Ratios of microbial biomass carbon to total organic carbon in arable soils

The ratio of microbial biomass carbon (Cmic) to total organic carbon (Corg) was measured in soils from long-term agricultural experiments located in the temperate climatic zone of Central Europe. One hundred and thirty-four plots located in 26 experimental sites were analysed. This survey of Cmic-to-Corg ratios indicated no universal equilibrium constant. In contrast, a comparative regression analysis of permanent monoculture plots (M) with continuous crop rotation plots (R) showed both to be highly significantly different at the P = 0.001 level: the regression line of continuous crop rotations shows a steeper slope, suggesting that a higher concentration of microbial carbon is characteristic of the crop rotation. The regressions accounted for 87% (M) and 94% (R) of the variances in the data, respectively. Mean % Cmic in Corg amounted to 2.3 for permanent monocultures and to 2.9 for continuous crop rotations, respectively, of the simplest, the mineral fertilizer management. The differences in C-to-C ratios between the two management systems is thought to be caused by the difference in cropping management rather than by soil texture, i.e. clay content. These Cmic-to-Corg ratios rose to 4 or 3.7% in both plots under monoculture and plots under rotation, when the plots had received organic fertilizer the year prior to sampling. This rise in Cmic over Corg is seen as a transient rather than an absolute phenomenon and is believed to be due to the easily available carbon fraction of the introduced organic materia. Linearity of the Cmic-to-Corg relationship seems to be expressed only up to 2.5% Corg. Regressions of Cmic-to-Nt were as well highly significant (P =0.001) and in addition, the comparative regression analysis between M and R plots.