 kg N2O–N was emitted by each kilogram of fertilizer N applied.



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  		Fig. 8. (a) Relationship between N denitrified between no-till and plow till and (b) relationship between N denitrified and N rate from experiments conducted in the Argentine Pampa Region.

 
Crops under no-till usually receive more N fertilizer in pampean agroecosystems (Alvarez et al., 2000). Consequently, two different effects of no-till adoption on denitrification may be expected, the direct impact of no-till on N2O emission and the effect of higher N rates. In the humid portion of the Pampas wheat and corn usually receive around 20 kg fertilizer N ha–1 more than under plow tillage. The mitigation potential of no-till was estimated under a hypothetical scenario where C sequestration in soil occurred at a rate of 460 kg C ha–1 yr–1 during the 4 to 9 yr period from introduction, and where N2O emission increases 1 kg N2O–N ha–1 yr–1. This quantity was estimated for the common double crop wheat/soybean–corn–soybean rotation. It was assumed no differences in denitrification are produced during the soybean crop (because of no available data for the crop) and that N fertilizer rates are increased 20 kg N ha–1 in graminaceus crops. The impact of N on SOC was estimated as a 1 kg C ha–1 increase by 1 kg N ha–1 applied, using a regression model adapted to humid pampean scenarios (Alvarez, 2005). A C saving of 31 kg C ha–1 yr–1 due to reduced fuel consumption (West and Marland, 2002) was also assumed. Given these assumptions, the C mitigation potential of no-till is strongly reduced when estimates are extrapolated for a long time period (Fig. 9 ). After an initial phase of C sequestration in soil, when no-till would engender climate mitigation, the increases in N2O emissions might counteract this effect and lead to global warming beyond 35 yr from adoption.



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  		Fig. 9. Carbon mitigation potential under no-till in the Argentine Pampa Region. Estimations were performed assuming a C sequestration of 460 kg C ha–1 yr–1 under no-till between Years 4 to 9 from installation, a C saving of 31 kg C ha–1 yr–1 due to lower fuel consumption and an increase of 1 kg C soil kg–1 N fertilizer applied, with rates of 20 kg N ha–1 greater for corn and wheat under the wheat/soybean–corn–soybean rotation. An increase of 1 kg N–N2O emitted by denitrification under no-till was also assumed.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Although our results are in agreement with the general findings that no-till systems increase SOC compared with conventional tillage practices (Kern and Johnson, 1993; West and Post, 2002), there are some divergences about the SOC sequestration process and its global warming mitigation potential. Previous estimates of the effect of changing from plow till to no-till on SOC sequestration rate ranged from 325 to 480 kg C ha–1 yr–1 (Six et al., 2002; West and Post, 2002), with overall SOC sequestration potential ranging between 2.9 and 12 Mg C ha–1 when reaching a plateau (Alvarez, 2005; Paustian et al., 1997; Six et al., 2002). The SOC sequestration rate under no-till has been described as an S-shape phenomenon related to time, which reaches steady state conditions around 15 to 25 yr (Alvarez, 2005; West and Post, 2002). Other reports had described a linear tendency of the sequestration processes (Six et al., 2002) or that sequestration is not time related (Paustian et al., 1997). Climate, soil texture, and rotation did not improve the variance accounted for in the SOC sequestration process (Alvarez, 2005; Paustian et al., 1997; West and Post, 2002). In the Pampas, the SOC sequestration rate under no-till is within the ranges previously observed but with low SOC sequestration potential, and without a clear association with time since its adoption. Soil texture and rotation are not correlated with SOC sequestration. The greater relative impact of no-till on SOC in semiarid areas seems to be a consequence of higher soil water contents in this tillage system, leading to yield increases and greater amounts of residues returned to the soil in water-limited scenarios (Bono et al., 2004). Results from pampean experiments showed that estimations of SOC sequestration can vary deeply depending on the way data are calculated. No-till SOC sequestration potential was reduced on average by 40% when {Delta}SOC was calculated on equivalent mass basis compared with estimations based on fixed depths, with greater differences between both estimation procedures in the low range of {Delta}SOC values (50%) than in the high range (25%). This is the consequence of the increase in soil bulk density under no till that has a greater impact on small {Delta}SOC estimations. The potential of the region to sequester C is low and in many areas would not exceed 6% of soil C stocks in the 0- to 20-cm stratum. No-till leads to C savings of 24 to 61 kg C ha–1 yr–1 by reducing fuel consumption (Borin et al., 1997; Kern and Johnson, 1993; West and Marland, 2002) and higher C costs of 6 to 46 kg C ha–1 yr–1 from greater inputs of fertilizers and herbicides (West and Marland, 2002). An overall net saving of 31 kg C ha–1 yr–1 under no-till has been estimated in the USA (West and Marland, 2002), and this process continues after net C sequestration in soil ceases. This estimation was used as a gross approximation for C savings under no-till in the Pampas because there are no available local data. The main crops in the USA are corn, wheat, and soybean, cropped under rain-fed conditions on Mollisols, as in Argentina. Nitrogen fertilization increases SOC levels (Glendining and Powlson, 1991; Paustian et al., 1997) by increasing residues returned to the soil. Consequently, an estimation of the effects of N fertilizer on SOC was included in the mitigation potential estimation.

Emissions of N2O are greater under no-till due to soil compaction (Smith et al., 2000), a phenomenon that operates against the mitigation potential of the greenhouse effect produced by SOC sequestration. A two- to threefold increase in seasonal or yearly N2O emissions under no-till have been reported in many sites (Smith and Conen, 2004). In the Pampas, some of the studies reviewed shown higher N2O emissions under no-till that may be ascribed not only to soil compaction but to greater soil water contents in no-till. Denitrification is more intense as water-filled pore space increases (Sainz Rozas et al., 2001) and pampean soils generally had greater water content when cropped under no-till systems (Buschiazzo et al., 1996; Panigatti et al., 1998). Higher N fertilizer rates used in no-till agro-ecosystems may lead to higher N2O emissions (Mosier, 1998). Results from the Pampas also show increases in denitrification rates as fertilizer N increases. In our approach, we intended to include both effects on the estimation of N2O emitted, but results are rather uncertain. Available information in the Pampas suggests that after SOC sequestration in soil reaches equilibrium, C mitigation potential by adoption of no-till would be negated by associated N2O emissions.

Using the IPCC methodology it has been proposed that adoption of no-till will mitigate climate change both in the Pampas (Viglizzo et al., 2002) and in Brazil (Cerri et al., 2004) by accumulation of SOC. The Intergovernmental Panel on Climate Change (IPPC) methodology (IPCC, 1997) accounts for an estimated SOC rise of 10% with conversion from conventional tillage systems to no-till but it does not take into account increases in N2O emissions (Mosier et al., 1998). Our results showed an average SOC increase lower than the IPPC estimate (7%) but, more important, they also suggest that higher N2O emissions in no-till managed agroecosystems of the humid portion of the Pampas might counteract SOC sequestration in several decades, leading to global warming. As these estimations were performed extrapolating beyond the time frame of the data set used, they must be taken into account with caution. Future work must focus on this problem. Research is also needed on many other possible effects of no-till on environmental quality such as decreased soil erosion and improved water quality.


   ACKNOWLEDGMENTS
 
This study was supported by a grant UBACYT G033 and G004 from the University of Buenos Aires. We acknowledge three unknown reviewers that greatly improve the original version of the manuscript.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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