Abstract
The effects of six crop-pasture rotations were evaluated on properties of soil aggregates in a volcanic soil (Humic Haploxerand) of south-central Chile. Rotations that included intensive cropping without pastures, and crops with short- or long-term pastures were maintained for 12 years after which soil samples were taken at 0–5 and 5–10 cm depths for analysis of the C, N and S contents in the different grades of water-stable aggregates. The mean weight diameter of the aggregates was also determined as an indicator of structural stability. The results showed that the rotations which included long-term pastures of alfalfa (Medicago sativa L.) or white clover (Trifolium repens L.) had higher contents of C, N and S in soil and there was a higher concentration of these elements in macro-aggregates (>0.5 mm) than in micro-aggregates (<0.5 mm). Additionally, the rotations with pastures also produced greater structural stability of the soil aggregates. Consequently, crop rotations that included pastures, particularly those of longer duration, improved the soil and were therefore a more sustainable use of the soil resource compared to the more intensive rotations. Additionally, there was evidence of hierarchical organization of the soil structure, which is a previously non-described feature of volcanic soils.
Keywords:
Introduction
The search for sustainability in agricultural systems and associated soil resources is a current theme of agronomic investigations. Crop rotations, management practices, and the quantity and quality of plant residues can all affect the content of organic matter in soil and, more specifically, the carbon (C), nitrogen (N) and suphur (S) contained therein (Harris et al., Citation1966; Angers et al., Citation1993; Gregorich et al., Citation1994; Haynes & Beare, Citation1996; Haynes, Citation1999a). In general, intensive crop rotations cause deterioration of the soil fertility and structure (Rennie et al., Citation1954; Hussain et al., Citation1999; Stuart et al., Citation2002) while less intensive rotations that include pastures tend to increase the contents of C, N and S in the soil, and also improve structural stability (Dexter, Citation1988; Haynes, Citation2000).
In general, there are two basic size classes of aggregates considered in a soil structure that has ‘hierarchical organization’ (where smaller structural units are joined to make progressively larger ones): macro-aggregates (>0.25 mm) and micro-aggregates (< 0.25 mm) (Tisdall & Oades, Citation1982; Oades, Citation1984; Elliott, Citation1986; Gupta & Germida, Citation1988; Oades & Waters, Citation1991; Golchin et al., Citation1995; Six et al., Citation2002; Mikha & Rice, Citation2004; García-Oliva et al., Citation2004; Bayhan et al., Citation2005). The relative increase or decrease of the number of macro-aggregates in soil can be used to infer the effect of crop rotations and management on structural stability, soil quality and general sustainability of agricultural systems (Carter et al., Citation2003).
Although previous investigations have established the effect of tillage and crop rotations on the contents of C, N and S in soil aggregates (Puget et al., Citation1995; Franzluebbers & Arshad, Citation1997; Six et al., Citation1998; Aguilera et al., Citation2002), the subject has been less studied in volcanic soils (andisols), probably because they represent only 1%, approximately, of world soils (Shoji et al., Citation1993). Therefore, the objectives of this study were to determine the effect of long-term rotations that include crops and pastures on the structural stability of a volcanic soil, and to measure the relative contents of C, N and S among the different sized aggregates of the soil.
Materials and methods
Soil
The crop-pasture rotations were established in 1991 on the grounds of the Santa Rosa experimental station (Quilamapu Regional Research Centre), Chillán-Chile (36°31′34′′ latitude S, 71°54′40′′ longitude W) on a volcanic soil Arrayán silt loam (medial, amorphic, thermic Humic Haploxerands) with a 0 to 1% slope. The physical-chemical characteristics of the soil (0–10 cm depth) were determined prior to the establishment of the rotations and were as follows: soil pH 5.98; N-NO3 8.87 mg kg−1; available P 17.05 mg kg−1; organic C 4.5%; exchangeable Ca, Mg, K and Na 5.75, 0.65, 0.33 and 0.48 cmolc kg−1, respectively; total soil N 0.45%; bulk density 1.05 g cm−3; field capacity 45.48%; and permanent wilting point 29.40% (Zagal et al., Citation2002). The water contents at field capacity and permanent wilting point were measured using a pressure plate apparatus (Klute, Citation1986); organic C by wet combustion; total N by the Kjeldahl method; available P by the Olsen method; exchangeable Ca, Mg, K and Na were determined by atomic absorption after soil extraction with NH4O2C2H3; soil pH using a ratio of soil to water of 1:2.5 (Sadzawka et al., Citation2002) and bulk density by the gravimetric core method (Blake & Hartge, Citation1986).
