Effects of Incorporating Medium and Trace Elements on the Mineralization Characteristics and Soil Organic Carbon Components of Aeolian Sandy Soil

LIU Jiayue¹, KOU Wei¹, YUAN Jianqiang², XUE Shaoqi¹, WANG Xudong¹

¹College of Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China
²Hongliulin Mining Company, Binhe New District, Shenmu City, Yulin City, Shaanxi Province, Shenmu 719300, China

Abstract

To investigate the mineralization characteristics of organic fertilizer amended with medium and trace elements in aeolian sandy soil and its effects on soil organic carbon components, indoor incubation experiments and field trials were conducted. We examined the decomposition and residual rates of organic fertilizer with and without trace elements, as well as the impacts of different application rates on soil organic carbon (SOC), active organic carbon, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), microbial biomass carbon (MBC), and the contents of amino sugars including glucosamine, galactosamine, and muramic acid. The results demonstrated that compared with organic fertilizer alone, the addition of trace elements in incubation experiments reduced SOC mineralization. In field trials, trace element-amended organic fertilizer further increased active organic carbon (1.79%–7.95%), low-activity organic carbon (2.20%–4.91%), MAOC (3.89%–7.95%), MBC (1.21%–13.32%), muramic acid (2.41%–6.14%), and total amino sugars (3.46%–6.32%). However, it decreased high-activity organic carbon (0.71%–1.48%) and POC (4.91%–5.86%) content, though these differences were not statistically significant. These findings indicate that trace element-amended organic fertilizer can mitigate mineralization of organic fertilizer in aeolian sandy soil, enhance the contents of labile organic carbon, recalcitrant organic carbon, MAOC, and MBC, and promote organic carbon turnover and sequestration.

Keywords: organic fertilizer; mineralization; organic carbon; aeolian sandy soil; medium and trace elements; organic carbon components

Introduction

The soil carbon pool represents the largest terrestrial carbon reservoir, directly influencing global terrestrial ecosystem carbon balance and playing a crucial role in soil quality. Soil organic carbon (SOC) constitutes an important component of the soil carbon pool, exerting significant effects on ecosystem resilience and productivity. Mineralization represents the primary pathway for carbon release from soil to atmosphere. Under the context of global climate change, strengthening research and management of SOC not only helps mitigate climate change but also promotes healthy development of soil ecosystems and sustainable agriculture.

China's soils commonly suffer from low fertility. According to statistics, aeolian sandy soil covers 17.58% of national land area, with approximately 6.84×10⁶ km² distributed across northern China's desertified lands. As a typical soil type in northern regions, aeolian sandy soil is characterized by low clay content, high porosity, low fertility, and insufficient organic carbon, which affects agricultural ecological environments and sustainable development. SOC components include active organic carbon, particulate organic carbon, microbial biomass carbon, and mineral-associated organic carbon, which respond more rapidly to agricultural management practices than total SOC. Therefore, studying SOC components facilitates rapid assessment of overall organic carbon accumulation and stability.

Previous studies have improved aeolian sandy soil through organic fertilizer, biochar, and natural mineral applications. Organic fertilizer application represents the most direct method for increasing SOC content and components, and is considered an effective approach for improving soil quality. However, due to good soil aeration in aeolian sandy soil regions, exogenous organic materials such as straw and organic fertilizer readily undergo mineralization, resulting in low humification coefficients and slow organic carbon enhancement. Medium and trace elements such as calcium, manganese, zinc, and iron are essential plant nutrients involved in various life processes. Current research indicates that adding appropriate amounts of trace elements to organic fertilizer can increase soil soluble trace element content, improve bioavailability of exchangeable cations, and facilitate plant uptake. These elements can also strongly adsorb to soil components, exchange on soil surfaces, and form complexes with organic carbon, thereby inhibiting organic carbon decomposition and benefiting SOC sequestration.

Therefore, this study employed incubation and field experiments to investigate the mineralization characteristics of trace element-amended organic fertilizer in aeolian sandy soil and its effects on SOC components, providing theoretical support for addressing issues of low fertility and rapid organic carbon mineralization in aeolian sandy soil.

