Humus differentiation and correlation with other soil properties under pure forest in semi-arid loess hilly area

Long-term growth or continued planting of pure forests which composed of one single species would cause soil humus differentiation both in content and composition,so studying the characteristics of humus differentiation and its correlation with other soil properties may provide scientific basis for the management and reformation of pure forests. In this study,8 typical pure forest soils in Nanniwan forest station( which located at the semi-arid loess hill area) were sampled from 24 plots and their humus and other soil properties were measured systematically. The results showed that:(1) the soil from Platycladus orientalis forest had the highest humus content( 34. 61 g / kg) but moderate humification degree and stability; the soils from Ulmus pumila and Betula platyphylla had moderate humus content( 19. 69—23. 58 g / kg) but highest humification degree and stability; the soils from Acer ginnala and Populus simonii forests had both moderate humus content(20. 59—22. 53 g / kg) and composition; the soils from Pinus tabulaeformis,Hippophae rhamnoides and Robinia pseudoacia forests had both low humus content( 11. 77—13. 81 g / kg) and lowest humification degree and stability.( 2)According to correlation analysis among humus content and other soil properties,humic acid had significant( P 0. 05)positive interaction with CEC,N( total and available N),microbe biomass and protease activity( correlation coefficient0. 769—0. 926,the same below),but significant negative with available Cu(- 0. 793); fulvic acid had significant positive interaction with N,CEC,microbe biomass,sucrase and phosphatase activities( 0. 836—0. 955),but significant negative with available Cu(- 0. 822); humin had significant positive interaction with N,CEC,microbe biomass and phosphatase activity and available Zn(0. 766—0. 951),but significant negative with dehydrogenase activity(- 0. 784).(3) According to correlation analysis among humus composition and other soil properties,no significant correlation(P 0.05) among humus composition and other soil properties was observed. However,the ratio of humic acid / total humus acid would be enhanced by protease,sucrase and peroxidase to some extent,but restrained by dehydrogenase; the ratio of humic acid / fulvic acid would be enhanced by available K,CEC and urease to some extent,but restrained by dehydrogenase.(4)In general,humus content had more observable correlation with other soil properties than humus composition Increasing N fertilize and increasing K fertilize were helpful for the improvement of humus content and composition,respectively.Forming mixed forests or increasing the undergrowth was the essential measure to promote the soil humification process and to solve the problem of soil degradation.