Treatments
The experiment had a randomized complete block design with six treatments and four repetitions, in plots that measured 20×14 m (280 m2). The field investigation included sugar beet (Beta vulgaris L. subsp. vulgaris), wheat (Triticum aestivum L.), red clover (Trifolium pratense L.), beans (Phaseolus vulgaris L.), barley (Hordeum vulgare L.), corn (Zea mays L.), alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) which were used in the rotation treatments as follows: I. sugar beet-wheat-two year red clover (SB-W-RC2); II. sugar beet-wheat-beans-barley (SB-W-BN-BR); III. corn-wheat-two year red clover (C-W-RC2); IV. corn-wheat-beans-barley (C-W-BN-BR); V. sugar beet-wheat-corn-five year alfalfa (SB-W-C-A5); VI. sugar beet-wheat-corn-fiver year white clover (SB-W-C-WC5).
Soil samples and analysis of C, N and S
In October 2003, 12 years after initiating the field experiment, soil samples were collected from each rotation treatment. Four composite samples, each consisting of three sub-samples, were obtained per repetition at 0–5 and 5–10 cm soil depths. In the laboratory, the soil samples were air dried and sieved (2 mm mesh). These samples were used for determination of soil properties of the whole (or bulk) soil. Additionally, four undisturbed samples were also collected per repetition, at 0–5 and 5–10 cm depths, using a metal cylinder (15 cm diameter×5 cm height). In the laboratory, the soil samples were gently parted by hand, air dried, and passed through 8- and 4-mm sieves. The soil aggregates that were retained on the 4 mm sieve were subsequently wet-sieved (Yoder, Citation1936), as detailed below. The contents of C, N and S in the whole soil and aggregates were measured using dry combustion in a total elemental analyser (Elementar Model Variomax CNS).
Distribution of water-stable aggregates (WSA) and their mean weight diameter (MWD)
Four 100 g samples of air-dried aggregates (8–4 mm) were brought to saturation and placed on the top of six nested sieves ( 4.0, 2.0, 1.0, 0.5, 0.25 and 0.05 mm) that were in a container holding de-ionized water. The soil and sieves were subsequently submerged in the water (35 mm depth) and agitated for 30 min at 25 cycles per min−1 (Yoder, Citation1936; Kemper & Chepil, Citation1965; Kemper & Rosenau, Citation1986).
The water-stable aggregates that were distributed in the different sized sieves were placed in a drying oven at 105°C for 24 h, and afterwards weighed to determine the total weight of aggregates within each size range. The relative stability of the soil aggregates for each treatment was evaluated by their mean weight diameter (MWD) which was calculated using the following formula:
Statistical analysis
The data were analysed using ANOVA-GLM for a randomized complete block design (SAS Institute, Citation1999), and the rotation treatment was used as the principal factor in the GLM model for each soil depth. The different size classes of aggregates were considered as additional split plots for statistical analysis of C, N and S contents among the aggregate size classes. The separation of treatment means was performed using Tukey's test with a significance level of p=0.05.
