Introduction

Climate change is the most serious global environmental problem facing humanity in the world today. The main reason for the global greenhouse effect is the greenhouse gases (GHG), including carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) (Tian et al. 2016) produced by agricultural production activities, such as land-use change, agriculture and waste management. According to United States Environmental Protection Agency (USEPA 2006), CH₄and N₂O emissions from agricultural land use activities account for 50% and 75% of total global emissions, respectively. Therefore, agricultural GHG emission reduction plays an important role in controlling global climate change. Soil is a huge carbon pool in terrestrial ecosystems. Soil can be carbon source or carbon sink and affects the concentration of CO₂ in the atmosphere (Duiker and Lal 1999). Many studies around the world have shown that soil carbon sequestration can be facilitated by appropriate land use and agricultural management (Poeplau and Don 2015; Lange et al. 2015; Gao et al. 2018). According to the Intergovernmental Panel on Climate Change (IPCC), the technological potential of global agricultural emissions reduction is as high as 5500–6000 Mt CO₂ equivalent per year (Smith et al. 2007). Of these, 89% came from reducing soil CO₂ emissions (e.g., soil carbon sequestration). Therefore, improving fertilizer use efficiency through reasonable farming measures and increasing carbon sequestration in crop soil systems are important ways to reduce GHG emissions in agriculture.

The carbon sink capacity of crop systems has attracted the attention of researchers. As crop soil ecosystems are net source or net sink of atmospheric CO₂? Many studies have reported the carbon sink function of crops, such as wheat-maize-soybean rotation systems in black soils (Liang et al. 2012), paddy field system for long-term application of organic fertilizer (Hu et al. 2017), and no-till corn/soybean ecosystem (Bernacchi et al. 2005). Studies have also demonstrated that most of Chinese cropping systems are net source of GHG emissions (Wen et al. 2010). Hence, there are large variations of net GHG balance between different cropping systems or cultivation management. Thus, the carbon source/sink balance of tobacco soil ecosystem is rarely studied.

It is generally believed that application of organic fertilizer is an effective way to improve the soil and promote the growth of plants. The application of organic fertilizer can increase the number of soil microorganisms (Jannoura et al. 2014), improve the physical and chemical properties of soil, and regulate soil carbon pools (Lazcano et al. 2013; Liu et al. 2013; Wei et al. 2019). However, there are complex trade-offs between soil organic carbon (SOC) sequestration and GHG emissions, and the greenhouse effects of organic fertilizers remain controversial for field crops. Although organic fertilizers play an important role in increasing soil carbon sequestration and crop yield, their effects on GHG emissions cannot be ignored. Studies have shown that the application of organic fertilizer can promote the emission of CO₂ and N₂O, and significantly enhance GHG warming potential (Zhu et al. 2013; Das and Adhya 2014). There are also reports on the use of organic fertilizers to increase soil carbon sinks (Sekhon et al. 2009; Yang et al. 2015; Hu et al. 2017). Therefore, the carbon sink effect of organic fertilizer varies among different crops and environment and few reports are available on the effect of organic fertilizers on soil carbon and nitrogen balance and GHG emissions in tobacco fields. To this end, the effects of different fertilization treatments on carbon and nitrogen gas emissions and comprehensive greenhouse effects of tobacco soil ecosystems are studied under the field condition, to provide a basis for tobacco field soil cultivation and GHG emission reduction.

Materials and Methods

Experimental site and materials

The experiment was conducted in Xiangcheng county, Xuchang city, Henan Province, China (N33°51′, E113°25′), from 2016 to 2017. The soil type was cinnamon with basic physical and chemical properties with pH 7.89, organic matter 17.20 mg·kg^-1, total nitrogen 1.10 mg·kg^-1, alkali nitrogen 76.54 mg·kg^-1, available phosphorus 23.16 mg·kg^-1, and available potassium 102.22 mg·kg^-1. The variety of flue-cured tobacco tested was Zhongyan 100. The fertilizer applied was a special one for local tobacco, organic fertilizer (sesame cake fertilizer, 5.0% N content, 1.0% K₂O content), compound fertilizer (N:P₂O₅:K₂O=10:10:10), and potassium sulfate (50% K₂O content).

