Microbial Phosphorus Immobilization slows Soil Phosphorus Cycling in Tropical Secondary Succession

Abstract:

Tropical secondary forests regenerating on formerly deforested land act as carbon sinks and are important for biodiversity conservation, but their regrowth potential can be constrained by the availability of soil phosphorus (P). However, detailed knowledge of how soil P cycling changes during tropical secondary succession, especially with regards to interactions with soil microbes, is lacking. Soil microbes can solubilize P from pools unavailable for plants, but they also compete for and immobilize P in their biomass, which often increases during succession. The activity of soil phosphatase enzymes, which are released by plants and microbes in order to cleave P from unavailable organic sources, can serve as indicator for potential changes in P demand with progressing succession. In order to study how soil microbes interact with P availability during tropical secondary succession, we conducted a chronosequence study in the lowland moist tropics of Panama, consisting of 58 secondary forest sites ranging from 0 to 39 years of regrowth and 5 mature forests. We measured successional changes in microbial stoichiometry and soil exoenzyme activities and set them in relation to changes in soil total, microbial and available P pools. Soils across all successional stages were very low in P, with total P below 250 ppm and available P below 0.3 ppm on average. Forest regrowth led to increases in soil organic carbon (C) while soil available P decreased with succession. The decrease in soil P was accompanied by increases in microbial biomass P, so that the contribution of microbial P to total P increased from 6 to 12%. Surprisingly, microbial biomass P increased independently of microbial biomass C, leading to decreases in microbial C:P ratios. The decrease in available P was correlated to phosphatase activity, which increased 2.3-fold during succession, while enzymes involved in carbon and nitrogen acquisition didn’t change. Our findings suggest a strengthening of biological P demand during secondary succession. As succession proceeds P availability for plants decreases and microbes are in increasing control over P cycling. Therefore, in late-successional forests, plants may primarily depend on P released from microbial turnover or on accessing P from organic sources.