School of Engineering, Environmental Systems Program, University of California Merced, Merced, CA, USA.
Department of Biological Sciences, Florida International University, Miami, USA.
Oecologia. 2020 Mar;192(3):591-601. doi: 10.1007/s00442-020-04602-2. Epub 2020 Jan 28.
Leaf-cutter ant nests are biogeochemical hot spots where ants live and import vegetation to grow fungus. Metabolic activity and (in wet tropical forests) soil gas flux to the nest may result in high nest CO concentrations if not adequately ventilated. Wind-driven ventilation mitigates high CO concentrations in grasslands, but little is known about exchange for forest species faced with prolonged windless conditions. We studied Atta cephalotes nests located under dense canopy (leaf area index > 5) in a wet tropical rainforest in Costa Rica, where wind events are infrequent. We instrumented nests with thermocouples and flow-through CO sensing chambers. The results showed that CO concentrations exiting leaf-cutter ant nests follow a diel pattern with higher values at night. We developed an efflux model based on pressure differences that evaluated the observed CO diel pattern in terms of ventilation by (1) free convection (warm, less dense air rises out the nest more prominently at night) and (2) episodic wind-forced convection events providing occasional supplemental ventilation during daytime. Average greenhouse gas emissions were estimated through nest vents at about 78 kg COeq nest year. At the ecosystem level, leaf-cutter ant nest vents accounted for 0.2% to 1% of total rainforest soil emissions. In wet, clayey tropical soils, leaf-cutter ant nests act as free convection-driven conduits for exporting CO and other greenhouse gases produced within the nest (fungus and ant respiration, refuse decay), and by roots and soil microbes surrounding the nest. This allows A. cephalotes nests to be ventilated without reliable wind conditions.
切叶蚁蚁巢是生物地球化学热点，蚂蚁在这里生活并输入植被来培育真菌。如果通风不足，代谢活动和（在湿润的热带森林中）向蚁巢输送的土壤气体通量可能导致蚁巢中 CO 浓度升高。风驱动的通风可以减轻草原上的高 CO 浓度，但对于面临长时间无风条件的森林物种，其交换情况知之甚少。我们研究了位于哥斯达黎加湿润热带雨林中茂密树冠（叶面积指数 > 5）下的 Atta cephalotes 蚁巢，那里风事件很少发生。我们使用热电偶和气流 CO 感应室对蚁巢进行了仪器化处理。结果表明，切叶蚁蚁巢排出的 CO 浓度呈现出昼夜节律模式，夜间浓度较高。我们开发了一种基于压力差的排放模型，该模型根据（1）自由对流（较温暖、密度较低的空气在夜间更显著地从蚁巢中上升）和（2）偶发的风强迫对流事件在白天提供偶尔的补充通风，来评估观察到的 CO 昼夜节律模式。通过蚁巢通风口估算的温室气体平均排放量约为 78 kg COeq 蚁巢年。在潮湿的粘性热带土壤中，切叶蚁蚁巢充当着自由对流驱动的通道，将蚁巢内产生的 CO 和其他温室气体（真菌和蚂蚁呼吸、废物腐烂）以及蚁巢周围的根系和土壤微生物排出。这使得 A. cephalotes 蚁巢能够在没有可靠风条件的情况下进行通风。
