SOIL ORGANISMS 



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 trihacia 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 pro- 

 nounced, has been found upon the rabbit rests on basalt blocks in 

 southern France. 



The nitrophilous bark communities of lichens, mosses, and algae 

 have been carefully examined by Ochsner and Frey (1925, 1928). 



Prasiolefum 

 stipifafae 



Fama/fneTum 

 sfreps/V/'s 



\ Rama/inefum 

 \polymorp/iae 



Gyrophoretum 

 artjcae 



)A/ectorietum 

 cha/jbeiformfs 



IGyrop/forefum 

 cy/indr/'cae 



Fig. 120. — Belting of the lichen associations according to decreasing nitrate content 

 of the habitat. A, Resting place of birds of prey in the high mountains of Central 

 Scandinavia. (After Gams.) 5, in High Tatra. {After Motyka.) 



Ochsner distinguishes two markedly nitrophilous bark associations: 

 the hght-loving Physcietum and the Parmelietum acetabulae, com- 

 prising several sub-associations, which prefers older trees with fur- 

 rowed bark. 



Den-trification. — Over against the nitrate formers stand the denitri- 

 fying 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. 



The living conditions for nitrifying and denitrifying bacteria must 

 be somewhat similar, for the denitrifiers occur only where nitric acid is 

 being formed. The soil conditions favoring one or other of these 

 groups of bacteria determine whether construction or destruction of 