Materials and Methods

1.1 Study Area and Soil Properties

Soil samples were collected from Shenmu City in northern Shaanxi Province (109°40′–110°54′E, 38°13′–39°27′N), which features a temperate semi-arid continental climate with an average annual temperature of 8.5°C, annual precipitation of 423.2 mm, and 2876 hours of sunshine. The experimental soil was aeolian sandy soil with light loam texture, containing 6.0 g·kg⁻¹ SOC, 0.48 g·kg⁻¹ total nitrogen, 0.28 g·kg⁻¹ total phosphorus, 21.14 g·kg⁻¹ total potassium, 8.52 mg·kg⁻¹ available phosphorus, 165.67 mg·kg⁻¹ available potassium, and pH 8.59. The cropping system was annual single-crop corn (variety DF899).

1.2.1 Experimental Materials

Two organic fertilizer materials were selected: (1) OF, fermented and decomposed crop straw; and (2) OF+ME, sheep manure compost supplemented with medium and trace elements at ratios of calcium nitrate, ferrous sulfate, manganese sulfate, and zinc sulfate. All fertilizers were provided by Inner Mongolia Hengsheng Environmental Protection Technology Engineering Co., Ltd., with main nutrient contents shown in Table 1 [TABLE:1].

1.2.2 Experimental Methods

Incubation Experiment: 500 g of air-dried soil sieved through a 2 mm mesh was placed in 500 mL plastic bottles. Treatments included: (1) OF (organic fertilizer), (2) OF+ME (organic fertilizer with trace elements), and (3) CK (control without organic fertilizer). Each treatment was replicated three times. Organic fertilizers were mixed into the soil at 6 g·kg⁻¹ (soil basis) for OF and OF+ME treatments. Soil moisture was adjusted to 60% of field water holding capacity, and bottles were covered with perforated lids and incubated at 25°C for 360 days. Moisture was replenished every 5 days by weight, and destructive sampling was performed at 30, 60, 90, 120, 150, 180, 240, and 360 days to determine SOC content and calculate decomposition and residual rates.

Field Experiment: Conducted in aeolian sandy soil with two organic fertilizers applied at four rates: 7.5 t·hm⁻² (OF1, OF1+ME), 15 t·hm⁻² (OF2, OF2+ME), 22.5 t·hm⁻² (OF3, OF3+ME), and 30 t·hm⁻² (OF4, OF4+ME), plus a no-fertilizer control (CK). The experiment used a randomized block design with 66.7 m² plots (10 m × 6.67 m). Soil samples were collected before corn sowing (May 1) and after harvest (October 1), air-dried at room temperature, sieved, and analyzed for SOC and its components.

1.2.3 Soil Sample Analysis

SOC content was determined by the K₂Cr₂O₇-H₂SO₄ heating method. Active organic carbon was fractionated using three KMnO₄ concentrations (33, 167, and 333 mmol·L⁻¹) to oxidize different activity levels, with colorimetric measurement at 565 nm, dividing into high-activity, medium-activity, and low-activity organic carbon. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) were separated by sodium hexametaphosphate dispersion, with the >53 μm fraction as POC and <53 μm fraction as MAOC. Microbial biomass carbon (MBC) was measured by chloroform fumigation extraction. Amino sugars were determined as aldononitrile acetate derivatives by gas chromatography following Zhang et al. (1996), yielding three amino monosaccharides: glucosamine, galactosamine, and muramic acid.

1.3 Calculations and Data Processing

The residual rate of organic carbon from organic materials was calculated as:

$$CX = \frac{g_1C/gC - g_2C/gC}{gC} \times 100$$

where $CX$ is the organic carbon residual rate (%), $g_1C/gC$ is SOC content after soil-organic fertilizer mixture incubation (g·kg⁻¹), $g_2C/gC$ is SOC content in control soil after incubation (g·kg⁻¹), and $gC$ is the initial organic carbon content of added organic fertilizer (g·kg⁻¹). Corrections were made for SOC decomposition in control soil and mineralization of added materials.

