How do contrasting tree size and irrigation dynamics influence the soil microbial biomass in a flood-irrigated Pecan Orchard in Tornillo, Texas?

Compared to all other terrestrial components, soil comprises the largest organic C stocks such that knowing the controlling factors and underlying processes can help to utilize its potential. Owing to the substantial spatio-temporal interplay at soil-plant-microbial interface, we have insufficient information to answer how different soil types and tree biomass interactions influence the surface layer soil microbial dynamics. Recognizing the uneven distribution of different soil fractions and temporal dynamics in moisture, soils of dryland agroecosystems are expected to have unknown trends in microbial biomass C. To understand how spatially variable tree biomass and temporal soil moisture trends affect soil microbial biomass, we performed periodic sampling runs in a flood irrigated Pecan orchard, located in Tornillo, Texas. The concerned study site has some unique spatial contrast developed by fluvial deposits on the regionally coarse soil fractions. The intriguing mosaic of contrasting soil textures has resulted in pecan trees with substantially variable tree biomass and created a spatial pattern of large and small trees adjoining the contrasting soil textures. In the present investigation, we have taken advantage of these features and chosen two pecan trees of contrasting biomass and collected four samples each on the north and south side of trees at a distance less than 5m from the tree base. The campaign involved surface soil sampling (0-10 cm) at every four-days interval for a period of 36 days and covering two irrigation events. Analysis of soil samples involved gravimetric soil moisture content, microbial biomass C and N. We found that soils associated with lower tree biomass (addressed as Pecan-fine) recorded significantly higher microbial biomass over the larger trees (addressed as Pecan-coarse). Furthermore, the regression analysis of soil microbial biomass versus gravimetric soil moisture content exhibited weak dependency of soil microbial biomass on the soil moisture variations. Overall, these results conclude that Tree size have substantial influence on the soil microbial biomass whereas the moisture dynamics have weakly affected the microbial biomass C.

相较于所有其他陆地组分，土壤拥有规模最大的有机碳库，因此明确其调控因子与内在过程，有助于挖掘土壤的应用潜力。由于土壤-植物-微生物界面存在显著的时空交互作用，目前我们仍缺乏足够信息来阐明不同土壤类型与树木生物量的交互作用如何影响表层土壤微生物动态。鉴于不同土壤组分的分布不均以及土壤水分的时间动态变化，旱地农业生态系统中的土壤微生物生物量碳（microbial biomass C）变化趋势仍未明确。为明确空间异质性的树木生物量与土壤水分时间变化趋势如何影响土壤微生物生物量，我们在位于美国德克萨斯州托尼洛（Tornillo）的一处漫灌碧根果（Pecan）果园开展了周期性采样工作。该研究区域的独特空间异质性源于河流沉积物对区域粗质地土壤组分的改造。这种质地迥异的土壤镶嵌分布格局，使得园内碧根果树的生物量存在显著差异，形成了与不同土壤质地相邻的大小树空间分布模式。本研究利用这一特征，选取了生物量差异显著的两株碧根果树，并在每株树的北侧与南侧距离树干不足5米的位置各采集4份土壤样品。本次采样周期为36天，每4天采集一次表层（0-10 cm）土壤样品，覆盖两次灌溉事件。土壤样品分析指标包括重量含水量（gravimetric soil moisture content）、微生物生物量碳（C）与微生物生物量氮（N）。研究发现，与低生物量树木相关的土壤（命名为“碧根果-细质地”组）的微生物生物量显著高于高生物量树木相关的土壤（命名为“碧根果-粗质地”组）。此外，土壤微生物生物量与重量含水量的回归分析结果表明，土壤微生物生物量对土壤水分变化的依赖性较弱。综上，本研究结果表明，树木大小对土壤微生物生物量具有显著影响，而土壤水分动态对微生物生物量碳的影响相对微弱。