Comparative Study On Soil Nitrogen Transformation Between Dry-hot Valley And Non-dry-hot Valley

Nitrogen is an essential element for plant growth and photosynthesis.In the process of ecological restoration,plants absorb more nitrogen,and nitrogen often becomes a key factor affecting vegetation restoration.Under the influence of intense water-heat contradiction,dry-hot valley becomes the most typical fragile ecosystem in southwest China.Once destroyed,it is difficult to recover.The restoration of degraded ecosystem in this region is the key and difficult point of ecological restoration project in southwest China.Research on soil nitrogen cycle in dry-hot valley can provide theoretical reference for land resource integration and ecological restoration project in this area.In this paper,the responses of soil net nitrogen transformation and gross nitrogen transformation to different land use types in dry-hot valley and non-dry-hot valley were studied by comparing the soil nitrogen transformation process and microbial nitrogen use efficiency of different land use types.The nitrogen supply capacity and nitrogen retention capacity of dry-hot valley were evaluated.The main results are as follows:(1)Soil organic carbon and total nitrogen in dry-hot valley were lower than those in non-dry-hot valley,and inorganic nitrogen in dry-hot valley was mainly ammonium nitrogen.The soil organic carbon in dry-hot valley(19.69g/kg)was 23.2%higher than that in dry-hot valley(15.97g/kg).The total soil nitrogen in non-dry-hot valley(1.82g/kg)was 33.1%higher than that in dry-hot valley(1.37g/kg).In dry-hot valley,the main form of soil inorganic nitrogen is ammonium nitrogen,accounting for 88.4%of soil inorganic nitrogen,while in non-dry-hot valley,the proportion of ammonium nitrogen and nitrate nitrogen is similar,accounting for 60.3%of ammonium nitrogen.The difference of organic carbon and total nitrogen in different land use types was primary forest>secondary forest>shrubland>farmland.(2)The nitrogen supply capacity of dry-hot valley is weaker than that of non-dryhot valley.The soil gross nitrogen mineralization rate in dry-hot valley was 1.42mgN·kg-1day-1,while that in non-dry-hot valley was 1.99 mgN·kg-1day-1,and that in non-dry-hot valley was 39.8%higher than that in dry-hot valley,indicating that the dry-hot valley had a weaker nitrogen supply capacity than that in non-dry-hot valley.The nitrification rate of primary nitrogen was 7.15 mgN·kg-1day-1 in dry-hot valley and 6.94 mgN·kg-1day-1 in non-dry-hot valley.Combined with the characteristics of low mineralization and high nitrification of nitrogen in dry-hot valley,the risk of nitrogen loss in dry-hot valley is greater than that in non-dry-hot valley.The rate of primary mineralization showed that agricultural land<shrub and grassland land<secondary forest≤primary forest,that is,ecological restoration can improve the nitrogen supply capacity of the ecosystem.(3)Dry-hot valleys have a lower risk of nitrogen restriction.The microbial nitrogen use efficiency of dry-hot valley is 53%,while that of non-dry-hot valley is 59.6%.Under different land use types,the microbial nitrogen use efficiency of secondary forest and primary forest(Savanna)in dry-hot valley was lower than that of agricultural land and shrub land,which meant that secondary forest and primary forest had lower risk of nitrogen limitation.However,the non-dry-hot valley had higher microbial NITROGEN utilization efficiency and higher risk of nitrogen restriction in primary forest stage.Our study shows that there are significant differences in nitrogen cycle between dry-hot valley and non-dry-hot valley.Dry-hot valley has a more open nitrogen cycle and has a greater risk of nitrogen loss.In contrast,adjacent non-dry-hot valleys exhibit nitrogen-limited characteristics.Our study shows that the dry-hot valley nitrogen cycle has its own characteristics,it needs more attention.