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 E,ythrina 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 of 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). Respect the first objective, the impact of Erythrina poeppigiana on soil characteristics, at three different positions relative to the shade tree and from three different soil depths, were evaluated in five paired coffee farms (organic and conventional) in central Costa Rica in 2000 and 2004. In both years, C and N concentrations at 0-5 cm were similar for all positions in the organic system probably due to an even distribution of pruning residues; but in the conventional system, higher C and N concentrations were found close to the shade tree vs. the positions 2 m from the trunk. This finding highlighted the importance of E. poeppigiana in maintaining SOM levels. 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. Organic systems had similar nutrient concentrations (P, K, Ca, Mg and Zn and NO3-) to conventional farms at 0-5 cm. A balance of nutrient inputs-outputs in each farm within both systems appears to explain the absence of differences between systems. Respect the second objective, two field trials were set up to evaluate the effects of E. poeppigiana pruning residue addition on size-density SOM fractions (2002 and 2004). Within the first field trial, the effects of the addition of 10 Mg ha-1 of E. poeppigiana pruning residues on three size density fractions, were evaluated over a 330-day period. In the second field trial, the effects of the addition of E. poeppigiana pruning residues were evaluated in a 105-day trial, with soil samplings every 15 days to determine if there were early effects of treatments on SOM fractions. Additionally, the macroorganic fraction (> 150 μm), total C in the 2002 trial and POM (>53 μm) in 2004 were evaluated as indicators of changes due to pruning residue management. 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. The amount of C in the light fraction appeared to be the best indicator of changes in labile soil C due to pruning residue management. However, an economic evaluation of the costs of size density-fractionation is recommended because evaluations of macroorganic C or dry weight of LF (as opposed to C content of LF) may be less expensive indicators of short-term labile SOM decomposition. Carbon in macroorganic matter after 330 days was reduced by 33% at CATIE and 44% at Pejivalle in the residue only treatment, and by 62 and 52% in the respective bare soil controls, highlighting the importance of pruning residue additions in organic coffee farms to maintain the active soil C components. Respect the third objective, in the 2002 and 2004 field trials (see preceding paragraph), the effects of E. poeppigiana pruning residue additions on soil K, NO3-or NH/ concentrations, with and without microbial inocula or earthworms additions, were tested. An additional greenhouse trial was set up to evaluate the effects on maize seedling growth, of pruning residues with microbial inocula applications. The positive effects of pruning residue additions on soil K concentrations were detected in both the field and the greenhouse trials. No effects of microbial inocula or earthworm additions on soil K, NO3-or NH/ 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). Microbial mixture and earthworm applications also should be considered ineffective due to their insignificant and temporary impact on labile SOM in 2002, and because no significant effects were detected when disturbance factors were strictly controlled in the 2004 trial.