Researchers affiliated with the Federal Technological University of Paraná, the University of São Paulo, the Federal University of Viçosa, the Federal Rural University of the Semi-Arid, and other Brazilian institutions set out to quantify how maize stover, the shredded residue left on fields under no-till farming, alters the environmental behavior of S-metolachlor. Their work, published in the journal Discover Soil, focused on two chemically contrasting soils collected from the state of Paraná: a clayey Oxisol from Pato Branco and a sandy Ultisol from Paranavaí. These two soil orders dominate much of Brazil’s arable land, and they differ sharply in texture, organic matter, and water retention, making them an ideal natural experiment for testing how residue management interacts with soil type.

The team amended both soils with maize stover at rates ranging from zero to one percent by weight, equivalent to field applications of zero to 20 tonnes per hectare. They then ran a battery of standardized tests. Sorption and desorption were measured using the batch equilibrium method recommended by the Organisation for Economic Co-operation and Development, in which soil samples are shaken with herbicide solutions and the amount of chemical remaining in the liquid phase is quantified. Leaching was assessed in 30-centimeter PVC columns packed with soil and covered with stover, then subjected to 60 millimeters of simulated rainfall over two hours, a scenario based on average January rainfall at the collection sites. In every case, S-metolachlor concentrations were measured by high-performance liquid chromatography, with the analytical methods validated for selectivity, linearity, detection limits, and recovery following Brazilian regulatory guidelines.

The results showed that the stover did far more than sit on the surface. Adding it measurably changed the chemistry of both soils, raising pH, increasing organic carbon, and boosting levels of phosphorus and potassium. In the Oxisol, organic carbon climbed from 2.38 percent in the unamended soil by 16 to 36 percent depending on the stover rate, while potassium rose by as much as 61 percent. The researchers attribute these shifts to the nutrient content of maize residue itself, which is known to contain appreciable amounts of nitrogen, phosphorus, potassium, sulfur, and calcium. The residue also appears to exert a mild liming effect, with its basic cations reacting with exchangeable acidity and nudging pH upward by 11 to 18 percent across the two soils.

Those chemical changes translated directly into stronger herbicide retention. Freundlich sorption coefficients for S-metolachlor ranged from 3.24 to 5.98 in the Oxisol and from 1.48 to 2.61 in the Ultisol, with the highest values generally occurring at the greatest stover rates. At the one percent amendment rate, roughly 64 percent of the applied herbicide was sorbed in the Oxisol and about 37 percent in the Ultisol. The clayey soil consistently outperformed the sandy one, a pattern the authors attribute to its 88 percent clay content and higher organic carbon, both of which provide abundant sorption sites. Desorption, meanwhile, declined as stover rates increased, meaning that once the herbicide bound to the amended soil, it was less inclined to let go. The hysteresis coefficients all exceeded one, indicating negative hysteresis and a degree of reversibility in sorption that the researchers link to the herbicide’s moderately hydrophobic character interacting with nonpolar functional groups in the plant residue.

The leaching experiments revealed an equally striking story. Without any stover, more than 70 percent of the applied S-metolachlor stayed within the top five centimeters of soil in both soils, indicating limited vertical mobility under the tested rainfall. In the Oxisol, adding stover at rates of 5 to 20 tonnes per hectare trapped 9 to 30 percent of the herbicide in the residue layer itself, with the remainder concentrated near the surface; no herbicide was detected in the leachate at all. The sandy Ultisol behaved differently. There, the herbicide appeared in the leachate at stover rates of 5, 10, and 15 tonnes per hectare, with roughly 50 to 63 percent of the applied chemical recovered from the collected drainage water, though it vanished from the leachate at the highest, 20-tonne rate and in the unamended controls.

That soil-dependent behavior is one of the study’s most important takeaways. The authors caution that the simulated rainfall of 30 millimeters per hour represented a high-intensity event that may have promoted rapid water movement and preferential flow, particularly through the sandy Ultisol, so the leaching patterns they observed should be interpreted within that specific scenario. They also note that lipophilic constituents of maize residue can alter sorption-desorption equilibria in ways that sometimes enhance vertical movement, echoing earlier findings that wheat residue increased the mobility of atrazine. In other words, crop residue is not a simple shield; it is an active chemical participant whose effects depend on the soil it covers.

For weed management, the implications cut in a promising direction. Herbicide held in the stover layer and near the soil surface may be released gradually into the soil solution, potentially extending the window of activity against emerging weeds during early crop establishment. The residue itself adds a physical barrier that delays weed emergence, reducing the need for repeat applications. At the same time, stronger sorption and reduced desorption lower the risk that S-metolachlor will migrate into aquifers, addressing a long-standing environmental concern for a compound whose sorption coefficient normalized to organic carbon typically falls between 100 and 200 milliliters per gram and whose soil half-life ranges from 15 to 50 days. The researchers emphasize that soils with higher organic carbon and intrinsic sorption capacity are better equipped to retain the herbicide safely.

The authors are careful to frame their conclusions as laboratory findings that still require validation under field conditions, where factors such as residue decomposition, microbial activity, and variable rainfall will complicate the picture. Even so, the study makes a compelling case that the residue, the herbicide, and the soil must be considered as a single interacting system rather than in isolation. Adjusting stover rates to match soil-specific conditions, they suggest, could simultaneously improve herbicide performance, sustain weed control, and shrink the environmental footprint of one of agriculture’s most indispensable chemicals. In an era when both water quality and herbicide resistance demand smarter solutions, the humble corn stalk may turn out to be one of the cheapest tools available.

Subject of Research: The effect of maize stover amendment on the sorption, desorption, and leaching of S-metolachlor herbicide in Brazilian Oxisol and Ultisol soils

Article Title: Sorption, desorption, and leaching of S-metolachlor in two soils amended with maize stover

Article References: da Silva Borges, M. P., Trezzi, M. M., da Costa Lima, A., Araújo, G. R., Junior, V. B. P., Paiva, A. C. P., Silva, D. V., da Silva Brochado, M. G., & Mendes, K. F. (2026). Sorption, desorption, and leaching of S-metolachlor in two soils amended with maize stover. Discover Soil, 3(1), Article 178. https://doi.org/10.1007/s44378-026-00333-7

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00333-7

Keywords: S-metolachlor, maize stover, herbicide sorption, leaching, Oxisol, Ultisol, soil organic carbon, no-till farming, Freundlich isotherm, water contamination, weed management, soil chemistry

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