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Article

Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic

by
Ondřej Sedlář
*,
Jiří Balík
,
Jindřich Černý
,
Pavel Suran
,
Martin Kulhánek
and
Tetiana Bihun
Department of Agro-Environmental Chemistry and Plant Nutrition, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague, 16500 Praha, Czech Republic
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(6), 1277; https://doi.org/10.3390/agronomy15061277
Submission received: 11 April 2025 / Revised: 16 May 2025 / Accepted: 21 May 2025 / Published: 22 May 2025
(This article belongs to the Special Issue Humic Substances: Chemistry and Multidimensional Role in Agricultural Systems and Pollution Management)
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Abstract

:
Humic substances affect both soil fertility and carbon sequestration. This study aimed to evaluate the effect of straw return on the quality of soil organic matter on arable land commonly farmed by private farmers at 65 different sites between 2012 and 2022 in the Czech Republic (central Europe). In this study, most of the carbon supply was applied in straw (67% of the carbon input on average). No significant correlation between the total carbon input and both parameters of soil organic matter quality and soil organic carbon content was found. The ratio of optical absorbance at 465 to 665 nm (E4/E6) and humification index correlated most significantly with organic fertilization. However, the E4/E6 ratio was more significantly affected by the altitude of the experimental site compared to the organic fertilization. When the weighted mean C/N ratio of organic fertilizers applied exceeded the value of ca. 65, there was a decrease in the E4/E6 ratio in fluvisols and luvisols and an increase in the humification index in loamy soils, sandy loamy soils, and silt loamy soils compared to the C/N ratio ≤ 50. Leguminous cultivation revealed no significant effect on soil organic matter quality.

1. Introduction

The role of soil processes in global carbon and climate models is entering a period of growing attention and increasing maturity. These activities in turn reveal the severity of soil-related issues at stake for the remainder of this century—the need to rapidly regain a balance to the physical and biological processes that drive and maintain soil properties, and the societal implications that will result if we do not [1].
Managing soil organic matter and increasing the content of humic substances in soils the right way will be of central importance for sustainable agriculture [2] because organic amendments increase soil aggregate stability, nutrients, and moisture content [3]. Furthermore, despite the limited potential of organic amendments to mitigate climate change in low clay soil, even small carbon accrual in these soils may be important for climate change adaptation, food security, and soil health improvements [4].
Due to the central role of humic substances in the high soil organic carbon stocks in soils, there are efforts to add humic substances to soils [2,5]. Humic substances are classified as humic acids, fulvic acids, and humins based on their solubility in water, acidic, or alkaline solutions. The ability of humic acids to withstand degradation for long periods and their amphiphilic properties enable them to form complex cations. Humic acids can positively affect soil physical, chemical, and biological characteristics, including texture, structure, water-holding capacity, cation exchange capacity, pH, soil carbon, enzymes, nitrogen cycling, and nutrient availability [6].
The potential of carbon sequestration in soil is determined by its carbon storage capacity and its stability in soil [7]. A more stable complex of humic acids with soil particles has long-term implications in soil carbon sequestration for posterity [8,9]. On the contrary, more labile carbon can easily be lost to the atmosphere as CO2 [10]. But how to quantify the stability of organic matter? Wang et al. [11] state that an increase in soil organic carbon, humin carbon, humic acid carbon, the ratio of humic acid carbon to fulvic acid carbon (CHA/CFA), and the percentage of humic acid carbon (CHA/CHS) leads to an improvement in soil organic matter structure and stability. Another parameter of soil organic matter quality is the ratio of optical absorbance at 465 to 665 nm (E4/E6). A lower E4/E6 ratio reveals greater stability and humification of humic carbon [8,9].
Carbon stability expressed by the DOC/SOC ratio is an indicator of dissolved organic carbon (DOC) desorption or soil organic carbon (SOC) stability [12]. A lower DOC/SOC ratio could indicate reduced carbon loss, due to the strong mobility of DOC. A lower proportion of labile carbon in the SOC could enhance the SOC accumulation and stabilization [13].
All the above-mentioned soil organic matter quality parameters could be simply expressed based on our analyses, and their suitability was assessed in on-farm trials. The on-farm trials were based solely on conventional agricultural practices because, as Datta et al. [9] state, conventional agriculture systems have the potential to store lower amounts of organic carbon in the stable humic acid fraction and maintain the sustainability of the cereal-based system compared to conservation agriculture. Furthermore, labile and humified organic matter are better protected in grassland soils and are consequently less vulnerable to mineralization compared to arable land [14].
Some materials, such as those produced by carbonization, have high sorption capacity and the ability to stabilize soil carbon [15], but we focus on the quality of soil organic matter in our study. Although carbon retention of straw and green manure is lower in comparison with animal-derived amendments [4,5], both straw and manure amendments are responsible for soil organic carbon sequestration [16]. Sadly, agriculture in the Czech Republic is characterized by an imbalance between livestock and crop production due to reduced animal husbandry (there are areas completely devoid of livestock production), which leads to low organic manure inputs [17]. Horacek et al. [18] compared archive samples from the 1960s with the present ones collected at the same sites in chernozems also in the Czech Republic revealing deterioration of soil qualitative parameters (lower humic acid to fulvic acid ratio, higher E4/E6 ratio, and lower humification rate) over time. On the other hand, soil organic carbon content showed contrary results: the amount of total soil organic matter at the same sites is higher now than it used to be about 50 years ago. It can be concluded that the current decline in SOM quality in chernozems is partly compensated by the higher accumulation of SOM in the soils [18].
As straw is usually the dominant carbon-containing fertilizer in the Czech Republic, often completely replacing animal manures, the effect of straw return on soil organic carbon stability was assessed in fields commonly farmed by private farmers. A prerequisite for achieving this goal was to find a parameter of soil organic carbon stability significantly related to organic fertilization under different soil conditions. The quality of soil organic matter was expressed based on the fractionation of soil organic carbon.

