Glomalin-related soil protein (GRSP) is believed to be produced by arbuscular mycorrhizal fungi (AMF). However, this fraction of organic matter is influenced by various soil and land-use parameters and its link with AMF has been questioned. The aim of this study was to advance the understanding of the origin of GRSP and its value as a marker of AMF activity by focusing on the effects of soil P status. Archived soils from two phosphorus fertilization field trials on sandy soils under maize cultivation in the south-west of France were studied. Trends in GRSP and soil organic carbon (SOC) were compared. Grain yield and available P (Olsen-P) were monitored and compared to assess P sufficiency/limitation. The time trends of GRSP for each site were not significant. No significant P-fertilization effect on GRSP was observed for the P-sufficient continuously cropped soil, for which the crop yield increase was small. For the P-deficient, former forest soil, P fertilization led to a marked increase in crop production and a significantly larger GRSP content. These trends are coherent with GRSP input linked to crop C-inputs, including the incorporation of crop residue.