Results and discussion
Total carbon (C), nitrogen (N) and sulphur (S) in the soil
The total C content in the soil, after 12 years under the different crop-pasture rotations, is presented in . Interestingly, none of the rotation treatments decreased the content of soil C below the initial amount of 4.5%, possibly because the Arrayán soil is nearly level and not susceptible to erosion. Additionally, volcanic soils in general have higher levels of organic matter compared to non-volcanic soils that may tend to buffer excessive depletion of soil C. Nevertheless, at 0–5 cm depth, soil C contents were highest in the crop rotations that included five years of pasture (SB-W-C-A5 and SB-W-C-WC5). However, the contents of soil C in these treatments (5.58 and 5.45%, respectively) were significantly higher (p < 0.05) compared only to the intensive rotation SB-W-BN-BR, that had the lowest content of soil C (4.78%). The soil C content at 5–10 cm depth was lower, in general, compared with the 0–5 cm depth and there were no significant differences (p=0.05) among the rotation treatments. However, the rotations SB-W-C-A5 and SB-W-C-WC5 similarly had somewhat higher contents of soil C (5.23 and 5.26%, respectively) compared to the other rotations.
Table I. Total contents of C, N, S in the volcanic soil Arrayán silt loam, after 12 years under crop-pasture rotations of different intensities.
The total N content of soil at the 0–5 cm depth varied between 0.44 and 0.52% (). In general, the soil N contents among the rotation treatments had similar trends to the C contents. The rotations that included five years of pasture (SB-W-C-A5 and SB-W-C-WC5) had somewhat higher values (0.52 and 0.50%, respectively) compared to the other rotations, and were significantly higher (p<0.05) than the rotation SB-W-BN-BR which had the lowest content of soil N (0.44%) of all the rotation treatments. At 5–10 cm depth, soil N content in the rotations SB-W-C-A5 and SB-W-C-WC5 was somewhat lower than was measured for these rotations at 0–5 cm depth, and was only slightly higher than was measured in the other rotations at 5–10 cm depth.
The total content of soil S at 0–5 cm depth was highest in those crop rotations that included five years of pasture (SB-W-C-A5 and SB-W-C-WC5), but was significantly higher (p < 0.05) compared only to the rotation SB-W-BN-BR (). However, at 5–10 cm depth there were no significant differences among the rotation treatments, but the general tendency was that the rotations which included five years of pasture also had somewhat higher values of soil S. Overall, the total content of soil S at 5–10 cm depth was slightly lower than was measured at 0–5 cm depth.
The foregoing results verified a tendency that has been noted in other investigations whereby reduced tillage combined with greater crop diversity in rotations resulted in higher contents of soil C, N and S compared to more intensive systems of crop production (Doran, Citation1980; McConkey et al., Citation2003, Sainju et al., Citation2006). In this investigation, the least intensive rotations (SB-W-C-A5 and SB-W-C-WC5) had the greatest increases of soil C, N and S over time probably because during the pasture periods the soil was undisturbed, which permitted greater development of root systems, accumulation of organic residues, and lower oxidation of soil organic matter (Haynes, Citation1999b; Zagal et al., Citation2002). Similarly, legumes in symbiosis with bacteria (Rhizobium spp.) in the pastures facilitated N fixation which also increased the content and availability of N in the soil (Schnitzer & Khan, Citation1978; Shah et al., Citation2003). In contrast, the lowest levels of C, N and S in the soil were measured in the most intensive rotation (SB-W-BN-BR) that did not include pasture and where the soil was intensively tilled prior to planting sugar beet. Also, sugar beet does not produce significant amounts of crop residue that are returned to the soil (Sierra & Rodriguez, Citation1986).
Size distribution of soil aggregates
The distribution of water-stable aggregates in the five size-range classes was determined for the rotations (). In the 0–5 cm soil depth, over all the treatments, there was a predominance of aggregates (mean 66.57%) > 0.25 mm. Specifically, the rotation C-W-RC2 had the most aggregates >0.25 mm (72.35%) which was significantly greater (p<0.05) than the rotation C-W-BN-BR which had the fewest soil aggregates of this size (59.40%). All rotations that included pasture (SB-W-RC2, C-W-RC2, SB-W-C-A5 and SB-W-C-WC5) favoured the formation of macro-aggregates 4–2 mm (mean 51.04%) compared to the rotations without pasture (mean 19.46%). However, the rotations with pastures had relatively lower proportions of aggregates 2–0.25 mm (mean 18.55%) compared to those without pastures (mean 41.05%).
