Soil is one of the planet’s largest carbon reservoirs, holding more carbon than the atmosphere and all vegetation combined. But not all soil carbon is created equal. Scientists now distinguish between two fundamentally different pools. Particulate organic carbon, or POC, consists of partially decomposed plant fragments—bits of straw, root debris, and leaf litter—that are physically visible but chemically vulnerable. Microbes can digest this material relatively easily, releasing carbon dioxide back into the atmosphere within years to decades. Mineral-associated organic carbon, or MAOC, by contrast, consists of small organic molecules that bind tightly to clay and silt particles, forming complexes that can persist in soil for centuries or even millennia. Because MAOC is the more durable reservoir, the ratio between these two fractions is a critical indicator of how stable a soil’s carbon stock really is.

To determine which pool different farming practices actually build, the research team ran a seven-year field experiment comparing straw incorporation, plastic film mulching, and nitrogen fertilization, alone and in combination, against conventional tillage. What makes the study unusual is its forensic approach to carbon sources. Rather than simply measuring total soil organic carbon, the researchers deployed three molecular biomarkers, each tracing a different pathway of carbon entry. Phospholipid fatty acids, or PLFAs, revealed the size and composition of living microbial communities. Amino sugars served as fingerprints of microbial necromass—the accumulated remains of dead bacteria and fungi. And lignin phenols, compounds that plants produce in their cell walls but microbes do not, marked carbon that arrived directly from crop residues.

The yield results were striking. Nitrogen fertilization proved the most powerful productivity lever, increasing annual wheat grain yield by 45 percent. Straw incorporation raised yields by 23 percent per year, while plastic film mulching delivered a 14 percent boost. These numbers alone would encourage any farmer to adopt all three practices simultaneously. All three treatments also increased soil organic carbon contents relative to conventional tillage, seemingly confirming the widely held assumption that intensive management simultaneously improves productivity and sequesters carbon. But when the researchers decomposed that carbon into its particulate and mineral-associated fractions, a more complicated picture emerged.

Straw incorporation emerged as the most carbon-friendly of the three practices. By returning crop residues to the soil, it substantially increased plant carbon inputs and stimulated the proliferation of bacteria and saprotrophic fungi—the decomposer organisms that break down organic matter. This microbial surge was accompanied by higher enzyme activities, and the combined effect raised the accumulation of both lignin phenols and microbial necromass carbon in the soil. As a result, particulate organic carbon contents increased by 1.5-fold compared with conventional tillage, and total soil organic carbon rose by 17 percent. Crucially, straw incorporation enriched both the POC and MAOC fractions with plant-derived and microbial-derived carbon, meaning it fed both the fast and slow carbon pools at once.

Plastic film mulching told a different story. The practice, which involves covering soil with thin polyethylene sheets to conserve moisture and warm the seedbed, did increase both POC and total SOC contents. Yet beneath the surface, the mineral-associated fraction was being eroded. Mulched soils showed reduced contents of amino sugars and lignin phenols within the MAOC fraction, along with lower concentrations of microbial PLFAs there—evidence that the living microbial community and its necromass contributions to the stable carbon pool had declined. Consequently, MAOC content itself decreased under mulching. The film appears to alter the soil microclimate in ways that shift carbon cycling away from the microbial pathways that build durable, mineral-bound carbon, even as it boosts the total stock through particulate accumulation.

Nitrogen fertilization produced perhaps the most consequential trade-off of all. While it increased POC content, it decreased MAOC content and reduced the contribution of mineral-associated carbon to total SOC under both straw incorporation and film mulching. The mechanism, according to the biomarker evidence, runs through the microbial community: nitrogen addition reduced amino sugar content and microbial PLFA concentration, weakening the microbial carbon pump that converts plant carbon into necromass and ultimately into mineral-stabilized forms. In other words, the fertilizer that delivers the biggest yield gains also suppresses the very organisms and biochemical processes responsible for locking carbon into soil’s most stable reservoir.

