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Article

The Structural Quality of Soil Organic Matter under Selected Soil Fertility Management Practices in the Central Highlands of Kenya

by
Miriam Githongo
1,
Lucy Ngatia
2,
Milka Kiboi
3,4,
Anne Muriuki
5,
Andreas Fliessbach
6,
Collins Musafiri
3,
Riqiang Fu
7 and
Felix Ngetich
3,8,*
1
Rosewood Futures, Nyahururu P.O. Box 2171-20300, Kenya
2
College of Agriculture & Food Sciences, Florida A&M University, Tallahassee, FL 32307, USA
3
Cortile Scientific Limited, Nairobi P.O. Box 34991-00100, Kenya
4
Division of Research Innovation and Outreach, KCA University, Nairobi P.O. Box 56808-00200, Kenya
5
National Agriculture Research Laboratories, Kenya Agricultural and Livestock Research Organization, Nairobi P.O. Box 14733-00800, Kenya
6
Department of Soil Sciences, Research Institute of Organic Agriculture (FIBL), Ackerstrasse 113, 5070 Frick, Switzerland
7
National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA
8
School of Agricultural and Food Sciences, Jaramogi Oginga Odinga University of Science and Technology (JOOUST), Bondo P.O. Box 210-40601, Kenya
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(8), 6500; https://doi.org/10.3390/su15086500
Submission received: 21 December 2022 / Revised: 3 April 2023 / Accepted: 4 April 2023 / Published: 11 April 2023
(This article belongs to the Special Issue The Sustainability of Agricultural Soils)
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Abstract

:
As influenced by agricultural practices, soil organic matter (SOM) stability is imperative in maintaining soil fertility and crop production. Integrated soil management practices have been recommended for soil fertility improvement by enhancing soil organic matter. We examined the SOM stability under integrated soil management practices for six consecutive cropping seasons in the high agricultural potential area of the Central Highlands of Kenya. The experimental design was a complete randomized block design with fourteen treatments replicated four times. The treatments were minimum (Mt) and conventional tillage (Ct) combined with sole mineral fertilizer (Mf), crop residue combined with mineral fertilizer (RMf), crop residue combined with mineral fertilizer and animal manure (RMfM), crop residue combined with animal manure and Dolichos Lablab L. intercrop (RML), crop residue combined with Tithonia diversifolia and animal manure (RTiM), and crop residue combined with Tithonia diversifolia and phosphate rock (Minjingu) (RTiP), as well as a control (no inputs). SOC was higher in treatments with organic inputs and a combination of organic and inorganic inputs. Treatments with sole mineral fertilizer and no input recorded lower SOC amounts. The C functional groups followed the sequence: alkyl C (53%) > O-alkyl C (17%) > aromatic C (9%) > carboxyl C (8%) > methoxyl C (7%) > phenolic C (6%). The alkyl C proportion was higher in organic inputs treatments, while O-alkyl C was higher in organic and inorganic fertilizer treatment combinations. Methoxyl C, aromatic C, and phenolic C proportion of SOC was greater in crop residue and mineral fertilizer combination, while carboxylic C was lower than the control in most treatments. In addition, the organic inputs treatments had a higher alkyl C/O-alkyl C ratio, increased aliphaticity, and higher hydrophobicity. Applying organic fertilizers individually or in combination with inorganic fertilizers could potentially increase C storage in the soil, thereby enhancing SOC stocks.