Table II. Distribution of water stable aggregates (%) and the mean weighted diameter (MWD) of aggregates in the volcanic soil Arrayán silt loam, after 12 years of crop-pasture rotations of different intensities.
At the depth of 5–10 cm, the rotations SB-W-RC2 and SB-W-C-WC5 had significantly higher (p<0.05) amounts of aggregates >0.25 mm (67.71% and 68.33%, respectively) compared to the rotation SB-W-BN-BR (57.53%) (). However, there were no significant differences among the rest of the rotation treatments, although there was a tendency that the rotations which included pastures had a greater percentage of macro-aggregates 4–2 mm (mean 39.86%) compared to those without pastures (mean 24.85%).
In general, the rotations that included short-duration pastures (red clover for two years) and long-duration pastures (alfalfa or white clover for five years) had approximately 50% macro-aggregates 4–2 mm in the soil surface (0–5 cm depth) and 40% in the subsurface (5–10 cm depth) (). The rotations without pastures generally favoured a higher proportion of smaller-sized aggregates (<2 mm).
The mean weight diameter (MWD) of soil aggregates in the 0–5 cm depth was greater (p<0.05) in all of the rotations that included pastures (SB-W-RC2, C-W-RC2, SB-W-C-A5 and SB-W-C-WC5) compared to those without pastures (SB-W-BN-BR and C-W-BN-BR) (). Similarly, the MWD at 5–10 cm depth was generally lower for the more intensive rotations without pastures (SB-W-BN-BR and C-W-BN-BR) compared to the other rotation treatments. However, only the rotation SB-W-BN-BR had MWD that was significantly lower (p<0.05) compared to the rotations SB-W-RC2, C-W-RC2 and SB-W-C-WC5. In general, structural stability of soil is increased when there is a greater MWD of soil aggregates and, consequently, a higher proportion of water-stable macro-aggregates (Tripathy & Singh, Citation2004). Therefore, considering that the MWD was generally higher in the rotations that included pastures, and lower in the more intensive rotations without pastures, it was concluded that MWD is a useful indicator of structural stability in this volcanic soil. In previous investigations, crop rotations that included four to five years of pastures increased the total organic matter and promoted the formation of macro-aggregates in the soil (Haynes et al., Citation1991; Yoo & Wander, Citation2006), and rotations that included legumes increased the overall structural stability of soils (Haynes & Francis, Citation1990).
C, N and S contents in the size classes of water-stable aggregates (WSA)
Comparison of the contents of soil C, N and S (averaged over all treatments) within aggregates (0–5 cm depth) revealed that as the size of the water-stable aggregates increased from 0.05 to 4.0 mm there were greater contents of C, N and S (p<0.05) (). The concentration of C, N and S was significantly greater in the aggregates ≥ 0.5 mm diameter compared to all of the smaller aggregates, and similarly the aggregates ≥ 1mm in diameter had higher C, N and S concentrations than all aggregates smaller than this size. A similar tendency was found in the soil depth of 5–10 cm.
Table III. Total contents of C, N and S in the volcanic soil Arrayán silt loam, of the different sizes of water-stable aggregates averaged over all the treatments of crop-pasture rotations of different intensities.
