NITROGEN IN FOREST SOIL. 439 



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 cer- 

 tain 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 spe- 

 cies, 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, how- 

 ever, 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 nitro- 

 gen, which is of great interest to foresters. Professor Henry has 

 proved by experiments that the loss of nitrogen in forest soil is con- 

 stantly 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 lime- 

 stone, 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 

 i 



