Soil Systems · 2026
Characterization of Soil Organic Matter in Agricultural Soils Under Various Tillage Practices Using Fluorescence Spectroscopy
Vázquez-Ortega, A., Franks, M., Kieffer, K.
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Characterization of Soil Organic Matter in Agricultural Soil…
Abstract
Conventional tillage, a soil preparation practice used to produce a fine seedbed, can disturb the soil profile by promoting soil compaction and soil organic matter (SOM) degradation. In contrast, conservation tillage, such as no-till, has the potential to sustain or increase SOM. This study aimed to (1) quantify soil organic carbon (SOC) content under conservation tillage and conventional tillage practices, (2) describe the degree of aromaticity of bioavailable SOC using fluorescence spectroscopy, and (3) correlate SOC quantity with nitrogen and phosphorus retention in soils. Fluorescence spectroscopy is a sensitive and non-destructive tool that allows for the assessment of bioavailable SOC quality related to the molecular structure, degree of aromaticity (cyclic molecules with carbon double bonds), and recalcitrance (difficulty of decomposition) of organic compounds. This study employed fluorescence excitation–emission matrices combined with parallel factor analysis (EEM-PARAFAC) to identify humic-like, fulvic-like, and protein-like substances. Data on agricultural management practices were collected from spring 2014 until fall 2017. We obtained soil samples (fall 2017) from farms in the Western Lake Erie Basin, Ohio, and performed geochemical characterization in the bulk soil and aqueous extraction. Our results showed that no-till and minimal tillage fields consistently had greater SOC and fluorescence intensity in the humic-like acids region when compared to conventional tilled fields (no-till: 34,000 mg TOC kg−1; tilled six times: 16,000 mg TOC kg−1). No-till enhanced SOC stabilization. In addition, conservation tillage practices retained the largest total nitrogen (no-till: 2800 mg TN kg−1; tilled six times: 1350 mg TN kg−1) and total phosphorus (no-till: 470 mg TP kg−1; tilled six times: 250 mg TP kg−1) concentrations at all studied depths (0–30 cm) when compared to conventional tilled fields. Conservation tillage promotes the accumulation of highly aromatic organic compounds favoring high cation exchange capacity, and NO3− and PO43− retention and plant bioavailability.
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Conventional tillage, a soil preparation practice used to produce a fine seedbed, can disturb the soil profile by promoting soil compaction and soil organic matter (SOM) degradation. In contrast, conservation tillage, such as no-till, has the potential to sustain or increase SOM. This study aimed to (1) quantify soil organic carbon (SOC) content under conservation tillage and conventional tillage practices, (2) describe the degree of aromaticity of bioavailable SOC using fluorescence spectroscopy, and (3) correlate SOC quantity with nitrogen and phosphorus retention in soils. Fluorescence spectroscopy is a sensitive and non-destructive tool that allows for the assessment of bioavailable SOC quality related to the molecular structure, degree of aromaticity (cyclic molecules with carbon double bonds), and recalcitrance (difficulty of decomposition) of organic compounds. This study employed fluorescence excitation–emission matrices combined with parallel factor analysis (EEM-PARAFAC) to identify humic-like, fulvic-like, and protein-like substances. Data on agricultural management practices were collected from spring 2014 until fall 2017. We obtained soil samples (fall 2017) from farms in the Western Lake Erie Basin, Ohio, and performed geochemical characterization in the bulk soil and aqueous extraction. Our results showed that no-till and minimal tillage fields consistently had greater SOC and fluorescence intensity in the humic-like acids region when compared to conventional tilled fields (no-till: 34,000 mg TOC kg−1; tilled six times: 16,000 mg TOC kg−1). No-till enhanced SOC stabilization. In addition, conservation tillage practices retained the largest total nitrogen (no-till: 2800 mg TN kg−1; tilled six times: 1350 mg TN kg−1) and total phosphorus (no-till: 470 mg TP kg−1; tilled six times: 250 mg TP kg−1) concentrations at all studied depths (0–30 cm) when compared to conventional tilled fields. Conservation tillage promotes the accumulation of highly aromatic organic compounds favoring high cation exchange capacity, and NO3− and PO43− retention and plant bioavailability.
