Iranian Journal of Soil and Water Research

Iranian Journal of Soil and Water Research

Sensitivity Assessment of Soil Organic Carbon Fractions to Fertilization Managements in a Wheat–Maize Rotation

Document Type : Research Paper

Authors
fardin manbari 1
Hossein Mirseyed Hosseini 2
Farhad Moshiri 3
Hossein Ali Alikhani 4
1 soil science, soil science department, faculty of agricultural Engineering , University of Tehran-Karaj- Iran
2 Soil Science Department, - Faculty of agricultural engineering - University of Tehran- Karaj- Iran
3 Soil and Water Research Institute (SWRI), Agricultural Research, Extension and Education Organization
4 professor of soil science, soil science department, faculty of agricultural Engineering , University of Tehran
10.22059/ijswr.2026.414212.670151
Abstract
This study evaluated the sensitivity of various soil organic carbon (SOC) fractions, compared and ranked carbon pools based on their responsiveness to combined chemical and organic fertilizer applications, and identified the most sensitive SOC fraction for monitoring SOC dynamics under semi-arid conditions. The experiment was conducted at the Soil and Water Research Institute of Iran's research station in Meshkin Dasht, Karaj, using a randomized complete block design with three replications. Over seven years (2017–2024), six fertilizer treatments were continuously applied: an unfertilized control (CO), chemical NPK fertilizer, cattle manure (MF), municipal waste compost (CF), cattle manure combined with 75% of the recommended nitrogen rate (MNF), and municipal waste compost combined with 75% of the recommended nitrogen rate (CNF). In 2024, after the wheat harvest, soil samples were collected from a depth of 0–30 cm to measure active and passive fractions, as well as microbial biomass carbon. Results showed that organic fertilizers and their combined use with chemical nitrogen fertilizers significantly increased various SOC fractions (P ≤ 0.01). Integrated treatments (especially CNF and MNF) exhibited the highest sensitivity indices (SI), calculated as the percentage increase in each carbon fraction relative to the reference soil (RE), while the control treatment showed the lowest values. The relative sensitivity of SOC fractions followed this order: dissolved organic carbon (DOC) (3.93) > particulate organic carbon (POC) (1.94) > light fraction organic carbon (LFOC) (1.61) > passive carbon (PC) (1.08) > microbial biomass carbon (MBC) (0.70) > permanganate-oxidizable carbon (POXC) (0.61). This demonstrates the efficiency of dynamic indices for short-term monitoring of fertilizer management effects. Therefore, dissolved organic carbon can be considered a rapid and cost-effective indicator for assessing changes in SOC dynamics within crop rotation systems.
Keywords
Fertilizer management, Soil labile carbon, Soil organic carbon, Wheat&ndash
Corn rotation
Subjects

Introduction

Soil organic carbon (SOC) plays a fundamental role in soil fertility and ecosystem sustainability. However, due to the slow response of total SOC to short-term management changes, modern approaches emphasize the evaluation of different carbon fractions (labile and stable pools). The concept of "carbon fraction sensitivity" has emerged as an index for assessing the responsiveness of various carbon pools to agricultural management practices. Despite the importance of this topic, comparative and quantitative studies on the relative sensitivity ranking of different SOC fractions in long-term experiments, particularly in semi-arid ecosystems, remain limited. Therefore, this study was conducted with three main objectives: (1) to determine the response of different soil organic carbon fractions to fertilizer treatments in a long-term wheat-maize rotation; (2) to assess and rank the sensitivity and relative sensitivity of each fraction; and (3) to identify the most sensitive carbon fraction for monitoring carbon dynamics under semi-arid climatic conditions. The main hypothesis was that labile carbon fractions would show higher sensitivity to integrated organic-mineral fertilizer management compared to total SOC.

