Acid sulfate (AS) soils cause severe environmental hazards in their recipient watercourses worldwide. Different soil water management practices can help prevent the hazards. Because long-lasting field experiments are expensive and site specific, sophisticated water flow simulation models can be used, for example, to estimate the effects of different management practices on the soil and runoff water quality. In AS soils, postglacial land uplift and the reclamation of soil for agricultural use have caused ripening of the soil. This study examined soil ripening and its effect on soil hydraulic properties, such as saturated hydraulic conductivity (K-sat), the water retention curve (WRC), macroporosity and air entry values, in different soil horizons of Nordic AS soils. The properties were determined from soil core samples in the laboratory, and single and dual-porosity models were fitted to the measured WRC data. The differences in WRCs at different depths were the result of the different ripening status of the soil horizons. The proportion of macroporosity was found to decrease with depth in the soil, and air entry values were higher in deep horizons than in the upper ripe horizons. According to the determined WRCs, the required soil water potential for the ripening of hypersulfidic parent sediments can be produced by plants only. The dual-porosity van Genuchten model provided a good fit to the measured data; the sensitivity of the model to its parameters varied between study sites due to the differences in underlying WRC measurements. In the long-term simulation of the development of water quality in AS soil areas, the effect of ripening on the evolution of soil hydraulic properties should be taken into account.

Irrigated agriculture provides 40% of the world's crop-based foods but often with a negative impact on the environment. It is important to recognize that in addition to providing food and fibre, irrigation and controlled drainage systems can be optimized to provide additional ecosystem services and mitigate climate change by using resources in a more efficient way. Contemporary case studies were identified from around the world, including flood control by paddy fields in Japan, water quality enhancement and wastewater reuse in South Africa and Taiwan, micro-/meso-climate regulation in Ethiopia and Japan, controlled drainage and sub-irrigation to maximize carbon sequestration and minimize leaching in Finland, and groundwater table management to reduce irrigation water and pumping requirements in Turkey. Irrigation infrastructure, such as rice paddy terraced landscapes (Japan) and large dams and canals (Australia), have also achieved notable additional ecotourism job creation. Case studies were analysed in terms of funding opportunities and compared using the Common International Classification of Ecosystem Services system. It is recommended that planning frameworks be developed that seek to optimize ecosystem services such as the ones discussed above. Policy should be updated to recognize these services and provide incentives to irrigators and water management entities accordingly.

Land drainage lowers the soil water table, which triggers enhanced oxidation of iron sulfide minerals in acid sulfate soils, causing acidification of fields and posing environmental hazards to aquatic ecosystems. The objective of this study was to develop a model for investigating the effects of oxidation of iron sulfide minerals and the cation exchange capacity (CEC) on the long-term evolution of pH, redox potential and solute concentrations in soil water and drain discharge. Two electron acceptors (oxygen and ferric iron) were included in the model. The model was implemented in the HP1 modeling platform (coupled HYDRUS-1D and PHREEQC-3) and applied to an acid sulfate soil profile of an agricultural field in Finland. The geochemical part of the model was tested against field observations. A comparison of different model versions showed that CEC had an essential role in mimicking the observed pH profile of soil. The drain discharge water quality was similar in the model versions both with and without CEC, but the buffering effect of CEC could be seen as lower iron concentrations. The oxidation of metastable iron sulfide minerals was three times faster than the oxidation of pyrite. Descriptions for the oxidation of both two minerals and for CEC were required to conduct long-term simulations that reproduce the observed geochemical state of a drained agricultural field. The HP1 modeling platform facilitated the testing of alternative chemical transformation processes of an AS soil profile in response to increased drainage efficiency after subsurface drainage installation.

The off-site effects of agricultural organic soils include the leaching of N, P, and organic carbon (OC) to watercourses and CO2, CH4, and N2O emissions into the atmosphere. The aim of this study was to quantify how the thickness of organic layers affects these loads. A 19.56-ha experimental field drained by subsurface pipes was established in Ruukki, northwestern Finland. Three plots had a 60-80 cm-thick sedge peat layer and three others had a thickness of 20 cm or less. The drainage pipes lie in mineral soil that, in this field, contains sulfidicmaterial. This study documents the experimental settings and reports on the leaching of substances in the first two years, as well as CO2, CH4 and N2O emissions during eight weeks in one summer. Total N (TN) and OC loads were higher from the thicker peat plots. The mean TN loads during a hydrological year were 15.4 and 9.2 kg ha(-1) from the thicker and thinner peat plots, respectively, with organic N representing 36% of TN load. Total P (TP) load averaged 0.27 kg ha(-1) yr(-1). Dissolved P load represented 63 and 36% of TP in the thicker peat area and only 23 and 13% in the thinner peat area, and was thus increased upon peat thickness. These N and P loads through the subsurface drainage system represented roughly 83% of TN and 64% of TP loads from this field. There were no clear differences in greenhouse gas emissions among the plots during the eight-week monitoring period. Slowly oxidizing sulfide in the subsoil resulted in annual leaching of 147 kg S ha(-1), almost ten times that of non-sulfidic soils. Our first results emphasize the effect of the peat thickness on the leaching of substances and warn about considering all organic soils as a single group in environmental assessments. (C) 2021 The Authors. Published by Elsevier B.V.

