Research Article

The Effect of Different Organic and Inorganic Fertilizer Applications on Soil Carbon Budget of the Soybean Crop Under Long-Term Field Conditions

Seher ILHAN, Veysi AKŞAHIN^* and Ibrahim ORTAS

Department of Soil Science and Plant Nutrition, Faculty of Agriculture, University of Cukurova, Adana, Turkey 

*Corresponding author e-mail: veysiaksahin@gmail.com

(Received: 29/08/2025; Revised: 13/10/2025; Accepted: 25/10/2025; Published: 20/12/2025)

ABSTRACT

The study aims to investigate the effects of different organic and inorganic fertilizer applications on SOC content.

A long-term field experiment was conducted in 1996 at the Cukurova University field site (37° 0’55.53″N 35° 21’20.56″E), in the same experiment, inorganic fertilizer (NPK), animal manure (25 t ha^-1), compost (25 t ha^-1), and mycorrhizal compost (10 t ha^-1) were applied as a regular procedure. To verify the hypothesis, the same trial area (long-term organic and inorganic fertilizer applications) was repeated with different plants (soybean planted) in 2023 and after harvest, soil samples were taken (from 0-15 and 15-30 cm soil depth) and total and soil organic carbon analysis was performed. The results showed that SOC concentrations in soybean-planted plots were measured at depths of 0-15 cm and 15-30 cm % 1.18 and % 1.11 as in control plots, respectively, while carbon concentrations at the same depths in animal manure-applied plots were 1.71% and 1.48%, respectively. Overall, more SOC was retained in soybean plots treated with compost and compost+mycorrhiza. Statistical analysis, which also accounted for differences between experimental subjects, revealed significant differences in total C and SOC concentrations. At the 0-15 cm soil depth, compared to the control, the contributions were 23% for mineral fertilizer, 38% for animal manure, 36% for compost, and 43% for compost + mycorrhiza. Similar patterns were observed with lower percentages at 15-30 cm soil depth. Spore counts were significantly higher than those obtained with the control and mineral fertilizers. The animal manure application counted 44 spores/10g of soil at depths of 0–15 cm and 40 spores/10g of soil at depths of 15–30 cm, respectively, while the mycorrhiza+compost application counted 77 spores/10g of soil and 33 spores/10g of soil, respectively. P₂O₅ was highest in the mineral fertilizer application (189.73 kg ha^-1) at the surface and in the animal manure application (141.50 kg ha^-1) at the lower depth. There is a positive correlation between spore counts and organic carbon values. This indicates that organic practices increase microbial activity, especially spore numbers. Increased microbial activity, as is well known, improves soil health, which indirectly reduces the adverse effects of climate change. Evaluation of organic fertilizers, including compost, compost + mycorrhiza, and animal manure, reveals significant benefits for sustainable agriculture and climate change mitigation.

Keywords: Soybean, Organic Carbon, Organic and inorganic fertilizers, Long-term experiment, Total Carbon