1. Introduction

Declining soil fertility has pressured food security in sub-Saharan Africa [1,2]. This decline in soil fertility has been revealed by reduced soil organic carbon (SOC) and nutrient contents [3,4]. In the Central Highlands of Kenya, soil fertility decline has been exacerbated by continuous cropping, nutrient mining, and lack of nutrient replenishment [5,6]. Soil organic matter (SOM) is the major reservoir of organic C on the earth’s surface and represents a significant C pool [7]. The SOM comprises physical, chemical, and biological processes that improve soil fertility, increase soil moisture, and improve quality [8,9]. To maintain soil fertility, crop production, performance, and environmental stability, SOM storage and stability are prudent [8]. The stability of SOM is its resistance to decomposition or mineralization [10]. Therefore, to understand SOM sensitivity, properties, and functions, it is necessary to know SOM composition and structure [11]. This understanding provides comprehension of SOM’s heterogeneous and complex nature in soil [12]. Given the multifaceted interactions that occur through the soil matrix, it is necessary to understand how the stability of SOM is affected by agricultural management practices and how this influences SOM’s chemical structure.
Soil organic matter (SOM) contains about 1550 Pg of C globally [13] and about 60% C by mass [14]. Most or all of this pool is assumed to be SOC, except for small amounts of inorganic C in some soils [15]. Using soil inputs has impacted SOC and its stability by enhancing SOC content and storage [16]. Furthermore, studies have shown that adding organic resources provides fresh organic residues and releases soil minerals for organo-mineral complexes that enhance soil C storage [17]. In the Central Highlands of Kenya, smallholder farmers have used organic inputs to improve SOC in the soil, but they contain relatively low nutrients compared with inorganic fertilizers; therefore, a combination of organic and inorganic fertilizers is strongly encouraged [5,18,19]. Additionally, organic and inorganic fertilizer inputs increase yields and provide plant and root biomass C in the soil [20], positively affecting the soil’s physical and chemical properties [21]. Studies have shown different influences on the SOC amounts when soil inputs, organic and inorganic, are added to the soil. For example, sole organic input or in combination with inorganic fertilizers tends to increase SOC concentrations [22], whereas sole inorganic fertilization yields inconsistent outcomes [23]. However, the use of soil inputs has exhibited increased SOC in the soil; therefore, it is plausible to evaluate the influence of integrated soil management practices on SOC and SOC composition in the Central Highlands of Kenya.
Using molecular techniques to assess the chemical structure of SOM helps explain the founding paths, decay, and interfaces with espoused agricultural management systems [24]. One such technique is solid-state cross-polarization magic angle spinning 13C nuclear magnetic resonance spectroscopy (CP-MAS 13C-NMR), a fast, accurate, and precise method [25] that has been used to characterize natural organic matter (OM) and SOC [26]. CPMAS 13C NMR spectroscopy can accurately and precisely quantify different C functional groups in mixtures of important SOM source compounds and probably also in SOM of organic surface and mineral topsoil horizons [25]. It is plausible to use CPMAS 13C NMR spectroscopy to understand the complex nature of SOM in the soil under different inputs and tillage in the Central Highlands of Kenya.
Only a few studies in the Central Highlands of Kenya have reported SOC amounts in soils amended with various organic and inorganic soil inputs under conventional tillage (Mucheru-Muna et al. [5]) and minimum tillage (Kiboi et al. [27]). These studies have significantly advanced our understanding of SOC amounts improvement when integrated soil management practices are used. While soil carbon is an indicator of soil health and productivity, limited research has been done in the Central Highlands of Kenya on the stability of soil organic carbon under different soil inputs in maize cropping systems; thus, the impact of SOC contribution to soil quality is lacking. Further, information is scanty about the chemical structure of SOM under these management strategies in the Central Highlands of Kenya. This study’s objective was to investigate the quality of SOM in soils under integrated soil management practices in the Central Highlands of Kenya using the 13C cross-polarization magic angle spinning (CP-MAS) NMR.

2. Materials and Methods

2.1. Site Description

We carried out the study in Meru south sub-County, Tharaka Nithi County. The experimental site was at Kangutu Primary School (00°98′ S, 37°08′ E) farm (Figure 1). The sub-County lies 1468 m above sea level (a.s.l) and is located on the Upper Midland 2 (UM2) agroecological zone (AEZ) [28].
The area experiences two rainy seasons annually; March to May (long rains) and October to December (short rains). It receives an annual average rainfall of 1500 mm, while the annual temperatures range from 18.2 to 20.6 ℃. [28]. The predominant soils in the area are Humic Nitisols which are well-drained, extremely deep, dusky red to dark reddish-brown, friable clay with humic topsoil [28]. However, due to continuous cropping and nutrient mining, these soils require amendments to improve and sustain their fertility [29]. The soil properties of the experimentation site are presented in Table 1.

