2000 years of paddy soil development - gain and loss of soil carbon
Abstract
The long-term impact of lowland rice growing on C stocks in soils was evaluated in the coastal region of subtropical China. During the past 2000 years new farmland was created through consecutive land reclamation by protective dikes, providing a unique chronosequence of soil formation under agricultural use. Parts of the land were used for paddy rice, other parts for a variety of non-irrigated crops, allowing study of the C dynamics of paddy soil development in direct comparison to soils not used for lowland rice cultivation (non-paddy). Beside one soil profile at the mudflat, which represents the parent material, soil profiles of the chronosequence were sampled by horizon in triplicate, ranging from 50 to 2000 years of paddy soil use and from 50 to 700 year old non-paddy soils. All samples were analysed for 14C concentrations and for bulk density, total C (TC) as well as organic C (OC) and inorganic C (IC) concentrations, and the corresponding stocks were calculated. In addition, topsoils were subjected to a physical fractionation procedure to evaluate changes of OC saturation in particle size fractions during soil development. In the first 300 years, rice cultivation did not significantly change TC stocks of the soils compared to that of non-paddy soils. The fast decalcification in paddy soils was compensated by a similar gain of OC in the topsoil. Higher 14C concentrations indicate the original C of the sediment in paddy topsoils was replaced faster by recently photosynthesized C than in non-paddy topsoils. OC accumulation in paddy topsoils appears to be complete after 300 years. High increases of OC in paddy topsoils may be due to high OC inputs associated with retarded decomposition during flooding of paddy fields. After 300 years, the decalcification in paddy soils was not compensated anymore by further enrichment of OC in the topsoil. In addition, the paddy soils lost OC in the subsoil. The dense plough pan decreases OC transport from the paddy topsoil into the subsoil, while OC transport in non-paddy soils is unimpeded. Therefore, the replacement of the old sedimentary C by new OC was slower in the paddy subsoils than in the non-paddy soils as indicated by 14C data. The enrichment of OC in paddy topsoils in the first 300 years can be directly related to increasing OC contents and saturation levels in the silt- and clay-sized fractions. However, ongoing redistribution of OC between silt- and clay- sized fractions after 300 years indicates that small-scale processes of OC storage are still proceeding.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.B41H0409K
- Keywords:
-
- 0486 BIOGEOSCIENCES / Soils/pedology