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Understanding the dynamics of organic carbonmineralization is fundamental in forecasting biosphere to atmosphere net carbon ecosystem exchange (NEE). With this perspective, we developed 3D-CMCC-PSM, a new version of the hybrid process based model 3D-CMCC FEM where also heterotrophic respiration (Rh) is explicitly simulated. The aim was to quantify NEE as a forward problem, by subtracting ecosystem respiration (Reco) to gross primary productivity (GPP). To do so, we developed a simplification of the soil carbon dynamics routine proposed in the DNDC (DeNitrification-DeComposition) computer simulation model. The method calculates decomposition as a function of soil moisture, temperature, state of the organic compartments, and relative abundance of microbial pools. Given the pulse dynamics of soil respiration, we introduced modifications in some of the principal constitutive relations involved in phenology and littering sub-routines. We quantified the model structure-related uncertainty in NEE, by running our training simulations over 1000 random parameter-sets extracted from parameter distributions expected from literature. 3D-CMCC-PSM predictability was tested on independent time series for 6 Fluxnet sites. The model resulted in daily and monthly estimations highly consistent with the observed time series. It showed lower predictability in Mediterranean ecosystems, suggesting that it may need further improvements in addressing evapotranspiration and water dynamics. [ABSTRACT FROM AUTHOR]

The amount of soil organic carbon (SOC) released into the atmosphere as carbon dioxide (CO), which is referred to as heterotrophic respiration (Rh), is technically difficult to measure despite its necessity to the understanding of how to protect and increase soil carbon stocks. Within this context, the aim of this study is to determine Rh in two Mediterranean forests dominated by pine and oak using radiocarbon measurements of the bulk SOC from different soil layers. The annual Rh was 3.22 Mg C ha y under pine and 3.13 Mg C ha y under oak, corresponding to 38 and 31% of the annual soil respiration, respectively. The accuracy of the Rh values was evaluated by determining the net primary production (NPP), as the sum of the Rh and the net ecosystem production measured by eddy covariance, then comparing it with the NPP obtained through independent biometric measurements. No significant differences were observed, which suggested the suitability of our methodology to infer Rh. Assuming the C inputs to soil to consist exclusively of the aboveground and belowground litter and the C output exclusively of the Rh, both soils were C sinks, which is consistent with a previous modeling study that was performed in the same stands. In conclusion, radiocarbon analysis of bulk SOC provided a reliable estimate of the average annual amount of soil carbon released to the atmosphere; hence, its application is convenient for calculating Rh because it utilizes only a single soil sampling and no time-consuming monitoring activities. [ABSTRACT FROM AUTHOR]

The measurement of soil CO 2 fluxes and their isotopic composition is increasingly used to study and distinguish respiration sources in the soil profile. The influence of non-steady state processes on soil respiratory δ 13 CO 2 values, such as diffusive fractionation, remains unclear but is suggested to be strong, especially in arid ecosystems. In this study, an in situ Fourier transform infrared spectrometer was coupled to automated soil flux chambers and installed in an arid grassland in Italy. Hourly flux and concentration measurements of CO 2 and its isotopic composition were taken over bare soil and senescent grass material. CO 2 fluxes were stable at night (∼1 μmol m −2 s −1 ), and showed peak emissions during the day (up to 10 μmol m −2 s −1 ). Respiratory δ 13 CO 2 values showed a diel pattern. Daytime respiratory δ 13 CO 2 values were on average −24.8‰ and nighttime values were on average −26.9‰, but showed significantly more depleted values during nights with strong increase in atmospheric CO 2 . We hypothesize that the observed diel variation in respiratory δ 13 CO 2 values was a consequence of the non-steady state process “diffusive fractionation” induced by the increasing atmospheric CO 2 concentrations at night. [ABSTRACT FROM AUTHOR]