Effects of Straw and Biochar Returned to the Soil on Soil Physical Properties and pH Value in Cold Rice Region

Yuefeng CUI, Hongru SHI, Aonan GUO, Guocai SUN, Guiyan WANG, Jian WANG, Wenjia HUANG, Tiegang LU*

1. Tieling Academy of Agricultural Sciences, Tieling 112616, China; 2.Liaoning Agricultural Development Service Center, Shenyang 110034, China; 3.Tieling Municipal Bureau of Agriculture and Rural Affairs, Tieling 112608, China

Abstract [Objectives] In order to explore the feasibility of using straw and biochar returned to the soil to improve soil physical properties and pH value in cold rice regions of China. [Methods] the effects of straw directly returned to the soil and charred straw (biochar) returned to the soil on soil bulk density, porosity, temperature and pH value of cold paddy soil were studied in this paper. [Results] The results showed that compared with conventional production, straw (6 t/ha), a small amount of biochar (2 t/ha) and a large amount of biochar (40 t/ha) returned to the soil reduced paddy soil bulk density at different growth stages by 6.02%-11.86%, 2.69%-6.67% and 8.58%-11.32%, respectively, increased total porosity by 7.41%-14.93%, 3.19%-8.38% and 9.81%-14.27%, respectively, and increased aeration porosity by 22.28%-192.11%, 17.80%-92.11% and 52.44%-157.11%, respectively. Straw and a small amount of biochar returned to the soil had no significant effect on soil temperature and pH value of paddy field, but a large amount of biochar returned to the soil could significantly increase soil temperature by 5.13%-8.79% and pH value by 3.15%-5.96% in the later stage of rice growth. [Conclusions] The straw and biochar returned to the soil could reduce soil bulk density, increase total porosity and aeration porosity, and only a large amount of biochar returned to the soil could significantly increase soil temperature and pH value.

Key words Straw, Biochar, Cold rice region, Soil physical properties, Soil pH value

1 Introduction

China is rich in agricultural and forestry waste resources, with an annual output of 700 million tons of all kinds of straw, accounting for about 20% to 30% of the world’s straw output, ranking first in the world. More than 50% of the straw is used as firewood or discarded and burned, which not only causes a waste of resources, but also pollutes the environment. Therefore, the comprehensive treatment and efficient utilization of straw has become the key to solving the problem of sustainable development of soil, resources, environment and agriculture. Straw is an important part of biomass energy resources and organic fertilizer sources in China. Straw decomposition in soil after being returned to the soil can improve soil structure and physical properties, increase the content of organic matter, and play an important role in maintaining and improving soil fertility. The study shows that after straw is returned to the field, the soil bulk density decreases and the porosity increases, which is beneficial to improving the soil structure and aeration condition, strengthening the soil’s absorption and transformation of light radiation, with the effect of increasing temperature. It is mainly reflected in the 0-5 cm soil layer in the short term and in the 0-15 cm soil layer in the long term.

Biochar, which comes from biomass materials such as straw, has the characteristics of rich pore structure, large specific surface area and strong adsorption, which can play a good carrier role in retaining water and nutrients, reduce soil bulk density and increase porosity, and improve soil aeration conditions. The decrease of soil pH value is an important index of the decline of agricultural soil fertility quality, while biochar can reduce the acidity of the soil with lower pH value and the alkalinity of the soil with higher pH value, and is not limited by making materials. Other studies have shown that the mechanism of soil improvement by biochar is to improve soil properties and fertility by affecting soil pH value, so as to promote crop growth.

However, the current research on straw carbonization is mostly concentrated in warm and humid tropical or subtropical areas, while there are few studies on rice fields in cold areas in the north. Due to cold water irrigation and cold climate factors, the soil temperature of paddy field in northern China is lower, the soil viscosity is high, and there are few aggregate structures. The porosity is low, the permeability of water and air is poor, and the activity of microorganism is weak. Especially due to the long-term application of a large amount of chemical fertilizer, the acidification of paddy soil is intensified and the soil productivity decreases. Therefore, it is necessary to study the application effect of straw and biochar in this eco-climatic region. This experiment focused on the effects of biochar returned to the soil on soil physical properties and pH value of cold paddy field, in order to provide reference for soil improvement and quality improvement in cold rice region of Northeast China.

