The effect of different tillage systems on the size distribution of aggregates and organic carbon distribution and storage in different size aggregates in a Hydragric Anthrosol were studied in a long-term experiment in Chongqing, China. The experiment included three tillage treatments: conventional tillage with rotation of rice and winter fallow (CT-r) system, no-till and ridge culture with rotation of rice and rape (RT-rr) system, and conventional tillage with rotation of rice and rape (CT-rr) system. The results showed that the aggregates 0.02–0.25 mm in diameter accounted for the largest portion in each soil layer under all treatments. Compared with the CT-r system, in the 0–10 cm layer, the amount of aggregates > 0.02 mm was larger under the RT-rr system, but smaller under the CT-rr system. In the 0–20 cm layer, the organic carbon content of all fractions of aggregates was the highest under the RT-rr system and lowest under the CT-rr system. The total organic carbon content showed a positive linear relationship with the amount of aggregates with diameter ranging from 0.25 to 2 mm. The storage of organic carbon in all fractions of aggregates under the RT-rr system was higher than that under the CT-r system in the 0–20 cm layer, but in the 0–60 cm soil layer, there was no distinct difference. Under the CT-rr system, the storage of organic carbon in all fractions of aggregates was lower than that under the CT-r system; most of the newly lost organic carbon was from the aggregates 0.002–0.02 and 0.02–0.25 mm in diameter.
Soil aggregates, the basic unit of soil structure, have a great influence on the physical, chemical,
and biochemical processes of soils (Lu and Li, 2002). Highly aggregated soil structure is the most
desirable condition for plant growth because it has a beneficial impact on soil moisture status, nutrient
dynamics, and soil tilth (Oades, 1984). Distribution of organic carbon in soil aggregates is one of the
most important properties of soil geochemistry (Pan, 1999). Binding of organic carbon to aggregates
in soils may be controlled by the soil organic mineral and biotic interaction (Six et al., 2004; Wen and
Guan, 2004).
The protection mechanism of soil aggregation may explain the effect of soil organic carbon sequestration
(Pan et al., 2003). Organic carbon in micro-aggregates was found to be more stable than that in
macro-aggregates (Puget et al., 2000), and the content of organic carbon, especially the newly formed,
is sensitive to the changes in land use and cultivation (Six et al., 1998; John et al., 2005). Aggregates
0.02–0.25 and 0.25–2 mm in diameter were found to be the main carrier of organic carbon in paddy soils
of the Taihu Lake region and the Paludalfs of the north Huaihe River region, China (Li et al., 2000a, b;
Zhang et al., 2001). Therefore, it is important to study the mechanism of organic carbon sequestration
based on the transformation of organic carbon at micro-aggregate level in cultivated soils.
Appropriate soil physical management is one of the key factors to maintain or improve its agricultural
productivity and/or to prevent soil and environment degradation (Lal, 2000); inappropriate management
(e.g., intense ploughing and not using cover crops) can cause rapid soil deterioration. Cultivation causes
reduction of macro-aggregates, but it does not affect micro-aggregate stability. Peixoto et al. (2006)observed that the content of organic carbon and macro-aggregates increase under the no-till system.
Guggenberger et al. (1999) found that there is a strong correlation between tillage intensity and the
turnover rate of soil organic carbon and aggregates. The content of organic carbon under no-tillage
system is higher than that under the conventional tillage, which results from the reduced turnover rate
of macro-aggregates in the no-till system. Huang et al. (2006) observed that the ridge culture with no-till
system increases the organic carbon storage in a purple paddy soil (Hydragric Anthrosol). However,
there are few investigations about the effect of tillage system on the size distribution of soil aggregates
and the soil organic carbon distribution and storage in different size aggregates. Luo et al. (2003)
showed a correlation between the amount of water-stable aggregates and the tillage intensity in a purple
soil. The objective of this study was to compare the effects of different tillage systems on the size
distribution of soil aggregates and the organic carbon distribution in different size aggregates based on
a long-term experiment.
It was clear that in the Hydragric Anthrosol in Chongqing area, which developed from the parent
materials of purple stone, the dominant portion of soil aggregates was those 0.02–0.25 mm in diameter,
and the lowest portion was those >2 and < 0.002 mm. Tillage systems obviously influenced the distribution
of aggregates and the organic carbon storage in the soil. The no-till and ridge culture (RT-rr)
system increased the amount of aggregates 0.02–0.25 and 0.25–2 mm in diameter in the 0–10 cm soil
layer, but the conventional tillage (CT-rr) system decreased the amount of these two fractions of aggregates.
Organic carbon was enriched in each fraction of aggregates in the soil surface layer (0–20 cm)
under the RT-rr system, but decreased under the other conventional tillage systems. It was evident that
the RT-rr system was more beneficial for the accumulation of organic carbon in the soil surface layer
and enhanced the formation of aggregates 0.02–0.25 and 0.25–2 mm in diameter, which implicated the
rotation of rice and rape under the no-till and ridge culture system as an efficient tillage practice for
organic carbon sequestration in Hydragric Anthrosols of Sichuan Basin of China.
