Increasing tree diversity reduces spatial heterogeneity of soil organic carbon and promotes carbon storage in subtropical forests

Plant diversity’s effect on ecosystem functioning is well-established, but its effect on the spatial heterogeneity of soil carbon (C) stocks and microbial responses with increasing diversity is still unclear. Here we propose that increasing tree diversity (TD) would reduce the spatial heterogeneity of soil C stocks while increasing soil C storage, but with inconsistent effect on soil microbial distribution. We test these hypotheses in a subtropical forest comprising three plots each of a monoculture, 10-species, and 30-species plantations. Spatially explicit analysis along distance matrices was conducted on measured soil C fractions and microbial distribution using geostatistical modeling. We show that increasing TD, especially with 30 species reduced the sill value and spatial heterogeneity of soil organic C (SOC) (nugget ratio (NR): >75%) and dissolved organic C (DOC) (NR: 25–75%). Increasing TD reduced the standard deviation in SOC, total dissolved C (TDC), and dissolved inorganic C (DIC) values, while significantly (p<0.01) increasing their concentrations (SOC=16.8–19.2%, DOC=11.2–28.6%, DIC=21.7–34.0%, and TDC=13.1–28.0%) compared to the monoculture. The insignificant change in SOC after increasing TD from 10 to 30 species indicates the importance of litterfall quality and not quantity on SOC accumulation. Besides, the increase in soil pH and reduction in bulk density with increasing TD promoted soil microbial abundance, while inducing a strong spatial distribution (NR<25%) of most soil fungal groups, compared to the bacterial groups. Structural equation modeling indicated negative relationships between DOC and soil microbes in the 30 species mixture, while a positive relationship ensued in the monoculture. Thus, monocultures could accelerate SOC loss through higher DOC-microbes interaction. Therefore, diverse subtropical forests accumulate more SOC with less spatial heterogeneity and higher stability than monocultures. Promoting forest diversity would be valuable for enhancing C sequestration and ecosystem restoration, and measures of its C stocks can be scaled up to predict ecosystem processes with higher accuracy than in monocultures.