A causal evidence regarding improved soil traits in citrus by addition of purified glomalin-related soil protein.

• Purified EE-GRSP solid was added into soil to analyze GRSP function. • EE-GRSP/DE-GRSP increased with EE-GRSP addition but decreased with the time. • EE-GRSP addition elevated SOC and the fraction of WSA at 0.25‒4 mm size. • High addition of EE-GRSP increased soil NH 4 N, Olsen-P, and available K levels. • A direct evidence regarding EE-GRSP function was provided. Glomalin-related soil protein (GRSP) is a glycoprotein metabolite produced by arbuscular mycorrhizal fungi. Although GRSP has favorable effects on soil, there is a lack of causal evidence to support these claims. In this study, different amounts of purified easily extractable GRSP (EE-GRSP) solid (0.25‒1 g/kg) were mixed with soil and buried under the canopy of field Lane late navel orange trees in 0.45-μm nylon mesh bags. The impacts of EE-GRSP addition on soil traits, including endogenous GRSP levels, soil fertility, and the distribution and stability of soil water-stable aggregate (WSA) were studied. At 7 and 15 months of EE-GRSP addition, the levels of easily extractable, difficultly extractable, and total GRSP increased to varying extents with the increase of EE-GRSP addition but decreasing over time. The addition of EE-GRSP also led to an increase in the fraction of WSA at the size of 0.25‒4 mm, and the effect was more pronounced with higher amounts of EE-GRSP addition, resulting in a significant improvement in WSA stability (24.7 % to 91.3 % at 7 months and 36.2 % to 121.4 % at 15 months). Furthermore, high amounts (1 and 2 g/kg) of EE-GRSP significantly increased the levels of available K, NH 4 N, and Olsen-P in the soil. Soil organic carbon levels also showed a tendency to increase with increasing EE-GRSP addition, with increases ranging from 66.0 % to 246.0 % at 7 months and 14.5 % to 402.4 % at 15 months. Overall, this work provides the first evidence that adding EE-GRSP to soil can enhance soil fertility and aggregate stability in the citrus orchard. [ABSTRACT FROM AUTHOR]