Glucosamine (GluN) and muramic acid (MurA) were used as biomarkers for fungal and bacterial residues:

$$\text{Bacterial necromass carbon (BNC)} = 45 \times \text{MurA-C}$$

$$\text{Fungal necromass carbon (FNC)} = \left(\frac{\text{GluN-C}}{179} - \frac{2 \times \text{MurA-C}}{251}\right) \times 179 \times 9$$

where MurA-C is muramic acid carbon content (mg·kg⁻¹), GluN-C is glucosamine carbon content (mg·kg⁻¹), and 45 and 9 are conversion factors to bacterial and fungal necromass carbon. Microbial necromass carbon is the sum of bacterial and fungal necromass carbon.

Data were analyzed using Microsoft Office Excel 2016 and SPSS 25.0 software for variance analysis and significance testing (α = 0.05).

Results

2.1 Organic Carbon Residual Rate Under Different Organic Fertilizer Treatments in Incubation

Compared with organic fertilizer alone, trace element addition increased the residual rate of organic carbon. As shown in Figure 1 [FIGURE:1], both organic fertilizer treatments exhibited rapid initial decomposition followed by slower decomposition. Decomposition rates were similar during the first 180 days. After 180 days, the trace element-amended treatment showed slowed decomposition with increasing time, with decomposition rate significantly reduced by 4.86% and 2.99% at 240 and 360 days, respectively (P < 0.05). The residual rate of organic carbon in the trace element treatment increased by 1.22% and 1.50% at 180 and 360 days compared with organic fertilizer alone, though differences were not significant (Figure 2 [FIGURE:2]).

2.2 Changes in Soil Organic Carbon Content Under Different Field Application Rates

Organic fertilizer with trace elements increased SOC content compared with organic fertilizer alone. As shown in Figure 3 [FIGURE:3], SOC content in all treatments increased significantly with increasing fertilizer rate compared with CK, with increases of 25.45% (OF4) and 27.33% (OF4+ME) at the highest rate (P < 0.05). At the same application rate, trace element addition increased SOC content by 0.44% (OF1+ME), 0.87% (OF2+ME), 1.50% (OF3+ME), and 1.22% (OF4+ME) compared with organic fertilizer alone.

2.3 Changes in Soil Active Organic Carbon Under Different Field Application Rates

Trace element addition increased active organic carbon content while decreasing high-activity organic carbon content, though differences were not significant. As shown in Figure 4 [FIGURE:4], soil active organic carbon fractions followed the order: low-activity > medium-activity > high-activity. With increasing fertilizer rates, both organic fertilizer and trace element-amended treatments increased high-, medium-, and low-activity organic carbon content (P < 0.05). At the same application rate, trace element treatments increased medium-activity organic carbon by 1.98–2.72% and low-activity organic carbon by 2.20–4.91%, while decreasing high-activity organic carbon by 0.71–1.48% compared with organic fertilizer alone, though differences were not significant.

2.4 Changes in Soil POC, MAOC, and MBC Under Different Field Application Rates

Trace element addition increased MAOC and MBC content while decreasing POC content (Figure 5 [FIGURE:5]). Compared with CK, both organic fertilizer treatments increased POC and MAOC content with increasing rates, with MAOC content increasing significantly by 10.42% (OF4) and 16.67% (OF4+ME) (P < 0.05). MBC content increased significantly by 10.83% (OF4) and 9.27% (OF4+ME) (P < 0.05). Trace element addition further increased MAOC and MBC content compared with organic fertilizer alone, with increases of 5.10–8.10% for MAOC and 4.91–5.86% for MBC at the same application rate, though POC differences were not significant.

The proportions of SOC fractions changed accordingly (Table 2 [TABLE:2]). Trace element addition increased the MAOC/SOC ratio by 0.51–6.99% and MBC/SOC ratio by 4.45–26.18%, while decreasing the POC/SOC ratio by 0.22–7.20% (P < 0.05).

2.5 Changes in Soil Amino Sugars Under Different Field Application Rates

Trace element addition increased total amino sugars and individual amino monosaccharides (Figure 6 [FIGURE:6]). Compared with CK, organic fertilizer and trace element treatments significantly increased total amino sugars and all three monosaccharides (P < 0.05). Trace element addition further increased these contents compared with organic fertilizer alone, with total amino sugars increasing by 3.46–6.32%, glucosamine by 1.21–13.32%, galactosamine by 2.41–6.14%, and muramic acid by 3.61–6.62% at the same application rate (P < 0.05).