2 Materials and methods

2.1 Test materials

The experiment was carried out in the rice experimental field of Tieling Academy of Agricultural Sciences, which is located in the northern part of Liaoning Province, China (42°14′ N, 123°48′ E). It has a typical semi-humid continental monsoon climate. The annual average temperature is 6.3 ℃, the active accumulated temperature from April to September is 3 496.9 ℃, the precipitation is 643.5 mm, the number of sunshine hours is 1 357.6, and the average temperature is 19.1 ℃. In the experimental field, rice has been planted continuously for more than 40 years, and the irrigation water comes from the cold water at the depth of 30 m. The content of nutrition index of soil plough layer (0-20 cm soil layer) is as follows: the content of total nitrogen (1.06 g/kg); the content of total phosphorus (0.85 g/kg); the content of total potassium (17.24 g/kg); the content of available nitrogen (93.64 mg/kg); the content of available phosphorus (38.28 mg/kg); available potassium (75.06 mg/kg); organic carbon (10.73 g/kg); pH value (6.36).

The tested variety is Shennong 265 (northern super japonica rice), the number of leaves on the main stem is 15, which has the characteristics of compact plant type, strong tillering ability and erect panicle type. The straw was crushed into 0.5-1.0 cm small segments. Biochar was produced by Liaoning Biochar Engineering and Technology Research Center under the condition of pyrolysis and anoxia at 400-500 ℃. About 1/3 straw was made into biochar with a grain size of 1.5-2.0 mm. The main physical and chemical properties of straw and biochar are shown in Table 1.

Table 1 Main physicochemical properties of straw and biochar

2.2 Experimental design

There are 4 treatments in the experiment. Local conventional fertilization: 46% urea (456.5 kg/ha), 12% calcium superphosphate (875 kg/ha), 52% potassium sulfate (202 kg/ha), denoted as CK. Straw returned to the soil: 6 t/ha straw was applied on the basis in CK, denoted as S. A small amount of biochar returned to the soil: 2 t/ha biochar was applied on the basis in CK (calculated according to the conversion of straw to 30% biochar), denoted as C1. A large amount of biochar returned to the soil: 40 t/ha biochar was applied on the basis in CK, denoted as C2.

The experiment adopted the planting method of seedling transplanting. It was sown on April 18, 2013, transplanted on May 28, and harvested on October 8; it was then sown on April 14, transplanted on May 27 and harvested on October 9, 2014. The specification of rice seedlings for transplanting is 30.0 cm×13.3 cm, with 3 seedlings per hole. Random blocks were arranged and each treatment was repeated 3 times. There were a total of 12 plots, each with an area of 21 m. Ridging was done for each plot, equipped with inlet channel and drainage channel, for separate irrigation and drainage. Nitrogen fertilizer was applied based on weight ratio of base fertilizer∶tillering fertilizer∶panicle fertilizer=5∶3∶2, and straw and biochar were applied at one time before rice transplanting, evenly scattered on the soil surface, and then mixed evenly by rotary tillage. Calcium superphosphate was applied 100% as base fertilizer, potassium sulfate was applied 50% as base fertilizer and applied 50% as ear fertilizer. Other cultivation and management measures were carried out in accordance with the conventional rice field production rules.

2.3 Collection and determination of soil samples

0-20 cm soil samples were excavated vertically by soil drill at tillering stage, jointing stage, heading stage, filling stage and maturity stage. Three points were randomly selected from each plot, and the pH value of the sample was determined after natural air drying. When the thermometer was buried in each plot, the temperature of the soil at the downward depth of 5 cm was read when the soil was taken. At the same time, the bulk density and porosity of soil were measured by cutting ring method. The specific determination method is based on

Method

of

Soil

Agrochemical

Analysis

edited by Lu Rukun.

