| As the largest terrestrial ecosystem organic carbon C pool, soil has a direct effect on atmospheric CO2 concentrations in the global carbon cycle. The soil carbon pool also affects soil fertility, and is critical for soil quality and crop production. Therefore, both the protection and increasing the amount of soil organic carbon is important in order to decrease the impact of global climate change, and also for the sustainable development of agriculture and food security. Returning plant material to soil is one of the most important measures to sequester C, and is widely practiced all over the world. To better understand the process of soil organic carbon accumulation and decomposition processes and mechanism in response to various factors, Huangnitian, a paddy soil in a double cropping area of Central China was selected to determine the dynamics of total carbon and soil organic carbon mineralization characteristics and biological mechanisms involved. A series of incubations were conducted under different conditions. The main experiments and results were as follows:(1) Impacts of moisture content (40% vs.100% of water holding capacity (WHC)) and N fertilizer (urea, U vs. pig manure, M) on C and N transformations were investigated. Results indicated that Water Holding Capacity (WHC) showed the greatest effect on C transformations, followed by N fertilizer, with its effects decreasing with time. The cumulative CO2 emissions were in the order 100%WHC-M> 100%WHC-U> 40%WHC-M> 40%WHC-U treatments, regardless of temperature regimes. Additionally, total organic carbon (TOC) contents and net TOC changes were smallest in 35?-100%WHC-U (P< 0.01) aned largest in 15?-40%WHC-M (P< 0.01). Thee material decomposition ratio during the 0-7 d,0-28 d and 0-105 d incubation periods, calculated by CO2 release, showed the maximum value weas 35?-100%WHC-U while the minimum value was 15?-40%WHC-M. Additionally, net nitrification with 100%WHC-U was higher than witeh 100%WHC-M, but treatments incubated at 40%WHC showed slight nitrification. Soil microorganisms are adaptated to a wide range of temperatures for N mineralization, the magnitude of C- and N- mineralization mainly depend on the availability of soil water and N resources.(2) Impacts of a microbial inoculum on C and N mineralization were evaluated during the decomposition of rice straw in soil, the results implied the inoculum, especially fungi, would adjust to temperature and moisture conditions and N fertilization in regulating organic C mineralization, with water potential having a great role in regulating substrate and nutrient availability. Experiments were conducted at 15?-70%,25?-40%,25?-70%,25?-100% and 35?-70% of water holding capacity (WHC) with adequate N, supplied as urea or manure, respectively. Treatments at 25?-70%WHC,25?-100%WHC and 35?-70%WHC generally achieved significant higher CO2 evolution while treatment at 25?-40%WHC had the least. This was more evident with added manure compared to urea (P< 0.05). The inoculum generally increased the decomposition of C inputs and the largest increases were in the initial 28 days of treatments at 25?-70%WHC,25?-100% WHC and 35?-70% WHC; only the 25?-40%WHC treatment actually immobilized C. The CO2 release rates were positively correlated with dissolved organic C (DOC), but rates were different between treatments. Despite equivalent N application rates, manure treatments produced significantly less NO3", NH4+ than those with urea. Incubation at 25?-40%WHC decreased soil pH the least, probably due to relative low moisture causing delayed nitrification.(3) Changes in priming effects and the taxonomy of soil microbial communities identified and compared using the next generation sequencing, after amendment with plant feedstock and its corresponding biochar were investigated. Feedstock addition resulted in a positive priming effect of 1,999 mg C kg-1 soil (increase of 253.7% compared to control) while biochar gave negative primed C of -872.1 mg C kg-1 soil (-254.3%). Linear relationships between mineralized substrates and mineralized soil C were detected. Predominant phyla were affiliated to Acidobacteria, Actinobacteria, Chloroflexi, Gemmatimonadetes, Firmicutes, Planctomycetes, Proteobacteria, Verrucomicrobia, Ascomycota, Basidiomycota, Blastocladiomycota, Chytridiomycota, Zygomycota, Euryarchaeota and Thaumarchaeota during decomposition. Cluster analysis demonstrated separate phylogenetic grouping of feedstock and biochar. Bacteria (Acidobacteria, Firmicutes, Gemmatimonadetes, Planctomycetes), fungi (Ascomycota) and archaea(Euryarchaeota) were closely correlated to primed soil C (R2=-0.98,-0.99,0.84,0.81,0.91 and 0.91, respectively). The quality of plant materials (especially labile C) shifted microbial community (specific microbial taxa) responses, resulting in a distinctive priming intensity, giving a better understanding of the functional role of the soil microbial community as an important driver of priming effects.(4) Maize feedstocks or biochar and artificial root exudates (pulse or single inputs) were added to soil to create plant material- soil- root exudates (three-C-sources) system. The aim was to evaluate the effects on soil organic carbon changes by means of the stepping iterative method. Results showed that exudates pulse inputs caused regular increased in ?13C abundance during the incubation; the values of treatments applied maize feedstock were enriched. Compared to 40 d, exogenous biomass C contents were 0.34 and 3.62 mg C kg-1 at 80 d, while the maize feedstock caused a larger decline than than biochar. Treatment interactions on primed C were mostly negative especially just after pulse exudates were applied, indicating that the interaction between exudate inputs and plant materials decreased the release of CO2 from soil organic carbon. Maize feedstock and biochar had quite different interactive effects on C changes, which may be because of the differences in labile C, nutrients, and pore structures. This method can fractionation C sources narrowing the range of values of soil-derived C, although it is most likely that there are interactions between the other two sources. |
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