| Land use change is a key component of global changes,and different land use types affect inputs of net primary productivity and soil organic matter directly by changing vegetation types and indirectly by changing soil properties.These changes significantly impact terrestrial ecosystem nitrogen(N)cycling processes and microbial functioning.However,the underlying mechanism on different land use types N transformation processes and microbial functioning remains unclear.In this study,we conducted the experiments to explore soil N transformation processes and microbial mechanisms of denitrification in three land use types including abandoned land,shrubland and woodland in Danjiangkou reservoir area,by using the methods of PVC top-tub method,cultivation method,static-gas chromatographic method,acetylene inhibition technique and functional gene sequencing.The main conclusions are as(1)Impafollows:ct of different land use types on soil N mineralization,ammonification and nitrificationBoth the net N mineralization and nitrification rates were higher in the woodland(0.016 ± 0.015 mg kg-1 d-1,-0.006 ± 0.005 mg kg-1 d-1)and shrubland(0.052 ± 0.022 mg kg-1 d-1,0.007 ± 0.004 mg kg-1 d-1)compared to the abandoned land(-0.006 ±0.013 mg kg-1 d-1,-0.025 ± 0.005 mg kg-1 d-1),which were due to the greater soil organic matter in the surface of shrubland and woodland could provide adequate substrate for N transformation processes,and nutrients for microbial growth,and hence increased microbial activities and enhanced soil N transformation processes.Meanwhile,microbial activities could be inhibited by high temperature and drought,leading to low net mineralization and nitrification rates in summer and consequently caused lower both the net mineralization and nitrification rates in summer(-0.039±0.004 mg kg-1 d-1,-0.012 ± 0.002 mg kg-1 d-1,-0.023 ± 0.003 mg kg-1 d-1)compared to winter(0.115±0.021 mg kg-1 d-1,0.122 ± 0.007 mg kg-1 d-1,-0.008 ± 0.008 mg kg-1 d-1).However,after relieving drought stress,soil N potential net mineralization and nitrification were significantly higher in summer compared to winter.(2)Impact of different land use types on soil N denitrificationThe denitrification rate was significantly higher in abandoned land(0.311±0.025 mg m-2 h-1)than woodland(0.236±0.015 mg m-2 h-1),and it was positively related to moisture and NO3-concentration,indicating that the soil exchangeable NO3-and moisture were main drivers on the changes in denitrification following different land use types.The N denitrification rateDominican s were significantly higher in winter compared to summer.Meanwhile,nirS and nirK,denitrifying gene,varied with season.The CHAO 1 index.ACE index and Shannon index with nirS were significantly higher in winter compared to summer in abandoned land and shrubland.There were not significant differences in a diversity index between seasons in woodland.The CHAO 1 index,ACE index and Shannon index with nosZ were significantly higher in winter compared to summer in three different land use types,with no significant differences between the Simpson index with nosZ.The abundance of nirS and nosZ,moisture and NO3-were key factors in regulating denitrification under seasons.(3)Impact of different land use types on N2O fluxThe N2O flux was higher in woodland(0.010 ± 0.001 mg m-2 h-1)and shrubland(0.007 ± 0.001 mg m-2 h-1)than abandoned land(0.011 ± 0.001 mg m-2 h-1),for the larger cover of vegetation could provide adequate substrate for soil microbes and hence increase soil N cycling processes in woodland.Our results indicated that nitrification was a main route to N2O flux in this study area.In N-limited soils,the N will be preferentially utilized for plant and microbial growth and soil organic matter accumulation,and then it will be altered to gases released into atmosphere.The N2O flux was significantly lower in spring than the other seasons,possibly due to N utilized for the rapid growth of microbes and plants.In summary,there were differences in soil N transformation processes following different land use types.Both the net N mineralization and nitrification rates were higher in the woodland and shrubland compared to the abandoned land,while the denitrification rate was significantly higher in abandoned land than woodland.The moisture and NO3-concentration were main drivers on denitrification following different land use types,whereas the denitrifying gene(nirS and nosZ),moisture and NO3-were key factors on denitrification under seasons.In addition,we found that the nitrification was a main route to N2O flux in this study area. |
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