Different composition and distribution patterns of mineral‐protected versus hydrolyzable lipids in shrubland soils

Abstract

AbstractMineral protection is known as an important mechanism stabilizing soil organic carbon (SOC). However, the composition, sources, and variations of mineral‐protected SOC remain poorly constrained. To fill this knowledge gap, we used hydrofluoric acid to demineralize soil matrix and compared the sources and distribution of mineral‐protected lipids (ML) versus hydrolyzable lipids (HL) of four typical Chinese shrubland soils. ML was found to represent a sizable fraction (9–32%) of total aliphatic lipids (including n‐alkanols; n‐alkanoic acids; α,ω‐alkanedioic acids; hydroxyalkanoic acids; and midchain‐substituted acids) in all soils. Based on carbon chain length and branch positions, microbe‐ and plant‐derived lipids were distinguished. No significant difference was found in the ratio of microbe‐ to plant‐derived lipids in ML versus HL, implying that plant and microbial inputs are equally important for the mineral‐associated soil lipids. However, ML contained a higher proportion of nonspecific lipids, especially at depths. Furthermore, to evaluate key environmental variable(s) controlling the distribution of different lipid components, a multiple stepwise regression analysis was conducted. Notably, ML was mainly affected by SOC‐to‐nitrogen ratio instead of mineralogical properties, implying that the accrual of mineral‐associated soil lipids relies strongly on organic matter properties. Collectively, our findings provide novel insights on sources and accumulation mechanisms of mineral‐protected soil lipids. SOC decomposition and subsequent accretion of degradation products appear to be vital for the sequestration of mineral‐associated soil lipids and warrant better recognition in the investigations of stable soil carbon accumulation mechanisms.