Tree roots exert greater impacts on phosphorus fractions than aboveground litter in mineral soils under a Pinus sylvestris var. mongolica plantation

Abstract

Detritus inputs from aboveground litter and belowground roots, as well as roots exudates sustain soil phosphorus (P) supply in forests, but how the aboveground litter and roots differentially and interactively drive soil P transformations and availability is poorly understood. We experimentally removed aboveground litter, and then excluded roots in litter intact/removed plots in a nutrient-poor Mongolian pine (Pinus sylvestris var. mongolica) plantation in northern China. Phosphorus fractions with different bioavailability (labile P, moderately labile P, stable P, and occluded P), P adsorption characteristics and related biochemical properties in mineral soils were examined five years after the establishment of the experiment. Root exclusion significantly reduced labile organic P by 56%, and did not affect other recalcitrant organic P fractions and total organic P in both the litter intact and removed plots. Root exclusion significantly elevated all inorganic P fractions by 21%–26% in the litter removed plots, but not in the litter intact plots. Litter removal alone did not change soil P fractions except that it slightly increased labile organic and stable inorganic P concentrations. Maximum P adsorption capacity was improved by litter removal, but maximum P buffer capacity was neither affected by litter removal nor by root exclusion. Soil organic P fractions were correlated closely with soil microbial biomass, acid phosphatase activities and soil organic carbon, while inorganic P fractions were correlated significantly with exchangeable Ca2+ concentration and pH. The results revealed the predominant role of roots in driving the mobilization of inorganic, as well as mineralization-immobilization of organic P in this Mongolian pine plantation, and highlighted the facilitation of aboveground litter on root driving mobilization of soil inorganic P.