The classic view of weathering as a stabilizing feedback traces back to influential work in the 1980s and 1990s. Robert Berner and colleagues formalized how silicate weathering responds to atmospheric carbon dioxide and temperature, creating a negative feedback loop: more carbon dioxide warms the planet and acidifies rain, which accelerates weathering, which in turn removes carbon dioxide from the atmosphere. Later, Maureen Raymo and William Ruddiman proposed that the uplift of mountain ranges such as the Himalaya may have cooled the Cenozoic Earth by exposing fresh rock to erosion. Francis Macdonald and colleagues later connected arc volcanism and associated weathering to major climatic transitions. These frameworks cemented the idea that mountain building and rock exposure are, in broad strokes, allies of a cool climate.

But as Bufe emphasizes, drawing down carbon dioxide is only one side of the ledger. Weathering and erosion also release carbon. The oxidation of fossil organic carbon buried in sedimentary rocks, and the oxidation of sulfide minerals such as pyrite, both deliver carbon dioxide to the atmosphere, in some landscapes at rates that rival or exceed the silicate weathering sink. Recent work by Bufe, Jeremy Rugenstein and Niels Hovius, published in Science in 2024, compiled global evidence that mountain ranges act not merely as carbon sinks but as carbon sources as well, with the balance between drawdown and release depending on what the rocks are made of, how fast they erode, and how water moves through them. In the Southern Alps of New Zealand, for example, rapid erosion of carbon-rich sedimentary rock can push the net carbon balance toward emission, whereas volcanic arcs dominated by fresh basalt tend to favor drawdown.

The picture grows still more complicated when biology and hydrology enter the equation. Susan Brantley and colleagues showed in 2023 that the depth and structure of the critical zone, the weathered skin of Earth where rock, soil, water and life interact, exert fundamental control on how much mineral surface area is available for reactions. Vegetation, microbial communities and soil organic matter can both accelerate mineral dissolution and shield it from infiltrating water. Meanwhile, organic carbon itself is constantly being eroded from soils, transported by rivers, buried in floodplains and deltas, or oxidized back to carbon dioxide. Studies led by Robert Hilton and A. J. West have mapped this biospheric carbon loop, and work by Guillaume Soulet and colleagues has quantified how efficiently the biosphere exchanges carbon with the atmosphere through erosion, complicating any simple accounting of weathering as a one-way carbon pump. Sue Tank and colleagues have further shown that in high-latitude permafrost landscapes, the fate of weathering-derived carbon in streams and lakes can shift dramatically as climates warm.

Spatial scale matters profoundly. A weathering flux measured on a single hillslope may not scale linearly to a whole catchment, and catchment-scale estimates may not extrapolate to mountain belts or continents. Supply of fresh mineral surface, runoff, temperature and lithology each impose their own dependencies, and these dependencies can switch sign across environments. In hot, wet tropical basaltic terrains, weathering rates can be extraordinarily high and largely supply-limited. In cold, arid or transport-limited settings, the same mineralogy may weather sluggishly. Time introduces its own twists: pulses of tectonic uplift or glacial grinding expose fresh rock that weathers rapidly at first, then slows as easily dissolved minerals are exhausted. Over million-year timescales, these transients mean that today’s weathering fluxes partly reflect climate and tectonics from deep in the past, making it treacherous to read modern weathering rates as a simple response to modern conditions.

Disciplinary boundaries compound the challenge. Geochemists who trace dissolved ions in rivers, geomorphologists who measure erosion and sediment transport, soil scientists who study profile development, ecologists who quantify vegetation dynamics and carbon cycling, and climate modelers who simulate global feedbacks each hold pieces of the puzzle. Bufe argues that only research spanning spatial scales, temporal scales and disciplines can resolve how weathering modulates the carbon cycle in a warming world. As the climate changes, weathering itself will change: rising temperatures and altered hydrology will accelerate some reactions, thawing permafrost will expose new mineral surfaces and release ancient organic carbon, and intensifying storms will mobilize sediment in ways that models are only beginning to capture. Whether the geological thermostat will help blunt anthropogenic warming, and on what timescale, remains an open and consequential question.

It is against this backdrop of genuine scientific uncertainty that Bufe levels his sharpest critique. Enhanced rock weathering, the practice of spreading crushed silicate rock such as basalt on farmland to accelerate carbon dioxide drawdown, has surged in popularity as a carbon removal strategy. A growing industry now sells carbon credits based on the assumption that a quantifiable and permanent fraction of the applied rock dissolves, and that the resulting bicarbonate represents durable carbon sequestration. Commercial rankings of the best enhanced weathering credit projects have proliferated, and the market is expanding faster than the underlying science can constrain it. In a companion review in Nature Reviews Earth & Environment, Maximilian Schiedung and colleagues catalogued the substantial uncertainties in measuring, reporting and verifying enhanced weathering outcomes, from dissolution kinetics in real soils to secondary carbonate formation that can return carbon to the atmosphere.

