Soil microbial tests for discriminating between different cropping systemsand fertiliser regimes

The aim of this study was to evaluate a set of microbial soil tests for the ir ability to discriminate between different agricultural practices. For th is purpose three sites included in the Swedish Long-Term Soil Fertility Exp eriments were chosen. The fertility experiments were designed to compare di fferent cropping systems (simulating farming with and without livestock), P K-fertiliser and N-fertiliser regimes. Six different microbial tests were u sed to derive nine variables describing: (1) basal microbial activity (B-re s), (2) potential microbial activities (substrate induced respiration, SIR; potential NH4+ oxidation, PAO; potential denitrification activity, PDA; an d alkaline phosphatase activity, Alk-P), (3) specific microbial growth rate s (mu (res) and mu (PDA)) and (4) nutrient-limited respiration rates (maxim al P-limited respiration, Max-P; and maximal N-limited respiration, Max-N?) . Among the individual microbial variables B-res, SIR, mu (res) and mu (PDA ) were the best discriminators of the two different cropping systems. All o f them, except mu (PDA), showed some degree of interaction between differen t treatments. However, the best discriminators between cropping systems wer e the components [principal component (PC)1 and 2] from a PC analysis (PCA) . In all three soils PC 1 discriminated well between the two cropping syste ms. In addition, PC 1 and PC 2 reflected the P-fertilisation rate. Max-P al one had the best potential to reflect the microbially available P in the so il and thereby indirectly the plant-available P. The mu (res) was also usef ul when assessing available P in the soil. The N-fertilisation rate seemed to be the most difficult treatment to assess with the microbial activity va riables. In addition, PCA revealed a consistent functional relationship in all three soils between the potential activity variables (SIR, PAO, PDA, an d Alk-P).