Last year, my little guy - then 10 - and I went out and did a species survey on our lawn - not identifying all of them, since we are hardly competent to do that - and counted more than 100 kinds different plant organisms, including a large number of seedling trees that may have resulted from our hiking and planting activities. We, um, don't do weed killers generally at my house, since we don't believe in the existence of "weeds" except when they appear in the driveway or if the plant happens to be carbon dioxide loving poison ivy. (We kill poison ivy usually by burying it under grass clippings.)
And, again, we're always putting tree seeds of various types in the ground - somewhat haphazardly - to see if we can get new trees. (For example, I stick all of my fruit pits in the ground. It would appear that many of them sprout as well. I can recognize the cherry trees, but the rest are kind of obscure. I may live to find out what they are; you never know.)
This year, with very little rain all summer, and lots of oppressively hot days, the grass is all brown, and certainly the species count is down - not that we've checked - and regrettably one species that seems to have suffered the least is a variety of grass that we know is an introduced species and is rapidly becoming a problem in this area.
Anyway. Trees inspire my wonder, and although I have no particular expertise in the area, droughts, infestations, and other threats to trees have always concern me and leave me asking myself how trees work. During this drought, I've been asking myself why particular branches die and others don't, and what chemical signals are involved in the transitions in leaves still in August to yellow, redden or simply brown. Are the early browning trees dying, or is it merely a kind of defense involving an early autumn. Will all of the browned trees, or at least some come back? If they remain alive, what mechanisms are involved?
These questions intrigue me.
Thus, once in a while, while exhausted - and I've been very tired and very fed up lately - I'll wander over to a nice scientific journal I like to browse, Tree Physiology. Needing some distraction, and with these questions in mind, I decided to check out the titles in this month's issue. As it happens, by coincidence, this month's issue is relevant to the very questions that have been troubling me. So I downloaded a number of relevant articles.
Interestingly, this month's issue touches on an aspect of tree physiology about which I had not thought much: The capacity of trees to store nitrogen. I am always interested in nitrogen flows and once, in this space, spent a lot of time musing - somewhat unsuccessfully - about industrial nitrogen fixation and its relationship to the existing biosphere.
I'm not big on fertilizing my property - if you must know - because I am concerned about nitrogen and phosphorous run-off and its effect on surface waters. As far as I can tell, my trees do not suffer much as a result of my concern, but it is possible that that one reason is that my neighbors - like most suburbanites - are big on fertilizers, and I get enough wind blown and water based runoff on my property to fertilize my grounds for free, which is good because, among other things, I'm a cheapskate as well as an enviromentalist.
I have also always assumed that smog based nitrogen, as well as industrial nitrogen is present in rainwater, even if phosphorous isn't.
The paper I will be discussing is this one: Tree Physiology 30, 1083–1095. The title of the paper is "Nitrogen storage and remobilization by trees: ecophysiological relevance in a changing world." The article is an invited review for a special journal issue devoted to tree nutrition. The authors are Peter Millard of the Macaulay Land Use Research Institute, in Aberdeen, Scotland, and Gwen-Aelle Grelet of the University of Aberdeen.
The authors begin with an argument that although many people assume that the carbon budget is the most important issue in understanding how trees grow, it may be that nitrogen budgets may be not merely as important but are actually more important.
Quoth the authors:
Resource use by trees has often been considered using carbon (C) as a basic currency. Because C3 photosynthesis is not CO2 saturated at current atmospheric concentrations and C constitutes about half the dry mass of plants, physiologists have assumed that plant functioning can be considered in terms of the C ‘cost’. The underlying assumption to this approach is that the ability of trees to assimilate and allocate C ultimately regulates their use of other resources and their growth. This C-centric view of tree physiology has tended to dominate our thinking of how trees will respond to aspects of global environmental change, such as rising atmospheric CO2 levels (as discussed by Körner 2006), temperature (e.g., Adams et al. 2009) or drought (e.g., McDowell et al. 2008). However, this approach assumes that tree growth and functioning are limited by the availability of C. Is this assumption correct?