2. Materials and Methods

The effect of an 11-year (2012–2022) incorporation of straw into soil on organic carbon stability was studied.

2.1. Study Area

On-farm trials were performed in the Czech Republic (central Europe). The studied land consists of a set of 69 observation plots on arable land with various soil-climatic conditions (Figure 1 and Figure 2), because carbon sequestration is strongly influenced by soil pH, climate, content of mineral nitrogen, etc. The plots were located on land conventionally farmed by private farmers. The use of both organic and mineral fertilizers was recorded by farmers throughout the experiments. Organic fertilizer rates were converted to carbon rates. Mineral fertilizers were used based on the experience of individual farmers, following current legislation.
At the beginning of vegetation in 2023, three subsamples were taken within 15 m of the target point in each field. Each subsample consisted of seven partial probes. Only topsoil samples (0–30 cm) were collected because Song et al. [19] state that humic acids play an important role in soil organic carbon stock in the topsoil under long-term fertilization. Although subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC concentrations, a depth of ploughing determines the SOC sequestration by enlarging the storage space for SOC-rich material [20]. Skadell et al. [21] state that approximately 20% of the impact of agricultural management on SOC stocks occurs in the subsoil (30–50 cm). The soil and climate conditions of the experimental sites are presented in Table 1, and the quartile boundaries of 25% and 75% are also provided to give an idea of the range of values.

2.2. Chemical Analysis of Soil

Soil organic carbon (SOC) was measured using a modified Dumas combustion method, employing a CHNS Vario MACRO Cube Analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany) after carbonate digestion with hydrochloric acid and subsequent drying [22].
For dissolved organic carbon (DOC) determination, a 0.01 mol/L CaCl2 (Lach-Ner, s.r.o., Neratovice, Czech Republic) extraction according to [23] was used. The DOC content in soil samples was analyzed using segmental flow analysis with infrared detection, utilizing a Skalar Plus System (Skalar Analytical B.V., Breda, The Netherlands).
To calculate the carbon input, the following carbon contents in fresh matter were used: cereal and canola straw 42% C, beet tops 5% C, digestate from biogas stations 2.3% C, farmyard manure 9% C, slurry 0.03% C, compost 12.5% C, and sewage sludge 32% C. These values, which are usual in the conditions of the Czech Republic, were compiled from legislative materials of the Crop Research Institute in Prague [24].

Fractionation of Soil Organic Carbon

Soil organic carbon fractions were determined in a mixture of 0.1 mol/L Na4P207 (Thermo Fisher Scientific Chemicals Inc., Waltham, MA, USA) and 0.1 mol/L NaOH solution (Lach-Ner, s.r.o., Neratovice, Czech Republic). The E4/E6 absorbance ratio was measured directly from the filtrate using a Lambda 25 visible light spectrophotometer (PerkinElmer, Inc., Shelton, CT, USA), based on specific absorbance values at 465 nm and 665 nm [19].
The carbon content in humic substances (CHS) was measured from the neutralized solution. Carbon in fulvic acids (CFA) was assessed using the acidified filtrate. The remaining precipitate, containing humic acid carbon (CHA), was dissolved in hot 0.05 mol/L NaOH. Before iodometric titration of all samples, the dry residue obtained by evaporation was dissolved in a mixture of 0.067 mol/L K2Cr2O7 (PENTA s.r.o., Prague, Czech Republic) and concentrated H2SO4 (PENTA s.r.o., Prague, Czech Republic). A more detailed description of the method is given by Sedlar et al. [5].