2.2. Experimental Design

We implemented a randomized complete block design with fourteen treatments (Table 2), replicated four times. The plot size was 6 m by 4.5 m, and the test crop was maize (Zea mays L.), variety H516. The plant spacing was 0.75 m between the rows and 0.5 m between plants. We did land preparation two weeks before the onset of the rainy season. Where land plowing in minimum tillage plots, only planting holes were plowed. We incorporated organic inputs (Tithonia diversifolia and animal manure) into the soil in the planting holes. In conventional tillage plots, we prepared the land by hand hoeing up to a depth of 15 cm, broadcasted Tithonia diversifolia and animal manure on the plots, and incorporated it into the soil using a hand hoe. We planted two weeks after the incorporation of Tithonia diversifolia and animal manure. The nutrient quality of Tithonia diversifolia and animal manure are described in Table 3. We planted three seeds per hill and later thinned to two seedlings two weeks after emergence resulting in the recommended plant density of 53,333 per hectare [31]. We surface-applied crop residue after thinning at a rate of 5 Mg ha−1 per season in treatments with crop residue (Table 2). We applied nitrogen (60 kg N ha−1) per season to meet the recommended nitrogen content for maize in the study area [32]. We used an equivalence of 30 kg N ha−1 from organic inputs and 30 kg N ha−1 per season in treatments with organic inputs and mineral fertilizer. Inorganic P was supplied using triple superphosphate at 90 kg ha−1 per season in treatments with mineral fertilizers during planting. We hand-pulled weeds in treatments under minimum tillage and weeded using a hand hoe in conventional tillage treatments.

2.3. Soil Sampling

We collected the soil samples at the end of the long rains 2019 season, after six consecutive cropping seasons since the experiment’s establishment. We sampled approximately 50 g of soil from each plot from 0–15 cm depth using the Eijkelkamp gouge auger. We put the sample into 60 mL plastic vials, labelled them, and shipped them to the National High Magnetic Field Laboratory, USA, for nuclear magnetic resonance (NMR) analysis. We also sampled for the soil’s physical properties by collecting approximately 1 kg of soil from each of the plots at a depth of 0–20 cm using the Eijkelkamp gouge auger.

2.4. 13C-CPMAS NMR Characterization

The solid-state 13C CPMAS ssNMR spectra were collected using a Bruker 300 MHz DRX NMR spectrometer. The spectrometer had a Bruker 4.0 mm double resonance MAS NMR probe. Samples were placed in a zirconium oxide rotor with a diameter of 4 mm and Kel-F caps. A spinning speed of 9.5 kHz ±3 Hz was applied, and recycle delay was set to 2 s at RT using Bruker pneumatic MAS control unit. A ramped 1H pulse (from 35 to 50 kHz) by 13C RF field where a 4.0 μs 1H π/2 pulse followed by a 1H spin-lock field of 45 kHz for 1 ms contact time enhanced all 13C signals from 1H. Depending on the sensitivity of the sample, the number of accumulated scans varied between 10,000 and 500,000. Carbon chemical shifts were referenced to the carbonyl carbon signal of glycine at 176.4 ppm. For quantification of different C species, the 13C NMR spectra were divided into six chemical shift regions which were assigned to specific C functional groups according to Knicker [33]; alkyl (0–45 ppm), methoxyl (45–60 ppm), O-alkyl (60–110 ppm), aromatic (110–140 ppm), phenolic (140–160 ppm), and carboxyl (160–220 ppm). The percent C of the functional group was converted to g functional group C kg−1 sample using organic C values of the soil samples.

2.5. Statistical Analysis

We analyzed the data using one-way ANOVA in SAS version 9.4 Software. We used the least significant difference (LSD) to compare the means of the soil variables under different soil management practices treatments at p ≤ 0.05.

3. Results

3.1. Effect of Different Soil Inputs and Tillage on Soil Organic Carbon

Soil organic carbon (SOC) was significantly (p < 0.0001) higher under CtRML, CtRTiM, MtRML, MtRTiM, CtRMfM, MtRMfM, CtRMf, MtRTiP, MtRMf, CtRTiP, and MtMf by 108, 87, 87, 69, 56, 51, 50, 39, 36, 7, and 3% compared with the control (CtC) (Figure 2). In contrast, CtMf (22.65 g kg−1C) treatment decreased SOC content by 16% compared with CtC (27.11 g kg−1C).

