Speciation and microscale distribution of phosphorus compounds accumulated in continuously fertilized greenhouse soils

Abstract

The overuse of phosphate fertilizer results in the accumulation of surplus phosphorus (P) compounds in soil, and this trend is particularly intense in greenhouse farming. We aimed to characterize P compounds in greenhouse soils by comparing their speciation in soil samples collected from greenhouses and an open field. Two soil types with different phosphate sorption abilities, namely, Ultisol and Andisol, were considered. Phosphorus compounds in the soil were characterized by bulk soil analysis via acid extraction; molecular-scale analyses by solid-state ³¹P nuclear magnetic resonance and P K-edge X-ray absorption near edge structure and spatially resolved analyses by electron probe microanalyzer and micro-X-ray absorption near edge structure. Spectroscopic results showed that a larger proportion of P compounds associated with inorganic P compounds associated with calcium  (Ca-P) was present in the greenhouse soil than in the open-field soil. Some P compounds in the greenhouse Andisol were Ca-P, even though Andisol has a high phosphate sorption ability due to an abundance of aluminum-bearing sorbents. Heterogeneously distributed spots of Calcium phosphate, most likely hydroxyapatite or tricalcium phosphate, were found at the microscale in the greenhouse Ultisol soil grain. In addition to the detection of poorly-soluble Ca-P, a larger proportion of Ca-P was potentially plant available. Spatially resolved analyses showed that organic amendments containing Ca-P were incorporated into the Andisol soil grains. Our study demonstrated that Ca-P with different solubilities can serve as a source of phosphate even in soils of high phosphate sorption ability under greenhouse conditions.