Biogas slurry reshapes soil carbon dynamics in dryland maize fields

Biogas slurry, the liquid byproduct of anaerobic digestion, has long been valued as a substitute for mineral fertilizer in crop production. A new multi-year field experiment conducted at the National Field Scientific Observation and Research Station of Dryland Farmland Ecosystem in Shouyang, Shanxi Province, China, shows that its influence on soil extends well beyond nutrient supply. Published in Carbon Research, the study reports that topdressing maize with biogas slurry reshapes the quantity, composition and vertical distribution of soil dissolved organic carbon (DOC), one of the most dynamic and mobile fractions of the soil organic carbon pool, in ways that depend strongly on both crop growth stage and soil depth.

The research team, led by Xiaoyang Liang and colleagues, set up six treatments between 2021 and 2023: an unfertilized control receiving only basal fertilizer, a pure chemical fertilizer topdressing treatment, and four replacement levels in which 25, 50, 75 or 100 percent of the topdressed chemical nitrogen was substituted with biogas slurry delivered through drip irrigation. Topdressing nitrogen was applied at 150 kilograms per hectare, split between the six-leaf and twelve-leaf maize stages, on top of a standardized basal fertilization of 130:40:30 kilograms per hectare of nitrogen, phosphorus pentoxide and potassium oxide. Eighteen plots of 4 by 12 meters, three per treatment, were randomly arranged in the field.

To track DOC dynamics from reproductive growth to physiological maturity, the researchers sampled soil at three maize stages in 2023: tasseling (VT) in late July, milk ripening (R3) in late August, and maturity (R6) in early October. Samples were taken at depths of 0–10, 10–20 and 20–40 centimeters near the drip irrigation emitters, using a five-point composite sampling method with three replicates per treatment. DOC was extracted with ultrapure water at a 1:5 soil-to-water ratio and quantified with a total organic carbon analyzer, while its optical and structural properties were characterized using ultraviolet–visible spectroscopy, excitation-emission matrix (EEM) fluorescence spectroscopy with fluorescence regional integration, and Fourier transform infrared spectroscopy.

The headline finding is a striking stage-dependent reversal. Compared with pure chemical fertilizer topdressing, biogas slurry topdressing increased soil DOC content at the tasseling stage by 16.74 percent on average, with higher slurry ratios producing larger increases. Yet by the milk stage, the direction had flipped: DOC content under slurry treatments was 20.19 percent lower on average than under chemical fertilizer. The authors interpret this pattern as a sequence of rapid soluble carbon enrichment after slurry application, followed by biological consumption as roots and microorganisms exploited the newly available labile substrates during the peak demand period of maize growth. Across all layers and stages, topdressing of any kind increased DOC content by 14.90 percent relative to the unfertilized control.

The vertical dimension of the story proved equally consequential. During tasseling and milk ripening, DOC content declined linearly with soil depth under all treatments. At maturity, however, the slurry treatments changed the temporal and depth patterns observed under chemical fertilizer: in the surface 0–10 centimeter layer, slurry accelerated the post-tasseling decline in DOC, with the pure slurry treatment showing a rate of decrease 3.66 times higher than chemical fertilizer, while in deeper layers it shifted the temporal pattern from an initial rise and fall to either a linear decrease or a decrease followed by an increase. The authors suggest this reflects enhanced downward redistribution of DOC through residue decomposition and late-season leaching.

Spectral indices added a layer of mechanistic detail. The ultraviolet–visible slope ratio, a proxy for molecular size, rose by 81.51 percent at tasseling and 185.82 percent at milk ripening under slurry topdressing relative to chemical fertilizer, indicating a shift toward smaller, more mobile DOC fractions, but fell by 31.12 percent at maturity. Colored dissolved organic matter and the aromaticity and hydrophobicity indices SUVA254 and SUVA260 decreased during tasseling, pointing to microbial consumption of chromophoric, aromatic and hydrophobic fractions rather than simple accumulation of aromatic carbon. Fluorescence indices told a complementary tale: the fluorescence index, freshness index and biological index, which track DOC source and microbial renewal, increased on average by 2.08, 3.80 and 3.98 percent under slurry, while the humification index rose by 2.83 percent, suggesting that slurry simultaneously stimulates fresh microbial production and partial conversion of carbon into humified forms.

The compositional analysis revealed what the slurry itself carried and what the soil did with it. Raw biogas slurry was dominated by humic acid-like substances, which made up 89.36 percent of its fluorescence signal, with fulvic acid-like substances accounting for 7.02 percent. In soil, slurry topdressing increased the proportions of humic acid-like substances by 5.86 percent and soluble microbial by-products by 4.97 percent relative to chemical fertilizer, while decreasing aromatic protein I, aromatic protein II and fulvic acid-like fractions by 12.52, 20.81 and 7.41 percent. Infrared spectroscopy showed that slurry increased the relative abundances of aromatic carbon, C≡C and O–H/N–H functional groups by 5.92, 13.00 and 3.98 percent respectively, and raised the ratio of carbon-containing to other functional groups, a shift the authors link to selective preservation of chemically more stable carbon structures.

Perhaps the most consequential result concerns which factor governs this system. The team’s hypotheses had cast biogas slurry as a key driver of DOC change, but partial least squares path modeling told a different story. Soil depth emerged as the most direct and significant factor regulating the EEM indices, organic components and DOC content, with the strongest total effects including soil depth on the EEM index (0.734), soil depth on organic components (0.556) and soil depth on DOC content (−0.520). Maize growth stage dominated the ultraviolet–visible indices with a total effect of 0.744, while slurry-derived DOC input acted mainly by modifying the quantity and quality of exogenous carbon entering the system. Principal coordinate analysis reinforced this hierarchy: the relative importance of factors regulating DOC composition followed the order soil depth, then growth stage, then treatment.

The authors are careful about the limits of their inference. The transformation pathways they describe were deduced from spectroscopic characterization and lack direct validation at the molecular level or over long-term continuous dynamics, and they call for future work combining long-term monitoring, high-resolution mass spectrometry and functional validation. Even so, the study offers dryland agriculture a practical reframing of biogas slurry management. Because the slurry carbon did not simply accumulate but was rapidly consumed, transformed or redistributed, the value of slurry topdressing lies in timing and profile-scale retention: application schedules should consider not only the fertilizer replacement ratio but also the capacity of different soil layers to hold or transport dissolved carbon. In a system where water scarcity constrains production and mineral fertilizer prices keep rising, treating slurry as a tool for regulating carbon turnover, rather than merely a nitrogen substitute, may be its most underappreciated contribution.

Subject of Research: Field experiment on how biogas slurry topdressing regulates soil dissolved organic carbon in dryland maize

Article Title: Effects of biogas slurry topdressing on soil dissolved organic carbon: a field experiment

Article References: Liang, X., Wang, C., Wang, H., Wen, Y., Wang, J., Qin, J., & Chen, B. (2026). Effects of biogas slurry topdressing on soil dissolved organic carbon: a field experiment. Carbon Research, 5(1), Article 49. https://doi.org/10.1007/s44246-026-00295-z

Image Credits: AI Generated

DOI: 10.1007/s44246-026-00295-z

Keywords: biogas slurry, dissolved organic carbon, soil carbon, maize, dryland agriculture, fluorescence spectroscopy, UV-Vis spectroscopy, FTIR, humification, soil depth, nutrient recycling, field experiment