Phosphorous fractions in soils of rubber-based agroforestry systems: Influence of season, management and stand age.

Rubber-based agroforestry system is a vital management practice and its productivity is often controlled by soil phosphorus (P) nutrient, but little information is available on P fractions dynamics in such system. The aim of this study was to examine the seasonal, management and stand age effects on P fractions, acid phosphatase activity, microbial biomass P, other physical-chemical properties and litter and roots in four systems: 10-year-old rubber mono- (YM) and intercropping (YI) with N-fixing species (NFS), 22-year-old mono- (MM) and intercropping (MI) in Xishuangbanna, Southwestern China. Most P fractions varied seasonally at different depths, with highest values in the fog-cool season (i.e. labile P at 5-60cm, non-labile P and total P at 30-60cm). By contrast, moderately labile P varied little over time, except in MI that had lower values in the rainy season. Compared with their monoculture counterparts, YI doubled resin-Pi concentration but decreased NaHCO3 -extractable P, HCl-Pi and residual-Po at the 0-30cm depth, whereas MI had hardly any changes in P species at the 60-cm depth. Surprisingly, residual-Po was enriched down to the deepest soil (30-60cm) in both YI and MI in the fog-cool season. All P fractions, except NaOH0.1 -Pi , were greatly reduced with increasing stand age. In addition to plants uptake, these changes can be explained by seasonality in soil environments (e.g. moisture, temperature, pH and microbial activity) and decomposition of litter and roots. Moreover, YI decreased labile Po stock, but MI increased moderately labile Pi at the 60-cm depth across seasons. The results imply that a large amount of residual-Po exists in acidic Oxisol from China and that they can be reasonably exploited to reduce the application of P fertilizers, highlighting the importance of Po pool. Taken together, intercropping mature rubber plantation with NFS appears to be an effective way to enhance productivity while maintaining adequate soil P concentration over the long run.