Short-term precipitation exclusion alters microbial responses to soil moisture in a wet tropical forest

Many wet tropical forests, which contain a quarter of global terrestrial biomass carbon stocks, will experience changes in precipitation regime over the next century. Soil microbial responses to altered rainfall are likely to be an important feedback on ecosystem carbon cycling, but the ecological mechanisms underpinning these responses are poorly understood. We examined how reduced rainfall affect- ed soil microbial abundance, activity, and community compo- sition using a 6-month precipitation exclusion experiment at La Selva Biological Station, Costa Rica. Thereafter, we ad- dressed the persistent effects of field moisture treatments by exposing soils to a controlled soil moisture gradient in the lab for 4 weeks. In the field, compositional and functional re- sponses to reduced rainfall were dependent on initial condi- tions, consistent with a large degree of spatial heterogeneity in tropical forests. However, the precipitation manipulation sig- nificantly altered microbial functional responses to soil mois- ture. Communities with prior drought exposure exhibited higher respiration rates per unit microbial biomass under all conditions and respired significantly more CO2 than control soils at low soil moisture. These functional patterns suggest that changes in microbial physiology may drive positive feed- backs to rising atmospheric CO2 concentrations if wet tropical forests experience longer or more intense dry seasons in the future.

占全球陆地生物量碳库（terrestrial biomass carbon stocks）四分之一的湿润热带森林，将在未来百年间经历降水格局（precipitation regime）的改变。土壤微生物对降雨变化的响应，可能是生态系统碳循环的重要反馈，但支撑这类响应的生态学机制仍未得到充分解析。我们依托哥斯达黎加拉塞尔瓦生物站（La Selva Biological Station）开展了为期6个月的减雨试验（precipitation exclusion experiment），以此探究降雨减少对土壤微生物丰度（soil microbial abundance）、活性及群落组成（community composition）的影响。此后，我们将经野外水分调控处理的土壤置于实验室的受控土壤水分梯度（soil moisture gradient）环境中培养4周，以此探究野外水分处理的持续效应。野外实验结果显示，群落组成与功能对降雨减少的响应依赖于初始环境条件，这与热带森林中广泛存在的空间异质性特征相符。不过，降水调控试验显著改变了微生物群落对土壤水分的功能响应模式。经历过野外干旱预处理的微生物群落，在所有实验条件下的单位微生物生物量呼吸速率均更高；在低土壤水分条件下，其释放的CO₂量也显著高于对照土壤。上述功能特征表明，若未来湿润热带森林遭遇更长或更强的旱季，微生物生理特性的改变可能会对大气CO₂浓度升高产生正反馈效应。