The Impacts Of Afforestation On Changes Of Soil Organic Carbon And Major Nutrients In The Mid-watershed Of "One River And Two Tributaries" In Tibet

Since 1980s,a sustainable afforestation was conducted in the mid-watershe of“One River and Two Tributaries”.It was significance for improving ecological situation,local economic developemen,the farmers'working and living environment.The soil is a base for plant growing and propagation.But there's no systematic research on the change of soil physicochemical properties before and after afforestation in this special area.Exploring the soil carbon sequestration and nutrient change of the afforestaion has significance for accurate evaluate the ecological contribution for the Tibetan afforestation activities in Tibet plateau.It could fill the gap of afforestation impacts on soil carbon and soil nutrient in Tibet,and provided a certain theoretical basis for reasonable afforestation and promoting sustainable management for artificial forest.This research took the afforestation area in the region of“Two River and Four Streams”Afforestation Project,the artificial forest and control plots in the mid-watershed of“One River and Two Tributaries”as study objects.524 afforestation areas and 54 couples of stands with control plots were surveyed,acquiring nearly a thousand soil samples.Aimed to master the soil nutrients and fertility characteristic of the Tibet's afforestation areas,the soil physicochemical properties,stands,environment,etc.were systematic measured and researched,researching the soil carbon?SOC?and nutrients's change law,trend and its'influencing factors after afforestation in this important afforestation area.The main results and conclusion of this study were as follows:1.The soil nutrient and fertility characteristics of afforestation areas in Tibet.The soil nutrient level and synthetic fertility?Ps?were totally low with afforestation areas in Tibet,wihch general lacked soil phosphorus,potassium,organic matter,etc.Each kind of soil nutrient and its fertility level were gradually well from the northwest Ngari to southeast Nyingchi,showing the obvious horizontal zonal law.The afforestation areas of Shigatse,Shannan,Lhasa had a certain geographical common character,with many soil nutrients content similar.The soil nutrient contents and its fertility of afforestation areas in the mid-watershed of“One River and Two Tributaries”were lower than the Tibetan average level.2.The soil nutrient characteristic before or after afforestation in the mid-watershed of“One River and Two Tributaries”.The mid-watershed of“One River and Two Tributaries”is the important area for Tibetan afforestation,with widespread lack of soil nutrient content.After afforestation,the content and nutrient levels of soil bulk density?BD?,soil mass water content?MWC?,TN,TP,AP,soil organic carbon?SOC?changed siginificant in soil profile of 0-80cm layer?P<0.01?.After afforestation,BD becamed lower,MWC came higher,the content of SOC?OM?,TN increased with the soil abundance degree and sub-fertility becoming well,but the TP and AP changed oppositely.P_s,pH and TK changed little.The soil abundance degree and sub-fertility of AP changed most obvious with highest percentage.Secondly,they were TP,TN and OM.After afforestation,the soil lacked AP mostly.3.The changes of soil nutrient in soil layers before or after afforestaion in the mid-watershed of“One River and Two Tributaries”.?1?The storage of SOC,TN,TP,TK,AN,AP,AK in the 0-80cm soil layer of artificial forest were 66.18Mg hm^-2?7.55 Mg hm^-2?5.8 Mg hm^-2?118.62 Mg hm^-2?53.16 g m^-2?4.72g m^-2?95.42 g m^-2,respectively.Before or after afforestation,there was no significant difference for pH,AK and soil particle-size distribution in each soil layer.Afforestation had most obvious change in the 0-20cm soil layer,which controled the change of total soil profile.The content of AP decreased significance after afforestation,with the biggest relative change rate in each soil layer,which was the smallest for TK.The evaluating indexes of AP's change were siginificant different after afforestation in all soil layers.?2?The soil nutrient changed with the depth of soil layer,showing the same change trends with or without afforestation.In deeper soil layer,BD increased,MWC decreased,the content and storage of SOC,TN,TP,AN,AP,AK decreased.The soil nutrient had obvious properties of surface-aggregation,which was more aboundant in top layer than sub layer.4.The storage change of soil nutrient in different landuse and stand age.?1?There was significant difference of soil nutrient storage in different landuse.Arable land had the highest storage in all nutrients,and that were general decreased after afforestion?5to 15a?.The storage of SOC,TN were accumulated after afforestation on barren