2.3. Computations

Humification indices and carbon lability were calculated as follows [25,26,27,28]:
degree of polymerization: CHA/CFA
humification index: HI = CHA/SOC
humification rate: HR = CHS/SOC
percentage of humic acid carbon: PHA = CHA/CHS × 100 (%)
C lability = DOC/SOC × 100 (%)
where CFA is the fulvic acid carbon (g/kg), CHA is the humic acid carbon (g/kg), CHS is the humic substances carbon (g/kg), SOC is a soil organic carbon (g/kg), and DOC is the dissolved organic carbon (g/kg).

2.4. Statistical Analysis

A one-way analysis of variance (ANOVA) using the Tukey test was used. Pearson’s correlation coefficients were employed to examine the relationships between the studied variables within each soil type and texture category. A p-value of 0.05 or lower was regarded as statistically significant. Data analysis was performed using Statistica ver. 13 software (TIBCO Software Inc., Palo Alto, CA, USA).

3. Results

A correlation matrix (Table 2) shows that the total amount of carbon supplied in organic fertilizers had no significant effect on the qualitative parameters of soil organic matter (SOM). Straw accounted for the largest proportion of the mass of organic fertilizers applied and its rate correlated significantly with the E4/E6 absorbance ratio. The proportion of carbon applied in straw correlated significantly with the humification index and the E4/E6 ratio. The weighted average of the C/N ratio of organic fertilizers correlated significantly with the CHA/CFA ratio and humification index.
The organic fertilization most significantly affected the E4/E6 ratio and humification index. However, all significant correlations between fertilization and SOM quality were only weak.
Furthermore, the correlation matrix was focused on the relationships between the SOM quality and altitude, average annual air temperature (2012–2022), and average annual rainfall (2012–2022). Although the E4/E6 ratio correlated significantly with the straw application, this parameter was more significantly affected by altitude (moderate correlation). The E4/E6 ratio correlated with annual air temperature as well as with straw C input. The CHA/CFA ratio showed a weak but statistically significant correlation with the weighted mean C/N ratio of the organic fertilizers applied, as well as with annual air temperature and rainfall. The humification rate significantly correlated only with the mean C/N ratio of organic fertilizers, with no influence from site-specific characteristics. The humification index exhibited the strongest correlation with the C/N ratio of organic fertilizers, while annual air temperature, annual rainfall, and proportion of carbon applied in straw correlated less significantly with the humification index. Soil organic carbon content correlated significantly only with annual air temperature, and this correlation was weak and negative.
Neither the percentage of humic acid carbon (PHA) nor the DOC/SOC ratio correlates with the fertilization or selected site conditions.
Table 3 giving the results of univariate significance tests shows that the E4/E6 ratio and CHA/CFA ratio were more significantly affected by soil type, while the humification index differed more significantly among soil texture categories compared to the soil type. A variation in the humification rate was equally significant among both soil types and categories of soil texture. Therefore, the relationships between soil organic matter and the organic fertilization method were investigated separately for soil types in the case of the E4/E6 ratio, CHA/CFA ratio, and humification rate, and separately for soil texture categories in the case of the humification index and humification rate.
To assess the effect of organic fertilization, values of parameters of soil organic matter quality were compared between the two levels of organic fertilization. Both these levels of organic fertilization were chosen so that their average values were as similar as possible across soil types and soil texture categories, respectively (Table 4 and Table 5).

3.1. SOM Quality in Individual Soil Types

As follows from Table 4, the straw return improved the E4/E6 ratio: an average carbon rate delivered in straw of 11.0–11.6 t/ha/11 years resulted in a reduction in the E4/E6 ratio compared to an average rate of 1.49–3.14 t C/ha/11 years in straw. This phenomenon was observed in cambisols and luvisols.
In fluvisols and chernozems, a reduction in the E4/E6 ratio was found when, on average, all the carbon supplied in organic fertilizer came from the straw. When the proportion of carbon supplied by straw averaged 54.5–69.2%, the E4/E6 ratio was increased in comparison with all carbon supplied in straw, which is undesirable from the point of view of soil organic matter quality.
A weighted mean C/N ratio of organic fertilizers applied of 42.2–50.1 resulted in an increased E4/E6 ratio compared to an average C/N ratio of ca. 70 in fluvisols and luvisols.
The humification rate differed least significantly between the different levels of organic fertilization (Table 4 and Table 5).

