Nitrogen Fixation in Salix and Populus (willows and poplars)
Here is the third entry in a series exploring new and orphaned knowledge of nitrogen cycling to update the view of the forest gardener – there are many articles to be written so stay tuned.
In this article, we’ll discuss the nitrogen fixing ability of Salix and Populus.
Both of these genuses are members of the Salicaceae family.
Salix contains some 400 species, mostly from cold and temperate areas of the Northern hemisphere and growing in damp soils. Willows have been used for thousands of years for medicinal purposes, weaving and as a flexible, durable timber (e.g. for making cricket bats). Willow stems can be soaked in water to make ‘willow water’ – an effective homemade hormone rooting liquid.
Populus (the poplars) are a group of 25-30 species also native to the Northern hemisphere. Included are the aspens and cottonwoods. The cottonwoods are typically found growing in wetlands and riparian areas, often on nutrient-poor gravelly or sandy soils. Poplars are often grown for timber production (most commonly used in fibre-board), and as shelterbelts plantings.
Both willows and poplars are well-known pioneer species, colonizing nutrient poor soils (such as stony and sandy riparians areas with practically no organic material for obtaining fixed nitrogen) while still maintaining rapid growth rates. They are both excellent choices for stream and slope stabilization, nurse planting, absorbing agricultural runoff, for animal fodder (being of slightly higher digestibility than grass) and biomass energy due to their fast growth and energy density in temperate regions.
They are excellently behaved species in short-rotation coppice (SRC). In UK conditions, willows can supply approximately 9 dry tons of biomass per hectare and poplars approximately 6.3 dry tons of biomass per hectare (the poplars seemed more susceptible to rust infections). Precipitation was positively correlated with increases in yields. Up to 30 dry tons of biomass per hectare were reported in Washington state on better alluvial soils. [1][2]
ASIDE – Salicylic acid the plant hormone and willow mulch
Willows contain a large amount of salicylic acid (varying per species) – which functions as a pain reliever in humans as well as a plant hormone involved in triggering flowering of some plants and disease resistance. Salicylic acid signals to plants to ramp up their ‘immune systems’ – which includes things such as release of antifungal, insecticidal and antiviral compounds and cell-wall thickening. Trials from the Bartlett Tree Research and Diagnostic Laboratory based at the University of Reading have demonstrated white willow (Salix alba) mulch applied to apple scab sensitive trees resulted in significantly less apple scab at the end of the growing season (important to note they did not perform a control with other mulch).
Salicylic acid applied foliarly has also been shown to reduce Phytophthora symptoms in rubber trees (Hevea brasiliensis) by 40%, and to confer strong resistance to Glomerella leaf spot fungus in a susceptible apple variety. Research has hardly started in this area – so we can hopefully expect a lot of new exciting things from this ‘immune-boosting’ approach. [3][4][5][6][7]
Research summary
Nitrogen fixation was first detected in the wetwood (perpetually water-saturated wood, usually heartwood) of poplars in the 1980s using acetylene reduction assays, but it was not explored further at the time. [8][9]
Fertilizer response studies have also long noted that poplars often don’t perform much better with supplemental nitrogen. A 1993 study by Heilman and Xie found that 500 kg N/ha/y applications over 4 years only resulted in 21% better growth versus non-nitrogen applied crops. They also noted the top 3 unfertilized poplar clones produced more biomass than the worst-performing fertilized clones.
Many other studies have also found little effect from nitrogen addition to poplar and willow growth rates. [10][11]
In 2005 – Doty et al isolated nitrogen fixing bacteria from within both hybrid cottonwood (Populus trichocarpa × P. deltoides) and Populus deltoides plant stem tissues, after these tissues were strongly surface sterilized. These bacteria (identified as a variant of Rhizobia tropici) were found reliably, from cottonwoods from different sources, but not from other known nitrogen fixing plant species (such as Robinia or Leucaena). This bacteria was found to grow rapidly. [12]
An interesting study from 2010 tried to increase long-rotation poplar growth rates using nitrogen-fixing sea buckthorn (Hippophae rhamnoides). After 5 years, there were no significant differences between the two plots. However after 15 years, the mixed plots contained 40% more aboveground biomass and 100% more litter nitrogen (115kg N/ha vs 54.6kg N/ha). [13]
A 2011 review from the German Institute for Forest Genetics summarizes the case for nitrogen fixation in poplars and willows to date, and concludes: “Thus far, it is unknown if diazotrophic bacteria are present in all Salicaceae species. It can be expected that fast growing poplar and willow species adapted to riparian habitats with sandy soils poor in N availability are able to fix N.” [11]
A study from 2014 performed a few experiments to determine the extent that endophytes supplied Populus with nitrogen. The first experiment inoculated a sterile Populus clone (Nisqually-1) with groupings of endophytes isolated from wild Washington-state Salix and Populus species, and found that these inoculated cuttings grew more than twice as much as uninoculated controls after 31 days.