2.4 Statistics and analysis

All the test data were sorted out by Excel 2010. The difference was analyzed by single factor analysis of variance with DPS7.05 software, and multiple comparison (

LSD

) was used to judge the difference between treatments (

P

<0.05). All the measured data were expressed in the form of mean±standard deviation.

3 Results and analysis

3.1 Effects of straw and biochar on soil physical properties

3.1.1

Effects of straw and biochar on soil bulk density. Bulk density is one of the important indexes to measure soil physical properties, which has a great influence on soil ventilation, water content and nutrient transport function. It can be seen from Fig.1 that the soil bulk density basically increased at first and then decreased, but the difference was not very obvious in the whole growing season. The bulk density in CK was 1.41-1.48 g/cm, the bulk density in treatment S was 1.27-1.39 g/cm, and the bulk density in treatment C1 and C2 was 1.35-1.40 g/cmand 1.27-1.39 g/cm, respectively. The bulk density in each treatment was 1.27-1.43 g/cmat tillering stage, the bulk density in treatment S, C1 and C2 decreased by 6.95%, 3.34% and 10.89% respectively compared with that in CK, and there was a significant difference between S, C2 and CK; the bulk density in each treatment at jointing stage was 1.31-1.48 g/cm, and the bulk density in treatment S, C1 and C2 decreased significantly by 6.02%, 3.99% and 11.32% respectively compared with that in CK; the bulk density in each treatment at heading stage was 1.29-1.44 g/cm, the bulk density in treatment S, C1 and C2 decreased significantly by 6.67%, 2.69% and 10.02% respectively compared with that in CK, and there was a significant difference between S, C2 and CK; during the filling stage, the bulk density in each treatment was 1.30-1.48 g/cm, and the bulk density in treatment S, C1 and C2 decreased significantly by 11.86%, 6.67% and 10.18%, respectively compared with that in CK; in the maturity stage, the bulk density in each treatment was 1.27-1.41 g/cm, and the bulk density in treatment S, C1 and C2 decreased significantly by 10.00%, 4.57% and 8.58%, respectively compared with that in CK. It can be seen that straw and biochar returned to the soil could significantly reduce the bulk density of paddy soil at all growth stages, the soil bulk density in the treatment of returning straw to soil was 6.02%-11.86% lower than that in CK, and the decrease was even greater in the later growth stage. The soil bulk density in the treatment of returning a small amount of biochar to the soil decreased by 2.69%-6.67% compared with that in CK, and the difference also reached a significant level in the later stage. The soil bulk density in the treatment of returning a large amount of biochar to the soil decreased by 8.58%-11.32% compared with that in CK, and the decrease was more obvious in the early growth stage.

Note: Different lowercase letters above the column indicate significant differences among treatments in the same growth period (P<0.05), the same in the following.

3.1.2

Effects of straw and biochar on soil porosity. Soil porosity is closely related to soil water permeability, thermal conductivity and compactness. It can be seen from Table 2 that there were some differences in soil total porosity among different treatments at different growth stages. The total porosity in treatment S, C1 and C2 at tillering stage was 8.12%, 3.91% and 12.74% higher than that in CK, respectively, there was a significant difference between S, C2 and CK, but there was no significant difference between C1 and CK; the total porosity in treatment S, C1 and C2 at jointing stage was significantly higher than that in CK by 7.41%, 5.08% and 14.27%, respectively; at heading stage, the total porosity in treatment S, C1 and C2 increased by 8.03%, 3.19% and 11.85% compared with that in CK, S and C2 were significantly different from CK, while C1 was not significantly different from CK; at filling stage, the total porosity in treatment S, C1 and C2 significantly increased by 14.93%, 8.38% and 12.81%, respectively; at maturity stage, the total porosity in treatment S, C1 and C2 significantly increased by 11.44%, 5.23% and 9.81%, respectively. It can be seen that returning straw to the soil could significantly increase the total porosity of paddy soil by 7.41%-14.93%, and the increase in the late growth stage was greater than that in the early stage; a small amount of biochar returned to the soil had a certain effect on improving the total porosity of paddy soil, with an increase of 3.19%-8.38%, especially in the filling stage; a large amount of biochar returned to the soil could significantly increase the total porosity of paddy soil by 9.81% to 14.27% during the whole growth period, and played an efficient role since the tillering stage.

