Quantitative Characterization, Controlling Factors And Reconstruction Of Pore Space In Red Soils Derived From Quaternary Red Clay

Soil porosity and pore size distribution are important soil quality indicators. The total volume, size distribution, and shape of soil pore spaces determine many soil processes and functions, such as water storage and transmission, gas diffusivity, microbial activity and soil mechanical resistance to root penetration. Red soils formed on Quaternary red earth widely cover the subtropical and tropical regions of China and is one of the most common soil types to be used in crop production. However, there is limited information available about pore structure and its response to soil management. The quantification of pores over a wide range of pore sizes is very useful to improve our knowledge about the structural stability, water movement, gas diffusivity, root penetration and microbial activities, In the present work, different methods, including water characteristics curve, mercury intrusion porosimertry and nitrogen adsorption isotherm (NAI), was used to evaluate the porosity, pore size distribution and pore sizes characteristic of red soils. The main results were as followings:1. Total porosity values (TPV) of these soils, calculated from bulk density, ranged from44.9to57.4cm3100cm-3with an average of50.2m3100m-3. TP percentage of these soils, calculated from the water characteristic curve of red earth, ranged from40.8%to62.4%with an average of48.8%. Wide variability in porosity measured between0.05-15bar pressure (0.2-60μm) of soils varied between a minimum of2.3%and a maximum of13.6%. Wide variability in total MIP porosity (0.003-100μm) of soils was observed, varied between a minimum of33%and a maximum of45%. The differential PSD curves of the soils exhibit three distinct peaks from0.01to0.05μm,0.1to2μm and above70μm along the entire range of investigated pores. The multi-modal PSD in the red soils represented the existence of a more heterogeneous pore system. The pore size classification indicated that ultramicropores (0.1-5μm) are the dominant pore class (on average35%), followed by cryptopores and macropores, representing30%and23%pore fraction, respectively. The mesopores and micropores shared5%and7%pore fraction, respectively. The pore range measured by NAI is from nanometers to tens of nanometers, which could provide valuable information about nano-scale pores in the soils. The soils showed an average pore volume of0.0387cm3g-1with highest pore volume of0.0607cm3g-1and least pore volume of0.0196cm3g-1. The soils had greatest amount of pore volume existed in the pore size diameter of0.01-0.05μm, accounted for59%of total NAI pore volume. The pore fraction of<0.01μm contributed27%for total NAI pore volume.2. The correlation analysis and principal component analysis was used to describe the dependence of the changes in porosity on the soil properties and constitutes. A positive and strong correlation was observed between iron oxide (Fed) and pore volumes of0.01-0.1μm and<0.01μm, whereas negative correlation was found between Fed and pore volume of0.1-5μm. A positive and highly significant linear correlations between the pore volumes of<0.01and0.01-0.1μm and clay fraction were also found. Therefore, it can be concluded that the Fed and clay contributed greatly to the pore volume of crytomicropores in the red soils. PCA analysis indicated a close association between Fed and clay and the crytopores (<0.1μm). The<0.1μm pore volume determined by MIP and NAD are found to correlate linearly with the Fed and clay content. Soil organic carbon (SOC) had no significant correlation with porosity. The iron oxides, rather than SOM, are the dominant cementing agents for aggregation in red soils and the relationship between SOM and porosity might not be as strong in these soils as in2:1clay dominated soils.3. Red soil pot experiment was carried out with the soil ameliorant lime and charcoal.The results showed that, charcoal and lime application increased wheat yield, with the maximum yield doubled compared with control. Applying ameliorant could enhance pH of soil, and extremely significantly reduce the exchangeable aluminum content in soil and aluminum poison. The sludge biochar and the straw biochar were able to reduce bulk density, and increase soil porosity. Applying sludge biochar could enhance the equivalent pore of these red soils.