Determination of Organic Carbon Content of Corn Stalk:A Casestudy in Laiyuan County, Hebei Province

. Organic carbon is an important component of straw, which plays an important role in the hydrothermal conditions, physical and chemical properties of straw. In order to discuss the transformation of corn straw in straw and the influence on straw fertility, this paper selects 6 villages in Laiyuan County, Hebei Province as the study area


1.Introduction
Straw is an important part of natural ecosystem and plays an important role in maintaining ecological balance, maintaining ecological balance and promoting agricultural development. Straw organic carbon is an important index of straw fertility, and its content directly affects the properties of straw and plant growth. Therefore, the determination of organic carbon content of straw is of great significance for agricultural production and land use.
Due to the different factors such as region and straw type, the collected samples have obvious differences, so the determination methods of organic carbon content of straw are different. At present, the laboratory commonly used determination methods are potassium dichromate oxidation method, sulfuric acid-potassium dichromate oxidation method, potassium permanganate oxidation method, alkaline potassium persulfate method, acetonitrile extraction -potassium dichromate oxidation method [1].Liu Meihong et al [2] The organic carbon of crop straw in Ningqiang county of Shaanxi Province was determined by potassium dichromate oxidation method, and the estimation method of organic carbon of crop straw was put forward. Su Xiujuan et al [3] .used potassium dichromate oxidation method to determine the organic carbon content of straw on both sides of Weihe River in Baoji City, Shaanxi Province, and studied its spatial variability. Liu Xinguang et al [4] The organic carbon content of crop straw in Tongyu county, Jilin Province was determined by using potassium dichromate oxidation method, and the suitable method was determined by comparing with the measured value.
Taking Laiyuan County, Hebei Province as an example, taking corn straw as the research object, the organic carbon content of corn straw in each township in this county is determined by potassium dichromate oxidation method in the laboratory, and the spatial distribution and seasonal variation trend of organic carbon content of corn straw in this county are analyzed, aiming to provide scientific basis for agricultural production and land use in this county. In this study, the spatial variability of organic carbon content of corn stalk in Laiyuan County is analyzed by geostatistics method at county scale, aiming to provide scientific basis for the utilization of corn stalk in the whole county.

Overview of the study area
Laiyuan County is located in the northeast of Hebei Province, located in the eastern foot of Taihang Mountain and the north foot of Yanshan junction, between east longitude 114°15 '~115°25', north latitude 39°40 '~42°34' between, Laiyuan County is a temperate continental semihumid monsoon climate area, the average annual temperature 11.7℃, the average annual rainfall 481.8mm . There are four straw types in Laiyuan County: brown soil, dark brown soil and brown soil. Among them, brown soil has the largest area and the widest distribution. The straw types of cultivated land in the county are Heiyuan soil and lousen soil, and the distribution range of Heiyuan soil is as follows: Heiyuan soil is concentrated in the central valley area about 1100m above sea level; The lignite soils are distributed in the central valley at an altitude of about 1000m. Hailu soil is mainly in the western and northern mountainous areas (figure1).

Experimental design
The field trial began in September 2022 under a continuous maize cropping system. In order to facilitate statistical analysis, each large division was divided into 3 cells with roughly equal area during sampling. In each cell, 1m*1 construction samples of 3 sample points were collected according to the diagonal method. After mixing, the samples were taken as one sample, with the collection depth ranging from 0 to 20 cm. The fresh roots collected were air-dried to a plastic state and then gently broken apart along the natural cracks. After the samples were completely air-dried, they were measured and treated [5].
After weighing 0.05-0.5g (accurate to 0.0001g, weighing the sample according to the content of organic matter in the sample), add 0.4mol/L (1/6K2Cr2O7) 10ml accurately into 150ml triangle bottle, turn the test tube to mix evenly, and then insert a glass funnel into the mouth of the triangle bottle. Put the triangular bottles into the enamel tray one by one, and then put the tray into the oven that has been preheated to 170-180 ° C, after putting the oil temperature down to 170-180 ° C, and so on when the solution in the triangular bottle boils, the timing must be controlled at this moment, the temperature of the oven must not make the solution boil violently, and maintain it at 170-180 ° C, 5±0.5min after the tray is removed from the oven. Cool for a while, rinse the funnel and the inner wall of the triangle bottle with water, so that the total volume of liquid in the triangle bottle is controlled to 50-60ml, add 3-4 drops of phenanthrene indicator, titrate with 0.1mol/L FeSO4 standard solution, and the color of the solution changes from orange to blue and green until it becomes brick red .If the volume of ferrous sulfate solution used for titration is less than 1/3 of the volume of ferrous sulfate consumed by the blank, the soil weight should be reduced and remeasured.

