Soil respiration is an important component of the carbon (C) cycle and a major contributor to total ecosystem C efflux. Knowledge of the factors that drive soil respiration in drylands is limited, despite these regions represent more than 40% of the Earth land’s surface. In these environments, biocrusts play an important role in CO₂ exchange towards and from soils. However, the temporal dynamics and main drivers of CO₂ efflux in biocrusts compared to other representative dryland covers such as bare soil and soil under perennial grasses, has not been fully investigated. In this study, we measured the soil CO₂ molar fraction (χ_c) at 2 and 5 cm depths in representative surface covers (cyanobacteria and lichen dominated biocrusts, soil under the alpha grass Macrochloa tenacissima and bare soil) from a semiarid area in SE Spain (Tabernas desert, Almeria) using small solid-state CO₂ sensors, during one hydrological year. We determined the CO₂ efflux (F_s) from the 0 to 5 cm soil profile using the gradient method. Our results show that soil χ_c and F_s were low in all surface covers (on average, 464 ppm at 5 cm) during dry soil periods (soil moisture <0.05 m³ m⁻³). χ_c and subsequent F_s rapidly increased after rainfall, and showed the highest values in the soil under grass (M. tenacissima) and lichen biocrusts. Time series analysis of F_s allowed identifying periodic patterns in F_s strongly related to moisture and temperature periodicities. Moisture was the main driver for F_s on timescales of weeks and months, while temperature was the main driver on daily scales. Moisture exerted a greater influence on F_s in lichen and soil under grass, while temperature had a greater effect on F_s in cyanobacteria and bare soil. Estimated annual CO₂ efflux was 633 g CO₂ m⁻² y^-1 in the soil under M. tenacissima, 450 g CO₂ m⁻² y^-1 in the lichen biocrust, 268 g CO₂ m⁻² y^-1 in the cyanobacterial biocrust and 188 g CO₂ m⁻² y^-1 in the bare soil. On the whole, we demonstrate the suitability of automated F_s measurements for characterising rapid changes in C efflux from dryland surfaces due to changing environmental conditions, which can help improve C predictions in drylands under current climate change.

土壤呼吸是碳 (C) 循环的重要组成部分，也是生态系统碳排放总量的主要贡献者。尽管这些地区占地球陆地表面的 40% 以上，但对驱动旱地土壤呼吸的因素的了解是有限的。在这些环境中，生物结皮在进出土壤的CO ₂交换中起着重要作用。然而，与其他代表性旱地覆盖物（如裸土和多年生草本土壤）相比，生物结皮中 CO ₂外排的时间动态和主要驱动因素尚未得到充分研究。在本研究中，我们测量了土壤 CO ₂摩尔分数（χ _c) 在 2 和 5 厘米深度的代表性表面覆盖物（蓝藻和地衣占主导地位的生物结皮、阿尔法草Macrochloa tenacissima下的土壤和裸土）来自西班牙东南部（塔贝纳斯沙漠，阿尔梅里亚）的半干旱地区，使用小型固态 CO ₂传感器，在一个水文年。我们使用梯度法确定了0 到 5 cm 土壤剖面的 CO ₂流出 ( F _s )。我们的结果表明，在干燥土壤期间（土壤水分 <0.05 m ³ m ^-3），所有地表覆盖物的土壤 χ _c和F _s都很低（平均，5 cm 处为 464 ppm ）。χ _c和随后的F _s降雨后迅速增加，并在草（M. tenacissima）和地衣生物结皮下的土壤中显示出最高值。F _{s 的}时间序列分析允许识别与湿度和温度周期性密切相关的F _{s 中的}周期性模式。在数周和数月的时间尺度上，水分是F _s的主要驱动因素，而温度是日尺度上的主要驱动因素。水分对地衣和草下土壤中的F _{s 的}影响较大，而温度对蓝藻和裸土中的F _{s 的}影响较大。估计每年的 CO ₂流出量为 633 g CO ₂m ^-2 y ^-1在M. tenacissima下的土壤中，450 g CO ₂ m ^-2 y ^-1在地衣生物外壳中，268 g CO ₂ m ^-2 y ^-1在蓝藻生物外壳中和 188 g CO ₂ m ^{- 2} y ^-1在裸土中。总体而言，我们证明了自动化F _s测量适用于表征由于环境条件变化而从旱地表面流出的 C 的快速变化，这有助于改进当前气候变化下旱地 C 的预测。