Grouping the water-stable aggregates (0–5 cm depth) based on the statistically distinct ranges of 4–1, 1–0.5 and <0.5 mm () illustrated that the concentrations of C, N and S increased with greater size of the aggregates, as expected (). The contents of C, N and S in aggregates 1–0.5 mm were, on average, 16% higher than in the aggregates <0.5 mm. Similarly, the C, N and S contents in aggregates 4–1 mm were approximately 7% higher than in the 1–0.5 mm aggregates. Additionally, the rotations that had the highest contents of C, N and S in the whole soil (SB-W-C-A5 and SB-W-C-WC5) () generally had greater concentrations of these elements within the water-stable aggregates (). The intensive rotation SB-W-BN-BR that had the lowest contents of C, N and S in the whole soil () also had lower concentrations of these elements in all sizes of the water-stable aggregates ().
Table IV. Contents of C, N and S in aggregates 4–1, 1–0.5, and < 0.5 mm (0–5 cm depth), in the volcanic soil Arrayán silt loam, after 12 years of crop-pasture rotations of different intensities.
The C content of the water stable-aggregates within the size ranges 4–1, 1–0.5 and < 0.5 mm, and whole soil (0–5 cm depth) was calculated using the following formula:
Table V. Total calculated C content in different size ranges of water-stable aggregates (4–1, 1–0.5 and < 0.5 mm) in the volcanic soil Arrayán silt loam, after 12 years of crop-pasture rotations of different intensities.
Although it is often assumed that soil aggregates have uniform properties regardless of their size, it has been demonstrated that aggregate properties can vary according to their size, and may be different when compared to ‘average properties’ of the whole soil (Garey, Citation1954). Recent research has found that the total C content was greater in macro-aggregates than micro-aggregates of an inceptisol and two alfisols and was indicative of hierarchical organization of the soil structure (Bronick & Lal, Citation2005). However, in another investigation of oxisols, the contents of C and N were similar in the different size classes of aggregates and hierarchical organization of the soil structure was therefore not supported (Zotarelli et al., Citation2005). In this investigation, the increased contents of C, N and S in larger aggregates, regardless of the rotation, may also be explained by a hierarchical ordering of the soil structure, which is a previously non-described feature of andisols.
In general, micro-aggregates have a larger external surface area (m2 g−1) than macro-aggregates, and are stabilized by persistent aromatic humic material that is associated with amorphous Fe and Al compounds. Macro-aggregates, however, are stabilized by transient bonding agents such as roots, hyphae and polysaccharides that are derived from plants and microorganisms (Tisdale & Oades, Citation1982; Brady, Citation1990). The total content of C, N and S in macro-aggregates of the Arrayán volcanic soil would consequently be due to the organic matter (persistent aromatic humic material) that is involved in the formation of micro-aggregates, but also that which acts as a ‘transient’ bonding agent (roots, hyphae etc.) between the micro-aggregates to form the macro-aggregates. The lower contents of C, N and S that were observed in the more intensive rotations were probably caused by the destruction of soil macro-aggregates which, in turn, accelerated the oxidization, or mineralization, of freshly exposed organic matter that was previously occluded within the macro-aggregates.
In conclusion, after 12 years there were higher contents of C, N and S in soil, and greater structural stability, in soils under the crop rotations that included pastures (especially five-year pastures). There was also evidence of hierarchical organization of the soil structure which previously has not been documented in volcanic soils. Overall, the results indicated that the inclusion of longer-term pastures in crop rotations not only improved the soil, which favoured its sustained use, but also promoted C sequestration within the macro-aggregates.
Acknowledgements
This research was funded by the Chilean Institute for Agricultural Investigations (CRI-INIA) and the Soils Department, Faculty of Agronomy of the University of Concepción, Chile, and was part of a project for a doctoral thesis of the Environmental Science Programme (EULA) of the University of Concepción. The authors sincerely thank Pablo Undurraga, Research Agronomist, for assistance in laboratory analysis of soil samples.
References
- Aguilera , M. , Mora , M. , Bori , G. , Peirano , P. and Zunino , M. 2002 . Balance and distribution of sulphur in volcanic ash-derived soils in Chile . Soil Biology & Biochemistry , 34 : 1355 – 1361 .
