Litter, root, and mycorrhiza manipulations and seasonal effects on soil physicochemical properties and microbial communities in a subtropical coniferous and broad-leaved mixed forest

Organic inputs from aboveground litter and underground roots are an important factor affecting nutrient cycling in forest ecosystems. However, we still know little about the seasonal effects of the interaction between aboveground and underground organic inputs on soil organic carbon, nutrients and microorganisms after vegetation restoration in degraded red soil. Therefore, we focused on a mixed forest dominated by Schima superba and Pinus massoniana that had been restored for 27 years on eroded and degraded red soil in a subtropical region. Five treatments were set as follows: retaining aboveground litter + retaining root + retaining mycorrhizae (LRM, control treatment), doubling aboveground litter + retaining root + retaining mycorrhizae (DLRM), removing aboveground litter + retaining root + retaining mycorrhizae (NRM), removing aboveground litter + removing root + retaining mycorrhizae (NNM), and removing aboveground litter + removing root + removing mycorrhizae (NNN). After more than three years of treatment, DLRM, NRM, NNM, and NNN treatments reduced soil moisture content by 32.0–56.8 % in the rainy season compared with the LRM treatment. Soil total nitrogen and ammonium nitrogen concentrations were the highest in the DLRM treatment. Soil ammonium concentration and pH were higher in the rainy season than those in the dry season, while soil nitrate concentration was higher in the dry season. Soil available phosphorus concentration in the dry season decreased by 64.5 % in the DLRM treatment, while they were 2.0–10.7 times of those in the LRM, NRM, NNM, and NNN treatments compared to the rainy season. Soil microbial communities were dominated by bacteria across treatments, accounting for 74.0–75.5 % of the total phospholipid fatty acid (PLFA) of soil microbes, and there was no significant difference among treatments. Except for fungi, the total PLFAs of soil microorganisms and the PLFA content of each microbial taxon were higher in the dry season than those in the rainy season. The F/B value in the rainy season was higher than that in the dry season. The PLFA contents of gram-positive bacteria and actinomyces in the DLRM and NRM treatments were higher than those in the NNM treatment, and PLFA contents of both in the dry season were 1.5 and 1.6 times of those in the rainy season, respectively. Soil total phosphorus and pH had the highest contribution to soil microbial community changes in rainy and dry seasons, respectively. Comprehensive evaluation showed that double aboveground litter addition was more conducive to soil quality improvement. In conclusion, litter, roots and mycorrhiza manipulations affected the PLFA contents of soil microorganisms through the regulation of soil physicochemical properties, rather than the proportions of each microbial taxon in the total PLFAs, which was related to the season. The results can provide a theoretical basis for soil quality improvement as driven by soil microorganisms during the restoration of degraded red soil.