Besides causing acidification, acid sulfate (AS) soils contain large nitrogen (N) stocks and are a potential source of N loading to waters and nitrous oxide (N2O) emissions. We quantified the stocks and flows of N, including crop yields, N leaching, and N2O emissions, in a cultivated AS soil in western Finland. We also investigated whether controlled drainage (CD) and sub-irrigation (CDI) to keep the sulfidic horizons inundated can alleviate N losses. Total N stock at 0–100 cm (19.5 Mg ha−1) was smaller than at 100–200 cm (26.6 Mg ha−1), and the mineral N stock was largest below 170 cm. Annual N leaching (31–91 kg N ha−1) plus N in harvested grain (74–122 kg N ha−1) was 148% (range 118–189%) of N applied in fertilizers (90–125 kg N ha−1) in 2011–2017, suggesting substantial N supply from soil reserves. Annual emissions of N2O measured during 2 years were 8–28 kg N ha−1. The most probable reasons for high N2O emission rates in AS soils are concomitant large mineral N pools with fluctuating redox conditions and low pH in the oxidized subsoil, all favoring formation of N2O in nitrification and denitrification. Although the groundwater level was higher in CD and CDI than in conventional drainage, N load and crop offtake did not differ between the drainage methods, but there were differences in emissions. Nitrogen flows to the atmosphere and drainage water were clearly larger than those in non-AS mineral soils indicating that AS soils are potential hotspots of environmental impacts.

The trenchless (T0) and trencher (T1) drainage installation methods are widely applied in Finland. There is an ongoing debate and a lack of science-based information about the performance differences between the methods. The objective was to assess drainage performance differences between T0 and T1 by analyzing groundwater table observations from field sections drained with the two methods. The differences were studied by using statistical analysis over a two-year period after the drainage installation. An experimental field in middle-Finland was divided into four T0 sections and four T1 sections. The groundwater level was manually measured about twice a week from seven locations in each section. Automatic recording was installed in one T0 section and one T1 section. The manual observations formed 56 time series, which were tested between the same-method plots (T0-T0 and T1-T1) and the different-method plots (T0-T1). Automatic data was used to validate the manual observations. In the T0 sections, 60-90% of the groundwater level observations were higher than those in the T1 sections. These observations had an average difference of 0.14-0.25 m. The variation in the groundwater level time series was larger between the T0 sections than between the T1 sections. Statistically significant differences between the same method field sections indicated that other factors also affected the groundwater table (soil type, etc.). However, the differences between T0 and T1 were stronger than those between the same-method sections, and the differences were clearest when the groundwater levels were above the drain depth (1.0 m). In the seasonal time series, the biggest differences were found during the autumn and winter periods. The average differences between T0 and T1 might not be significant in practice, but occasional larger ( > 0.4 m) differences may have a short-term influence on field activities and crop growth.

Large amounts of sulfate and divalent iron (Fe) are released into the pore water of acid sulfate (AS) soils upon oxidation of sulfidic materials. The simultaneously produced acidity dissolves metals from the soil matrix. Reduced horizons of AS soils commonly have a large mineral nitrogen (N) stock in the form of NH4+-N, which is a potential source of N leaching and gaseous emissions. This study was carried out at three AS soil sites in Finland. Cation composition of pore water was monitored in a monolithic lysimeter experiment. Timothy samples grown in an AS soil were analysed for mineral elements and the soil was investigated for zinc (Zn) distribution in different chemical species at four depths down to 85cm. The composition of pipestems formed in previous root channels was investigated by SEM, X-ray EDX and XRD. Emissions of CO2 and N2O were measured in an AS field which had a peaty topsoil. Monitoring by closed chambers was carried out at three sites differing in the depth of peat layer (15, 30 and 60cm). In the sulfuric horizon, large amounts of calcium and magnesium were mobilized. Reflooding stopped the mobilization of those elements but resulted in abundant dissolution of Fe, which became the dominant cation in the pore water. Timothy growing in an AS soil showed only small deviations from the average composition measured in Finland with the exception of Zn that was at the deficiency level. As Zn in the root zone had been dissolved and subsequently leached, it was recovered in easily soluble forms in the subsoil. Pipestem composition indicated accumulation of Fe and formation of a new solid phase probably mostly in the form of schwertmannite even though jarosite was also detected. The annual CO2 emissions from the peaty AS soil were about 6000kgCha−1 but the N2O emissions were relatively small, with a magnitude more typical of mineral rather than organic soils. Thus, large N stock of an AS soil do not necessarily contribute to abundant gaseous N emissions.