Materials and Methods

This study was conducted as a seven-year long-term experiment (2017–2024) at the Research Station of the Soil and Water Research Institute of Iran in Meshkin Dasht, Karaj (semi-arid climatic conditions). The experiment was arranged in a randomized complete block design with three replications in permanent plots. Six fertilizer treatments were applied: unfertilized control (CO), chemical NPK fertilizer, cattle manure compost (MF), municipal waste compost (CF), cattle manure + 75% recommended nitrogen (MNF), and municipal waste compost + 75% recommended nitrogen (CNF). Additionally, an undisturbed plot (RE) was considered as reference soil to determine baseline conditions. After wheat harvest in the seventh year, soil samples were collected from the 0–30 cm depth, and various carbon fractions including total organic carbon (SOC), dissolved organic carbon (DOC), particulate organic carbon (POC), light fraction organic carbon (LFOC), microbial biomass carbon (MBC), permanganate-oxidizable carbon (POXC), and passive carbon (PC) were measured. The sensitivity index (SI) relative to reference soil (RE) and relative sensitivity index (RSI) relative to total SOC were calculated. Data were analyzed using analysis of variance and Tukey's mean comparison test at 1% and 5% probability levels.

Results

Analysis of variance results showed that fertilizer management had significant effects (P≤0.01) on all carbon fractions and grain yield. Integrated treatments (especially MNF and CNF) showed the highest increases in all carbon pools; the highest grain yield (6.94 Mg ha⁻¹) and maximum values of SOC, DOC, POC, and LFOC were recorded in these treatments. Pearson correlation analysis also showed highly significant positive correlations among fractions, with the highest correlation between DOC and LFOC (0.98) and DOC and MBC (0.94).

Based on the sensitivity index (SI) results relative to reference soil, integrated treatments CNF and MNF showed the highest responsiveness. The mean sensitivity index of fractions was ranked as follows: DOC (274.30) > LFOC (141.82) > POC (135.07) > PC (83.20) > SOC (76.10) > MBC (41.59) > POXC (40.20).

Also, based on the relative sensitivity index (RSI), where SOC was considered equal to 1, the responsiveness order of fractions to fertilizer management was: DOC (3.93) > POC (1.94) > LFOC (1.61) > PC (1.08) > MBC (0.70) > POXC (0.61).

These values indicate that dissolved organic carbon (DOC) was on average about 4 times more sensitive than total SOC to management changes.

Discussion and Conclusion

The findings of this long-term study confirmed that integrated application of organic and mineral fertilizers (simultaneous supply of energy through organic carbon and nutrients through chemical fertilizers) not only increases soil carbon stocks but also strongly enhances carbon cycle dynamics. The sensitivity ranking of fractions showed that these pools are not equally efficient for monitoring soil changes. Dissolved organic carbon (DOC), due to its mobile nature, independence from aggregate protection, and direct association with root exudates and microbial activity, was identified as the most sensitive and rapid indicator for short-term monitoring of fertilizer management effects. Following that, light fraction (LFOC) and particulate organic carbon (POC) showed high discrimination power as intermediate indicators. In contrast, MBC and POXC indices, despite their active nature, showed lower relative sensitivity compared to DOC and POC.

Practical Implications: The results of this study provide a scientific basis for the fertilizer industry and crop management. Practical recommendations include: prioritizing the production and consumption of integrated organic-mineral fertilizers, applying approximately 20 Mg ha⁻¹ of organic fertilizer annually for wheat-maize rotation in semi-arid regions, controlling salinity quality in municipal waste composts, and using dissolved organic carbon (DOC) and particulate organic carbon (POC) as practical, rapid, and cost-effective indicators for field evaluation of fertilizer product efficiency and soil health monitoring.

Funding

This research was financially and morally supported by the Soil and Water Research Institute and the Vice Chancellor for Research, University of Tehran.

Authorship contribution

Conceptualization, H. Mirseyed, F. Moshiri.; methodology, F. Mambari, H. Mirseyed, F. Moshiri.; All authors contributed as required to the conceptualization of the article and writing of the original and subsequent drafts.

Declaration of Generative AI and AI-assisted technologies in the writing process

There is no use of any type of artificial intelligence-based technology in this paper.

Data availability statement

 Data from this study are available upon request from the authors.

Acknowledgements

The author would like to thank the cooperation and assistance of the various departments of the Iranian Soil and Water Research Institute who played an important role in conducting this research. We would also like to express our gratitude to the Department of Soil Science and Engineering, University of Tehran, for providing the facilities and context for this scientific cooperation.

Ethical considerations

The authors avoided data fabrication, falsification, and plagiarism, and any form of misconduct.

Conflict of interest

The authors declare no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 57, Issue 7
September 2026
Pages 1559-1575