Study On Soil Ammonia Oxidation Process In Agricultural Reclaimed Peatland

Peatlands play an important role in the global carbon cycle and climate change.They are an important component of the global terrestrial carbon pool.Natural peatlands are generally nitrogen-limited ecosystems.However,peatlands have been reclaimed into paddy fields due to agricultural use recent years.Peatlands are increasingly exposed to nitrogen inputs due to increased agricultural fertilizer application.This may cause changes in the carbon to nitrogen ratio of peatland soils and seriously threaten the carbon pool function of peatlands.Therefore it is important to study the transformation process of N in peatland soils after agricultural reclamation.In this study,we selected the Jinchuan Peatland in Jinlin Province as our study area,and soil samples were collected from paddy-peatland,natural peatland and ordinary rice paddy.We investigated the abundance and community structure of ammonia-oxidizing microorganisms in agricultural reclaimed peatlands through real-time quantitative PCR and high-throughput sequencing techniques.And we preliminary determined the relative contributions of ammonia-oxidizing archaea and ammonia-oxidizing bacteria in the process of ammonia oxidation in agricultural reclaimed peatlands through field survey and microcosm incubation experiment.The main conclusions are as follows:（1）Agricultural reclamation significantly affects the physicochemical properties of peatland.The soil p H and ammonium nitrogen contents of the paddy-peatland were significantly higher than natural peatland,but the total nitrogen,organic carbon,soluble organic carbon and nitrate nitrogen contents were significantly lower than natural peatland.However,compared with the ordinary rice paddy,the soil p H and nitrate nitrogen contents were not significantly different.The total nitrogen,organic carbon,carbon to nitrogen ratio and ammonium nitrogen contents of the paddy-peatland were significantly higher and the soluble organic carbon content was significantly lower than that of the ordinary rice paddy.（2）Agricultural reclamation significantly reduced the absolute abundance of total archaea,ammonia-oxidizing archaea and nitrite-oxidizing bacteria,while the absolute abundance of total bacteria and ammonia-oxidizing bacteria significantly increased in paddy-peatland.However,the absolute abundance of all these microorganisms was significantly lower than that of the ordinary rice paddy.This may be related to the increase in p H and ammonium nitrogen concentration and decrease in total nitrogen and organic carbon content of the paddy-peatland after agricultural reclamation.The species composition and diversity of the AOA community did not change significantly after the peatland was reclaimed into paddy-peatland,but the relative abundance of dominant species in the AOB community increased and the species diversity decreased.In addition,the species diversity of AOA community in paddy-peatland was higher than that in ordinary rice paddy,but the species diversity of AOB community was lower than that in ordinary rice paddy.This is mainly due to the changes in nutrients and physicochemical properties of paddy-peatland after agricultural reclamation.And the increase in the concentration of ammonium nitrogen in paddy-peatland may be the main reason for the increase in the relative abundance of dominant species in AOB community.In addition,the impact of agricultural reclamation on the AOA community structure of paddy-peatland was small,probably because the p H of the reclaimed paddy-peatland soil was still acidic and the paddy-peatland still maintained a high organic matter content.（3）The ammonia oxidation potential and net nitrification rate of paddy-peatland were significantly higher than those of natural peatland and ordinary rice paddy.The results of correlation and RDA analyses showed that ammonia oxidation potential and net nitrification rate were significantly and positively correlated with soil p H,suggesting that the increase in soil p H due to agricultural reclamation may be the main reason for the increase in ammonia oxidation potential and net nitrification rate in paddy-peatland.In the microcosm incubation experiment,ammonium sulfate addition significantly increased the net nitrification rate of paddy-peatland and ordinary rice paddy,but ammonium sulfate addition had no significant effect on ammonia oxidation and net nitrification rate of natural peatland.This may be due to the low activity of the natural peatland’s own ammonia-oxidizing microorganisms,which makes the soil ammonia oxidation weak.In addition,since strong mineralization occurred in both natural peatland and paddy-peatland and ordinary rice paddy,the net nitrification rate of paddy-peatland and ordinary rice paddy was significantly increased by the addition of ammonium sulfate may also be due to the increased NH₃produced by soil mineralization,which promoted soil ammonia oxidation.（4）Microcosm incubation experiments showed that soil N₂O emissions increased after reclamation of natural peatland into paddy-peatland,and the cumulative N₂O emissions were significantly and positively correlated with soil ammonium nitrogen concentration（P<0.05）.This indicates that the increase in soil ammonium nitrogen concentration due to fertilizer application after agricultural reclamation may be responsible for the increase in N₂O emissions from paddy-peatland.（5）Microcosm incubation experiments preliminary showed that N₂O emissions from paddy-peatland and ordinary rice paddy soils were mainly contributed by ammonia oxidation,and the contribution of AOB was higher than AOA.However,N₂O emissions from natural peatland soils are mainly contributed by other processes such as denitrification.The contribution of AOB to N₂O emissions in paddy-peatland was higher than AOA after agricultural reclamation,further suggesting that AOB may play a dominant role in soil ammonia oxidation in paddy-peatland.This may be related to the increase of ammonium nitrogen content and p H and the decrease of organic matter content in soil after agricultural reclamation.In addition,this study preliminary determined that the ammonia oxidation process in paddy-peatland and ordinary rice paddy soils after agricultural reclamation may be mainly dominated by AOB,while AOA may play a dominant role in the soil ammonia oxidation process in natural peatland.