Abstract
BACKGROUND: Enhanced nitrogen (N) load was considered a critical factor influencing phosphorus (P) availability and P-cycling in marsh soils. However, information on the links between soil P availability and microbial genes involved in P-cycling processes under N enrichment conditions remains scarce. METHODS: A field N load experiment with four treatments (N(0), N(low), N(medium), and N(high)) was conducted in Cyperus malaccensis marsh of the Min River estuary, and soil P availability, the relative abundances of P-cycling functional genes and their regulatory roles on P availability were investigated. RESULTS: The total phosphorus (TP) contents in soils were significantly positively correlated with N load levels (p < 0.05). Compared with the N(0) treatment, the TP in the N(low), N(medium) and N(high) treatments increased by 8.97%, 17.34% and 15.21%, respectively. With increasing N load levels, the proportions of easily- and moderately-available P in TP contents noticeably increased, suggesting that N additions enhanced soil P availability. Metagenomic sequence analyses showed that N enrichment markedly altered the relative abundances of P-cycling functional genes. Briefly, the abundances of inorganic P solubilization genes (particularly ppa and ppx) increased substantially with increasing N load levels. The total abundances of organic P mineralization genes in the N(low) and N(medium) treatments decreased markedly, while those in the N(high) treatment increased greatly. The abundances of genes coding for phytase (phy and appA) markedly increased with increasing N load levels, implying that phytase was more sensitive to N enrichment. Furthermore, enhanced N load noticeably reduced the abundances of genes participated in P transportation (particularly ugpABEC) and those involved in P-assimilating process (e.g., phoR, phoB, pstABCS and pit). As affected by enhanced N load, the contents of easily-available P showed strong correlations with the abundances of genes involved in inorganic P solubilization while those of moderately-available P (particularly Sonic-P(i), Sonic-P(o) and NaOH-P(i)) were positively correlated with the abundances of genes involved in P regulation and transportation, indicating strong linkages between P-cycling functional genes and soil P availability. CONCLUSIONS: This paper found that, under N enrichment conditions, the increased inorganic P solubilization potential and the weakened microbial P immobilization capacity were beneficial to increasing soil P availability.