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a Department of Soil Science, Federal University of Santa Maria, Zip Code 97119-900, Santa Maria, RS, Brazil
b Department of Soil Science, Federal University of Rio Grande do Sul, Zip Code 90001-970, Porto Alegre, RS, Brazil
c Agricultural Research and Rural Extension Organization for the State of Santa Catarina (Epagri), Zip Code 88034-901, Florianópolis, SC, Brazil
d Center of Experimentation and Research Fundacep, 98100-970, Cruz Alta, RS, Brazil
* Corresponding author (tamado{at}smail.ufsm.br)
Received for publication June 10, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CPI, carbon pool index • CT, conventional tillage • NT, no tillage • POC, particulate organic carbon • POCPI, particulate organic carbon pool index • RT, reduced tillage • SOM, soil organic matter • TLCC, tropical legume cover crop • TOC, total organic carbon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Southern Brazil had a very quick shift in land use from forest and native grassland to agriculture. In 1969, the cropland area in the state of Rio Grande do Sul was 0.8 million ha, while it reached 4 million ha in 1977 (Mielniczuk, 2003). During that time, southern Brazil cropland relied on a soil management system based on intensive tillage (two plows + four to six disks per year), soybean–wheat double-cropping system, wheat straw burning, and large areas with winter fallow. This soil management system caused severe soil degradation due to widespread erosion. In the 1970s, it was estimated that for each 1 kg of soybean harvested in southern Brazil, approximately 10 kg of fertile topsoil were eroded (Gianluppi et al., 1979; Mielniczuk, 2003).
To counteract this unsustainable management, the farmers of southern Brazil started to adopt NT practices. Farmers adapted NT to tropical and subtropical environments mainly by increasing the amount of straw by using winter cover crops and summer crop rotation to compensate for the rapid residue decomposition. The farmers called this system "NT in the straw" due to the high amount of mulch left on the soil surface at seeding time. In the mid-1980s, NT was rapidly adopted in southern Brazil, achieving 80% of total cropland (approximately 10 million ha) in 2004–2005. From the 1970s to the mid-1980s, this region was estimated to be a source of carbon dioxide to the atmosphere, but it has transformed since then into a major C sink thanks to the adoption of NT and other soil conservation practices (Mielniczuk et al., 2003; Bayer et al., 2006b).
In addition to favorable conditions for biological decomposition of SOM, southern Brazil has a well-distributed rainfall pattern through the year allowing for the development of intensive cropping systems, with three or even four crops per year, that can result in high addition of crop residues to the soil surface (Mielniczuk et al., 2003; Bayer et al., 2006b). These systems increase the input of biomass C to the soil and, consequently, lead to the accumulation of C in soils under conservation tillage systems as verified in tropical Cerrado (Bayer et al., 2000a, 2006b; Amado et al., 2001) and temperate soils (Lal et al., 1999).
No-till improves physical protection of SOM within soil aggregates. Particulate fractions of SOM occluded into soil aggregates have longer turnover times than free fractions (Feller and Beare, 1997; Lal et al., 1999; Six et al., 1998, 1999, 2000). However, few studies have been performed in soils from southern Brazil. Variable charge minerals could improve this mechanism of SOM stabilization and as a consequence contribute to C accumulation in NT soils (Parfitt et al., 1997; Hassink and Whitmore, 1997; Balesdent et al., 2000; Baldock and Skjemstad, 2000). In general, the less oxidative environment in NT soils increases the lability of SOM, as evidenced by spectroscopic techniques (Bayer et al., 2000a, 2001) and by the ratio between labile and non-labile pools of SOM (Diekow et al., 2005b).
In this study four of the oldest NT experiments in the southern Brazil were sampled to evaluate the effect of crop rotation and cover crops on the enhancing the potential of organic C accumulation and the lability of SOM in soils with different texture and mineralogy. Further, the physical fractionation of SOM was performed in a sandy clay loam soil to evaluate a magnitude of the physical protection as a mechanism of SOM stabilization in NT soils.
