Variation in nitrogen fixation and foliar microbial communities driven by conifer host species and site fertility

Mature temperate and boreal forest productivity is frequently nitrogen (N) limited. Sources of N to balance losses are not well understood, but include symbiotic N₂ fixation in mosses and understory species, sedimentary bedrock N, and deposition of N pollution near areas of intensive land use. Further, some tree species may have N-fixing symbionts allowing them to establish and persist in low N sites. Recently, we showed that foliar N₂-fixing endophytic bacteria have the potential to provide treeline limber pine trees with a small, but direct source of atmospheric N. To determine whether other pine species host N₂-fixing endophytes and how fixation rates and bacterial communities vary with soil fertility, we used acetylene reduction to measure foliar N₂ fixation in bishop pine (Pinus muricata) and lodgepole pine (Pinus contorta ssp. bolanderi and ssp. contorta) along a soil age and fertility gradient in Mendocino, CA. We also experimentally exposed pine seedlings to ¹⁵N₂ to quantify N₂ fixation rates across tissues.

Over multiple sampling dates, we confirmed endophytic N₂ fixation in both conifer species. As expected, N₂ fixation rates were higher in the youngest site with high available soil phosphorus (P), suggesting that P availability may constrain atmospheric N inputs to these pines. Contrary to our expectation of no difference between pine species, we found significantly higher N₂ fixation rates in bishop pine, but only in mid-wet season. Our ¹⁵N₂ experiment confirmed N₂ fixation in both species, but showed no difference between them as seedlings. ¹⁵N accumulation was greater in roots than foliage and in young vs. old needles, suggesting that growing roots and leaves are sinks for fixed N, or that microbes in these tissues are more active N₂ fixers. As expected, the foliar microbial community was dominated by Acetobacteraceae, however this group was negatively correlated with N₂ fixation in lodgepole pine at the mid-fertility site, implying other taxa may be fixing N. While additional work is required to pinpoint N₂-fixing taxa in these trees, small but consistent and direct provision of atmospheric N to Pinus species by foliar- and root-associated bacteria appears to be an important aspect of the Pinus N economy, particularly in sites with ample P but low soil N.