Statistic analysis
Microsoft Office Excel 2007 was used for data processing, SPSS Statistics 17.0 software was used for statistical analysis, and trend surface analysis (Kriging) and spatial interpolation method (ArcGIS) were used for statistical analysis. The spatial distribution characteristics of the samples were analyzed by Kriging interpolation method(figure2).

Spatial variability of organic carbon content in straw
Straw organic carbon content has obvious spatial heterogeneity, and its spatial variation is mainly affected by topographic factors, with obvious spatial autocorrelation [6][7][8]. The organic carbon content of straw in different towns is different (table1). The higher the altitude, the higher the organic carbon content, and the lower the organic carbon content of the towns below 500m. In the 0~20 cm soil layer, the organic carbon content in the northern area of Laiyuan County is the highest, followed by the southern area, and the western area is the lowest. The organic carbon content of straw increased gradually from northeast to southwest. From the northwest to the southeast, the organic carbon content of straw gradually decreases [9].

The organic carbon content of straw was different at different altitudes
The organic carbon content of straw was ranked from high to low as north > Middle > West > East. In the 0-100 cm soil layer, the organic carbon content of straw increased by 0.016 for every 100m elevation increase, and 0.014 for every 100m elevation increase. This is mainly due to the fact that the topographic index decreases by 0.027 for every 100m increase in altitude, resulting in a 0.027 decrease in topographic index for every 100m increase in altitude. This may be due to the fact that corn stalks require more nutrients and water during the growth process, while the western area of Laiyuan County is a hilly township with a higher altitude and relatively low straw nutrient content [10].

Spatial distribution characteristics of or ganic carbon in straw
This may be due to the complex topography, high altitude, relatively steep terrain, strong evaporation of straw water, and dry climate with little rain in the northern region, resulting in low surface moisture content of straw. The organic matter content of straw in central and eastern regions is relatively high. This may be due to the higher elevation of the central and eastern regions, the relatively gentle terrain, and the high level of vegetation cover on the surface. At the same time, the terrain of these areas is relatively flat and the straw fertility condition is good [11].
The western region has low elevation, flat terrain, slow terrain, low vegetation coverage, and relatively low content of straw organic matter. The reason for the highest organic carbon content of straw in western China may be that the decomposition of organic matter in straw in this region mainly depends on the leaching effect of rain, which increases the organic matter content of straw [12].

Response of organic carbon of maize stalk to environmental factors
In 0-100 cm soil layer, the organic carbon content of straw decreased with the increase of straw depth. Straw organic carbon content was positively correlated with straw temperature and straw water content (P<0.01), but had no significant correlation with straw pH value (P>0.05) [13].
Since the organic matter content of straw is an important factor affecting the carbon pool of straw, the influence of organic carbon content should be more important than its relationship with other environmental factors. It was found that with the deepening of soil layer, the organic matter content gradually decreased, indicating that the organic carbon in the straw gradually decreased, which was because the corn straw needed more nutrients and water in the growth process [14].

4.conclusion
In Laiyuan County, the soil organic carbon content of corn straw showed a trend of "high in northeast and low in southwest". From the perspective of topography, elevation and slope have a certain effect on soil organic carbon content, and elevation has a greater effect on soil organic carbon content. The soil organic carbon content of corn stalk is higher in low mountain hills and mountain valleys with an altitude of 2 700~3 300m. Slope had little effect on soil organic carbon content. From the perspective of seasonal variation, the soil organic carbon content of corn straw was the highest in August and September, 50.49% and 57.25%, respectively. In this paper, the SOC of maize straw in Laiyuan County, Hebei Province has been measured by routine method. Because of the lack of a detailed description of the test procedure, it's hard to say whether or not there is an error. To increase the measuring precision, we need to choose suitable means and devices in the course of the experiment. Moreover, the SOC measurement of maize stalks with different characteristics, characteristics and degrees were also presented. Therefore, it is necessary to take into account the SOC of each class in the research region.