Straw and biochar build soil carbon in salty farmland by two different routes

On salt-degraded farmland in China’s Yellow River Delta, a two-year field experiment has shown that the route a soil takes to store more organic carbon depends sharply on which organic amendment farmers add alongside manure. Combining manure with maize straw pushed soil organic carbon up mainly by feeding labile carbon pools and stimulating microbial enzyme activity, while combining manure with biochar achieved similar gains largely by reshaping the soil’s physical architecture, building larger aggregates and better-connected pore networks that physically shield carbon from decomposition. The study, published in Biochar, was led by researchers working in Dongying City, Shandong Province, where saline degradation coincides with critically low organic matter and limited crop productivity.

The experiment was set up in 2022 as a randomized block design with four treatments and three replicates, each plot measuring 100 square meters. The treatments were compound fertilizer alone, compound fertilizer plus manure, compound fertilizer plus manure plus straw, and compound fertilizer plus manure plus biochar. All organic amendments were applied at an equivalent carbon input rate of 5,250 kilograms of carbon per hectare, incorporated into the top 20 centimeters of soil at the start of the experiment, with no additional organic amendments in 2023. The biochar was produced by pyrolyzing maize straw at 500 degrees Celsius under oxygen-limited conditions, and the maize variety Denghai 605 was cultivated over two consecutive growing seasons at a density of 67,500 plants per hectare. The initial soil at the site contained 8.67 grams of organic carbon per kilogram, with a pH of 8.23.

After two growing seasons, both combined treatments had outperformed manure alone. In 2023, soil organic carbon content under manure plus straw and manure plus biochar was 18.8 to 19.4 percent higher than under manure alone, a statistically significant difference, and both treatments also increased the soil organic carbon stock and the carbon sequestration rate. Both treatments lowered soil pH relative to manure alone, by 7.8 percent for biochar and 6.5 percent for straw, and both reduced bulk density while increasing total nitrogen. The reductions in salinity and pH matter because osmotic stress and ion toxicity from salt restrict crop growth and depress residue returns, which in turn accelerates the decline of organic carbon storage in these soils.

The functional divergence between the two strategies emerged clearly in the biochemical measurements. Manure plus straw increased the activity of carbon-cycling enzymes, including beta-glucosidase, beta-xylosidase, cellobiohydrolase, and leucine aminopeptidase, with increases of 9.7 to 32.6 percent in 2023. It also raised the labile carbon pools: microbial biomass carbon rose by 30.7 to 74.9 percent and easily oxidizable organic carbon by 60.1 percent compared with manure alone. The treatment also increased vector length in enzyme stoichiometry analysis, indicating greater microbial carbon limitation, and raised the half-saturation constant Km for several enzymes, which the authors interpret as reduced enzyme-substrate affinity, possibly reflecting microbial allocation toward high-throughput, low-affinity enzymes when labile carbon is abundant.

Microbial necromass carbon, measured from amino sugar biomarkers, told a complementary story. Manure plus straw increased microbial necromass carbon by 10.8 percent and fungal necromass carbon by 11.2 percent relative to manure alone, and necromass fractions were positively correlated with soil organic carbon content. The authors suggest that straw-derived carbon, processed through intensified microbial anabolism, was being channeled into persistent fungal residues, which contributed disproportionately more to soil organic carbon than bacterial necromass did. They note that fungi assimilate organic substrates with greater efficiency, form stable complexes with plant polyphenols, and possess enzymatic machinery suited to degrading recalcitrant plant residues. Across all treatments, phosphorus limited microbial metabolism more than nitrogen did, a pattern the authors attribute to the alkaline pH of saline-alkali soils promoting calcium-phosphate precipitation that reduces bioavailable phosphorus.

