Dynamics of density fractions of macro-organic matter after forest conversion to sugarcane and woodlots, accounted for in a modified Century model

Soil organic matter (SOM) is the major controlling factor of soil fertility for low external input agriculture. However, most models describing SOM dy namics are based upon pools which are not directly measurable. We developed a SOM submodule for the CENTURY model based on Ludox particle size density fractions. The turnover rates of these easily measurable fractions were de termined by assessing their C-13 isotope signatures in a chronosequence und er surgane after rainforest conversion. The net monthly decomposition rates of light (L), intermediate (I) and heav y (H) fractions of macro-organic matter (150 mu m - 2 mm size) under sugarc ane cultivated for 2-10 years following forest removal ranged from 0.0162 a nd 0.0154 month(-1) for forest-derived L and I fractions to 0.0118 month(-1 ) for H fractions, while for unfractionated forest soil organic matter it w as 0.0068 month (1). The soil carbon of the CENTURY model was reconstructed and the 'slow' (SOM2) pool was divided into L, I, H and R fractions, where the R (resistant) fraction represents the 50-150 mu m size fraction. The m odified CENTURY model simulated the dynamics of L, I and H fractions as wel l as total organic carbon (C%) under sugarcane with a coefficient of determ ination (R-2) of 0.90, 0.95 and 0.98. respectively. Without further adjustm ents the model was applied to woodlots of Gliricidia sepium and Peltophorum dasyrrachis. The model accounted for 60% of the variation in measured ligh t (L) fraction in the 0-5 cm layer under Gliricidia and Peltophorum, but on ly for 40% of the variation in the I and H fraction data. Results thus show some progress in linking SOM models to measurable soil organic matter frac tions, but are not yet satisfactory for the heavier fractions, more strongl y associated with mineral particles. Experimental data for these fractions show a considerable spatial variability, possibly linked to activity of soi l fauna, not covered by the model.