Conventional tillage, a soil preparation practice used to produce a fine seedbed, can disturb the soil profile by promoting soil compaction and soil organic matter (SOM) degradation. In contrast, conservation tillage, such as no-till, has the potential to sustain or increase SOM. This study aimed to (1) quantify soil organic carbon (SOC) content under conservation tillage and conventional tillage practices, (2) describe the degree of aromaticity of bioavailable SOC using fluorescence spectroscopy, and (3) correlate SOC quantity with nitrogen and phosphorus retention in soils. Fluorescence spectroscopy is a sensitive and non-destructive tool that allows for the assessment of bioavailable SOC quality related to the molecular structure, degree of aromaticity (cyclic molecules with carbon double bonds), and recalcitrance (difficulty of decomposition) of organic compounds. This study employed fluorescence excitation–emission matrices combined with parallel factor analysis (EEM-PARAFAC) to identify humic-like, fulvic-like, and protein-like substances. Data on agricultural management practices were collected from spring 2014 until fall 2017. We obtained soil samples (fall 2017) from farms in the Western Lake Erie Basin, Ohio, and performed geochemical characterization in the bulk soil and aqueous extraction. Our results showed that no-till and minimal tillage fields consistently had greater SOC and fluorescence intensity in the humic-like acids region when compared to conventional tilled fields (no-till: 34,000 mg TOC kg−1; tilled six times: 16,000 mg TOC kg−1). No-till enhanced SOC stabilization. In addition, conservation tillage practices retained the largest total nitrogen (no-till: 2800 mg TN kg−1; tilled six times: 1350 mg TN kg−1) and total phosphorus (no-till: 470 mg TP kg−1; tilled six times: 250 mg TP kg−1) concentrations at all studied depths (0–30 cm) when compared to conventional tilled fields. Conservation tillage promotes the accumulation of highly aromatic organic compounds favoring high cation exchange capacity, and NO3− and PO43− retention and plant bioavailability.
Findings
What changed what, as the paper reports it. Grouped by the measured property. The largest groups are open; open the others as you need them.
bulk soil total phosphorus (TP)
↑ 13
- increase
conservation practices (Fields1,2,7,8) vs. conventional tillage (Fields3,4,5,6)significance not stated · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm; agreeing conservationvariants versusconventional3–6; observational/pseudoreplicated; roundedabstractillustrativenumbers notgroupmeans, depthunspecified
Numbers
- treatment 470 vs. control 250 mg TP kg-1 (+88 %) · Abstract illustrative no-till versus six-times-tilledfield, depthnotstated
Source · p. 6 “With respect to TP, the fields managed under conventional practices (Fields 3, 4, 5, and 6) had the lowest content at 15–30 cm depth.”
Source · p. 12 “In general, fields managed under conservation tillage practices retained the highest bulk SOC, TN, and TP concentrations at all studied depths (0–30 cm)”
- increase
with bulk soil total organic carbon (TOC)significant · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm withinfielddepthprofiles; significantpositiveFields1,3,4,6,7 only. Field2r.07 andField5r.83 unstarred; not assertedpositive significant. Field8negativeexception separatelyrecorded. Paperreported.05 withcaptionpairedt-testcaveat.
Numbers
- r = 0.93 · Field1
- r = 0.98 · Field3
- r = 0.97 · Field4
- r = 0.99 · Field6
- r = 0.92 · Field7
Source · p. 7 “for Fields 1, 3, 4, 6, and 7 were significantly positively correlated, ranging from 0.92 to 0.99 (p-values ≤ 0.05).”