(If you are interested, by the way, I happen to know as a result of something I was doing professionally, though periphally, about McDowell's very interesting work on the implication of drought on trees. This work is being conducted at Los Alamos National Laboratory and, no, it does not
involve nuclear energy. McDowell's work involves the carbon storage by nonstructural carbohydrates, including interestingly, not just non-structural sugars, but also lipids - a point to which the current paper refers. Although people do not often consider lipids in trees, trees can be an important source of dietary fats, particularly in nuts. Walnuts for instance are an important source of the "omega 3" fatty acid eicosopentenoic acid, which - albeit having nothing to do with walnut eating - is thought to account for the low rate of heart disease among Inuit people. Much of the western world's dietary
fat - not necessarily a good
fat like eicosopentenoic acid - comes from palm trees grown in monoculture plantations grown on rototilled rain forest.)
I was talking about nitrogen and trees.
The authors write further as follows:
Tree growth can use N and C which are derived from several
possible sources (Figure 1), broadly categorized as external or internal resources. External N can come from mineralization of soil organic matter (or fertilizers), microbial fixation of atmospheric N2, organic N transferred from mycorrhizal symbionts to tree roots or, in some ecosystems, atmospheric N deposition. External C comes primarily from assimilation of atmospheric CO2 through photosynthesis. Internal resources are derived from storage through the physiological processes of remobilization and recycling (Figure 1). If a tree is replete with a particular resource, there can also be sequestration, which represents a metabolic dead-end, thereby precluding further use. This review first considers the differences in the physiological processes regulating N and C storage by trees and then discusses the ecological significance of N storage, along with attempts to quantify the contribution N remobilization makes to annual nutrient demand by the tree.
Most people who are concerned with climate change - even those who give lip service to such concern but are otherwise more concerned with eliminating the world's largest, by far, source of climate change gas free primary energy - talk about carbon dioxide. Better educated persons who reflect on climate change gases, may also discuss methane, the chief component of dangerous natural gas. Methane is the second largest contributor to climate change, to be sure. Some atmospheric methane comes from the industrial use of dangerous natural gas, and some from agricultural practices, and other anthropogenic sources such as sewage treatment. However the third largest climate change gas - it also deleteriously effects the ozone layer in a major way - contains no carbon whatsoever. It's nitrous oxide, ironically also known as "laughing gas." Nitrous oxide accumulations in earth's atmosphere are, however, no laughing matter. Even if we were to manage carbon dioxide and methane, it is very possible that nitrous oxide would continue to increase to ever more dangerous levels.
We hear a lot of trash talk - and trust me, it is trash - about "carbon sequestration" but we almost say nothing about "nitrogen sequestration." One reason that this is the case is that most people realize that most of earth's atmosphere is mostly nitrogen, and thus naturally think of said air as the ultimate sink for nitrogen. To some extent, this is, in fact, true. Many nitrogen oxides do ultimately decompose to give nitrogen gas in the atmosphere. In fact, for billions of years this process resulted in a fairly constant equilibrium. Another reason is that people are simply unaware of the environmental implications of nitrogen chemistry. It just doesn't get publicized, and its not the sort of thing that people can pretend will go away by posting cute litte meaningless pictures of solar cells and wind turbines on the Internet, because if the fixed nitrogen goes away, food goes away. Almost all fertilizers end up their lives as atmospheric nitrogen oxides.
Certain industrial catalysts like platinum - often alloyed with palladium ruthenium and rhodium - also work to restore the equilibrium between nitrogen oxides and nitrogen and oxygen gas by decomposing nitrogen monoxide and nitrogen dioxide to their constituent elements. The most common example of such a platinum/ruthenium/rhodium/palladium catalyst is the automotive catalytic converter. Generally I have nothing good to say about cars or the car CULTure, but one silver lining on the car CULTure tragedy is that catalytic converters, albeit via diffusion, do work to eliminate some nitrogen oxides resulting from agriculture even when the car isn't running.