3.2. SOM Quality in Individual Soil Texture Categories

As shown in Table 5, the humification rate differed less frequently between the different levels of organic fertilization compared to the humification index.
An average straw carbon input of 0.32–1.93 t/ha/11 years resulted in a decreased humification index compared to an average input of 13.1–14.0 t C/ha/11 years in clay loamy soils and sandy loamy soils (Table 5). However, when the carbon applied in straw achieved a proportion ≤ 33.2%, the humification index was reduced compared to applying almost all the carbon in straw.
A weighted mean C/N ratio of organic fertilizers applied less than 42.8 resulted in a decrease in the humification index compared to the C/N ratio of 66.7–75.0 except for clay loamy soils.
As leguminose were not grown at all studied sites, two groups of plots with similar site characteristics were selected to test the effect of leguminous cultivation on soil organic matter quality (Table 6): one group of plots had leguminous crops as a part of the cropping system, the other group of plots did not. Fertilizer inputs differed between the two groups of plots only in the amount of straw applied and annual rainfall. Yet, no difference in soil organic matter quality was found between the plots with and without leguminous cultivation.

4. Discussion

Although we do not have values of soil properties from the beginning of the on-farm trials, it can be assumed that after 11 years of the trials, the effect of previous organic fertilization is negligible. Fernandez et al. [29] state that despite important differences in the initial chemical and thermal properties of two types of sewage sludges, the chemical and thermal properties of the soil humic acids were quite similar to one another after 3 years of amendment. Gondek et Mierzwa [30] recorded higher values of the CHA/CFA ratio in soils treated with compost compared to only mineral NPK treatment 1 year after the fertilization. However, 3 years after the application, no differences between the treatments were found [30].
Since straw accounted for the largest proportion of the weight of organic fertilizers applied in our study, only the effect of straw application was assessed. Low nutrient availability in green manure and straw amendments, as indicated by high phosphatase and N-acetylglucosaminidase activities, may indicate a reduction in carbon retention of organic matter inputs due to the microbial mixing of nutrients with plant-derived amendments [4]. Nevertheless, plant-based fertilizers improve soil health (respiration-CO2), organic matter content, and nutrient availability [31].
Probably due to low carbon input in compost, manure, slurry, digestate, beet tops, and sewage sludge, the application of these fertilizers had no positive effect on soil organic matter. In contrast, Sedlar et al. [5] found in long-term field experiments that the humification index, humic acid carbon content, and the CHA/CFA ratio were more frequently elevated in the manure-fertilized treatment compared to the straw-alone treatment. This discrepancy may be explained by the fact that in the study of Sedlar et al. [5] conducted under similar soil-climatic conditions, fully 78% of carbon was applied in manure compared to carbon supplied in straw. However, in our study, over 11 years, an average of 7.16 t C/ha was applied in straw, while in manure it averaged 2.14 t C/ha, i.e., only 28% of carbon was supplied with farmyard manure compared to the straw. In addition, on 38 of the 69 plots, farmyard manure was not applied at all. Liang et al. [32], who reported conclusively higher soil organic carbon content after applying decomposed sheep manure compared to straw return in a 4-year field trial, applied almost the same amount of carbon in straw as in farmyard manure (24.2 t C/ha and 22.5 t C/ha, respectively), which is again a much higher farmyard manure input than in our on-farm trials. On the other hand, the incubation experiment of Zhang et al. [33] with an application of 3 wt% of cow manure achieved the same soil organic carbon content with the same amount of applied maize straw.
No significant correlation between the total carbon input and both parameters of SOM quality and soil organic carbon content was found. Also, Bai et al. [34] found different effects of different organic amendments on the stability of soil organic carbon. Castellano et al. [35] point out that the amount of microbial residue stabilization is ultimately determined by physicochemical protection mechanisms operating within the mineral soil matrix. Thus, litter quality should not affect the stable SOM stocks or the rate of SOM stabilization when a soil has no additional capacity for SOM stabilization (i.e., when the soil is C-saturated). Indeed, some studies report an effect of litter quality on total and/or physicochemically stabilized SOC pool size or the litter-C to SOM-C conversion rate, while others do not. Therefore, based on our results, carbon saturation of the studied soils cannot be assumed.
The most significant differences between the organic fertilization levels and SOM quality parameters were found in the case of the humification index, followed by the E4/E6 ratio. In addition, the E4/E6 ratio was more significantly affected by organic fertilization than the CHA/CFA ratio. These findings are consistent with those of Oktaba et al. [36], who observed a significant influence of various fertilizers on the E4/E6 ratio but reported no impact on the CHA/CFA ratio. Balik et al. [28] also did not observe a significant effect of organic fertilization in combination with different doses of mineral fertilizers on the CHA/CFA ratio or humification rate, but the effect of fertilization on the E4/E6 ratio was significant.
