Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA.
Ecology. 2013 Mar;94(3):739-50. doi: 10.1890/12-0278.1.
Biological nitrogen fixation (BNF) is a key ecological process that can restore nitrogen (N) lost in wildfire and shape the pace and pattern of post-fire forest recovery. To date, there is limited information on how climate and soil fertility interact to influence different pathways of BNF in early forest succession. We studied asymbiotic (forest floor and soil) and symbiotic (the shrub Ceanothus integerrimus) BNF rates across six sites in the Klamath National Forest, California, USA. We used combined gradient and experimental phosphorus (P) fertilization studies to explore cross-site variation in BNF rates and then related these rates to abiotic and biotic variables. We estimate that our measured BNF rates 22 years after wildfire (6.1-12.1 kg N x ha(-1) x yr(-1)) are unlikely to fully replace wildfire N losses. We found that asymbiotic BNF is P limited, although this is not the case for symbiotic BNF in Ceanothus. In contrast, Ceanothus BNF is largely driven by competition from other vegetation: in high-productivity sites with high potential evapotranspiration (Et), shrub biomass is suppressed as tree biomass increases. Because shrub biomass governed cross-site variation in Ceanothus BNF, this competitive interaction led to lower BNF in sites with high productivity and Et. Overall, these results suggest that the effects of nutrients play a larger role in driving asymbiotic than symbiotic fixation across our post-fire sites. However, because symbiotic BNF is 8-90x greater than asymbiotic BNF, it is interspecific plant competition that governs overall BNF inputs in these forests.
生物固氮（BNF）是一种关键的生态过程，它可以恢复野火中损失的氮，并塑造火灾后森林恢复的速度和模式。迄今为止，关于气候和土壤肥力如何相互作用影响早期森林演替中不同 BNF 途径的信息有限。我们研究了美国加利福尼亚州克拉马斯国家森林六个地点的非共生（林下地被物和土壤）和共生（灌木 Ceanothus integerrimus）BNF 速率。我们使用组合梯度和实验磷（P）施肥研究来探索 BNF 速率的跨站点变化，然后将这些速率与非生物和生物变量相关联。我们估计，我们在野火后 22 年测量的 BNF 速率（6.1-12.1 kg N x ha(-1) x yr(-1)）不太可能完全替代野火造成的氮损失。我们发现，非共生 BNF 受到 P 的限制，尽管 Ceanothus 中的共生 BNF 并非如此。相比之下，Ceanothus 的 BNF 主要受来自其他植被的竞争驱动：在高生产力和高潜在蒸散量（Et）的地点，随着树木生物量的增加，灌木生物量受到抑制。由于灌木生物量控制了 Ceanothus BNF 的跨站点变化，这种竞争相互作用导致生产力和 Et 较高的地点的 BNF 较低。总的来说，这些结果表明，在我们的火灾后地点，养分的影响在驱动非共生固定方面比共生固定更为重要。然而，由于共生 BNF 是非共生 BNF 的 8-90 倍，因此种间植物竞争决定了这些森林中整体 BNF 的输入。