The broader implication is that the two most celebrated pathways for building soil carbon—straw incorporation and film mulching—achieve their gains primarily through POC accumulation while simultaneously weakening the mineral-associated stabilization pathways that make carbon persistence possible. Nitrogen fertilization amplifies this effect. Particulate carbon is not worthless; it feeds soil food webs, improves aggregation, and can eventually be processed into more stable forms. But it is inherently transient, vulnerable to changes in temperature, moisture, and tillage. Recent global research has underscored this vulnerability, with studies showing that mineral-associated organic carbon across the world’s drylands is itself sensitive to climate warming—raising the stakes for management decisions that determine how much carbon sits in each pool.

For the rapidly expanding practice of agricultural carbon crediting, the findings carry a warning. Many carbon markets and national inventories currently reward measured increases in total soil organic carbon without distinguishing between its fractions. A field that gains carbon through straw incorporation and mulching may look like a sequestration success on paper, while the added carbon sits in a labile pool that could be released within a decade. The study’s authors suggest that quantifying the contributions of POC and MAOC to SOC—ideally with the biomarker toolkit they employed—reduces uncertainty in predicting how soil carbon will respond to management and should inform which practices are genuinely climate-positive. As wheat systems across China’s Loess Plateau and similar dryland regions worldwide face intensifying pressure to raise yields while storing carbon, this seven-year experiment makes clear that the two goals can pull in opposite directions, and that the stability of soil carbon, not just its quantity, must become the yardstick of success.

Subject of Research: Effects of nitrogen fertilization, straw incorporation, and plastic film mulching on wheat yield and the stability of soil organic carbon fractions

Article Title: Nitrogen fertilization, straw incorporation, and plastic film mulching increase wheat yield and soil organic carbon but reveal trade-offs in carbon stability

Article References: Zhao, Q., Abdo, A. I., Zhang, K., Li, H., Zhou, C., Chen, J., Wang, L., & Kuzyakov, Y. (2026). Nitrogen fertilization, straw incorporation, and plastic film mulching increase wheat yield and soil organic carbon but reveal trade-offs in carbon stability. Plant and Soil. https://doi.org/10.1007/s11104-026-09044-y

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09044-y

Keywords: soil organic carbon, wheat yield, nitrogen fertilization, straw incorporation, plastic film mulching, particulate organic carbon, mineral-associated organic carbon, microbial necromass, lignin phenols, phospholipid fatty acids, carbon sequestration, soil microbiology

The experiment compared annual applications of biochar at a rate of 2.625 tonnes per hectare with maize stover incorporation at approximately 7.5 tonnes per hectare, alongside an untreated control plot. After ten crop seasons, researchers sampled soil at depths of 0 to 20, 20 to 40, and 40 to 60 centimeters to capture how carbon had moved and stabilized throughout the profile. Both amendments significantly increased soil organic carbon across the entire 0 to 60 centimeter depth range, confirming their value as carbon-building tools. But the similarities largely ended there. In the topsoil, the two amendments performed almost identically, with carbon gains of 49.70 percent for biochar and 48.87 percent for stover. Below the surface, however, the paths diverged dramatically.

Maize stover proved far more effective at pushing carbon into deeper soil layers. In the 20 to 40 centimeter horizon, stover increased soil organic carbon by 105.90 percent, compared with 72.81 percent for biochar. In the deepest layer measured, 40 to 60 centimeters, stover delivered a 32.35 percent gain while biochar managed only 4.74 percent. These contrasting depth patterns indicate that carbon accumulation depends on amendment-specific transport and stabilization processes rather than on the sheer quantity of carbon added. Stover-derived dissolved organic carbon showed stronger vertical movement through the soil column, carrying plant-derived compounds downward, whereas biochar contributed more stable carbon directly to the layers where it was incorporated.

To understand what was happening at the molecular level, the research team deployed an impressive analytical arsenal. Dissolved organic carbon was characterized using Fourier transform ion cyclotron resonance mass spectrometry, a technique capable of resolving thousands of individual molecular formulas in complex environmental samples. This was complemented by lignin phenol analysis to trace plant-derived carbon, amino-sugar measurements to quantify microbial necromass, soil aggregate fractionation, and statistical modeling to tie the pools together. The combination allowed the investigators to assess not just how much carbon was present, but where it came from and how vulnerable it was to decomposition.

