Enzymatically Labile Phosphorus Forms in Humin from Forest Soils

Humin plays a key role in various environmental processes like nutrient retention, carbon sequestration together with contaminant migration. Due to the resistance of humin to microbial degradation, exploring the mineralization process of phosphorus associated with humin is relevant to evaluate the lability of organic phosphorus and to develop a sustainable nutrient management strategy in ecosystems. Herein, humin was obtained from three climatically contrasting forest soils, including brown coniferous forest soil (Humic Cambisol), dark brown soil (Haplic Luvisol), as well as yellow brown soil (Haplic Luvisol), by treatment via a mixture of HF-HCl after soluble humic components were removed. The lability of phosphorus bound to humin and humic acid was examined by hydrolysis reaction with phytase from wheat, acid phosphatase from potato, and nuclease P1 from Penicillium citrinum alone or in combination. The results demonstrated that humin contained labile inorganic phosphorus as well as organic phosphorus. In the labile organic phosphorus, phytate-like phosphorus displayed the greatest proportion followed by simple monoester phosphorus and polynucleotide-like phosphorus. The labile total phosphorus was lower in humin than in humic acid. Among the three soils, labile total phosphorus was greater in brown coniferous forest soil than in yellow brown soil for humin and in dark brown soil than in yellow brown soil for humic acid. The labile total phosphorus was positively correlated well with Mn and Zn contents in humin and humic acid. Moreover, the lower proportion of labile total phosphorus in yellow brown soil could also be attributed to its higher pH value and silt and clay-size fractions compared to brown coniferous forest soil and dark brown soil. Our results can provide new information on the potential role of humin in P cycling and nutrition and are of significance for sustainable nutrient management in ecosystems.