Browsing by Subject "agroecology/Soil Science"

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  • Kalu, Subin (Helsingin yliopisto, 2022)
    Biochars are highly stable porous carbon-rich substances that, when added to soils, have high potential to increase soil carbon (C) sequestration, enhance soil fertility and crop yield, as well as bring other environmental benefits such as reducing emissions of greenhouse gases (GHG) and leaching of nutrients, and remediation of soils contaminated with heavy metals. The potential of biochars to provide agricultural and environmental benefits had led to an exponential increase in the number of studies on the effects of biochars since the beginning of this century. However, the long-term effects of a single application of biochars are not well known. In addition, the beneficial effects of biochars have been observed mostly in (sub-) tropical regions dominated by highly weathered, nutrient-poor acidic soils with low C contents. On the other hand, only a few studies have been conducted on boreal soils that typically have higher C contents. Therefore, this research aimed to investigate the long-term effects of wood-based biochars when combined with different fertilizers in boreal agricultural soils, in terms of i) plant growth and nutrient uptake, ii) soil physical properties, iii) nitrogen (N) dynamics, and iv) GHG emissions. For this, data were collected from four field experiments in Finnish soils, where biochars were applied two to eight years prior to this research, as well as from a greenhouse experiment. Over the eight years of field experiments, the biochars had minor effects on plant growth and nutrient uptake in both nutrient-poor and nutrient-sufficient soils. Throughout this period, the biochars increased plant growth only on two occasions. On both occasions, the fields were cropped with nitrogen-fixing plants in the previous growing season, thus suggesting that the result may be explained by pre-crop effects. The biochars notably increased plant potassium (K) uptake while reducing plant aluminum (Al) and sodium (Na) uptake, indicating that biochars can ameliorate plant K deficiency, and reduce Al and Na toxicity stress. The biochars increased the contents of several nutrients in plant biomass with time, suggesting a possible long-term fertilization effect either through the slow release of nutrients initially contained in biochars or via the enhanced nutrient holding capacity of biochars as they weather in the field. On the other hand, the biochar reduced plant manganese (Mn) content with time in a nutrient-poor soil, suggesting that immediately after application, the biochar increased plant availability of Mn (either present in the biochar or soil), which decreased over the years. After six or seven years of application, the biochars did not affect the physical or hydrological properties of topsoil. Immediately following the application, the biochar had increased plant available water in coarse-textured soil. However, this effect disappeared with time, which could be caused by the loss of the biochar or the movement of biochar down the soil profile. The biochars were shown to have nitrate (NO3-) retaining capacity in both the short-term greenhouse experiment and in the field experiment in clayey soil where spruce and willow biochars were applied two years before. The increased N use efficiency, increased plant N uptake, and reduced N leaching by biochars in these experiments were most likely due to the increased retention of NO3- by the biochars. The spruce biochar was better than the willow biochar in NO3- retention, most likely because of the higher specific surface area. The 15N labeling greenhouse experiment suggested that biochars could induce ammonia volatilization that leads to the loss of fertilizer ammonium (NH4+) because of increased soil pH. On the other hand, the ability of biochars to retain NO3- increases the soil retention and plant uptake of NO3-. Furthermore, there was an indication that biochars increased the plant N availability via increased mineralization of soil organic N in the short-term. The biochars increased carbon dioxide (CO2) efflux from two out of the four field experiments. In addition to the increased soil microbial activity, the increased plant growth might have contributed to increased CO2 efflux. However, there were no clear effects of biochars on the emissions of nitrous oxide (N2O) and methane (CH4) in any of the fields. Despite this, the spruce and willow biochars tended to reduce N2O emission during the peak emission period after two years. The potential of biochars to reduce N2O emission appeared to be dependent on soil silt content and initial soil C content. Interestingly, the wood-based biochar reduced the yield-scaled emissions of non-CO2 GHG in the field experiment on coarse-textured soil, even after seven years of application. The reduction was mostly due to the increased crop yield, which could be a result of the increased availability of plant water by biochar during the extremely dry growing season. Overall, no negative effects of biochars were observed in the greenhouse or the longer-term field experiments in boreal soils. Therefore, this research supports the concept that biochars as soil amendment materials are a safe and practical way to increase soil C sequestration. However, achieving consistent noticeable agronomic and other environmental benefits after several years of a single application of wood-based biochars is implausible in boreal soils. Thus, subsequent application of nutrient-rich biochars, such as co-composted biochars likely provides a more reasonable alternative for a consistent increase in soil C sequestration, as well as agronomic benefits. Future biochar research should focus on this direction.​