Relationship between macropores and soil organic carbon fractions under long-term organic manure application

Greenhouse vegetable production systems converted from cereal fields often experience a decrease in soil aeration due to intensive land use. Organic fertilization is a method commonly used to solve this problem. Although previous studies have considered soil organic carbon (SOC) or soil pores under organic cultivation, the relationship between SOC fractions and soil macropores is unknown. This study analysed the long-term progressive variations in soil macropore (>50 mu m) and SOC fraction characteristics in organic greenhouse vegetable plantations (1-year-old, 9-year-old, and 14-year-old fields) converted from rice-wheat rotation fields and established the relationship with X-ray computed tomography scanning and C-13 solid-state nuclear magnetic resonance spectroscopy. The results suggested that the total macroporosity increased from 7.95% to 16.36% throughout the whole organic plantation, and a similar trend occurred in small (50-500 mu m) and medium (500-1,000 mu m) macroporosity. Besides, the total SOC content and the contents of its fractions increased with organic planting time. With the total macroporosity exhibiting significant associations with SOC (r = .81*), the relative proportion of O-alkyl C (r = -.94*), and the relative proportion of aromatic C (r = .89*), a lower O-alkyl C proportion of SOC might help to increase macroporosity. A similar relationship was found between the relative proportion of SOC fractions and the macroporosity. The O-alkyl C proportion had significantly negative relations with aromatic C (r = -.94*) and SOC (r = .84*). Overall, both SOC and its fractions can help to improve soil macroporosity, but the effects of different fractions on these pores vary.