From the point of view of different growth stages, the soil capillary porosity in CK treatment was 37.66%-40.46%, the soil capillary porosity in treatment S was 36.30%-40.23%, the soil capillary porosity in treatment C1 was 36.35%-40.68%, and the soil capillary porosity in treatment C2 was 38.20%-40.33%. There was no significant difference among the treatments in the whole growth stage, which showed that the straw or biochar returned to the soil had no significant effect on soil capillary porosity, and it was not affected by the amount of biochar application.

There were some differences in soil aeration porosity among different treatments at different growth stages. The soil aeration porosity in treatment S, C1 and C2 at tillering stage increased significantly by 60.64%, 36.86% and 52.44% respectively compared with that in CK; at jointing stage, the soil aeration porosity in treatment S and C1 increased by 22.28% and 43.25% respectively compared with that in CK, the difference was not significant, and the soil aeration porosity in treatment C2 increased significantly by 86.83% compared with that in CK; the soil aeration porosity in treatment S, C1 and C2 at heading stage increased by 53.64%, 17.88% and 80.96%, respectively, there was no significant difference between S, C1 and CK, but there was a significant difference between C2 and CK; the soil aeration porosity in treatment S, C1 and C2 at filling stage increased significantly by 192.11%, 92.11% and 157.11% respectively compared with that in CK; the soil aeration porosity in treatment S, C1 and C2 at maturity stage increased significantly by 81.03%, 36.60% and 59.98% respectively compared with that in CK. It can be seen that the straw and a small amount of biochar returned to the soil could significantly increase the porosity of paddy soil in the early and later stages, especially at the filling stage, up to 192.11% and 92.11%, respectively; a large amount of biochar returned to the soil could significantly improve the porosity of paddy soil during the whole growth period, with an increase of 52.44%-157.11%, especially during the filling period.

3.1.3

Effects of straw and biochar on soil temperature. Soil temperature is an important ecological factor for the growth of rice, which has an important effect on the absorption of water and nutrients by rice roots. It can be seen from Fig.2 that the soil temperature in each treatment had little difference from tillering stage to heading stage, and decreased greatly at and after filling stage. The soil temperature in each treatment at tillering stage was 24.8-25.8 ℃, and the temperature in treatment S, C1 and C2 increased by 4.03%, 0.54% and 2.01% respectively compared with that in CK, and the difference among treatments did not reach a significant level; at jointing stage, the soil temperature in each treatment was 24.7-25.3 ℃, and the soil temperature in treatment S, C1 and C2 increased by 0.68%, 1.08% and 2.70% respectively compared with that in CK, and there was no significant difference among treatments; the range of soil temperature at heading stage was between 23.5 ℃ and 24.3 ℃, the soil temperature in treatment S, C1 and C2 increased by 0.02%, 0.71% and 3.55% respectively compared with that in CK, and there was no significant difference among treatments; during the filling period, the soil temperature range in each treatment was 19.5-20.5 ℃, the soil temperature in treatment S, C1 and C2 increased by 2.91%, 3.25% and 5.13% respectively compared with that in CK, and there was a significant difference between C2 and CK; during the maturity period, the soil temperature range in each treatment was 13.3-14.5 ℃, the soil temperature in treatment S, C1 and C2 increased by 2.50%, 3.75% and 8.75% respectively compared with that in CK, and there was a significant difference between C2 and CK. It can be seen that the straw and a small amount of biochar returned to the soil had no significant advantage in improving the soil temperature of rice fields at all growth stages, while a large amount of biochar returned to the soil had little effect on soil temperature in the early stage, but could significantly increase soil temperature by 5.13%-79% in the late growth stage.

