Critical limits for microbial indicators in tropical Oxisols at post-harvest: The FERTBIO soil sample concept

The FERTBIO soil sample concept considers the advantages that standardization of the sampling time and pretreatment procedures for soil chemical fertility (FERT) and microbiological (BIO) analyses offer to farmers and commercial laboratories. This study examined the effects of two soil sampling/handling strategies (flowering/field-moist soil samples (FFM) and post-harvest/air-dried soil samples (PHAD)) on the critical limits (CLs) for microbial biomass carbon (MBC) and soil enzymes (β-glucosidase, arylsulfatase, acid phosphatase and dehydrogenase). Soil samples (0 to 10 cm depth) were collected in 2013 and 2015 from 24 treatments from three long-term experiments on a Cerrado clayey Red Oxisol. Reduced Major Axis (RMA) regressions showed that PHAD sampling was representative of FFM. Despite the significant reductions in MBC (11%), β-glucosidase (55%), arylsulfatase (49%), acid phosphatase (54%) and dehydrogenase (46%), PHAD sampling preserved the ranking of treatments and did not alter the adjustment of regression models between soil organic carbon (SOC) and microbial indicators. Critical limits specific to post-harvest air-dried soil samples were established for each microbial indicator. The interpretative table generated using the FERTBIO soil sample concept represents an important step in research efforts to incorporate microbial indicators into routine commercial soil analyses in Brazil. In this regard, soil enzyme determinations are more advantageous, as they can be performed directly on air-dried soil samples.