Department of Plant and Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel.
Tree Physiol. 2020 Oct 29;40(11):1595-1605. doi: 10.1093/treephys/tpaa092.
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.
根系分泌物是根分泌物过程的一部分，而根分泌物过程是植物根系释放到土壤中的有机碳（C）的主要来源。这种 C 通量被认为对生态系统中的 C 和养分循环有深远影响。根系分泌物的数量取决于植物物种、全年的时间以及外部生物和非生物因素。由于在野外条件下很难收集成熟树木的根系分泌物，因此对其通量知之甚少，特别是在水资源有限的生态系统中，例如季节性炎热干燥的地中海灌木丛。在这里，我们通过每月一次的时间分辨率在整个一年中对森林中的 DNA 鉴定的根系进行 48 小时培养，收集了来自裸子植物柏木和常绿被子植物黄连木的分泌物。我们检查了根系分泌物 C 通量与土壤（含水量、水势、温度）和大气（蒸气压亏缺、温度）的非生物参数之间的关系。我们还研究了与叶片 C 通量的关系，叶片 C 通量是树木 C 平衡的指标。根系分泌物排放率沿年度变化显著，在两个物种的雨季中，从每个根系每天 6μg C cm -2 增加到旱季中的 4 到 11 倍。逐步线性混合效应模型表明，三个土壤参数对分泌率的影响最大。在生物因素中，柏木的分泌率与叶片同化呈显著负相关，黄连木的分泌率与叶片呼吸呈显著负相关。我们观察到在旱季分泌物通量增强，这表明在野外，分泌物动力学对旱季树木 C 供应不足的敏感性较低。这两个关键的地中海森林物种似乎对根际的季节性变化做出反应，例如干燥和升温，因此在季节性干旱下投资于根际。
