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Similar to higher plants, microbial autotrophs possess photosynthetic systems that enable them to fix CO2. To measure the activity of microbial autotrophs in assimilating atmospheric CO2, five paddy soils were incubated with14C-labeled CO2for 45 days to determine the amount of14C-labeled organic C being synthesized. The results showed that a significant amount of14C-labeled CO2incorporated into microbial biomass was soil specific, accounting for 0.37%–1.18% of soil organic carbon (14C-labeled organic C range: 81.6–156.9 mg C kg−1of the soil after 45 days). Consequently, high amounts of C-labeled organic C were synthesized (the synthesis rates ranged from 86 to 166 mg C m−2d−1). The amount of atmospheric14CO2incorporated into microbial biomass (14C-labeled microbial biomass) was significantly correlated with organic C components (14C-labeled organic C) in the soil (r = 0.80,p < 0.0001). Our results indicate that the microbial assimilation of atmospheric CO2is an important process for the sequestration and cycling of terrestrial C. Our results showed that microbial assimilation of atmospheric CO2has been underestimated by researchers globally, and that it should be accounted for in global terrestrial carbon cycle models. [ABSTRACT FROM AUTHOR]

Crop-assimilated carbon (C), an important source of soil organic carbon (SOC), represents a key linked component of the C cycle in the soil-plant-atmosphere continuum. In the farmland ecosystem, however, the quantitative characterization and mechanism involved in the distribution and transformation of the assimilated C in soil over the plant's life cycle are problems relatively easily ignored. Research in this area is therefore indispensable for a thorough understanding of the process and characteristics of the organic C cycle in farmland soil. This paper provides an overview on: (1) the distribution, transformation rules, and structural features of crop-assimilated C in soil and its contribution to SOC and the function of microorganisms in the transformation of assimilated C. (2) The chemical compositions and structural features of the assimilated C after its entry into soil organic matter. (3) The relationship between assimilated C, rhizosphere deposition, and C isotope technology. Based on the findings, we consider that further research on the distribution of crop-assimilated C in the soil-crop system and the quantitative relations of several C-transformation steps such as crop input, transformation, protection, and stabilization in different ecosystems should be conducted. Moreover, the component and structure of 'new C' input into the soil by rhizosphere deposition in C assimilation and its relationship with oxidation and mineralized stability should also be accounted for. [ABSTRACT FROM AUTHOR]

Soil soluble organic nitrogen plays a significantly important role in nitrogen biogeochemical cycling. The aim of this study was to investigate the amount of soluble organic and inorganic nitrogen pools extracted by water, salt solutions, and centrifugal-drainage technique from different horticultural management system soils. Approximately 5.4-16.6, 4.4-46.5, 7.1-18.2, 8.8-27.8, 1.8-5.6, and 1.7-4.4 mg kg - 1 soluble organic nitrogen were obtained by 1M KCl, 0.5M K2SO4, 10 mM CaCl2, 1/15 mM phosphate buffer (pH 7.0), water, and centrifugal-drainage technique, respectively, in the 0-20 cm horticultural soils. Large variation in soluble organic nitrogen pools was observed across the sites in the present study and the volumes of soluble organic nitrogen pools generally followed the order: organic production system soil > conventional production system soil > transitional production system soil. In relative contrast to soluble organic nitrogen, organic management, known primarily for the absence of artificial fertilizers and pesticides, resulted in significantly lower levels of both soluble inorganic nitrogen and total soluble nitrogen in the soil. The degree of soluble inorganic nitrogen recovery appears highly dependent on the different chemical extractants and generally followed the series: phosphate buffer (pH 7.0) > KCl≈K2SO4 > CaCl2 > water > centrifugal-drainage technique. The soluble organic carbon and nitrogen in all soil extracts were all positively related to soil total carbon, total nitrogen, and electric conductivity value. This suggested that soil total carbon and nitrogen played a central role in the retention of soluble organic nitrogen in soils. Further studies on investigating the soluble organic nitrogen degradation pathway and its bottleneck are warranted. [ABSTRACT FROM AUTHOR]

The effects of varied amounts of fertilization on yield, fruit quality, and nitrogen (N) uptake of muskmelons (Cucumis melo L. var reticulatus Naud) grown under both organic and conventional farming conditions were evaluated. Organic fertilizer (0.0, 0.55, 1.1, and 2.2 kg m-2) and mineral fertilizers containing the same amounts of estimated plant available nutrients [N, phosphorus (P), and potassium (K)] were applied to organic and conventional farming plots, respectively, in both the spring and autumn seasons of 2005. In comparison to conventional farming conditions, muskmelons grown under organic farming conditions had the same yield, total soluble solids (TSS) and soluble sugar contents in both growing seasons, and fruit pulp nitrate content was significantly reduced by 12% on average in spring and 16% on average in autumn. At harvest maturity the aboveground plant N concentration was significantly higher in the conventional treatments than in the organic treatments. At the vine growth stage, the plant N concentrations were similar in all treatments in both seasons. The ratios of nitrate N to total N amount in aboveground biomass were higher in conventional and high fertilized organic treatments than in low or not fertilized organic treatments under limited N supply from the soil. Muskmelon plants absorbed mainly inorganic N, and the protein N fraction in the xylem sap was larger than the amino acid N fraction. Plants grown in the organic system had a higher proportion of organic N in their xylem sap, especially when manure input was low. [ABSTRACT FROM AUTHOR]