Decomposition of Different Organic Materials and Variation of Active Organic Carbon and Nitrogen Contents in Tobacco-planted Soil

Yao LU, Bo ZHENG, Zongyan DUAN*, Zuhong LI, Yan XIE, Xueli QIU, Jianxin WANG

1. Institute of Agricultural Resources & Environment, Yunnan Academy of Agricultural Sciences, Kunming 650205, China;

2. Tobacco Technology Research Center, Qujing Branch of Yunnan Provincial Tobacco Company, Qujing 654000, China

The organic materials contain rich mineral nutrients and organic carbon. When returned to soil, the organic materials will be decomposed by the soil microorganisms, thus the nutrients will be releases to soil and then absorbed and utilized by plants. The decomposition rate is an important indicator for evaluating the role of organic material in maintaining and improving soil organic matter distribution and soil fertility[1].Currently, under field conditions, a lot of researches have been conducted on the decomposition rate of different organic materials and the effects of different utilization pattern on the decomposition rate.Among different organic materials, the decomposition rates ranked generally as leguminous cropstraw＞graminaceouscropstraw[2];graminaceous crop straw ＞rapeseed straw[3-4]; corn straw ＞wheat straw ＞rice straw[5-6];straw ＞stubble[7];oilseed cake ＞straw[8];crop straw ＞animal feces[2,5,9].The effects of returning-to-soil pattern on the decomposition rate of organic materials are reflected as follows:burying ＞mulching[3];5 cm burying ＞15 cm burying ＞open mulching ＞water soaking[10-11]. At the same time,numerous studies have shown the returning of organic materials to soil plays an important role in the soil active organic C accumulation and the improvement of soil N supply capability[12-14].However,the variation of active organic C and N contents and active organic C/N ratio in the residues and the relationships between active organic C and N contents,active organic C/N ratio and the decomposition rate of organic materials have not been studied thoroughly. However, there are rare reports on the decomposition of organic materials in tobacco-plantedsoil.

Yunnan Province is a large producer of flue-cured tobacco. The continuous cropping has highlighted the soil disorder. It has been proven the returning of organic materials to soil can increase the organic C content in soil[15], thereby inhibiting the deterioration of soil quality and stabilizing fields’ productivity[16]. In this study, the oilseed cake, vetch, rapeseed straw,wheat straw and corn straw, which were common in Yunnan Province,were selected. They were placed in nylon mesh bags. When the tobacco seedlings were transplanted,the nylon mesh bags were directly buried in soil among tobacco seedlings. During the growth period of flue-cured tobacco,the buried organic materials were sampled regularly. After rinsed and dried, the samples were weighed. In addition, the active organic C and N contents in the residual materials were analyzed at different periods,aiming at revealing the decomposition characteristics of oilseed cake, rapeseed straw, vetch, wheat straw and corn straw in tobacco-planted soil,the variation of organic C and N contents and the relationship between organic C and N contents and decomposition rate.This study will provide a theoretical basis for the management of tobacco-planted soil and the reasonable utilization of straw resources.

Material and Methods

Test time and site

The test site (26°06′07″N,104°08′65″E)was located in Gengtun Village,Banqiao Town, Xuanwei County,Qujing City,Yunnan Province.The test site has an altitude of 2 000 m,an annual average temperature of 13.4 ℃and an annual rainfall of 935 mm. It is dry land of the dam area. The lastseason crop was vetch. The fertilization,the transplanting of flue-cured tobacco seedlings and the burying of test samples were completed on April 26th, 2009. The collection of all the samples was completed till August 15th.

Test treatments and design

The in-situ nylon mesh bags soil burying method was adopted. There were a total of 5 treatments: oilseed cake (OC), vetch (VC), rapeseed straw (OS), wheat straw (WS) and corn straw(CS).There were 3 duplications for each treatment. The rapeseed, wheat and corn straws were cut into segments in about 5 cm long.The vetch and oilseed cake were crushed into crumbs.Then the organic materials were placed into the 400-mesh nylon bags in size of 15 cm × 15 cm (20 g/bag). After sealed, the nylon bags were buried in 20 cm ditches between 2 adjacent tobacco ridges. The plant and row spacing of flue-cured tobacco seedlings was 0.55 m×1.2 m.

The test soil was calcareous red soil, specific for planting tobacco. It has a higher fertility. After the transplanting of tobacco seedlings, but before the fertilization, the soil samples(0-30 cm of depth) were collected with the S-shaped 5 points sampling method. The soil basic agrochemical characters were as follows: pH 7.11,organic matter 48.6 g/kg, total N 2.0 g/kg, alkali-hydrolyzable nitrogen 161.46 mg/kg, available phosphorus 49.23 mg/kg, available potassium 146.11 mg/kg.

