Reduce revenue vs. increase expenditure: Fires and plant invasion drive soil carbon loss with different mechanisms in a Mediterranean shrubland

Fires and plant invasions pose substantial threats to Mediterranean
ecosystems, particularly in the context of a changing climate. Our study
utilized a data-model integration approach to assess the response of soil
organic carbon (SOC) to fires and plant invasion under three Shared
Socio-Economic Pathway (SSP) scenarios (SSP1-26, SSP2-45, and SSP5-85). We
parameterized the CLM-Microbe model and then investigated the individual
and interactive impacts of fires and plant invasion on soil C by comparing
factorial simulations of initialization (fire/no wildfire in 2021), fire
module on/off, and with and without plant invasion during 2023-2100 in a
Mediterranean ecosystem. The simulations indicated a marked C loss due to
the 2021 wildfire, projected fires, and plant invasion across all future
climate scenarios. Specifically, the 2021 wildfire, projected fires, and
plant invasion reduced the SOC (0-30 cm) by 0.12, 0.26, and 0.15 kg C m-2
under SSP1-26, 0.12, 0.30, and 0.12 kg C m-2 under SSP2-45, and 0.12,
0.24, and 0.13 kg C m-2 under SSP5-85, respectively. However, fires and
plant invasion decreased SOC through distinct mechanisms. The effects of
the 2021 wildfire occurred due to its negative legacy on the soil
microbial community and, thus, litter accumulation, suppressing the
formation of soil carbon via decomposition. Influences of projected fires
occur via consuming fuel, suppressing carbon input to soils. In contrast,
the impacts of plant invasions were due to enhanced microbial respiration,
leading to C loss. These findings emphasize the need for tailored C
sequestration strategies considering the disparate impacts of fires and
plant invasions in the Mediterranean climate.