Soil heterogeneity influences microbial access to substrates and creates habitats varying in substrate concentrations, thus leading to local variations in carbon (C) dynamics. Based on theoretical considerations, we expected that higher heterogeneity would decrease microbial activity. To test this hypothesis, we modified substrate spatial heterogeneity using 3D-printed cylinders with four compartments (either preventing or allowing diffusion between compartments). The same total amount of glucose (1.5 mg glucose C per cylinder) was added either to one compartment (highest local concentration, 2.0 mg glucose C g⁻¹ soil, and highest heterogeneity), to two (medium concentration, 1.0 mg glucose C g⁻¹ soil, and intermediate heterogeneity), or to four compartments (lowest local concentration, 0.5 mg glucose C g⁻¹ soil, and equivalent to homogeneous conditions). Thus, we experimentally created a gradient of substrate spatial heterogeneity. The 3D cylinders containing soil were transferred into standard calorimetry ampoules and were incubated in isothermal calorimeters to monitor soil heat dissipation rates as a proxy of soil microbial activity over 51 h at 18 °C. When diffusion among compartments was prevented, the most heterogeneous treatment showed the lowest heat dissipation rates, despite having the highest local substrate concentration. Compared to homogeneous conditions, the heat dissipation rate from the most heterogeneous treatment was 110% lower at the beginning of the experiment (12.7 μJ g⁻¹ soil s⁻¹) and 50% lower when heat dissipation rates reached a peak (72.6 μJ g⁻¹ soil s⁻¹). Moreover, the peak was delayed by approximately 2 h compared to the most homogeneous treatment. When diffusion among compartments was allowed, the effect of substrate spatial heterogeneity on microbial activity was strongly diminished. Our findings emphasize the influence of substrate spatial heterogeneity on soil microbial dynamics, highlighting the importance of including it in C cycling models for a better understanding of soil C dynamics.

土壤异质性影响微生物对底物的接触，并造成底物浓度变化的生境，从而导致碳（C）动态的局部变化。基于理论上的考虑，我们预计更高的异质性将降低微生物活性。为了检验这一假设，我们使用具有四个隔室的3D打印圆柱体（防止或允许隔室之间扩散）修改了基材的空间异质性。将相同总量的葡萄糖（每瓶1.5 mg葡萄糖C）添加到一个隔间（最高局部浓度，2.0 mg葡萄糖C g ^-1土壤，并且最高异质性），添加到两个隔间（中等浓度，1.0 mg葡萄糖C g ^-1土壤和中等异质性），或分为四个部分（最低局部浓度，0.5 mg葡萄糖C g ^-1土壤，并相当于均质条件）。因此，我们通过实验创建了衬底空间异质性的梯度。将装有土壤的3D圆柱体转移到标准量热安瓿瓶中，并在等温量热仪中孵育，以监测土壤的散热速率，以此作为18°C下51 h内土壤微生物活性的指标。当防止隔室之间扩散时，尽管具有最高的局部底物浓度，但最不均匀的处理显示出最低的散热率。与均匀条件相比，最不均匀处理的散热率在实验开始时降低了110％（12.7μJg^-1土s ^-1）并在散热率达到峰值（72.6μJg ^-1土s ^-1）时降低50％。而且，与最均匀的处理相比，峰延迟了大约2小时。当允许在隔室之间扩散时，基质空间异质性对微生物活性的影响被大大降低。我们的发现强调了基质空间异质性对土壤微生物动力学的影响，强调了将其纳入碳循环模型以更好地了解土壤碳动力学的重要性。