Manure plus biochar followed a different path altogether. Compared with manure alone, it increased the proportion of large macroaggregates larger than 2 millimeters by 40.9 percent, raised the mean weight diameter by 31.9 percent and the geometric mean diameter by 39.3 percent, and increased aggregate-associated carbon in large and small macroaggregates by 21.2 and 17.1 percent respectively. X-ray computed tomography of macroaggregates revealed that biochar increased the proportion of pores larger than 100 micrometers by 16.4 percent, decreased micropores, and raised total porosity by 25.3 to 45.8 percent compared with the fertilizer-only and manure-only treatments. The authors propose that biochar’s porous structure acts as a skeleton that adsorbs fine particles, counteracts the clay dispersion caused by sodium ions, and strengthens inter-particle cohesion, while its recalcitrant carbon compounds and high surface area facilitate organo-mineral interactions and cation bridging that stabilize aggregates.

Microbial community analysis reinforced the mechanistic split. Manure plus straw increased the bacterial Shannon diversity index and enriched Proteobacteria and Acidobacteria while suppressing Actinobacteria and Chloroflexi, whereas manure plus biochar increased fungal Shannon diversity by 2.8 percent. Co-occurrence network analysis identified four bacterial and four fungal ecological modules. Under manure plus straw, the relative abundance of bacterial modules dominated by Actinobacteria declined, and soil organic carbon was negatively correlated with those modules. Under manure plus biochar, a fungal module dominated by Ascomycota increased in relative abundance, and soil organic carbon was positively correlated with it. The authors are careful to note that these correlations do not imply direct causation and highlight potential mechanistic pathways requiring further validation.

Partial least squares path modeling tied the threads together statistically. Under manure plus straw, the treatment had direct positive effects on labile carbon fractions, microbial resource limitation, and enzyme activities, with path coefficients of 0.63, 0.64, and 0.73; labile carbon fractions influenced bacterial necromass, which in turn affected soil organic carbon, while enzyme activities influenced fungal necromass and then soil organic carbon. Under manure plus biochar, the treatment positively affected soil properties and soil aggregates with path coefficients of 0.53 and 0.72, and soil aggregates influenced bacterial necromass and soil organic carbon, while the fungal module showed a direct positive effect on soil organic carbon. The model’s fit was assessed using multiple criteria including goodness of fit, average variance extracted, and standardized root mean square residual.

The study’s scope and limitations deserve attention. It ran for only two years, which the authors acknowledge cannot establish long-term equilibrium conditions, and it did not include isolated straw or biochar treatments, so the comparison centers on what supplementing manure with either material achieves rather than on standalone applications. The network analyses can identify putative keystone taxa but cannot distinguish direct metabolic interactions from habitat-filtered coexistence without isotope probing or synthetic community manipulations. Environmental factors such as temperature, moisture, crop growth stage, and salinity fluctuations may strongly influence microbial metabolism and carbon turnover, and the authors call for multi-season and long-term observations to fully understand the dynamics of carbon sequestration in these amended coastal saline soils.

Even with those caveats, the findings carry practical weight for a region where the Yellow River Delta spans roughly 500,000 hectares of salt-affected land. Both strategies offered viable, mechanism-dependent pathways for improving carbon management and fertility restoration in coastal saline ecosystems, but they are not interchangeable. Where the goal is rapid biochemical turnover and microbial processing of fresh plant residues, straw appears to be the more direct lever; where the goal is durable physical protection of carbon within stable aggregates and well-connected pore networks, biochar’s structural properties seem to do the heavy lifting. For farmers and land managers reclaiming saline soils with manure as the base amendment, the choice of supplement may determine not just how much carbon the soil gains, but how long that carbon stays put.

Subject of Research: Mechanisms of soil organic carbon sequestration under manure combined with straw or biochar in coastal saline soils

Article Title: Divergent pathways of straw versus biochar integration with manure to enhance SOC sequestration in coastal saline soils

Article References: Bai, X., Zhou, J., Dong, R., Wang, S., Chen, M., Wang, J., Li, X., Li, H., Colinet, G., Zhang, H., & Li, Y. (2026). Divergent pathways of straw versus biochar integration with manure to enhance SOC sequestration in coastal saline soils. Biochar, 8(1), Article 139. https://doi.org/10.1007/s42773-026-00656-2

Image Credits: AI Generated

DOI: 10.1007/s42773-026-00656-2

Keywords: soil organic carbon, biochar, straw, manure, coastal saline soils, microbial necromass, soil aggregates, pore structure, enzyme kinetics, microbial networks, carbon sequestration, Yellow River Delta