Effects Of Nitrogen And Phosphorus Addition On The Litter Decomposition And Soil Properties With Salix Cupularis In An Alpine Desert Ecosystem

The litter is the main carrier of nutrient flow and element circulation in terrestrial ecosystem.The decomposition of litters plays an important role in improving soil quality and soil fertility.The precipitation of atmospheric N and P is an important way for the increase of the content of N and P in each component of the terrestrial ecosystem.Due to the intensification of human activities,the precipitation of atmospheric N and P gradually increases,which may affect the decomposition of litters and thus affect the development,succession and stability of the ecosystem.Previous studies that simulated the deposition of N and P have shown that the addition of N and P will affect the decomposition process of litters,which is not consistent with the climate,the type of ecosystem and the addition concentration.As a biological material for ecological restoration in alpine sandy land,the salix cupularis shrub has been widely used.Currently,there is relatively little research on the impact of N and P addition on litter decomposition in alpine ecosystem.Therefore,studying the litter decomposition and soil nutrient change characteristics of salix cupularis shrub under N?P settlement can help to reveal the maintenance mechanism of nutrient transformation and productivity balance during the ecological restoration of unstable sandy land ecosystem.In this paper,N and P deposition in the northwest Sichuan plateau was used to be the background,with no N and P addition treatment as the control(CK).We set three types of fertilization:adding N and P separately and adding N and P jointly,three fertilization gradients:low concentration(N1 P1 N1P1),medium concentration(N2 P2 N2P2)and high concentration(N3 P3 N3P3).The effects of different treatments and levels of N and P on the decomposition process of litters,the contents of C,N and P in soil and the enzyme activity with salix cupularis shrub in the alpine sandy land of northwest Sichuan were studied.The main research results are as follows:(1)The low concentration of N addition(N1:3 g/m².a)significantly promoted the decomposition of litters(P<0.05),and the medium and high concentration of N addition(N2:6 g/m².a;N3:9 g/m².a)was inhibited in the early stage of treatment,which was alleviated with the extension of time and gradually transformed into a promoting effect.The addition of P and N+P significantly promoted the decomposition of litters(P<0.05),and the increase of the concentration promoted the decomposition.At the same concentration level,there was a significant difference in the mass residual rate of litters among treatments of N addition,P addition and N+P composite addition(P<0.05).The mass residual rate of litters showed that N+P composite addition<P composite addition<N addition,and N+P composite addition was more conducive to the decomposition of litters.(2)The addition of N at low concentration significantly promoted the decomposition of lignin in litters,while the treatment at medium and high concentration inhibited lignin decomposition of litters,and the inhibition increased with the increase of concentration.The addition of N significantly promoted the decomposition of cellulose,and the promotion increased with the increase of N concentration.P addition and N+P composite addition significantly promoted the decomposition of lignin and cellulose,and both of them enhanced with the increase of concentration.At the same concentration level,compared with N and P addition,N+P composite addition was more conducive to promoting the decomposition of lignin and cellulose.(3)The addition of N,P and N+P at low concentration significantly promoted the release of total carbon from litters,and the promotion increased with the increase of concentration.The N and N+P addition at low concentration both significantly promoted the release of total nitrogen of litters.In the early stage of N addition treatment with high concentration,total nitrogen release was promoted,which was weakened with the extension of time and gradually converted into inhibition,while N+P addition promoted with the increase of concentration.The P addition at low concentration had an inhibitory effect on the release of total nitrogen,which was transformed into a promoting effect with the increase of concentration.The N addition of low concentration significantly promoted the release of total phosphorus,which decreased with the increase of concentration.The release of total phosphorus was inhibited by the P and N+P addition at low concentration.The inhibition increased with the increase of the concentration of P addtion,At the same concentration level,the N+P addition of high concentration had the strongest promoting effect on total carbon and total phosphorus of litters,while the P addition at high concentration had the greatest promoting effect on total nitrogen.(4)The content of EOC,DOC,NO₃^--N,NH₄⁺-N and AP treated by N,P and N+P addition increased significantly.The contents of EOC,DOC,NO₃^--N,NH₄⁺-N and AP were significantly higher than CK(P<0.05),the contents of DOC,NO₃^--N and AP increased significantly with the increase of concentration,while the contents of EOC and NH₄⁺-N decreased with the increase of concentration.There was no significant change in TN and TP.The contents of EOC,DOC,NO₃^--N,NH₄⁺-N and AP increased significantly by the N+P addition at each concentration gradient.With the increase of concentration,the contents of TOC,NO₃^--N and AP increased significantly,while the contents of EOC,DOC and NH₄⁺-N decreased.At the same concentration level,the content of soil organic carbon,nitrate nitrogen and available phosphorus increased the most at N+P addtion,but there was no significant change in soil total nitrogen and total phosphorus.(5)The activities of invertase,cellulase,polyphenol oxidase,urease and phosphatase treated with N,P and N+P addition all increased significantly.The activities of invertase,cellulase,polyphenol oxidase,urease and phosphatase in the N and N+P addition at low concentration(N1:3 g/m².a,N1P1:3 N g/m².a+1 P g/m².a)were significantly higher than CK(P<0.05).The activities of invertase and polyphenol oxidase decreased with the increase of N concentration,while the activities of cellulase,urease and phosphatase increased significantly.The activities of invertase,cellulase and phosphatase increased significantly with the increase of P concentration,while the activities of urease and polyphenol oxidase decreased.The activities of sucrase,cellulase,urease,phosphatase and polyphenol oxidase decreased with the increase of N+P addition.