Effects of Plastic Film Mulching on Soil Carbon Fixation Capacity and Fertility Level in Eastern Qinghai

Kai YANG, Qinglan HU, Jingui WANG,2*

1. College of Agriculture and Animal Husbandry, Qinghai University, Xining 810016, China; 2. State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, China

Abstract [Objectives] To explore the effects of single application of chemical fertilizers on soil carbon fixation capacity and soil fertility under plastic film mulching conditions in eastern Qinghai, and to provide a theoretical basis for realizing the sustainable development of film mulching planting method in this area. [Methods] The effects of single application of chemical fertilizer cultivation mode under film mulching conditions on the soil organic carbon (SOC), labile organic carbon (LOC), carbon management index (CMI), extractable humus carbon (CHE), humic acid carbon (CHA), and fulvic acid carbon (CFA) in the cultivated layer (0-20 cm) were studied through three consecutive years of field experiments on dryland maize farmland in the eastern Qinghai. [Results] Under the film mulching condition, the SOC, LOC and CMI of the single application of chemical fertilizer cultivation mode were lower than that of the open field control. CHE, CHA and CFA increased with the increase of planting years, but the degree of increase was generally less than that of the open field control. With the increase of planting years, by 2020, the soil LOC/SOC value of film mulching decreased by 4.97% compared with before the start of the experiment, while the open field control increased by 1.11%; the organic carbon oxidation stability coefficient (KOS) of the film mulching was higher than that of the open field control; the soil CHA/CFA value and PQ value were higher than that of the open field control. [Conclusions] Under the condition of single application of chemical fertilizers, the continuous film mulching cultivation mode reduces the soil carbon fixation capacity, and soil organic carbon tends to be stable, which is not conducive to biological utilization and could reduce the soil fertility and degrade the soil quality, causing adverse effects on the stability of crop yield and sustainable production in the long run.

Key words Soil, Carbon fixation capacity, Fertility level, Organic carbon, Humus, Plastic film mulching

1 Introduction

Soil is not only the source of the global carbon cycle, but also the sink of the carbon cycle. Its small changes will cause considerable fluctuations in the CO2concentration in the atmosphere, thereby affecting global environmental changes[1]. Some studies have shown that the carbon pool content in the soil accounts for about 2/3 of the total terrestrial carbon pool, twice that of the atmospheric carbon pool, and the total amount is up to 1 550 Pg[2]. According to recommendations of the Intergovernmental Panel on Climate Change (IPCC), improving soil carbon fixation capacity can reduce agricultural greenhouse gas emissions by 89%[3]. Related studies also indicate that carbon fixation in agricultural soil is a green environmental protection measure to control climate change[4]. Global environmental changes and soil fertility level changes are closely related to the cycle and transformation of soil organic carbon (SOC)[5]. Soil humus plays a very important role in the global carbon balance process[6]. Formed under the leading action of soil microorganisms, soil humus is used to characterize the biological fertility of the soil[7]. Soil humus is the main source of nitrogen and phosphorus in the soil; it can form a good soil structure by improving soil structure and colloidal conditions, thereby increasing soil water storage, fertilizer retention and buffering properties[8]. In addition, the extractable humus can absorb and complex pollutants, thereby reducing the pollution to the soil[9]. Although soil humus accounts for a small proportion of the total soil mass, it plays an important role in the formation of soil fertility and environmental protection.

The input of soil organic matter is closely related to agricultural management measures, and different management conditions have different effects on the decomposition and transformation of soil organic carbon and the formation of humus. The plastic film mulching changes the soil water, heat, and other conditions, leading to a series of changes in soil physical, chemical and biological properties[10]. In this study, we explored the effects of single application of chemical fertilizers on the characteristics of soil organic carbon and humus composition under the condition of plastic film mulching in eastern Qinghai, to provide a scientific basis for judging the effects of single application of chemical fertilizer and plastic film mulching cultivation mode on soil carbon fixation capacity and soil fertility level in this area.

