Spectral Exploration of Calcium Accumulation in Organic Matter in Gray Desert Soil from Northwest China

Little attention has been paid to the accumulation of soil organic matter (SOM) in the fringes of the mid-latitude desert. In this paper, soil samples from a long-term field experiment conducted from 1990 to 2013 at a research station in Urumqi, China by different fertilizer treatments, were used to determine soil properties and soil dissolved organic matter (DOM) by chemical analysis, fluorescence excitation emission matrix (EEM) spectroscopy, and high resolution-transmission electron microscopy (HR-TEM). The binding features of DOM under the addition of Ca²⁺ were analyzed using a two-dimensional (2D) Fourier transform infrared (FTIR) spectrometer further to explore the response of the DOM to increasing concentrations of Ca²⁺. Long-term application of chemical fertilizers and goat manure increased soil organic carbon (SOC) by 1.34- and 1.86-fold, respectively, relative to the non-fertilized control (8.95g.kg^-1). Compared with the control, application of chemical fertilizers and manure significantly increased the concentrations of Ca, Mg, Si, humic and fulvic acid-like substances in DOM but decreased the amounts of trivalent metals (Al and Fe) and protein-like substances. Although crystalline Al/Fe nanoparticles and amorphous or short-range-order Si/Al nanoparticles existed in all DOM samples, crystalline Ca/Si nanoparticles were predominant in the samples treated with goat manure. Although organic matter and Si-O-containing nanoparticles were involved in the binding of Ca²⁺ to DOM, application of chemical fertilizers weakened Ca²⁺ association with components of the amide II group (1510 cm^-1) and Si-O linkage (1080 cm^-1), whereas application of goat manure enhanced the affinity of Ca²⁺ for Si-O linkage. Our results suggested that the enrichment of Ca in gray desert soil possibly helps accumulate SOM by forming crystalline Ca/Si nanoparticles in addition to Ca²⁺ and organic matter complexes.