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MATERIAL AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The third experiment was performed at the Center of Experimentation and Research Fundacep in Cruz Alta (RS) for 19 yr (1985–2004). The soil is a clay Rhodic Hapludox with a clay content of 570 g kg–1 in its upper horizon. It is referred here as a clay Hapludox. The average precipitation is 1727 mm yr–1, without dry season. The average annual temperature is 19.2°C. The climatic classification is wet subtropic Cfa in Koeppen classification (Koeppen, 1948). The experiment was performed in 60- x 40-m plots, which were split in three replications of 30 x 13.3 m to soil sampling. The experimental design is a randomized block, considering the subsamples in the plots as replications. The treatments selected were: (i) wheat–soybean under CT, (ii) wheat–soybean under NT, (iii) black oat–soybean–black oat + vetch–maize–radish oil (Raphanus sativus L.)–wheat–soybean, designated as Intensive Cropping System II under CT, (iv) Intensive Cropping System II under NT, and (v) native grass. The experiment was amended with lime and fertilized with N, phosphorus, and potassium following soil analysis. Campos et al. (1995) reported results for this field experiment.
The fourth experiment was performed at the Epagri Experimental Station in Campos Novos (SC) for 7 yr (1994–2001). The soil is a clay Hapludox with a clay content of 760 g kg–1 in its upper horizon. The soil is referred herein as a high clay Hapludox. The average precipitation is 1964 mm yr–1, without dry season. The average annual temperature is 16.5°C. The climatic classification is wet temperate Cfb in Koeppen classification (Koeppen, 1948). The experimental design is a randomized block with 6- x 6-m plot size and three replicates. The treatments selected were: (i) triticale (Triticosecale spp. Wittm)–rye (Secale cereale L.)–soybean–common vetch–maize–black oat–black bean–buckwheat (Fagopyrum esculentum Moench) or radish oil, designated as Intensive Cropping System III, with residues removed under CT, (ii) Intensive Cropping System III under CT, (iii) Intensive Cropping System III under NT, and (iv) native grass. The experiment was fertilized with N, phosphorus, and potassium following soil analysis. Veiga et al. (2000) showed the experimental data of this field.
Soil Sampling
Soil samples for organic C analysis were manually collected in the following depth increments: 0 to 2.5, 2.5 to 5.0, 5.0 to 7.5, 7.5 to 10.0, 10.0 to 15.0, and 15.0 to 20.0 cm. Soil samples were air dried at room temperature, crushed with a wood roll to pass through a 2-mm open mesh sieve, and stored in plastic pots. Only the results from 0- to 5- and 0- to 20-cm layers will be presented in this paper.
Soil sampling to densimetric fractionation of SOM was performed only at the 0- to 5-cm depth. The soil was sampled in cores, manually disturbed to pass through a 9.51-mm open mesh sieve, air dried, and stored in plastic pots.
Analysis
Total Organic Carbon
The TOC in the first two sites was determined through the Walkley–Black dichromate oxidation method with external heating (Walkley and Black, 1947), while in the third and fourth sites the Mebius modified method (Yeomans and Bremner, 1988) and wet combustion with CO2 capture method (Nelson and Sommers, 1982) were used, respectively. To perform a mathematical adjustment of the results from the different methods, a comparison among them was performed. This comparison showed a close relationship among the methods and the TOC results obtained from Mebius (Walkley–Black = 0.99173 x Mebius, n = 6, r2 = 0.99, P < 0.0001) and wet combustion with CO2 capture methods (Walkley–Black = 1.08776 x wet combustion, n = 6, r2 = 0.99, P < 0.0001), which were mathematically adjusted to the Walkley–Black method. The TOC results were expressed in equivalent soil depth (Bayer et al., 2000b), where the soil bulk density was determined with soil cores sampled by volumetric rings (Embrapa, 1997).
Particulate Organic Carbon
The physical fractionation of SOM followed the procedure described by Cambardella and Elliot (1992). The C contents were determined in the >53-µm size fraction. The results of POC were expressed only at the 0- to 5-cm layer, which was the layer more affected by the NT system. Treatment effect on POC in the deeper layers followed the same trend of the 0- to 5-cm layer, but with lower magnitude (data not shown).
Calculation of Carbon Pool Indexes and Lability of Soil Organic Matter
The carbon pool index (CPI) and the particulate organic carbon pool index (POCPI) were calculated, respectively, through the ratio between TOC or POC stocks in each treatment and the TOC or POC stocks in the native grassland reference system (Blair et al., 1995; Diekow et al., 2005b). The lability of SOM was estimated through the ratio between the labile pool and the non-labile pool of SOM, where the labile pool was the POC stock and the non-labile pool was calculated by difference between TOC and POC stocks (Bayer et al., 2002; Diekow et al., 2005b).
