| Sequestration of soil organic carbon (SOC) is main developed object in the process of reducing the emission of green house gases (GHGs) and sustainable development under the global change situation. In addition, a lot of people pay attention to the effect of agricultural soils in carbon storage and development including the carbon cycle at the same time. It is not clear to the potential and way of carbon sequestration in agricultural soils and GHGs reducing till now. While, the mechanism of carbon sequestration is also need to be studied. Paddy soil is a typical and main soil type in China soils. The main plant in the south of China is rice. Thus, paddy soils play an important role in the south of China. Moreover, the potential of carbon sequestration in paddy soils is high. Generally speaking, the pH was acidic, and the amount of SOC was high in some paddy soils in the south of China. However, the mechanism of carbon sequestration in these paddy soils is not clear now. We hypothesis that the carbon sequestration in paddy soils is conducted by interacted with iron oxides and convert to a stable status, and stabilize on molecular structure under the physical protection of aggregates. Therefore, three typical paddy soils developed from different pedogenesis in the south of China were used in this study. Moreover, a marsh soil without cultivated and developed from Yangtze River valley was also used in order to compared to paddy soils. Laboratory incubation experiment with maize straw for30,90and180days was conducted in three paddy soils and a marsh soil.13C stable isotope trace technique and off-line in the presence of tetramethylammonium hydroxide (TMAH) and followed by gas chromatography mass spectrometry (Pyr-TMAH-GC-MS) were used in order to study the mechanism of carbon sequestration in paddy soils and a marsh soil. Then, some new material could be provided for studying the mechanism of carbon sequestration. The main conclusions were as follows:1. Sequestration of SOC in whole soils and aggregates of paddy soils and marsh soilThe changes of amounts of SOC incubated with maize straw for30,90and180days was studied in paddy soil TP, developed from clayey lacustrine deposits in the Tai Lake plain, paddy soil RP, derived from red earth in the rolling red soil area, paddy soil PP, formed on Jurassic purple shale and sandstone and marsh soil MS, developed from Yangze River valley. The increase in SOC was higher in soil TP and soil RP than in soil PP and soil MS. While, the C/N ratio significantly correlated to the increased SOC. Maybe the C/N ratio related to the capacity of carbon sequestration in soils. Moreover, increase in SOC of particle size fractions was mainly in coarse fraction (2000-200μmn and200-20μm). The distribution of SOC in particle size fractions transferred from coarse fraction (2000-200μm) to fine fraction (20-2μm) in paddy soils with time, but SOC mainly distributed in coarse fractions (2000-200μm) of soil MS.In addition, different sources of SOC, such as SOC derived from paddy soils and maize straw, were also studied by13C isotope trace. It showed that there were more maize straw organic carbon sequestrated in soil TP and soil RP than in soil PP and soil MS, and there were more SOC derived from paddy soil in soil TP and soil RP too. It showed that the capacity of carbon sequestration in soil TP and soil RP is higher than in soil PP and soil MS. The maize straw organic carbon was mainly sequestrated in coarse fraction (2000-200μm) in results of13C isotope trace.2. Effect of free iron oxide (Fed) on carbon sequestration in paddy soils and marsh soilA positive correlation between Fe/Al-OC and free iron oxide (Fed) showed that the free iron oxide related to carbon sequestration. Both the Fe/Al-OC and humin were the main fractions of soil organic matter (SOM). The sum of them was more than90%in SOM. However, the percentage of Fe/Al-OC was decreased, but it was increased in humin with time. A positive correlation between increased SOC and increased Fe/Al-OC was found in30days of incubation with maize straw, but a positive correlation between increased SOC and increased humin was found in180days of incubation with maize straw. It showed that the convert from Fe/Al-OC to humin existed in carbon sequestration and was the process of carbon sequestration and stability. The distribution of the fractions of SOM was different in particle size fractions. The maize straw organic carbon was mainly sequestrated as Fe/Al-OC and humin in coarse fractions (2000-200μm and200-20μm) of paddy soils and marsh soil.3. Effect of hydrophobic organic matter on carbon sequestrationThe results of Pyr-TMAH-GC-MS analysis showed that increase in hydrophobic organic matter (lignin and lipids) and hydrophilic organic matter (polysaccharides) was found in this study. Then, both of them decreased with time, but the decrease in hydrophobic organic matter was slower than in hydrophilic organic matter. While, hydrophobic materials (lignin and lipids) were more stable than hydrophilic material (polysaccharides). Compared to30days of incubation with maize straw, the percentage of hydrophobic organic matter was higher in180days of incubation. It also showed that the hydrophobic organic matter was more stable than hydrophilic organic matter. Clay content, pH and amounts of free iron oxide were the effect factors of the amounts of hydrophobic materials in this study. There were more hydrophobic materials in soil TP and soil RP than in soil PP and soil MS. Maybe it was a reason for the high stability of SOC in soil RP. Moreover, the sequestration of hydrophobic organic matter in particle size fractions of various types of paddy soils was different. Hydrophobic materials mainly accumulated in coarse fractions (2000-200μmå'Œ200-20μm) when new organic matters were input in paddy soils and marsh soil. Then the increased amounts of hydrophobic organic matter decreased in the fractions of2000-200μm but, trended towards accumulating in other fractions of paddy soils with time. In contrary, the amounts of hydrophobic organic matter decreased with time in particle size fractions of2000-200μm,200-20μm, and20-2μm.4. Analysis of Fe/Al-OC by FTIR spectroscopy and13C CPMAS-NMRHydrophobic functional groups such as phenol groups were stable fractions of Fe/Al-OC analyzed by Fourier transfer infrared (FTIR) spectroscopy. It played a significant role in carbon sequestration of paddy soils and marsh soil. While, it may was an important reason for the stability of hydrophilic materials which was protected by hydrophobic materials. In addition, the Fe/Al-OC extracted from soil TP and soil RP contained more alkyl groups than in soil PP analyzed by13C CPMAS NMR in the end of incubation. The content of aromatic groups of Fe/Al-OC was high in2000-200μm and20-2μm fractions in soil TP and soil RP, but it was high were in200-20μm and<2μm fractions in soil PP. If we consider the increase in hydrophobic organic matter together, the changes of aromatic and alkyl groups of Fe/Al-OC may relate to the stability of SOC in particle size fractions.In summary, crop derived organic carbon was mainly sequestrated in coarse aggregates (2000-200μm). The amounts of Fed related to the carbon sequestration in paddy soils. Maybe the SOC were more stable in soil TP and soil RP because of the more Fed and hydrophobic organic matter. Carbon sequestration of SOC in paddy soils was that SOC interacted to Fed and converted to a stable status, and was stabilized on molecular structure under the protection of aggregates at the same time. |
PDF Full Text Request