Carbon dioxide flux and microbial responses under multiple-nutrient manipulations in a subtropical forest soil

Many studies have explored the responses of soil carbon dioxide (CO2) emission to altered supplies of carbon (C) and nitrogen (N), while limited information exists on phosphorus (P). However, much less is known about its responses to altered supplies of other elements in soils. This study evaluated the response patterns of CO2 emissions to five levels of alkalinity and nine elements: C, N, P, potassium (K), sulphur (S), calcium (Ca), magnesium (Mg), aluminium (Al), and iron (Fe) by using an incubation study. Changes in dissolved organic C (DOC), microbial biomass C (MBC), and microbial structure were also investigated. The result showed that lower C addition (1000-2000 mg kg-1) insignificantly influenced CO2 emission for up to 55 h, unlike its immediate stimulation with higher C levels. Increasing N addition increased CO2 emission and DOC but decreased MBC. The initial stimulation of CO2 emission, followed by a rapid decline at higher nutrient concentrations, particularly under Al and Fe (acidic conditions), indicated an initial increase in microbial metabolism to adjust to unfav-ourable soil conditions. Compared to the control treatment, MBC was reduced by high N, S, Ca, and alkalinity levels, while an increase was observed with K and Al addition. Soil bacteria showed resilience to extreme nutrient addition levels. However, the stimulation of soil fungi under high C, N, and Al additions contributed significantly to CO2 emission. The dynamic patterns of CO2 emission, DOC, and microbial responses to increasing nutrient supply indicate the complexity of predicting soil processes; hence, the need for further research that accounts for the complex interactions occurring in forest soils. However, our investigation provides insight into the short-term response patterns of soil C under varied nutrient levels, which can provide a reference for land C models for subtropical forest soils and other organic soils.