| Soil elements such as carbon(C),nitrogen(N),and phosphorus(P)are important components of soil fertility,and soil total nitrogen and phosphorus contents reflect the potential ability of soil to provide nutrients.Soil carbon,nitrogen and phosphorus elements are coupled with each other in the process of ecological cycle,and their stoichiometric ratios are important indicators for predicting ecosystem structure and function.Soil carbon,nitrogen and phosphorus contents and their stoichiometric ratios are affected by various environmental factors.Knowledge of distribution characteristics and influencing factors of soil carbon,nitrogen and phosphorus contents and their stoichiometric ratios in different land use types is of great significance for understanding soil quality and nutrient cycling.This study was conducted in the north of the Yangtze River in Wanzhou District,Chongqing,which is located in the Three Gorges Reservoir area.A total of 2303 topsoil samples were collected from dry land,paddy field and orchard.The distribution characteristics of soil organic carbon,total nitrogen and total phosphorus contents and their stoichiometric ratios(C:N,C:P,N:P)in the three land use types were analyzed.Environmental variables included climate(annual average temperature and annual precipitation),topography(elevation,slope,aspect and topographical wetness index(TWI)),vegetation(normalized difference vegetation index(NDVI)),parent material,and soil p H.Kruskal Wallis test,Spearman correlation analysis and random forest techniques were applied to explore the effects of environmental factors on the variability of soil C,N and P contents and their stoichiometric ratios in different land use types.The main findings were as follows:(1)Differences in soil C,N and P contents and their stoichiometric ratios were observed among different land use types.The order of soil organic carbon and total nitrogen contents was paddy field>orchard>dry land.Soil total phosphorus content was similar in dry land and orchard,and higher than that in paddy field.The order of soil C:N was paddy field>dry land>orchard.Soil C:P and N:P showed similar trend with the highest in paddy field and the lowest in dry land.The soil C,N and P contents and their stoichiometric ratios presented moderate variability in the three land use types.The results of soil nutrient classification showed that the proportions of soil C,N and P contents in dry land at deficient and relatively deficient levels were 89.41%,49.25%and 49.60%,respectively,and the proportions of soil C,N and P contents in paddy field at deficient and relatively deficient levels were 71.36%,18.93%and 75.21%,respectively,and the proportions of soil C,N and P contents in orchard at deficient and relatively deficient levels were 89.17%,47.08%and 55.83%,respectively.This indicated that soil organic carbon content had a higher deficient proportion in dry land,paddy field and orchard,and the average values of soil C,N and P stoichiometric ratios in the three land use types were lower than those of the global and Chinese soils.(2)The correlation between soil C,N and P contents and environmental variables varied with land use types.In dry land,organic carbon content was significantly negatively correlated with soil p H,annual average temperature,NDVI and slope,and closely significantly positively correlated with elevation and TWI.Total nitrogen content was significantly negatively correlated with soil p H,annual average temperature and slope,and closely significantly positively correlated with elevation and TWI.Total phosphorus content was closely significantly positively correlated with soil p H,and closely significantly negatively correlated with elevation.In paddy field,organic carbon content was closely significantly positively correlated with soil p H,elevation and TWI,and significantly negatively correlated with annual average temperature,annual precipitation,NDVI and slope.Total nitrogen content was closely significantly positively correlated with soil p H,elevation and TWI,and significantly negatively correlated with annual average temperature,annual precipitation,slope and aspect.Total phosphorus content was significantly positively correlated with soil p H,TWI and NDVI,and closely significantly negatively correlated with elevation.In orchard,organic carbon content was closely significantly negatively correlated with soil p H and annual average temperature,and significantly positively correlated with elevation and TWI.Total nitrogen content was closely significantly negatively correlated with soil p H and annual average temperature,and significantly positively correlated with elevation,TWI and NDVI.Total phosphorus content was closely significantly positively correlated with soil p H and slope,and significantly negatively correlated with aspect and elevation.