Table 2 Change of soil porosity in the treatment of returning straw and biochar to the soil

3.2 Effects of straw and biochar on soil pH value

As can be seen from Fig.3, during the whole growth period of rice, the soil pH value in each treatment basically showed a wavy trend of first decreasing, then rising and then falling. The soil pH value in each treatment at tillering stage was 6.77-7.00, and the soil pH value in S and C1 was 1.23% and 0.25% higher than that in CK, respectively, which was not significantly different from that in CK, while the soil pH value in C2 increased significantly by 3.45% compared with that in CK; at jointing stage, the soil pH value in each treatment was 6.65-6.92, the soil pH value in S and C1 was 2.51% and 2.26% higher than that in CK, respectively, with no significant difference, and the soil pH value in C2 increased significantly by 4.01% compared with that in CK; the soil pH value in each treatment at heading stage was 6.88-7.10, and the pH value in S and C1 was 2.18% and 1.70% higher than that in CK, respectively, with no significant difference, while the soil pH value in C2 increased significantly by 3.15% compared with that in CK; during the filling stage, the soil pH value in each treatment was 6.43-6.80, and the soil pH value in S and C1 increased by 1.55% and 3.37% respectively compared with that in CK, with no significant difference, while the soil pH value in C2 increased significantly by 5.70% compared with that in CK; in the maturity stage, the soil pH value in each treatment was 6.15-6.52, and the soil pH value in S and C1 increased by 0.81% and 2.44% respectively compared with that in CK, with no significant difference, while the soil pH value in C2 increased significantly by 5.96% compared with that in CK. It can be seen that straw and a small amount of biochar returned to the soil increased the pH value of paddy soil in the whole growth stage of rice, but the difference was not significant compared with CK. A large amount of biochar returned to the soil could significantly increase the pH value of the paddy soil by 3.15%-5.96%, and the increase would be greater in the later stage so that the soil was in a near-neutral state.

Fig.2 Effects of straw and biochar returned to the soil on soil temperature

Fig.3 Effects of straw and biochar returned to the soil on soil pH value

4 Conclusion and discussion

The physical properties of soil mainly include bulk density, porosity, water content and temperature, soil structure, aeration and organic matter content, which can affect soil fertility and the growth of plant roots in soil. Soil bulk density and porosity are important indicators to reflect the characteristics of soil structure, and there is generally a negative correlation between them. After returning straw to the field, the soil bulk density decreased and the porosity increased, which made the soil loose, aerated and permeable, thereby promoting the activity of soil microorganisms and enhancing the supply of soil nutrients.

Through the long-term study of returning straw to field in cold region, Dong Guijun

et

al

. thought that with the increase of time of returning straw to the soil, the soil bulk density decreased from 1.32 g/cmto 1.25 g/cm(a significant decrease of 0.07 g/cm). Research by Li Shizhong

et

al

. on returning straw to the field in the Yellow River Irrigation Area of Ningxia Hui Autonomous Region showed that the soil bulk density decreased by 10.9% and the soil porosity increased by 7.3% compared with the same period of last year. The results showed that returning straw to the field significantly reduced the bulk density of 0-20 cm soil at different growth stages by 6.02%-11.86%, while the total porosity increased by 7.41%-14.93%, and the aeration porosity increased by 22.28%-192.11%, but the effect on capillary porosity was not significant. Returning straw to the field can affect the absorption and transformation of light radiation and heat conduction of soil, and has the double effect of "heating" at low temperature and "cooling" at high temperature. Studies have shown that returning straw to the field combined with application of chemical fertilizer increased soil temperature at 08:00 and 20:00 when the air temperature was low, but decreased soil temperature at 14:00 when the air temperature was high. The experiments of Xiao Guohua