Hevia et al. [37] reported that the E4/E6 ratio did not correlate with the climatic conditions nor with soil texture or management. This phenomenon was not confirmed in our study because the effect of altitude on the E4/E6 ratio was stronger than the effect of organic fertilization. Furthermore, the E4/E6 ratio correlated as strongly with organic fertilization as with the annual temperature in our study. On the other hand, the results of Li et al. [38], who found that the E4/E6 ratio increased with increasing rainfall, were not confirmed by our results.
The CHA/CFA ratio correlated with altitude and annual rainfall as significantly as with the weighted mean C/N ratio of organic fertilizers applied, but more significantly than with other expressions of organic fertilization. The CHA/CFA ratio and humification index negatively correlated with annual rainfall, concurring with the findings of Larionova et al. [39] and Radmanovic et al. [40]. Blonska and Lasota [41] recorded a decrease in the CHA/CFA ratio along with the rising soil moisture, which may suggest an increase in carbon mobility in soils.
Higher values of the weighted mean C/N ratio of organic fertilizers applied led to a decrease in the E4/E6 ratio in fluvisols and luvisols and an increase in both the humification index and percentage of humic acid carbon in loamy soils, sandy loamy soils, and silt loamy soils. This finding complies with the results of Sedlar et al. [5], who recorded a significant effect of the weighted mean C/N ratio of organic fertilizers applied.
Maize straw return also led to a significant increase in the CHA/CFA ratio and humification index at the 0–20 cm soil depth, regardless of the nitrogen rate in mineral fertilizers [42,43,44]. In a winter wheat and summer maize rotation, straw return significantly enhanced the humification index [45]. However, these findings were not confirmed for the CHA/CFA ratio by our results.
Wu et al. [46] state that straw addition largely accelerated soil organic carbon mineralization and resulted in a positive priming effect, primarily because of the increased microbial biomass and β-glucosidase and cellobiohydrolase activities. However, the newly formed straw-derived soil organic carbon overcompensated soil organic carbon losses, resulting in a net soil carbon gain (positive carbon balance) [46]. Although Rahman et al. [3] state that labile carbon was found significantly higher in rice straw treatment than that of poultry manure and cow dung, there were no significant differences in the carbon stock, carbon sequestration index, and carbon sequestration rate. Both rice straw and farmyard manure increased SOC stocks in the 0–20 cm layer, but straw achieved lower carbon sequestration efficiency compared to the farmyard manure treatment, especially in N-deficient soil in 3-year field trials in China [16]. Either way, Koishi et al. [4] state that in a stockless management of SOC, cereal straw restitution offers a viable alternative solution to animal manure to increase and stabilize soil organic carbon.
Banach-Szott et al. [47] highlight the influence of soil properties on humification. Therefore, the investigated soils were divided according to soil types. However, not only soil type but also soil texture significantly affects the quality of soil organic matter [47,48,49]. Sand content might be an important factor for controlling the characteristics and distribution of SOM in soil profiles [50].
In our study, the E4/E6 ratio and CHA/CFA ratio were more significantly influenced by soil type, while the humification index differed more significantly between soil texture categories compared to the soil type. The variation in the humification rate was equally significant among both soil types and categories of soil texture.
Qualitative parameters of the SOM, soil texture, and mineralogical composition play a very important role in carbon fixation in soil. Furthermore, the quality of soil organic matter appears to depend mainly on the mineralogy of the soil [51].
A lower E4/E6 ratio reflects a greater ratio of humic acids to fulvic acids [52], which is consistent with the findings of Balik et al. [28], who recorded a strong correlation of the E4/E6 ratio with the humification index and humic acid carbon in luvisols. The lowest values of the E4/E6 ratio of 4.12–4.62 were found in chernozems in our study. The limit for high-quality humic substances is less than 4; the range of the E4/E6 ratio of 4.2–4.7 indicates a middle quality of soil organic matter [53].
Leguminous cultivation can significantly increase carbon and nitrogen preservation [54]. Arlauskiene et al. [55] recorded an increase in stable humic substances using the mass of legume swards in combination with cereal straw. Cultivation of leguminous plants positively influences humification. Arenosols may be used in crop rotation with approximately 40% leguminous plants to maintain a stable humus balance [56]. In our study, which was, however, carried out at just 24 sites, organic fertilization differed only in the amount of applied straw and annual rainfall. Despite this, there was no significant difference in soil organic matter quality between plots with and without leguminose in crops grown. Regarding the soil organic matter quality, it can be concluded that straw can compensate for the absence of leguminose in crops grown.