Densimetric Fractionation of Soil Organic Matter
To evaluate the importance of physical protection on the organic C accumulation in NT soils from southern Brazil, soil surface samples (0- to 5-cm layer) of the sandy clay loam Paleudult were submitted for a densimetric fractionation of SOM (Golchin et al., 1994) using a 2.0 g cm–3 sodium polytungstate (SPT) solution. Ten grams of <9.5-mm soil aggregates were immersed in 80 mL of SPT in a centrifuge tube. The tube was covered with plastic and manually inverted slowly five times to avoid disruption of aggregates, and then allowed to stand for 60 min before centrifuging at 2000 x g for 90 min. After removal of light free fraction (LFF) of SOM, SPT solution was added to residual soil and the volume of the suspension adjusted and sonified using a total energy of 250 J mL–1 to break soil aggregates and liberate the occluded fraction (LOF) of SOM. The tube was allowed to stand for 60 min before centrifuging at 2000 x g for 90 min. The free and occluded fractions were obtained through filtration of the overflow, and they were washed with deionized water, dried at 60°C for 24 h, and analyzed in relation to C concentrations by dry combustion in a TOC analyzer (Shimadzu, Kyoto, Japan). These C results were not adjusted to the Walkley–Black method. As the results of LFF and LOF did not compare or relate to the TOC or POC results, they will be used only to evaluate the importance of physical protection of SOM in the NT clay loam Paleudult.
Carbon Accumulation Rates
The C accumulation rates were calculated for the 0- to 20-cm depth using the following equation:
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Statistical Analysis
Analysis of variance was performed using the SAS statistical package (SAS Institute, 2001). Each field site was analyzed separately, according to experimental design. The third experiment results were analyzed according to a randomized block design, with three replications (subsamples). The Tukey test was applied at p < 0.05 level of probability.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The soil management systems modified the ratio of TOC (0- to 5-cm layer) to TOC (0- to 20-cm layer) verified in native grass (Table 2). Conventionally tilled soil had a decrease in this ratio, showing that the distribution of TOC in the soil profile under CT was more uniform than in native grassland soil, most likely due to the annual soil disturbance and deposition of residue into the soil tillage layer. Except for the high clay Hapludox, the NT soils that were combined with intensive cropping systems and systems with tropical legume cover crops were able to maintain or even increase this ratio compared to native grass, which is related to preferential C accumulation on the soil surface where the crop residues are placed.
Particulate organic carbon (POC), mainly composed of crop residues and roots in initial decomposition stages, has been considered as a more sensitive indicator of soil management than TOC (Bayer et al., 2001, 2002; Conceição and Amado, 2002). The carbon pool index (CPI) proposed by Blair et al. (1995) was calculated to the 0- to 5- and 0- to 20-cm depths and adapted to particulate organic carbon (POCPI). These indexes comparing the highest (improved soil management system) and the lowest (poor soil management system) TOC soil management treatments for each soil were the following: sandy loam Paleudalf, 2.9 and 10.7; sandy clay loam Paleudult, 2.3 and 7.0; and high clay Hapludox, 1.3 and 1.7, respectively, for CPI and POCPI. These results show that POCPI was a more sensitive indicator for soil management than CPI, regardless of the soil type. The highest CPIs were found in intensive cropping systems and with tropical legume cover crops under NT; on the other hand, the lowest CPIs were found in the treatments with the combination of absent or low biomass input and CT. The effect of soil management on POCPI has consequences for soil quality due to the role of this labile organic C fraction in nutrient cycling, biologic activity, and many related soil properties (Blair et al., 1995; Diekow et al., 2005b). In the highest clay content soil, the CPI (0- to 20-cm layer) showed low sensitivity to soil management, while CPI (0- to 5-cm layer) and POCPI were able to discriminate among the soil management systems. Generally, POC had a higher coefficient of variation than TOC, except for the high clay Hapludox (Table 2).
The treatments with tropical legume cover crops such as pigeon pea–maize (sandy clay loam Paleudult) and velvet beans–maize (sandy loam Paleudalf) had, respectively, 53 and 71% higher POC stocks than the native grass treatment. Good soil management practices, such as cover crops, minimum soil disturbance, N fertilization, and legume cover crops increased POC stocks. In contrast, in the sandy clay loam Paleudult, the POC stocks under double-cropping systems (black oat–maize) with RT and CT reached only 56 and 68%, respectively, of the reference treatment. These results indicate that treatments with improved soil management practices increase POC stocks and treatments with poor soil management promote decrease of this C fraction in relation to native grass (Table 2).
