| The paper studied the relations of biomass of fine root,physical properties of soil, water-stable aggregate content and organic matter content to soil anti-erodibility in three types of forest-grass compound mode in Danling county of Sichuan province.Three modes of forest-grass compound are Alnus formosana—Lolium multiflorum(AF—LM)forest land, Alnus formosana—natural grass(AF—natural grass)forest land and Alnus cremastogyme—natural grass(AC—natural grass)forest land(CK).The result is as followings:(1)Distribution and growth characteristics of fine root and grass root in forest land①Biomass of fine root of AF—LM forest land,AF—natural grass forest land and AC—natural grass forest land is 5.7796t/hm2,3.8005t/hm2,3.6948t/hm2, respectively.Biomass of grass root is 0.9074t/hm2,1.8606t/hm2,1.8827t/hm2,respectively.②Vertical distribution of root biomass of three-mode forest-grass root in soil is up layer＞middle layer＞down layer.Proportions of fine-root biomass of upper-,middle- and down-layer in three-mode forest land are almost same,while the proportions of grass-roots biomass are different.The proportion of fine roots biomass of upper layer,middle layer and down layer in AF—LM forest land is 41:33:26,79:21:0 of that of biomass of grass roots; 40:38:22 of fine root biomass in AF—natural grass forest land,56:25:19 of biomass of grass roots;40:36:24 of fine root biomass in AC—natural grass forest land,57:24:19 of that of grass root biomass.③Biomass of fine root with diameter≤1mm,1～2mm is 2.7770t/hm2,3.0026t/hm2 respectively in AF—LM forest land.The percentages of upper-,middle- and down-layer in soil of fine root with diameter≤1mm are 52%,43%,48%;1.9187t/hm2,1.8818t/hm2 of fine roots biomass with diameter≤1mm,1～2mm in AF—natural grass forest land,65%,45%,35%in percentage in soil layers of fine root with diameter≤1mm;1.9800t/hm2,1.7148t/hm2 of fine roots biomass with diameter≤1mm,1～2mm in AC—natural grass forest land,64%,50%,42%in percentage in soil layers of fine roots with diameter≤1mm.④Live biomass of fine root with diameter≤1mm,1～2mm in AF—LM forest land accounts for 73%,81%of total biomass of fine root with the same diameter level.That of AF—natural grass forest land accounts for 90%,77%;That of AC—natural grass forest land accounts for 91%,74%in total.⑤Change in month of totle biomass of root presents dual-apex mode in three-mode forest land,apex values present in June to July and September,lowest value presents in January. Biomass of upper-layer root culminates most and presents obvious changing trend,which has most effect in changing curve in month of total biomass of root.⑥Vertical distribution and changing dynamics in month of accumulation value in fine root with diameter≤1mm and grass root present obvious changing in rule,which limits vertical distribution and month-changing rule of total biomass of root.But vertical distribution and month-change rule of 1～2mm fine root seldom presents in rule.(2)Soil physical and chemical properties change in forest land①Soil bulk density increases by following of soil layer deepen,vertical changing trend of capillary porosity and total porosity is in controverse,in three-mode forest land. Month-change trend of soil bulk density and porosity is not very apparent.②In three-mode forest land,vertical change trend of water-stable aggregation content and total water-stable aggregation content presents upper layer＞middle layer＞down layer.③The maximum value of water-stable aggregation content shows in September,the minimum value shows in January to May.The max value is 207.22%,the average value per month is 189.74%in AF—LM forest land.The max value is 208.64%,the average value per month is 187.80%in AF—natural grass forest land.The max value is 205.29%,average value per month is 183.76%in AC—natural grass forest land.④＞5mm water-stable aggregation content increased in Sept.Contents of three-mode forest land average 111.45%in little discrepancy.⑤The effect of fine roots with diameter≤1mm is obvious,and effect of grass roots is also important,to increase＞5mm,3～5mm water-stable aggregation content and total content and decrease 1～2mm water-stable aggregation content.⑥In three- mode forest land,vertical distribution of soil oganic matter content upper layer＞middle layer＞down layer.Total content of organic matter in whole year is 327.37g/kg in AF—LM forest land,the max value presents in Sept.(84.37g/kg).Total content of organic substance in whole year is 311.10g/kg in AF—natural grass forest land,the max value presents in July(71.5g/kg).Total content of organic substance in whole year is 337.10g/kg in AC—natural grass forest land,the max value presents in Sept.(80.70g/kg).⑦Relative coefficient of biomass of≤1mm roots and soil organic substance is the max.(0.832**).Positive relativity presents in content of organic substance,water-stable aggregation with diameter＞5mm,3～5mm and total content of water-stable aggragation, negative relativity to water-stable aggregation with diameter of 0.25～0.5mm,1～2mm.(3)Evelation of soil anti-erodibility in different forest-grass compound mode forest landBy analysis of main components of indexes pertinent to soil anti-erodibility,complex index of anti-erodibility of multi-layered soil in three Type forest land is upper layer＞middle layer＞lowlay.Average comprehensive exponent of soil anti-erodibility is "AF—LM forest land" (1.041)＞"AF—natural grass forest land"(-0.248)＞"AC—natural grass forest land" (-0.793).The forest-grass compound model of"AF—LM" has good developing trend to promote anti-erodibility of soil.In conclusion,competation was lower in underground space distribution in young forest growth period of five-year growth AF—LM forest land,which has apparent effect in improving soil and developing soil anti-erodibility.The result can be developed in engineering of foresting without farming and eco-revetalization in western region of Sichuan. Developing acticities arranges in suitable habitat region.
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