Effects Of Different Land Uses On Soil Organic Carbon And Nitrogen Fractions In Jinyun Mountain

The National Nature Reserve-Jinyun Mountain located in the northwest of Chongqing, which is the typical subtropical mountain in southwest China, a variety of national precious and rare plants grow there. However, in the process of historical development, with the increasing of population, largr-scale deforestation activities occurred, large areas of virgin forest land reclaimed for sloping farmland and orchard, human activities have greatly changed land use patterns in this region.In this study, we collected4typical utilization patterns in Jinyun Mountain, including:subtropical evergreen broad-leaved forest (abbreviation:forest), sloping farmland, orchard and abandoned land to study the impacts on SOC and TN contents and storages as well as active organic carbon (included Labile Organic Matter-LOM, Dissolved Organic Carbon-DOC, Microbial Biomass Carbon-MBC, Light Fraction Organic Carbon-LFOC and Particulate Organic Carbon-POC) and active organic nitrogen (included Dissolved Organic Nitrogen-DON, Microbial Biomass nitrogen-MBN, Light Fraction Organic nitrogen-LFON and Particulate Organic Nitrogen-PON). Results were showed as following:1. The impacts of different land use on SOC and TN content and storage(1) The contents of SOC decreased with the increase of soil depth under different land use. In the whole soil layer (0-60cm), the order of SOC contents was Abandoned Land> Forest> Orchard> Sloping Farmland. The SOC storage of different land use types showed significant difference (p<0.05), and the order of SOC storage of different land uses was Abandoned Land> Forest> Orchard> Sloping Farmland. The results indicated that abandoned land was more conductive to the accumulation of SOC, and forest was more advantageous to the accumulation of SOC than orchard and sloping farmland.(2) TN contents and storage of abandoned land were significantly higher than other three land uses (p<0.05), but there was no significant difference of TN content and storage between forest, orchard and sloping farmland (p>0.05). Abandoned land significantly increased soil TN content than sloping farmland, while there was no significant influence on TN content between forest, orchard and sloping farmland.2. The impacts of different land use on soil active organic carbon and nitrogen(1) The contents of soil labile organic matter decreased with the increase of soil depth under four land use types. In the whole soil layer (0-60cm), the order of LOM contents was Abandoned Land> Forest> Orchard> Sloping Farmland, indicating that the conversion from forest into orchard or sloping farmland, the contents of LOM decreased, and conversion from sloping farmland into abandoned land, the contents of LOM increased. There was no significant difference of ERs between four land uses by ANOVA (p>0.05), which showed that ERs were not sensible to the change of land use. Three CMIs were all Abandoned Land> Forest> Orchard> Sloping Farmland, revealing that forest reclamation resulted in the degradation of soil quality, and abandonment of sloping farmland would improve soil quality.(2) Soil DOC and DON contents had similar profile changes, both of them had obvious variances in the vertical distribution profile under different land use types. Compared with the other three land use patterns, abandoned land increased DOC content significantly (p<0.05), while the impact of DON content was not significant under different land use patterns (p>0.05).(3) The contents of MBC and MBN under four land use patterns decreased with the increase of soil depth. In the whole soil layer (0-60cm), MBC content ranked in the following order:Forest> Sloping Farmland> Abandoned Land> Orchard, and the order of MBN content was Abandoned Land> Orchard> Sloping Farmland> Forest. The single factor variance analysis showed that four different land use patterns had no significant influence on MBC and MBN content (p>0.05).(4) The average ratio of soil light fraction under different land uses ranged from3.45%to10.89%. LFOC and LFON contents obviously decreased in the whole soil profile. The differences of LFOC and LFON contents in the same soil layer among the four land uses were largest in the0-10cm soil layer and the lowest in50-60cm soil layer. The LFOC and LFON contents of forest, abandoned land and orchard were significantly higher than sloping farmland (p<0.05), reclaiming forest to sloping farmland would accelerate the decomposition of soil LFOC and LFON.(5) In the0-60cm soil layer, soil POC and PON contents decreased with the soil depth increased. The average ratio of soil particulate fraction under different land use ranged from12.24%to50.20%, analysis of variance showed that the soil particulate fraction ratio of forest was significant higher than other land use types (p<0.05). Abandoned land significantly increased soil POC and PON contents than other land uses (p<0.05), while there was no significant influence on POC and PON content between forest, orchard and sloping farmland (p>0.05), abandoned sloping farmland would improve POC and PON contents.(6) Total organic carbon and nitrogen contents were significantly positive correlated with each active organic carbon and nitrogen fractions (p<0.01), indicating that there were close relationships between soil carbon and nitrogen components. To a certain degree, all of them characterized the content of soil active carbon and nitrogen and they reflected the effectiveness of soil organic matter. Sensitivity index used to represent the sensitive soil carbon and nitrogen components after land-use changing, the POC and PON had the highest sensitivity indexes, which could reflect the impact on soil quality when land use patterns changed.3. The impacts of different land use on soil physical and chemical properties(1) Soil bulk density was Sloping Farmland> Orchard> Abandoned Land>Forest, soil bulk density increased significantly after reclaiming forest into orchard and sloping farmland (p<0.05). Soil total phosphorus (TP), available phosphorus (AP), Alkali-hydrolyzed nitrogen (Alkali-N), NO3--N, NH4+-N, available sulfur (AS) and available potassium (AK) had obvious variances in the vertical distribution profile under different land uses. The TP content of abandoned land was significant higher than the other land-uses (p<0.05), while the sloping farmland and orchard AP contents were characterized higher. The Alkali-N contents of forest and abandoned land were significant higher than orchard and sloping farmland (p<0.05). The forest had the highest NO3--N, NH4+-N and AS contents, they all significantly higher than the other land use patterns (p<0.05). There was no influence on soil AK content between different land use patterns (p>0.05).(2) Soil carbon and nitrogen components were significantly negative correlation with soil bulk density (p<0.01). The soil bulk density was higher, the less carbon and nitrogen accumulated. Soil AP, Alkali-N, NO3--N, NH4+-N, and AK had a closer relationship with Soil carbon and nitrogen components, all of them had significant positive correlation with each other (p<0.05). TP had close relationships with SOC, TN, DOC, MBN, POC and PON, but had no obvious correlations with other carbon and nitrogen components. Soil pH and AS had no obvious relationships with soil carbon and nitrogen fractions.