Response Of Soil Organic Carbon In Deep Layer To Land Use Changes On The Hilly Loess Plateau

Land use change plays an important role in the global carbon balance and climate change. In the past several years, lots of fruitful researches were conducted on SOC pool in the Loess Plateau region, although most of the researches focused on shallow organic carbon. Recent studies had shown that huge amount of SOC was accumulated in deep soil layers(>1m) under ground. So far SOC change with the deforestration process from natural forest into plantations and cropland was rarely reported in the Loess Plateau. Limited knowledge was available for understanding the effect of land use change on deep SOC in the region. In this thesis, taking Ziwuling forest zone of the hilly Loess Plateau as study area, selecting five land use types including natural woodland, natural shrubland, artificial woodland, revegetated grassland, cropland as study object, response of SOC and organic carbon fractions to land use change in deep soil(100-200cm) were explored through field investigation and sampling. For further understanding of the source of deep organic carbon, on-site infiltration experiment was conducted to clarify the contribution of dissolved organic carbon to deep SOC. The main results were as follows:1. In Ziwuling forest zone on the hilly Loess Plateau, SOC storages of natural woodland, natural shrub, artificial woodland, revegetated grassland and cropland was 134.65t?hm-2, 96.23t?hm-2, 124.98t?hm-2, 76.03t?hm-2, 82.75t?hm-2, respectively. SOC storages in deep soil layers(100-200cm) contributed 33%, 29%, 31%, 34%, 42% to total SOC storages in the whole profile(0-200cm).2. SOC content in the whole profile(0~200cm) was varied significantly between different land use types. Among them, SOC content of natural woodland and artificial woodland was highest, followed by natural woodland, the lowest was revegetated grassland and cropland. SOC content in shallow layer was varied significantly between different land use types, however no obvious difference was observed of SOC content in deep soil layers in all of the investigated land use types.3. Land use changes showed significant effect on SOC storages, and changes of SOC storages in shallow soil layers were more sensitive than deep soil layer. From natural woodland to artificial woodland, natural woodland to cropland, natural shrubland to revegetated grassland and natural shrubland to cropland, SOC storages in 0-100 cm profile decreased by 2%-48%, while the deep layer decreased by 12%-22%.4. SOC storage in cropland decreased with the time of conversion from natural shrubland to cropland. After 30 years of cultivation, SOC content decreased rapidly by 31%. SOC of the shallow layer(0-100cm) declined slightly after 30-50 years of cultivation by 8%. After 50 years of cultivation, SOC content keep a relatively stable level. SOC of deep soil layer(100-200cm) declined by 4%-15% after the landuse change.5. Soil active organic carbon content in deep soil layer(60-200cm) accounted for 8%-51% and 71%-84% of soil active organic carbon in the profile(0-200cm) under different land-use types. Soil active organic carbon content in shallow layer(0-60cm) and deep layer(60-200cm) was affected by landuse changes. From natural woodland to artificial woodland, natural woodland to cropland, natural shrubland to revegetated grassland and natural shrubland to cropland, labile organic carbon(LOC) in the deep layer decreased by 1%-21%, and micro biomass carbion(MBC) in the deep layer reduced by 8%-25%.6. Soil dissolved organic carbon(DOC) content decreased with the increase of depth in soil profile, which was significantly correlated with SOC content. DOC content in shallow layer(0-100cm) and deep layer(100-200cm) accounted for 58% and 42% of DOC in the profile(0-200cm), respectively. After 10 d infiltration, DOC content increased by 23% in the profile(0-200cm), while after 30 d infiltration, DOC content in shallow layer(0-100cm) and deep layer(100-200cm) only increased by 5% and 4%.