Compositional variation of glomalin-related soil protein in different forest stands and farmland

Glomalin-related soil protein (GRSP), a glycoprotein produced by the hyphae of arbuscular mycorrhizal fungi, is well known for its significant ecological role in soil function, while lack of cognition of GRSP chemical structural characteristics has largely restricted the ecological status and application prospect of GRSP. In this study, five tree species and adjacent farmland have been investigated to uncover the structural characteristic of GRSP and the inter-specific differences through a variety of technical methods. We found that GRSP contained five functional groups, i.e. O—H and N—H stretching, C—H stretching, CO and COO— stretching,symmetric COO— stretching, C—H bending, C—O and Si—O—Si stretching. Ultraviolet spectrum showed that the peak wavelengh of GRSP was 225-248 nm and OD value was 0.9-2.0. XRD data showed that GRSP contained seven mineral substances (quartz, montmorillonite, illite, plagioclase, illite &, montmorillonite mixed-layer, and two unrecognizable substances), and their relative crystallinity was quite low with average grain size 25.83, 17.93, 14.50, 21.45, 28.45, 28.10 and 17.40 nm, respectively. Moreover, XPS data found 11 elements of C1s, Al2p, O1s, Si2p, P2p, Ca2p, K2p, N1s, Fe2p, Mg1s and Na1s in GRSP, and their average percentages were 28.9%, 7.0%, 43.4%, 9.8%, 0.4%, 1.0%, 0.6%, 1.4%, 2.1%, 0.7% and 7.8%, respectively. Tree species and farmland practices strongly affected GRSP composition. The biggest difference of function group was C—H stretching between Mongolian oak and birch (5.5 fold), while the minimum difference was O—H and N—H stretching between Manchurian ash and birch(1.3 fold). Juglans mandshurica had the highest Uv-OD values. Mineral types of GRSP were also different. Manchurian ash (Fraxinus mandshurica) and Mongolian oak (Quercus mongolica) forests contained plagioclase, illite &, montmorillonite mixed-layer, and two unrecognizable substances, and more soil mineral substances were found in GRSP from other tree species and farmland. The 5.2-fold difference of quartz grain size was found in different vegetations, while plagioclase had the minimum inter-species differences(17%). Relative crystallinity of montmorillonite in larch plantation was 5.1 fold higher than that in Manchurian ash, while very smaller differences in two unrecognizable substances (4% variation of b substance) were found between Manchurian ash and Mongolian oak forests. C1s, O1s, Si2p, N1s and Na1s could be found in GRSP from all forests and farmland, and their variation was 1.1-31.6 fold. Our study confirms that GRSP composition varies greatly among different vegetational types, and soil minerals could also affect the composition owing to variable soil minerals in GRSP. Clarification of the influencing mechanism of GRSP through revealing the GRSP compositional characteristics in this study is helpful for restoring soil ecological functions of farmland and forest soils in this region.