Effects of Erythrina poeppigiana pruning residues on soil organic matter in organic coffee plantations

Abstract

The effects of Erythrina poeppigiana pruning residue effects on soil organic matter were investigated with three different objectives. Firstly, to study the effects of proximity of this shade tree on soil characteristics in organic and conventional coffee farms in Costa Rica, additionally a comparison between soil characteristics in organic and conventional farms was carried out (Chapter III). Secondly, to analyze the effects of Erythrina poeppigiana pruning residue additions on the size-density fractions, and on other fractions of soil organic matter (SOM) (Chapter IV), and thirdly, to investigate the effects on the application of microbial inocula or earthworm treatments on pruning residue decomposition, in order to evaluate the possibility of improving nutrient release from these pruning residues in the topsoil of organic coffee farms (Chapter V). A trend to higher total C and N concentrations for organic farms in comparison to conventional farms was found, but these differences were not significant in either of the two study years. No significant temporal changes in soil C or N concentrations were found between 2000 and 2004. In the 2004 study, significantly higher soil K and Ca concentrations were found near the shade tree due to the high contents of nutrients in E. poeppigiana pruning residues. All of the analyzed SOM fractions reflected SOM decomposition since they showed higher values when plant residue decomposition in the soil was incomplete (90 or 105 days). All the indicators also showed lower values or tended to have lower values at the end of each decomposition period (180 days). Total soil C was not a good indicator of short-term changes in labile SOM. No effects of microbial inocula or earthworm additions on soil K, NO3- or NH4+ concentrations were detected in the field trial. The microbial applications should be considered ineffective in increasing nutrient availability in field conditions and also in greenhouse conditions, due to weak temporary effects on maize seedling growth which were only observed in the first two weeks of the experiment on poor soils (taken from the 10-20 cm layer).