Contribution of GRSP to Soil Organic Carbon and Nitrogen Varied with Altitude and Soil Depth in a Lowland Tropical Forest

Glomalin-related soil protein (GRSP) is pivotal for sustaining soil organic carbon (SOC) stability in tropical forest ecosystems. Nevertheless, how SOC stability responds to altitude and soil layer variations, as well as the quantitative contributions of GRSP to SOC and soil nitrogen, remain poorly understood in lowland tropical forests. Here, we conducted a field-based investigation to characterize the distribution patterns of SOC, readily oxidizable organic carbon (ROC), GRSP, and other key soil physicochemical properties across different soil layers. We further comprehensively evaluated the SOC stability and quantified the contributions of GRSP to soil C and N pools at two altitudes in a lowland tropical forest. The results revealed that altitude exerted a significant effect on SOC contents, with substantially higher SOC stability observed at the high altitude (400 m) relative to low altitude (100 m). Vertically, SOC and ROC contents were significantly higher in the surface soil layer (0–20 cm) than in the deep layer (20–100 cm), while deep soil exhibited stronger SOC stability compared with surface soil. The contributions of GRSP to SOC and total N (TN) varied distinctly across altitudes and soil layers: GRSP contributed more substantially to SOC and TN at low altitude than at high altitude, and its contribution was also greater in surface soil than in deep soil. Both easily extractable (EE-GRSP) and total GRSP (T-GRSP) were significantly positive correlated with ROC, SOC, and TN (p < 0.05). Redundancy analysis (RDA) further indicated that the ROC served as the primary regulatory factor governing the variation in SOC, EE-GRSP, and T-GRSP at both altitudes. Additionally, soil available phosphorus (AP) and C/P also drove the SOC and GRSP at low altitude, while pH, TN, and C/N regulated the SOC and GRSP at high altitude. Therefore, we propose that the altitudes and soil layers synergistically modulate the SOC stability and the contributions of GRSP to SOC in lowland tropical forests. These findings highlight the necessity of incorporating altitude and soil layer heterogeneity into evaluation and modeling of carbon cycles in tropical forests.