MICROBIAL BIOMASS AND ACTIVITY IN SILT AND SAND LOAMS AFTER LONG-TERMSHALLOW TILLAGE IN CENTRAL GERMANY

Forms of ploughless cultivation, such as shallow tillage, in central G ermany, are associated with N deficiency symptoms in crops which may b e 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 matt er, biomass, activity and community structure of the soil microflora o f a silt loam (Orthic Luvisol) and a sand loam (Dystric Cambisol). Mic robial parameters at the study site were compared directly with that o f 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 cultivato r 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 xyla nase activity were markedly reduced, indicating deteriorated condition s for soil microorganisms. The significant increase in the ergosterol to biomass C ratio in the surface layers, indicated a change of the mi crobial decomposer community towards fungi after termination of invert ing tillage. As indicated by a decline in the metabolic quotients qCO( 2) and qNH(4), i.e., the biomass-specific CO2 production and NH4 miner alization rates, the reduction in biomass and activity of soil microor ganisms must have been caused by a reduced input of plant material. Th us, the decline in microbial activity was probably due to energy defic iency, and not due to stress caused by reduced oxygen concentrations i n the compacted 7-30 cm layers of the unploughed sites. (C) 1998 Elsev ier Science B.V. All rights reserved.