Soil microbial tests for discriminating between different cropping systems and fertiliser regimes

The aim of this study was to evaluate a set of microbial soil tests for their ability to discriminate between different agricultural practices. For this purpose three sites included in the Swedish Long-Term Soil Fertility Experiments were chosen. The fertility experiments were designed to compare different cropping systems (simulating farming with and without livestock), PK-fertiliser and N-fertiliser regimes. Six different microbial tests were used to derive nine variables describing: (1) basal microbial activity (B-res), (2) potential microbial activities (substrate induced respiration, SIR; potential NH4+ oxidation, PAO; potential denitrification activity, PDA; and alkaline phosphatase activity, Alk-P), (3) specific microbial growth rates (mu (res) and mu (PDA)) and (4) nutrient-limited respiration rates (maximal 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 of them, except mu (PDA), showed some degree of interaction between different treatments. However, the best discriminators between cropping systems were 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 systems. In addition, PC 1 and PC 2 reflected the P-fertilisation rate. Max-P alone had the best potential to reflect the microbially available P in the soil and thereby indirectly the plant-available P. The mu (res) was also useful when assessing available P in the soil. The N-fertilisation rate seemed to be the most difficult treatment to assess with the microbial activity variables. In addition, PCA revealed a consistent functional relationship in all three soils between the potential activity variables (SIR, PAO, PDA, and Alk-P).