5. Conclusions

Although there are numerous methods to determine the quality of soil organic matter, only organic carbon fractionation was used in our study. Based on 11-year observations of conventionally farmed arable land, the following conclusions were expressed:
  • No significant correlation between the total carbon input and both parameters of SOM quality and soil organic carbon content was found.
  • Neither a percentage of humic acid carbon (PHA), carbon stability (DOC/SOC ratio), nor soil organic carbon (SOC) content correlated significantly with organic fertilization expressed by straw carbon input, proportion of carbon applied in straw, and weighted mean C/N ratio of organic fertilizers applied. Conversely, the E4/E6 ratio and humification index (HI) showed the strongest correlation with organic fertilization. However, the E4/E6 ratio was most significantly affected by the altitude of the experimental site.
  • The E4/E6 ratio and CHA/CFA ratio were more significantly different among soil types (cambisol, fluvisol, chernozem, and luvisol), while the humification index differed more significantly among soil texture categories (clay loam, loam, sandy loam, and silt loam) compared to the soil type. The variation in the humification rate was equally significant among soil types and among categories of soil texture.
  • Increased average doses of carbon applied in straw from less than 3.3 t C/ha/11 years to ca. 12.0 t C/ha/11 years caused an increase in the humification index in clay loamy soils and sandy loams and a decrease in the E4/E6 ratio in cambisols and luvisols.
  • When almost all carbon (97–100%) was applied in straw, a decrease in the E4/E6 ratio was observed in fluvisols and chernozems compared to the average carbon applied in straw of 0–70%. An increase in the proportion of carbon applied in straw from 0–40% to 97–100% resulted in an increase in the humification index in loamy soils and sandy loamy soils.
  • When the weighted mean C/N ratio of the fertilizers applied exceeded the value of ca. 65, there was a decrease in the E4/E6 ratio in fluvisols and luvisols and an increase in the humification index in loamy soils, sandy loamy soils, and silt loamy soils compared to the C/N ratio ≤ 50.
  • Comparison of 12 plots with an average of 33.5% leguminose content in the crop rotation with 12 plots completely without leguminose cultivation did not reveal significant differences in soil organic matter quality, i.e., E4/E6 ratio, CHA/CFA ratio, humification index, and humification ratio.

Author Contributions

Conceptualization, O.S.; methodology, J.Č. and J.B.; validation, M.K. and J.B.; investigation, T.B. and P.S.; data curation, J.Č.; writing—original draft preparation, O.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Czech Ministry of Agriculture through the research project QK21010124, “Soil Organic Matter—Evaluation of Selected Quality Parameters”, and the project QK23020056, “Development and Verification of Model Systems for Long-Term Carbon Sequestration in the Czech Republic”.

Data Availability Statement

All data can be obtained from the corresponding author upon request.

Acknowledgments

We extend our gratitude to the team at the Central Institute for Supervising and Testing in Agriculture in Brno for their assistance in coordinating the soil sampling.

