Biological effects of plant residues with constrasting chemical compositions on plant and soil under humid tropical conditions

A study on plant residues with contrasting chemical compositions was conducted under laboratory, growth chamber and humid tropical field conditions to understand the function of the soil fauna in the breakdown of plant residues, the cycling of nutrients, in particular nitrogen, and the performance of maize as a test crop. Leaves from ten agroforestry and fallow plant species with a wide range of chemical compositions (different C/N ratios and contents of lignin and polyphenols) were selected for laboratory incubations. Of the ten species, prunings from Acioa barteri , Gliricidia sepium and Leucaena leucocephala , maize ( Zea mays ) stover and rice ( Oryza sativa ) straw were selected for field and growth chamber experiments.

Laboratory incubation showed that N mineralization was significantly correlated with initial N, polyphenol and lignin content of plant residues. Litterbag studies in the field showed that decomposition was also correlated with the activity of soil fauna. In the field the effect of polyphenols on the degradation of plant residues was less prominent than in the laboratory. In both growth chamber and field trials the breakdown of plant residues and nutrient release were enhanced in the presence of earthworms and millipedes.

Application of plant residues increased populations of earthworms and ants. Stepwise regression illustrated populations of earthworms were negatively correlated with lignin:N ratio; populations of ants were positively correlated with N content of plant residues. Application of high C/N ratio and lignin content plant residues attracted termites. Millipedes were not influenced by the quality of plant residues.

Addition of plant residues as mulch ameliorated the soil microclimate by lowering soil temperature and maintaining soil moisture. A strong mulching effect on the soil microclimate was associated with application of plant residues with high C/N ratio and lignin content.

Increase in the amount of mineral N in the soil solution was recorded with application of plant residues with "high" or "low quality", but probably caused by different mechanisms. The N contribution of "high quality" materials mainly originated from their decomposition, whereas mulching effects on microclimate, which presumably promoted the degradation of soil organic matter, mainly accounted for the N contribution of "low quality" materials. There is a need to both consider the direct and indirect contributions of plant residues applied as mulch.

Markedly improved crop performance was achieved with the application of plant residues with "high" or "low quality". Materials with "intermediate quality" had no significant impacts.

Results of the study indicate the possibility to influence nutrient cycling by manipulation of soil faunal activity.

It is concluded that a keen choice of plant residues in terms of nutritional effects and mulching-induced effects on soil microclimate is a prerequisite for the application of soil fauna-mediated decomposition towards synchronization of soil nutrient supply and crop nutrient demand.