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In drylands, where water scarcity limits vascular plant growth, much of the primary production occurs at the soil surface. This is where complex macro‐ and microbial communities, in an intricate bond with soil particles, form biological soil crusts (biocrusts). Despite their critical role in regulating C and N cycling in dryland ecosystems, there is limited understanding of the fate of biologically fixed C and N from biocrusts into the mineral soil, or how climate change will affect C and N fluxes between the atmosphere, biocrusts, and subsurface soils. To address these gaps, we subjected biocrust–soil systems to experimental warming and drought under controlled laboratory conditions, monitored CO2 fluxes, and applied dual isotopic labeling pulses (13CO2 and 15N2). This allowed detailed quantification of elemental pathways into specific organic matter (OM) pools and microbial biomass via density fractionation and phospholipid fatty acid analyses. While biocrusts modulated CO2 fluxes regardless of the temperature regime, drought severely limited their photosynthetic C uptake to the extent that the systems no longer sustained net C uptake. Furthermore, the effect of biocrusts extended into the underlying 1 cm of mineral soil, where C and N accumulated as mineral‐associated OM (MAOM<63μm). This was strongly associated with increased relative dominance of fungi, suggesting that fungal hyphae facilitate the downward C and N translocation and subsequent MAOM formation. Most strikingly, however, these pathways were disrupted in systems exposed to warming, where no effects of biocrusts on the elemental composition of the underlying soil nor on MAOM were determined. This was further associated with reduced net biological N fixation under combined warming and drought, highlighting how changing climatic conditions diminish some of the most fundamental ecosystem functions of biocrusts, with detrimental repercussions for C and N cycling and the persistence of soil organic matter pools in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Quantifying carbon dioxide (CO2) fluxes between soil and atmosphere is key in understanding net ecosystem C exchange and biogeochemical C cycling in plant‐soil systems. In ecosystems with low primary production and sparse vegetation, for example, dry lands or subpolar regions where C fluxes are small, measurement sensitivity is key—even so when measurements are combined with isotopic labeling. Here, we present a simplified gas sampling system developed to facilitate sampling and measurement of low soil CO2 fluxes as well as in situ 13CO2 labeling in the same setup. The capacity of the system was tested in a set of feature tests along with gas measurements of dryland soil‐biocrust systems. The system's sensitivity to capture minor changes in CO2 concentration was confirmed in respiration and photosynthesis measurements of soil‐biocrust systems, where fluxes down to 0.1 μmol CO2 m−2 s−1 were quantified. A balloon, implemented to counterbalance underpressure build‐up during gas withdrawal, mitigated 72% of pressure differences at sampling. The overall system volume was reduced to a minimum to limit contamination caused by residual air, and the design enabled one‐step flushing and evacuation of system compartments and gas sample bags, successfully ruling out cross‐contamination between samples. Ultimately, this system offers a flexible and accessible solution for CO2 measurements that can be applied not only on arid soils with low biological activity and turnover rates, but also on plant‐soil systems. The modifications enabled larger, and thereby more representative, sample volumes to be collected while limiting incubation, contamination, and pressure effects on the intact soil system. Plain Language Summary: Measurements of carbon dioxide (CO2) fluxes between soil and atmosphere are crucial to understand the terrestrial carbon cycle. In dry regions, these fluxes are rather small as both the release of CO2 via microbial respiration and uptake of CO2 via photosynthesis is limited by lack of water and sparse vegetation cover. We have developed a simplified CO2 gas sampling setup suitable for incubation experiments of soils with low biological activity and turnover rates, for example, soils from dry lands or subpolar regions, using only cost‐effective, easily available and replaceable system compartments. We tested system parameters in a set of feature tests and could rule out leakage and cross‐contamination. We further confirmed the suitability of the system for capturing small CO2 fluxes in photosynthesis and respiration measurements of soils with biocrusts. A balloon was installed to counterbalance pressure build‐up during sampling, allowing for larger gas volumes to be collected, which resulted in a mitigation of 72% of the underpressure build‐up. This system offers a straightforward and accessible solution for CO2 measurements, with features limiting contamination and pressure effects on the research sample. Key Points: We present a simplified and low‐cost gas sampling system, developed for quantifying CO2 fluxes in soil incubation experimentsRespiration rates down to 0.1 μmol CO2 m−2 s−1 were quantified, confirming the suitability of the system for low‐CO2 flux measurementsA balloon was installed, mitigating 72% of pressure changes in the headspace during sampling [ABSTRACT FROM AUTHOR]