2.6 Contribution of Soil Amino Sugars to Organic Carbon

Trace element addition increased the contribution of amino sugar carbon to SOC (Figure 7 [FIGURE:7]). Compared with CK, total amino sugar carbon, bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and microbial necromass carbon increased significantly (P < 0.05). Trace element addition further increased these contributions, with total amino sugar carbon increasing by 2.41–6.14%, BNC by 2.94–6.96%, FNC by 3.25–6.62%, and microbial necromass carbon by 2.70–4.99% compared with organic fertilizer alone (P < 0.05). The contribution rate of microbial necromass carbon to SOC increased by 3.81–4.99% across treatments (P < 0.05).

Discussion

3.1 Effects of Trace Element Addition on Organic Fertilizer Mineralization Under Incubation Conditions

Numerous studies have shown that exogenous organic matter type affects mineralization. Metal (oxyhydr)oxides are key factors controlling SOC stability. In this study, both organic fertilizers exhibited rapid initial decomposition followed by a slow phase, consistent with Chen et al. [21] in red paddy soil. This pattern likely reflects a shift from readily available, active organic carbon to more recalcitrant carbon sources as decomposition progresses, leading to relatively stable mineralization rates [35]. Compared with organic fertilizer alone, trace element addition mitigated mineralization and increased residual rates, possibly because the added elements supplemented soil nutrients, formed oxides that complexed with SOC, and inhibited decomposition. Iron, calcium, zinc, and manganese can adsorb or chelate organic carbon, inhibiting hydrolytic enzyme activity and SOC mineralization, thereby affecting carbon turnover and sequestration [36, 37].

3.2 Effects of Trace Element Addition on Active Organic Carbon in Field Trials

Returning straw and applying organic fertilizer are important measures for increasing SOC. This study found that both organic fertilizers significantly increased SOC and all active organic carbon fractions compared with CK, consistent with Wang et al. [43] in red soil. The increased SOC was primarily in active fractions. Trace element addition enhanced this effect, increasing SOC, MAOC, and MBC while decreasing POC. This may occur because organic fertilizer provides nutrients and active organic carbon that stimulate microbial growth, with subsequent decomposition adding new organic materials that increase active carbon pools [38]. The enriched organic matter increases cation exchange capacity [39], enhancing adsorption of element cations. Trace elements may adsorb or chelate with highly oxidizable organic carbon, reducing its content.

3.3 Effects of Trace Element Addition on Organic Carbon Components in Field Trials

Adding trace elements to organic fertilizer increased SOC, total amino sugars, individual amino monosaccharides, and microbial necromass carbon (Figure 7). This likely occurs because organic fertilizer provides nutrients and energy for microbes, altering microbial activity and stimulating metabolism, which promotes necromass accumulation [31, 32]. Trace element input further promotes microbial necromass accumulation, which can be physically protected within soil aggregates and chemically protected through interactions with iron oxides and minerals, converting to stable SOC [33, 34]. Trace element addition increased the MAOC/SOC ratio while decreasing POC/SOC, possibly because trace elements improve substrate availability and elemental composition [40], promoting formation of micro-aggregates that increase microbial numbers, diversity, and growth rates, thereby increasing MAOC content. Fungal necromass carbon contributed more to SOC than bacterial necromass carbon [55], indicating fungal dominance, likely because fungal residues decompose slowly and can be strongly adsorbed to soil minerals [56, 57]. POC may be more vulnerable to microbial decomposition due to lack of physicochemical protection, leading to transformation from particulate to mineral-associated forms [58].

Conclusion

This study investigated the effects of trace element-amended organic fertilizer on mineralization characteristics and SOC components in aeolian sandy soil, yielding the following main conclusions:

1) Adding trace elements to organic fertilizer can reduce mineralization rate of organic fertilizer in aeolian sandy soil and increase SOC content.

2) Trace element addition increased microbial carbon content, glucosamine, galactosamine, muramic acid, microbial necromass carbon, and their contribution rates, promoting further turnover of exogenous organic fertilizer under microbial action.

3) Trace element addition increased active organic carbon, low-activity organic carbon, MAOC, and MBC contents and their proportions of SOC, facilitating complexation of newly formed humus with soil minerals to form stable SOC and promote carbon sequestration.

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