Study region: Six countries within the Baltic Sea Basin: Denmark, Sweden, Finland, Lithuania, Poland and Germany Study focus: During transport from the field to the sea nitrogen undergoes natural reduction, but with large spatial variations. Mapping this variation would allow more optimal regulation strategies, by imposing most restrictions in areas with low natural reduction. In the present study, a map for spatially variable nitrate reduction in groundwater is developed covering six countries in the Baltic Sea Basin. In contrary to previous studies within the Baltic Sea Basin, the present study provides an independent estimate of the nitrogen reduction in groundwater, based on review of national data and studies. Depending on availability, different approaches were used for the countries ranging from national modelling to expert judgement. Through a joint workshop the assessments were, to the extent possible, harmonised. New hydrological insights for the region: The review revealed large variations in the hydro-geochemical conditions important for transport and degradation of nitrogen in groundwater. This includes the hydrogeology, the reducing conditions of the subsurface, and the fraction of water transported by drainage systems bypassing the reducing subsurface environments. Significant variations in groundwater reduction between the countries and within most of the countries were thus found, indicating that strategies for nitrogen regulation and mitigation measures may be optimised, if variation in the natural reduction of nitrate is considered.

Acid sulfate soil leachates deteriorate the aquatic ecosystems of their recipient waters around the world. In Finland, AS soils are located mainly on the coast of the Baltic Sea, where rivers and estuaries suffer from acid leachates and waters do not meet with the criteria of good water quality set by the EU. Field drainage of cultivated AS soils is attributable to leaching of acidity, but regardless of various mitigation measures, the acidity of discharge water in these areas has not decreased significantly. In order to better understand the pathways involved in the formation of acidity, the redox status of 56 Finnish AS soil fields was examined using redox potential and pH data measured down to 2m. The findings indicated that the oxidation of soils has occurred at depths below the drainage pipes, with the median being at a depth of 1.6m. In fields cultivated for a long time, soil texture had a stronger effect on the depth of the redox interface than the drainage method; open ditch drainage and subsurface drainage; oxidation being faster in sandy and silty soils than in clayey soils. The isostatic land uplift also seems to affect the depth of the redox interface in the long run. Most of the studied fields had been cultivated for at least 30years prior to the study. However, the pH values of the soils were still very low, probably due to actual and retained acidity. The prevention of oxidation of sulfidic materials in subsoils is important, but measures for neutralizing the acidity are needed. Without them it seems that the leaching of acidity will continue and may decrease only slowly. However, severe droughts during summers and the reclamation of unripe AS soils for any purpose will increase the leaching of acidity.

3 Foreword 5 List of original publications and participation 10 Symbols and abbreviations 12

The cultivated acid sulfate soils on the coast of Gulf of Bothnia in Western Finland are highly valued for their excellent crop yields. However, there is an urgent need to find management solutions that minimize the load of acidity and dissolved metals from these soils to watercourses due to oxidation of sulfidic materials in their subsoil. In this 4-year study, possibilities to manage the groundwater with controlled subsurface drainage (CD) and subsurface irrigation (CDI), including a vertical plastic sheet to prevent by-pass flow was investigated. The groundwater in the conventionally subsurface drained field (ND) dropped down into the sulfidic horizon for almost three months in summers. CD delayed the groundwater drop, shortening the time to less than two months. In CDI system, the groundwater could be kept above the sulfidic horizon nearly throughout the summers, thereby preventing oxidation of sulfides in the lower subsoil. The acidity of drainage waters was lowest in the CDI system. However, the effect was not substantial because the acidity was still high in all the drainage systems.

