Dynamics of size–density fractions of soil organic matter following the addition of tree litter to organic coffee farms

The addition of organic matter to soil is frequently viewed as a vital intervention to maintain soil quality. The aim of this study was to investigate the temporal response of the soil macroorganic fraction to different organic coffee farming practices (e.g., plant residue, earthworm and microbial inocula addition). Three density fractions of macroorganic matter (>150 μm) were studied during 1 year after adding shade tree (Erythrina poeppigiana) pruning residues to the soil (5 t ha−1 twice at 6 monthly intervals). Soil macroorganic matter represented only a small proportion of total soil organic matter (SOM) (3–6% of total). Even though the total amount of SOM did not change over time, significant temporal changes in the size of the macroorganic fraction were observed that appeared to be largely independent of the management regime. The light density fraction seemed to be the most responsive fraction and this study suggests that it may provide a qualitative indicator of the ‘active’ fraction of SOM; the size of the macroorganic fraction did not provide a reliable indicator of the rate of litter decomposition or nutrient release. The addition of microbial inoculants and earthworms had only a small and inconsistent effect on macroorganic matter dynamics and these practices appeared to offer little agronomic benefit. This study highlights the need for continued organic matter inputs to maintain soil C reserves and preserve soil organic quality in tropical organic farming systems.