Of course cars also form nitrogen oxides while running, and in fact, the brown gas in automotive smog - one can easily see this most days on the beaches around Los Angeles - is nitrogen dioxide. However the effects of the aforementioned catalysts has largely worked to mitigate - if not eliminate - this gas from automotive sources, assuming that the converters don't fail. If a converter does fail, this will most often be detected in many states - including California and my home state of New Jersey - within one year during a required inspection of the car's emission system although the car will emit nitrogen oxides until the failure is discovered.
Despite the modicum of success realized by automotive catalytic converters, they only treat a two of the world's three most problematic nitrogen oxides. There is no common catalyst that destroys the third, nitrous oxide, N2O. (A recent publication, Catalysis Communications 10 (2008) 132–136, reports a cesium doped cobalt oxide catalyst that can catalyze this reaction in the presence of steam and oxygen, but this is obviously energy intensive and would not work on atmospheric N2O.) Compared to nitrogen oxide, NO, and nitric oxide, NO2, nitrous oxide is fairly inert and does not decompose quickly into nitrogen and oxygen with or without a catalyst. (It will however, support combustion at high temperatures - it is a mild oxidant - and fires can be a nitrous oxide sink under some circumstances.) Nitrous oxide's main sink - and it is part of the largely biological nitrogen cycle- isn't even biological, although the main source is biological, that is, bacterial. Because denitrifying bacteria produce N2O (as well as nitrogen gas) from nitrates and nitrites formed from ammonia and nitrate based fertilizers, and increase in the use of these fertilizers - which has been on going for about a century - leads to an increase in N2O levels. However no organism is known to destroy N2O. The main sink for nitrous oxide is radiation, space radiation, particularly from our sun.
This may sound like a winner - everybody gets all weepy eyed over solar stuff in an almost primitivist way way beyond the point of reason - and it is, in fact, a winner. Without the destruction of N2O by the solar radiation flux, biological activity would have overheated the earth long ago, but even so it's not quite as simple as it seems. First of all, the change in the rate of production of N2O by anthropogenic forces is ever shifting the equilibrium position higher. Secondly, as mentioned previously, the destruction of N2O in the atmosphere works my a mechanism that is not unlike that in the destruction of CFC's - the process catalyzes the destruction of the ozone layer. Here is a pop article on the subject: Nitrous Oxide Fingered As Monster Ozone Slayer.
In the stratosphere, nitrous oxide, during its decomposition, works to produce free radicals that work very much like chlorine radicals from CFC's. In fact, since the banning of CFC's under the Montreal Protocol - the first truly international environmental success story - N2O has become the predominant cause of ozone depletion and is expected to remain so for the entire 21st century. Eventually it will become worse than the situation obtained via CFC's. For the record, melanoma rates are still rising and that's not the least of it.
Isn't that special?
Anyway. The point of this amusing little N2O diversion was to make it clear that in fact, "nitrogen sequestration" - the sequestration of fixed nitrogen, is important and the role that our stressed trees play in this affair needs examination, since the industrial Haber process - used to make everything from fertilizer to rocket fuel - is not going to be abandoned anytime soon.
So it matters whether trees sequester nitrogen, I guess. But do they?
The Tree Physiology paper offers a "definitions" section in which a distinction is made between sequestration and storage. Nitrogen is sequestered, according to this definition, if it cannot be remobilized for use in another plant tissue from the tissue in which it is originally used. An example is given. The amino acid arginine is the most nitrogen rich of all the 20 coded proteinaceous amino acids owing to a guandinyl functionality on its side chain. This functionality contains three nitrogen atoms. Because the "amino acid" portion adds one more nitrogen, it requires four nitrogen atoms to make a single arginine molecule. Of the other 19 coded amino acids, only four, histidine, asparagine, glutamine, and lysine contain more than one nitrogen: Histidine, with an imidazolium ring, has three nitrogens. Each of the remaining has two. As it happens, certain coniferous trees - in response to nitrogen excesses - collect arginine in their needles in such a way that the amino acid is never released. They sequester nitrogen. Ordinary deciduous leaves however collect nitrogen, but when they fall off in autumn, this nitrogen collects on the soil and is made available for reabsorption of the nitrogen. These leaves store nitrogen but do not sequester it.