3.2. Effect of Different Soil Inputs and Tillage on Soil Organic Matter Structure

The results of CPMAS NMR are shown in Figure 3. Briefly, alkyl C was significantly (p < 0.0001) higher under MtRML, CtRML, MtRTiM, MtRMfM, CtRTiM, MtRTiP, CtRMf, CtRMfM, MtC, and MtMf by 118, 116, 95, 69, 68, 57, 42, 36, 6, and 4% compared with the control. Methoxyl C was significantly (p < 0.0001) higher under all the treatments with CtRMf, MtMf, MtRTiP, MtRML, MtRTiM, CtRMfM, CtRML, CtRTiM, CtMf, MtRMfM, MtRMf, CtRTiP, and MtC higher by 71, 41, 39, 35, 34, 31, 31, 29, 19, 18, 18, 14, and 14% compared with the control. O-alkyl was significantly (p < 0.0001) higher under all the treatments with CtRTiP, MtRMf, CtRMf, CtRMfM, CtRTiM, CtMf, MtMf, CtRML, MtRTiP MtRMfM, MtRML, MtRTiM, and MtC higher by 234, 232, 142, 127, 88, 83, 61, 45, 25, 17, 16, 11, and 4% compared with the control. Aromatic C was significantly (p < 0.0001) higher under MtRMf, CtRTiP, CtRMfM, CtMf, CtRMf, CtRMf, CtRTiM, MtMf, and CtRML by 174, 139, 71, 46, 35, 28, 21, and 10 % compared with the control. Phenolic C was significantly (p < 0.0001) higher under MtRMf, CtRTiP, CtRMfM, CtMf, CtRTiM, and CtRMf by 83, 44, 19, 18, 6, and 1% compared with the control. In the carboxylic C group, the control treatment was significantly (p < 0.0001) higher than all the other treatments except for CtRTiP, which was 1% higher than the control. In summary, CtRTiP was highest in O-alkyl C and carboxylic C, MtRMf was highest in aromatic C and phenolic C, MtRML was highest in alkyl C, and CtRML was highest in the methoxyl C carbon group.

3.3. Effect of Different Soil Inputs and Tillage on Carbon Group Ratios

The aliphatic/aromatic ratio was lowest under MtRMf (4.27) and highest under MtRTiM (34.27) was the highest. The alkyl C/O-alkyl C ratio was lowest under CtRTiP (4.78) and highest under MtRML (40.34). The Hydrophobic C/Hydrophilic C ratio was the lowest under CtRTiP (2.30) and highest under MtRML (6.86) (Figure 4).

4. Discussion

The soil’s organic carbon content and alkyl C were significantly increased in treatments with organic inputs, indicating the importance of organic and inorganic inputs in increasing soil organic carbon, thereby impacting soil fertility. The carbon functional groups of methoxyl C, aromatic C, and phenolic C were higher in the treatment combination of crop residues and mineral fertilizer. This shows the importance of mineral fertilizer in the soil’s carbon stability.
In this study, the soil carbon content varied under integrated soil inputs and tillage treatments compared to the control. Treatments with crop residues, animal manure, and legume intercropping had higher SOC content than control. This is attributed to organic matter input from the legume intercrop, crop residue, and manure, leading to high C input and increasing the SOC [34]. The legume intercrop promoted N inputs and increased substrate efficiency as an energy source for soil biology [35], thereby increasing microbial activity and SOC. Crop residue, Tithonia diversifolia, and animal manure treatments had higher SOC as they provided an organic C source to the soil. The animal manure had a higher lignin content than Tithonia diversifolia, which slowed decomposition (Table 3). Thus, the humus of the slowly decomposing manure may preserve released nutrients from the fast-decaying Tithonia diversifolia within the rhizosphere, fostering a C increase in the soil. This agrees with a study by Margenot et al. [36], who indicated that organic matter inputs increased microbial enzymatic activity, potentially increasing stable SOC stocks.
Crop residues, mineral fertilizer, and animal manure treatments had significantly higher SOC content than the control attributing this to organic carbon addition to the soil [37]. Further, soil N from the inorganic fertilizer addition enhanced C storage in the soil [38]. This agrees with Guo et al. [39] in Eastern China, which reported higher SOC amounts in treatments combining organic (cattle manure) and inorganic fertilizers than in treatments with sole chemical fertilizer and control. Treatments with crop residue, Tithonia diversifolia, and rock phosphate had higher SOC than the control endorsing this to the addition of organic C from Tithonia diversifolia, which could have influenced the structural properties by enmeshing primary soil particles via root action and enhancing microbial activities by producing cementing agents [40].
We observed higher SOC from crop residues and mineral fertilizer treatments. The addition of carbon through crop residue and root biomass due to N addition from mineral fertilizer in these treatments increased SOC. The SOC amounts in convention and minimum tillage treatments were similar except for the CtMf treatment, which was lower than the control (Figure 2). The reduction of carbon in CtMf could result from mineral N immobilization and N mining from SOM by the microorganisms, as the microbes utilized the available mineral N as an energy source in contrast with the control where the root biomass provided the essential plant-derived OM.
The amount of soil inputs added to the soil could have influenced OC content in the soil and no-tillage [41]. Dimassi et al. [42] suggested that soil inputs produced similar SOC amounts under different tillage practices (conventional and minimum tillage), as in our study, except for CtMf treatment. Though studies showed conventional and minimum tillage practices do not significantly influence SOC [43,44], others revealed decreased SOC when conventional tillag. For instance, Assunção et al. [45] reported that minimum tillage increased the amount of SOC compared to conventional tillage.

