Changing soil physicochemical properties, reducing net global warming potential and increasing maize silage yields by one-time high-dose incorporation of vegetable residue in drylands

Technical Summary: One-Time High-Dose Incorporation of Vegetable Residue in Drylands

Problem Statement
China generates over 340 million tons of vegetable residue annually, representing the second-largest source of agricultural residues after staple crop straws. While these residues contain significant nutrient reserves (N, P, K) and organic matter, their high moisture content (>85%) and low dry matter content (<15%) hinder the efficiency of traditional field incorporation methods. Improper disposal in trading markets and production regions causes severe environmental pollution. Furthermore, the Loess Plateau, a major vegetable production region, faces soil degradation due to intensive cultivation and inadequate fertilization, with current yields often below 60% of their potential. There is a critical need to determine optimal strategies for recycling vegetable residues that balance environmental mitigation (greenhouse gas emissions) with agricultural productivity in semi-arid ecosystems.

Methodology
A two-year field experiment (2020–2022) was conducted at the Experimental Station of Semi-Arid Ecosystem on the Loess Plateau (Gansu Province, China). The study utilized a randomized block design with silage maize (Zea mays L.) as the test crop. The experimental treatments involved a factorial combination of:

• Incorporation Rates (W): Three levels of one-time high-dose vegetable residue application: W1 (800 t·ha⁻¹), W2 (1600 t·ha⁻¹), and W3 (2400 t·ha⁻¹).
• Incorporation Depths (D): Three depths: D1 (10 cm), D2 (20 cm), and D3 (30 cm).
• Control: A no-incorporation treatment (NW) where the topsoil was plowed without residue amendment.

The vegetable residues consisted of a mixture of baby cabbage, cauliflower, celery, and asparagus lettuce collected from a local trading center. In 2021, maize was grown under no-tillage conditions; in 2022, ridge-furrow mulching technology was introduced.

Key measurements included:

• Soil Physicochemical Properties: Temperature, moisture content, pH, redox potential (RP), dissolved organic carbon (DOC), and inorganic nitrogen (NH₄⁺-N, NO₃⁻-N).
• Greenhouse Gas (GHG) Emissions: CO₂, N₂O, and CH₄ fluxes were monitored using static chambers and gas chromatography.
• Climate Impact Metrics: Net Global Warming Potential (NGWP) and Greenhouse Gas Intensity (GHGI) were calculated, integrating direct soil emissions, indirect emissions from agricultural operations, and carbon sequestration potential from residue incorporation.
• Crop Productivity: Silage maize biomass yield was measured at the physiological milky stage.
• Statistical Analysis: Data were analyzed using ANOVA, Structural Equation Modeling (SEM), Pearson correlation, and Random Forest analysis to determine factor importance.

Key Results

• Soil Physicochemical Properties: High-dose incorporation significantly altered soil conditions. Both incorporation rate and depth positively correlated with soil temperature and moisture content. Residue incorporation induced a biphasic response in soil pH (initial increase followed by decline) and a sustained decrease in redox potential. DOC and NH₄⁺-N concentrations peaked 15–30 days after incorporation and were strongly dose-dependent, increasing up to 4.8 times (DOC) and 139.9 times (NH₄⁺-N) relative to the control. Conversely, NO₃⁻-N accumulation showed an inverse trend with incorporation intensity.
• GHG Emissions: Increasing the incorporation rate significantly elevated CO₂, N₂O, and CH₄ emissions (5.15–12.77, 25.54–70.67, and 0.92–2.24 times the control, respectively). CO₂ emissions exhibited a tri-modal temporal pattern, peaking at day 20. N₂O emissions were primarily influenced by incorporation depth, with deeper incorporation suppressing emissions at lower rates (W1, W2) but increasing them at the highest rate (W3). CH₄ fluxes shifted from positive to negative (sink) after day 150.
• Net Global Warming Potential (NGWP) and GHGI: Despite increased GHG emissions, the high-dose treatments resulted in negative NGWP values, indicating a net carbon sink effect. There was a strong negative correlation between incorporation rate and NGWP (r = -0.91, P < 0.01). The carbon sequestration benefits from the massive organic carbon input outweighed the associated emissions.
• Crop Yield: Silage maize yields responded parabolically to the incorporation rate. Moderate rates (W1 and W2) significantly increased yields (up to 112.4% relative to control), while the highest rate (W3) caused yield reductions (17.7–41.1% below control), attributed to secondary salinization and excessive nutrient accumulation. Yield showed a positive linear relationship with incorporation depth.
• Optimal Strategy: The combination of 1600 t·ha⁻¹ application rate (W2) and 30 cm incorporation depth (D3) was identified as optimal. This treatment maximized crop productivity benefits while minimizing environmental impacts, achieving a balance between carbon sequestration and yield.

Significance and Claims
The paper claims that one-time high-dose incorporation of vegetable residue is a feasible strategy for the semi-arid Loess Plateau to address both waste management and soil fertility issues. The study demonstrates that while such incorporation initially stimulates GHG emissions, the substantial carbon sequestration potential and the resulting increase in crop productivity (in moderate treatments) effectively offset these emissions, leading to a net reduction in global warming potential.

The authors assert that the practice can enhance soil fertility, partially replace synthetic fertilizers, and improve crop yields, thereby promoting sustainable resource utilization. However, they caution that excessive application rates (e.g., 2400 t·ha⁻¹) can lead to soil salinization and yield decline, highlighting the necessity of identifying optimal rates and depths. The study provides critical insights for balancing carbon sequestration goals with agricultural productivity, suggesting that controlled vegetable residue incorporation, particularly when combined with agronomic interventions like ridge-furrow mulching, represents a sustainable management strategy for arid regions. The authors note that long-term monitoring is required to fully assess the sustainability of these strategies regarding soil carbon saturation and continued GHG emissions.