2 Materials and methods

2.1 Overview of the study areaThe study area is located in Minhe County, Haidong City, Qinghai Province (35°45′-36°26′ N, 102°26′-103°04′ E). It has a plateau continental arid climate, an average annual temperature of 9 ℃ and an average annual rainfall of 360 mm, frost-free period of 198 d. The soil types are mainly chestnut soil, chernozem and limestone soil. The elevation is in the range of 1 650-4 220 m, with the average of 2 200 m. The county’s crop sown area is 44 500 ha, of which the planting area of film mulched maize exceeded 26 700 ha in 2020, making it the largest film mulching maize planting area in Qinghai Province. The experimental field of this study is in Qijia Village (36°13′ N, 101°38′ E), Lierbao Town, with an elevation of 1 924 m. The film mulched maize planting in this area is large and representative. Before the start of the experiment, the soil physical and chemical properties of the cultivated layer (0-20 cm) were: pH 8.03, organic matter 17.63 g/kg, alkali hydrolyzed nitrogen 79.07 g/kg, available phosphorus 18.05 mg/kg, and available potassium 118.29 mg/kg.

2.2 Experimental settingIn this experiment, we set two cultivation modes: plastic film mulching cultivation (FM) and open field cultivation (OF). The plot is 4 m×5 m=20 m2, we set 3 repetitions for each treatment. The test maize variety was Jinkai 3 (maize hybrid), with a row spacing of 55 cm and a plant spacing of 35 cm. No basal fertilizer was applied in each plot, only maize formula fertilizer was used once when sowing. Fertilizer analytic formula is 18-12-15 (N-P2O5-K2O), total nutrient is ≥45%, the application amount was 300 kg/ha, and other managements were the same as field measures. The experiment was carried out for three consecutive years in 2018, 2019 and 2020, with 2018 as the control (CK).

2.3 Sample collection and analysis

2.3.1Sample collection. After the maize was harvested each year, soil samples of the cultivated layer (0-20 cm) in each plot were collected using the "S" type 5-point sampling method. The soil samples of each point were mixed evenly, then removed the roots, residual film and stones and other debris, and left about 1 kg to place in a ziplock bag and brought to the laboratory, dried naturally, ground and sieved for later use.

2.3.2Sample analysis. The total organic carbon content of the soil was determined by the K2Cr2O7external heating method[11], the specific steps are as follows: mix 0.8 mol/L K2Cr2O7with H2SO4at 1∶1, boil with the samples for 5 min at 170-180 ℃, then measure theSOC(a). The soil labile organic carbon (LOC) was determined by the 333 mmol/L KMnO4oxidation method proposed by Loginowetal.[12], the specific steps are as follows: weigh 1.5 g air-dried soil (screened through a 0.25 mm sieve), place in a 100 mL centrifuge tube, add 25 mL 333 mmol/L KMnO4, centrifuge at 2 000 r/min for 5 min, dilute the supernatant with ionized water at 1∶250 and then colorimetrically measure at 565 nm, and repeat 3 times. Finally, calculate theLOCcontent (b) according to changes in KMnO4concentration (1 mmol/L KMnO4consumes 0.75 mmol/L or 9 mg carbon during the oxidation process). The oxidation stability coefficientKOSis used as an indicator to measure the oxidation stability of organic carbon, and it is calculated by Formula (1):

KOS=(a-b)/b

(1)

whereais the SOC,bis the LOC, anda-bis non-labile organic carbon.

The extraction and separation of soil humus adopt the method of Kumadaetal.[13], using a mixture of 0.1 mol/L sodium pyrophosphate and 0.1 mol/L sodium hydroxide (pH=13) as the extractant. The extractable humus carbon (CHE) and humic acid carbon (CHA) are determined by potassium dichromate oxidation method and fulvic acid carbon (CFA) is obtained by subtraction, as shown in Formula (2)-(3):

CFA=CHE-CHA

(2)

PQ=CHA/CHE

(3)

The impact of different land management measures on the quantity and quality of soil organic carbon pools can be comprehensively indicated by the carbon management index (CMI)[14-15]. TheCMIof different cultivation mode soils are based on the soil before the start of the experiment in 2018 as the reference soil, and the corresponding calculation formulas are as follows[16-17]:

CMI=CPI×LI×100

(4)

Carbon pool index (CPI)=Total organic carbon content of the sample (g/kg)/Total organic carbon content of the reference soil (g/kg)

(5)

Carbon pool lability (L)=Labile organic matter in the sample (g/kg)/Non-labile organic matter in the sample (g/kg)

(6)

Lability index (LI)=Carbon pool lability of the sample (L)/Carbon pool lability of the reference soil (LO)

(7)

2.3.3Data processing. The experimental data were analyzed by SPSS 16.0 software, and multiple comparisons were carried using Duncan method, and Excel 2013 was used for chart plotting.

