Popular Science Monthly/Volume 62/March 1903/The Source of Nitrogen in Forest Soil
|THE SOURCE OF NITROGEN IN FOREST SOIL.|
BUREAU OF FORESTRY, U. S. DEPT. OF AGRICULTURE.
NORMAL development and growth of forest plants is possible only when the plants are able to obtain a sufficient amount of nutritive substances from their medium.
Among the substances indispensable for the nutrition of plants nitrogen occupies a conspicuous place. On an average, sixteen per cent, of this element is included in the composition of albuminous matter. Nitrogen is necessary for the formation of protoplasm of vegetative cells, and when it is absent no formation of protoplasm can take place; hence, no development of organic life in general is possible. Plants derive nitrogen from the soil, where it is found in a free state (air), or combined in the form of nitrogenous compounds.
If plants had the capacity of absorbing atmospheric nitrogen and assimilating it like carbon, the question as to the presence of nitrogen in the soil would have no interest, since the air would be a sufficient source of nitrogen. While some plants have the capacity of absorbing atmospheric nitrogen, the majority draw their supply from the salts of nitrogen found in the soil.
The consumption of atmospheric nitrogen has been fully determined for the wild acacia (black locust) and for the white and black alders. Thus in quartz sand completely free from nitrogen, seeds of black locust, while developing into seedlings, increased their contents of nitrogen from 0.0024 gr. to 0.092 gr., or more than 38 times, between May 1 and September 10. The locust in this respect resembles all the other leguminosæ, which have long been known to agriculturists as accumulators of nitrogen. On examining plants which absorb the free nitrogen contained in the soil, it was found that the roots of these plants have tubercles inhabited by organisms. These organisms are bacteria, which, attracted by the excretions of the roots, immigrate from the soil and cause the formation of root-tubercles which are invariably present in the papilionaceæ. The bacteria absorb the atmospheric nitrogen and transmit it to the plant. A great many bacteria cause tubercles on the roots; they can be classified according to the species which they choose as their host, more conveniently than by their external appearance. Each species of the leguminosæ seems to possess its own race of bacteria, which can be made serviceable to other species only by gradual adaptation. Leguminosæ which lack proper bacteria are backward in development.
If, for instance, black locusts are grown in soil which contains no bacteria or only bacteria taken from the tubercles of leguminosæ somewhat less closely allied to them, they thrive only moderately; whereas under the influence of the locust bacteria they develop luxuriantly. The organism living in the root tubercles of the alder is also, according to Hiltner, instrumental in the absorption of nitrogen. It displays its activity particularly whenever the available nitrogen of the soil begins to be exhausted on account of its vigorous consumption by the tree. The fungi which cover the suction rootlets of oaks, beeches, birches, and other cupuliferæ, as well as conifers and many other woody plants, probably also assist the trees in the taking up of nitrogen. They do not, however, absorb the atmospheric nitrogen, but only decompose, among other organic substances, the nitrogenous compounds of humus, and thus convert them into chemical combinations more accessible to the tree.
Thus, excepting the leguminosæ and a few species of other families, plants depend for their supply of nitrogen upon the nitrogenous compounds in the soil. And it is with these plants that the question as to the source of nitrogen in the soil becomes important, inasmuch as there are no minerals containing nitrogen so widely distributed as to satisfy the demand of trees for this element. Atmospheric precipitations and the product of the decay of animal and vegetable refuse are usually supposed to be the sources of nitrogenous compounds in the soil. The chemical compounds in which nitrogen occurs in the soil are chiefly nitrates, ammonia, and organic nitrogenous compounds resulting from the decomposition of organic substances. The best sources of nitrogen for green vegetation are generally supposed to be the nitrates; but for forest trees, at least, these seem to be of small account, because, as Ebermayer says, in forest and marshy soils nitrates are present, if at all, only in small traces. Also, in the interior of stems nitrogen is found in the form of nitric acid only when they are grown on a soil containing nitrates; for instance, on cultivated land. Among the compounds of nitrogen which are supplied to the soil through atmospheric precipitation, some nitric acid is found, but two to five times as much ammonia is present. The whole amount of nitrogen which is held in combination and supplied to the soil in this manner through snow and rain amounts, according to Ramann, to nine pounds per year per acre, and even less. This is insufficient to supply the demands of trees for nitrogen, as they store up a much greater quantity. Ebermayer states the average amount of nitrogen taken up by various kinds of trees per year and acre as follows:
|Silver fir forest,||"||35||"||"||"|
|Scotch pine forest,||"||30||"||"||"|
Thus, only the ammonium salts and the organic compounds of nitrogen formed in the process of decay are available for the roots as sources of nitrogen. The amount of nitrogen supplied to the soil through atmospheric precipitation, either in the form of nitrates or ammonia, is not sufficient to supply the needs of trees for nitrogen.
There remains still another source, and this is the organic compounds of nitrogen formed in the process of decay of litter. In fact, Ebermayer has recorded strongly developed roots of spruces and firs on the Bavarian Alps that grew in pure humus one meter thick, from which he concludes that the dark forest humus furnishes all the nitrogen and other mineral nourishment required by trees.
