Spatial variability of topsoil C and N in relation to soil erosion along a cultivated black soil toposequence

Soil erosion is a crucial process leading to soil organic carbon (SOC) and total nitrogen (TN) redistributions in the topsoil in agricultural ecosystems, yet it remains ambiguous how the spatial variability of topsoil SOC and TN respond to soil erosion. The objective was to characterize the spatial structures of topsoil SOC, TN and their stochiometric ratio, and to disentangle their spatial relationships with soil erosion in a landscape. On a toposequence spanning ∼ 3,500 m in the black soil region of northeast China, soil erosion rate (ER), and SOC, TN and the C/N ratios at the depths of 0–10 and 10–20 cm were investigated in an interval of 40 m. Unlike the highly variable ER presenting a long-range exponential structure, both the contents and stocks of topsoil SOC and TN were moderately variable and displayed spherical structures with much shorter correlation ranges between 1,000 and 1,400 m. The C/N ratios were even less variable, and exhibited nearly random distributions. Except for the C/N ratio at 0–10 cm depth, all variables were significantly negatively associated with ER at the scale of investigation (p < 0.05), as erosion tends to remove the fertile topsoil, especially the fine particulate organic matter rich in C, and to promote SOC decomposition via aggregate disintegration. The corresponding correlation coefficients ranged from −0.260 to −0.527. Applying multivariate variational mode decomposition (MVMD), however, insignificant and even significantly positive correlations were detected at different spatial scales between some variables and ER. The wavelet coherency corroborated the scale-dependent relationships of each variable with ER, and specified the sections displaying various correlations. These findings demonstrate the intricate spatial relationships of topsoil C and N with erosion, and hold important implications for sustainable agricultural management.