Fungal and bacterial growth in soil with plantmaterials of different C/N ratios

Abstract:
Fungal (acetate-in-ergosterol incorporation) and bacterial (leucine/thymidine

incorporation) growth resulting from alfalfa (C/N = 15) and barley straw

(C/N = 75) addition was studied in soil microcosms for 64 days. Nitrogen

amendments were used to compensate for the C/N difference between the

substrates. Fungal growth increased to a maximum after 3–7 days, at five to eight

times the controls, following the addition of straw, and three to four times the

controls following the addition of alfalfa. After 20–30 days, the fungal growth rate

converged with the controls, resulting in a cumulative fungal growth two to three

times the controls following straw addition and about 20% higher than

the controls following alfalfa addition. The bacterial growth rate reached rates five

times the controls following alfalfa addition and twice that of the controls

following straw addition after 3–7 days. It remained elevated after 64 days. The

cumulative bacterial growth was two and four times the controls following straw

and alfalfa addition, respectively. A negative correlation was found between

N addition and bacterial growth, while N stimulated fungal growth. Thus, the

C/N ratio of the additions (substrate and extra N) could not entirely explain the

different results regarding fungal and bacterial growths. Respiration was not always

related to the combined growth of the microorganisms, emphasizing the requirement

for a better understanding of growth efficiencies of fungi and bacteria.