Heat, water, and chemical transport in soils

Transport processes of heat, water, and chemicals in soils are very important for managing the root zone for maximum crop production and controlling soil and water quality for minimum degradation and pollution. New and simple analytical or approximate solutions to the corresponding transport problems are presented in this research. Analytical or approximate solutions are further manipulated to estimate the corresponding transport properties. An analytical solution to coupled conduction and convection heat transfer problem under field conditions is obtained by Fourier transformation. The analytical solution can predict field observations of infiltration and temperature well. Three new methods for estimating transport properties in soils are developed in this study. They are: soil water diffusivity determination by general similarity, soil hydraulic property estimation by integral method, and solute transport parameter estimation by boundary layer theory. The general similarity method for water diffusivity determination only requires measuring advance of wetting front with time. The general similarity diffusivities for five soils compare well to those determined by Boltzmann transformation. The integral method estimates soil water characteristic curve and unsaturated hydraulic conductivity simultaneously. The estimated hydraulic properties for six soils ranging from sandy loam to clay loam by the integral method are in good agreement with independently determined values. A boundary layer method for estimating dispersion coefficient and retardation factor is developed. The boundary layer method is applicable both to soil columns and field soils. All of the methods developed in this dissertation present simplifications in application and they are less time consuming than current methods

Effect of Polyacrylamide Applications on Soil Hydraulic Characteristics and Sediment Yield of Sloping Land

The objective of this study is to determine how PAM applications to a fraction of the surface affect hydraulic parameters of flow, soil water infiltration and soil sediment yield during rainfall on steep sloping land. Five PAM application rates (PAMR) 0, 0.4, 0.7, 1.0 and 1.3 g kg-1 were used for 5 sloping plots in this study. As PAMR increased from 0 to 1.3 g kg-1 at the rainfall intensity (RI) of 1.58 mm min-1, the Froude numbers decreased from 34.7 to 9.1, the Reynolds numbers (Re) decreased from 568 to 305, and the Darcy–Weisbach coefficients increased from 0.0028 to 0.041. The total runoff values were 33.8, 35.9, 31.6, 25.6 and 18.1 mm when the PAMR were 0, 0.4, 0.7, 1.0 and 1.3 g kg-1, respectively. The cumulative sediment increased rapidly with the rainfall time. In conclusion, PAM applications to a fraction of soil surface can be effective at reducing the erosion of steep sloping land

Spatial variations in soil-water carrying capacity of three typical revegetation species on the Loess Plateau, China

Re-vegetation is a necessary control measure of soil erosion in the Loess Plateau. However, excessive re-vegetation can aggravate soil water shortage, which can in turn threaten the health and services of restored ecosystems. An optimal plant cover or biomass (i.e., soil-water carrying capacity for vegetation, SWCCV) is important for regional water balance, soil protection and vegetation sustainability. The objective of this study was to determine the spatial distribution of SWCCV for three non-native tree (Robinia pseudoacaia), shrub (Caragana korshinskii) and grass (Medicago sativa) species used in the re-vegetation of the Loess Plateau. The dynamics of actual evapotranspiration (AET), net primary productivity (NPP) and leaf area index (LAI) were simulated using a modified Biome-BGC (Bio-Geochemical Cycles) model. Soil and physiological parameters required by the model were validated using field-observed AET for the three plant species at six sites in the study area. The validated model was used to simulate the dynamics of AET, NPP and LAI for the three plant species at 243 representative sites in the study area for the period 1961–2014. The results show that spatial distributions of mean AET, NPP and LAI generally increased from northwest to southeast, much the same as mean annual precipitation (MAP) gradient. In terms of maximum LAI, the ranges of optimal plant cover were 1.1–3.5 for R. pseudoacaia, 1.0–2.4 for C. korshinskii and 0.7–3.0 for M. sativa. The corresponding SWCCV, expressed as NPP were 202.4–616.5, 83.7–201.7 and 56.3–253.0 g C m−2 yr−1. MAP, mean annual temperature, soil texture and elevation were the main variables driving SWCCV under the plant species; explaining over 86% of the spatial variations in mean NPP in the study area. Further re-vegetation therefore needs careful reconsideration under the prevailing climatic, soil and topographic conditions. The results of the study provide a re-vegetation threshold to guide future re-vegetation activities and to ensure a sustainable eco-hydrological environment in the Loess Plateau

