Effects of fertilizer types and water quality on carbon dioxide emissions from soil in wheat-maize rotations

Science of the Total Environment 698 (2020) 134010

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Effects of fertilizer types and water quality on carbon dioxide emissions from soil in wheat-maize rotations Yanbing Chi, Peiling Yang ⁎, Shumei Ren, Ning Ma, Jing Yang, Yao Xu College of Water Resources & Civil Engineering, China Agricultural University, No. 17 Tsinghua East Road, Haidian District, Beijing, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Difference of CO2 emissions from soil under reclaimed water and fresh water is insignificant. • The variation in soil CO2 emissions is not influenced by fertilizer types. • The types of fertilizercan impact soil nutrient conditions. • Slow-released fertilizer have a lower CO2 emission and better yield.

a r t i c l e

i n f o

Article history: Received 15 April 2019 Received in revised form 14 August 2019 Accepted 19 August 2019 Available online 27 August 2019 Keywords: CO2 emission Reclaimed water Forms of fertilizer Soil properties Wheat-maize rotation Yield

a b s t r a c t The use of fertilizers as addition inputs in agricultural systems can increase the yield of wheat and maize, while also stimulating the emission of carbon dioxide from soil that the main man-made greenhouse gas. Our objectives focused on the impact of different types of synthetic fertilizers and water quality. The purposes were to determine the feasibility of using wasted water for irrigation and to relate CO2 fluxes to the yield of maize and wheat, as well as to select the best fertilizer type with low CO2 emission and high yield. The experiment consisted of a double factors test focusing on four forms of fertilizer (urea, amine and slow release fertilizer) and the quality of water (reclaimed water and underground water). The results showed that the reclaimed water was not significant on the CO2 discharge rate, the maize-wheat yield or the soil properties in 2014 or 2015; however, the CO2 emission increased slightly in 2015. Focusing on fertilizer treatments, the reclaimed water & amine fertilizer treatment (CAF) that had higher cumulative CO2 emissions was 32.75 t·ha−1 in 2014 and 33.86 t·ha−1 in 2015. According to the ratio CO2/Y, the slow released fertilizer that reduces CO2 emissions and keeps the yield high is the preferred choice. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The rising concentrations of greenhouse gases (GHGs) have been implicated with global climate change (Edenhofer and Seyboth, 2013). ⁎ Corresponding author. E-mail address: [email protected] (P. Yang).

https://doi.org/10.1016/j.scitotenv.2019.134010 0048-9697/© 2019 Elsevier B.V. All rights reserved.

Agriculture plays an important role in the global flux of GHGs; most research has found that CO2 is the most abundant GHG in soil and has an emission flux that is more than a hundred times that of N2 O, CH4 , etc. (Abalos et al., 2014; Chen et al., 2010; Price et al., 2015; Ruser et al., 2006). Water and N fertilizer have been shown to be the driving factors in the emission of CO2 from soil (Abalos et al., 2014; Darwish et al., 2006). These affect soil CO2 emission by

