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In order to investigate the carbon sequestration potential of biochar on soil, citrus orchard soils with a forest age of 5 years was taken as the research object, citrus peel biochar (OBC) and magnesium-modified citrus peel biochar (OBC-mg) were selected as additive materials, and organic carbon mineralization experiments were carried out in citrus orchard soil. OBC and OBC-Mg were applied to citrus orchard soils at four application rates (0, 1, 2, and 4%), and incubated at a constant temperature for 100 days. Compared with CK, the cumulative mineralization of soil organic carbon decreased by 5.11% with 1% OBC and 2.14% with 1% OBC-Mg. The application of OBC and OBC-Mg significantly increased the content of soil organic carbon fraction, while the content of soil organic carbon fraction was higher in OBC-Mg treated soil than in OBC treated soil. Meanwhile, the cumulative mineralization of soil organic carbon was significantly and positively correlated with the activities of soil catalase, urease and sucrase. The enzyme activities increased with the cumulative mineralization of organic carbon, and the enzyme activities of the OBC-Mg treated soil were significantly higher than those of the OBC treated soil. The results indicated that the OBC-Mg treatment inhibited the organic carbon mineralization in citrus orchard soils and was more favorable to the increase of soil organic carbon fraction. The Mg-modified approach improved the carbon sequestration potential of biochar for citrus orchard soils and provided favorable support for the theory of soil carbon sink in orchards.

Decomposition of forest litter regulates the cycling of nutrients in forest ecosystems and influences the net carbon storage for terrestrial ecosystems.Laccase enzyme mainly produced by basidiomycetes plays a central role in lignin degradation and thus contributes to the cycling of soil organic matter.Compared to many studies of basidiomycetous laccase gene diversity in pure broad-leaved or coniferous forests,comparative data on diversity of this gene from different forest types are still rare,especially from forests exhibiting significant differences in vegetation and soil types.This study investigated the diversity and community structure the basidiomycete laccase gene in litter(O horizon) and surface soil(A horizon,0 to 20 cm) from subtropical karst primeval forest(a mixed evergreen and deciduous broad-leaved forest) and artificial masson pine forest in red soil(a coniferous forest) by constructing cDNA libraries at mRNA level.The diversity,richness and evenness of the laccase gene were higher in the primeval forest than those in the masson pine forest for the O horizon,while the reverse trend in these indices was observed for the A horizon.For the same forest ecosystem,the values of the aforementioned indicators from O horizon were correspondingly higher,but apparently lower,than those from A horizon in the primeval forest and the masson pine forest,respectively.With regard to taxonomy at the order level,Agaricales accounted for 90.2% of the clone library for the O horizon of the masson pine forest,and dominated both horizons of the two forests.Polyporales and Gomphales were abundant in the O horizon of the primeval forest and A horizon of the masson pine forest.Russulales was only detected in the A horizon of the primeval forest,accounting for 12.4% of the clone library.Regarding the community structure,the distributions of basidiomycetous populations containing the laccase gene in the A horizon of the primeval forest and O horizon of the masson pine forest were more uneven than those in the O horizon of the primeval forest and A horizon of the masson pine forest.One and two absolutely dominant basidiomycetous species(accounting for ≥20% of the total clones in each clone library),showing high amino acid similarity to Agaricus sp.,Mycena sp.and Pleurotus sp.,were found in the O horizon of the primeval forest and A horizon of the masson pine forest,respectively.Moreover,the laccase activity and the contents of carbon,nitrogen and lignin were significantly higher in the primeval forest than those in the masson pine forest,and were significantly higher in the O horizon than those in the A horizon of a same forest.These results indicate that there are great discrepancies in basidiomycetous laccase gene diversity and community structure in different soil horizons between subtropical broad-leaved forest and coniferous forest in China.These discrepancies may derive from intricate effects closely related to the availability and diversity of microbe-utilized substrates and soil pH.