The lability of SOM, calculated as the ratio between POC and TOC stocks at 0 to 5 cm, varied from 0.20 to 0.48 in soils under native grass. The management systems that combined low crop residue addition and intensive soil disturbance (CT) led to a decrease in SOM lability (POC to TOC ratio varying from 0.08 to 0.27). In contrast, NT soils with high addition of crop residues promoted an increase in the SOM lability as evidenced by the higher POC to TOC ratios (varying from 0.18 to 0.35). Lability is an important property of SOM because it affects the fluxes of C and energy in soil through microbial activity, which, in turn, provides a positive feedback on soil quality (Blair et al., 1995; Conceição and Amado, 2002; Diekow et al., 2005b).
Rates of Carbon Accumulation
Tillage Effect
The NT soils showed a range of 0.12 to 0.43 Mg C ha–1 yr–1 of C accumulation relative to the CT soil (Table 4). In the sandy clay loam Paleudult, NT had 0.12 Mg ha–1 yr–1 C higher accumulation than CT under the double-cropping system (black oat–maize). In this soil, the TOC increment rates were 26.7 (0- to 5-cm layer) and 6.0 (0- to 20-cm layer) kg ha–1 cm–1 yr–1, indicating that C accumulation in the NT soils occurred preferentially in soil surface layer.
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The highest rates of C accumulation in NT compared to CT were found when the intensive cropping systems replaced the traditional double-cropping system used in southern Brazil. In the clay Hapludox soil NT had 0.25 Mg ha–1 yr–1 more C accumulation than CT under Intensive Cropping System II. There was an increase of 56% in NT C accumulation rate due to the more intensive in cropping system. The TOC increment rates were 48.4 (0- to 5-cm layer) and 12.5 (0- to 20-cm layer) kg ha–1 cm–1 yr–1.
In the high clay Hapludox, the NT had 0.43 Mg ha–1 yr–1 more C accumulation than CT under Intensive Cropping System III. The TOC increment rates were 54.3 (0- to 5-cm layer) and 21.5 (0- to 20-cm layer) kg ha–1 cm–1 yr–1. Therefore, the C accumulation rates of NT compared to CT under an intensive cropping system were greater in the high clay Hapludox than in the clay Hapludox (0.25 Mg ha–1 yr–1). Considering the lower crop residue additions in the high clay Hapludox than in the clay Hapludox (Table 3), higher rates of C accumulation in the high clay soil are most likely be related to the shorter duration of the experiment (7 yr) compared to the clay soil (19 yr). Higher C accumulation rates are expected to occur in the first couple years of NT adoption in SOM-depleted soils.
Cropping System Effect
The highest rate of C accumulation (0.59 Mg ha–1 yr–1) was found in the velvet beans (tropical legume cover crop)–maize system compared to winter fallow–maize under NT, in the sandy loam Paleudalf (Table 4). The TOC increment rates were 53.2 (0- to 5-cm layer) and 29.5 (0- to 20-cm layer) kg ha–1 cm–1 yr–1. No-till with tropical legume increased C accumulation compared to double-cropping system (rye–maize) by 0.43 Mg ha–1 yr–1. However, it should be stressed that the sandy loam Paleudalf had the lowest TOC stocks among the soils investigated.
In the sandy clay loam Paleudult the pigeon pea (tropical legume cover crop)–maize had 0.38 Mg ha–1 yr–1 more C accumulation than the black oat–maize (double-cropping system) in NT. The TOC increment rates were 80.4 (0- to 5-cm layer) and 19.0 (0- to 20-cm layer) kg ha–1 cm–1 yr–1. The C accumulation rate in the sandy loam Paleudalf under NT was 55% higher than in sandy clay loam Paleudult, which must be related to the different reference systems used (winter fallow in sandy loam Paleudalf and black oat in the sandy clay loam Paleudult). In both cropping systems under NT, the highest gain of C was verified in the shallow soil layer. The tropical legume cover crops provide high inputs of C and N from aboveground biomass and roots. Also, the following maize crop is expected to produce higher biomass due to improvement in N soil availability from the previous legume cover crop (Amado et al., 2001; Bayer et al., 2000a, 2000b, 2006a; Diekow et al., 2005a).
The NT rate of C accumulation in the intensive cropping system compared to double-cropping system was higher in the clay Hapludox (0.34 Mg ha–1 yr–1) than in the sandy clay loam Paleudult (0.25 Mg ha–1 yr–1). This result is likely related to the increased stability of the SOM in the clay soil when compared to sandy clay loam, despite the lower additions of plant biomass to the clay Hapludox. Under NT, the difference in C addition between an intensive cropping system and double-cropping system was greater in the clay Hapludox (1.84 Mg C ha–1 yr–1) than in sandy clay loam Paleudult (1.20 Mg C ha–1 yr–1) (Table 3). Also, the study on the clay Hapludox was performed for 4 yr longer than on the sandy clay loam Paleudult.