4.1. Effect of Integrated Soil Inputs and Tillage on SOM Structure

The stability of soil organic matter depends on the mechanisms of interaction of organic carbon in the soil [46]. In our study, alkyl C (0–45 ppm) dominated soil organic C, followed by O-alkyl C (61–110 ppm), as also found by Schweizer et al. [47]. Organic input addition accelerated the decomposition of labile C compared to recalcitrant C components [48]. Plant litter contained high amounts of cutins in leaf cuticles and suberins found outside roots [49,50]. These alkyl C components are preserved during SOM decompositions [51,52]. Thus, adding organic inputs lower th excessive decomposition of soil hydrophobic components, increasing SOM amounts similar to [53,54].
Microbial community preferentially decomposes labile C, and the products contributing to the formation of stable C [55]. The alkyl C from organic and inorganic combination was contrary to Zhang et al. [56], who attributed their findings to slower decomposition of the organic inputs in the treatment combinations. The slower decomposition results from applying organic manure and fresh plant litter from Tithonia diversifolia [57]. Microbial community structure and its microorganisms lead to a higher alkyl C in minimum tillage treatments, as alkyl C is the most persistent stable fraction derived from soil microorganisms’ metabolic products [58].
The methoxyl C was higher in crop residue, and mineral fertilizer treatment combination compared with the control. The higher methoxyl C proportion from crop residue and mineral fertilizer treatment combination was attributed to the fact that the methoxyl C chemical structure is recalcitrant, thus, not prone to degradation by microorganisms [59]. The mineral fertilizer provided mineral N input, which encouraged plant root growth. Therefore, it is possible that plant roots were the main organic input, and this would explain the recalcitrant lignin accumulation. However, the higher O-alkyl C proportion of CtRTiP, MtRMf, CtRMf, and CtRMfM treatments could be ascribed to the presence of certain defense mechanisms of O-alkyl C constituents [60]. In addition, the inorganic fertilizer stimulates Mf decomposition due to increased microbial activity [61], increasing hydrolytic enzymatic activities, and reduced oxidative enzymes, decreasing the decomposition of more resistant OM and possibly increasing SOC storage [60]. Further, the high O-alkyl C proportions under integrated organic and inorganic fertilizer addition could influence soil physical and biological properties and clay mineral adsorption of polysaccharides [23]. This, in turn, could impact aggregation and soil particles’ structural activity, leading to soil C sequestration [62].
The findings on aromatic C proportion of SOC agreed with Luan et al. [16], who reported that aromatic C was lower in the soils that received organic inputs (20.8%–23.9%) and greater in the soils that received sole mineral fertilizer (26.6%) than the control. In contrast, a previous study by Zhang et al. [63] indicated that aromatic C was lower in treatments with the addition of organic inputs than those without the addition of organic inputs. The variances could explain the varying results in the quality of organic inputs, soil type, and environmental conditions [64]. However, the higher phenolic C in MtRMf and CtRTiP treatments could be attributed to lignin degradation as phenolic C in the soil is derived from microbial degradation of lignin in dead plant remains [65]. Phenolic C is selectively preserved by the microorganisms, and thus resistant to degradation [66].