- decrease
with bulk soil total organic carbon (TOC)significance not stated · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm Field8 residue-retaining conservationfield; negativeexception topositiveheadline, Table2−.84star. Paperreportedalpha.05, not exactcorrelationp recalculated; captiont-testcaveat.
Numbers
- r = -0.84 · Field8
Source · p. 6 “In general, TOC was positively correlated with TP, except for Fields 2 (r = 0.07) and 8 (r = −0.84).”
Source · p. 7 “8 | 0.99 * | − 0.84 * | 0.57 | | 0.73”
bulk soil total nitrogen (TN)
↑ 12
- increase
conservation practices (Fields1,2,7,8) vs. conventional tillage (Fields3,4,5,6)significance not stated · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm; agreeing conservationvariants versusconventional3–6; observational/pseudoreplicated; roundedabstractillustrativevalues notgroupmeans, depthunspecified
Numbers
- treatment 2,800 vs. control 1,350 mg TN kg-1 (+107.4 %) · Abstract illustrative no-till versus six-times-tilled field, withnostateddepth
Source · p. 6 “The fields managed under conservation practices (Fields 1, 2, 7, and 8) consistently had a larger content of TOC and TN at 15–30 cm depth.”
Source · p. 12 “In general, fields managed under conservation tillage practices retained the highest bulk SOC, TN, and TP concentrations at all studied depths (0–30 cm)”
- increase
with bulk soil total organic carbon (TOC)significant · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm; withinfield depthprofiles, all8fields. PaperreportsPearson r andsignificance.05; Table2caption describespairedone-tailedt-test ratherthanstandardcorrelationtest; retainedasreported, not recalculated. Composite samples/analyticaltriplicates are notindependent fieldreplicates.
Numbers
- r = 0.95 · Field1
- r = 0.99 · Field2
- r = 0.99 · Field3
- r = 0.99 · Field4
- r = 0.99 · Field5
- r = 0.98 · Field6
- r = 0.96 · Field7
- r = 0.99 · Field8
Source · p. 6 “Bulk TOC was positively correlated with TN in all investigated fields, with correlation coefficients ranging from 0.95 to 0.99 (p-values ≤ 0.05)”
bulk soil total organic carbon (TOC)
1
- increase
conservation practices (Fields1,2,7,8) vs. conventional tillage (Fields3,4,5,6)significance not stated · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm (0–5,5–15,15–30); Fields1,2no-till and7,8residueconservation versus3–6conventional. Observational, confoundedmanagement/soil andpseudoreplication. Illustrativenumbersroundedabstractvalueswithnostateddepth, not groupmeans.
Numbers
- treatment 34,000 vs. control 16,000 mg TOC kg-1 (+112.5 %) · Abstract illustrative no-till versus six-times-tilled field; exact fieldIDs/depth notstated with these roundednumbers
Source · p. 6 “The fields managed under conservation practices (Fields 1, 2, 7, and 8) consistently had a larger content of TOC and TN at 15–30 cm depth.”
Source · p. 11 “In the upper 5 cm, no-till and 30% crop residue fields (Fields 1, 2, 7, 8) consistently had greater TOC than the conventionally tilled fields (Fields 3, 4, 5, 6)”
Source · p. 12 “In general, fields managed under conservation tillage practices retained the highest bulk SOC, TN, and TP concentrations at all studied depths (0–30 cm)”
humic-like acid RegionV fluorescence intensity
1
- increase
conservation practices (Fields1,2,7,8) vs. conventional tillage (Fields3,4,5,6)significance not stated · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm; raw aqueousRegionV fluorescence; no pairwisemarksinFigure2B; observationalfieldcomparison, not modeledPARAFACscore or directhumiccarbonmass
Source · p. 8 “Field 1 (no-till) had the greatest fluorescence intensity in Regions III and V across all depths”
Source · p. 8 “Fields 2, 7, and 8 (no-till and 30% soil surface with crop residue) had the second greatest fluorescence intensity in Region V.”