3 Results and analysis

3.1 Changes inSOCandLOCcontent under different cultivation modesAs indicated in Table 1, different cultivation modes had different effects on the accumulation ofSOC. With the increase in planting years, theSOCcontent increased by 0.29% in the OF cultivation mode, while theSOCcontent decreased by 0.69% in the FM cultivation mode, but the difference was not significant (P<0.05).

Table 1 Changes in SOC content in the cultivated layer under different cultivation modes (g/kg)

The soilLOCis a direct participant in the soil nutrient cycle and structural composition, and can respond more sensitively to the impact of land use and management measures[14,16], so as to better indicate changes in soil quality and organic carbon pools[18-19]. As shown in Fig.1A, with the increase of planting years, compared with 2018, the soilLOCcontent of the OF cultivation mode increased by 1.0% by 2020, while that of the FM cultivation mode decreased by 1.7%, but the difference was not significant (P>0.05). For the soilKOS, with the increase of planting years, its changes were opposite to that of soilLOC(Fig.1B). Specifically, compared with 2018, the soilKOSof the OF cultivation mode decreased by 0.9% by 2020, while that of the FM cultivation mode increased by 1.6%, the difference was not significant (P>0.05).

Note: Different lowercase letters indicate significant differences between treatments (P<0.05), the same as below.

3.2 Effects of different cultivation modes onCMILefroy and Blair[20]calculated and proposed the Soil Carbon Management Index (CMI) on the basis of the establishment of theLOCcarbon pool index (CPI) in 1993. Since theCMIcombines the two aspects of soilCPIand soil carbon pool lability under the influence of man-made management measures, it can more comprehensively reflect the dynamic changes of soil organic matter properties due to changes in external conditions. TheCMIcan reflect the degree to which agricultural measures have reduced or renewed soil quality[21-22]. The increase inCMIindicates that farming has the effect of fertilizing the soil and the soil properties have developed in a benign direction; the decrease inCMIindicates that fertilization and farming have reduced soil fertility and soil properties have developed in a bad direction, which means that management and fertilization measures are not scientific[23].

As indicated in Table 2, different cultivation modes and planting years had different effects on theCMIof the cultivated soil. Specifically, with the increase of planting years, theSOC,LOC,CPI,LIandCMIwere gradually decreased compared withCKunder the condition of FM cultivation mode. By 2020, the soilLIandCMIunder the condition of FM cultivation mode decreased by 5.62% and 6.36%, respectively compared with the open fieldCK; compared with theCK, that of the OF cultivation mode increased by 2.09% and 2.39%, respectively.

Table 2 Effects of different cultivation modes and planting years on the CMI of the cultivated soil

The ratio ofLOCto total soil organic carbon (LOC/SOC) and its ratio to non-LOC(carbon pool labilityL) can reflect the quality and stability of soil organic carbon to a certain extent. The higher the ratio, the easier the organic carbon is decomposed and mineralized by microorganisms, the shorter turnover period or the higher the lability; the smaller ratio indicates that the soil organic carbon is relatively stable and not easily used by organisms[18,24]. With the increase of planting years, the soilLOC/SOCvalue andLvalue of the FM cultivation mode in 2020 decreased by 4.97% and 5.96%, respectively compared with that at the start of the experiment, while the soilLOC/SOCvalue andLvalue of the OF cultivation model increased by 1.11% and 2.09%, respectively, compared with that at the start of the experiment.

3.3 Changes in the carbon content of various components of soil humus under different cultivation modesSoil humus can be divided into humic acid (HA), fulvic acid (FA) and humin (Hu)[25]. Among them, HA is only soluble in alkali, FA is soluble in both acid and alkali, and Hu is an inert substance. HA has the highest CEC content, is slightly acidic, has the highest lability in humus, and is the main participant in the formation of soil structure[5]. FA has higher lability and smaller molecular weight. It is the primary product of HA formation and decomposition, and affects the accumulation and renewal of HA[5].

