Paddy soils continuously cultivated for hundreds to thousands of years still sequester carbon

Abstract

Soil organic carbon (SOC) dynamics in paddy soil profiles along a rice cultivation chronosequence were investigated to explore the effects of long-term rice cultivation on SOC dynamics in soil profiles. The rice cultivation chronosequence included the time periods of approximately 50, 100, 500, 700, 1000 and 2000 years, and the selected field sites were located in Cixi City, Zhejiang Province, China (50-year paddy field: 30°11′03″N, 121°21′19″ E; 100-year paddy field: 30°09′50″N, 121°20′57″E; 500-year paddy field: 30°12′25″N, 121°07′45″E; 700-year paddy field: 30°10′25″N, 121°09′06″E; 1000-year paddy field: 30°09′45″N, 121°06′57″E; 2000-year paddy field: 30°08′13″N, 121°21′36″E). The results showed that the cultivation periods had no significant impact on the SOC contents in the top horizons of the paddy fields after 50 years of rice cultivation. The SOC contents below the depth of approximately 40 cm for the older paddy soils were higher in comparison to paddy soils cultivated for 50–500 years. SR was the ratio of the SOC density in the 0–40 cm layer to that in the 40–80 cm layer. Across the chronosequence, SR values of the paddy fields decreased as the rice cultivation periods increased in duration. The difference in δ13C between the bottom layers and topsoil decreased as the duration of the cultivation periods increased. Below the soil depth of about 60 cm, the difference in δ13C between SOC and the rice-derived residue also decreased as the duration of the cultivation periods increased. The C/N ratios at the depth of approximately 40–60 cm began to increase gradually with the duration of rice cultivation periods after 500 years of rice cultivation. The changes in SR, δ13C and C/N ratios in soil profiles indicated that long-term rice cultivation is able to cause weakly decomposed and low δ13C rice-derived organic carbon to be accumulated in subsoil. Increasing amounts of organic carbon were sequestrated in the subsoil as the duration of the rice cultivation periods increased. These findings support the fact that paddy soils cultivated for hundreds to thousands of years were still potential C sequestrators.