# Heterotrophic Soil Microbes at Work: Short-Term Responses to Differentiated Fertilization Inputs

## TL;DR

Organic fertilizers boost soil microbial activity more than inorganic ones, with high organic nitrogen content showing the strongest effect.

## Contribution

The study reveals that organic fertilizers, especially those rich in organic nitrogen, significantly enhance microbial density and enzymatic activity in soil.

## Key findings

- Organic fertilizers increased bacterial density and dehydrogenase activity more than inorganic ones.
- High organic nitrogen treatments showed the highest bacterial density (6.25 log10 CFU g−1 soil) and early DHA peaks.
- A strong positive correlation (r = 0.79) exists between bacterial density and dehydrogenase activity.

## Abstract

This study compared the effects of inorganic and organic fertilizers with different compositions on soil bacterial density and dehydrogenase activity (DHA), two key indicators of soil microbial activity. Results showed that organic fertilizers promoted higher microbial density and enzymatic activity than inorganic fertilizers. The strongest response occurred in the treatment containing a high proportion of organic nitrogen, where bacterial density reached 6.25 log10 CFU g−1 soil in May, followed by a decline in July. DHA, however, responded more rapidly, peaking earlier in April at 42.75 µg TPF g−1 soil, suggesting that enzymatic activity increased before microbial density increase. In the other organic fertilizer treatments, both parameters peaked in June, though at lower levels, and remained relatively stable or decreased slightly toward the end of the experiment. The results indicate that the quality and biodegradability of organic matter strongly influence microbial growth and metabolic activity. Overall, organic fertilization enhanced soil biological functioning compared with inorganic fertilization, primarily through the greater availability of carbon and nitrogen substrates that support microbial metabolism. These findings highlight the role of organic inputs in sustaining soil health and promoting long-term agricultural productivity.

The interaction between organic and inorganic nutrients, bacterial communities, and soil fertility has been well documented over time. Conventional agricultural systems heavily utilize both inorganic and organic fertilizers, each exerting distinct effects on soil microbial dynamics and plant growth. The objective of our experiments was to identify the most effective fertilization strategy for improving the biological quality of a microbiologically impoverished and low-productivity soil. To this end, four fertilization strategies were evaluated: (i) organic fertilizers characterized by a high content of organic carbon (Fertil 4-5-7—variant 1); (ii) organic fertilizers with 12% organic nitrogen from proteins (Bio Ostara N—variant 2) (iii) combined inorganic–organic fertilizers (P35 Bio—variant 3) and (iv) mineral (inorganic) fertilizers (BioAktiv—variant V4). This study aimed to assess the short-term effects of fertilizers with varying chemical compositions on the density of cultivable heterotrophic bacteria and their associated dehydrogenase (DH) activity in a petrocalcic chernozem soil containing pedogenic carbonates. Soil sampling was conducted according to a randomized block design, comprising four replicates per treatment (control plus four fertilizer types). The enumeration of cultivable bacteria was performed using Nutrient Agar and A2R Agar media, whereas dehydrogenase activity (DHA) was quantified based on the reduction of 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) to 1,3,5-triphenyl-tetrazolium formazan (TPF) by bacterial dehydrogenase enzymes. Marked differences were observed in both parameters between the plots amended with inorganic fertilizers and those treated with organic fertilizers, as well as among the organic fertilizer treatments of varying composition. The most pronounced increases in both bacterial density and dehydrogenase activity (DHA) were recorded in the plots receiving the fertilizer with a high organic nitrogen content. In this treatment, the maximum bacterial population density reached 6.25 log10 CFU g−1 dry soil after approximately two months (May), followed by a significant decline starting in July. In contrast, DHA exhibited a more rapid response, reaching its peak in April (42.75 µg TPF g−1 soil), indicating an earlier DHA activation of microbial metabolism. This temporal lag between the two parameters suggests that enzymatic activity responded more swiftly to the nutrient inputs than did microbial biomass proliferation. For the other two organic fertilizer variants, bacterial population dynamics were broadly similar, with peak densities recorded in June, ranging from 5.98 log10 CFU g−1 soil (V3) to 6.03 log10 CFU g−1 soil (V1). A comparable trend was observed in DHA: in V3, maximum DHA was attained in June (30 µg TPF g−1 soil), after which it remained relatively stable, whereas in V1, it peaked in June (24.05 µg TPF g−1 soil) and subsequently declined slightly toward the end of the experimental period. Overall, the temporal dynamics of bacterial density and DHA demonstrated a strong dependence on the quality and biodegradability of the organic matter supplied by each fertilizer. Both parameters were consistently lower under inorganic fertilization compared with organic treatments, suggesting that the observed increases in microbial density and activity were primarily mediated by the enhanced availability of organic substrates. The relationship between the density of culturable heterotrophic bacteria and dehydrogenase (DH) activity was strongly positive (r = 0.79), indicating a close functional linkage between bacterial density and oxidative enzyme activity. This connection suggests that the culturable fraction of the heterotrophic microbial community plays a key role in the early stages of organic matter mineralization derived from the applied fertilizers, particularly in the decomposition of easily degradable substrates.