High expectations to harness agricultural soils as sinks for atmospheric carbon (C) necessitate methods for verifying changes in soil organic C (SOC) stocks and improving the accuracy of regional SOC stock estimates specific to various land‐use and management types. In this study we used a sample set included in the Finnish national soil monitoring network (n = 125) to determine the SOC stock and its vertical distribution in Finnish agricultural mineral soils using fixed depth (FD) and equivalent soil mass (ESM) methods. Further, we explored the within‐field variation in SOC stock and minimum sample size in the context of SOC sequestration. Boreal agricultural mineral soils were found to be rich in SOC. The average SOC stock ranged between 8.4 and 9.8 kg m−2 in the top 30 cm depending on cropping system and soil type. Cultivation of perennial crops tended to enrich the C close to the soil surface in comparison to dominantly annual crops, which exhibited a less steep C gradient with depth. The commonly used van Bemmelen factor of 1.724 to convert results of loss on ignition to SOC was found to be appropriate for coarse‐textured Finnish agricultural soils. However, the results showed that in clay‐rich soils, it is critical to take into account the structural water bound in clay particles to avoid overestimation of the SOC content. The study also demonstrated that the minimum sample size required to detect the change of 0.1 kg C m−2 at field level is typically several hundreds. Therefore empirical soil sampling is not at all cost‐effective enough for verifying the changes in SOC stocks with regard to compensation of C sequestration for farmers. Thus, alternative verification methods, such as SOC modelling in combination with field‐level data collected, for example, using remote sensing should be developed further. Highlights Boreal agricultural mineral soils are rich in C Rotations where perennial crops dominated tended to enrich C in the soil surface Soil clay content should be taken into account when converting SOM to SOC The sample size required to verify SOC stock changes for C trading is extremely high

Abstract. Anthropogenic activities on the Indo-Gangetic Plain emit vast amounts of light-absorbing particles (LAPs) into the atmosphere, modifying the atmospheric radiation state. With transport to the nearby Himalayas and deposition to its surfaces the particles contribute to glacier melt and snowmelt via darkening of the highly reflective snow. The central Himalayas have been identified as a region where LAPs are especially pronounced in glacier snow but still remain a region where measurements of LAPs in the snow are scarce. Here we study the deposition of LAPs in five snow pits sampled in 2016 (and one from 2015) within 1 km from each other from two glaciers in the Sunderdhunga Valley, in the state of Uttarakhand, India, in the central Himalayas. The snow pits display a distinct enriched LAP layer interleaved by younger snow above and older snow below. The LAPs exhibit a distinct vertical distribution in these different snow layers. For the analyzed elemental carbon (EC), the younger snow layers in the different pits show similarities, which can be characterized by a deposition constant of about 50 µg m−2 mm−1 snow water equivalent (SWE), while the old-snow layers also indicate similar values, described by a deposition constant of roughly 150 µg m−2 mm−1 SWE. The enriched LAP layer, contrarily, displays no similar trends between the pits. Instead, it is characterized by very high amounts of LAPs and differ in orders of magnitude for concentration between the pits. The enriched LAP layer is likely a result of strong melting that took place during the summers of 2015 and 2016, as well as possible lateral transport of LAPs. The mineral dust fractional absorption is slightly below 50 % for the young- and old-snow layers, whereas it is the dominating light-absorbing constituent in the enriched LAP layer, thus, highlighting the importance of dust in the region. Our results indicate the problems with complex topography in the Himalayas but, nonetheless, can be useful in large-scale assessments of LAPs in Himalayan snow.

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<div> <p><span data-contrast="auto">Cultivated peatlands are a major source of greenhouse gas (GHG) emissions and water pollution in northern Europe, and their future management is a key issue on the path to carbon neutral societies. Conventional cultivation requires drainage, and above the drainage depth all peat is prone to decomposition with the implication that these soils have the highest emission rates per area compared to any other land use. Paludiculture is a management option in which wet-tolerant crops are produced with raised ground water levels. It is thus a GHG mitigation method that allows for slowing down peat decomposition in drained peatlands while still maintaining agricultural income for the landowner. There are tradeoffs to consider when implementing paludiculture: 1) methane emissions rise with the switch of aerobic to anaerobic decomposition, 2) slowing down decomposition reduces nutrient mineralisation from the peat and compromises productivity and 3) harvesting reduces the potential to sequester carbon to the ecosystem compared to natural wetlands.</span><span data-ccp-props="{"201341983":0,"335559739":160,"335559740":259}"> </span></p> </div> <div> <p><span data-contrast="auto">We experimented paludiculture at a highly degraded peat site in southern Finland with plots of willow, forage, and mixed vegetation (set-aside). We recorded the yields, emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and auxiliary environmental data for four years, as well as nutrient content of the soil water for two years. We will present the results of these measurements, including estimates on the net ecosystem carbon balance of each crop based on empiric models. </span><span data-ccp-props="{"201341983":0,"335559739":160,"335559740":259}"> </span></p> </div> <div> <p><span data-contrast="auto">Raising the ground water level to the desired depth (-20 cm) turned out to be challenging. The mean annual ground water table levels during the four study years were about 80, 40, 40 and 30 cm (the measurements of the last year are still ongoing). The preliminary results suggest that even a slight raise of the ground water level was able to slow down CO2 emissions from soil respiration, while an increase in CH4 emission partly counteracted this benefit especially when the ground water level was above 30 cm. Nitrous oxide emissions were extremely high after the initial disturbance of the site but remained at a relatively low level after that. The results will be compared to an adjacent site with an annual crop, and paludiculture as a mitigation measure discussed. </span><span data-ccp-props="{"201341983":0,"335559739":160,"335559740":259}"> </span></p> </div>