The lowest C accumulation rates across cropping systems were verified under CT. For example, in the clay Hapludox, the Intensive Cropping System II under CT experienced a C increase rate of 0.26 Mg ha–1 yr–1 higher rate of C accumulation than the double-cropping system. Under NT, the difference extended to 0.34 Mg C ha–1 yr–1. The smaller C accumulation observed under CT in the clay Hapludox is related to the smaller C additions observed under this system (Table 3). The TOC increment rates for CT were 9.5 (0- to 5-cm layer) and 12.9 (0- to 20-cm layer) kg ha–1 cm–1 yr–1. There was a slight increase in soil C storage under CT when the cropping system was improved. The approximately uniform increment in C through the soil depths probably is related to use of a plow disc and tandem disc in CT system. These disc implements promote a more uniform distribution of residues through the soil profile than the moldboard plow, which inverts the soil layers, or NT, which keeps the residues on soil surface (Sá et al., 2001).
Tillage and Cropping Effect
In the sandy clay loam Paleudult, NT under Intensive Cropping System I (four crops yr–1) had a C addition rate of 7.83 Mg ha–1 yr–1 compared to 6.67 Mg ha–1 yr–1 for CT under double-cropping (two crops yr–1) (Table 3). The NT under Intensive Cropping System I on the sandy clay loam Paleudult had an annual C accumulation rate that was 0.37 Mg ha–1 yr–1 higher than for CT under double cropping. This rate is approximately three times higher than the rate found when comparing NT to CT alone (0.12 Mg ha–1 yr–1). It should be stressed that in this case there are two effects combined: minimum soil disturbance provide by NT and higher residue addition due to the Intensive Cropping System I. The TOC increment rates, comparing NT under Intensive Cropping System I with CT under double-cropping system in the sandy clay loam Paleudult, were 76.0 (0- to 5-cm layer) and 18.5 (0- to 20-cm layer) kg ha–1 cm–1 yr–1. Carbon accumulation was approximately four times higher in the first than the second soil depth.
In the clay Hapludox, the Intensive Cropping System II under NT had a C accumulation of 0.51 Mg ha–1 yr–1 higher than the double-cropping system under CT. The difference in annual C inputs between these two systems was 2.20 Mg ha–1 yr–1 of C in favor of the Intensive Cropping System II under NT (Table 3). In general, the combined use of NT and an intensive cropping system resulted in higher C accumulation rates. The single effect of tillage under an intensive cropping system (NT x CT = 0.25 Mg ha–1 yr–1) was lower than the single effect of a cropping system under NT (Intensive Cropping System II x double-cropping = 0.34 Mg ha–1 yr–1). Thus, under an intensive cropping system, NT had 0.69 Mg ha–1 yr–1 more C addition than CT in the same cropping system. The intensive cropping system had 1.84 Mg ha–1 yr–1 more C addition than the double-cropping system, both under NT (Table 3). These results stress the importance of crop residue addition to enhance the potential of C sequestration in NT soils.
Physical Protection of Organic Matter in Tillage Systems
The densimetric fractionation of SOM was performed at the 0- to 5-cm soil depth in the sandy clay loam Paleudult to evaluate the effect of the physical protection on C accumulation in this southern Brazil soil. Figure 2 shows the effect of NT on C stocks in the free and occluded light fractions of SOM in comparison to CT soil. It was observed that the C stock in the free light fraction was 3.5 times higher in NT than CT soil. This result was probably a consequence of the decrease in soil temperature mainly in the surface layers, and lower residue–soil contact in NT compared to CT. The NT also increased by 3.5 times the C stock in the occluded fraction in comparison to CT. These tillage systems did not change the proportion of free and occluded fraction in TOC. In addition, the increase of C stock in the occluded light fraction of SOM (3.55 g C kg–1 of soil) represented approximately 74% of the total increase in the light fraction (4.78 g C kg–1 of soil) observed under NT relative to CT. These results suggest that physical protection of organic matter into soil aggregates was an important mechanism in the increase of C stocks in the NT sandy clay loam Paleudult (subtropical soil). These results were similar to those obtained in temperate soils (Carter et al., 1994; Franzluebbers and Arshad, 1997; Six et al., 2000). The importance of physical protection to C accumulation in NT soils also depends on soil texture and mineralogy, effects that need to be better understood in tropical and subtropical soils.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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