Water N/P ratios influence emergent plant nutrient uptake and growth by altering the microbial activities of rhizosphere soil

Pollutant water discharge greatly alters surface water stoichiometric characteristics, resulting in potential risks for aquatic ecosystems. Few studies have been conducted on the effects of increased water nitrogen (N) and phosphorus (P) ratios (N/P) on nutrient uptake by emergent plants (EPs), which generally obtain nutrients from soil, as well as their mechanisms. A small-scale study was conducted to identify the effects of the influent water N/P ratio on nutrient absorption by reed (Phragmites australis), rhizosphere soil activity and the abundance of N- and P-related functional genes. Sterilization significantly decreased reed biomass and the accumulation of N and P, suggesting the importance of microbes in the rhizosphere and soil. Compared with the low-N/P treatment, the high-N/P treatment generally resulted in significantly greater reed biomass and reed N and P accumulation, as well as lower soil available N (AN) and available P (AP) and significantly greater soil enzyme activities of invertase and phosphatase (p < 0.05). Linear mixed model analysis suggested that reed N and P accumulation was determined mainly by rhizosphere soil AN and AP (negative correlation), respectively. The increased influent water N/P ratios clearly promoted the abundance of the phosphatase gene (phoD), phosphate-binding protein-encoding gene (pstS) and ammonia monooxygenase (amoA) in rhizosphere soil. Thus, although reeds obtain nutrients mainly from soil, an increased influent N/P ratio can increase reed biomass, N and P accumulation by improving the nutritional absorptive capacity and effectiveness of reeds.