Using a comprehensive model for cropland types in relationships between soil bulk density and organic carbon to predict site-specific carbon stocks

Site-specific modeling with consideration of land-use types is required to evaluate the effectiveness of different farming practices on organic carbon (OC) sequestration by soil. In this study, a comprehensive model for different cropland types fitting to the relationship between bulk density (Bd) and OC was used to evaluate carbon sequestration by soil.

Six cropland sites for crop cultivation (i.e., lettuce, lisianthus, pineapple, multigrain, tea, and coffee) in southwest Taiwan were utilized for illustration. The linear (L) and exponential decay (ED) models f_L(x) and f_ED(x) were fitted to the relationship between Bd and OC. The integrations of f_L(x) and f_ED(x) associated with site-specific boundaries D_L^* and D_ED^* were in expression as F_L^*(x) and F_ED^*(x), respectively to predict OC stock (OCS) for each cropland site.

The results showed a significantly negative correlation between Bd and OC. According to the validation results, the site-specific predictions of OCS by both F_L^*(x) and F_ED^*(x) were reliable for the cropland sites. Also, further OC sequestration to increase OCS by 0.4% in a 20-cm depth of soil would more efficiently occur in deeper soil than in upper soil. Current practices for cultivating lisianthus, tea, and coffee, increasing OC would be more conservational and result in a significantly decreasing Bd trend.

In contrast, croplands for the cultivation of lettuce, multigrain, and pineapple, which have much less OC saturation, could efficiently increase OCS by 0.4% in a 20-cm soil depth for further OC sequestration.