Nitrogen Cycling Processes And Microbial Regulation Mechanisms In Limestone And Acid Red Soils Of Subtropical Forests

To reveal differences in soil nitrogen(N)supply and N leaching risks of forest soils in subtropical monsoonal zone,a series of studies were conducted with a calcareous forest soil and an acid forest red soil in northern Guangxi.1)Soil was incubated with different nitrification inhibitors to determine the nitrification potentials of ammonia oxidation archaea(AOA),ammonia oxidation bacteria(AOB)and heterotrophic nitrifiers(Heterotrophic),and q PCR was applied to estimate the gene abundances of AOA,AOB and complete ammonia oxidizing bacteria(Comammox).2)¹⁵N labeling experiment was applied to estimate soil N transformation rates.3)Soil and its aggregates were incubated with N addition to estimate the net transformation rates.Combining with the enzyme activities involving in carbon(C)and N turnovers as well as the community structure of bacteria and archaea through 16S r RNA,the differences in N transformation processes and their microbial regulation mechanisms of different soil types,soil depths and soil aggregates were explored;the key factors were identified;and the mechanisms for the N transformations in calcareous and acidic red forest soils were revealed.The main conclusions of this study are as follows:(1)The nutrient contents of soil C,N and phosphorus(P)were higher in limestone soil than in acidic red soil,and decreased with the increase of soil depth.The nutrient contents of the two soils were highest in the aggregates of<0.25 mm,indicating that the aggregates of<0.25 mm had had a stronger adsorption capacity for nutrients due to a larger specific surface area.(2)The variation of soil nitrification potential depended mainly on soil type,and had a great correlation with soil p H and C,N,P contents.The nitrification potential(PN),ammonia oxidation potential(PAO)and nitrite oxidation potential(PNO)of limestone soil were significantly higher than those of acidic red soil,and decreased with the increase of soil depth.AOB was the main contributor to PN in limestone soil.AOA was the main contributor to PN in acidic red soil.PAO was the limiting step of nitrification potential.The abundance of amo A gene in soil AOA,AOB and Comammox and the abundance of nxr B gene in nitrite oxidizing bacteria(NOB)were consistent with the variation of the potentials,i.e.,the values were higher in the limestone soil than the acid red soil;and gradually decreased with soil depth.The gene abundance of AOA in limestone soil was significantly higher than that of AOB,but AOB had a greater contribution to soil nitrification potential,indicating that the activity of AOB had low correlation with its gene abundance.In addition,the gene abundance of AOB in acidic red soil was close to undetectable,but there was a high gene abundance of Comammox,and AOB had a greater contribution to soil nitrification potential,indicating that the main nitrifying bacteria in acidic red soil was Comammox.(3)The results of ¹⁵N labeling experiment show that soil N transformation rates varied greatly between the two soil types,and had a great correlation with soil p H and total calcium(TCa).The N transformation rates of limestone soil were significantly higher than those of acidic red soil.Nitrate(NO₃^-)in both soils was mainly produced via heterotrophic nitrification.Because limestone soil is characterized as calcium-rich and alkaline,its higher p H is beneficial to the accumulation of soil organic matter and the growth and reproduction of microorganisms,thus promoting the total soil mineralization rate(M_Norg),total nitrification rate(O_NH4),autotrophic nitrification Rate(AN),heterotrophic nitrification rate(HN),and nitrate dissimilatory rate to ammonium(DNRA)As comparison,the availability of inorganic nitrogen was lower in acidic red soil soils.The C,N,P contents,p H and TCa were the dominant factors affecting N transformation processes.(4)The incubation experiments of different soil aggregates with NO₃^-addition show that net ammonia rate(NAR),net nitrification rate(NNR)and net mineralization rate(NMR)were significantly different among soil types and aggregates.The rates in limestone soil was significantly higher than those in acidic red soil.The NAR and NNR rates decreased gradually with the decrease of soil aggregate sizes.The NMR showed the opposite trend.In general,the2?4 mm soil aggregates in limestone soils showed a higher risk of NO₃^-leaching.(5)The activities of amylase(Amy),sucrase(Suc),?-glucosidase(BG),urease(Ure),protease(Pro),acetylaminosidase(NAG)and leucine aminopeptidase(LAP)in limestone soil were higher than those in acidic red soil.,The enzyme activities decreased with soil depth.The stoichiometry of enzyme activities show that the enzyme activities in both soils were N-limited.Between them,the N limitation in limestone soil was greater.(6)The diversity of bacteria in limestone soil was significantly higher than that in acidic red soil.The diversity of archaea in limestone soil was lower than that in acidic red soil.At the Phylum level,Proteobacteria,Acidobacteria,Actinobacteria,Verrucomicrobia were the dominant bacteria in both soils.The dominant archaea were Thaumarchaeota,Euryarchaeota and Crenarchaeota.Chemoheterotrophy and aerobic chemoheterotrophy were the most dominant functional groups in bacteria,both abundances were significantly higher in limestone soil than acidic red soil,and decreased with soil depth.The aerobic ammonia oxidation and nitrification were the most dominant functional groups in archaea,both abundance were significantly higher in limestone soil than in acidic red soil,and decreased with the soil depth.In conclusion,compared with acidic red soil,limestone soil has higher capacity transformation rates,higher N availability,greater capacity of N supply and higher risk of NO₃^-leaching.Among them,the 2?4 mm aggregates in limestone soil have higher potential risk of NO₃^-leaching.