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changing the soil environment, such as soil aeration, soil pH, soil moisture, C/N ratio of substances, etc. In irrigated agroecosystems, irrigation and fertilizer application practices and changes in crops have contributed to changes in water-filled pore space (WFPS), as well as carbon (C) and nitrogen (N) dynamics, and thereby to changes in CO2 emissions (Abalos et al., 2014; Snyder et al., 2009; Ward et al., 2017; Weiler et al., 2018). For example, Franzluebbers and Iqbal considered that soil microbial activity and the decomposition processes that transform plant-derived C to soil organic matter (SOM) and CO2 can be impacted significantly by these soil environment characteristics (Franzluebbers et al., 1995; Iqbal et al., 2008). Due to the different of distinct climate, soil conditions and agricultural management, the degree of influencing factor is different. Iqbal observed a highly positive correlation between soil CO2 flux and soil temperature & WFPS interaction in a subtropical pinus plantation of Qianyanzhou in southeastern China (Iqbal et al., 2009), however Lee found there is no positive correlation between them(Lee et al., 2007). To guide farmers and to formulate policies for future CO2 emission reductions in agriculture, it is paramount to understand how water quality and fertilization affect the emission of these gases and need a detailed analysis for the specific environment. With the decrease of water as a resource, in recent years, many regions have begun to grope for irrigation using reclaimed water. Globally, approximately 20 million ha of land is now irrigated with reclaimed water; this approach has become a key strategy in fighting water shortages (Evanylo et al., 2010; Hamilton et al., 2007). Reclaimed water contains some nutrient elements, such as nitrogen; thus, its application to agricultural fields may bring additional benefits to soil and reduce the need for fertilizer application (Chen et al., 2015). On the other hand, reclaimed-water irrigation may also introduce various nutrient substances and microorganisms that can bring about soil properties changes, such as changes in soil pH (Banón et al., 2011), the addition of salt-related environmental threats (Hamilton et al., 2007), the increase of soil nutrients (organic carbon and nitrogen) (Pereira et al., 2011) and the decrease in hydraulic conductivity (Levy et al., 1999). The stability of reclaimed water is reported in these cases, as well as modest reclaimed water irrigating. Thus, it is valid to consider that reclaimed water could impact the CO2 emission from soil by affecting the soil environment and soil respiration; however, there are few reports in previous studies. Table 1 shows the water quality standards of reclaimed water irrigation, the limits for the physiochemical indexes and the hygienic indexes of the reclaimed water for irrigation (Xu et al., 2016). In general, N fertilizer is always an important factor in agricultural management; previous studies focused on the effect of the amount (Wang et al., 2013; Ward et al., 2017) and types of N fertilizer (organic and synthetic) (Ding et al., 2007; Lee et al., 2007) on CO2 emission from soil. These studies have demonstrated that there are significant differences in soil respiration due to the levels of N fertilizer and the form of N fertilizer; these studies have also shown that N fertilizer application is succeeded by temporary increases in CO2 concentrations. As we know, many choices of synthetic N fertilizer that improve the crop yield are available for fertigation in agricultural management. Urea and amines are very water-soluble, slow-release fertilizers that can achieve balance among releasing nutrients, supplying soil nutrients, Table 1 Basic control items indicating quality of reclaimed water for irrigation. Index