With the exception of conifers, most deciduous trees do not sequester nitrogen. Most nitrogen remobilization and reuptake actually takes place in the autumn, when the trees appear to be in hibernation but are actually quite alive with activity.
This has an interesting consequence, now that I think of it, and offers one distinction between suburban trees and forest trees. One of my many eccentricities in contrast to my neighbors is that I do not pile my leaves on the side of the road to be hauled away by the town. While I do rake them up. I usually collect them under one of the tree sections of my property where I let them rot. Thus I do not seem to need fertilizers for my trees. This also explains why forests do not seem to need nitrogen fertilizers to survive.
My neighbors who do let the town haul away the leaves are in quite a different position. The hauling of leaves is hauling away nitrogen, an important nutrient, thus requiring synthetic fertilizers, irrespective of all the trouble said fertilizers create.
I always suspected as much.
All this said, the growth of large trees obviously ties up more nitrogen than does, say, grass. The nitrogen may be remobilized, but it is doing something. Large trees also sequester and store carbon, the former via structural carbonhydrates, and the latter in non-structural carbohydrates. The structural portions of trees actually contain very little nitrogen.
My trees I suspect get some of their nitrogen from the rain around here - if we have rain - because air pollution results in nitrates, usually present as nitric acid, being present in it.
But again, we haven't had much rain here. I was, in fact, disappointed that we didn't get anything at all from Hurricane Earl which just passed off our coast, not a drop. No rain is forecast for weeks in my area.
Although we have had several rainy years recently, I suspect that part of this situation derives from climate change, at least the part involving the extreme heat that did so much damage to forests around here as well as suburban trees. Bayesian mathematics is notoriously difficult and sort of wiggly as mathematics goes, but the weather is noticably extreme of late here.
If one checks the drought monitor as of this writing, one can see that about one third of the United States is abnormally dry, D1. My area is D2, "moderate drought" although from the state of trees around here, it doesn't seem all that moderate to me. I check this thing frequently - at least once a week - and my qualitative perception is that there have been much worse recent years than this one for the country as a whole. A few years back, Atlanta's water supply was down to just a few weeks, and a massive drought struck the midwest recently as well, killing off lots of corn, including that which was supposed to help save the doomed car CULTure. The submerged Glen Canyon played a fan dance with humanity in the last decade when the Colorado was severely stressed (when isn't it stressed, damn dams!?!). The question is not whether these droughts are consistent, but that they seem erratic. One may expect regular droughts in California, but the eastern portion of the United States is largely forest climates. That's not the case this year. The question is not just one of trends, but also one of instability. Trees have lifetimes that last centuries. It follows that things which kill them off - even in brief states -have long term consequences. Since it took so much attention here on my shady lawn to save just a few sapling oaks, I suspect that in the forests themselves, almost no new trees will survive. Things are so bad that even the poison ivy is dying. These wide swings year to year cannot be good for the forest flora.
So much for my concern.
The current issue of Tree Physiology, by the way, apparently contains lots of articles on the questions that trouble me, including phosphorous dynamics and the effect of drought on forests. (I once wrote about phosphorous in this space in a diary called Another Happy Story About Agricultural Resource Depletion: Phosphate, Nauru, and Your Toilet.) This might be a good way to spend the holiday weekend, trying to understand what may happen to the trees and forests around here by reading papers in Tree Physiology.
In any case, with the exception of some pine needles, trees do not sequester nitrogen, although they do temporarily store it. Live trees of course, do sequester carbon, but only if they are healthy.