Environmental hazards caused by acid sulphate (AS) soils are of worldwide concern. Among various mitigation measures, waterlogging has mainly been studied in subtropical and tropical conditions. To assess the environmental relevance of waterlogging as a mitigation option in boreal AS soils, we arranged a 2.5-year experiment with monolithic lysimeters to monitor changes in the soil redox potential, pH and the concentrations of aluminium (Al), iron (Fe) and sulphur (S) in pore water in response to low and high groundwater levels in four AS soil horizons. The monoliths consisted of acidic oxidized B horizons and a reduced C horizon containing sulphidic material. Eight lysimeters were cropped (reed canary grass, Phalaris arundinacea) and two were bare without a crop. Waterlogging was conducive to reduction reactions causing a slight rise in pH, a substantial increase in Fe (Fepw) and a decrease in Al (Alpw) in the pore water. The increase in Fepw was decisively higher in the cropped waterlogged lysimeters than in the bare ones, which was attributable to the microbiologically catalysed reductive dissolution of poorly ordered iron oxides and secondary minerals. In contrast to warmer climates, Fepw concentrations remained high throughout the experiment, indicating that the reduction was poised in the iron range, while sulphate was not reduced to sulphide. Therefore, the precipitation of iron sulphide was negligible in the environment with a low pH and abundant with poorly ordered Fe oxides. Increased Fe in pore water counteracts the positive effects of waterlogging, when water is flushed from fields to watercourses, where re-oxidation of Fe causes acidity and oxygen depletion. However, waterlogging prevented further oxidation of sulphidic materials and decreased Alpw to one-tenth of the initial concentrations, and even to one-hundredth of the levels in the low water table lysimeters.

Acid sulphate (AS) soils are most prevalent in the tropics, but the acidic discharge from cultivated AS soils also threatens water bodies under boreal conditions. Feasible options to reduce the acid load are needed. In this study, the groundwater of an AS field was monitored for 3.5years, and the efficiency of waterlogging in mitigating the environmental risks caused by acidic discharge was investigated in a 2.5-year experiment with 10 monolithic lysimeters taken from the same field. In order to unravel the transferability of the results from lysimeters to the field scale, the Al, Fe and S concentrations in discharge water from the lysimeters were compared with those in the groundwater of the AS field (pedon and field scale), and in pore water (pedon and horizon scale). In the waterlogged bare lysimeters (HWB), the Al, Fe and S concentrations in discharge waters were broadly similar to those measured in the groundwater and followed the changes in the pore water. In the waterlogged cropped (reed canary grass, Phalaris arundinacea) lysimeters (HWC), in contrast, the discharge waters were markedly higher in Fe and lower in Al than the groundwater in the field. This outcome was attributable to the reduction of Fe3+ to the more soluble Fe2+ and the reduction-induced increase in pH, which enhanced the formation of Al3+ hydroxy species. Lowering of the water table (LWC) caused soil ripening, which resulted in increased saturated hydraulic conductivity and porosity and enhanced the oxidation of sulphidic materials and acid formation. The responses of Al, Fe and S in drainage waters from HWC and LWC lysimeters resembled previous findings in AS soils. Based on this and the similarity between dissolved element concentrations in the discharge water of HWB lysimeters and groundwater in the field, we conclude that our monolithic lysimeters yielded realistic results concerning the efficiency of various methods in mitigating environmental risks related to cultivated AS soils.

Acid sulphate (AS) soils along the Baltic coasts contain significant amounts of organic carbon and nitrogen in their subsoils. The abundance, composition, and activity of microbial communities throughout the AS soil profile were analysed. The data from a drained AS soil were compared with those from a drained non-AS soil and a pristine wetland soil from the same region. Moreover, the potential production of methane, carbon dioxide, and nitrous oxide from the soils was determined under laboratory conditions. Direct microscopic counting, glucose-induced respiration (GIR), whole cell hybridisation, and extended phospholipid fatty acid (PLFA) analysis confirmed the presence of abundant microbial communities in the topsoil and also in the deepest Cg2 horizon of the AS soil. The patterns of microbial counts, biomass and activity in the profile of the AS soil and partly also in the non-AS soil therefore differed from the general tendency of gradual decreases in soil profiles. High respiration in the deepest Cg2 horizon of the AS soil (5.66 μg Cg(-1)h(-1), as compared to 2.71 μg Cg(-1)h(-1) in a top Ap horizon) is unusual but reasonable given the large amount of organic carbon in this horizon. Nitrous oxide production peaked in the BCgc horizon of the AS and in the BC horizon of the non-AS soil, but the peak value was ten-fold higher in the AS soil than in the non-AS soil (82.3 vs. 8.6 ng Ng(-1)d(-1)). The data suggest that boreal AS soils on the Baltic coast contain high microbial abundance and activity. This, together with the abundant carbon and total and mineral nitrogen in the deep layers of AS soils, may result in substantial gas production. Consequently, high GHG emissions could occur, for example, when the generally high water table is lowered because of arable farming.