Source · p. 10 “No-till (Fields 1, 2) and 30% crop residue sites (Fields 7, 8) consistently had greater fluorescence intensity summations for humic-like acids (Region V) when compared to conventionally tilled fields (Fields 3, 4, 5, 6)”
humification index (HIX)
= 11
- no effect
no-till (Fields1and2) vs. till (Fields3–8)p = 0.774 · 0–30 cm · fall2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0–30cm samples; actual two-group ANOVA no-tillFields1–2 versusALLtilled3–8, includingresidueconservation; samplingpseudoreplication. HIXnull qualifies generalhumificationclaim.
Numbers
- treatment 5.62 vs. control 5.44 (+3.309 %)
Source · p. 8 “There were no significant differences between average HIX or FI values by field, or when comparing till (fields 3–8) vs. no-till (fields 1–2) (Table 4).”
Source · p. 10 “HIX | 5.62 | | 1.23 | 5.44 | 1.25 | 0.085 | 0.774”
fluorescence index (FI)
= 11
- no effect
no-till (Fields1and2) vs. till (Fields3–8)p = 0.574 · 0–30 cm · fall 2017 · eight farm fields in the Western Lake Erie Basin, Ohio · 0-30 cm; actual two-group comparison of no-till fields1-2 versus all tilled fields3-8, including residue-conservation fields7-8. Farm fields are observational and management/soil confounded; analytical triplicates and composited depths do not provide independent field replication.
Numbers
- treatment 1.19 vs. control 1.2 (-0.8333 %)
Source · p. 8 “There were no significant differences between average HIX or FI values by field, or when comparing till (fields 3–8) vs. no-till (fields 1–2) (Table 4).”
Source · p. 10 “FI | 1.19 | | 0.0071 | 1.2 | 0.051 | 0.33 | 0.574”
Set-up
Treatments
- tillage practicesTillage system
- conservation practices (Fields1,2,7,8)Conservation tillageconventional tillage (Fields3,4,5,6)Conventional tillageno-till (Fields1and2)Zero tillagetill (Fields3–8)Tillage system
Measured
- bulk soil total organic carbon (TOC)Soil organic carbon
- Unit
- mg kg-1
- Method
- High-temperatureoxidation withIRCO2 detection, ShimadzuTOC-VCSH/SSM5000A; composited field-depth samples
- bulk soil total nitrogen (TN)Total Nitrogen
- Unit
- mg kg-1
- Method
- USEPA353.2, copperizedcadmium nitrate reduction and520nm spectrophotometry asreported
- bulk soil total phosphorus (TP)Total Phosphorus
- Unit
- mg kg-1
- Method
- Alkaline persulfate digestion withcolorimetric detection
- humic-like acid RegionV fluorescence intensityHumic substance carbon
- Method
- SumofEEMfluorescence inRegionV of1:10 soil:water extract; proxyforhumic-like character, not directhumiccarbonmass. Aqualog200–450nmEx/250–650nmEm
- humification index (HIX)DOM fluorescence humification index
- Method
- Ratioofemission300–345nm to435–480nm at254nm excitation, asreported
- fluorescence index (FI)DOM fluorescence index
- Method
- Aqualog excitation-emission matrix fluorescence spectroscopy; FI as defined by the paper, indicating SOM sources.
Sites
eight farm fields in the Western Lake Erie Basin, Ohio
- Location
- Maumee River watershed, Western Lake Erie Basin, northwestern Ohio, USA (Fields1,2,7,8)Sandusky River watershed, Western Lake Erie Basin, Ohio, USA (Fields3–6)
- Land use
- actively farmed fieldsArable land
- Management on the whole site
- crop rotation
| Depth | Texture | Clay | pH | Organic C | Bulk density |
|---|---|---|---|---|---|
| 0–30 cm | – | – | 5.9–7.1 | – | – |
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