4.2. Effect of Different Soil Inputs and Tillage on Carbon Group Ratios

The alkyl/O-alkyl (AL/OA) ratio has been used to indicate the relative degree of SOC decomposition or the degree of humification of the SOM [59], and a low value indicates a better quality of SOC [65]. The low soil alkyl C/O-alkyl C ratio in CtRTiP and MtRMf treatments suggest that SOC decomposed more slowly. This may be a result of the addition of carbohydrates and cellulose structures to the soil organic matter. A study by Schoebitz and Vidal [67] reported that combining organic and inorganic soil inputs led to efficient nutrient usage, enhancing soil C. Organic and inorganic inputs integration limits carbon mineralization, possibly due to the formation of compounds resistant to microbial activity, leading to increased C storage [55]. Thus, these fertilization combinations could delay the decomposition process, leading to C sequestration in the soil. The organic inputs treatment consumed more labile C, leading to increased recalcitrant C [68]. This was contradictory to a study by Wang et al. [20], who found that adding mineral fertilizers consumed more labile C, resulting in a higher alkyl C/O-alkyl C ratio than organic inputs treatments.
The aliphatic C/aromatic C (AL/AR) ratio predicts SOM chemical composition’s complexity [59]. The highest aliphaticC/aromatic C ratio in MtRTiM and lowest in MtRMf treatment indicated that the MtRTiM treatment had C functional groups with a simpler molecular structure, less condensation, and a less aromatic structure. High aliphaticity indicates that the treatment may improve the decomposition potential of SOC [61,69]. The increased aliphaticity under MtRTiM and MtRML treatments may result from the selective preservation of lignin structures during microbial degradation when specific microorganisms and oxidative enzymes degrade lignin and have low biomass or weak enzymatic activity [60]. Zhao et al. [70] found that smaller fungi populations and weak oxidative enzymatic activities in soils under organic inputs treatments. This contradicts a study by Chang et al. [71], who observed that aliphatic C/aromatic C increased with increasing disturbance intensity. The findings could be attributed to the presence of recalcitrant C and that microbial community structure that could have decreased alkyl C with increasing soil disturbances.
The high hydrophobic C/hydrophilic C ratio under crop residue, animal manure, and legume intercrop treatment combination could be attributed to the addition of organic inputs, indicating that SOM was highly protected from microbial decomposition [72] as free lipids formed a hydrophobic coating around the soil organic matter [73]. This contradicts Zhang et al. [67], who reported a lower hydrophobicity of SOM in treatments under organic inputs than in chemical fertilizer treatment. Studies by Zhang et al. [69] have indicated that a higher value of aliphatic C/aromatic C and hydrophobic C/hydrophilic C ratio suggested that the soil organic matter was more aliphatic and hydrophobic. Most of the minimum tillage treatments had higher than conventional tillage treatments. The higher hydrophobic C/hydrophilic C ratio in minimum tillage could be explained by SOM aggregation due to reduced tillage, temporarily cementing material from simple polysaccharide secretion to hyphae and roots and leading to short-term stability and stable complex aromatic substances [74].

5. Conclusions

Our results showed treatment with crop residues, animal manure, legume intercrop, and Tithonia diversifolia (CtRML, CtRTiM, MtRML, and MtRTiM) unswervingly increased the amount of OC in the soil. This underscores the importance of organic inputs in improving SOC in the soil and the intricate nutrient synergy in the combination of organic and inorganic inputs treatments, which also increase SOC. We showed that organic inputs increase alkyl C, and combining organic and inorganic inputs increases O-alkyl C. This underscores the importance of both sole organic and combination organic/inorganic inputs in SOC stability. Smallholder farmers in the Central Highlands of Kenya could increase yields and improve SOC stocks by adding organic and inorganic inputs. We further established that adding crop residues as mulch and mineral fertilizer increased methoxyl C, aromatic C, and phenolic C functional groups. This shows the importance of mineral fertilizer addition with mulch to improve the decomposition of SOM. In our study, organic inputs treatments had a higher value of aliphatic C/aromatic C and hydrophobic C/hydrophilic C ratio and, therefore, were more aliphatic and hydrophobic. This underpins the importance of organic inputs in SOC stability and storage. A combination of organic and inorganic soil inputs in the study showed a low alkyl C/O-alkyl C ratio showing a better quality SOC that did not decompose readily. Therefore, this study highlights the possibility of improving crop production by improving SOC stability in the soil, which would increase soil fertility through different soil fertility management practices. There is a need to encourage smallholder farmers to embrace different soil fertility management practices to improve the soil fertility status and crop performance and productivity.

Author Contributions

Conceptualization, M.G. and M.K.; methodology, M.G., L.N., M.K., R.F. and F.N.; validation, C.M.; formal analysis, M.G., R.F., L.N. and F.N.; investigation, M.G., M.K. and F.N.; resources, F.N., A.M. and A.F.; data curation, M.G.; writing—original draft preparation, M.G. and F.N.; writing—review and editing, C.M., A.F., L.N., F.N. and A.M.; visualization, M.G. and F.N.; supervision, F.N., A.F. and M.K.; project administration, A.M. and M.K.; funding acquisition, A.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Swiss Agency for Development and Cooperation (SDC) and the Swiss National Science Foundation (SNSF) in the Swiss Programme for Research on Global Issues for Development (r4d programme), Grant No. 152224. The APC was partially funded by the Swiss National Science Foundation (SNSF).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

