The influence of dissolved organic matter (DOM) composition on DOM-aluminium interaction in Podzols: a column experiment

Author(s)

Sara Ramos dos Santos

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2020-12-09

Examining board members

Pablo Vidal Torrado; Carlos Eduardo Pellegrino Cerri; Judith Schellekens; Wilson Tadeu Lopes da Silva

• Advisor: Pablo Vidal Torrado

Abstract

Dissolved organic matter (DOM) can be linked to metals by the formation of organometal complexes. Therefore, DOM plays an important role in many environmental processes, such as metals transport and soil carbon stability. Podzols, or spodosols, are a good example of this process because the formation of their Bh horizon is based on the association of DOM with Al and/or Fe through a process called podzolization. It is well-known that functional groups play an essential role in the binding mechanisms of DOM with metals. However, the role of molecular structure in this process has received little attention. To understand the role of functional groups and molecular architecture, the molecular composition of DOM was analysed in an experimental setting with several associations with Al. The hypotheses of this work are that i) complexation of DOM with Al is selective, and ii) precipitation of DOM in sand without Al addition also occurs and is not selective. Column experiments were carried out to study the retention of DOM under controlled conditions. Different sources of DOM collected in a podzol area under Restinga vegetation were used, including water from two streams (Stream-1 and Stream-2), Litter and Char. Each DOM solution passed through a column containing material from a podzol E horizon with and without an added Al source (Al(NO3)3.9H2O and kaolin), and material from a podzol Bh horizon. These column materials were selected to reflect the effects of precipitation, complexation, adsorption and desorption, respectively. The leachates from the different DOM types that passed through the column were qualified and quantified to verify its retention in the soil and evaluate possible selectivity during these processes. The results indicate distinct behaviour of DOM types. Leachates from B columns rom Stream-1 and Stream-2 had increases in C content compared to the initial solution, with 159% and 263% of DOC, respectively. This was similar to leachates with only addition of water and reflect a net desorption; in contrast, the B columns treated with DOM from Litter and Char showed a net retention with a decrease of C of 13% and 40 %. In the E horizons (with and without Al), the treatments with DOM from Stream-2 presented the greatest retention with 46% on average, compared to 34% and 39% for DOM from Litter and Char, respectively. The treatment with DOM from Stream-2 even showed 63% retention in the columns with kaolin, reflecting the largest retention via adsorption. The E columns treated with DOM from Char presented the lowest retention with 39% on average, with exception of the column without Al addition (47% retained). The fact that the highest retention of DOM from Char occurred in the treatment without Al addition suggest that (in addition to desorption, complexation and adsorption) retention through precipitation was also selective. In addition, comparing all DOM types for E columns without Al, leachates from Char had significantly higher retention of C, jointly with those from Stream-2 (p<0,05). The column materials also clearly influenced the retention of C, with the columns with kaolin being more efficient than the columns with Al in solution, being 42% and 27% on average, respectively (Stream-1 excluded). This different effect of column type on C retention suggests a process-dependent selectivity in DOM retention. To evaluate this selective retention of DOM, the composition of (leachates of) each DOM source was evaluated using FTIR, Py-GC/MS, the proportion of humic acids to fulvic acid (HA/FA), and elemental analyses (CHN). (AU)

• FAPESP's process: 18/07094-4 - The selectivity in the complexation of the dissolved organic matter (DOM) in the formation of B horizon of Podzols
• Grantee: Sara Ramos dos Santos
• Support Opportunities: Scholarships in Brazil - Doctorate