Experimental design

The two-year randomized block experimental design was consistent. The experiment was comprised of three treatments: no fertilizer (CK), single application of chemical fertilizer (T1), combined application of chemical fertilizer and organic fertilizer (T2). The single application of chemical fertilizer was 375 kg·hm^-2 as compound fertilizer and 225 kg·hm^-2 as potassium sulfate. The combined application of chemical and organic fertilizer was 375 kg·hm^-2 as organic fertilizer, 375 kg·hm^-2 as compound fertilizer and 225 kg·hm^-2 as potassium sulfate. The experiment used three replications, and the plot size of 100 m².

All fertilizers were applied once before ridging. In 2016, fertilization and ridging were carried out on May 9, GHG collection began on May 10, and tobacco seedling transplanting was carried out on May 11. In 2017, the fertilization and ridging were performed on May 1, GHG collection on May 2, tobacco seedlings were transplanted on May 3. Other field management carried out was in accordance with local high quality tobacco production practices.

Sample collection and analysis methods

GHG were collected and measured using static chamber-weather chromatography. The volume of the static box base (length × width × height) was 60 cm × 50 cm × 30 cm, and the volume of the box (length × width × height) was 60 cm × 50 cm × 15 cm. Gas collection began after fertilization and ridging, which was carried out at 9:00–11:00 am at day 1, 3, 5, 7, 9, 11, 13, 15, 30, 45, 60, 75, 90, and 115. The CO₂ and N₂O concentrations of the samples were determined using a GC7890 gas chromatograph (Agilent, U.S.A.).

Soil ammonia volatilization was measured by continuous chamber evacuation followed by titration. The closed chamber was made of transparent plexiglass material with an inner diameter of 20 cm and a device height of 30 cm. There were two vent holes at the top of the chamber. One of the vents (30 mm in diameter) was connected to the vent tube containing the boric acid absorbing liquid filter bottle to reduce the influence of surface air exchange on the determination of ammonia volatilization. The other vent (11 mm in diameter) was connected to a gas cylinder containing boric acid absorbing liquid. The air in the sealed chamber was passed through a boric acid absorbing liquid by means of suction and decompression to absorb and fix the ammonia therein, and the collected solution was measured by a standard dilute sulfuric acid titration method. Samples were collected every day at 9:00–11:00 a.m. and 3:00–5:00 p.m. after fertilization. The cumulative amount of ammonia volatilization for a total of 15 days was used as the total emission.

At the maturity, samples of roots, stems and leaves of the plants were collected, dried and pulverized, and the carbon content of the plants was analyzed by Vario MARCO cube elemental analyzer (Elementar, Germany), and then the carbon fixation of the whole plant was calculated.

Index calculation method

The amount of change in CO₂ was calculated using the formula:

ΔSOC = the carbon output - the carbon input

Where, the carbon output is CO₂ emission (kg·hm^-2); the carbon input is carbon from organic fertilizer (kg·hm^-2).

Organic fertilizer carbon input (in terms of CO₂) =

(amount of organic fertilizer applied × carbon content in organic fertilizer × 44) / 12.

Nitrogen emissions = ammonia volatilization + N₂O emissions

The overall greenhouse effect of CO₂ and N₂O production in this study was characterized by CO₂ equivalent, calculated as:

CO₂-eq (kg·hm^-2) = N₂O emission (kg·hm^-2)×298+ ΔSOC (kg·hm^-2).

The greenhouse gas emission intensity (GHGI) is the CO₂ equivalent per unit of production, and the formula is:

GHGI (kg·kg^-1) = CO₂-eq (kg·hm^-2) / crop yield (kg·hm^-2).

