Effects Of Different Cultivation Years On The Variation Of Soil Nitrogen Components And Enzyme Activities In Alpine-cold Grassland In Northwest Of Sichuan

Northwest of Sichuan is located in the eastern edge of the Qinghai-Tibet Plateau.It is the largest peat swamp wetland in the world and one of the five largest pastoral areas in China.Quantitative study of the impact of cultivation on alpine-cold grassland in northwestern Sichuan is of great significance to the protection of the ecosystem and is conducive to provide a scientific basis for the rational development and utilization of the ecosystem.Soil nitrogen is not only one of the most important nutrients necessary for plant growth and reproduction,but also one of the important factors limiting the productivity of grassland ecosystem.The study of soil nitrogen cycling transformation plays an important role in revealing the mechanism of soil fertility reduction in grassland cultivation and maintaining the sustainable development of grassland.However,at present,the study of soil nitrogen composition in cultivated grassland is mainly in agro-pastoral ecotone,arid and semi-arid regions,and there are few reports on alpine grassland cultivation.Soil enzymes can promote the growth and reproduction of microorganisms in the soil and the mineralization decomposition of nitrogen and other important elements.It is of great significance to study soil enzyme activities for evaluating the mode of cultivation.However,at present,the research on soil enzyme activities in some extreme environments is very limited,especially the study on the effects of alpine-cold grassland cultivation on soil enzyme activities.Therefore,it is necessary to study the effect of cultivation on nitrogen composition and enzyme activity in the soil of alpine-cold grassland.In order to reveal the effect of cultivation on the characteristics of soil nitrogen components and enzyme activities in alpine-cold grassland,this paper studied the northwestern Sichuan alpine-cold grassland after 1,3,10,16,27,and 40 years of cultivation.The native grassland was taken as the control.Soil sampling and analysis were used to study the soil nitrogen composition of alpine-cold grassland in northwest Sichuan:total nitrogen(TN),available nitrogen(AN),ammonium nitrogen(NH₄⁺-N),nitrate nitrogen(NO₃^--N),total acid hydrolysable nitrogen(THAN),acid solution ammonium nitrogen(ASAN),amino sugar nitrogen(ASN),amino acid nitrogen(AAN),acid hydrolysable unknown nitrogen(AHUN),non acid hydrolysable nitrogen(NAHN)and microbial biomass nitrogen(MBN);Enzyme activity:Urease,Sucrase,Phosphatase,and Catalase activity characteristics.The results show:(1)During the cultivation of alpine-cold grassland in northwestern Sichuan,with the increase of cultivation years,the contents of soil TN,AN,NH₄⁺-N,NO₃^--N,MBN,TAHN,ASAN,ASN,AAN,AHUN and NAHN decreased significantly(P<0.05).In the 0-20 cm soil layer,the soil nitrogen decreased continuously and the greatest decrease occurred.After40 years of cultivation,the soil TN,AN,NH₄⁺-N,NO₃^--N,MBN,TAHN,ASAN,ASN,AAN,AHUN and NAHN The content decreased by 69.00%,73.37%,35.07%,42.80%,49.62%,72.15%,62.72%,66.08%,63.44%,94.00%and 51.78%,respectively.A similar pattern of variation appeared in 20-40 and 40-60 cm soil layers,and the content and decrease of soil nitrogen were less than 0-20 cm.In the 0-20 cm soil layer,the loss rate of nitrogen was fastest in the early stage of the cultivation,and the contents of TN,AN,NH₄⁺-N,NO₃^--N,MBN,TAHN,ASAN,ASN,AAN,AHUN and NAHN in the soil were in the range of 0-1 years.Respectively reduced 113.67%,11.73%,13.93%,11.30%,12.81%,10.51%,11.90%,13.89%,6.82%,11.93%and 30.91%,significantly higher than the loss rate in the middle and late stages of the cultivation.(2)In the process of alpine-cold grassland cultivation in northwestern Sichuan,after 40years of cultivation in 0-20 cm soil layer,the decrease of TN in soil nutrients was 69.00%,which was lower than that of AN,which was 73.37%.The decline of NH₄⁺-N in soil inorganic nitrogen was significant.35.07%was lower than that of NO₃^--N with a decrease of 42.80%;soil organic nitrogen had a decrease of 72.15%,62.72%,66.08%,63.44%,94.00%and 51.78%in THAN,ASAN,ASN,AAN,AHUN and NAHN,respectively.The proportion of organic nitrogen components in TN was changed by the cultivation,and the proportion of organic nitrogen components in the natural grassland before the cultivation accounted for TN:AAN>ASAN>AHUN>NAHN>ASN.After 40 years of cultivation,the distribution of organic nitrogen components has become:AAN>ASAN>NAHN>ASN>AHUN.The proportion of AAN,ASAN,and NAHN in TN increased as a result of the cultivation,which increased by 4.73%,4.98%,and 8.88%,respectively.The proportion of AHUN in TN decreased by 19.23%.The ratio of ASN to TN changed little.(3)In the process of alpine-cold grassland cultivation in northwestern Sichuan,soil urease,invertase,phosphatase and catalase activities decreased significantly(P<0.05)with the increase of cultivation years.In 0-20 cm soil layer,the activity of soil enzymes decreased continuously and the greatest decrease occurred.After 40 years of cultivation,soil urease,sucrose enzyme,phosphatase and catalase decreased by 41.80%,57.01%,54.22%and46.53%,respectively.A similar pattern of variation was observed in 20-40 and 40-60 cm soil layers,and soil urease,sucrose,phosphatase,and catalase activities were less than 0-20 cm.In the 0-20 cm soil layer,the enzyme activity decreased most significantly in the early stage of planting,and the activities of urease,sucrase,phosphatase,and catalase decreased by11.17%,3.18%,20.09%,and 11.73%in the years 0-1,respectively.It is higher than the rate of decline in the middle and late stages of cultivation.(4)In the process of alpine-cold grassland cultivation in northwestern Sichuan,the contents of powder,clay,soil water content(SWC),urease,sucrase,phosphatase,catalase and soil nitrogen showed significant(P<0.01)positive correlation;There was a significant(P<0.05)or extremely significant(P<0.01)negative correlation among sand content,bulk density and soil nitrogen components.Among them,the correlations among TN,AN,NH₄⁺-N,NO₃^--N,TAHN,ASAN,ASN,AAN,AHUN,NAHN and SWC in soil physical properties were the highest;and the correlation between MBN and clay content in soil physical properties was the highest.The correlation of NO₃^--N in urease,phosphatase and nitrogen components was the highest;the correlation of THAN in sucrase and nitrogen components was the highest;the catalase had the highest correlation with ASN in the nitrogen component.