Diurnal and seasonal carbon budget of subtropical moss-dominated biocrusts

Aims: Soil mosses play an important role in carbon sequestration. We aimed to provide accurate estimates of CO₂ flux and identify the drivers in subtropical moss-dominated biocrusts at diurnal and seasonal scales in subtropical China. Methods: We used an automated, continuous carbon flux measurement system with transparent chambers to measure net ecosystem exchange of CO₂ (NEE) of the miniature ecosystem of moss-dominated biocrusts from April 2017 to March 2018. Results: NEE during the study period ranged from -1.65 μmol m⁻² s⁻¹ to 1.77 μmol m⁻² s⁻¹, with a cumulative NEE of 95.14 g C m⁻² y⁻¹. On a daily basis, moss-dominated biocrusts acted as a carbon sink at daytime in the spring and winter but they acted as a carbon source at night-time throughout the year. Negative NEE (i.e., net carbon gain) was observed on 9% of the measurement days over the year, mostly when soil moisture was high. In the summer and autumn, moss-dominated biocrusts mostly acted as a carbon source throughout the entire day. Photosynthetically active radiation and soil moisture were the most important predictors of NEE variation. Daily NEE positively related to soil temperature, while soil moisture had a negative relationship with daytime NEE but not nighttime NEE. Conclusion: The strong temporal variation of NEE at the daily scale across different seasons emphasizes the importance of continuous flux measurements. This study also underscores the crucial role of soil moisture in affecting the carbon sink function of moss-dominated biocrusts in subtropical areas.