SUITMA 2005 Cairo - Soil organic matter composition in ancient and prehistoric paddy soils from the lower Yangtze
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Historical sites theme of the SUITMA 2005 symposium. |
| Heike Knicker,i | Z. H. Cao,ii |
| Z.Y. Hu,ii | Ingrid Kögel-Knabner.i |
- i - Lehrstuhl für Bodenkunde, Technische Universität München, Freising-Weihenstephan, Germany
- ii - Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China.
China is a country of long rice cultivation history which can be dated back to the Neolithic age. Nowadays, rice (Oryza Sativa L) is still one of the major crop and irrigated fields account for more than 50% of the total area for rice production. Examination of prehistoric irrigated rice fields can increase our knowledge about ancient agricultural history and improve understanding of paddy soils and the processes involved in their formation and development. We concentrated on the characterization of SOM in archeologically ancient and recent paddy soils by solid-state 13C NMR spectroscopy. The used materials derived from an excavation site close to Suzhou, China in the Yangtze River Delta. Here, in an open area of 300 m2 prehistoric irrigated rice fields were discovered at a depth of 100 cm. Each rice field was surrounded with ridges that were connected via outlets to ditches and ponds to control the water level. Earthenware pots used as irrigation tools and carbonized cultivated rice grains with an age (14C) of 3903 BC were recovered in these fields or ponds.
The upper part of the studied soil profile (0-42 cm) consists of the present paddy soil developed on material, deposited by floods above a different ancient paddy soil, possibly about 1320 BC. The prehistoric rice field below this ancient soil expands from 57 cm to a depth of 160 cm and overlays the parent loess deposition (160 -200 cm).
NMR analyses of SOM in the recent soil showed a typical pattern expected for arable soils with a dominance of O-alkyl C. Down to the depth of 42 cm, the proportion of aromatic C doubled (45%), possibly due to the accumulation of charred residues produced during former burning of harvest residues. The ancient paddy field, too, disclosed an increase of aromatic C starting from 40% of the organic C in the layer 42-57 cm to 66% at the depth 75-100 cm. The low contribution of O-alkyl C and alkyl C (16%) may either be caused by the preferential degradation of uncharred SOM during the last 1000 years, or could indicate that most of the SOM precursors were of pyrogenic origin i.e. carbonized rice corns and hulks. The high proportion of carboxyl C agrees with a high oxidation degree of the aromatic moieties. The SOM of the prehistoric rice field revealed a feature comparable to that of the deeper ancient paddy soil (75 to100 cm). However, here, the aromatic proportion decreased with soil depths. Aromatic moieties comprising between 40 and 50% of the SOM were also identified in the parent material, indicating leaching of charred residues through the soil profile. The high carboxyl C content of the aged char may have supported dissolution and transport, but also subsequent retention by adsorption to the mineral phase. Comparing the ancient/prehistoric SOM with that of recent paddy fields reveals a considerable long-term impact of frequent burning on its quality which is reflected by an alteration to predominantly aromatic structures. Transport of such structures with the changing water table may explain the relatively high C content (6 mg g-1 soil) and considerable aromaticity in the parent material occurring at a depth below 160 cm.
