| In subtropics, the diversities and fluctuations of different land-use types significantly affect the soil nutrient supplies, components and spatial distribution of soil organic carbon pools (SOC). Over the past few decades, the way of land use tremendously changed due to the rapidly developed socio-economic, which result in a strong influence on SOC pool in the scale of regional and national. Moreover, the indeterminancy of temporal and spatial variations of SOC pools was caused by frequent land use fragmentation and interference. In this study, six landtypes were selected:natural secondary forest, Cunninghamia lanceolata plantation forest, Phyllostachy edulis plantation, nursery garden, aerid farmland, and paddy field., To properly evaluate the component, dynamic characteristics, and ecological significance in soil carbon pool; improving the mechanism of the dynamic balance in soil carbon circulations; implement the sustainable soil resources; and further understanding the mechanisms of soil organic matter degradation, the characteristics of spatial temporal variations of soil nutrient and the component of SOC pools were studied.. The main results were summarized as following:1. Soil bulk density was 1.14～1.58g/cm3 at the depth of 0-15cm, and 1.24～1.47g/cm3 at the depth of 0-30cm. Compared with evergreen secondary forest L. glaber and Cyclobalanopsis glauca, soil bulk density increased 12.64%,10.49%,17.66%, and 10.59% under Cunninghamia lanceolata plantation forest, nursery garden, arid farmland, and paddy field, and decreased 0.80% under Phyllostachy edulis plantation at the depth of 0-30cm. The sample soil has typical features of clayey texture in south red soil hilly region. Soil of paddy field was silt loam, others were clay loam. Under the effects of frequent human activities, soil silt and clay content obviously decreased, while fine sand and silt content increased. Fractal dimension of soil particle size significantly different from different land-use types. L. glaber+ Cyclobalanopsis glauca evergreen secondary forest has the best soil, followed by Phyllostachy edulis plantation and nursery garden, and Cunninghamia lanceolata plantation forest, dry farmland, and paddy field were the worst. Paddy field contented the highest soil water, Cunninghamia lanceolata plantation forest, dry farmland and nursery farmland contented less water when compared to paddy filed, L. glaber+ Cyclobalanopsis glauca evergreen secondary forest and Phyllostachy edulis plantation had the least water. Seasonal dynamics of soil water content under six land-use types were basically same. Different land-use types had significant effects on soil physical properties. When natural secondary forest changed to farmland, soil bulk density increased, soil particle distribution and pore structure obviously coarsened, physical clay content and fractal dimension decreased.2. Sample soil was acidic, pH value was between 4.6 and5.5. Farmland and nursery garden had the highest pH, paddy field and Phyllostachy edulis plantation were second, L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest and Cunninghamia lanceolata plantation forest were lowest. Seasonal dynamics of soil pH were basically the same under six land-use types except arid farmland. Significant difference of total nitrogen (TN), hydrolysable nitrogen (HN), total phosphorus (TP), available phosphorus (AP), total potassium (TK) and available potassium (AK) appeared in the top soil. Different land-use types had significant effects on soil TN, HN, TP, AP, AK, and soil supply intensity. Lland-use types significantly affected the soil nutrient accumulation and supply. Total and available nutrient contents had obvious seasonal dynamics under different land-use types. However, land-use types only had significant effects on seasonal dynamics of soil TN, and HN, and had little effects on seasonal dynamics of soil TP, AP, TK, and AK. Soil HN, TP, TK, AK contents were deficient in the hilly region of central Hunan; especially TP and TK. Soil nutrient comprehensive index was 1.87～9.32, in the order of arid farmland (9.32)> paddy field (4.29)> L. glaber+ Cyclobalanopsis glauca evergreen secondary forest (3.35)>Phyllostachy edulis plantation (2.89)> Cunninghamia lanceolata plantation forest (2.50)> nursery garden (1.87). Correlations between soil nutrient and pH were not exactly the same whether under the whole research area or six land-use types. Under different land-use types, synergism and coupling was represented among TN, HN and TP, AP, AK, AP and AK. Antagonism was represented between TN, TP and TK.3. Four soil enzyme activities decreased with soil depth increasing. Different enzyme activities had different decreasing level, and had insignificant difference. Under different land-use types, soil catalase and acid phosphatase differed slightly, urease and invertase differed obviously. Phyllostachy edulis plantation had the highest activity of four kinds of enzyme (except urease at the depth of 0-15cm). L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest had the higher activity of acid phosphatase, urease and invertase. It shows that land-use types less affected catalase, acid phosphatase activity, and greatly affected urease and invertase activity. When natural secondary forest changed to Cunninghamia lanceolata plantation forest and farmland, soil enzyme activity variously decreased. Soil enzyme activity in Phyllostachy edulis plantation increased through moderately farming and fertilizing. Seasonal dynamics of acid phosphatase had very significant difference among different land-use types. Catalase activity had different seasonal trends. Seasonal dynamics of urease represented as single peak curve, but the peak value and time varied with land-use types. Invertase had same seasonal trends under different land-use types. It shows that land-use types had significant effects on seasonal dynamics of soil acid phosphatase, catalase and urease activity, and affected seasonal trend of invertase activity less obviously. Land-use types just changed the fluctuation range of invertase activity. There were significant correlations between soil catalase, urease, acid phosphatase, invertase activity,soil pH, and contents of N, P, K. It shows that soil enzyme activities had a closer correlation with soil nutrient circulation and energy conversation in hilly region of central Hunan. And soil enzyme activitiesindicated the evolution characteristics of soil properties under different land-use types.4. Soil organic carbon density (SOCD) decreased with soil depth increasing in six