Evidence that Northern Pioneering Pines with Tuberculate Mycorrhizae are Unaffected by Varying Soil Nitrogen Levels

Abstract

Tuberculate mycorrhizae on Pinus contorta (lodgepole pine) have previously been shown to reduce acetylene, but an outstanding question has been to what degree these structures could meet the nitrogen requirements of the tree. We compared the growth, tissue nitrogen contents, and stable nitrogen isotope ratios of P. contorta growing in gravel pits to the same species growing on adjacent intact soil. Trees growing in severely nitrogen deficient gravel pits had virtually identical growth rates and tissue nitrogen contents to those growing on intact soil that had nitrogen levels typical for the area. δ15N values for trees in the gravel pits were substantially lower than δ15N values for trees on intact soil, and isotope ratios in vegetation were lower than the isotope ratios of the soil. The form of soil nitrogen in the gravel pits was almost exclusively nitrate, while ammonium predominated in the intact soil. Discrimination against 15N during plant uptake of soil nitrate in the highly N-deficient soil should be weak or nonexistent. Therefore, the low δ15N in the gravel pit trees suggests that trees growing in gravel pits were using another nitrogen source in addition to the soil. Precipitation-borne nitrogen in the study area is extremely low. In conjunction with our other work, these findings strongly suggests that P. contorta and its microbial symbionts or associates fix nitrogen in sufficient amounts to sustain vigorous tree growth on the most nitrogen-deficient soils.

Nitrogen Fixation Outside and Inside Plant Tissues

“…Engelm. ), a commercially important gymnosperm species indigenous to western North America, is capable of growing in very rocky substrates and is notable for its ability to thrive on nutrient poor, N-limited soils (Weetman et al, 1988;Chapman & Paul, 2012). Based on earlier work with lodgepole pine suggesting that rhizospheric BNF contributed only small amounts of N to seedlings (Chanway & Holl, 1991) as well as reports that BNF in sugarcane was endophytic, we searched for endophytic diazotrophs in lodgepole pine as a possible explanation for the ability of this species to grow on N-deficient substrates.…”

Feast not famine: Nitrogen pools recover rapidly in 25‐yr‐old postfire lodgepole pine

“…Several studies have suggested that N can accumulate in young pines in the absence of known symbiotic N fixers (Bormann et al. , Chapman and Paul ), although sources of so‐called “occult nitrogen” (Binkley et al. ) have proven challenging to find.…”

“…, Reed et al. , Chapman and Paul ), suggesting the potential for associative N fixation in these young postfire stands.…”

“…Our hypotheses assumed minimal N inputs to these stands because species known to fix N were absent or at very low abundance in our study sites, and atmospheric deposition is low in this region. However, symbiotic and nonsymbiotic N fixation has been reported in some young pine stands (Wei and Kimmins 1998, Bormann et al 2002, Reed et al 2011, Chapman and Paul 2012, suggesting the potential for associative N fixation in these young postfire stands.…”

A Spatially Explicit, Empirical Estimate of Tree‐Based Biological Nitrogen Fixation in Forests of the United States

“…The extent and rates of BNF in these trees are not widely known, though Wurzburger () extrapolated the rates reported by Moyes et al () and found that the flux for pines was negligible. Reports also suggest BNF in Populus trichocarpa , via endophytes (Doty et al, ), and Pinus contorta , via bacteria that live within their tuberculate mycorrhizae (Chapman & Paul, ; Paul et al, ). We use the abundance of Pinus contorta and Populus trichocarpa along with published BNF rates (Paul et al, ) to examine their ecosystem level relevance (see Text S4 in the supporting information).…”