| Phosphorus(P)is an essential nutrient for plant and microorganism growth in subtropical forest soils.It is generally accepted that P availability is highly limited in subtropical forest soils,where is rich in oxides such as iron and aluminum,for P is easily adsorbed and fixed by soil minerals.Additionally,nitrogen deposition in subtropical regions of China is as high as 40 kg N ha-1 y-1,which may indirectly affect soil Pfractions and availability by changing abiotic and biological processes.is essential to reveal mechanisms that sustain and enhance ecosystem function in subtropical forest.We conducted an experiment to simulated N deposition with different gradients of N addition[control(0 kg N·hm-2·a-1),low N(20 kg N·hm-2·a-1),middle N(40 kg N·hm-2·a-1),and high N(80 kg N·hm-2·a-1)]in a subtropical Phyllostachys pubescen forest and sampled after three years of experiment addition.The soil properties,and soil P fractions,and microbial community characteristics and some plant factors were measured.aims to explore the influence of nitrogen addition on soil P fractions and microorganisms,and try to uncover the mechanism of microorganisms in the Phyllostachys pubescens forest ecosystem maintaining soil P supply in the background of nitrogen addition,and provide a scientific basis for improving the quality of subtropical soil.Compared with the control,the low nitrogen treatment significantly reduced the content of soluble inorganic P in soil by about 39.00%;the low nitrogen treatment and high nitrogen treatment significantly reduced the content of labile organic P by about 37.15%and 47.90%.The ratio of soil organic carbon to organophosphorus was much greater than200,indicating that nitrogen addition aggravated the P limitation in this study area.In addition,nitrogen addition significantly increased the content of P in amorphous oxides and secondary minerals,indicating that nitrogen addition enhanced the soil's ability to fix P by changing the morphology of iron and aluminum oxides.Under nitrogen addition,there was significant positive correlation between soluble inorganic P and unstable inorganic P,closed storage P and total P,and between unstable organic P and secondary mineral P,but not between unstable inorganic P and other P components.The ratio of soil organic carbon to organophosphorus was far greater than 200,indicating that nitrogen addition aggravated the P limitation in our study area.In addition,nitrogen addition significantly increased the content of amorphous oxides and secondary minerals P,indicating that nitrogen addition may enhance the soil's ability to fix P by changing the morphology of iron and aluminum oxides.There was significant positive correlation between soluble inorganic P and labile inorganic P,occluded P and total P,and between labile organic P and secondary mineral P,but not between labile inorganic P and other P components under N addition.Short-term nitrogen addition significantly improved soil nitrogen availability,but significantly reduced soil carbon and P availability.Short-term nitrogen addition significantly promoted the growth of Phyllostachy pubescens and the absorption of P by fine roots(the content of P of fine roots increased by 0.11 to 0.27 times under nitrogen addition treatment).Moreover,the nitrogen addition significantly reduced the microbial biomass P about 30.16%to 44.35%,and increased the ratio of microbial biomass nitrogen to P about 30.84%to 83.55%,indicating that nitrogen addition increased the demand for P of microorganisms.Nitrogen addition also significantly increased the carrier angle and length of microbial enzymes,indicating that nitrogen addition caused carbon and P limitation of microorganisms.However,the enzyme activities associated with carbon and P acquisition and the ratio of these enzymes to microbial biomass were significantly increased,whereas the change of nitrogen acquisition enzymes was reversed,suggesting that microbes adapted to environmental changes and alleviate their carbon and P limitations by enhancing enzyme synthesis and metabolism and adjusting their nutrient acquisition strategies,i.e.,reducing resource allocation to nitrogen acquisition enzymes to improve the synthesis of carbon and P acquisition enzymes.And the response strength of microorganisms under low nitrogen treatment is greater than high nitrogen treatment.Short-term nitrogen addition had no significant effect on the diversity and community composition of bacteria and fungi,indicating that microorganisms have strong adaptability to short-term nitrogen inputs.However,nitrogen addition significantly affected the function of microorganisms,for example,improving the relative abundance regulatory genes of bacterial,genes relate to organophosphorus mineralization(genes encoding for acid phosphatase,and phytase,and alkaline phosphatase)by functional prediction results,and reducing the relative abundance of undefined saprophytes in fungi,which was significantly negatively correlated with soil acid phosphatase activity.These results indicated that the P cycle functional potential of microorganisms was improved.The variation partition analysis showed that and the effects of biological factors(such as microbial characteristics,phosphatase activity,plant factors,etc.)on P fractions were greater than that of abiotic factors(available nutrient content,iron and aluminum oxide morphology,etc).Among them,the most important influencing factors were the relative abundance of pho R genes,and acid phosphatase activity and the content of microbial biomass P.A random forest model analysis further revealed that the most important influencing factors were the relative abundance of pho R genes,and acid phosphatase activity and the content of microbial biomass P.Among them,The structural equation model showed that nitrogen addition increased the demand of microorganisms for P,and indirectly affected the relative abundance of phosphate mineralization genes and activity of enzymes related to phosphorous cycle by stimulating the abundance of microbial regulate genes(P starvation)by functional prediction,which was an important reason explained the mineralization of soil organic P.The results showed that nitrogen addition decreased soil P availability in subtropical bamboo forest and increased microbial P limitation,and microorganisms promote P transformation(especially organophosphorus mineralization)by changing their P conversion genes and phosphatase metabolism rather than community structure to meet the increased P demand.These results are helpful to clarify the microbial mechanism of soil P availability in subtropical forest ecosystem under the background of short-term nitrogen deposition,and provide parameters for further exploring the influence of nitrogen deposition on soil P cycle in subtropical forest in the future,and provide reference for the development of regional forest management measures. |
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