SMR01 Konza Prairie grassland soil microbial responses to long-term management of N availability

Anthropogenic actions have significantly increased biological nitrogen (N) availability on a global scale. In tallgrass prairies, this phenomenon is exacerbated by land management changes, such as fire suppression. Historically, tallgrass prairie fire removed N through volatilization, but fire suppression has contributed to increased soil N availability as well as woody encroachment. Because soil microbes respond to N availability and plant growth, these changes may alter microbial composition and important microbially-mediated functions. Grassland management affects the soil environment on multiple time scales including short (fertilization or fire event), seasonal (growing vs. non-growing season), and long-term (decadal plant turnover and nutrient accumulation), therefore my goal was to understand community variability at different time scales affecting the population and community dynamics of soil microbes. I predicted soil microbes would be sensitive to environmental changes at all time scales, seasonal variation would reflect increased plant rhizodeposit-supported populations during summer and decomposers during winter, and long-term fire suppression and chronic fertilization would drive soil microbial community turnover associated with accumulation of plant litter and N. Soil microbial responses to short-term fire/fertilization events were minimal, while microbial population sizes fluctuate seasonally and synchronously, and microbial community composition varied more with management history than at shorter time scales. Bacterial populations increased 10x during growing-season plant rhizodeposition, while fungal populations were less dynamic, but decreased in fall, possibly reflecting a shift to subsistence on soil organic matter. In contrast, microbial community composition was seasonally stable, but distinct between long-term management treatments, which may indicate accumulation of niche-defining plant or soil properties over decades. Prokaryotic communities responded to altered N availability via both fertilization and loss due to fire, with the highest abundance of "copiotrophic" (r-selected) taxa in unburned, fertilized soils. Fungal communities responded to N fertilization with higher abundance of arbuscular mycorrhizal fungi, pathogens, and saprotrophs, possibly due to changes in nutrient stoichiometry and litter availability in fertilized plots. However, fungal response to fire was largely independent of N availability, and plant community differences were correlated with fungal, but not bacterial, community composition, highlighting the likely nutritional codependence of fungi and plants, and fungal competitive advantages for plant litter substrates. The timing of changes in soil microbial communities is critical for plant nutrition and nutrient cycling in prairies, and this novel dataset on the temporal resolution of microbial responses to environmental variability contributes to the broader understanding of ecosystem responses to global change.人为活动已在全球范围内显著提升了生物有效氮（N）的可获得性。在高草草原生态系统中，火抑制等土地管理方式的改变进一步加剧了这一现象。历史上，高草草原的野火通过挥发作用移除土壤氮素，但火抑制措施不仅提升了土壤有效氮含量，还促进了木本植物的入侵。由于土壤微生物对氮素可获得性与植物生长存在响应，上述变化可能会改变微生物群落组成及其重要的微生物介导生态功能。草原管理会在多个时间尺度上影响土壤环境：包括短期（施肥或野火事件）、季节尺度（生长季与非生长季）以及长期（数十年的植物周转与养分累积）。因此本研究旨在明确不同时间尺度下的群落变异如何调控土壤微生物的种群与群落动态。本研究假设：土壤微生物对所有时间尺度下的环境变化均存在敏感性；季节变异会体现为夏季植物根际沉积支撑的微生物种群增多，冬季则以分解者种群为主；长期火抑制与长期施肥会驱动与植物凋落物及氮素累积相关的土壤微生物群落更替。土壤微生物对短期野火/施肥事件的响应相对微弱，但微生物种群规模会呈现季节性同步波动；且微生物群落组成的差异更多受管理历史影响，而非短期尺度的环境变化。在生长季植物根际沉积期间，细菌种群数量增长10倍；而真菌种群的动态变化相对平缓，但在秋季会出现数量下降，这可能反映了真菌转向以土壤有机质为营养来源的生存策略。与之相对，微生物群落组成在季节尺度上保持稳定，但在不同长期管理处理间存在显著差异，这可能表明数十年间植物或土壤的生境定义属性发生了累积性变化。原核生物群落对施肥导致的氮素提升以及野火导致的氮素流失均存在响应：在未受野火干扰且施加了肥料的土壤中，“富营养型（r-选择型）”类群的丰度最高。真菌群落对氮素施肥的响应体现为丛枝菌根真菌、病原菌与腐生生物的丰度提升，这可能与施肥样地内养分化学计量比及凋落物可获得性的变化有关。但真菌对野火的响应基本不受氮素可获得性的影响；植物群落的差异与真菌群落组成显著相关，却与细菌群落组成无关，这凸显了真菌与植物间可能存在的营养共生关系，以及真菌在植物凋落物底物利用上的竞争优势。土壤微生物群落变化的时间节律对草原植物营养获取与养分循环至关重要；本研究针对微生物对环境变异的时间响应分辨率所构建的新型数据集，有助于更全面地理解生态系统对全球变化的响应机制。