Spatial variations of aggregate-associated humic substance in heavy-textured soils along a climatic gradient

Abstract

Humic substances (HS), as important soil components, are crucial to soil fertility and ecology. The composition of HS is interlinked within the formation of aggregates, and jointly influences soil biogeochemical processes. However, little detailed and quantitative information is available on the association of the HS composition with aggregate distribution for soils spanning a broad range of climatic gradients (e.g., temperature and precipitation). Herein, the spatial distribution of HS and its chemical composition (humic acids, HAs; fulvic acids, FAs; iron-linked humin, HMi; clay-combined humin, HMc; and insoluble humin residue, HMr) associating with different sizes of aggregates (>0.25, 0.25∼0.05, and <0.05 mm) from six typical zonal soils (two Luvisols, Alisols, Acrisols, Plinthosols, and Ferrosols) of a heavy texture (silty clay loam, silty clay, and clay) under a constant land use from temperate to tropical climate in central-southern China (34°48′N ∼ 19°52′N), were investigated. Results indicate that soil weathering degree (characterized by free iron oxides Fed) increased with the increase of annual temperature and precipitation along the latitude transect, which indirectly reflects the influence of climatic gradient. The spatial distribution of HS was significantly influenced by soil type, profile horizon, and its interaction (p < 0.001), while topsoil with higher proportion of macroaggregates (>0.25 mm) and higher concentration of HS was more significantly affected by climatic conditions than subsoil (F > 21, p < 0.001). Additionally, the composition of HS was dominated by insoluble humin residue (3.4 ∼ 14.38 g kg−1), which tended to decrease with the increase of weathering gradient except for Rhodi-Humic Ferrosols, whereas fulvic acids in the second rank (0.9 ∼ 5.96 g kg−1) demonstrated an opposite spatial trend along the latitude transect. With respect to the concentration of aggregate-associated HS, it was higher in macro- and micro- aggregates, especially for Luvilsols, and had a positive correlation to its concentration in bulk soils (r>0.52, p < 0.01) and the proportion of its corresponding aggregate fraction (p < 0.05). Moreover, aggregate-associated fulvic acids were positively correlated with sesquioxides, clay content, 14 nm intergrade mineral and kaolinite, but negatively correlated with soil pH, bulk density, silt content, and vermiculite (p < 0.05). However, these physicochemical properties had an opposite effect on the variation of humic acids in macro- and micro- aggregates, and made different contributions to humins up to the subdivided composition and the associated aggregate size fraction. These results would contribute to the prediction accuracy of HS storage across scales, especially when a wide size-range aggregation and various geochemical factors are considered.