Simulation of accumulation and mineralization (CO₂ release) of organic carbon in chernozem under different straw return ways after corn harvesting

Highlights

• “Straw mixing” return increased soil organic content.

• “Straw mixing” return improved the composition of micro-aggregate.

• CO₂–C emissions with “straw mixing” were less than that of “straw mulching”.

• Corn straw return mixed with soil is a better method for fertility and environment.

Abstract

Straw return is the main way of fertility improvement of cultivated chernozem in Songnen Plain of Northeast China. To reveal the effect of “corn straw return mixed with soil or ploughing” (“straw mixing” for short) and “corn straw return to the surface of soil” (“straw mulching” for short) on fertility and environment of soil, we focused on the effects of different straw applications on the release of CO₂ and accumulation of soil organic carbon. A laboratory simulative incubation experiment was conducted with chernozem, and corn stalk. Under the constant condition of 28 °C, all curves of the emission rate of CO₂ had appeared a peak of the emission of CO₂ on the 4th or 6th day of the culture, and then stepped into the first slow emission platform on the 13th–52nd day and stepped into the second slow emission platform on the 52nd–91st day. With the same amount of straw, the peak value of CO₂ emission rate of “straw mixing” (23.00, 43.64, 79.70 mg kg⁻¹ d⁻¹ for straw 2%, 4%, and 6%, respectively) was apparently greater than that of “straw mulching” (22.94, 39.76, 58.71 mg kg⁻¹ d⁻¹ for straw 2%, 4%, and 6%, respectively). However, after entering the slow emission platform, the average emission rate of CO₂ of “straw mixing” would be less than that of “straw mulching”. There was an apparent intersection point (about 31 d for straw 2%, 4% and 47 d for straw 6%) on the curves of both the CO₂ emission rate of “straw mixing” and “straw mulching”. Before the intersection point, the accumulated amount of CO₂–C emission of “straw mixing” was greater than that of “straw mulching” and then lower than that of “straw mulching” gradually after the point. The analysis of the result about the soil organic carbon and the composition of micro-aggregate showed that with the same amount of straw addition, “straw mixing” could increase soil organic content and improve the composition of micro-aggregate better than “straw mulching”. The results indicated that it might have a better effect on fertility and environment of soil than the return of straw for surface of soil if the means of ploughing in autumn and rotary tillage, etc. could be used in the process of the returning of straw mixed with soil or cultivation.

Introduction

Chernozem is an important agricultural soil source in Songnen plain which has some problems such as poor organic matter, light texture and bad water retention or fertilizer conservation capacity, etc. (Jilin Soil and Fertilizer Station, 1998). Organic fertilizer application and crop stubble return are the traditional improvement method of chernozem. In recent years, with the continuous decrease of organic fertilizer application, corn stubble and straw has become the dominant technical measures for improving soil fertility. At present, “straw overturning” and “straw surface mulching” are the two important returning methods. For using the different harvesting machineries, “straw overturning” has varied ways. Some harvesting machineries can break corn straw into piece and spread them on the surface of the soil. When farmers plowing the soils, the small pieces of straw will be turn into the soil. Some machines only overwhelmed the straws in the situ. Then, rotary tillage machines will be used to break corn straw and stubble into piece into the soil. Some farmers simply burn straws in order to facilitate sowing. In recent years, corn stubble and straw return have become main technical measures for improving the soil fertilization. Numerous studies have demonstrated that the return of straw or stubble had significant effects on improving soil organic matter content, promoting soil water retention or fertilizer conservation capacity (Dou and Jiang, 1988, Jiang et al., 1988, Zhao, 1996, Sommer et al., 2011, Liu et al., 2014a, Liu et al., 2014b). With the environmental problems becoming more and more important, the reports about effects of the application of organic material on soil carbon emissions have increased day by day (Lal, 2004, Anders et al., 2013, Li et al., 2013). In recent years, “Straw surface mulching” is one of the important protective tillage method introduced into Northeast of China, which has been still in the stage of experimental demonstration. According to our investigation and observation, although the technology has certain protective effect on soil erosion, a large number of CO₂ may be released to the atmosphere by directly exposing to the soil surface, aerobic mineralization. On the other hand, the content increase of soil organic matter was not obvious. Different tillage might alter the SOM decomposition environment by aerating the soil, breaking the aggregates, incorporating residue into the plow layer, and therefore, changing soil and crop residue contact. Therefore, there may be the different SOC storage and CO₂ emissions between “straw overturning” and “straw surface mulching”. Relatively few studies have been conducted in the China Corn Belt of chernozem region relating SOC storage and CO₂ emissions under a range of long-term tillage management. Monitoring CO₂ emissions and changes in SOC for different tillage treatments are important to identifying the management practices that maintain soil productivity, increase C storage, and contribute to mitigate the greenhouse effects. Since the 1980s, agricultural researchers in China have carried out a lot of researches about the effect of non-decaying materials like crop straw on soil fertility (Lin et al., 1980, Cheng et al., 1981, Jiang et al., 1988). But these results were all about the mixing of organic materials and soil condition. There were not many comparative researches about the effect of “straw return mixed with soil or ploughing” and “straw return to the surface of soil “on soil fertility and environment. Many theoretical questions still had not been elucidated. This research chose chernozem from Songnen Plain as research object, simulated incubation experiments of corn straw covered and straw mixed with soil, and studied effects of two ways of straw returning on the emission of CO₂ and accumulation of organic carbon. From the perspective of increasing organic matter and reducing CO₂ emission, the aim of this study is to reveal the different effects of the two straw return ways on agriculture and environment. The result will provide theoretical support for constructing the environment friendly soil fertility and tillage system.