Increase in N input for a longer time, soil salinity, pH, osmotic potential and ammonia fertilizer may affect soil microbial C, N and P adversely. Please consult the following literature that may provide you the reasons.

1. Microbial biomass, fungal and bacterial residues, and their relationships to the soil organic matter C/N/P/S ratios (2016). Geoderma 271, 115-123.

2. Long-term effects of mineral fertilizers on soil microorganisms - A Review. (2014). Soil Biology & Biochemistry 75, 54-63.(Geisseler and Scow)

Thanks so much, I am greatful

Just check your controls! We expereinced initially a lot of this problem during the fumigation-incubation.

If this is ruled out, then it could be excessive fertilization that could have depressed microbial activity compared to the control treatment. One interesting paper of Bardget et al. (1999) may be referred. Seasonal changes in soil microbial communities along a fertility gradient of temperate grasslands. Soil Biol. Biochem.

http://dx.doi.org/10.1016/S0038-0717(99)00016-4

If your control soil is similar to the treatment, then pH and salinity can be ruled out. Unless, you are testing across locations with varying soil types. Try to keep the control of the same location to remove this artefact that can creep in.

Thanks Desouza Blaise, I am analyzing soils of rainforest trees which is not fertilized but realized some of my controls greater than the fumegated