The Impacts Of Extreme Drought On Soil Nitrogen Mineralization In Inner Mongolia Steppe

In the context of the dramatic changes in the global climate,extreme drought events have occurred on a global scale,which has caused severe impacts on the structure and function of terrestrial ecosystems.Extreme drought refers to the occurrence of a drought event with a probability of less than 10%in a certain region.Multiple climate prediction models indicate that the extreme drought events in Inner Mongolia grasslands show a sharp increase in intensity and frequency.Soil nitrogen is an important factor limiting the productivity of grassland ecosystems,and process of soil nitrogen mineralization is very sensitive to drought and presents a large spatial heterogeneity.Unlike general droughts,extreme droughts have longer drought periods and stronger drought intensity,which may have a deeper impact on soil nitrogen mineralization processes.However,the research on the effect of extreme drought on soil nitrogen mineralization in different grasslands is not enough to unveil the whole picture for now.Compared with the traditional single-point study,the networked study applied in this study adopts a unified control factor,sampling and determination methods at the regional or global scale,so it can better evaluate the response of soil nitrogen mineralization processes in different grassland ecosystems to extreme drought events more accurate.Our study based on the Inner Mongolia grassland extreme drought network research platform based,and six research plots have been established from the west to the east according to the gradient of precipitation in the Inner Mongolia steppe with a span of 2500 kilometers,which covering desert grassland,typical grassland and meadow grassland.And we set unified treatments in six sites that control?Control,CON?,a reduction of 66%of the rainfall in the growing season?May-August??Chronic drought,CHR?and two treatments in the growing season for two consecutive months of drought?June and July??Intense drought,INT?.In this study,the effect of extreme drought on the soil nitrogen mineralization process was investigated by measuring soil ammonia-nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?content,above-ground biomass and cover of vegetation.The result is as follows:1.Compared with CON,two extreme drought treatments?CHR and INT?increased NH₄⁺-N,NO₃^--N and inorganic nitrogen?Inorganic-N?contents in soil.With the increase of the index of aridity,the response coefficients of NH₄⁺-N and Inorganic-N to CHR and INT increased at first and then decreased,while the response coefficients of NO₃--N to CHR and INT decreased first and then increased.The results indicated that soil NH₄⁺-N and Inorganic-N content were not sensitive to the two extreme drought treatments at the more arid and humid areas,but were more sensitive at moderate drought index areas,but the response of NO₃--N concentrations to extreme drought were opposite.2.Compared with CON,both CHR and INT resulted in a decrease in soil net ammoniation and net nitrification rates in six sites.The response coefficient of net ammoniation rate to the two extreme drought treatment decreased first and then increased with the increase of the index of aridity in July of 2016 and 2017,while it increased first and then decreased in August of 2016.The relationship between the response coefficient of soil net nitrification rate and drought index was opposite to the relationship between the response coefficient of net ammoniation rate and drought index.It shows that the effect of extreme drought treatment on soil net ammoniation rate is stronger in arid and humid areas,while the impact on net nitrification rate is stronger in moderate drought index areas.Overall,the response rate of net ammoniation rate and net nitrification rate to CHR was higher than that of INT,demonstrates that soil ammoniation rate and net nitrification rate are more sensitive to CHR.3.At the regional scale,the contents of soil NH₄⁺-N,NO₃^--N and Inorganic-N decreased with the increase of biomass and showing a significant negative correlation?P<0.05?.While the soil net ammoniation rate and net nitrification rate increased with the increase of above-ground biomass,showing a significant positive correlation?P<0.05?,and the above relationship was not affected by site and extreme drought treatment.