Impact of mixed straw on soil hydraulic properties

Abstract: To explore the impact of crop residues on soil hydraulic properties and water retention characteristics, a column experiment of soil water movement properties, which mixed crop residues into the soil, was carried out in August 2014, at Henan Key Laboratory of Water-saving Agriculture, North China University of Water Resources and Electric Power, Zhengzhou, China. There were 9 treatments with 2 kinds of crop residues, wheat straw and corn stalk. Proportions of mixed straw was calculated as the percentage of dry soil mass, which varied from 0 as control and 1.0%, 1.6%, 2.25%, 3.2%, respectively. Using a rice and wheat thresher, winter wheat straw and summer maize stalks were cut into pieces by once and third times. Those straw pieces in length less than 5 cm were collected and mixed with dry top-soil from test area in designated proportions. And then they were filled into the soil column as 1.4 g/cm3 soil bulk density layer by layer. First, water filled into soil column from its surface with soil infiltration instrument (Guelph) and lasted for 1 hour. Then, soil water retention was measured by the unsaturated hydraulic conductivity meter (Ku-Pf). It was implemented that the soil-hydraulic functions of van Genuchten, who used the statistical pore-size distribution model to obtain a predictive equation for the unsaturated hydraulic conductivity function in terms of soil water retention parameters. At last, the ratio of easily available water, which defined as easily used soil water content divided by saturated soil water content, was analyzed. Results showed that the van-Genuchten model was suitable for soil water retention under the condition of mixed wheat straw and corn stalk into the soil. For soils mixed with straw, soil moisture was lower than that of control in low suction section (soil water suction less than 150 cm H2O). In middle and high suction section (soil water suction more than 150 cm H2O), at a same suction, soil water content of mixed 1% wheat straw was lower than that of control, moisture of mixed 1.6% wheat straw was near to the control, and its mixed 2.25 %, 3.2% wheat straw was higher than that of the control. Descending order of soil mixed straw water content was 3.2%, 2.25%, 1.6% and 1%. For summer maize stalk, soil water content of soil mixed stalk was higher than that of the control in all treatments. Increasing the ratio of mixed corn stalk, soil moisture was increased firstly and then decreased. The highest soil water content appeared at 2.25% summer maize stalk mixed into soil. But soil water content of 3.2% treatment was lower than that of mixed 2.25% stalk. Soil moisture decreased from the ratio of stalk 2.25%, 3.2%, 1.6% to 1.0% treatment by sequence. Soil hydraulic parameters of van-Genuchten model were affected by mixed crop residue. Parameter α, reciprocal of intake air value was smaller than the control. The shape coefficient n increased with the mixing amount of straw from 1.0% to 2.25%, but it decreased at 3.2%. Mixed straw into soil affected the residual soil water content and the saturated soil water content in the soil-hydraulic functions of van Genuchten too. Soil saturated soil water content were smaller than the control after mixing two kinds of straw. The residual soil water content changed as the shape coefficient values. The residual soil water content was lower slightly than the control after adding winter wheat straw, while slightly higher than the control after adding summer maize stalk. The soil gravity water decreased and the easily available water increased after mixed crop straw. Soil gravity water reduced 19.3%-73.3%. The maximum ratio of the easily available soil water were 26.3%, 30.6%, which appeared at 3.2% winter wheat straw and 2.25% summer maize stalk mixed into soil, respectively. It could significantly increase soil water conserving properties by mixed straw into soil.