Soil organic carbon (SOC) is one of the main carbon reservoirs in the terrestrial ecosystem. It is important to study SOC dynamics and effects of organic carbon amendments in paddy fields because of their vest expansion in south China. A study was carried out to evaluate the relationship between the SOC content and organic carbon input under various organic amendments at a long-term fertilization experiment that was established on a red soil under a double rice cropping system in 1981. The treatments included non-fertilization (CK), nitrogen-phosphorus-potassium fertilization in early rice only (NPK), green manure (Astragalus sinicus L.) in early rice only (OM1), high rate of green manure in early rice only (OM2), combined green manure in early rice and farmyard manure in late rice (OM3), combined green manure in early rice, farmyard manure in late rice and rice straw mulching in winter (OM4), combined green manure in early rice and rice straw mulching in winter (OM5). Our data showed that the SOC content was the highest under OM3 and OM4, followed by OM1, OM2 and OM5, then NPK fertilization, and the lowest under non-fertilization. However, our analyses in SOC stock indicated a significant difference between OM3 (33.9 t ha−1) and OM4 (31.8 t ha−1), but no difference between NPK fertilization (27 t ha−1) and non-fertilization (28.1 t ha−1). There was a significant linear increase in SOC over time for all treatments, and the slop of linear equation was greater in organic manure treatments (0.276–0.344 g kg−1 yr−1) than in chemical fertilizer (0.216 g kg−1 yr−1) and no fertilizer (0.127 g kg−1 yr−1).
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