(3)The correlation between soil C,N and P stoichiometric ratios and environmental variables differed with land use types.In dry land,soil C:N was significantly negatively correlated with NDVI,and significantly positively correlated with TWI.Soil C:P and N:P were significantly negatively correlated with soil p H,annual average temperature,NDVI and slope,and significantly positively correlated with elevation and TWI.In paddy field,soil C:N was closely significantly positively correlated with soil p H and TWI,and significantly negatively correlated with annual precipitation,NDVI and slope.Soil C:P and N:P were significantly negatively correlated with soil p H,annual average temperature,NDVI and slope,and closely significantly positively correlated with elevation.Soil C:P was significantly positively correlated with TWI.In orchard,soil C:N was significantly negatively correlated with soil p H and annual precipitation.Soil C:P and N:P were closely significantly negatively correlated with soil p H,annual average temperature and slope,and significantly positively correlated with elevation and aspect.(4)Significant differences in soil C,N and P contents and their stoichiometric ratios were found among different parent materials under the three land use types.In dry land,the soils developed from Ziliujing Formation had the highest mean values of organic carbon,total nitrogen and phosphorus contents.In paddy field,the soils developed from Xujiahe Formation had the highest average value of organic carbon content,and the soils developed from Ziliujing Formation had the highest average values of total nitrogen and phosphorus contents.In orchard,the soils developed from Ziliujing Formation had the highest mean values of organic carbon and total nitrogen contents,and the soils developed from Suining Formation had the highest mean value of total phosphorus content.In dry land,the soils developed from Ziliujing Formation had the highest mean value of C:N,and the soils developed from Penglaizhen Formation had the highest mean values of C:P and N:P.In paddy field,the soils developed from Xujiahe Formation had the highest average values of C:N and C:P,and the soils developed from Penglaizhen Formation had the highest mean value of N:P.In orchard,the soils developed from Ziliujing Formation had the highest mean values of C:P and N:P,while there was no significant difference in soil C:N among parent materials.(5)The random forest model had different explanatory abilities for the spatial variability of soil C,N and P contents and their stoichiometric ratios in the three land use types.The R~2 of the models for dry land,paddy field and orchard were 0.24～0.61,0.25～0.48 and 0.39～0.72,respectively.The results indicated that more variations of soil C,N and P contents and their stoichiometric ratios in orchard than that in dry land and paddy field could be explained by the random forest model with the considered environmental factors.(6)The random forest model was used to calculate the importance of environmental factors on soil C,N and P contents and their stoichiometric ratios in different land use types.The results showed that the environmental factors influencing soil C,N and P contents and their stoichiometric ratios were different among the three land use types.In dry land,the main factors were annual average temperature,elevation and TWI for organic carbon content,were annual average temperature,p H and elevation for total nitrogen content,and were p H,elevation and aspect for total phosphorus content.In paddy field,the main factors were p H,annual average temperature and slope for both organic carbon and total nitrogen contents,and were p H,annual average temperature and elevation for total phosphorus content.In orchard,the main factors were annual average temperature,TWI and p H for organic carbon content,were annual average temperature,p H and NDVI for total nitrogen content,and were p H,aspect and slope for total phosphorus content.In dry land,the main factors were annual average temperature,elevation and NDVI for soil C:N,were p H,elevation and annual average temperature for both soil C:P and N:P.In paddy field,the main factors were annual average temperature,elevation and NDVI for soil C:N,were p H,elevation and slope for soil C:P,and were p H,elevation and NDVI for soil N:P.In orchard,the main factors were TWI,elevation and annual average temperature for soil C:N,and were p H,annual average temperature and slope for both soil C:P and N:P.The results showed that soil organic carbon and total nitrogen contents in dry land and orchard were significantly affected by climatic factors,while total phosphorus content in dry land and orchard and soil C,N and P contents in paddy field were significantly influenced by soil factors.Soil C:N in dry land and paddy field was significantly affected by climatic factors,soil C:N in orchard was significantly influenced by topographic factors,and soil C:P and N:P in the three land use types were significantly affected by soil factors. |
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