Soil respiration (Rs) is the largest source of atmospheric CO₂, and an accurate understanding of the relationship between near-surface winds, CO₂ release from the soil surface, and measurement methods is critical for predicting future atmospheric CO₂ concentrations. In this study, the relationship between wind speed and soil CO₂ fluxes is elucidated on a global scale through meta-analysis, and the flux measurement methodology is further explored in conjunction with the results of a controlled trial to clarify the uncertainty of the measurement results. The results indicate that near-surface wind speed is positively correlated with soil CO₂ release and that near-surface winds result in increased soil CO₂ gas release. Wind disturbance affects both the concentration gradient and gas chamber measurements, and the lower calculated soil CO₂ release conflicts with the notion that the wind pump effect and Bernoulli effect of negative pressure cause a greater surface gas exchange. The results of the log-response ratios indicate that near-surface winds lead to an underestimation of 12.19–19.75% in widely-used gas chamber method measurements. The results of this study imply that some of the current Rs measurements are biased and that the influence of near-surface winds on Rs measurements needs to be urgently addressed to assess the terrestrial carbon cycle more accurately and develop climate change response strategies.

Molar diffusion mechanism is generally considered to be the main physical process of gas transport at the soil–atmosphere interface. However, the advection mechanism in porous medium can considerably affect soil gas transport. Barometric pressure fluctuations caused by pressure pumps is one of the main factors that affects the advection mechanism. Most of the existing studies are overly focused on the construction of complex mathematical models and cannot exclude other environmental factors from interfering. In the present study, a simple attenuation form of barometric fluctuations was explored as a “minimum unit” of pressure wave in laboratory. A pressure attenuation model (PAM) was developed to verify the relationship between pressure difference and gas emission from soil surface by measuring the change in pressure attenuation. The effect of pressure fluctuations on soil surface gas fluxes was then quantified based on the calculated fluxes. In addition, the relationship between the physical properties of the soil medium and the change in pressure was also analyzed. The results show that fluctuations in air pressure can cause a change in soil CO₂ fluxes by an order of magnitude (change of 1 Pa can result in approximately 100% change in flux for sandy loam). The sensitivity of different soil medium to pressure differences was positively correlated with soil gas permeability, which is the main physical property of soil that influences the response of gas to pressure fluctuations. These results provide important prerequisites for quantifying more complex pressure fluctuations in a future study.

Purpose: The soil air diffusion coefficient (D_s) is particularly important for the study of soil gas diffusion movement, and there are still many uncertainties in the widely used methods; as such, a method was designed to in situ measure the soil gas diffusion coefficient. Methods: Four different soil media were selected and studied by means of a designed in situ measurement of soil gas diffusion coefficients, and these were compared and analyzed with the predictions of several commonly used prediction models. In addition, they were combined with gas transport models to validate the results of the empirical models that were obtained by the in situ measurements. Results: The results of the data indicate that increasing the volumetric soil moisture content decreases the soil gas diffusion coefficient, with changes in the soil gas diffusion coefficient for the small-grained quartz sand medium being similar to those predicted by the Buckingham model. The soil gas diffusion coefficients for the large-grained quartz sand were similar to the Millington and Quirk model predictions at low humidity; for increased humidity, it was instead similar to the Buckingham model predictions. The soil gas diffusion coefficients of the two active media were closer to those of the SWLR model with high C_m. In addition, the R² of the measured data was verified, by empirical modeling, to be greater than 0.54, and inversion experiments were conducted to verify that the results were consistent with those of the SWLR model. Conclusion: When measuring relative diffusion coefficients in the field, we recommend the in situ measurement method, which is more reflective of the actual situation in natural environments and provides more accurate data support for soil carbon flux studies.

Forest soil CO₂ flux measurements are important for studying global climate change. Current monitoring methods are based on closed gas chambers, which block the wind pumping effect of near-surface winds in the measurements, resulting in biased values. Therefore, in this study, the effects of near-surface winds on chamber-monitored fluxes were investigated. The CO₂ flux was quantified using a designed flux reference system with different CO₂ concentrations, and the monitoring performance of the closed chamber was studied. Wavelet coherence was used to analyze the response relationship between near-surface winds and soil gas, and was combined with a flux calculation model to explore the relevant factors influencing gas chamber measurement-produced bias. The data indicate that at near-surface wind speeds greater than 0.8 m·s⁻¹, gas transport enhancement was significant and further increased the deviation of the gas chamber-monitored CO₂ fluxes. The monitoring error of the flow chamber (NSF) increased from 7% to 30% in soils with low carbon content, but did not vary significantly (3–7%) in soils with high CO₂ concentrations. The flux measurement bias of the non-flow chamber (NSNF) was positively correlated with the soil carbon content, with the measurement error expanding by 16–24% with increasing soil CO₂ concentrations. The measurement errors of the exponential and linear models in a windless environment were 9.8% (Exp) and 18.7% (Lin), respectively. The estimation errors of both models were positively correlated with both the time of a single monitoring event and the wind-induced coefficient D_w. Therefore, flux calculation models should be improved by considering environments with wind disturbances to reduce the effect of wind on measured values, which will help improve the accuracy of ecosystem carbon budgets.

Urban expansion has triggered significant changes in soil organic carbon (SOC), profoundly affecting the global carbon cycle. The accurate prediction of the global distribution of urban SOC and assessment of the impact of future urban expansion on SOC are essential for urban soil carbon management. By using data from 377 urban locations, this study estimated the global distribution of urban SOC and projected future SOC changes under two socioeconomic scenarios: SSP126 and SSP585. The results showed that compared with the urban SOC density in 2000 (64.75 Mg C ha^− 1), the global urban SOC density by 2100 decreased by 1.19% and 4.72% under the SSP126 and SSP585 scenarios, respectively. However, urban SOC stocks increased by 66.13% and 178.75% by 2100 under SSP126 and SSP585, respectively, reflecting a “carbon transfer” from natural ecosystems to urban land use. This study provides critical insights into the dynamics of urban SOC and underscores the necessity to balance urban expansion with ecosystem protection to reduce carbon loss and stabilise the global carbon balance.