From Table 3, it can be known that in the three consecutive years of experiment, the soilCHEunder the OF and FM cultivation modes accounted for 37.0%-40.4% and 35.4%-37.9% of the total soil carbon, respectively; theCHAaccounted for 12.5%-13.2% and 13.2%-14.0% of total soil carbon, respectively; theCFAaccounted for 24.5%-27.3% and 22.2%-23.9% of total soil carbon, respectively. Under the two different cultivation modes, the carbon content of the extractable humus components of the cultivated layer was higher than that of humic acid, andCHE,CHAandCFAall gradually increased with the increase of planting years. In 2018-2020, the soilCHEcontent of the OF and FM cultivation models increased by 9.5% and 6.4%, respectively. Among them, the increase of the OF cultivation mode was significantly different (P<0.05), while the difference of FM cultivation mode was not significant (P>0.05); theCHAcontent of two modes increased by 5.5% and 5.2% respectively, and the difference in the increase did not reach a significant level (P>0.05); theCFAcontent of two modes increased by 11.6% and 7.0% respectively, and the difference was not significant (P>0.05).

Table 3 Changes in the relative content of soil humus components under different cultivation modes (g/kg)

3.4 Changes in the relative proportion of soil humus carbon composition under different cultivation modesWith the changes in the carbon content of the soil humus components, its relative proportion also shows a certain change rule (Table 4 and Fig.1). Previous studies usedCHA/CFAvalue orPQvalue as an important indicator to measure the humification degree of soil humus and the quality of soil humus[5]. The greater theCHA/CFAvalue orPQvalue, the higher the soil humic acid content, the larger the molecular weight, and the more complex molecular structure, which means the higher the humus lability and the better the quality[26]. Therefore, through analyzing the change rules of these relative proportions byCHA/CFAvalue orPQvalue, it is possible to characterize the effects of different treatments on the ability and quality of soil humus to a certain extent.

As shown in Fig.2, both theCHA/CFAvalue andPQvalue under the OF cultivation mode increased first and then decreased with the planting years, while theCHA/CFAvalue andPQvalue under the FM cultivation mode first decreased and then increased with the planting years, and the change trend of theCHA/CFAvalue andPQvalue was basically the same. Specifically, the soilCHA/CFAof the OF cultivation mode was 0.48-0.52, and the soilCHA/CFAof the FM cultivation mode was 0.57-0.59, and the range of change was not significant (P>0.05). The soilPQvalue of the OF and FM cultivation modes was 0.32-0.34 and 0.36-0.37, respectively, and the range of change was not significant (P>0.05). The results of one-way analysis of variance showed that the soilCHA/CFAof the FM cultivation mode was significantly higher than the OF cultivation model in 2018 and 2020 (P<0.05), while the difference in 2019 was not significant (P>0.05); thePQvalue did not change significantly between years and under different cultivation modes. In summary, the soilCHA/CFAandPQvalues of the FM cultivation mode were greater than those of the OF cultivation mode. It can be inferred that the FM cultivation mode is conducive to the renewal and activation of soil humus in this experimental stage, thereby improving the humus quality.

Fig.2 Changes in the relative proportion of soil humus carbon composition under different cultivation modes

4 Conclusions

Compared with the open field cultivation mode, theSOC,LOC, and soilCMIof the film mulching cultivation mode are all reduced;CHE,CHAandCFAare all increased with the increase of the planting years, but all are small than that under the open field cultivation mode. If film-mulching cultivation model with single application of fertilizer is used, it may have an adverse effect on the soil carbon balance and nutrient cycling conversion in the area, and ultimately adversely influence the sustainable production of the cultivation mode and the stability of crop yield.

5 Discussion

5.1 Impact of plastic film mulching on soil organic carbonAccording to the results of this experiment, bothSOCandLOCcontents under the film mulching cultivation mode decreased with the increase of the planting years, while both of them increased with the increase of the planting years under the open field cultivation mode, because the increase in soil temperature after film mulching accelerates the activation of soil nutrients, and improves the availability of soil nutrients and water use efficiency, and the mulching avoids direct impact and beating of rain on the soil, so that the soil can keep loose and not easy to compact, and has good air and water permeability[27]. Compared with the open field cultivation mode, plastic film mulching has better water and heat conditions and aeration, which changes the micro-ecological environment, thereby reducing the decomposing residual rate of organic materials in the soil, accelerating the decomposition of soil organic matter, accordingly leading to the decrease ofSOCandLOCunder mulching cultivation condition. These results are basically consistent with findings of Wang Jingkuanetal.[28]and Liu Yingchuanetal[29]. The decline of soilLOCmeans higherSOCoxidation stability, that is, the larger theKOS, which may have a negative impact on soil nutrient cycling and structural composition.

