Abstract Reliable estimates of saturated hydraulic conductivity (Ks) are usually difficult to obtain, as Ks is regulated by a variety of soil processes acting at different spatial scales that may obscure each other's impacts. We hypothesized that these scale‐specific relationships could be well characterized with the aid of the noise‐assisted multivariate empirical mode decomposition (NA‐MEMD), thereby serving as a solid foundation for an accurate Ks prediction. The objective was to evaluate whether the incorporation of NA‐MEMD could improve the estimation of Ks based on the multiscale associations it unraveled. On a typical slope transect of 860 m in the black soil region of northeast China, Ks, mollic epipedon thickness, bulk density, soil organic carbon content, total (ϕ) and effective porosities (ϕe), and particle size distribution were investigated at every 20 m. Prior to NA‐MEMD, Ks was most strongly correlated with ϕe, and the linear regression models based on ϕe solely were satisfactory for Ks estimation at the scale of investigation. Adding other predictors significantly improved Ks prediction in calibration, but impaired it in validation. Upon decomposition by NA‐MEMD, Ks was found to be significantly associated with each attribute at two scales of oscillation at least. Summing up the estimates of each Ks component derived from the properties at the equivalent oscillation scales, the results outperformed the traditional multiple linear regressions made at the investigation scale, when the same sets of predictors were used. The application of NA‐MEMD, moreover, could save the tedious measurements of ϕe and ϕ. Excluding these two porosity‐related properties, Ks estimates obtained by incorporating NA‐MEMD were statistically similar or even better than those involving them before NA‐MEMD. These findings demonstrate the great potential of NA‐MEMD in untangling scale‐dependent relationships of Ks with various processes and hold important implications for future estimations of Ks and other hydraulic properties in the black soil region of northeast China and similar regions.

Soil erosion is a major cause of soil degradation, leading to soil quality deterioration. The resulting alternation of soil properties would in turn change the soil's responses in subsequent soil erosion events later, which, however, has rarely been studied. The objective was to examine the effects of such erosion-induced degradation on subsequent soil erosion and sediment size distribution of a cultivated black soil under heavy rainstorms. The plough layers of the black soil that had suffered from 0, 10, 30, 50, and 70 years of soil erosion in northeast China were artificially constructed, corresponding to five erosional degradation levels of none, slight, moderate, severe, and very severe degradations, successively. A simulated heavy rainfall at the 75 mm h(-1) intensity was performed, and runoff and sediment samples were collected every 6 min and analyzed for particle size distribution. Owing to the increasingly coarse source soil and thereby enhanced hydraulic conductivity, the steady-state runoff rate decreased from 1.06 to 0.77 mm min(-1), as the degradation level increased from none to very severe degradation. However, the fine-textured noneroded soil had a better water retention capacity, which decreased runoff, especially at the beginning of the rainfall. The maximum runoff in total, that is, 103.61 mm, was consequently observed in the slightly degraded soil. The sediment concentrations and yields here were also significantly greater than those of the other treatments (p <0.01), suggesting a relatively higher soil erodibility. Sediment sorting was observed at each treatment especially during the first half of the rainfall, except for the slightly eroded soil where the elevated runoff was believed sufficient to equally transport different-sized particles. The clay particles were enriched in the sediments eroded from the non- and moderately degraded soils, as the corresponding enrichment ratios (ERs) were typically above 1. Whereas for the severely and very severely degraded soils, ERs were around 0.5, indicating considerable depletion of these fine particles. These results demonstrate the diverse erosional responses of the soils exhibiting varying erosion-induced degradation levels and hold important implications for agricultural management and soil and water conservation in the black soil region of northeast China as well as other similar regions.

Straw mulching has been widely demonstrated as a useful practice for soil and water conservation, but its effectiveness in controlling rill erosion has rarely been systematically studied. The objective of the current study was to evaluate the effects of straw mulching on rill development and rill network evolution. A cinnamon soil was mulched with corn straw segments at five different rates of 0, 2.1, 4.2, 6.3 and 8.4 t ha-1, to attain the surface coverages of 0%, 10%, 20%, 30% and 40%, respectively, and received four 30-min simulated rainfalls at the intensity of 90 mm h-1. The structure from motion (SfM) photogrammetry was applied to measure soil surface elevation changes upon each rainfall. Straw mulching was found to retard runoff initiation, but decreased soil losses were typically acquired at the mulched treatments with 20% or more straw coverages. Nevertheless, this conservation effect did not monotonically increase with mulching rate, because a larger number of straw segments were more likely to induce rill erosion via surface flow convergence, thereby increasing total soil loss. Straw mulching also changed the course of rill development. Head retreat dominated rill erosion at the control without mulching throughout the entire experiment, suggesting an early stage of rill erosion. With the straw segments around and at the bottom, the initially equivalent contributions by headcutting, bed scouring and sideward erosion quickly shifted to the predominance of bed incision and then to sidewall expansion, succes-sively. The rapid transition of rill erosion subprocess in this way, together with the dendritic and stabilized networks, indicate a mature stage of rill development by the end of the experiment. The rill depths and main channel widths, as a consequence, were typically smaller at the mulched treatments. These findings demonstrate the effectiveness of adequate straw mulching in stabilizing rill network in addition to reducing soil erosion at the plot scale, and hold important implications for agricultural management in slope farming systems.

Saturated hydraulic conductivity (Ks) is controlled by a suite of soil processes operating at various scales. Little is known about the spatial variability of Ks along a slope and how it is changed through the planting operation process and the specific scales at which the controlling factors function. The objective was to characterize the scale-dependent relationships between Ks and various soil processes and thereby to identify the primary controlling factors of Ks. On a typical slope transect of 900 m in the black soil region of Northeast China, Ks and six soil properties of bulk density (BD), wet-aggregate stability (WAS), surface roughness (SR), soil organic carbon content, sand and clay contents were investigated in an interval of 20 m before and after mechanical sowing in the spring. Statistical correlation analysis, its combination with noise-assisted multivariate empirical mode decomposition (NA-MEMD) and wavelet coherence were employed to examine the interactions between Ks and each factor at different spatial scales. The BD was admittedly the dominant factor controlling Ks, followed by WAS and SR before and after sowing, respectively. The statistical correlation analysis only detected the significant interactions between Ks and BD, whereas the essential influence of WAS and SR were revealed upon frequency component decompositions by NA-MEMD and biwavelet coherency. Applying multiple wavelet coherence, BD was believed adequate in explaining Ks variability before sowing, whereas the combination of BD and SR was suggested in the case after sowing. These findings hold important implications for precise Ks estimations in the black soil region of Northeast China as well as other similar regions, and also demonstrate the effectiveness of NA-MEMD and wavelet coherence in untangling complex scale-dependent relationships among soil processes.