Doctoral thesis, 2017
Strategic nitrogen management in stockless organic cropping systemsRåberg, Tora
AbstractAgriculture faces the challenge of producing high yields to feed a growing world population, while simultaneously addressing environmental problems such as eutrophication, emissions of greenhouse gases, loss of biodiversity and soil degradation. Organic farming can be part of the solution, as it promotes biodiversity, uses less energy for fertiliser production and often has higher inputs of organic matter to soil than conventional farming. However, yields are often lower, partly due to asynchrony in mineralisation of organic nitrogen (N) and crop acquisition. Growing legumes for protein production and input of biological N2 fixation to supply the cropping system with N is a common practice on organic farms. The addition of reactive N to the agroecosystem via legumes may, just as with synthetic fertilisers, lead to N surpluses and environmentally harmful N losses. It is therefore important to improve N cycling within agricultural cropping systems. This thesis assessed the effects of strategic redistribution of residual biomass on productivity, crop quality, N balance, N and carbon (C) turnover, eutrophication potential and global warming potential in a stockless organic cropping system. A field experiment was established to test three strategies for recirculating N in residual biomass within a six-year crop rotation; 1) leaving crop residues in situ at harvest (IS), 2) biomass redistribution as silage to non-legume crops (BR) or 3) anaerobic digestion of the silage before redistribution (AD). A soil incubation experiment in a controlled environment was also performed, to measure mineralisation of N, soil respiration and greenhouse gas emissions from incorporation of fresh and anaerobically digested grass clover ley. Moreover, energy balance, greenhouse gas emissions and eutrophication potential in BR and AD were compared with those in IS in a life cycle assessment (LCA). Results from the field experiment showed that the BR and AD strategies maintained the same yields as IS, but resulted in higher N2 fixation in the legumes and consequently a more positive N balance. The soil incubation experiment showed that total C losses during 90 days after soil application of ley were higher than from digested ley. A major energy gain was achieved in AD, and a decrease in global warming potential compared to BR. There was a reduction in eutrophication potential with the strategic redistribution of silage and digestate (BR and AD), compared with IS. In conclusion these results show that strategic redistribution of biomass-based digestate can improve the N balance of crop rotations and produce a surplus of bioenergy, which are key elements for enhancing the sustainability of stockless organic cropping systems.
Keywordsbioenergy, biomass management, crop rotation, ecological intensification, green manure, life cycle assessment, nitrogen cycling, organic agriculture, soil incubation, stockless cropping systems
Published inActa Universitatis Agriculturae Sueciae
2017, number: 2017:102
ISBN: 978-91-7760-094-7, eISBN: 978-91-7760-095-4
Publisher: Department of Biosystems and Technology, Swedish University of Agricultural Sciences.
Swedish University of Agricultural Sciences, Department of Biosystems and Technology
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