- Angers , D.A. , Samson , N. and Legégé , A. 1993 . Early changes in water-stable aggregation induced by rotation and tillage in soil under barley production . Canadian Journal of Soil Science , 73 : 51 – 59 .
- Bayhan , A.K. , Isildar , A.A. and Akgül , M. 2005 . Tillage impacts on aggregate stability and crop productivity in a loam soil of a dryland in Turkey. Acta Agriculturae Scandinavica Section B . Soil and Plant Science , 55 : 214 – 220 .
- Blake , G.R. and Hartge , K.H. 1986 . “ Bulk density ” . In Methods of Soil Analysis, Part 1 , 2nd edn , Edited by: Klute , A . 425 – 442 . ASA, Madison, WI : Agronomy Monograph 9 .
- Brady , N.C. 1990 . The nature and properties of soils , 10th edn , 750 New York : Macmillan .
- Bronick , C.J. and Lal , R. 2005 . Manuring and rotation effects on soil organic carbon concentration for different aggregate size fractions of two soils in north-eastern Ohio, USA . Soil & Tillage Research , 81 : 239 – 252 .
- Carter , M.R. , Kunelius , H.T. , Sanderson , J.B. , Kimpinski , J. , Platt , H.W. and Bolinder , M.A. 2003 . Productivity parameters and soil health dynamics under long-term two-year potato rotations in Atlantic Canada . Soil & Tillage Research , 72 : 153 – 168 .
- Dexter , A.R. 1988 . Advances in characterization of soil structure . Soil & Tillage Research , 11 : 199 – 238 .
- Doran , J.W. 1980 . Soil microbial and biochemical changes associated with reduced tillage . Soil Science Society of America Journal , 44 : 765 – 771 .
- Elliott , E.T. 1986 . Aggregate structure and carbon, nitrogen and phosphorus in native and cultivated soils . Soil Science Society of America Journal , 50 : 627 – 633 .
- Eynard , A. , Shumacher , T.E. , Lindstrom , M.J. and Malo , D.D. 2005 . Effects of agricultural management systems on soil organic carbon in aggregates of Ustolls and Usterts . Soil & Tillage Research , 81 : 253 – 263 .
- Franzluebbers , A.J. and Arshad , M.A. 1997 . Soil microbial biomass and mineralizable carbon of water-stable aggregates . Soil Science Society of America Journal , 61 : 1090 – 1097 .
- García-Oliva , F. , Oliva , M. and Sveshtarova , B. 2004 . Effect of soil macro-aggregates crushing on C mineralization in a tropical deciduous forest ecosystem . Plant and Soil , 259 : 297 – 305 .
- Garey , C.L. 1954 . Properties of soil aggregates: I. Relation to size, water stability and mechanical composition . Soil Science Society of America Proceedings , 19 : 16 – 18 .
- Golchin , A. , Clake , P. , Oades , J.M. and Skjemstad , J.O. 1995 . The effects of cultivation on the composition of organic matter and structural stability of soils . Australian Journal of Soil Research , 33 : 975 – 993 .
- Gregorich , E.G. , Carter , M.R. , Angers , D.A. , Monreal , C.M. and Ellert , B.H. 1994 . Towards a minimum dataset to assess soil organic matter quality in agricultural soils . Canadian Journal of Soil Science , 74 : 367 – 385 .
- Gupta , V.V.S.R. and Germida , J.J. 1988 . Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation . Soil Biology & Biochemistry , 20 : 777 – 786 .
- Harris , R.F. , Chesters , G. and Allen , O.N. 1966 . Dynamics of soil aggregation . Advances in Agronomy , 18 : 108 – 169 .
- Haynes , R.J. and Francis , G.S. 1990 . Effects of mixed cropping farming systems on changes in soil properties on the Canterbury Plains . New Zealand Journal of Ecology , 14 : 73 – 82 .
