In temperate forest ecosystems, phosphorus (P) leached into the mineral soil largely adsorbs to pedogenic iron (Fe) and aluminum (Al) oxides. This raises the question as to which extent adsorbed P in mineral soil can be recycled and whether soil P scarcity promotes microbial communities better adapted to use these P sources. To investigate the mobilization of P bound to hydrous Fe oxides under natural conditions, goethite with adsorbed orthophosphate (OP) and phytic acid (PA) was buried for 35 months in beech ( Fagus sylvatica ) forest soils at three sites in Germany varying in geogenic P supply. We quantified total and surface P losses by X-ray fluorescence spectrometry and photoelectron spectroscopy and assessed changes in bioavailable P by sequential extractions (resin and NaHCO 3 ). Surrounding soil and goethite samples were analyzed for amino sugars, acid phosphomonoesterase activity, and microbial metagenomic properties. Total losses of OP and PA ranged between 1% and 58%, with lower losses of PA due to stronger sorption complexes. Resin- and NaHCO 3 -extractable P declined by 48%–94% and 31%–75%, respectively. Besides P desorption caused by concentration gradients, microbial P mining explained higher P losses under soil P scarcity. Based on stocks of P bound to pedogenic Fe and Al oxides, mineral P saturation levels, and in-situ P release rates, we infer that oxide-bound P in a P-poor sandy soil could likely not meet the P requirement of the vegetation, despite higher P losses. This result supports the idea that vegetation under P-deficient conditions depends more on P recycling in forest floor layers than under conditions of high P availability. Conversely, at the loamy P-rich sites, release of P bound to Fe and Al oxides was high enough to contribute to forest nutrition. Our results indicate that P, especially OP, when leached into mineral soil and sorbed to Fe and Al oxides, is not fully passivated but partly recycled at high P saturation levels, thus contributing to biological assimilation or downward P translocation.
Mikutta et al. (Fri,) studied this question.