Nitrogênio em Braun-Blanquet (1932)

In seeking a substitute for evaporation, Meyer (1926) argued that air humidity as well as temperature regulates the amount of evaporation. He therefore proposed the so-called “N. S. quotient” (Niederschlag und Sättigungsdefizit) —precipitation by saturation-deficit Quotient. In spite of its imperfections, appreciated by Meyer, the latio (quotient) appears to give significant results, and it has the advantage of the abundant air humidity data which are of record. The larger features of the distribution of both vegetation and climatic soil types agree with the results of this factor, as seen in an N. S. [145] quotient map of the United States by Jenny (1930) (Fig. 79). [Nota de rodapé 1: A similar climatic map of North America based on precipitation, effectiveness and temperature efficiency has been constructed by Thornwaite (1931).]. Jenny has also taken advantage of this to show graphically the relations between the nitrogen content of the soil and the climate along two isotherms in the United States. The one line (11°C.) belongs to the temperate region and includes territory in Colorado, Kansas, Missouri, Illinois, Indiana, Ohio, and New Jersey; the other (19°C.) passes through the subtropical states of Texas, Louisiana, and Mississippi (Fig. 80). (Braun-Blanquet 1932:143/5)

Soil Flora.—The microflora of the soil plays a much more significant role than the animals in soil economy. This flora is composed of countless bacteria, fungi, and algae, which live preferably in the root layer of the soil and are exceedingly active there. Their significance is indicated by their function of nitrogen fixation, as well as by the fact [235] that they make more available many substances already in the soil but difficult of assimilation by higher plants. (Braun-Blanquet 1932:234-5)

The discovery of the anaerobic schizomycete, Clostridium pastoriaium, by Winogradsky, has furnished us the key to the scientific explanation of the long-recognized fixation of atmospheric nitrogen by plants. With adequate aeration this schizomycete fixes 0.0025 to 0.003 g. of atmospheric nitrogen for each gram of glucose consumed. A great increase in the nitrogen content of the soil is followed by the cessation or retardation of the nitrogen-fixing activity of this bacterium. (Braun-Blanquet 1932:235)

Anaerobic and aerobic nitrogen fixers often occur associated together in the same soil, and with them certain fungi (Aspergillus, [235] Phoma, Gymnoascus, Alternaria, etc.) and algae. All of these seem able to fix free nitrogen. Some such fungi and bacteria seem to stand in a symbiotic relation to each other. (Braun-Blanquet 1932:235-6)

The tubercle bacteria (Bacterium radicicola, B. beijerinckii) probably fix nitrogen only in symbiosis with higher plants (mostly Leguminosae but also Podocarpus, Alnus, Elaeagnus, and Hippophae). The rôle they play in agriculture has been well known since the days of the classical investigations of Hellriegel and Wilfahrt, But we know nothing about their significance in the development of natural vegetation. It is probable that the tubercle bacteria make possible, or at least greatly favor, the usually luxuriant growth of numerous Leguminosae (Lupinus, Lotus, Astragalus, Ornithopus, etc.) upon the sandy soils of southern Europe and northern Africa. (Braun-Blanquet 1932:236)

The gain in nitrogen content of untilled, natural soils must be very considerable. Two fields in central England, which had been given no nitrogen fertilization for 22 and 24 years, showed, according to Russell (1927), a total increase in nitrogen content of 2,162 and 1,567 kg. per hectare. These gains of nitrogen were proportionate to the abundance of Leguminosae. (Braun-Blanquet 1932:236)

Nitrification Capacity.—The rate at which nitrification goes on seems to be independent of the quantity of organic nitrogen in the soil. But the nature of the humus nitrogen present probably affects the speed of ammonification very decidedly. In forests nitrification is [328] most active in the horizon of decomposition, in this respect agreeing with the activity of mycorhizal fungi. It decreases with depth. Rather inert nitrogen compounds slow up nitrification (Andre, 1921, p. 179). Olsen (1925) has determined that soils of fiat-moor communities with Carices, Molinia, and Deschampsia caespitosa, which did not show a trace of nitric acid at the first examination, showed 22 to 25 mg. per liter of soil after 25 days. From this it follows that the amount of nitrate present at any given time gives no indication of the nitrate supply of a plant community. Not only is nitric acid quickly assimilated by plants, but it is also leached out by rain and is often barely traceable. Thus the nitrate content of a soil is subject to considerable fluctuations from time to time. (Braun-Blanquet 1932:327-8)

In the mountains of central Europe the tall-herb association of the Adenostylion, which inhabits well-aerated, moist, weakly acid to basic soils, seems to be determined by the high nitrate content of the soil (Fig. 70). Nitrophilous communities are very widely distributed in dry, subtropical regions (Fig. 119). The most nitrophilous community of the Mediterranean region is the Silyhum-Urtica pilulifera association of which all the species show a high nitrogen content when young. (Braun-Blanquet 1932:240)

