Associative N₂ fixation (ANF) is widespread but poorly characterized, limiting our ability to estimate global inputs from N₂ fixation. In some places, ANF rates are at or below detection most of the time but occasionally and unpredictably spiking to very high rates. Here we test the hypothesis that plant phenology and rainfall events stimulate ANF episodes. We measured ANF in intact soil cores in switchgrass (Panicum virgatum L.) in Michigan, USA. We used rain exclusion shelters to impose three rainfall treatments with each receiving 60 mm of water over a 20-day period but at different frequencies. We concurrently established a treatment that received ambient rainfall, and all four treatments were replicated four times. To assess the effects of plant phenology, we measured ANF at key phenological stages in the ambient treatment. To assess the effects of rainfall, we measured ANF immediately before and immediately after each wetting event in each treatment involving rainfall manipulation. We found that the previous day’s rainfall could explain 29% of the variation in ANF rates within the ambient treatment alone, and that bulk soil C:N ratio was also positively correlated with ANF, explaining 18% of the variation alone. Wetting events increased ANF and the magnitude of response to wetting increased with the amount of water added and decreased with the amount of inorganic N added in water. ANF episodes thus appear to be driven primarily by wetting events. Wetting events likely increase C availability, promote microbial growth, and make rhizosphere conditions conducive to ANF.

结合性 N₂ 固定 （ANF） 广泛存在但特征不佳，限制了我们估计 N₂ 固定的全局输入的能力。在一些地方，ANF 发病率大部分时间处于或低于检测率，但偶尔会不可预测地飙升至非常高的比率。在这里，我们检验了植物物候和降雨事件刺激 ANF 发作的假设。我们测量了美国密歇根州柳枝稷 （Panicum virgatum L.） 完整土壤核心中的 ANF。我们使用防雨避雨棚实施了 3 次降雨处理，每次在 20 天内接收 60 毫米的水，但频率不同。我们同时建立了一个接受环境降雨的处理，所有四个处理都重复了四次。为了评估植物物候的影响，我们在环境处理的关键物候阶段测量了 ANF。为了评估降雨的影响，我们在涉及降雨操纵的每次处理中，在每次润湿事件之前和之后立即测量了 ANF。我们发现，前一天的降雨量可以单独解释环境处理中 29% 的 ANF 速率变化，并且大块土壤 C：N 比也与 ANF 呈正相关，仅解释了 18% 的变化。润湿事件增加了 ANF，对润湿的反应强度随着水中添加量的增加而增加，而对润湿的反应强度随着水中无机氮的添加量而减少。因此，ANF 发作似乎主要由湿润事件驱动。润湿事件可能会增加 C 的可用性，促进微生物生长，并使根际条件有利于 ANF。