Statistical analysis

Differences among different treatments were determined though one-way analysis of variance (ANOVA) and Least Significant Difference (LSD) test using S.P.S.S. 21.0 (S.P.S.S. software Inc., U.S.A.). Differences were considered significant at P < 0.05.

Results

Effect on soil CO₂ emission flux

Soil CO₂ emission flux was largely affected by fertilization (Fig. 1). In 2016, within the first 15 days after fertilization, soil CO₂ emission flux was at a relatively high level, and then decreased. The CO₂ emission flux increased gradually 45 days after fertilization, and reached the highest at 90 days after fertilization followed by a rapid decline. In 2017, within 15 days after fertilization, the soil CO₂ emission flux remained at a high level, and began to decline gradually after 75 days. The difference in soil CO₂ emission flux during the year may be closely related to temperature and rainfall differences. Compared with the control, soil CO₂ emission flux of chemical fertilizer treatment and combined chemical and organic fertilizer treatment increased significantly, and combined chemical and organic fertilizer treatment was slightly higher than chemical fertilizer treatment. Therefore, increasing application of organic fertilizer can promote soil CO₂ emissions to a certain extent.

Effect on soil carbon balance

Compared with the control, the soil CO₂ and C emissions of chemical fertilizer treatment and combined chemical and organic fertilizer treatment increased significantly, and the average increase reached 32.90% and 42.14% respectively (Table 1). Compared with chemical fertilizer treatment, both soil CO₂ and C emissions increased in combined chemical and organic fertilizer treatment, with an average increase of 7.0%, which may be closely related to organic fertilizers promoting soil microbial activity and improving soil physical and chemical properties. The carbon fixation of different treatments showed combined chemical and organic fertilizer treatment > chemical fertilizer treatment > no fertilizer, which was consistent for two years. In 2016, under no fertilizer condition, the tobacco soil ecosystem is a carbon source to the atmosphere (net carbon output), while under normal fertilization conditions, it is a weak carbon sink (net carbon input). In 2017, due to the increase of carbon emissions and reduction of plants carbon sequestration, the tobacco soil system showed a carbon source. Compared with the application of chemical fertilizer alone, adding organic fertilizer significantly increased the carbon sequestration by plants and significantly increased the system carbon input.

Effect on ammonia volatilization

The ammonia volatilization of soil is mainly concentrated within the first 15 days after fertilization. In this study, the ammonia volatilization of all fertilization treatments fluctuated within the first 15 days of fertilization (Fig. 2). Compared with control, the ammonia volatilization level of chemical fertilizer treatment and combined chemical and organic fertilizer treatment increased significantly. Five days after fertilization, the ammonia volatilization level of combined chemical and organic fertilizer treatment was significantly higher than chemical fertilizer treatment.

Effect on soil N₂O emission flux

The soil N₂O emission mainly occurred within the first 15 days after fertilization (Fig. 3). Compared with control, the soil N₂O emission fluxes from chemical fertilizer treatment and combined chemical and organic fertilizer treatment were significantly higher. There was an annual difference in the emission differences between chemical fertilizer treatment and combined chemical and organic fertilizer treatment. The N₂O emission from chemical fertilizer treatment was higher than combined chemical and organic fertilizer treatment in 2016, while the difference between the two was smaller in 2017.

Effect on soil nitrogen balance

Soil N emission mainly comes from N₂O and NH₃.Compared with control, both the soil N emissions of chemical fertilizer treatment and combined chemical and organic fertilizer treatment increased significantly. The average increase of N emissions reached 74.72% and 93.86%, respectively (Table 2). Compared with chemical fertilizer treatment, the N emission of combined chemical and organic fertilizer treatment was increased significantly, with an average increase of 11.79% in two years. The nitrogen accumulation of different treatments showed combined chemical and organic fertilizer > chemical fertilizer treatment > control, which was consistent during two years. In 2017, due to the weak growth of tobacco plants and the low dry matter accumulation, the nitrogen accumulation of plants was significantly lower than 2016.

Effect on soil carbon emission intensity