A second experiment tested using nitrogen dilution where the nitrogen was being fixed, and found “we infer that mineral N from the soil remains in the roots while nitrogen allocated to the leaves and stem is derived largely from BNF [biological nitrogen fixation] and suggests potential signaling mechanisms for optimized nitrogen allocation.”
They summarized this experiment with “Biological nitrogen fixation was estimated through 15N isotope dilution to be 65% nitrogen derived from air”
A third long term field experiment observed over 12 months showed approximately 40% increase in aboveground mass for the multi-species inoculated plants over the control plants – there was also no measured decrease in soil nitrogen over this period. [14]
A 2016 study from Doty et al attempted to quantify the amount of nitrogen fixed in stands of wild Populus trichocarpa.
They grew rooted cuttings on nitrogen-free hydroponic media and in a longer term experiment – nitrogen-free agar, and used a 15N dilution technique to estimate nitrogen fixation rates quantitatively: The poplar plants exposed to 15N2 for two weeks had a rate of N2-fixation of up to an average of 20.6 mg N/kg/day, if it is assumed that N2-fixation occurred uniformly throughout the 2-week period. The authors found highly variable amounts of nitrogen fixation (from 0.75 to 20.6 mg N/kg/day) and cautioned: Given the variability of N2-fixation within the wild poplar plants, it is not possible to accurately extrapolate from the data to whole-tree estimates of N acquired through biological N2-fixation by the diazotrophic endophytes, but general estimates may be calculated. [15]
I emailed asking about estimates but did not receive a reply.
Conclusions
Based on short-rotation coppice trials for willow performed in New Zealand, there is approximately 15t-27t of annual [wet] biomass production. Using a range of 10 – 20 mg N/kg/day fixing rate (average of measured fixing rates from 2016 study) and if we assume a uniform growing season of 180 days – then this is approximately 27 – 98 kg N/ha/y being fixed.
The 2014 study found that approximately 65% of the poplar’s nitrogen was fixed – assuming 6.3t of annual [dry] biomass (figures from UK), and a nitrogen percentage of 0.66% [of dry-weight] (estimated figures from 2014 study), then we also arrive at a figure of 27 kg N/ha/y.
These amounts are also likely to be underestimates as the root mass is not included in the annual biomass production.
These amounts are comparable to that of the rates estimated for pines. [16]
The study with mixed poplar and sea buckthorn gave very interesting results [that the sea buckthorn only increased the growth rate of the poplar well after 5 years]. I would like to speculate that there would be much smaller differences if the poplar was grown in short-rotation, as studies seem to indicate that it’s the stems and leaves contain the nitrogen fixing endophytes. Growing in long-rotation means the percentage of aboveground biomass formed of stems and leaves decreases every year, as more mass accumulates in woody tissue and there is not a corresponding increase in soft biomass.
How does this knowledge change the usage of poplars and willows in agroforestry systems?
Many of the studies found a low initial level of nitrogen fixation (possibly indicating a biological cost to establishing the symbiosis) in the weeks to months period. The sea buckthorn study indicated that poplar did not really benefit from actinorhizal nitrogen fixing (which are often obligate) until well after the 5th year. From these two ranges we can estimate that poplars and willows are mostly facultative nitrogen fixers, and can fix a significant amount of nitrogen for the first few years (enough to keep up with actinorhizal nitrogen fixers)- probably on the order of 50 kg N/ha/y. I hypothesize that this amount of nitrogen fixing can be extended through coppicing strategies – but this remains to be trialled.