Conflicts of Interest

The authors state that there are no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Agronomy 15 01277 g001
Figure 1. Monitored locations distribution in the Czech Republic with colour differentiation of soil type: chenozem—black, luvisol—red, fluvisol—blue, and cambisol—yellow.
Figure 1. Monitored locations distribution in the Czech Republic with colour differentiation of soil type: chenozem—black, luvisol—red, fluvisol—blue, and cambisol—yellow.
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Figure 2. Monitored locations distribution in the Czech Republic with colour differentiation of soil texture: loam—black, silt loam—brown, clay loam—red, and sandy loam—yellow.
Figure 2. Monitored locations distribution in the Czech Republic with colour differentiation of soil texture: loam—black, silt loam—brown, clay loam—red, and sandy loam—yellow.
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Table 1. Characteristics of experimental conditions (65 sites).
Table 1. Characteristics of experimental conditions (65 sites).
VariableQuartile 25%Quartile 75%AverageMedian
pH (CaCl2)5.757.106.346.20
altitude (m)323299240406
average daily air temperature (°C)9.589.589.0810.12
average annual precipitation (mm)568564511622
CEC (mmol+/kg)207190162244
CEC—cation exchange capacity in BaCl2.
Table 2. Relations between fertilization, site characteristics, and quality of soil organic matter—a correlation matrix (number of cases 69).
Table 2. Relations between fertilization, site characteristics, and quality of soil organic matter—a correlation matrix (number of cases 69).
C Input TotalStraw C Input% C Input in StrawWeighted C/N Ratio of org. fert.AltitudeAnnual Air Temperature Annual Rainfall
straw C input0.663 ***
% C input in straw−0.0200.479 ***
weighted C/N ratio of org. fertilizers0.0880.521 ***0.909 ***
altitude0.051−0.172−0.345 **−0.380 **
annual air temperature −0.0370.1210.316 *0.318 *−0.419 ***
annual rainfall −0.082−0.055−0.163−0.1480.068−0.505 ***
E4/E6−0.147−0.352 **−0.279 *−0.2540.563 ***−0.360 **0.134
CHA/CFA0.0240.1680.1860.261 *−0.288 *0.238−0.295 *
HR = CHS/SOC0.0580.1900.1590.279 *0.0730.157−0.204
HI = CHA/SOC0.0060.2430.312 *0.380 **−0.0320.276 *−0.316 *
PHA = CHA/CHS−0.1190.0300.1340.064−0.1250.116−0.045
DOC/SOC−0.111−0.0870.0760.0130.0510.1610.138
SOC0.0620.0960.0620.0410.099−0.270 *−0.123
The r-values marked with asterisks are significant at the significance levels * p < 0.05, ** p < 0.01, and *** p < 0.001. C—carbon, CHA—humic acids carbon, E4/E6—absorbance ratio at the wavelengths of 465 and 665 nm, CHA/CFA—humic to fulvic acid carbon ratio, HR—humification rate, HI—humification index, SOC—soil organic carbon, PHA—percentage of humic acid carbon, CHS—humic substances carbon, DOC—dissolved organic carbon.
Table 3. Univariate significance tests for differences in soil organic matter parameters among soil types (chernozem, cambisol, luvisol, fluvisol) and soil texture categories (loam, sandy loam, clay loam, and silt loam). The number of cases was 141.
Table 3. Univariate significance tests for differences in soil organic matter parameters among soil types (chernozem, cambisol, luvisol, fluvisol) and soil texture categories (loam, sandy loam, clay loam, and silt loam). The number of cases was 141.
E4/E6CHA/CFAHR = CHS/SOCHI = CHA/SOC
Soil type*********
Soil texture**n.s.****
The asterisks mark the significance levels * p < 0.05, ** p < 0.01, and *** p < 0.001, n.s.—non significant effect. E4/E6—absorbance ratio at the wavelengths of 465 and 665 nm, CHA/CFA—humic to fulvic acid carbon ratio, HR—humification rate, HI—humification index, SOC—soil organic carbon.
Table 4. Soil organic matter quality in two levels of carbon applied in straw, the proportion of carbon applied in the straw, and the weighted mean C/N ratio of organic fertilizers applied on plots with different soil types (n—number of cases in each group).
Table 4. Soil organic matter quality in two levels of carbon applied in straw, the proportion of carbon applied in the straw, and the weighted mean C/N ratio of organic fertilizers applied on plots with different soil types (n—number of cases in each group).
Average Straw C Input (t/ha/11 yrs)nE4/E6CHA/CFAHR
cambisol1.49156.64 b0.925 a0.297 a
11.6155.50 a1.123 a0.299 a
fluvisol2.52155.05 a1.366 a0.268 a
16.3125.51 a1.166 a0.261 a
chernozem3.33124.28 a1.568 a0.287 a
14.9154.52 a1.669 a0.336 b
luvisol3.14276.37 b0.890 a0.277 a
11275.85 a0.891 a0.289 a
Average % C applied in straw (%)
cambisol40156.20 a0.846 a0.313 a
99155.94 a1.202 b0.283 a
fluvisol54.5155.53 b1.341 a0.274 a