land-use types. The descending order of SOCD at the depth of 0-30cm were L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest (85.08t/hm2)> Cunninghamia lanceolata plantation forest (72.98 t/hm2)> Phyllostachy edulis plantation (65.69 t/hm2)> paddy field (64.72 t/hm2)> arid farmland (54.28 t/hm2)>nursery garden (40.94 t/hm2). Significant difference of total organic carbon (TOC), microbial biomass carbon (Cmic), dissolved organic carbon (DOC), readily oxidization carbon (ROC), and mineralization organic carbon (MOC) were observed. The effects were particularly distinct under L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest, Cunninghamia lanceolata plantation forest and Phyllostachy edulis plantation. Compared with L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest, contents of SOC, Cmic, DOC, ROC and MOC under Cunninghamia lanceolata plantation forest, Phyllostachy edulis plantation, nursery garden, arid farmland and paddy field decreased differently (except Cmic content under Phyllostachy edulis plantation and paddy field). The decreased range was larger under nursery garden and arid farmland, second larger was under paddy field and Cunninghamia lanceolata plantation forest, smallest one was under Phyllostachy edulis plantation. It shows that components of SOC pools significantly decreased when natural secondary forest changed to plantation forest and farmland. Among the components, contents of ROC and MOC decreased more significantly.Significant seasonal dynamics of TOC and Cmic contents under L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest, Cunninghamia lanceolata plantation forest, and Phyllostachy edulis plantation, which represented as higher in summer and lower in winter. Seasonal differences of TOC and Cmic were not significant under nursery garden and arid farmland, but significant under paddy field. DOC and ROC had significant seasonal changes and the pattern was similar under six land-use types. Seasonal trends of MOC were obvious, and similiar to TOC and Cmic. Seasonal dynamic, peak value and time varied with different land-use types. It shows that land-use types had significant effects on seasonal dynamics of TOC, Cmic, and MOC, but not significant on seasonal dynamics of DOC and ROC.Microbial quotient of paddy field was 5.38%-5.59%. Microbial quotient under other five land-use types was 2.26%～2.95%, Paddy field had the highest, second one was in Phyllostachy edulis plantation, lowest one was in arid farmland or nursery garden. The distribution proportion was 2.18%-2.86% at the depth of 0-15cm, was 2.50%～2.95% at the depth of 15-30cm. The order of DOC distribution proportion was opposite to the order of TOC content. Distribution proportion of soil ROC was 21.02%-32.54%, with the order of paddy field> L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest> Phyllostachy edulis plantation> darid farmland> nursery garden> Cunninghamia lanceolata plantation forest. The differences between the first three and the latter three were significant. Distribution proportion of MOC was 0.27%-0.47%, with the order of nursery garden> dry farmland> paddy field> Phyllostachy edulis plantation> L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest> Cunninghamia lanceolata plantation forest. It shows that land-use types had significant effects on the distribution proportion of soil active organic carbon components, and the difference dependent on the land use types.The turnover rate of Cmic at the depth of 0-30cm in hilly region of central Hunan was 0.50～1.90/a and the turnover period was 0.55-2.14a. Turnover rate of Cmic under paddy field was the highest, but the turnover period was the lowest.and the turnover was the highest (292.96g/m2.a1). Turnover rates of Cmic under nursery garden and dry farmland were the lowest, but the turnover period was the highest and the turnover was the lowest:24.81 g/m2.a1 and 30.17 g/m2.a1. It shows that, when SOC content was low under the paddy field, in order to supply abundant nutrients for crop growth,.accelerating turnover rate of Cmic and increasing the turnover can immobilize more nutrients and accelerate nutrient mineralization. The soil of L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest had a lower turnover rate of Cmic, and maintained a higher standing crop, which benefited for soil organic matter accumulation and nutrient fixation. Compared with L. glaber+ Cyclobalanopsis glauca evergreen broad-leaved forest, soil carbon management index decreased 18.37%,20.89%,44.05%,51.53% and 66.63% at the depth of 0-15cm under paddy field., Phyllostachy edulis plantation, Cunninghamia lanceolata plantation forest, dry farmland and nursery garden, the index decreased 11.61%, 9.14%,40.00%,48.58%, and 59.33% at the depth of 15-30cm. It shows that soil properties decreased, especially the top soil, when natural secondary forest changes to plantation forest, Phyllostachy edulis plantation or farmland.5. Whether under the whole research area or different land-use types, significant correlations among soil Cmic, DOC, ROC, MOC, and TOC were obtained. Coefficients between ROC, MOC and TOC were higher than that the value between Cmic, DOC and TOC. Different forms of soil organic carbon were strongly correlated with TN, HN, TP, urease, acid phosphatase, invertase, and were weakly correlated with TK, AP, AK. It shows that under different land-use types, soil Cmic, DOC, ROC, and MOC could be used as important components of soil active organic carbons, and content variation would cause TOC content changed. ROC and MOC contents have strong indicative functions. Different land-use types changed the soil TOC and active organic carbon contents, and they were also important to affect soil fertility changes. Under different land-use types, soil pH and water content had different effects on different forms of organic carbon. The management index of soil carbon pools was significantly correlated with TOC, Cmic, DOC, ROC, MOC, TN, HN, AP, urease, acid phosphatase, and invertase. It shows that differences of soil carbon pool and its management index were significant under agricultural or forestry management. The management index of soil carbon pools could reflect the effects of different management measures on soil carbon pools and soil fertility. When natural secondary forest changed to Cunninghamia lanceolata plantation forest, Phyllostachy edulis plantation, nursery garden and farmland, the content and quality of SOC decreased, and the management index of carbon pool andsoil fertility decreased too. |
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