Effect of vegetation type, wetting intensity, and nitrogen supply on external carbon stimulated heterotrophic respiration and microbial biomass carbon in forest soils

Abstract

By using packed soil-core incubation experiments, we have studied stimulating effects of addition of external carbon (C) (glucose, 6.4 g C m−2) on heterotrophic respiration and microbial biomass C of a mature broadleaf and Korean pine mixed forest (BKPF) and an adjacent white birch forest (WBF) soil under different wetting intensities (55% and 80% WFPS, water-filled pore space) and nitrogen (N) supply (NH4Cl and KNO3, 4.5 g N m−2) conditions. The results showed that for the control, the cumulative carbon dioxide (CO2) flux from WBF soil during the 15-day incubation ranged from 5.44 to 5.82 g CO2-C m−2, which was significantly larger than that from BKPF soil (2.86 to 3.36 g CO2-C m−2). With increasing wetting intensity, the cumulative CO2 flux from the control was decreased for the WBF soil, whereas an increase in the CO2 flux was observed in the BKPF soil (P < 0.05). The addition of NH4Cl or KNO3 alone significantly reduced the cumulative CO2 fluxes by 9.2%–21.6 % from the two soils, especially from WBF soil at low wetting intensity. The addition of glucose alone significantly increased soil heterotrophic respiration, microbial biomass C (MBC), and microbial metabolic quotient. The glucose-induced cumulative CO2 fluxes and soil MBC during the incubation ranged from 8.7 to 11.7 g CO2-C m−2 and from 7.4 to 23.9 g C m−2, which are larger than the dose of added C. Hence, the addition of external carbon can increase the decomposition of soil native organic C. The glucose-induced average and maximum rates of CO2 fluxes during the incubation were significantly influenced by wetting intensity (WI) and vegetation type (VT), and by WI×VT, NH4Cl×VT and WI×VT×NH4Cl (P<0.05). The addition of NH4Cl, instead of KNO3, significantly decreased the glucose-induced MBC of WBF soil (P<0.05), whereas adding NH4Cl and KNO3 both significantly increased the glucose-induced MBC of BKPF soil at high moisture (P<0.05). According to the differences in soil labile C pools, MBC and CO2 fluxes in the presence and absence of glucose, it can be concluded that the stimulating effects of glucose on soil heterotrophic respiration and MBC under temperate forests were dependent on vegetation type, soil moisture, and amount and type of the N added.