Bufe contends that issuing carbon credits for enhanced weathering at the present stage is premature. The very complexities that make natural weathering hard to quantify, the competing sources and sinks, the dependence on lithology, hydrology, biology and timescale, apply with full force to engineered deployments. Crushed rock applied to one field may behave very differently from the same material in another, and the net climate benefit must account for the carbon cost of mining, grinding and transporting rock, as well as for nitrous oxide and other greenhouse gas effects that soil amendments can trigger. Without robust, context-specific measurement of how much carbon is actually drawn down and for how long, credits risk monetizing an assumption rather than a verified outcome, potentially undermining confidence in carbon removal as a whole.

None of this argues that enhanced weathering should be abandoned, Bufe stresses. Rock weathering remains one of the most promising and scalable negative-emissions pathways, rooted in a process that has regulated Earth’s climate for eons. But realizing that promise responsibly demands investment in the fundamental science: long-term field experiments across climates and soil types, improved tracers of mineral dissolution and carbon fate in rivers and oceans, and models that honor the tangled interplay of tectonics, erosion, hydrology and life. The geological thermostat is real, but it is not a simple dial. Understanding its gears, and the directions in which some of them spin backward, is a prerequisite for turning rock weathering into a trustworthy tool against climate change rather than a cautionary tale about moving faster than the science.

Subject of Research: The complex role of rock weathering in Earth's carbon cycle and the risks of premature carbon crediting for enhanced weathering

Article Title: Rock weathering’s tangled role in Earth’s carbon cycle

Article References: Bufe, A. (2026). Rock weathering’s tangled role in Earth’s carbon cycle. Nature Water. https://doi.org/10.1038/s44221-026-00707-9

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00707-9

Keywords: rock weathering, carbon cycle, enhanced weathering, carbon credits, chemical weathering, silicate minerals, carbon dioxide removal, geomorphology, biogeochemistry, erosion, climate mitigation, Nature Water

The scale of the material is far from trivial. An adult horse of roughly 500 kilograms produces an estimated 20 to 31 kilograms of manure and bedding per day, amounting to some 9 to 11 tonnes annually. In regions with dense horse populations, those figures rival municipal bio-waste streams. The County of Wesel in Germany’s Lower Rhine basin, for example, counts more than 8,500 horses alongside 458,000 inhabitants; in 2024 the area collected 32,700 tonnes of bio-waste and 23,300 tonnes of green waste, while horse manure is estimated at around 75,000 tonnes. Yet because many stables sit far from farmland that could recycle those nutrients, manure increasingly generates disposal costs and storage constraints rather than agronomic value.

Chemically, horse manure is a distinctive feedstock. Pure manure contains about 1.0 to 1.7 percent nitrogen on a dry basis, much of it water-soluble and plant-available, along with 0.5 to 1.3 percent phosphorus as P2O5, roughly 1.1 percent potassium as K2O, and 84 to 95 percent organic matter. Its carbon-to-nitrogen ratio typically ranges from 20:1 to 32:1, but bedding materials such as straw or wood shavings can push mixtures above 50:1. That matters agronomically: high C:N materials trigger net nitrogen immobilisation in soil, reducing short-term plant-available nitrogen and potentially depressing yields. If manure is marketed as a fertiliser substitute, customers may perceive weak or even negative fertiliser effects, undermining willingness to pay. The bedding is therefore not a passive bulking agent but a design variable that shapes composting behaviour, nutrient dynamics, and ultimately market positioning.

Controlled composting resolves many of these problems. Thermophilic phases thermally degrade pathogens, parasite eggs, and weed seeds, while microbial activity stabilises labile nitrogen compounds into more predictable, plant-available forms. Finished horse manure composts reported in the literature contain total nitrogen of 1.7 to 2.3 percent, phosphorus up to 1.3 percent, potassium around 1.2 percent, and a near-neutral pH of 6.4 to 6.7, with the C:N ratio falling from an initial 27.3:1 to roughly 15.9:1 and moisture dropping to between 8 and 15 percent. Practical benchmarks, such as the German Organic Waste Ordinance, converge on sustaining temperatures above 55 degrees Celsius for more than two weeks. Studies of small-scale composting of manure with wood shavings show that weekly turning improves hygienisation uniformity, since outer pile layers can otherwise remain insufficiently sanitised. That quality assurance is what opens quality-sensitive horticulture, landscaping, and growing-media markets to manure-derived products.

