Abstract
There are large amounts of dissolved organic matter (DOM) released into the soil during spring thaw, but its bioavailability and components are still unknown. The quantity, composition and stability of DOM in water extracts of forest soils during thaw were studied after two-month freezing with 9 levels of soil moisture ranging from 10% to 90% water-filled pore space (WFPS), by measuring soil carbon dioxide (CO2) flux, biodegradable dissolved organic carbon (BDOC) and nitrogen (BDON), ultraviolet absorbance and parallel factor analysis of fluorescence excitation-emission matrices. Concentrations of BDOC, BDON, DOC and DON were lowest around 30% WFPS and relatively higher and lower soil moisture both increased DOM and BDOM concentrations in thawing soil. With increasing WFPS, the dominant component of soil DOM changed from humic acid-like substances to fulvic acid-like substances and the biological origin of DOM increased gradually. The protein-like component accounted for 8–20% of soil DOM and was affected by vegetation type and WFPS singly and interactively. The results implied that forest soils with more than 50% WFPS before winter freezing could release large amounts of fulvic acid-like DOM, which would be easily biodegraded and emitted as CO2 or run off with ground water during spring snow thaw.
Highlights
Future climatic change is likely to increase the occurrence of soil freezing-thawing events in high latitude and/or high altitude zones[1]
The variation of water-filled pore space (WFPS) significantly affected the concentrations of biodegradable dissolved organic carbon (DOC) (BDOC), non-biodegradable DOC (NBDOC), biodegradable Dissolved organic nitrogen (DON) (BDON) and non-biodegradable DON (NBDON) in water extracts of white birch forest (WBF) and Broadleaf and Korean pine mixed forest (BKPF) soils during thaw (p ≤ 0.001) (Fig. 1)
The concentration of biodegradable dissolved organic carbon (BDOC) was largest at 10% WFPS in the two forest soils, especially in the BKPF soil (p < 0.05), and it generally increased from 30% to 90% WFPS in the two forest soils (Fig. 1a,c)
Summary
Future climatic change is likely to increase the occurrence of soil freezing-thawing events in high latitude and/or high altitude zones[1]. The SUVA254, which indicates the aromaticity of DOM, is insufficient for understanding the complex components of DOM released during thaw[20, 21] It is noteworthy fluorescence excitation–emission (EEM) spectrophotometry combined with parallel factor (PARAFAC) analysis has been shown to be a sensitive, rapid and non-destructive technique to provide quantitative information on the components of DOM (e.g. humic-like, fulvic-like and protein-like DOM) from terrestrial and aquatic sources[22,23,24,25], and biodegradability of DOM can be measured by soil extracts plus inoculum incubation experiment[26, 27]. Combined with the environmental impacts (e.g. greenhouse gases fluxes and nutrient leaching) caused by the soil DOM released during spring thaw, it is important to understand the influence of antecedent soil moisture prior to freezing on the quantity, composition and stability of soil DOM during thaw, by using EEM spectrophotometry with PARAFAC and the DOM degradation incubation experiment. The results would be beneficial for our understanding of the properties and environmental functions of DOM released into the soil during spring thaw in cold temperate zones
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