Glomalin-related soil protein (GRSP), an operationally defined heat-stable soil protein pool originating largely from arbuscular mycorrhizal fungi (AMF), accumulates in soil aggregates during natural revegetation of abandoned Loess Plateau cropland and correlates with macroaggregate formation and the recovery of soil organic carbon (SOC), total nitrogen (TN), and recalcitrant carbon. Whether GRSP accumulation and its redistribution among aggregate size classes causally drives aggregate formation and long-term SOC/TN stabilization — rather than merely tracking these processes — remains unresolved, as do the molecular and microbial pathways by which recent versus cumulative GRSP becomes incorporated into the recalcitrant carbon pool of microaggregates, the role of phosphorus (P) limitation in modulating GRSP production and GRSP-mediated stabilization, and the validity of the projected ~40-year recovery of GRSP pools to natural-grassland levels. Manipulative pot-scale evidence (exogenous addition of crude or purified GRSP, hyphal disruption, AMF inoculation) supports a partial causal contribution, and a 2026 study decoded hyphae-secreted GRSP in a zero-carbon background as an Hsp60-homologous protein, but the soil GRSP pool remains an operationally defined mixed extract, no field-scale manipulative or long-term repeated-measures test exists, and a 2025 field-wide review explicitly calls for a shift from correlative to causal-mechanistic frameworks. The problem is open.

Background

Arbuscular mycorrhizal fungi (AMF) are soil fungi that form symbioses with most land plants; in 1998 Wright and Upadhyaya reported that a heat-stable glycoprotein produced on AMF hyphal walls, named glomalin, correlates strongly with soil aggregate stability across many soils. Because the substance is quantified by autoclaved citrate extraction followed by the Bradford assay (a colorimetric dye-binding protein assay) rather than by molecular identification, it is conventionally called glomalin-related soil protein (GRSP): an operational pool with a labile fraction (easily extractable GRSP, extracted once) and a cumulative fraction (total GRSP, extracted repeatedly from the same soil). GRSP binds soil particles and is unusually persistent, so it has been proposed as a key binding agent in the formation and stabilization of water-stable aggregates — soil clumps that resist disintegration in water, conventionally split into macroaggregates (>0.25 mm), microaggregates (0.053–0.25 mm), and silt+clay (<0.053 mm), with stability summarized by mean weight diameter (MWD), the mass-weighted average aggregate size after wet sieving. Aggregate structure in turn protects soil organic carbon (SOC): labile SOC fractions are preferentially held in macroaggregates, while recalcitrant carbon persists in microaggregates. In the Chinese Loess Plateau, decades of cropland abandonment under natural revegetation are the main passive restoration pathway, and recovery of aggregates and SOC is the central indicator of restoration success. Liu et al. (2020, Geoderma 357:113921) measured GRSP fractions, SOC lability fractions (obtained by graded sulfuric-acid oxidation, from labile Cf1C_{f1} to most-recalcitrant Cf4C_{f4}), total nitrogen (TN), and total phosphorus in each aggregate size class along a 0–32-year chronosequence of cropland abandonment (substituting sites of different ages for repeated measurement of one site, the space-for-time approach), with natural grassland as the reference. They found that rapid GRSP accumulation (years 22–32) coincided with macroaggregate formation, rising MWD, and increasing recalcitrant Cf4C_{f4} in microaggregates, and projected from these trends that GRSP pools would return to natural-grassland levels within roughly 40 years after abandonment. A foundational limitation of this and most adjacent work (reviewed by Rillig 2004) is that the evidence is correlative: co-occurring trends cannot distinguish whether GRSP drives aggregation and carbon stabilization, whether aggregation and carbon inputs promote GRSP retention, or whether both follow a third factor such as AMF hyphal growth, plant productivity, or microbial community shifts. GRSP is also an operationally, not molecularly, defined pool: extractions co-isolate proteins of non-AMF origin, including microbial necromass, which complicates attribution of any causal role to GRSP specifically. The identity question has recently been narrowed at the hyphal source: Liu, Deng, Zhou, Zou and Wu (2026, Soil Biology and Biochemistry 221:110233) grew mycorrhizal hyphae secreting GRSP in a zero-carbon background and reported homology of the secreted protein with Hsp60 — the 60-kDa heat-shock protein, a highly conserved molecular chaperone — together with its functional molecular architecture. This advance concerns hyphae-secreted GRSP, however; the soil-extracted pool that the causal question concerns remains a mixed operational extract whose non-AMF fraction is not eliminated.

Problem Statement

Establish whether glomalin-related soil protein (GRSP) accumulation and its redistribution among aggregate size classes causally drives soil aggregate formation and stabilization and the recovery and long-term stabilization of soil organic carbon (SOC, including the recalcitrant microaggregate carbon pool) and total nitrogen during natural revegetation of abandoned Loess Plateau cropland. The question concerns (i) whether experimentally altering GRSP production or retention (independently of, or alongside, the enmeshing effects of arbuscular mycorrhizal fungal (AMF) hyphae) changes aggregate formation rates and long-term SOC stabilization; (ii) the molecular and microbial-process pathways by which recent (easily extractable) versus cumulative (total) GRSP becomes incorporated into the recalcitrant carbon pool within microaggregates; and (iii) how phosphorus limitation and P-driven microbial interactions modulate GRSP production and GRSP-mediated SOC and TN stabilization during revegetation. The question also concerns whether the projected ~40-year recovery of GRSP pools to natural-grassland levels is supported.

References

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