Evolution Of Soil Characteristics And Soil Types After Land-use Conversion From Paddy Field To Upland

Paddy soil (stagnic anthrosol) is an important soil resource in China, it has great significance in maintaining food security and the quality of ecological environment. Over the last two decades, with the development of economy and the adjustment of structure of agriculture, large area of paddy field had converted into vegetable field, forest land and upland in the area of south China. To understand the effect of this conversion on the shifts of soil properties and soil classification, we selected Zhejiang province of south China as the study area, and chosen typical paddy soils and several chronosequences of upland soils, which were converted from the paddy soil for various years, as the research object. We collected four cultivated soil sequences, (i.e.blue clayey paddy soil with an inter peaty layer soil sequences, blue clayey paddy soil sequences, blue silt-clayey paddy soil sequences, and silt-loamy paddy soil sequences), and five soil profille sequences (i.e. blue clayey paddy soil with an inter peaty layer soil profile sequences, blue clayey paddy soil profile sequences, blue silt-clayey paddy soil profile sequences, powdery-muddy paddy soil profile sequences, and paddy soil on redeposit of gritty red soil profile sequences).The purposes of this study were to gain an insight into the soil physicochemical properties, microbial genetic diversity and community structure in response to land-use conversion and converted time for upland using combined method of physicochemical, PLFA and DGGE analyses with spatiotemporal substitution method for cultivated horizon soils, and to discuss the changes of soil morphology characteristics, physicochemical properties of soil profiles, the shifts of soil genetic characteristics and redox forms, and the evolutions of soil types and classification on the basis of genetic horizons and properties. We tried to assess the shifts of soil characteristics, predict the evolution of soil quality and types, and provide theoretical guidance for preventing soil degradation and maintaining sustainability of agricultural soils. The main conclusions are as follows:1. Changes of soil properties in cultivated horizonAfter land-use conversion from paddy field to upland, soil pH, organic matter,> 0.25mm water-stable aggregates, total nitrogen, alkaline hydrolysis nitrogen and base saturations generally decreased due to changes of soil moisture condition and cultivation management in the cultivated horizon. Moreover, soil particle composition and cation exchange capacity changed slightly. In contrast, soil total phosphorus, total potassium and available phosphorus increased significantly in all soil series owing to the increase of soil fertilization. Available potassium increased in all soil series except blue clayey paddy soil sequences.The results showed that after land-use conversion from paddy field to upland, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), ratio of MBC to soil organic carbon and basal respiration decreased significantly (P< 0.01) along with the shifts of some physical and chemical properties. Moreover, urease and phosphatase activities increased significantly (P< 0.05), however, catalase activities decreased significantly (P< 0.05). PLFA analysis indicated that total amount of PLFAs declined significantly (P< 0.01), soil microbial community structure changed, and nutritional stress in microbial communities enhanced significantly (P< 0.01). The ratio of Gram-negative bacteria to Gram-positive bacteria was decreased, furthermore, the ratio of aerobic bacteria to anaerobic bacteria was increased, and the relative abundance of bacteria, fungi, actinomycetes and protozoa PLFA shifted. Multivariate analyses of DGGE and PLFA data were in agreement with each other, suggesting that shifts in soil microbial genetic diversity and community structure were primarily attributed to land-use conversion rather than years of upland.2. Changes of soil properties in the soil profilesAfter land-use conversion from paddy field to upland, groundwater level declined, and the size of soil structures increased; surface and subsurface soil matrix color value remains unchanged, but the brightness of color increased; soil bulk density and firmness increased, in contrast, rate of porosity decreased; rusty root hole, rust streak and rust spot decreased significantly in surface and subsurface soil. After land-use conversion from paddy field to upland, no significant change was observed in total and free iron in the soil profile, while amorphous, complexation, ferrous and water-soluble iron decreased. Because manganese is more lively than iron, the change of manganese is more complex. Generally, total and free manganese increased in surface and subsurface soil. Because most of the free manganese was amorphous manganese, amorphous manganese generally increased in surface and subsurface soils; however, complexation manganese decreased in soil profiles along with the increase of converted time and the depth of profiles.3. The changes of soil typesAfter land-use conversion from paddy field to upland, soil genetic property changed, anthrostagnic moisture regime, anthrostagnic epipedon and hydragric horizon disappeared, so the classification position changed. The results showed that blue clayey paddy soils with an inter peaty layer belongs to typic hap-stagnic anthrosols in Chinese soil taxonomy, which converted into humic hapli-perudic argosols; Blue clayey paddy soils belongs to endogleyic Fe-leachi-stagnic anthrosols, which converted into typic hapli-perudic argosols; Blue silt-clayey paddy soil belongs to typic Fe-accumuli-stagnic anthrosols, which converted into typic hapli-perudic argosols; Powdery-muddy paddy soil belongs to typic hapli-stagnic anthrosols, which converted into typic hapli-perudic argosols; Paddy soil on redeposit of gritty red soil belongs to typic Fe-accumuli-stagnic anthrosols, which converted into mottlic ferri-udic argosols. After land-use conversion from paddy field to upland, different types of stagnic anthrosol shifted into the corresponding types of argosols, indicating the strong effects of human could make the soil types changed in a short time scale.