Characteristics of inorganic and organic phosphorus in Lake Sha sediments from a semiarid region, Northwest China: Sources and bioavailability

Phosphorus (P) is a key limiting nutrient in many lake ecosystems and the primary limiting nutrient for cyanobacterial blooms in most eutrophic lakes. Sources of organic matter, forms of inorganic P (Pi) and organic P (Po), their bioavailability were investigated in the sediments of Lake Sha located in the semiarid region of Ningxia Province, China. Sources of P in sediments were further explored, which can provide important information for better comprehensive control and management of external and internal loads of P in lakes. C/N ratio lower than 10 combined δ13C values (−28.5‰ to −26.4‰) and δ15N values (+4.2‰ to +5.2‰) indicated that the organic matter mainly resulted from mixed inputs of aquatic plants, bacteria and algae, and some terrestrial C3 plants (A plant in which the first product of photosynthesis is a 3-carbon acid, phosphoglyceric acid). Particulate matter from terrestrial C3 plants with recharged water from the Yellow River was likely an important source. The degradation rate of organic matter in sediments was likely relatively high due to the location of Lake Sha in a semiarid region. The contents and relative proportion of P forms in Lake Sha sediments decreased in the order Ca–P (calcium-bonded phosphorus) > Res-P (residual phosphorus) > Al–P (aluminum-bound P) ≈ Fe–P (iron-bound P) > NH4Cl–P (loosely sorbed P). Among these Pi forms, the total contents of NH4Cl–P, Fe–P, and Al–P with high bioavailability accounted for less than 9% of TP, while the content of Ca–P accounted for more than 50%. It should be noted that the wide distribution of Phragmites australis would result in the transformation of Ca–P to bioavailable P in Lake Sha. The chemical compositions of Po were further determined by 31P-nuclear magnetic resonance (NMR), and their relationships with sources of organic matter were analyzed. The rapid degradation of bioavailable diester P was one of the important sources for the replenishment of bioavailable P for the overlying water in Lake Sha. Labile monoester P and phytate in sediment were also potential sources of bioavailable P in the overlying water. Thus we suggest that the input of particulate matter with recharge water from the Yellow River was one of the important sources of bioavailable Pi and Po in Lake Sha. Particulate nutrients in Lake Sha in recharge water from the Yellow River should be evaluated or treated, such as by flocculent settling. Characterization of Pi and Po together aid in determining sources and bioav