BOD5/(mg/L) CODcr/(mg/L) SS/(mg/L) DO/(mg/L) pH

Types of crops to be irrigated Grain and oil crops in dry land

Textile crop

Outdoor vegetable

80 180 90 – 6–9

100 200 100

40 100 60 ≥0.5

Grain in paddy field 60 150 80

and assimilating nutriment of the plan; thus, their use in fertigation is widespread. (Dobbie and Simth, 2003; Fynn et al., 2003) studied the effect of different chemical types of N fertilizer on N2O emission and found urea treatment had the lowest N2O emission. However, little is known about how the form of N fertilizer affects soil CO2 fluxes. Understanding the factors responsible for soil CO2 flux is essential for predicting changes in this variable caused by changes in N fertilizer use. The objectives of this study were (1) to analyze the effect of reclaimed water on CO2 emission and soil properties, (2) to evaluate the effects of fertilizer types on CO2 emission with reclaimed water irrigation, and (3) to compare the effects of fertilizer types on yield and CO2/Y on wheat and maize yield in a typical growing area of China. 2. Materials and methods 2.1. Trial condition The field experiment was conducted at the China Agricultural University experimental point (116°41′2.31″ E, 39°41′6.93″ N) in Tongzhou, Beijing City, China. The climate is subtropical monsoon with four distinctive seasons. Summers are hot and humid, while winters are relatively cold and dry. The annual mean temperature is approximately 11.4–12.4 °C, the average annual rainfall is approximately 550–600 mm, and the frost-free period is 185 d. The soil in the experimental station is clay loam (0–70 cm) and topsoil (0–20 cm) (Table 2), which has an average TN of 2.92 g·kg−1, TP of 1.09 g·kg−1, and pH of 7.5. These conditions are suitable for wheat and maize growth. 2.2. Trial design 2.2.1. Investigation and research The main purpose of this survey was to understand the management model of fertilizer and irrigation water for grain crops with drip irrigation in the North China Plain during the warm temperate monsoon climate. Questionnaires that included the date of fertilizer application, the types and amounts of fertilizer, crops being raised, and crop yield, as well as the farmer's age, name, member of family, annual income, etc., were given to 260 farmers planting wheat and maize. To ensure the accuracy of the research, we conducted three levels: the survey district, sample size and the logic questionnaire. The survey district was centered on Beijing and included the periphery of Beijing, Hebei, Shandong, and Tianjin where maize and wheat were the main planting crops in the North China plain, with sample sizes of 50, 80, 80, and 50, respectively. These questionnaires were distributed to farmers between 30 and 50 years of age, which is the main food-producing labor force. The questions included the age, occupation, crop species, annual income, irrigation period, fertilization methods and fertilization time, etc. If the crop species did not match the annual income and if the irrigation method did not match the irrigation amount, we considered the planting experience did not meet our requirement. As a result, the survey was roughly consistent with some of the literature (Fang et al., 2010). The results showed that most people reported that the amount of fertilizer used was approximately 280 N kg·ha−1 during the wheat growth period in winter and approximately 300 N kg·ha−1 during the maize growth period in summer. Our study adopted a low-frequency irrigation method, where the rated flow of the drip irrigation pipe was 1.8 L/h. The period of fertilizer application was during the jointing stage and milk stage, aside from the application of the base fertilizer. In our investigation, the applied fertilizer was mainly urea, amine and slow-release fertilizer. To be suitable for the actual situation, our study applied fertilizer three times, the details of which are shown in Table 3. 2.2.2. The trial treatment All the experimental plots were 5 m wide by 30 m long in an existing long-term winter wheat-summer maize double-cropping field

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Table 2 Trial soil conditions. Depth (cm)

0–20 N20–40 N40–70 N70–100

Different size particle mass fraction (%) 0–0.005 (mm)

N0.005–0.050 (mm)

N0.05–1.00 (mm)

25.8 17.8 22.8 12.8

31.2 31.2 28.0 22.1

41.2 51.2 49.2 65.1

Soil texture

Volume density (g/cm3)

Field capacity (%)

Saturated hydraulic conductivity (cm/s)

Porosity

Clay loam Clay loam Clay loam Sandy loam

1.45 1.64 1.57 1.58

21.23 18.21 20.10 15.75

27.80 24.89 28.64 28.81

0.418 0.415 0.450 0.455

experiment. Measurements were carried out during the 2014–2015 growing seasons; the whole growth period was 486 d. The field experiment started in 2013 and had eight treatments in triplicate in a randomized complete block design. The quality parameters of reclaimed water are listed in Table 4. Treatments included four types of N fertilizer (urea, amine, slow-release and no fertilizer) and two kinds of water (underground water and reclaimed water); the overcross test combinations were UUF, UAF, USF, U0, RUF, RAF, RSF, and R0. Details on fertilizer application and crop husbandry are presented in Fig. 1. Straw was chopped (b5 cm) by an automated machine and returned to the field. The amount of soil N at the beginning of the experiment, October 2014, was as follows: 1.11 (C0), 1.08 (CUF), 1.09 (CAF), 1.07 (USF), 1.02 (U0), 1.09 (UUF), 1.10 (UAF), and 1.04 (USF) g/kg, with the maximum difference being 0.06 g/kg.

2.3. Sampling method The CO2 emission flux was determined by static chamber-gas chromatography using conventional methods (Buragienė et al., 2019; Oliveira Silva et al., 2019; Ding et al., 2007). The gas samples were analyzed by gas chromatography (Agilent GC-6820, Agilent Technologies Inc., Santa Clara, CA, USA) for CO2. The square chamber was 50 cm × 50 cm × 50 cm, and it was inserted to a depth of 5 cm in the soil. One chamber and stopwatch were inserted into each plot; 2–3 people sampled at the same time, and a gas collection was fulfilled in 10 s. The sampling procedure was repeated; the same N fertilizer was sampled simultaneously and all gas samples were finished in 1 h. Samples were taken between 14:00 PM and 16:00 PM during the wheat growth season and between 9:00 AM and 11:00 AM during the maize growth season. Soil temperature (0–5 cm depth) was measured once a week in each plot (24 plots in total). Measurements of CO2 emissions from the soil were performed once a week in each plot, aside from the 4 additional measurements that were taken every two days after fertilizer application. Soil properties (pH, electrical conductivity, organic C, soil N) were measured at approximately 10-15d intervals during the growing season from 2014 to 2015. During the entire sampling period, each index in each unit was collected twelve times a year. All analyses of soil properties were based on standard methods. The results were expressed in terms of 105 °C oven-dried weight.