Poplars and willows already have many uses in agroforestry systems (covered above), but should now be additionally considered as weak nitrogen fixers. I also briefly mentioned the potential usage of willow mulch as a plant ‘immune booster’ – I think it’s definitely worth trialling a willow coppice as a way to manage susceptible plants in a sustainable, eco-friendly way e.g. Phytophthora in avocados.
References
[1] Aylott, M. J., Casella, E. , Tubby, I. , Street, N. R., Smith, P. and Taylor, G. (2008), Yield and spatial supply of bioenergy poplar and willow short‐rotation coppice in the UK. New Phytologist, 178: 358-370. doi:10.1111/j.1469-8137.2008.02396.x
[2] New Zealand Poplar & Willow Research Trust. (n.d.). Why Plant Poplars and Willows on Farms. Retrieved April 8, 2020, from https://www.poplarandwillow.org.nz/documents/fact-sheet-1-why-plant-poplars-and-willows-on-farms.pdf
[3] Raskin I. (1992). Salicylate, a new plant hormone. Plant physiology, 99(3), 799–803. https://doi.org/10.1104/pp.99.3.799
[4] Orchard People. (2018, August 28). Episode 36: Willow Mulch for Fruit Trees [Audio podcast]. https://orchardpeople.com/willow-mulch-for-fruit-trees/.
[5] Percival, G, Bartlett Tree Research Laboratory. What’s New in Plant Protection. https://www.ltoa.org.uk/docs/LTOA-Mulches-Glynn_Percival.pdf
[6] Deenamo, N., Kuyyogsuy, A., Khompatara, K., Chanwun, T., Ekchaweng, K., & Churngchow, N. (2018). Salicylic Acid Induces Resistance in Rubber Tree against Phytophthora palmivora. International journal of molecular sciences, 19(7), 1883. https://doi.org/10.3390/ijms19071883
[7] Zhang, Y., Shi, X., Li, B., Zhang, Q., Liang, W., & Wang, C. (2016). Salicylic acid confers enhanced resistance to Glomerella leaf spot in apple. Plant Physiology and Biochemistry, 106, 64-72.
[8] Schink, B., Ward, J. C., & Zeikus, J. G. (1981). Microbiology of wetwood: role of anaerobic bacterial populations in living trees. Microbiology, 123(2), 313-322.
[9] Kamp, B. V. D. (1986). Nitrogen fixation in cottonwood wetwood. Canadian Journal of Forest Research, 16(5), 1118-1120.
[10] Heilman, p.e.; Xie, F. 1993. Influence of nitrogen on growth and productivity of short-rotation Populus trichocarpa x Populus deltoids hybrids. Canadian Journal of Forest Research. 23: 1863–1869.
[11] Wuehlisch, G. 2011. Evidence for Nitrogen-Fixation in the Salicaceae Family. Tree planters notes 54(2): 38-41
[12] Doty SL, Dosher MR, Singleton GL, Moore AL, Van Aken B, Stettler RF, Strand SE, Gordon MP. 2005. Identification of an endophytic Rhizobium in stems of Populus. Symbiosis 39: 27–35.
[13] Mao, R.; Zeng, D.H.; ai, g.Y.; Yang, D.; Li, L.J.; Liu, Y.X. 2010. Soil microbiological and chemical effects of a nitrogen-fixing shrub in poplar plantations in semi-arid region of Northeast China. European Journal of Soil Biology. 46: 325–329.
[14] Knoth JL, Kim SH, Ettl GJ, Doty SL (2014) Biological nitrogen fixation and biomass accumulation within poplar clones as a result of inoculations with diazotrophic endophyte consortia. New Phytol 201: 599–609. pmid:24117518
[15] Doty SL, Sher AW, Fleck ND, Khorasani M, Bumgarner RE, et al. (2016) Variable Nitrogen Fixation in Wild Populus. PLOS ONE 11(5): e0155979. https://doi.org/10.1371/journal.pone.0155979
[16] McIvor, I. (2007, November). Short rotation coppice willow as low carbon bioenergy farming. New Zealand Tree Grower. https://www.nzffa.org.nz/farm-forestry-model/resource-centre/tree-grower-articles/november-2007/short-rotation-coppice-willow-as-low-carbon-bioenergy-farming/