Microbial biomass and activity in silt and sand loams after long-term shallow tillage in central Germany

Forms of ploughless cultivation, such as shallow tillage, in central Germany, are associated with N deficiency symptoms in crops which may be tillage induced. On a farm in the hilly area around Eschwege, Hessia, we assessed the effects of 9 year shallow tillage with a horizontal axis rotary cultivator working to a depth of 7 cm on soil organic matter, biomass, activity and community structure of the soil microflora of a silt loam (Orthic Luvisol) and a sand loam (Dystric Cambisol). Microbial parameters at the study site were compared directly with that of the neighboring sites which had been ploughed by other farmers, but managed similarly with respect to crop rotation, plant protection and fertilization. Shallow tillage with a horizontal axis rotary cultivator led to compaction at the 7-30 cm soil depth. In this compacted part of the previous plough layer, the contents of microbial biomass C and ergosterol, the CO2 production rate, NH4 mineralization rate, and xylanase activity were markedly reduced, indicating deteriorated conditions for soil microorganisms. The significant increase in the ergosterol to biomass C ratio in the surface layers, indicated a change of the microbial decomposer community towards fungi after termination of inverting tillage. As indicated by a decline in the metabolic quotients qCO(2) and qNH(4), i.e., the biomass-specific CO2 production and NH4 mineralization rates, the reduction in biomass and activity of soil microorganisms must have been caused by a reduced input of plant material. Thus, the decline in microbial activity was probably due to energy deficiency, and not due to stress caused by reduced oxygen concentrations in the compacted 7-30 cm layers of the unploughed sites. (C) 1998 Elsevier Science B.V. All rights reserved.