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·ha⁻¹·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 8–90× greater than asymbiotic BNF it is interspecific plant competition that governs overall BNF inputs in these forests.

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【生态系统C循环对变暖的响应】Meng Lu Xuhui Zhou Qiang Yang Hui Li Yiqi Luo Changming Fang Jiakuan Chen Xin Yang and Bo Li. Responses of ecosystem carbon cycle to experimental warming: a meta-analysis. Ecology 2013 94:726–738. http://dx.doi.org-10.1890-12-0279.1