Dissolved Organic Matter Within Soil Aggregates in Forest Restoration: Insights From Optical Properties

Abstract

Forest restoration with Robinia pseudoacacia L. has many benefits, and dynamics of dissolved organic matter (DOM) might provide insight into soil restoration. Nevertheless, studies regarding the changes in the DOM within soil aggregates during artificial forest restoration are relatively limited in semiarid regions. Soil aggregates were collected from the topsoil, middle soil, and subsoil layers (i.e., 0–20, 30–50, and 80–100 cm) at six forests (i.e., R. pseudoacacia L.) lands with various restoration ages in a typical semiarid region. The variations of DOM properties in silt + clay size classes, microaggregates and macroaggregates (i.e., < 0.053 mm, 0.053–0.25 mm, and > 0.25 mm) were explored by UV–visible and three-dimensional excited emission matrix spectral. Overall, dissolved organic carbon (DOC) content (41.24–119.82 mg kg⁻¹) of soil aggregates generally increased with restoration age. The DOC content in topsoil aggregates was 3.11–7.13 times larger than those from the other two soil layers. The molecular weight, aromaticity degree, and humification degree had the same trends as that of DOC content with soil depth. The DOC content and aromaticity degree of DOM in macroaggregates were 1.26–4.27 times and 1.01–1.18 times higher than those in microaggregates and silt + clay size classes at most soil layers, respectively. These variations in DOM within soil aggregates were primarily attributed to the changes in soil texture and the densities of plant roots and litter during the forest restoration. The study reveals the positive impact of forest restoration on the increase of DOC content, providing scientific evidence for soil management.