et

al

. showed that no-tillage mulching with rice straw could increase the 0-5 cm soil temperature in early spring by 0.7-1.0 ℃. According to the study of Ramakrishna

et

al

., straw mulching mainly affected the shallow soil temperature within 10 cm, but had no significant effect on soil temperature below 10 cm. The results showed that although returning straw to the field had a certain effect on the temperature in paddy soil with a depth of 0-5 cm, but it was not significantly different from CK, which might be related to the way of straw returning, the time of investigation and the depth of soil.Biochar could affect soil properties because of its unique porous structure and physicochemical properties. The looseness of biochar could improve soil tightness, promote the formation of aggregates, reduce soil bulk density, increase microbial biomass, increase microbial activity, improve soil structure, increase total porosity, and improve soil aeration and water permeability. Studies of Oguntunde

et

al

. showed that after adding biochar, the soil bulk density decreased by 9%, while the total porosity increased from 45.7% to 50.6%. Githinji’s culture experiment of sandy loam by setting the volume ratio of biochar to soil showed that the soil bulk density decreased linearly with the increase of biochar (

R

=0.997). When the addition of biochar was 25.0%, 50.0%, 75.0% and 100.0%, the soil porosity increased by 10.0%, 22.0%, 38.0% and 56.0%, respectively compared with CK. The results showed that a small amount of biochar returned to the soil reduced the soil bulk density by 2.69%-6.67%, and the total porosity and aeration porosity increased by 3.19%-8.38% and 17.88%-92.11%, respectively.A large amount of biochar returned to the soil reduced the soil bulk density by 8.58%-11.32%, and increased the total porosity and aeration porosity by 9.81%-14.27% and 52.44%-157.11%, respectively, which was consistent with the research results of Eastman. The addition of biochar to the soil could darken the soil, and then affect the soil thermal conductivity and surface reflectance, resulting in the change of soil temperature. The long-term biochar positioning experiment of Zhang

et

al

. in farmland in North China showed that biochar could adjust the temperature fluctuation of soil layer with a downward depth of 5.0 cm, and had the function of de-peaking. The application of biochar could increase soil temperature by 0.6 ℃ at low temperature in winter. Ventura

et

al

. found that the application of biochar increased the surface temperature, but had no significant effect on the temperature of soil with a downward depth of 7.5 cm. The results showed that a small amount of biochar returned to the soil had no significant effect on soil temperature at each growth stage of rice, while a large amount of biochar returned to the soil could significantly increase the soil temperature by 5.13%-8.75% in the later stage of rice growth, possibly because there was sufficient water in the field in the early growth stage of rice, and biochar increased the water retention of soil, and the rate of soil warming would be greatly weakened due to high water content. In the later stage, under the condition of light, wet and dry irrigation, the soil moisture content decreased, and the dark soil was easier to absorb solar energy and reduce the soil surface reflectance, which highlighted the warming effect of a large amount of biochar.Soil pH determines soil acidity or alkalinity, directly affects the existing state and availability of soil nutrients, and plays an important role in soil microbial activity, mineral transformation and organic matter mineralization. Returning straw to the soil can regulate soil pH to a certain extent, while appropriate amount of biochar can increase soil pH, increase the redox potential of topsoil, reduce the total amount of reducing substances, and improve the supply of soil available nutrients. The results showed that straw and a small amount of biochar returned to the soil could increase the pH value of paddy soil during the whole growth stage of rice, but the difference was not significant, which was consistent with the research results of Zhou Yunlai

et

al

.. A large amount of biochar returned to the soil could significantly increase the pH value of paddy soil (3.15%-5.96%), and it would be greater at the later growth stage, which might be related to the pH of biochar itself (pH=9.02 in this experiment) and the carbonates (MgCO, CaCO) and organic acid radical (-COO-) formed in the production process.