| With the increasing severity of global warming,drought becomes a critical issue.Drought can have significant effects on the structure and function of ecosystems.Soil nitrogen cycle is one of the most basic and complex ecological processes in forest ecosystem,among which nitrogen mineralization is a key part.In this study,poplar plantation was selected as the research object.Through field simulation experiment of drought,field soil samples were cultured in situ,combined with indoor soil moisture control culture experiment,and soil physical and chemical properties were determined to study the seasonal dynamics and influencing factors of soil nitrogen mineralization rate in different soil layers under drought conditions.Meanwhile,the effects of long-term drought and short-term drought on nitrogen mineralization were analyzed.The field experiment site was selected in the poplar artificial forest of Dongtai Forest farm,Jiangsu Province,and the drought intensity was controlled(control,30% drought,50% drought)by reducing precipitation.Samples were collected from the surface layer(0-10cm),the middle layer(10-20cm)and the deep layer(20-30cm)in the test site in summer,autumn,winter and spring during 2021～2022.Soil moisture content,p H value,total carbon and nitrogen,ammonium nitrogen,nitrate nitrogen,microbial carbon and nitrogen,urease activity and other indicators were measured,and nitrogen mineralization rate was measured by the difference of nitrate nitrogen and ammonium nitrogen content in one month.Indoor through freeze-drying and adding pure water to control the water conditions(control,30% dry,50% dry),then constant temperature and humidity culture,measurement of each index.The results show that:1.Drought significantly reduced nitrogen mineralization rate(P<0.05),and the decreasing effect was strengthened with the increase of drought intensity.30% drought reduced soil nitrogen mineralization rate by 50%～126%,50% drought reduced soil nitrogen mineralization rate by 44%～250%.Soil ammonium nitrogen and nitrate nitrogen did not decrease due to the decrease of nitrogen ammonification rate and nitrification rate.Because drought reduced the absorption of ammonium nitrogen and nitrate nitrogen by plants,ammonium nitrogen showed an overall increasing trend,while nitrate nitrogen showed no significant overall change trend.Nitrogen mineralization rate is mainly positively correlated with microbial carbon,nitrogen and enzyme activities,but weakly correlated with soil p H,total carbon and nitrogen and other abiotic factors,that is,drought may affect soil nitrogen.In addition,nitrogen mineralization in the poplar forest was dominated by nitrogen nitrification.2.The microbial carbon and nitrogen content and nitrogen mineralization rate were the highest in hot and rainy summer and the lowest in winter(P<0.05).Drought and season had significant interaction on soil water content,total carbon,nitrate nitrogen,ammonium nitrogen and nitrogen mineralization rate(P<0.05).Summer drought had a higher effect on soil water content than the other three seasons,and season increased the negative effect of drought.Drought in summer and autumn had a positive effect on ammonium nitrogen content,while drought in winter and spring had a negative effect on ammonium nitrogen content.Summer drought had no significant effect on net nitrification rate and net mineralization rate,but drought in autumn,winter and spring significantly reduced nitrogen mineralization rate.3.The nitrogen mineralization rate of surface soil was significantly higher than that of deep soil(P<0.05).There was no interaction between soil layer and drought on nitrogen mineralization(P>0.05).The nitrogen mineralization rate of 10-20 cm soil layer was200%-600% lower than that of surface soil,and that of 20-30 cm soil layer was 200%-900%lower than that of surface soil.4.After short-term water stress,the content of microbial carbon and nitrogen in indoor cultivated soil decreased,and the rate of nitrogen mineralization decreased significantly(P<0.05),and the trend of microbial carbon and nitrogen content and the rate of nitrogen mineralization decreased more significantly with drought than that of outdoor soil.The content of ammonium nitrogen and nitrate nitrogen in indoor cultivated soil decreased under the effect of drought.In addition,there was no difference between indoor PVC pipe material and terrarium soil on nitrogen mineralization.In conclusion,drought has a negative effect on nitrogen mineralization rate,which increases with the increase of drought intensity.Nitrogen mineralization in the poplar forest was dominated by nitrification.In addition,there is an interaction between seasons and drought,and the nitrogen mineralization rate in summer has no significant response to drought compared with other seasons.Indoor drought still caused the decrease of nitrogen mineralization rate,and the decreasing trend was more significant.In the context of global change,these results are helpful for us to further understand the effects of drought on forest soil nitrogen mineralization process,and provide theoretical basis for sustainable management and management of poplar plantation. |
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