Uncovering scale- and location-dependent variations and drivers of soil nutrients along a southeast-northwest transect of the Qinghai-Tibetan plateau using wavelet analysis

Abstract

The spatial variability of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP), along with relationships with individual environmental factors, have been widely studied. However, the combined effects of multiple controlling factors remain underexplored across different scales and locations, particularly in complex environments like China’s Qinghai-Tibetan Plateau (QTP). This study aimed to identify the scale-specific factors controlling SOC, TN, and TP along a 1,800-km southeast-northwest transect of the QTP using wavelet coherence analysis. Results showed that SOC exhibited significant spatial variability at small (<100 km) and medium (100–200 km) scales, TN at small and medium scales, and TP at medium and large (>200 km) scales, particularly over the 500–1000 km segment of the transect (at a 95 % confidence level). The relationships between influencing factors and SOC, TN, and TP varied with spatial scale and transect location. Across all scales, bulk density (BD) emerged as the dominant factor explaining SOC and TN variability, with the largest average wavelet coherence (AWC) (0.55 for SOC and 0.53 for TN) and the percent area of significant coherence (PASC) (35.46 % for SOC and 36.16 % for TN). For TP variability, pH was the primary controlling factor (AWC=0.50, PASC=24.81 %). The best combinations of factors were pH and BD for SOC (AWC=0.85, PASC=62.13 %); pH, BD, and mean annual precipitation for TN (AWC=0.92, PASC=58.63 %); and pH, BD, and silt for TP (AWC=0.90, PASC=43.81 %). Adding additional factors did not consistently enhance explanatory power; a two-factor combination was sufficient for SOC, while a three-factor combination adequately explained TN and TP variability. Our findings clarify the spatial variations of SOC, TN, and TP, highlighting their scale- and location-specific dependencies on influencing factors along the southeast-northwest transect of the QTP. The insights gained from this study can support future modeling, mapping, management, and sampling strategies for SOC, TN, and TP in the alpine region.