Table 1 Decomposition amounts and rates of different organic materials at different periods

Sampling and analysis

The 1stsampling was conducted on May 9th.Then the proceeding samplings were conducted every 2 weeks.A total of 8 times of sampling were carried out.The samples were soaked in distilled water for 1 h. Then they were rinsed repeatedly.After dried,the samples were weighed. Based on the weights of residues, the decomposition amounts and decomposition rates were calculated. The samples were then smashed for determining the active organic C contents and total N contents in the residual organic materials.The active organic C content was determined with 333 mmol/L of KMnO4, accompanying 1h of oscillation atroom temperature[17]. The total N content was determined with the semi-micro Kjeldahl method[18].

Results and Analysis

Decomposition of different organic materials in soil

As shown in Table 1,the decomposition rates of oilseed cake and vetch were all high in first 2 weeks and then trended to decrease gradually.The decomposition rates declined to the lowest after the 6th-12thweeks.Then the decomposition rates were maintained at a relatively stable level.The decomposition rate of oilseed cake declined to the lowest in the 5th-6thweeks; the decomposition rates of corn straw and vetch declined to the lowest in the 7th-8thweeks;the decomposition rates of rapeseed straw and wheat straw declined to the lowest in the 11th-12thweeks. In the first 2 weeks, the decomposition amounts of wheat straw,corn straw,vetch,oilseed cake and rapeseed straw were 4.79,3.11, 7.31, 9.33 and 4.41 g, respectively with average decomposition rates of 0.34,0.22,0.52,0.67 and 0.32 g/d. The average decomposition rates of wheat straw, corn straw, vetch,oilseed cake and rapeseed straw in the 3rd to 4th weeks only accounted for 47.6%, 64.6%, 15.3%, 15.3% and 67.3% of those in the first 2 weeks. In the 5th-6thweeks, the average decomposition rate of oilseed cake declined to 0.02 g/d, which only accounted for 3.7% of that in the first 2 weeks. The average decomposition rate of corn straw declined to 0.07 g/d in the 7th-8thweeks, and it accounted for 29.5% of the average decomposition rate in the first 2 weeks. In the 7th-8thweeks, the average decomposition rate of vetch was decreased to 0.02 g/d,which only accounted for 3.9%of that in the first 2 weeks. In the 11th-12thweeks, the average decomposition rates of rapeseed straw and wheat straw were all decreased to 0.03 g/d, and they accounted for 8.8% and 8.5% of the average decomposition rates in the first 2 weeks,respectively.

Fig.1 showed the accumulative decomposition rates of oilseed cake and vetch were all increased gradually in the early period and then maintained at a stable level. Among the organic materials, the accumulative decomposition rates ranked as oilseed cake ＞vetch ＞ wheat straw and rapeseed straw ＞corn straw.After 16 weeks,the total accumulative decomposition rates of oilseed cake, vetch, wheat straw, rapeseed straw and corn straw were 63.2%, 62.6%, 60.2%, 60.47%and 54.9%,respectively.

Based on the results above,it can be concluded after returned to soil,the organic materials were decomposed rapidly in the early period and then slowly in the late period. The decomposition amounts and decomposition rates were all highest in the first 2 weeks. After the 6th-12thweeks, the decomposition amounts and decomposition rates were remained stable.Among the organic materials, oilseed cake had the highest decomposition rate,followed by vetch.The decomposition rates of wheat straw and rapeseed straw were equivalent. But the corn straw was decomposed most slowly.

Table 2 Analysis of active organic C content,total N content and C/N ratio in residual materials at different periods

Analysis of active organic C content,total N content and C/N ratio in the residual organic materials at different periods

As shown in Table 2,in the wheat straw residues, the active organic C content ranged from 81.8 to 121.8 g/kg,with average of 99.4 g/kg; the total N content ranged from 7 to 14.1 g/kg,with average of 9.8 g/kg; the active organic C/N ratio ranged from 7.8 to 12.7,with average of 10.1. At different periods,in the decomposition residues of corn straw, the active organic C content ranged from 86 to 123.2 g/kg,with average of 102.8 g/kg; the total N content ranged from 7.6 to 14.3 g/kg,with average of 11 g/kg;the active organic C/N ratio ranged from 7.9 to 12.1, with average of 9.4. In the vetch residues, the active organic C content ranged from 89.3 to 111.1 g/kg, with average of 996.6 g/kg;the total N content ranged from 23.8 to 32.2 g/kg,with average of 27.8 g/kg; the activeorganic C/N ratio ranged from 3.1 to 3.8, with average of 3.5. The active organic C content in the oilseed cake residues ranged from 55.7 to 77.3 g/kg at different periods with average of 63.2 g/kg; the total N content ranged from 33.9 to 57 g/kg, with average of 41.6 g/kg; the active organic C/N ratio ranged from 1.1 to 2 with average of 1.5. In the decomposition residues of rapeseed straw, the active organic C content ranged from 60.8 to 115.4 g/kg,with average of 91.7 g/kg; the total N content ranged from 7 to 16.1 g/kg with average of 12.2 g/kg; the active organic C/N ratio ranged from 6.5 to 10.5, with average of 7.5. In overall,the active organic C content and total N content in wheat straw, corn straw,vetch and rapeseed straw residues all trended to increase with the proceeding of decomposition. But the active organic C/N ratio trended to decrease.In the oilseed cake resides, the active organic C content and total N content trended to decrease with the proceeding of decomposition, but the active organic C/N ratio trended to increase.