The molecular analysis revealed that both amendments increased dissolved organic carbon concentrations, but they altered its properties in fundamentally different ways. Biochar lowered the nominal oxidation state of carbon in the dissolved fraction, a pattern associated with lower bioactivity and greater persistence in the environment. Stover, by contrast, produced dissolved organic carbon with higher bioactivity in the topsoil, consistent with a more readily metabolized carbon supply that fuels microbial activity. In essence, biochar appeared to deliver carbon in a chemically recalcitrant form destined for long-term residence, while stover fed the soil food web with labile substrates that were rapidly processed and redistributed.

The fate of carbon from each amendment also diverged at the level of microbial residues and plant-derived compounds. Biochar increased microbial necromass carbon while reducing plant-derived carbon in the 0 to 20 and 20 to 40 centimeter layers, a pattern the authors associate with enhanced decomposition of native plant carbon alongside the substantial input of stable biochar carbon. Stover increased both plant-derived carbon and microbial necromass carbon, particularly through active microbial processing of the incorporated residues. In other words, stover stimulated the biological machinery of the soil, generating microbial biomass that itself becomes a stable carbon pool, while biochar largely bypassed that machinery by depositing pre-stabilized carbon.

Soil structure played a decisive role in shaping these outcomes. Both amendments increased the proportion of small macroaggregates, the soil clumps that physically protect organic matter from decomposition, but stover exerted the stronger effect across the entire soil profile. Partial least-squares path modeling, a statistical technique for testing hypothesized causal chains, indicated that stover enhanced soil organic carbon through a coordinated pathway involving aggregates, plant-derived carbon, and microbial necromass carbon. Biochar operated primarily through direct stable-carbon input, with an indirect contribution from accumulating microbial residues. The two amendments, in effect, built soil carbon through entirely different architectural strategies.

The practical implications are significant for carbon management in agriculture. Biochar appears better suited to long-term carbon sequestration and the stabilization of persistent carbon pools, making it attractive for climate mitigation schemes that require durable offsets. Maize stover, meanwhile, supports active carbon cycling, microbial processing, and the retention of plant-derived carbon, functions that sustain soil fertility and nutrient supply. The results favor matching amendment choice to management objectives rather than assuming that any organic input will deliver the same carbon benefits. A farmer prioritizing durable carbon storage might favor biochar, while one seeking to revitalize soil biological activity might lean toward residue incorporation, or potentially combine both.

The authors are careful to note the limitations of their study. The experiment used three field replicates, and technical replicates were not performed for the mass spectrometry analysis because of high analytical costs. Initial soil properties were measured from a composite sample rather than separately by soil layer, and lignin phenols and amino sugars do not capture the entire spectrum of soil organic carbon, including highly processed organic matter and black-carbon-like materials. The proposed differences in microbial necromass turnover and dissolved organic carbon transport therefore require further direct testing. Future work should quantify the differential persistence of fungal and bacterial necromass carbon and clarify the mechanisms controlling dissolved organic matter movement into deeper soil layers, with longer-term monitoring across soils, climates, and cropping systems needed to establish how broadly these divergent carbon sequestration pathways apply.

Even with those caveats, the decade-long record provides rare empirical weight behind an increasingly important question: how agricultural soils can be managed as carbon sinks without compromising productivity. As carbon markets mature and governments seek verifiable soil-based climate solutions, understanding which amendment delivers which kind of carbon, and where in the soil profile it ends up, becomes essential information. This study demonstrates that the answer is not one-size-fits-all. Biochar and maize stover, applied to the same field under the same climate for ten years, sculpted the soil’s carbon inventory in measurably different ways, from the molecular composition of dissolved organic matter to the architecture of aggregates and the balance of plant and microbial residues. For researchers and policymakers alike, the message is clear: the route carbon takes into soil matters as much as the amount that goes in, and choosing the right route may determine whether soil carbon gains endure for years or fade within seasons.

Subject of Research: Divergent carbon sequestration pathways of biochar and maize stover in agricultural soil

Article Title: Biochar and maize stover take different routes to store carbon in soil

Article References: Biochar and maize stover take different routes to store carbon in soil. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: biochar, maize stover, soil organic carbon, dissolved organic carbon, microbial necromass, carbon sequestration, soil aggregates, lignin phenols, amino sugars, FT-ICR-MS, agricultural amendments, soil depth