land and grassland,and with a obvious fixed effect when above 15a of the stand age.The TP,TK,AP were comsumed on all landuse types after afforestation.That comsumed more remarkable on arable land.The BD decreased apparently in the top layer after afforestation on all landuse types.The MWC increased after afforestation on barren land and grassland,but only increased obviously in top layer after afforestation on arable land.?2?For artificial forests in different stand age,the storage of SOC,TN,AN increased as the longer stand age,and that of TP,AP were decreased.The SOC,TN increased more obvious when the stand age above 15a.AP was comsumed in all stand age stages.The accumulative rate of SOC storage was highest in the stand age of 15-30a.After afforestation,BD decreased and MWC increased apparently in the 0-20cm layer whatever the stand age.Only afforestation more than 30a,then the artificial forest could change the BD,MWC,and soil particle-size distribution apparently.?3?The storage of SOC,TN were increased almostly in all stand age stages when afforestation on barren land and grassland?stands with age 5a to 51a?,with significant increasing in top soil layer.Which increased the most in 15 to 30a after afforestation on grassland,and that increased the most above 30a after afforestation on barren land.When stand age above 15a,more older the stand age,more storage increased of SOC,TN after afforestation on barren land.The storage of TP,AP,TK almostly decreased in all stand age stages,on all landuse types,and in all soil layers.Under 15a of afforestation on grassland,the storage of AK increased obviously,and decreased when above 15a.That was increased in almostly soil layers when afforestation on barren land.All nutrients'storage decreased almost in each soil layer after afforestation?<15a?on arable land,and AP decreased obvious in all soil layers.5.The influence factors for the soil nutrient change after afforestation in the mid-watershed of“One River and Two Tributaries”.?1?The stand factors were not the main factor to influence the storage change of soil nutrients after afforestation in the research area,only interpreting 30.5%of the influence causes.The change of SOC,TN,AN were mainly influenced by stand factors.The change of SOC had positive correlation to the stand age and factors which standing for the growth of stand.TN in the upper soil layer related positively to stand growth factors,but with negative correlation in the under soil layer.The relative change rate of AN was similarly to it.After afforestation,the stand factors impact on the storage of SOC,AN,TN were stand age,coverage of undergrowth vegetation and canopy in sequence,with all positive correlation.But the storage change of AP,TK,AK,TP almost related negatively to the stand factors which standing for the stand growth.The change of AK,TP were weakly influenced by stand factors.?2?The environment factors were not the main factor to influence the storage change of soil nutrients after afforestation in the research area,only interpreting 19.7%of the influence causes.The change of soil nutrients mainly related to the temperature and altitude.The changes of AP,TK related negatively to the altitude,and related positively to the temperature.The changes of AN had negative correlation to temperature and with positive correlation to altitude in under soil layer.The environment factors had the strongest influence on AP's change.The impacts of altitude to AP was stronger than temperature.The changes of SOC,AK almost related seldom to the environment factors.?3?Different landuse had significant difference in initial soil physicochemical properties.The initial soil conditions were the main factors impact on soil nutrients'storage change,but with little impact on AK's change.The initial content of AP,TK and BD had great influence on the storage change of soil nutrients,which related negatively to the storage change of AP,TN,TP after afforestation.?4?After afforestation,the classes and influence degree of factors on the storage change of soil nutrients showed:initial soil conditions?landuse types?>stand factors>environment factors.Stand factors and enviroment factors influenced the soil physicochemical properties,the storage change of soil nutrients,and the ralative change rate stronger in upper soil layer than under soil layer.But that influenced the soil accumulative rate stronger in under soil layer.After afforestation,the main factors influencing the change of TN,TK,AP,TP were initial soil conditons,with negative correlation.The factors impact on SOC's change were stand factors and initial soil conditions in sequence,which positive correlated with stand age and average canopy,and negative correlated with initial content of TN,SOC.The change of AK was little influenced.