- Haynes , R.J. , Swift , R.S. and Stephen , R.C. 1991 . Influence of mixed cropping rotations (pasture-arable) on organic matter content, water stable aggregation and clod porosity in a group of soils . Soil & Tillage Research , 19 : 77 – 87 .
- Haynes , R.J. and Beare , M.H. 1996 . “ Aggregation and organic matter storage in meso-thermal humic soils ” . In Advances in Soil Science. Structure and Organic Matter Storage in Agricultural Soils , Edited by: Carter , MR and Stewart , BA . 213 – 262 . Boca Raton : CRC Lewis Publishers .
- Haynes , R.J. 1999a . Labile organic matter fractions and aggregate stability under short-term, grass-based leys . Soil Biology & Biochemistry , 31 : 1821 – 1830 .
- Haynes , R.J. 1999b . Size and activity of the soil microbial biomass under grass and arable management . Biology and Fertility of Soils , 30 : 210 – 216 .
- Haynes , R.J. 2000 . Labile organic matter as an indicator of organic matter quality in arable and pastoral soils in New Zealand . Soil Biology & Biochemistry , 32 : 211 – 219 .
- Heenan , D.P. , Chan , K.Y. and Knight , P.G. 2004 . Long-term impact of rotation, tillage and stubble management on the loss of soil organic carbon and nitrogen from a chromic luvisol . Soil & Tillage Research , 76 : 59 – 68 .
- Hussain , I. , Olson , K.R. and Edelhar , S.A. 1999 . Long term tillage effects on soil chemical properties and organic matter fractions . Soil Science Society of America Journal , 63 : 1335 – 1341 .
- Kemper , W.D. and Chepil , W.S. 1965 . “ Size distribution of aggregates ” . In Methods of Soil Analysis, Part 1 , Edited by: Black , CA . 499 – 509 . Madison, WI : America Society of Agronomy .
- Kemper , W.D. and Rosenua , R.C. 1986 . “ Aggregate stability and size distribution ” . In Methods of Soil Analysis, Part 1 , 2nd edn , Edited by: Klute , A . 425 – 442 . Madison, WI : Agronomy Monograph 9, ASA .
- Klute , A. 1986 . “ Water retention: laboratory methods ” . In Methods of Soil Analysis, Part 1. Physical and mineralogical , 2nd. edn , Edited by: Klute , A . 635 – 661 . Madison, WI : American Society of Agronomy .
- McConkey , B.G. , Liang , B.C. , Cambell , C.A. , Curtin , D. , Moulin , A. , Brant , S.A. and Lafond , G.P. 2003 . Crop rotation and tillage impacts on carbon sequestration in Canadian prairie soils . Soil & Tillage Reserch , 74 : 81 – 90 .
- Mikha , M.M. and Rice , C.W. 2004 . Tillage and manure effects on soil and aggregate-associated carbon and nitrogen . Soil Science Society of America Journal , 68 : 809 – 816 .
- Oades , J.M. 1984 . Soil organic matter and structural stability, mechanism and implementation for management . Plant and Soil , 76 : 319 – 337 .
- Oades , J.M. and Waters , A.G. 1991 . Aggregate hierarchy in soils . Australian Journal of Soil Research , 29 : 815 – 828 .
- Puget , P. , Chenu , C. and Balesdent , J. 1995 . Total and young organic matter distribution in aggregates of silty cultivated soils . European Journal of Soil Science , 46 : 449 – 559 .
- Rennie , D.A. , Truog , E. and Allen , O.H. 1954 . Soil aggregation as influenced by microbial gums, level of fertility and kind of crop . Soil Science Society of America Proceedings , 18 : 399 – 403 .
- Richards , L.A. and Weaver , L.R. 1964 . Moisture retention by some irrigated soils as related to soil moisture tension . Journal of Agricultural Research , 69 : 215 – 235 .
