Mixing tree species is a forest management strategy put forward to increase and stabilize primary productivity. Yet, little is known about soil organic carbon (SOC) storage in mixed species forests, particularly under water shortage. In this study, we used a tree diversity experiment in southwestern France to assess the interactive effects of water availability (via irrigation) and mixing tree species (monocultures of pine (Pinus pinaster) and birch (Betula pendula) versus mixed plots of pine-birch and pine-birch-oak (Quercus robur)) on SOC stocks in the forest floor and across five mineral soil layers (0–5, 5–15, 15–30, 30–60 and 60–90 cm deep). We found that SOC stocks were negatively affected by irrigation in the forest floor due to an increase in decomposition rates. However, SOC stocks were positively influenced by both the mixing of tree species and irrigation in the mineral soil, particularly at the 15–30-cm soil depth. This is because root niche partitioning in mixed plots and an increase in tree biomass in irrigated plots may have resulted in greater organic matter inputs through rhizodeposition and the incorporation of microbially-derived compounds. These preliminary results indicate that SOC dynamics and its response to biotic and abiotic factors are strongly dependent on soil depth. Our data further highlight that the positive effect of mixing trees on SOC stocks was higher in irrigated plots, thereby contradicting the idea that tree mixture effects are expected to be greater when environmental conditions are harsher. We conclude that mixing tree species can increase SOC stocks in the short term in temperate forests, even if the exact mechanisms remain to be identified.

中文翻译：

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在温带火山灰中，混合树种和水分对土壤有机碳储量的影响取决于深度

混合树种是为提高和稳定初级生产力而提出的森林经营策略。然而，对于混合物种森林中的土壤有机碳 (SOC) 储存知之甚少，尤其是在缺水情况下。在这项研究中，我们使用法国西南部的树木多样性实验来评估水的可用性（通过灌溉）和混合树种（松树（Pinus pinaster）和桦树（Betula pendula）的单一栽培与松树-桦树混合地块的交互影响和 pine-birch-oak ( Quercus robur)）在森林地面和五个矿质土壤层（0-5、5-15、15-30、30-60 和 60-90 厘米深）的 SOC 储量上。我们发现，由于分解率的增加，SOC 库存受到林地灌溉的负面影响。然而，土壤有机碳储量受到树种混合和矿质土壤灌溉的积极影响，特别是在 15-30 厘米的土壤深度。这是因为混合地块中的根生态位分区和灌溉地块中树木生物量的增加可能通过根际沉积和微生物衍生化合物的掺入导致更多的有机物质输入。这些初步结果表明，土壤有机碳动态及其对生物和非生物因素的反应强烈依赖于土壤深度。我们的数据进一步强调，在灌溉地块中混合树木对 SOC 库存的积极影响更高，因此与当环境条件更恶劣时树木混合效应预计会更大的想法相矛盾。我们得出结论，混合树种可以在短期内增加温带森林的 SOC 库存，即使确切的机制仍有待确定。