We would like to thank the Research Institute of Organic Agriculture (FiBL), for technical support through the farmer-driven organic resource management to build soil fertility (ORM4Soil) project. The authors would also like to thank the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida for the NMR analysis. We thank Kangutu Primary School in Meru south sub-County for providing the experimental site.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Map of the study area.
Figure 1. Map of the study area.
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Figure 2. Soil organic carbon under different soil inputs and tillage. CtC = Conventional tillage Control, CtMf = Conventional tillage + Mineral fertilizer, CtRMf = Conventionaltillage + Crop residues + Mineral fertilizer, CtRMfM = Conventional tillage + Crop residues + Mineral fertilizer + Animal manure, CtRML = Conventional tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), CtRTiM = Conventional tillage + Crop residues + Tithonia diversifolia + Animal manure, CtRTiP = Conventionaltillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu), MtC = Minimum tillage Control, MtMf = Minimum tillage + Mineral fertilizer, MtRMf = Minimum tillage + Crop residues + Mineral fertilizer, MtRMfM = Minimum tillage + Crop residues + Mineral fertilizer + Animal manure, MtRML = Minimum tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), MtRTiM = Minimum tillage + Crop residues + Tithonia diversifolia + Animal manure, MtRTiP = Minimum tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu). Treatment means were significantly different at p < 0.0001.
Figure 2. Soil organic carbon under different soil inputs and tillage. CtC = Conventional tillage Control, CtMf = Conventional tillage + Mineral fertilizer, CtRMf = Conventionaltillage + Crop residues + Mineral fertilizer, CtRMfM = Conventional tillage + Crop residues + Mineral fertilizer + Animal manure, CtRML = Conventional tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), CtRTiM = Conventional tillage + Crop residues + Tithonia diversifolia + Animal manure, CtRTiP = Conventionaltillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu), MtC = Minimum tillage Control, MtMf = Minimum tillage + Mineral fertilizer, MtRMf = Minimum tillage + Crop residues + Mineral fertilizer, MtRMfM = Minimum tillage + Crop residues + Mineral fertilizer + Animal manure, MtRML = Minimum tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), MtRTiM = Minimum tillage + Crop residues + Tithonia diversifolia + Animal manure, MtRTiP = Minimum tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu). Treatment means were significantly different at p < 0.0001.
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Figure 3. Dynamics of the relative abundances of alkyl C, methoxyl C, O-alkyl C, aromatic C, phenolic C, and carboxyl C in different soil inputs and tillage practices as determined by 13C CPMAS NMR.
Figure 3. Dynamics of the relative abundances of alkyl C, methoxyl C, O-alkyl C, aromatic C, phenolic C, and carboxyl C in different soil inputs and tillage practices as determined by 13C CPMAS NMR.
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Figure 4. The ratios of alkyl C/O-alkyl C; Aliphaticity and hydrophobicity of SOM under different soil management practices. Aliphatic/Aromatic = (Alkyl-C + O-alkyl-C)/Aromatics-C; Hydrophobic-C/Hydrophilic-C = (Alkyl-C + Aromatic-C)/ (O-alkyl-C + Carboxyl C).
Figure 4. The ratios of alkyl C/O-alkyl C; Aliphaticity and hydrophobicity of SOM under different soil management practices. Aliphatic/Aromatic = (Alkyl-C + O-alkyl-C)/Aromatics-C; Hydrophobic-C/Hydrophilic-C = (Alkyl-C + Aromatic-C)/ (O-alkyl-C + Carboxyl C).
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Table
Table 1. Soil properties of Kangutu experimental site.
Table 1. Soil properties of Kangutu experimental site.
Treatment 12 pHTotal N (g kg−1)TOC (g kg−1)P (mg kg−1)K (g kg−1)Ca (g kg−1)Mg (g kg−1)Mn (g kg−1)Cu (mg kg−1)Fe (mg kg−1)Zn (mg kg−1)Na (g kg−1)
CtC4.51.315.2382.99.520.113.94.227612.6
CtMf4.41.415.4823.41217.820.44.332632.6
CtRMf4.51.617.3773.3142417.23.628582.5
CtRMfM5.21.719.0717.43130.715.44.323662.8
CtRML5.11.718.2518.921.534133.520592.6
CtRTiM5.11.718.0509.826.528.612.53.721613
CtRTiP4.91.617.9746.51928.418.93.524614.1