If the supply of excreta is too great, only a few species can thrive, and even these are frequently dwarfed. According to their degree of nitrophily, the rock-lichen communities are often arranged in distinct [241] beltlike order. Upon the bird roosts and marmot rests of the Alps and the Tatra the surfaces of the overmanured rocks are occupied by the extremely nitrophilous Ramalinetum strepsilis, with Rinodina demissa, Xanthoria fallax, and Physcia tribacia as characteristic species. On somewhat sloping surfaces, to which the nitrogen compounds are washed by rain water, grows the Alectorietum chalybeiformis, with Gyrophora cirrhosa, Lecanora frustulosa, and L. melanophtalnia. The vertical surfaces are taken by the G. cylindrica-Cetraria noermoerica association (Fig. 120). Similar belt grouping, somewhat less pronounced, has been found upon the rabbit rests on basalt blocks in southern France. (Braun-Blanquet 1932:240-1)

Denitrification.—Over against the nitrate formers stand the denitrifying bacteria which destroy nitric acid (Bacterium denitrificans, B. stutzcri, etc.). Along with nitrogen-oxidizing processes we always find also nitrogen-reducing activities: nitrates are resolved into nitrites and these in turn reduced to free nitrogen by the destroyers of nitric acid. In case of direct denitrification, the free nitrogen escapes from the soil in gaseous form. (Braun-Blanquet 1932:241)

In addition to external factors, the nature of the decaying matter also affects humification. The nature of the humus is determined also by the destructibility, pH value, and buffering of decaying plant matter. The observations of Melin (1930) show that there is considerable variation of the rate of decay within a single species but that on the whole there is a distinct parallelism between the total nitrogen content and the rate of decomposition. In leaves of different species there is no correlation between nitrogen content and the rate of the primary decomposition. Nevertheless, the process of humus formation seems to be chemically the same in every case. According to the views of Michaelis and of Page (cited in Hesselman, 1927), perhaps the whole formation of humus, in so far as it is unaffected by the mineral matter, consists in a substitution of the metallic cations of acid compounds by H anions. The cations released are easily leached out in a moist climate and conducted into deeper soil layers, where they accumulate. (Braun-Blanquet 1932:244)

Humus and Total Nitrogen.—The total nitrogen of the soil runs approximately parallel to the humus content: the H:N ratio in a temperate climate is about 10:1 according to Waksman (1924). In a cool climate it is somewhat greater or about 12:1 ; in a warmer climate somewhat smaller—9:1 (Jenny, 1929) (see Fig. 80). The dependence of the nitrogen content, and therefore of the humus content of the soil, temperature and relative humidity (N.S. quotient, see p. 143) is brought out in the accompanying table by Jenny (1930). (Braun-Blanquet 1932:248)

With excessive amounts of manure the soil is stimulated to very great bacterial activity. This explains the rich development of nitrophilous dung plants which are able to store considerable quantities of nitrates in their tissues. According to the ability of the individual [277] species to endure the active nitrogen compounds, various degrees of nitrophily may be distinguished, and a whole series of floristically related, more or less nitrophilous “animal resting-place communities” may be recognized. These nitrophilous communities might very well be taken as indicators of the capacity of the soil for nitrification. It must not be forgotten, however, that where therophytes are not numerous, communities once established have considerable endurance and often maintain themselves in changed conditions. Since the establishment of the Swiss National Park (1911), the cattle yards there are no longer in use as such. Nevertheless, the floristic composition of the vegetation upon them has not changed in the least. The R. alpinus, Chenopodium bonus henricus, and Aconitum stands have withstood outside competition very well, even where the soil today has a strongly acid reaction. (Braun-Blanquet 1932:276-7)

The general climate of a region determines whether an accumulation of organic matter and nitrogen is possible and whether the upper layer of soil (root layer) is subject to increasing acidification or to an accumulation of salts. On this depends the whole course of the development of vegetation. (Braun-Blanquet 1932:310)

Zonation.—A girdle or beltlike arrangement of the units of vegetation, whether on a large or on a small scale, is caused by similarly arranged differences of important factors of the habitat: temperature, soil moisture, salt- or nitrogen-content of the soil, duration of snow cover, wind, etc. (Figs. 172, 173). The concentric vegetation zones of the earth (not to be confused with “regions of vegetation”) are due to the gradual increase in temperature from the poles to the equator. Decrease in humidity and precipitation toward the interior of continents causes zonation into belts of forest, grassland, steppe, and desert. (Braun-Blanquet 1932:345)

BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.