U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, Oxford, Mississippi, USA.
United States Department of Agriculture-Agricultural Research Service, National Sedimentation Laboratory, Oxford, Mississippi, USA.
Glob Chang Biol. 2023 Oct;29(19):5615-5633. doi: 10.1111/gcb.16905. Epub 2023 Aug 7.
Agriculture is the most dominant land use globally and is projected to increase in the future to support a growing human population but also threatens ecosystem structure and services. Bacteria mediate numerous biogeochemical pathways within ecosystems. Therefore, identifying linkages between stressors associated with agricultural land use and responses of bacterial diversity is an important step in understanding and improving resource management. Here, we use the Mississippi Alluvial Plain (MAP) ecoregion, a highly modified agroecosystem, as a case study to better understand agriculturally associated drivers of stream bacterial diversity and assembly mechanisms. In the MAP, we found that planktonic bacterial communities were strongly influenced by salinity. Tolerant taxa increased with increasing ion concentrations, likely driving homogenous selection which accounted for ~90% of assembly processes. Sediment bacterial phylogenetic diversity increased with increasing agricultural land use and was influenced by sediment particle size, with assembly mechanisms shifting from homogenous to variable selection as differences in median particle size increased. Within individual streams, sediment heterogeneity was correlated with bacterial diversity and a subsidy-stress relationship along the particle size gradient was observed. Planktonic and sediment communities within the same stream also diverged as sediment particle size decreased. Nutrients including carbon, nitrogen, and phosphorus, which tend to be elevated in agroecosystems, were also associated with detectable shifts in bacterial community structure. Collectively, our results establish that two understudied variables, salinity and sediment texture, are the primary drivers of bacterial diversity within the studied agroecosystem, whereas nutrients are secondary drivers. Although numerous macrobiological communities respond negatively, we observed increasing bacterial diversity in response to agricultural stressors including salinization and sedimentation. Elevated taxonomic and phylogenetic bacterial diversity likely increases the probability of detecting community responses to stressors. Thus, bacteria community responses may be more reliable for establishing water quality goals within highly modified agroecosystems that have experienced shifting baselines.
农业是全球最主要的土地利用方式，预计未来将增加，以支持不断增长的人口，但也威胁着生态系统的结构和服务。细菌在生态系统中介导着许多生物地球化学途径。因此，确定与农业土地利用相关的胁迫因子与细菌多样性响应之间的联系，是理解和改善资源管理的重要步骤。在这里，我们以密西西比河冲积平原（MAP）生态区为案例研究，该生态区是一个高度改造的农业生态系统，以更好地了解与农业相关的驱动溪流细菌多样性的因素和组装机制。在 MAP 中，我们发现浮游细菌群落受盐度的强烈影响。耐受类群随着离子浓度的增加而增加，可能驱动同质选择，占组装过程的~90%。随着农业用地的增加，沉积物细菌系统发育多样性增加，并受沉积物颗粒大小的影响，随着中值颗粒大小差异的增加，组装机制从同质选择转变为可变选择。在单个溪流中，沉积物异质性与细菌多样性相关，并且在颗粒大小梯度上观察到了养分供应与压力的关系。同一溪流中的浮游生物和沉积物群落也随着沉积物颗粒大小的减小而分化。在农业生态系统中通常升高的养分（包括碳、氮和磷）也与细菌群落结构的可检测变化相关。总的来说，我们的结果表明，两个未充分研究的变量，即盐度和沉积物质地，是研究农业生态系统中细菌多样性的主要驱动因素，而养分是次要驱动因素。尽管许多宏观生物群落对农业压力做出了负面反应，但我们观察到，由于盐化和沉积作用等农业压力的增加，细菌多样性也在增加。较高的分类和系统发育细菌多样性可能会增加检测群落对压力反应的可能性。因此，在经历基线变化的高度改造的农业生态系统中，细菌群落的响应可能更可靠，可用于建立水质目标。
