Soil biocrusts, formed from communities of microbes and their extracellular products are a common feature of dryland soil surfaces. Biocrust organisms are only intermittently metabolically active, but due to their ubiquity they make a significant contribution to the carbon cycle. Quantification of the controls and insights into the interlinked process of photosynthesis and respiration are essential to enhancing our understanding of the carbon cycle in the world’s drylands. Yet, there have been relatively few field studies investigating controls on both biocrust photosynthesis and respiration. We undertook field-based experiments at two dune sites during the dry season in Diamantina National Park in Queensland, Australia to determine how biocrust hydration and illumination affect soil CO₂ flux and photosynthesis. Static chambers and an infra-red gas analyser were used to quantify soil CO₂ flux, and a fluorometer and a CFImager were used to determine a range of photosynthetic parameters in the field and laboratory respectively. When dry, biocrust photosynthetic activity was not detected and soil CO₂ flux was very low irrespective of biocrust cover. Hydration led to a large and immediate increase in CO₂ flux, which was more pronounced in the presence of biocrusts and on the dune with thinner biocrusts. Hydration also initiated the onset of photosynthesis in some biocrusts, which was greatest under low light conditions and sustained with further hydration. There were only infrequent periods of net CO₂ uptake to the soil, occurring when CO₂ uptake due to photosynthetic activity was less than background soil CO₂ flux. Chlorophyll fluorescence imaging indicated biocrust spatial heterogeneity was evident at the cm scale where microtopography creates a myriad of environments for different crust organisms. Our findings demonstrate that biocrusts are highly spatially heterogenetic at both landscape and small scale, which suggests the maintenance of biocrust spatial diversity is likely to be key to imparting resilience to changing climate and disturbance. As well as reaffirming the importance of biocrusts for the carbon cycle in dryland dune soils the study demonstrates that biocrust respiration and photosynthesis respond differently to hydration and shading. This adds an unpredictability to the distribution of soil carbon stocks and the gaseous exchanges of CO₂ between the surface and atmosphere. Future changes to precipitation and increased temperatures are likely to reduce soil moisture across much of the Australian interior and consequently biocrusts may experience a decline in biomass, structure, and function which could have significant repercussions beyond carbon stocks.

由微生物群落及其细胞外产物形成的土壤生物结皮是旱地土壤表面的一个共同特征。生物外壳生物只是间歇性地代谢活跃，但由于它们无处不在，它们对碳循环做出了重大贡献。量化控制和洞察光合作用和呼吸作用相互关联的过程对于增强我们对世界旱地碳循环的理解至关重要。然而，调查对生物外壳光合作用和呼吸作用的控制的实地研究相对较少。我们在澳大利亚昆士兰州迪亚曼蒂纳国家公园的旱季期间在两个沙丘地点进行了实地实验，以确定生物地壳水化和光照如何影响土壤 CO ₂通量和光合作用。静态室和红外气体分析仪用于量化土壤 CO ₂通量，荧光计和 CFImager 分别用于确定现场和实验室中的一系列光合参数。干燥时，未检测到生物地壳光合活性，并且无论生物地壳覆盖情况如何，土壤 CO ₂通量都非常低。水合作用导致 CO ₂通量立即大幅增加，这在存在生物结皮和生物结皮较薄的沙丘上更为明显。水合作用还启动了一些生物结皮中的光合作用，这在低光照条件下最大，并随着进一步水合作用而持续。只有少数时期会出现净 CO ₂吸收到土壤中，当光合作用对CO _{2 的}吸收少于背景土壤 CO _{2 时发生}通量。叶绿素荧光成像表明生物地壳空间异质性在厘米尺度上很明显，其中微地形为不同的地壳生物创造了无数的环境。我们的研究结果表明，生物地壳在景观和小尺度上都具有高度的空间异质性，这表明维持生物地壳空间多样性可能是赋予对气候变化和干扰的适应能力的关键。除了重申生物结皮对旱地沙丘土壤中碳循环的重要性之外，该研究还表明生物结皮呼吸和光合作用对水合作用和遮荫的反应不同。这增加了土壤碳储量分布和 CO ₂气体交换的不可预测性在地表和大气之间。未来降水变化和温度升高可能会降低澳大利亚内陆大部分地区的土壤水分，因此生物结皮可能会经历生物量、结构和功能的下降，这可能会产生除碳储量之外的重大影响。