Phosphorus cycling during decomposition of plant residues in weathered soils from the tropics: Influence of plant factors

Incorporation of plant residues in tropical cropping systems has become an option to build up P availability. In addition to the C:P ratio, other plant factors potentially affect the ability of plant residues to supply P to subsequent crops. Incubation studies were conducted to evaluate the effect of selected plant characteristics on P availability and P transformations during residue decomposition. Plant residues of Sorghum bicolor and Crotalaria juncea with similar C:P ratio, but differing in biochemical factors (acid phosphomonoesterase), content of plant inorganic P (plant Pi) and quality of C substrates (contents of dissolved organic C, cellulose and lignin), were evaluated to determine their influence on resin P, microbial P and P associated with the particulate organic matter (POM-P). Sixty days after decomposition, soils incubated with crotalaria residue had on average 75% more resin P than soils incubated with sorghum residue. Soils incubated with sorghum had by contrast, 1.5 times larger immobilization of P and slower rates of microbial P losses. Changes in POM-P showed that a significant proportion of the microbially immobilized P occurred in the partially decomposed sorghum residues which was mediated by soil fungi. These results suggest that plant residue types might affect P availability by changing decomposer communities and turnover of microbial P. Quality of C compounds in the residues, as compared with plant Pi and phosphatase activity, seemed to be the main factor responsible for this substrate dependency. Hypothesis testing using a simulation model confirmed conclusions found in the incubation studies. These results suggest that factors, other than C:P ratio, should be considered when evaluating P availability from plant residues.