100124.92 a1.197 a0.254 a
chernozem69.294.49 b1.347 a0.275 a
100124.12 a1.155 a0.332 a
luvisol43.3186.48 a0.971 a0.300 a
97.1126.04 a0.832 a0.291 a
Average C/N ratio of fertilizers
cambisol35.6125.94 a0.869 a0.277 a
69.6155.93 a1.217 b0.315 a
fluvisol50.1155.53 b1.341 a0.274 a
75124.92 a1.197 a0.254 a
chernozem51.4124.40 a1.244 a0.284 a
75124.42 a1.683 a0.315 a
luvisol42.2276.35 b0.858 a0.282 a
67.9215.80 a0.891 a0.288 a
Mean values within the column that share the same letters do not differ significantly at the p < 0.05 level. E4/E6—absorbance ratio at the wavelengths of 465 and 665 nm, CHA/CFA—humic to fulvic acid carbon ratio, HR—humification rate HR = CHS/SOC, SOC—soil organic carbon, CHS—humic substances carbon.
Table 5. Soil organic matter quality in two levels of carbon applied in straw, the proportion of carbon applied in the straw, and the weighted mean C/N ratio of organic fertilizers applied on plots with different soil textures (n—number of cases in each group).
Table 5. Soil organic matter quality in two levels of carbon applied in straw, the proportion of carbon applied in the straw, and the weighted mean C/N ratio of organic fertilizers applied on plots with different soil textures (n—number of cases in each group).
Average Straw C Input (t/ha/11 yrs)nHIHR
clay loam1.93120.107 a0.202 a
13.1120.133 b0.299 b
loam1.91330.114 a0.291 a
12.9300.125 a0.281 a
sandy loam0.32120.110 a0.292 a
1490.153 b0.318 a
silt loam2.97150.142 a0.324 a
13180.144 a0.335 a
Average % C applied in straw (%)
clay loam38.6120.115 a0.238 a
100120.125 a0.263 a
loam33.2270.103 a0.266 a
97.6240.130 b0.290 a
sandy loam0150.112 a0.306 a
99.6120.140 b0.280 a
silt loam41.2120.136 a0.330 a
100150.167 a0.343 a
Average C/N ratio of fertilizers
clay loam39.5120.115 a0.238 a
75120.125 a0.263 a
loam33.2330.107 a0.271 a
70.9300.130 b0.297 b
sandy loam23.3150.106 a0.309 a
66.7120.140 b0.280 a
silt loam42.8180.121 a0.312 a
75210.151 b0.323 a
Mean values within the column that share the same letters do not differ significantly at the p < 0.05 level. HI—humification index HI = CHA/SOC, HR—humification rate HR = CHS/SOC, CHA—humic acids carbon, SOC—soil organic carbon, CHS—humic substances carbon.
Table 6. Comparison of experimental sites with an average proportion of leguminose in crops cultivated of 33.5% (group 1) and without leguminose at all (group 2). The number of cases was 24.
Table 6. Comparison of experimental sites with an average proportion of leguminose in crops cultivated of 33.5% (group 1) and without leguminose at all (group 2). The number of cases was 24.
Average Proportion of Leguminose in Crops Grown (%)12
33.5 b0.0 a
n1212
cereals in crops grown (%)42.9 a60.8 b
canola in crops grown (%)9.2 a14.1 a
root plant in crops grown (%)14.4 a25.1 a
altitude (m)388 a372 a
annual air temperature (°C)9.25 a9.39 a
annual rainfall (mm)600 b544 a
C input total (t/ha/11 yrs)7.32 a15.75 a
straw C input (t/ha/11 yrs)2.60 a7.64 b
average % C applied in straw (%)49.5 a50.9 a
weighted average of C/N ratio of org. fertilizers42.4 a47.8 a
E4/E66.35 a6.20 a
CHA/CFA0.944 a0.945 a
HI = CHA/SOC0.108 a0.132 a
HR = CHS/SOC0.282 a0.280 a
number of sites with a particular soil typeCambisol44
Fluvisol22
Luvisol24
Gleysol10
Planosol32
Mean values within the row that share the same letters do not differ significantly at the p < 0.05 level. C—carbon, CHA—humic acids carbon, E4/E6—absorbance ratio at the wavelengths of 465 and 665 nm, CHA/CFA—humic to fulvic acid carbon ratio, HI—humification index HI = CHA/SOC, SOC—soil organic carbon, HR—humification rate HR = CHS/SOC, CHS—humic substances carbon.
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Sedlář, O.; Balík, J.; Černý, J.; Suran, P.; Kulhánek, M.; Bihun, T. Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic. Agronomy 2025, 15, 1277. https://doi.org/10.3390/agronomy15061277

AMA Style

Sedlář O, Balík J, Černý J, Suran P, Kulhánek M, Bihun T. Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic. Agronomy. 2025; 15(6):1277. https://doi.org/10.3390/agronomy15061277

Chicago/Turabian Style

Sedlář, Ondřej, Jiří Balík, Jindřich Černý, Pavel Suran, Martin Kulhánek, and Tetiana Bihun. 2025. "Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic" Agronomy 15, no. 6: 1277. https://doi.org/10.3390/agronomy15061277

APA Style

Sedlář, O., Balík, J., Černý, J., Suran, P., Kulhánek, M., & Bihun, T. (2025). Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic. Agronomy, 15(6), 1277. https://doi.org/10.3390/agronomy15061277

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