- Sadzawka , R.A. , Flores , P.H. , Grez , Z.R. , Mora , G.M. , Carrasco , R. & Saavedra R. ( 2002 ). Programa de acreditación de laboratorios. Ronda normal 2003–1. Análisis de suelos ácidos. Comisión de Normalización y Acreditación, Sociedad Chilena de la Ciencia del Suelo . pp. 32 . ( In Spanish .)
- SAS Institute . ( 1999 ). SAS Release 8.1 . SAS Inst. , Cary NC .
- Schnitzer , M. and Khan , S.U. 1978 . Soil organic matter , Elsevier, Amsterdam, The Netherlands .
- Sainju , U.M. , Lenssen , A. , Caesar-Tonthat , T. and Waddell , J. 2006 . Tillage and crop rotation effects on dryland soil and residual carbon and nitrogen . Soil Science Society of America , 70 : 668 – 678 .
- Shah , Z.S.H. , Peoples , M.B. , Schwenke , G.D. and Herridge , D.F. 2003 . Crop residues and fertilizer N effects on nitrogen fixation and yields of a legume-cereal rotation and soil organic fertility . Field Crops Research , 83 : 1 – 11 .
- Shoji , S. , Nanzyo , M. and Dahlgren , R.A. 1993 . Volcanic ash soils, genesis, properties, and utilization, developments in soil science, No 21 , Elsevier, Amsterdam, The Netherlands, 288 pp .
- Sierra , C. & Rodríguez , J. ( 1986 ). Efecto del manejo del suelo en el suministro de N . Ciencia e Investigaciones Agrarias (Chile) , 13 , 229 – 237 . ( In Spanish .)
- Sisti , P.J. , Dos Santos , H.P. , Kohhann , R. , Alves , B.J. , Urquiaga , S. and Bodley , R. 2004 . Change in carbon and nitrogen stocks in soil under 13 years of conventional or zero tillage in southern Brazil . Soil & Tillage Research , 76 : 39 – 58 .
- Six , J. , Elliott , E.R. , Paustian , K. and Doran , J.W. 1998 . Aggregation and soil organic matter accumulation in cultivated and native grassland soils . Soil Science Society of America Journal , 62 : 1367 – 1377 .
- Six , J. , Conant , R.T. , Paul , E.A. and Paustian , K. 2002 . Stabilization mechanisms of soil organic matter : implications for C-saturation of soils . Plant and Soil , 241 : 155 – 176 .
- Stuart , A.G. , Gregory , A.P. and Erich , M.S. 2002 . Organic amendment and crop rotation effects on the recovery of soil organic matter and aggregation in potato cropping systems . Soil Science Society of America Journal , 66 : 1311 – 1319 .
- Tisdall , J.M. and Oades , J.M. 1982 . Organic matter and water-stable aggregates . Journal of Soil Science , 33 : 141 – 163 .
- Tripathy , R. and Singh , A.K. 2004 . Effect of water and nitrogen management on aggregate size and carbon enrichment of soil in rice-wheat cropping systems . Journal of Plant Nutrition and Soil Science , 167 : 216 – 228 .
- Yoder , R.E. 1936 . A direct method of aggregate analysis and a study of the physical nature of erosion losses . Journal of the American Society of Agronomy , 28 : 337 – 351 .
- Yoo , G. and Wander , M.M. 2006 . Influence of tillage practices on soil structural controls over carbon mineralization . Soil Science Society of America Journal , 70 : 651 – 659 .
- Zagal , E. , Rodrdríguez , N. , Vidal , I. & Quezada , L. ( 2002 ). Microbial activity in a volcanic ash soil under different agricultural management . Agricultura Técnica (Chile) , 62 , 297 – 309 . ( In Spanish .)
- Zotarelli , L. , Alves , B.J.R. , Urquiaga , S. , Torres , E. , dos Santos , H.P. Paustian , K. 2005 . Impact of tillage and crop rotations on aggregate-associated carbon in two oxisols . Soil Science Society of America Journal , 69 : 482 – 491 .