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Chemical and isotopic compositions have been measured for N2‐He‐rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle‐derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (l%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene‐Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to −27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and δ13CCO2 suggests a two end‐member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas‐water‐rock interaction.

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Carbon (C) plays an important role in the interaction between plant and rhizosphere microbial communities, but there is still limited information about how C source utilization soil microbial structure responds to soil fertility changes under the double‐cropping rice (Oryza sativa L.) system in Southern China paddy fields. Therefore, the effects of long‐term (33 years) fertilizer regimes on the characteristics of C utilization in both rhizosphere and nonrhizosphere soils under double‐cropping rice fields in Southern China were investigated by using the metagenome sequencing technology. The experiment began in 1986, and included five fertilizer treatments: without fertilizer input (CK), chemical fertilizer alone (MF), rice straw residue and chemical fertilizer (RF), 30% organic matter, and 70% chemical fertilizer (LOM), and 60% organic matter and 40% chemical fertilizer (HOM). The results showed that the relative abundance of Gemmatimonadetes and Planctomycetia in both the rhizosphere and nonrhizosphere soils was increased by application of rice straw residue and organic manure, whereas the relative abundance of Gammaproteobacteria and Nitrospira was promoted by application of inorganic fertilizers. The largest group of clusters of orthologous groups of proteins categories was “amino acid transport and metabolism” with 16.46% unigenes, followed by “general function prediction only” (12.23%). Regarding the gene ontology categories, biological process were the largest category (174 949, 46.40%), followed by cellular component (126 766, 33.62%), and molecular function (110 353, 29.26%). The principal coordinate analysis indicated that different parts of the root zone were the most important factors affecting the variation of C source utilization bacteria community, and the different fertilizer treatments were the second important factor affecting the variation of C source utilization bacteria community. As a result, the application of fertilization practices had significant effects on the abundance and community composition of C source utilization microbes in paddy soils. The results showed that the combined application of rice straw residue or organic manure with chemical fertilizer practices significantly increases the C source utilization of soil microorganisms in double‐cropping rice fields.

A B S T R A C TTo investigate the effects of urea nitrogen (N) and crop residues on soil organic carbon (SOC) decomposition, a batch of incubation experiments was carried out for 250 days by incorporating 15 Nlabeled urea and 13 C-labeled maize residue into soil. Adding maize residue alone or adding maize residue together with urea N had a significant priming effect on SOC. Furthermore, the direction of the priming effect changed over the incubation. This effect could be characterized by three stages. The first stage occurred just after maize residue addition when the substrate for microorganisms switched from native SOC to easily available maize C (lasting $7 days). The second stage showed a positive effect on the decomposition of native SOC (lasting $28-58 days). The third stage showed a negative effect on the decomposition of native SOC. In contrast, adding N alone caused a positive effect over the first 65 days of incubation, followed by a slight negative priming effect. The overall effect of maize residue C and urea N addition on the decomposition of native SOC was dependent on the balance between the inhibitory and stimulatory effects. At the end of the incubation, adding maize residue alone had little effect on the decomposition of native SOC; urea N addition alone increased SOC decomposition by 9.1%, while adding N to soil amended with maize residue decreased SOC decomposition by 9.5%. The amount of residueinhibited SOC decomposition per unit maize C mineralized was 0.21 AE 0.06 in the Maize + N treatment. Application of urea N significantly increased the mineralization rate of maize residue after 20 days of incubation. The increased N availability, microbial biomass and dissolved organic carbon (DOC) induced by the addition of N were responsible for the higher mineralization rate of maize residue. This indicates that the priming effect induced by maize residues could persist for a long time and involved not only one mechanism but a succession of processes. The response of the priming effect to the addition of maize residue and urea N differed depending on the microbial biomass, substrate C and N availability and the stage of decomposition. Adding N to soil amended with maize residue led to a more efficient use of maize residue at the slow mineralization stage.

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