| With the intense impact of human activities, it is complex terrain and mountainous wide distribution in Southwest of China. Jinyun Mountain is located in the southwest region of China, which belongs to Chongqing, and it’s also a national nature reserve and a growing location for many state-level rare and endangered plants. Changes of land use of the protected area will not only have a greater impact on soil fertility, but also change the growth environmental of these plants and animals which are living there. Therefore, in this thesis we collected soils of four typical land use types in Jinyun Mountain, including subtropical broadleaf woodland, abandoned land, orchards and sloping farmland, then analyzed the impact of different land use on soil aggregate stability, content and storage of organic carbon and total nitrogen in aggregates, active organic carbon and nitrogen fractions (labile organic carbon, dissolved organic carbon (DOC), dissolved total nitrogen (DTN), microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN)) and physical fractions of organic carbon (coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), intra-microaggregate particulate organic matter (iPOM), silt and clay within microaggregates (s+c_m) and silt and clay (s+c)) content and the distribution ratio. For this thesis we will provide some theoretical basis for the study of the impact of different land use on the sequestration ability of soil aggregates for carbon and nitrogen and soil nutrient cycling, but also a reference for effective land management policies. The results showed that:1The impact of different land use on soil aggregate stabilityDifferent land use leads to different proportion among soil aggregate size fractions. The main fracton in Woodland is macroaggregates, accounting for61.83%of the total fractions; There is a large part of small macroaggregate and silt and clay in Abandoned Land and Orchards, accounting for70.07%and69.25%, respectively. Sloping Farmland has a big part of silt and clay,which is about50%of the total. The order of MWD in four types of land use is Woodland (0.95mm)> Orchard (0.67mm)> Abandoned Land (0.53mm)> Sloping Farmland (0.47mm), and GMD is Woodland (0.54mm)> Orchard (0.28mm)> Abandoned Land (0.23mm)> Sloping Farmland (0.18mm). It means that taking Woodland into Orchard and Sloping Farmland will lead to a decrease of soil aggregates stability, while abandonment of Sloping Farmland will lead to a enhancemant of it.2The impact of different land use on organic carbon and total nitrogen in aggregates(1) The content of organic carbon and total nitrogen of aggregates in four land use decrease with soil depth increasing. The content of organic carbon in aggregates of Woodland and Abandoned Land are higher, followed by Orchards, and Sloping Farmland is minimum. And with the increasing size of aggregates, organic carbon content in Woodland increased, however Abandoned Land is opposite, which of that was gradually decreasing. And Abandoned Land has the highest content of total nitrogen except in silt and clay, followed by Woodland and Orchards, the lowest is in Sloping Farmland. The content of total nitrogen of four treatments in silt and clay is Woodland (1.08g/kg)> Orchard (0.85g/kg)> Abandoned Land (0.76g/kg)> Sloping Farmland (0.51g/kg). The conten of organic carbon and total nitrogen will be reduced by transforming Woodland into Orchards and Sloping Farmland, while the content has significantly increased after abandonment of Sloping Farmland.(2) After changing Woodland into Orchards and Sloping Farmland, storage of organic carbon in aggregates have a significantly reduced, and it declined by44.42%in macroaggregate of Orchard while by13.63%of Sloping Farmland. Compared with Woodland, organic carbon storage in two macroaggregates of Sloping Farmland decreased by74.86%and43.26%, respectively. While there is no significantly tend in microaggregates, and was significantly increased in silt and clay. It showed that after Woodland changing into Orchards and Sloping Farmland, the larger aggregates lost more organic carbon and total nitrogen, and the size of aggregates which storage organic carbon and total nitrogen is decreased, that is not beneficial for the sequestration of organic carbon and total nitrogen in soil. After abandonment, storage of organic carbon in macroaggregates increased by398.54%, while microaggregates increased by186.01%. The storage of total nitrogen in four size aggregates have also significantly increased, and their proportion is small macroaggregates (139.32%)> large macro aggregates (116.67%)> microaggregates (55.79%)> silt and clay (26.85%). That means the increase of total nitrogen occurs mainly in the larger aggregates, and the size of aggregates which has main component of the distribution of organic carbon and total nitrogen gradually shift to larger, and that is conducive to the accumulation of nutrients in soil.3The impact of different land use on active organic carbon and nitrogen fractions(1) Labile organic carbon content in four kinds of aggregates of four land use patterns are higher in Abandoned Land (3.78,2.80,2.33,1.23g/kg) and Woodland (1.43,1.82,2.04,2.83g/kg), followed by Orchards (1.00,1.58,1.39,1.07g/kg), Sloping Farmland (1.01,0.71,0.75,0.59g/kg) is minimum. Labile organic carbon is mainly distributed in large macroaggregate and small macro aggregate in Woodland, and after changing into Orchard and Sloping Farmland, the distribution ratio of large macroaggregate significantly decreased, while silt and clay significantly increased. And after abandonment, the proportion in silt and clay significantly reduced, while significantly increased in small macroaggregate. It showed that increasing of Labile organic carbon mainly occurs in larger aggregates, and smaller aggregates has a greater degree of protection than larger aggregates.