Mineral N stock and nitrate accumulation in the 50 to 200 m profile on the Loess Plateau

Nitrogen (N) stored in deep profiles is important in assessing regional and/or global N stocks and nitrate leaching risk to groundwater. The Chinese Loess Plateau, which is characterized by significantly thick loess deposits, potentially stores immense stocks of mineral N, posing future threats to groundwater quality. In order to determine the vertical distributions of nitrate and ammonium content in the region, as well as to characterize the potential accumulation of nitrate in the deep loess profile, we study loess samples collected at five sites (Yangling, Changwu, Fuxian, An'sai and Shenmu) through a 50 to 200 m loess profile. The estimated storage of mineral N varied significantly among the five sites, ranging from 0.46 to 2.43 × 104 kg N ha−1. Ammonium exhibited fluctuations and dominated mineral N stocks within the whole profile at the sites, except for the upper 20–30 m at Yangling and Changwu. Measured nitrate content in the entire profile at Fuxian, An'sai and Shenmu is low, but significant accumulations were observed to 30–50 m depth at the other two sites. Analysis of δ15N and δ18O of nitrate indicates different causes for accumulated nitrate at these two sites. Mineralization and nitrification of manure and organic N respectively contribute nitrate to the 0–12 and 12–30 m profile at Changwu; while nitrification of NH4+ fertilizer, NO3− fertilizer and nitrification of organic N control the nitrate distribution in the 0–3, 3–7 and 7–10 m layer at Yangling, respectively. Furthermore, our analysis illustrates the low denitrification potential in the lower part of the vadose zone. The accumulated nitrate introduced by human activities is thus mainly distributed in the upper vadose zone (above 30 m), indicating, currently, a low nitrate leaching risk to groundwater due to a high storage capacity of the thick vadose zone in the region

Erosion reduces soil microbial diversity, network complexity and multifunctionality

While soil erosion drives land degradation, the impact of erosion on soil microbial communities and multiple soil functions remains unclear. This hinders our ability to assess the true impact of erosion on soil ecosystem services and our ability to restore eroded environments. Here we examined the effect of erosion on microbial communities at two sites with contrasting soil texture and climates. Eroded plots had lower microbial network complexity, fewer microbial taxa, and fewer associations among microbial taxa, relative to non-eroded plots. Soil erosion also shifted microbial community composition, with decreased relative abundances of dominant phyla such as Proteobacteria, Bacteroidetes, and Gemmatimonadetes. In contrast, erosion led to an increase in the relative abundances of some bacterial families involved in N cycling, such as Acetobacteraceae and Beijerinckiaceae. Changes in microbiota characteristics were strongly related with erosion-induced changes in soil multifunctionality. Together, these results demonstrate that soil erosion has a significant negative impact on soil microbial diversity and functionality

The Accumulation of Organic Carbon in Mineral Soils by Afforestation of Abandoned Farmland

The afforestation of abandoned farmland significantly influences soil organic carbon (OC). However, the dynamics between OC inputs after afforestation and the original OC are not well understood. To learn more about soil OC dynamics after afforestation of farmland, we measured the soil OC content in paired forest and farmland plots in Shaanxi Province, China. The forest plots had been established on farmland 18, 24, 48, 100, and 200 yr previously. The natural 13C abundance of soil organic matter was also analyzed to distinguish between crop- and forest-derived C in the afforested soils. We observed a nonlinear accumulation of total OC in the 0–80 cm depth of the mineral soil across time. Total soil OC accumulated more rapidly under forest stands aged 18 to 48 yr than under forest stands aged 100 or 200 yrs. The rate of OC accumulation was also greater in the 0–10 cm depth than in the 10–80 cm depth. Forest-derived OC in afforested soils also accumulated nonlinearly across time, with the greatest increase in the 0–20 cm depth. Forest-derived OC in afforest soils accounted for 52–86% of the total OC in the 0–10 cm depth, 36–61% of the total OC in the 10–20 cm depth, and 11–50% of the total OC in the 20–80 cm depth. Crop-derived OC concentrations in the 0–20 cm depth decreased slightly after afforestation, but there was no change in crop-derived OC concentrations in the 20–80 cm depth. The results of our study support the claim that afforestation of farmland can sequester atmospheric CO2 by increasing soil OC stocks. Changes in the OC stocks of mineral soils after afforestation appear to be influenced mainly by the input of forest-derived C rather than by the loss of original OC