Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identified roots in the forest from the gymnosperm Cupressus sempervirens L. and the evergreen angiosperm Pistacia lentiscus L. by 48-h incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C flux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure deficit, temperature). We also studied relationships to C fluxes through the leaves as indicators of tree C balance. Root exudation rates varied significantly along the year, increasing from 6 μg C cm ⁻² root day⁻¹ in both species in the wet season to 4- and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-effects model showed that the three soil parameters were the most influential on exudation rates. Among biotic factors, there was a significant negative correlation of exudation rate with leaf assimilation in Cupressus and a significant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation flux during the dry season indicates that exudation dynamics in the field are less sensitive to the low tree C availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought.

中文翻译：

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干旱季节，地中海森林中成熟阔叶树和针叶树的根系分泌物增加。

根系分泌物是根状沉积过程的一部分，这是植物根系释放的土壤有机碳（C）的主要来源。人们认为这种碳通量对生态系统中的碳和养分循环具有深远的影响。根系分泌物的数量取决于植物种类，全年的时间以及外部生物和非生物因素。由于成熟树木的根系分泌物很难在田间条件下收集，因此对其通量知之甚少，尤其是在水分有限的生态系统中，例如季节性炎热干燥的地中海马奎斯。在这里，我们从裸子植物长柏（Cupressus sempervirens L.）和常绿被子植物黄连木（Pistacia lentiscus L.）的森林中经DNA鉴定的根中收集了分泌物。全年以每月时间分辨率48小时进行孵化。我们研究了根系分泌物C通量与土壤非生物参数（水含量，水势，温度）和大气（蒸气压亏缺，温度）之间的关系。我们还研究了通过叶子与C通量之间的关系，以此作为树木C平衡的指标。一年中根系渗出率变化很大，从两个物种的湿季根系分泌日均^6μgC cm ^-2上升到黄连木和柏树的4和11倍分别在旱季。逐步线性混合效应模型表明，三个土壤参数对渗出速率的影响最大。在生物因子中，柏树的渗出速率与叶片同化率呈显着负相关，黄连木与叶片呼吸呈显着负相关。我们对干旱季节渗出通量增强的观察表明，田间的渗出动态对干旱季节低树碳的有效性较不敏感。地中海的两种主要森林物种似乎对根际的季节变化（例如干燥和变暖）作出响应，因此在季节性干旱下将碳投入到根际。