Dynamic of organic matter in the heavy fraction after abandonment of cultivated wetlands

The abandonment of cultivated wetland soil increased the contents of light fraction organic matter (LFOM), heavy fraction organic matter (HFOM) and soil organic matter (SOM). The LFOM and HFOM content increased to 13.3 g kg(-1) and 62.4 g kg(-1) after 5 years whereas they were 8.4 and 47.9 g kg(-1) after 9 years of cropping, respectively. Fourteen years after abandonment, HFOM content increased to 104.3 g kg(-1). LFOM was positively correlated with HFOM (p<0.001). A Langmuir equation was used to calculate the highest HFOM value. The value for the natural wetland soil was closed to this theoretical value (140.8 g kg(-1)). After 14 years of abandonment, the HFOM maximum (HFOMMax) value was lower than the equilibrium value suggesting that a further increase in HFOM can occur after abandonment. Assuming a linear accumulation (3.87 Mg C ha(-1)yr(-1)), it would take approximately 24 years after the abandonment to reach the HFOMMax value.