Among the organic material residues,the active organic C contents ranked as corn straw ＞wheat straw ＞vetch ＞rapeseed straw ＞oilseed cake;the total N contents ranked as oilseed cake ＞vetch ＞rapeseed straw ＞corn straw ＞wheat straw;the active organic C/N ratios ranked as wheat straw ＞corn straw ＞rapeseed straw ＞vetch ＞oilseed cake.

Above all, in the organic material residues at different periods, a higher content of active organic C and a lower content of total N led to a relatively high active organic C/N ratio; in the oilseed cake residues, a lower content of active organic C and a higher content of total N led to a relatively low active organic C/N ratio; in the vetch residues, both the higher contents of active organic C and total N led to a relatively low active organic C/N ratio.With the prolonging of the decomposition time, the active organic C content and the total N content in the straw material residues, including wheat straw residue, corn straw residue,vetch residue and rapeseed straw residue, trended to increase in overall,but the active organic C/N ratio trended to decrease.However,the variation of active organic C content, total N content and active organic C/N ratio in the oilseed cake residue was on the contrary.

Table 3 Correlation analyses between accumulative decomposition rate and active organic C content,total N content and C/N ratio in different organic materials

Correlation analysis between decomposition rate and active organic C content, active organic N content and C/N ratio in different organic materials

Table 3 showed that there was a significantly positive correlation between the accumulative decomposition rate of organic materials and the total N content (r=0.584 5,P ＜0.01),and was a significantly negative correlation and the active organic C/N ratio(r =-0.661 6,P ＜0.01).But there was no significantly positive correlation between the accumulative decomposition rate and the active organic C content.

Discussion

In this study,the organic materials were decomposed rapidly in the early period but slowly in the late period after they were returned to soil. The decomposition rates of oilseed cake and vetch in the first 2 weeks were 25 times higher than those in the late period.The decomposition rates of wheat straw and rapeseed straw were more than 10 times higher than those in the late period. These study results were consistent with those of Li et al.[3]and Wu et al[19].In the early decomposition period, the straws are rich in soluble organic matters, such as polysaccharides, amino acids and organic acids and inorganic nutrients, which can be utilized by microorganisms. The number of microorganisms will be increased and their activity will also be enhanced. However, with the proceeding of decomposition, the soluble organic matters in the organic materials are decreased gradually. The remaining parts contain mainly a lot of difficultly-decomposed organic matters. Thus the activity of microorganisms will be reduced, thereby slowing the decomposition of organic materials. In this study, the 16-week accumulative decomposition rates of organic materials ranged from 54.9% to 64.2%. The study results are close to those of Li et al[3].They found the 100 d accumulative decomposition rates of wheat straw and rapeseed straw were 66.18%and 55.62%,respectively.

Among the organic materials, the decomposition rates rank as oilseed cake ＞vetch ＞wheat, rapeseed and corn straws and wheat straw ＞corn straw. These results are consistent with most of the findings[2-8]. However,in this study,the decomposition rate of corn straw is lowest and is lower than those of wheat straw and rapeseed straw,which are contrary with previous studies.The specific mechanism is still needed to study further. The decomposition rate of organic materials is greatly affected by their own chemical composition. In general, the low N content and high C/N ratio in organic materials are considered to be linked with low decomposition rate. In this study, the decomposition rate is proved to be negatively related to active organic C/N ratio (P＜0.01),but is positively related to total N content(P＜0.01). However, among the chemical components, organic C content, as well as total N content and active organic C/N ratio, is also considered to be closely related to the decomposition rate. There have been studies showing among the organic materials with similar content of lignin (inactive organic C), the higher the C/N ratio is, the higher the decomposition rate is; among the organic materials with similar C/N ratio, the lower the lignin content is, the higher the decomposition rate is. However, this study found there is no significant correlation between active organic C content and decomposition rate of organic materials.

In conclusion,in the early decomposition period of organic materials,the content of easily-decomposed active organic C that can be utilized by soil microorganisms is increased rapidly. However, with the proceeding of decomposition, the easily-decomposed active organic C content is reduced gradually. The residues contain mainly difficultly-decomposed inactive organic C. This will reduce the activity of microorganisms, thereby slowing the decomposition of organic materials.Among different organic materials,the decomposition rates rank as oilseed cake ＞vetch ＞wheat straw and rapeseed straw ＞corn straw. The decomposition rate is positively related to total N content(P ＜0.01),but is negatively related to active organic C/N ratio(P ＜0.01). There is no significant correlation between decomposition rate and active organic C content. But this is still needed to be studied further.

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