If, therefore, the source of nitrogen in forest soil is nitrogenous compounds resulting from the decay of the litter, one would expect in a forest which is managed on a business basis (that is, in which trees are removed when ripe), a gradual decrease of the contents of nitrogen in the soil, as occurs on a larger scale in agriculture. In agriculture, where the annual harvesting of crops deprives the soil of almost all the nitrogen which is assimilated by the plants, and returns to the soil only a small part of it by the decay of the roots of the plants, and where the easily soluble nitrates are washed out by rains and carried away from the fields, or deposited in layers inaccessible to the roots, the exhaustion of nitrogen in the soil sets in soon, and the artificial introduction of nitrogen becomes a necessity.
One of the most common ways of replenishing the nitrogen taken up by crops is manuring and the growing of leguminous plants which have the capacity of absorbing atmospheric nitrogen. These plants are plowed under during the period of blooming, and when they decompose they give their nitrogen to the soil. In the forest, it is true, a considerable part of the nitrogen is returned to the soil in the form of shed leaves, and only part of it, which is contained in the trunk of the tree, is removed. The washing out of nitrates from forest soil does not occur, because no nitrates are formed in it, and those which are brought in by atmospheric precipitation are decomposed under the influence of a special microorganism known as Bacillus dentrificans, which is formed in soils with acid reaction.
But forest soil, though it loses less nitrogen than does arable land, nevertheless loses it; and more remarkable yet, forest soils not only do not become poorer in nitrogen, but, on the contrary, become enriched with it, a fact readily demonstrated in poor soils planted to forests.
How this loss is compensated by nature was not known until recently. The introduction of nitrogen into forest soils artificially is not practicable, and therefore the enrichment of the soil with nitrogen must go on under the influence of other causes. A certain number of leguminous plants grow in forests, but these are by no means sufficient to compensate for the loss of nitrogen through the felling and removal of forest trees. In some arborescent species, as Alnus glutinosa, Robinia pseudacacia, and others, tubercles which stimulate assimilation of free nitrogen are found on the roots. It may happen that such species do not occur in the forest, or that the necessary bacteria do not develop in the soil, when the loss of nitrogen would not be replenished at all. This replenishing, however, always occurs, and some sources must be found to account for it.
Eecently, E. Henry, professor in the forest academy at Nancy, France, discovered a new source of enrichment of the soil with nitrogen, which is of great interest to foresters. Professor Henry has proved by experiments that the loss of nitrogen in forest soil is constantly repaired by means of absorption of atmospheric nitrogen by fresh forest litter.
In November, 1894, Professor Henry collected leaves only recently dead and still hanging on oaks and hornbeams (Carpinus betulus). The amount of nitrogen in these leaves was determined in per cent. of the dry substance. In this way it was found that the leaves of oaks contained 1.108 per cent, of nitrogen, and the leaves of the hornbeams 0.747 per cent. The oak leaves were placed in two zinc boxes. The bottom of one of the boxes was covered with limestone, that of the other with sandstone not containing lime. Both boxes were covered with a netting of galvanized wire. The leaves were dried in the laboratory, and 48.16 grams of their dry substance were placed in the first box, and 53.54 grams in the second. The leaves of the hornbeams were distributed in the same way. All four boxes were exposed to air, with necessary precautions against enriching the leaves with nitrogenous compounds. In December, 1895, the following year, Professor Henry determined the contents of nitrogen in the leaves taken from two boxes, whereby it was found that the oak leaves taken from the box with the limestone bottom contained 1.923 per cent, of nitrogen, and the leaves of hornbeam taken from the box with the sand bottom, 2.246 per cent. After making the necessary allowance for loss in weight of oak and hornbeam leaves owing to decomposition, Professor Henry computed the increase of nitrogen in the oak leaves at 4 per cent., in the hornbeams at.78 per cent. Thus it was proved by him that fresh leaves fallen from trees absorb atmospheric nitrogen in the process of decomposition.
The two other boxes remained exposed to the air for another year, and in May, 1896, fifty grams of fine forest soil were added to each box. On subjecting the leaves contained in them to a chemical analysis, Professor Henry found almost the same contents of nitrogen that had been found in the leaves of the first two boxes which were exposed to the air during only one year. From these results he concluded that the capacity of fallen leaves to absorb nitrogen from the air is retained only in leaves freshly fallen on old litter.
The capacity of forest litter to absorb nitrogen develops probably under the influence of special microorganisms, active only at the beginning of the process of decomposition of fallen leaves; later, however, when the process of decomposition of leaves goes on under the influence of exclusively inorganic agents, no increase of nitrogen i? observed; on the contrary, a loss is shown.
It is thus scientifically proved that forest litter is capable of enriching the soil with nitrogen, but only under the condition that the decomposition of freshly fallen leaves goes on. As to the assumtion that bacteria are developed in freshly fallen leaves, which, like Rhizobium leguminosarum Frk., possess the capacity of absorbing atmospheric nitrogen, it can only be said that as yet no bacteria have been found in forest litter.
A practical deduction from Professor Henry's scientific investigation is the advisability of planting cut-over areas as soon as possible, so that the young seedlings may find in the soil a quantity of nitrogen sufficient for their nourishment. The longer cut over areas remain unplanted, the less is success to be expected from planting, as the young trees develop poorly because of insufficient nourishment.
Thus, the forest not only furnishes timber and other products, prevents snow-and land-slides, and regulates the flow of rivers, but enriches the soil with nitrogen, one of the most essential nutritive elements of plants, and in this way transforms poor soils, fit only for tree growth, into rich agricultural lands.