Changes of soil organic carbon and its fractions in relation to soil physical properties in a long-term fertilized paddy

Soil organic carbon (SOC) has an important role in improving soil quality and sustainable production. A long-term fertilization study was conducted to investigate changes in SOC and its relation to soil physical properties in a rice paddy soil. The paddy soils analyzed were subjected to different fertilization practices: continuous application of inorganic fertilizers (NPK, N-P-K = 120-34.9-66.7 kg ha(-1) yr(-1) during 1967-1972 and 150-43.7-83.3 kg ha(-1) yr(-1) from 1973 to 2007), straw based compost (Compost, 10 Mg ha(-1) yr(-1)), a combination of NPK + Compost, and no fertilization (control). Soil physical properties were investigated at rice harvesting stage in the 41st year for analyzing the relationship with SOC fraction. Continuous compost application increased the total SOC concentration in plough layers and improved soil physical properties. In contrast, inorganic or no fertilization markedly decreased SOC concentration resulting to a deterioration of soil physical health. Most of the SOC was the organo-mineral fraction (<0.053 mm size), accounting for over 70% of total SOC. Macro-aggregate SOC fraction (2-0.25 mm size), which is used as an indicator of soil quality rather than total SOC, covered 8-17% of total SOC. These two SOC fractions accumulated with the same tendency as the total SOC changes. Comparatively, micro-aggregate SOC (0.25-0.053 mm size), which has high correlation with physical properties, significantly decreased with time, irrespective of the inorganic fertilizers or compost application, but the mechanism of decrease is not clear. Conclusively, compost increased total SOC content and effective SOC fraction, thereby improving soil physical properties and sustaining production. (C) 2009 Elsevier B.V. All rights reserved.