Response of soil microbial biomass dynamics to quality of plant material with emphasis on P availability

Soil microbial biomass is a major sink and source of plant available P and transformer of soil organic P. The dynamics of microbial biomass P (BP) and C (BC) and resin extractable P (Pex) in a sandy loam soil (Kandic Rhodustalf) after amending with plant materials of diverse quality were examined in a laboratory incubation study. Quality variables of the plant materials were described by the proportion of dry matter as total P, total N, lignin (LIG), and soluble polyphenolics (Pp). The materials were added to the soil at a rate equivalent to 10 t ha-1 (dry weight) and kept at 50% field moisture capacity and 25 degrees C. Included was an unamended control soil. During incubation of up to 56 d, soil samples were periodically analyzed for Pex, BP and BC. Quality variables affecting Pex, BP, BC and BP-to-BC ratio were total P, total N, N-to-P and C-to-P ratios. Total P appeared to be the most important quality factor affecting Pex, BP, BC and BP-to-BC ratio. There was no consistent significant correlation between Pex and BP or between BP and BC. This was attributed to the complexity of the dynamics of Pex and BP pools and the absence of a significant microbial turnover due to the short incubation periods assessed in the study. By virtue of the high P content and resulting in a low BP-to-BC ratio, leaves of the shrubby non-legume Tithonia diversifolia (Hemsley) A. Gray, had a distinct advantage of replenishing Pex in the short-term and yet maintain a high potential to release P through microbial turnover. The tree legume Sesbania sesban, on the other hand, resulted in a low BP-to-BC ratio but also had a low P content and may be useful for the build up of soil organic matter (SOM) and P release in the long term. Exploiting plant materials with quality variables exhibited by T. diversifolia might create the potentials for fast replenishment of solution P as well as improving SOM.