The great cycle of changes occurring in the soil whereby organic matter is gradually transformed and again made available as plant food is entirely dependent upon micro-organisms. Until a decade ago it was thought that bacteria were by far the most important group concerned in the bringing about of these changes, but recent studies have shown that, in at all events certain arcs of this great organic cycle, the fungi have, perhaps, an equal part to play. The life of fungi in the soil may, for our purposes, be considered from three points of view—their part in the decomposition of carbon compounds, their nitrogen relationships, and their work in the mineral transformations of the soil.
Carbon Relationships.
Of primary importance in the carbon relationships of soil fungi is the part played in the decomposition of the celluloses, which compose almost all the structural remains of plant tissues. Our first real knowledge of this subject was given by Van Iterson[28] in 1904 when he showed the wide extent of cellulose destruction by fungi, and devised methods whereby fifteen cellulose-decomposing forms, many of which have since proved to be common soil fungi, were isolated. Three years later Appel[1] published his account of the genus Fusarium, and showed that many of the species could destroy filter paper. A difficulty was introduced in 1908 by Schellenberg,[60] who, working with common soil forms, found that only hemicelluloses and not pure cellulose were destroyed. This has recently been supported by Otto,[48] but from the practical point of view the discussion is academic for the amount of pure cellulose in plants is insignificant.
In 1913 McBeth and Scales[43] showed that a considerable number of common soil fungi were most active cellulose destroyers, pure precipitated cellulose and cotton being readily attacked. This was supported by McBeth in 1916,[42] whilst Scales[59] has found that most species of Penicillium and Aspergillus decompose cellulose, especially where ammonium sulphate is the source of nitrogen. Waksman[65] tested twenty-two soil fungi and found that eleven decomposed cellulose rapidly and four slowly, whilst Dascewska,[16] Waksman,[66], [67] and others have concluded that soil fungi play a more important part in the decomposition of cellulose and in “humification” than soil bacteria. Schmitz[61] has recently shown that cellulose-destroying bacteria play no important part in the decay of wood under natural conditions.
In addition to the celluloses, practically all simple and complex organic carbon compounds are attacked by soil fungi, and in many cases the decomposition is very rapid.[26] Many Actinomycetes, Aspergilli and Penicillia are active starch splitters, and it is of interest to note that some of the strongest cellulose decomposers (Melanconium sp., Trichoderma sp., and Fusaria) secrete little diastase.[66] The Mucorales apparently do not attack cellulose, but can only utilise pectin bodies, monosaccharides, and partly disaccharides.[26] Dox and Neidig[19] have shown that various species of Aspergillus and Penicillium are able to attack the soil pentosans. Roussy,[58] Kohshi,[24] Verkade and Söhngen,[64] and many other workers have found that fats and fatty acids are readily used as food by soil fungi, and Koch and Oelsner[33] have recently shown that tannins are readily assimilated. Klöcker,[32] Ritter,[56] and others have shown that the utilisation of many carbon compounds is to a large extent determined by the source of nitrogen and its concentration in the pabulum.
There would seem, therefore, no doubt that the decomposition of celluloses and other carbon compounds is of primary importance in the life-activities of soil fungi.
Nitrogen Relationships.
In this section we shall consider the problems of nitrogen fixation and nitrification, of ammonification, and of the utilisation of nitrogenous compounds by soil fungi.
As soil fungi form so large a part of the soil population, the question of whether they can make use of the free nitrogen of the air is of primary importance. During the last two decades many investigators have attempted to solve the problem, often studying allied or identical species; but if one consults some thirty researches published during this period, opinion is found to be about equally divided. Even, however, in those studies where nitrogen fixation has been recorded the amounts are very slight, usually being below 5 mgrms. per 50 c.c. of solution, and often being obviously within the limits of experimental error. Latham,[37] however, working on Aspergillus niger, recorded variations ranging from a nitrogen loss of 42·5 mgrms. to a nitrogen fixation of 205·1 mgrms. per 50 c.c. of medium. Ternetz[63] found that different strains of Phoma radicis may fix from 2·5 mgrms. of nitrogen in the lowest case, to 15·7 mgrms. in the highest per 50 c.c. of nutrient solution. Duggar and Davis[20] report that Phoma betæ may fix nitrogen in quantities of 7·75 mgrms. per 50 c.c. of medium. The latter authors, in a very able critique of the problem, indicate certain possible sources of error in previous work, and if one examines the studies in which nitrogen fixation has been recorded in the light of these criticisms, it is difficult not to think that, with the exception of the genus Phoma, good evidence for nitrogen fixation by fungi is lacking. Phoma betæ is a common pathogen attacking beets, whilst P. radicis is a mycorrhizal form inhabiting various Ericales. Apart from these exact quantitative studies, which have given a negative verdict, there is a considerable amount of positive but indirect evidence for nitrogen fixation by mycorrhizal fungi,[55] and it is very unfortunate that more of these forms have not been investigated quantitatively. As the evidence stands to-day, one must conclude that the fungus flora does not play any part in the direct nitrogen enrichment of the soil.
Equally obscure is the question of nitrification and denitrification by soil fungi, but this is the result of a lack of study rather than of a plethora of indeterminate researches. Direct nitrification or denitrification has not been established, but the work of Laurent[38] and a few other workers appears to show that soil fungi can reduce nitrates to nitrites.