Organic Amendments Increase SOC in Semiarid Iraqi Soils (FTIR–PLS–RothC) - Bionatura journal

FTIR spectra exhibited clear and progressive differences in soil organic matter (SOM) functional groups across organic matter (OM) treatments and soil depths. In the surface layer (0–30 cm), increasing OM inputs intensified characteristic SOM-associated bands, including aliphatic C–H stretching (~2920–2850 cm⁻¹), aromatic/carboxyl C=O (~1630 cm⁻¹), and COO⁻ asymmetric stretching (~1420 cm⁻¹). Bands associated with polysaccharide structures and clay–organic complexes (1080–1000 cm⁻¹ and 790–670 cm⁻¹) were consistently present at all depths. Still, they decreased markedly in deeper layers, indicating reduced organic inputs and increased mineral dominance.

The 10% and 20% OM treatments displayed the strongest spectral responses, with clear enhancement of both labile (C–H stretching) and stabilised (C=O, COO⁻) carbon fractions, particularly in the 0–30 cm horizon.

The spectra display characteristic SOM and mineral-associated absorption bands, including aliphatic C–H stretching (2920–2850 cm⁻¹), aromatic/carboxyl C=O (~1630 cm⁻¹), COO⁻ asymmetric stretching (~1420 cm⁻¹), and Si–O/O/ O/clay-organic vibrations (1080–670 cm⁻¹). Peak intensities decrease with depth, indicating reduced organic inputs and increasing mineral dominance.

Slight increases in aliphatic and carbonyl functional groups are observed in surface soils compared with the control, reflecting modest SOM enrichment.

Surface soils show enhanced C–H and C=O bands, indicating early-stage SOM enrichment, while deeper horizons retain predominantly mineral spectral signatures.

Pronounced intensification of SOM-associated bands is observed in the 0–30 cm layer, differentiating this treatment from lower OM amendment levels.

Higher OM inputs substantially increase both labile (2920–2850 cm⁻¹) and stabilised (1630 cm⁻¹) SOM fractions, particularly in the surface layer.

The greatest increases in SOM-related absorption bands occur in the surface layer (0–30 cm), while deeper horizons remain dominated by mineral-associated features.

Organic matter inputs significantly increased surface soil organic carbon (SOC) relative to the untreated control (p < 0.05). SOC rose from 0.71% in the control plots to 1.52% under organic matter additions, 1.83%, and 2.11% under the 5%, 10% and 20% OM treatments, respectively. Similar trends were observed for total nitrogen (TN) and available phosphorus, indicating improved nutrient status under higher OM inputs.

Electrical conductivity remained within agronomically acceptable limits across all treatments, while soil pH decreased slightly with higher OM additions, consistent with organic acid release. Clay content was dominant at all depths, aligning with the strong SOM–mineral interactions observed in FTIR spectra.

Only SOC values explicitly obtained in the study are reported. Unmeasured depths are indicated with “–”. The data represent mean values from the long-term field experiment.

Partial Least Squares (PLS) regression accurately predicted SOC, TN, and other soil attributes using preprocessed FTIR spectra. Calibration and validation showed excellent model performance across most variables:

These findings confirm that PLS modelling is a robust, non-destructive approach for estimating SOC dynamics in semiarid soils amended with OM.

Metrics include R² for calibration and validation, root mean square error of calibration (RMSEC), root mean square error of prediction (RMSEP), bias, and ratio of performance to deviation (RPD). RPD values greater than 3 indicate excellent predictive accuracy.

RothC simulations closely reproduced the observed SOC trends across OM levels and soil depths. Predicted SOC values fell within −22% to +10% of measured values. Most carbon accumulation occurred in the top 30 cm, while deeper horizons showed limited changes, reflecting constrained organic matter movement and mineral protection at depth.

Humified and resistant carbon pools dominated long-term SOC stabilisation under 5–10% OM treatments, whereas lower OM levels resulted in greater contributions from labile pools.

Predicted SOC ranges and percentage deviations reflect model accuracy across horizons. A stronger model agreement is observed in the surface layer, where fresh organic inputs accumulate.

Climate variability strongly influenced organic matter decomposition and model behaviour. Annual fluctuations in temperature, rainfall, and relative humidity over the 14 years contextualise SOC responses to treatments.

Annual precipitation, temperature, and relative humidity illustrate the climatic constraints affecting SOC accumulation and decomposition in semiarid calcareous soils.

Strong agreement across datasets demonstrates the complementary strengths of FTIR–PLS modelling and RothC simulations for assessing long-term SOC dynamics.

Spectral evidence of SOM transformation and comparison with global FTIR studies
The enhanced intensities of aliphatic (2920–2850 cm⁻¹) and carbonyl (1630 cm⁻¹) bands under higher OM inputs are consistent with FTIR-based evidence from long-term manure trials in semiarid China and Spain, which also reported simultaneous accumulation of labile and stabilised SOM fractions ^1–3. The persistence of mineral-associated bands (1080–670 cm⁻¹) across depths supports the dominant role of clay–organic interactions in SOC stabilisation, a mechanism widely described for calcareous soils in Iran, Morocco, and Turkey ^4–6.

SOC response in the context of long-term global amendment trials
SOC gains in the surface layer (0–30 cm) followed patterns comparable to Mediterranean long-term trials, where moderate OM inputs often yield the highest increases in SOC due to optimal microbial turnover and aggregate stabilisation ^7–9. The SOC increment from 0.71% (control) to 1.52–2.11% under 5–20% OM aligns with studies in Spain and northern China, which observed SOC increases of 150–250% over 10–20 years under similar OM regimes ^{10, 11.}

Comparison of PLS results with international IR-chemometric studies
The exceptionally high RPD values (up to 41) obtained in this study surpass those commonly reported in global calibration datasets, where RPD values typically range from 2.5 to 10 ^15–18. This suggests that the biochemical contrast induced by sustained OM additions enhances spectral predictability, a phenomenon reported only in a few long-term European monitoring networks.

The identification of aliphatic, carbonyl, and clay-associated wavelengths as the primary predictors agrees with chemometric studies from Australia, Brazil, and Italy, where these regions consistently emerge as dominant predictors of SOC and nitrogen ^19–21.

Novelty: This is the first study in Iraq and among very few globally to:

RothC performance compared with international modelling studies
RothC reproduced surface SOC trends with deviations largely within −22 to +10%, closely matching evaluations from North Africa, China, and Europe, which report typical deviations of ±10–25% for semiarid systems ^22–24. The model’s limited performance at depth is consistent with the lack of measured SOC for calibration and with the known constraints of RothC in carbonate-rich soils, where decomposition dynamics differ from temperate systems ²⁵.

Integrated interpretation and regional significance
By triangulating FTIR functional groups, PLS predictive strength, and RothC carbon pools, this study demonstrates that long-term OM amendments enhance SOC mainly through increasing SOM fractions that undergo both biochemical transformation and mineral association.

Limitations and research needs
The primary limitation is the lack of measured SOC values at deeper horizons—common in field studies spanning decades—which restricts model calibration at depth. Complementary SOM characterisation techniques, such as ¹³C-NMR or pyrolysis-GC/MS, would enhance the interpretation of stabilised fractions. Future research should integrate microbial community dynamics, mineral surface reactivity, and depth-explicit SOC modelling to refine predictions for long-term carbon sequestration in semiarid agroecosystems.