Emissions, however, constitute the central technical trade-off. Composting generally suppresses methane relative to passive storage because aerobic conditions inhibit methanogenesis, but poorly managed piles can still release considerable methane. Meanwhile, ammonia volatilisation and nitrous oxide emissions can rise, eroding the product’s nitrogen content and contributing to eutrophication and climate forcing. Turning aerates the pile and cuts methane, yet can simultaneously increase ammonia losses by exposing ammonium-rich zones during thermophilic, alkaline phases. Research on dairy manure even shows that pile mixing can raise total measured greenhouse gas emissions while lowering methane alone. The authors conclude that good composting is not a compliance exercise but a core production competence: process control determines hygiene, nitrogen retention, odour, and customer-perceived value. Facilities with optimal aeration and exhaust gas treatment are preferable, which favours centralised processing where horse density or cooperative logistics allow efficient feedstock aggregation.

The systematic EU mapping reveals a structured, application-driven compost market in which soil improvement dominates, followed by fertilising functions and a smaller but meaningful segment for growing-media components. Solid composts prevail, while compost-biochar blends and vermicomposts are increasingly documented. Application fields extend beyond agriculture into horticulture, landscaping, urban greening, engineered soils, remediation, and green roof substrates. That breadth is strategically significant for horse manure, because peri-urban stables are often spatially closer to urban green infrastructure markets than to bulk agricultural outlets, reducing transport costs and enabling circularity branding that raises willingness to pay. The trade-off is that these markets demand higher product consistency and safety, reinforcing the case for controlled composting and rigorous quality management rather than passive pile storage.

Organisational form emerges as a decisive determinant of economic viability. Centralised private plants exploit economies of scale and professional quality assurance but require collection logistics and sufficient horse density. Municipal and public-private systems monetise composting partly through avoided disposal costs and internal use of compost in public green spaces, aligning well with the peri-urban geography of horse keeping. Decentralised models suit small holdings with limited investment capacity, where the business case rests on avoided container rental, haulage, and disposal contracts rather than product sales, though governance mechanisms are needed if products leave the site. Notably, the literature review found no documented cases of co-composting horse manure with municipal bio-waste, despite the apparent synergy: blending manure with kitchen and garden waste would raise nitrogen and potassium in the finished compost and could improve the economics of both streams.

Product differentiation offers the most promising frontier. Vermicomposting can unlock premium horticultural markets through higher microbial activity and nutrient availability, but it demands prior hygienisation and careful process management, raising complexity and risk. Compost-biochar blends present a more scalable strategy: biochar incorporated during composting improves nutrient retention and microbial habitat, and its carbon can persist in soils for decades to centuries, qualifying as a plausible carbon dioxide removal pathway. Voluntary carbon markets have begun recognising biochar-based removals, suggesting that carbon monetisation is more credible in compost-biochar models than in composting alone, provided monitoring, reporting, and verification frameworks are in place. Formulation design also follows application logic: fertiliser-oriented composts target C:N ratios of 8:1 to 12:1, soil improvers sit between 12:1 and 18:1, and carbon-storage or remediation blends exceed 18:1, often above 30:1 with woody biomass or biochar.

Regulation threads through every business model. Under the EU Fertilising Products Regulation (Regulation (EU) 2019/1009) and national frameworks such as Germany’s Fertilizer Act, Fertilizer Ordinance, and Bio-waste Ordinance, requirements for storage capacity, spreading periods, and waste classification shape what is legally and commercially feasible. Directive 2008/98/EC mandates separate biowaste collection, and the EU Soil Strategy for 2030 raises demand for organic matter inputs in degradation-prone regions such as Southern Europe. The authors’ central message is that viable models must treat regulatory conformity and quality assurance as core capabilities enabling market access, not external constraints. Horse-specific data on emissions, pharmaceuticals, and antibiotic resistance genes remain sparse compared with cattle and pig systems, and systematic evaluation of decentralised peri-urban systems is scarce. Future work combining horse-specific process monitoring with economic modelling across organisational structures would strengthen the evidence base for policy and investment. The larger conclusion is striking: Europe’s horse manure problem is really a design problem, and its solution lies in engineering purpose-built compost products, organisational models, and carbon strategies around the material’s distinctive chemistry.

Subject of Research: Business models for utilising horse manure through compost-based value chains in the European Union

Article Title: Business Models for Horse Manure Utilisation in the European Union: Compost-Based Products, Market Pathways, and Carbon Integration

Article References: Pleissner, D., Podßun, P., Hölscher, P., & Friege, H. (2026). Business Models for Horse Manure Utilisation in the European Union: Compost-Based Products, Market Pathways, and Carbon Integration. Waste and Biomass Valorization. https://doi.org/10.1007/s12649-026-03776-9

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03776-9

Keywords: horse manure, composting, circular economy, soil improver, biochar, carbon credits, peri-urban agriculture, EU Fertilising Products Regulation, waste valorization, growing media, greenhouse gas emissions, business models