Soil microbial biomass-C after 28 years of sewage sludge application vs. mineral fertilizers

Sewage sludge (SS) contains high levels of organic matter and nutrients, which become a source of energy for soil microbial communities. Soil microbes in turn are considered a soil health indicator, due to their sensitivity to changes through agricultural management practices. Although there are many scientific studies demonstrating the benefits of recycling SS in agriculture, few studies have been carried out to evaluate the effects that long-term application of this residue can have on soil microbial biomass-C (SMBC). We aimed to understand the impact of sewage sludge application rates vs. mineral fertilizers on SMBC. A long-term experiment (28 years) was carried out under field conditions in Jaboticabal, SP, Brazil. A randomized complete block design with four treatments and five replications was used. The treatments consisted of conventional mineral fertilization (CMF) and three accumulated rates of SS (120, 240, and 427 Mg ha^-1, dry basis). After SS was applied, U. ruziziensis was sown in the 2024/25 crop year, and soil samples were then collected in layers 0–0.1 m and 0.1–0.2 m deep. The soil samples were identified, refrigerated, and prepared to determine the C present in the microbial biomass, using the fumigation and extraction method with K₂SO₄ extractant at 0.5 mol L^-1. The results were subjected to descriptive analysis. After verifying normality using the Shapiro-Wilk test, analysis of variance was performed at a 5% significance level (F test), and the means were compared using the Tukey test (p≤0.05) in R core software. There was no significant difference between treatments, in which the rate CFM presented the highest amount of SMBC (177.90 μg MBC g^-1 soil), not differing from the SS treatment at the depth of 0–0.1 m. At the depth of 0.1–0.2 m, the highest averages of C in the biomass were observed in the treatments 240 and 427 Mg ha^-1, being 228.66 and 206.17 μg MBC g^-1 soil, respectively. Therefore, the highest rates of SS were able to provide an increase in SMBC, which in turn contributes to the greater C stocks (increase soil C sequestration and the mitigation of greenhouse gas emissions) and nutrient cycling for plants. Our results are aligned with the concepts of circular economy and the ‘Sustainable Development Goals’ (SDGs) of the UN’s ‘2030 Agenda’, with emphasis on ‘SDG2’, ‘SDG3’ and ‘SDG13’.