(2) After Woodland changing into Orchard, content of DOC in small macro aggregates, microaggregates and silt and clay were increased by25.04%,13.52%and9.86%, respectively, and DOC in microaggregates and silt and clay were increased by29.23%and15.03%, respectively, while chaning into Sloping Farmland, meanwhile the proportion of DOC in large macroaggregates and small macroaggregates significantly reduced, and significantly increase in microaggregates and silt and clay. And after abandonment, DOC content in large macroaggregates, small macroaggregates and microaggregates increased by195.98%,83.62%and62.25%, respectively, while the proportion of DOC in silt and clay reduced significantly. That means abandonment make larger aggregates accumulated more DOC, and the larger the size is, the greater effect on the enrichment of DOC.(3) After Woodland changing into Orchard, content of DTN in four aggregates has reduced by large macroaggregates (46.60%)> small macroaggregates (32.04%)≈microaggregates (32.58%)> silt and clay (18.27%), while changing into Sloping Farmland the small macroaggregate, microaggregates and silt and clay decreased by18.74%,41.86%,23.95%, respectively. And after abandonment, large macroaggregates, small macroaggregates, microaggregates and silt and clay is increased by75.36%, 17.19%,28.49%and76.08%. The proportion of DTN in woodland of four aggregates is orderd by microaggregates (37.88%)> small macroaggregates (30.08%)> large macroaggregates (18.11%)> silt and clay (13.93%), while Abandoned Land, Orchards and Sloping Farmland are orderd by silt and clay> microaggregates> small macroaggregates> large macroaggregates. It showed that reclaim of Woodland will result in a shift of DTN from larger aggregates to smaller aggregates. While abandonment will not lead the distribution ratio of DTN to a significantly change, indicating that this change does not result in a redistribution of DTN in these aggregates.(4) After Woodland changing, the content of MBC in large macroaggregates was significantly increased, but the other aggregates fractions were significantly reduced. Compared with Woodland, MBC content in Orchards and Sloping Farmland decreased by50.63%and25.86%in small macroaggregate, by24.11%and48.24%in microaggregates, and by47.94%and47.69in silt and clay. In addition to Woodland, MBC of the other three types are mainly distributed in silt and clay, the proportion reached about50%, while Woodland is mainly distributed in microaggregates. After abandonment, MBC concentrations in large macroaggregates are significantly increased by108.73%, while the others increased by10.41%,28.06%,10.01%, respectively. And the proportion of MBC in silt and clay and microaggregate was significantly decreased, while in large macroaggregates increased, but there is no significantly changes in distribution of MBC in four aggregates.(5) After Woodland changing, MBN content in small macroaggregates decreased by40.39%and48.99%, and microaggregates decreased by36.03%and50.29%, silt and clay decreased by26.24%and39.58%. And after abandonment, MBN content in large macroaggregates, small macroaggregates and micro-aggregates increased by549.63%,159.34%,41.91%, respectively, while silt and clay is decreased. MBN was mainly distributed in small macroaggregates and micro-aggregates in Woodland and Abandoned Land, while in silt and clay in Orchards and Sloping Farmland. After Woodland changing, the proportion of MBN in larger aggregates significantly reduced, while significantly increased in smaller aggregates. That means reclamation of Woodland will result in a shift of MBN into smaller aggregates, while it will be opposite after abandonment.4The impact of different land use on physical fractions of organic carbon(1) The results showed that the mean content of organic carbon in the fractions of SOC,cPOC,fPOC,iPOC,s+c_m and s+c fraction in the0-60cm soil depth of Woodland (9.02,3.14,1.61,0.33,0.42,3.53g/kg) was significantly higher than Orchard (3.27,0.93,0.27,0.10,0.24,1.73g/kg) and Sloping Farmland(2.58,0.51,0.10,0.12,0.08,1.77g/kg), which indicating that reclamation of Woodland causes the loss of SOC and its components; while Abandoned Land(14.90,5.17,2.36,0.42,0.59,6.36g/kg) was also significantly higher than Sloping Farmland, demonstrated that after the abandonment of cultivation in Sloping Farmland, SOC and its components can be effectively recovery and sequestration.(2) The iPOC has the lowest distribution ratio of organic carbon of the SOC fractions, and it’s about3%in four land use types. The cPOC and fPOC, being considered to be active in non-protected organic carbon pool, accounted the highest proportion of50%in Woodland and Abandoned Land. However, in Orchards(65.9%) and Sloping Farmland(71.6%), the distribution ratio of non-protected organic carbon pool was low, while it’s high in the s+c fraction in the pool of organic carbon which has chemical protection indicating that the activity of soil organic carbon in Woodland and Abandoned Land, which with higher soil fertility, is much larger than Orchards and Sloping Farmland.(3) And fPOC is the most sensitive indicators of SOC and its fractions, so it can be a good indicator of the impact on soil organic carbon pool by land use change. |
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