MtC4.61.415.8513.91421.817.74.526632.9
MtMf4.21.415.01472.7817.413.84.339643
MtRMf4.41.516.9734.114.521.515.23.931612.8
MtRMfM5.31.718.8479.152.532.717.53.519583.4
MtRML5.21.618.045921.535.818.73.817633
MtRTiM5.01.717.8676.421.530.715.94.426603.6
MtRTiP5.01.718.4757.8282613.5424663.1
1 Treatment: CtC = Conventional tillage Control, CtMf = Conventional tillage + Mineral fertilizer, CtRMf = Conventional tillage + Crop residues + Mineral fertilizer, CtRMfM = Conventional tillage + Crop residues + Mineral fertilizer + Animal manure, CtRML = Conventional tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), CtRTiM = Conventional tillage + Crop residues + Tithonia diversifolia + Animal manure, CtRTiP = Conventional tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu), MtC = Minimum tillage Control, MtMf = Minimum tillage + Mineral fertilizer, MtRMf = Minimum tillage + Crop residues + Mineral fertilizer, MtRMfM = Minimum tillage + Crop residues +Mineral fertilizer + Animal manure, MtRML = Minimum tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab), MtRTiM = Minimum tillage + Crop residues + Tithonia diversifolia + Animal manure, MtRTiP = Minimum tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu). 2 Soil chemical analysis done following Ryan et al. [30]: pH using pH meter; Nitrogen by Kjeldahl method; Organic carbon by modified Walkley and Black wet oxidation method; Phosphorus by Spectrophotometer; Potassium and Sodium by Flame Photometer; Calcium, Magnesium, Zinc, Iron, Copper, and Manganese by Atomic Absorption Spectrophotometry.
Table
Table 2. Field treatments implemented in the Kangutu experimentation site.
Table 2. Field treatments implemented in the Kangutu experimentation site.
TreatmentsAbbreviations
Minimum tillage ControlMtC
Minimum tillage + Sole Mineral fertilizerMtMf
Minimum tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab)MtRML
Minimum tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu)MtRTiP
Minimum tillage + Crop residues + Mineral fertilizer + Animal manureMtRMfM
Minimum tillage + Crop residues + Tithonia diversifolia + Animal manureMtRTiM
Minimum tillage + Crop residues + Mineral fertilizerMtRMf
Conventional tillage ControlCtC
Conventional tillage + Sole Mineral fertilizerCtMf
Conventional tillage + Crop residues + Animal manure + Legume intercrop (Dolichos Lablab)CtRML
Conventional tillage + Crop residues + Tithonia diversifolia + Phosphate rock (Minjingu)CtRTiP
Conventional tillage + Crop residues + Mineral fertilizer + Animal manureCtRMfM
Conventional tillage + Crop residues + Tithonia diversifolia + Animal manureCtRTiM
Conventional tillage + Crop residues + Mineral fertilizerCtRMf
Table
Table 3. Nutrient composition (%) of animal manure and Tithonia diversifolia applied in the soil at the Kangutu site.
Table 3. Nutrient composition (%) of animal manure and Tithonia diversifolia applied in the soil at the Kangutu site.
Organic InputNutrient Amount (%)
NPCC/NADF 1CelluloseLigninPolyphenols
Animal manure1.70 b20.53 a42.06 a16.30 a37.31 b21.75 a27.94 a0.18 a
Tithonia. diversifolia3.32 a0.14 a41.30 a11.41 b49.59 a18.08 a12.88 b0.14 a
1 ADF = Acid Detergent Fibre. 2 Means with the same letter in each column are not statistically different at p < 0.05.
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Githongo, M.; Ngatia, L.; Kiboi, M.; Muriuki, A.; Fliessbach, A.; Musafiri, C.; Fu, R.; Ngetich, F. The Structural Quality of Soil Organic Matter under Selected Soil Fertility Management Practices in the Central Highlands of Kenya. Sustainability 2023, 15, 6500. https://doi.org/10.3390/su15086500

AMA Style

Githongo M, Ngatia L, Kiboi M, Muriuki A, Fliessbach A, Musafiri C, Fu R, Ngetich F. The Structural Quality of Soil Organic Matter under Selected Soil Fertility Management Practices in the Central Highlands of Kenya. Sustainability. 2023; 15(8):6500. https://doi.org/10.3390/su15086500

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Githongo, Miriam, Lucy Ngatia, Milka Kiboi, Anne Muriuki, Andreas Fliessbach, Collins Musafiri, Riqiang Fu, and Felix Ngetich. 2023. "The Structural Quality of Soil Organic Matter under Selected Soil Fertility Management Practices in the Central Highlands of Kenya" Sustainability 15, no. 8: 6500. https://doi.org/10.3390/su15086500

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