Soil microbial properties and the assessment of available soil organic matter in a haplic Luvisol after several years of different cultivation and crop rotation

Adenosine triphosphate (ATP) contents, enzymatic activities, cellulose decomposition in situ, aggregate stability, total organic C contents, C- and N-mineralization potentials and amounts of water-soluble C compounds were measured in the plough layer of field plots from a haplic Luvisol at the ‘Ihinger Hof’ near Stuttgart, SW Germany. Plots had been treated differently for 14 years with regard to tillage (ploughing, rotary cultivation) and crop rotations (legume-cereals, rape-cereals). Soils were sampled from the 0–10 cm and 10–25 cm layer of the Ap horizon 4 times in 1990 during and after the vegetative growth of oats. On the basis of the input and distribution of crop and root residues during the previous 8 years a model for the decomposition of organic substances was used to estimate the amounts of the microbially-decomposable organic substances added to the plots (decomposable young soil organic matter, DYSOM). Our aim was to study the effects of management practices on soil microbial properties and to relate those effects to changes in the microbially-available part of the soil organic matter. The contents of total soil organic C, total N, ATP, and soil microbial activities in the 0–10 cm layer were mostly higher with rotary cultivation than with ploughing. In contrast, either no differences between the tillage systems or higher values with ploughing occurred in the second soil layer (10–25 cm). In the upper soil layer of the rape-cereals crop rotation soil microbial biomass was higher and more active than in the legume-cereals crop rotation. In the lower soil layer no uniform effects of the crop rotations were found. For the 0–10 cm layer there was an overlapping effect of cultivation and crop rotation on soil organic matter fractions and microbial biomass. Differences in soil microbial measurements were partly related to differences in humus contents. However, variations in microbial properties with different treatments were mostly larger than variations in total organic C content, sometimes occurring without any difference in total organic C contents. In these cases the contents of DYSOM and soil pH were found to have a dominant influence. Soil physical properties either showed only little differences (texture, bulk density), or were highly correlated to total organic C contents (water contents). There seemed to be a contradiction between a tendency for higher microbial activities and at the same time larger humus contents on plots with rotary cultivation compared to ploughing. This may be explained by a higher C efficiency in these plots, i.e. the soil microbial biomass-to-organic C ratio was relatively high and accompanied by small changes in biomass contents during the year.