Through analysis of theCMI, it is known that the OF cultivation mode can improve soil fertility, and the soil quality develops in a benign direction, while the film mulching cultivation mode reduces the soil fertility. A comprehensive analysis of indicators such asSOC,LOC,KOS, andCMIindicate that the mulching cultivation model is not conducive to soil fertility, which may lead to a decline in soil fertility, and long-term film mulching may change the soil carbon balance.

5.2 Impact of plastic film mulching on soil humusThe soil fertility level is closely related to the degree of decomposition and accumulation of soil humus, so the soil humus can be used to evaluate the soil fertility level[30]. Related studies have shown that long-term application of organic fertilizer or combined application of organic fertilizer and chemical fertilizer is conducive to the accumulation of soil humus and the improvement of soil fertility[31-33]. Findings of Zhang Fudao[34]indicate that application of organic fertilizer, no fertilizer and application of chemical fertilizer have the same effect on the accumulation of humus. According to some study, although long-term application of chemical fertilizers can basically maintain the balance of soil humus[34], the application of chemical fertilizers further increases the degree of condensation of soil humus molecules, which in turn affects the availability of soil nutrients[35]. There are also some different findings that long-term application of chemical fertilizers would reduce the content of soil humus[36]. These indicate that long-term application of chemical fertilizers has different effects on soil humus. According to our findings in this experiment, under the condition of single application of chemical fertilizers, the soil humus content in both the open field and film mulching cultivation modes will increase with the increase of planting years. However, compared with the open field cultivation mode, the soil humus content decreased after film mulching (Table 3), which may because the improvement of soil moisture, temperature and other conditions of the cultivated layer under the film mulching condition is conducive to the crop growth and consumes more soil nutrients.

The soil fertility properties are directly related to the content and proportion of each component in the soil humus[37]. In its components, humic acid is active and has a greater impact on soil fertility characteristics[38], and it plays an important role in the formation of soil aggregates[39]. Relevant studies have shown that long-term application of chemical fertilizers can increase soil HA/FA[8], while some research results have shown that long-term application of chemical fertilizers can reduce soil HA/FA[36]. The results of this study indicate that the soil HA/FA decreased with the increase of planting years under the two cultivation modes, but the difference was not significant (P>0.05); the soil HA/FA under the film mulching cultivation condition were higher than that of the open field cultivation mode, but the difference was not significant (P>0.05). According to some studies, the relative size of the amount of formation and decomposition determines the soil humus content[40], and whether the composition is based on humic acid or fulvic acid depends mainly on soil environmental conditions[41]. The results of this study show that the soil humus is mainly fulvic acid under the two cultivation modes, reflecting that in the soil of single application of chemical fertilizers, the extractable humus contains a large proportion of fulvic acid, its molecular weight is small, and the structure of humus is simpler, and the aggregation of soil structure is weakened, and the quality of soil organic matter develops in a bad direction, which is consistent with the previousCMIresult. In addition, the soil HA/FA can also reflect the degree of anthropogenic mellowing and fertility, that is, the higher the soil fertility and anthropogenic mellowing, the greater the ratio[42]. Generally, the HA/FA of highly anthropogenic mellowed soil is up to 1.4, that of moderately anthropogenic mellowed soil is about 0.5, and that of slightly anthropogenic mellowed soil is 0.2-0.3[43]. According to the results of this study, the soil HA/FA in the film mulching cultivation mode is 0.57-0.59, and the soil HA/FA in the open field cultivation mode is 0.48-0.52. These indicate that the degree of anthropogenic mellowing in plastic film mulching mode is higher than that of open field cultivation mode because the plastic film mulching may promote soil mellowing, which in turn may accelerate the consumption of soil nutrients by crops.