Agradecimentos: The author thanks the São Paulo Research Foundation FAPESP for support to this collaboration (2015/25406-5 and 2017/50184-1). The authors would like to thank the Brazilian Nanotechnology National Laboratory (LNNano) and the Brazilian Synchrotron Light Laboratory (LNLS) both at the Brazilian Center for Research in Energy and Materials (CNPEM) for the use of electron microscopy and synchrotron light facilities. This work was supported by grants from the strategic research area Biodiversity and Ecosystem Services in a Changing Climate (BECC), the Knut and Alice Wallenberg Foundation (2013.0073), and the Swedish Research Council (2016-04561 and 2017-04261). LG acknowledges support from the Crafoord Foundation (20180566). This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 731019 (EUSMI) Ver menos

Abstract: Components of dissolved organic matter (DOM) span from sub-nm molecules to colloidal aggregates of several hundred nm. The colloidal fraction is important for the transport of organic matter and associated elements in the environment, and for the stability of DOM constituents with respect to microbial decomposition. This study focuses on the colloidal properties of DOM extracted from spruce forest soils of a chronosequence. The DOM samples were obtained by common water extraction procedures at 21 and 100°C, respectively. We applied an experimental approach combining chemical analysis with light and X-ray scattering techniques that informed on the colloidal size, charge, and structure of DOM. Results showed that two main types of colloids were present: semi-flexible cylinders and fractal aggregates. The cylinders consisted of carbohydrates, presumably hemicelluloses, while the aggregates were a composite material containing a large fraction of carbohydrates together with aliphatics and clay particles. These fractal aggregates dominated the cold-water extracts whereas the strong increase in total organic carbon by hot-water extraction caused a concomitantly strong increase of semi-flexible cylinders, which became the predominant species. Comparison between the chronosequence soils showed that with increasing forest age, the amount of carbon extracted per gram of soil declined and the concentration of the semi-flexible cylinders decreased. Thus, the distribution between the fractal aggregates and cylinders in the forest soil DOM samples depends on the composition of the soil organic matter and the leaching temperature. Changes in this distribution may have important implications for the reactivity and stability of DOM colloids Ver menos