Table 3 The timing and amount of fertilizer application. Treatment 2014/2015 Winter-wheat 2014/2015 Summer-maize

Growth period Base fertilizer Jointing stage Milk stage Base fertilizer Jointing stage Milk stage

N fertilizer (N kg·ha−1) 100 90 90 100 100 100

2.4. Statistical methods All statistical analyses were carried out using SPSS V18 for Windows (SPSS Inc., Chicago, IL, USA). An analysis of variance (ANOVA) was used to determine treatment effects. The least significant difference procedure (LSD) and a probability level of 0.05 were used to determine significant differences between treatment means by using a t-test. In this study, average refers to the arithmetic mean. Cumulative soil CO2 emission was calculated by summing the products of the (averaged) two neighboring fluxes, multiplied by their interval time. 3. Results 3.1. Climate conditions During the seasonal growth period, cumulative rainfall and irrigation water in 2014 (414.4 mm) and in 2015 (444.3 mm) were minimally different than the historical average (550 mm). The schedule for the cropping system used is shown in Fig. 1 below. During the entire experiment, the air temperature ranged from 5 to 40 °C in 2014 and from 2 to 41 °C in 2015. The mean air temperature during the CO2 measurements in 2010 (15.6 °C) and 2011 (14.8 °C) were slightly lower than the historical average (16.1 °C). 3.2. Soil CO2 fluxes During the two-year sampling period, the CO2 emission flux between the underground and reclaimed water treatments were coherent: the soil CO2 flux increased gradually from the beginning of fertilization, with the peak of the CO2 emission flux occurring between the third and fifth days after fertilization (Fig. 2). In 2014–2015, the cumulative soil CO2 flux in all treatments during the maize growing season was significantly higher than during the wheat experimental period (Table 5). According to the irrigation and fertilization, the CO2 emission flux was higher between the jointing stage and the milk stage. The fluxes then declined and reached a minimum at the end of the mature period, followed by a gradual increase until the harvest of wheat and maize (Fig. 2). Compared with underground water, reclaimed water irrigation did not have any significant effects on the CO2 emission in 2014 and 2015 (Table 5), but the flux was higher. Our results showed that the difference between reclaimed water and fresh water was 1%–5% in 2014, and 5%–15% in 2015, with the biggest gap in the no-fertilizer treatment; Table 4 The quality of irrigation water. Water quality parameters CODcr BOD5 SS NH+ 4 -N TN TP

Reclaimed water (mg/L)

Fresh water (mg/L)

35.98 12.72 10.63 2.670 14.29 0.79

8.51 3.33 0 0.076 1.20 0.39

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Fig. 1. Schedule of field operations and cropping system used during the two years of CO2 measurements. indicates applied N fertilizer. The application dates were 2 Mar, 8 Apr, 26 Jun, 28 Jul, and 30 Aug in 2014 and 8 Apr, 10 May, 22 Jun, 26 Jul, and 22 Aug in 2015. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

this difference increased gradually with time. During the wheat experimental period in 2014, cumulative gas differed among treatments, ranging from highest to lowest in the following order: CUF ≈ CAF N UUF ≈ UAF N CSF N USF N C0 N U0 (Table 5). These results showed that fertilizer was an important factor on the CO2 emission from soil in the special agricultural management comparing with the quality of irrigation water. Throughout the trial period, a variation from 400 to 1800 mg·m−2·h−1 occurred when the additional N fertilizer was

terminated; meanwhile, the value of the cumulative crest could be even higher, reaching up to 80% of the total cumulative emission. In 2014, the CO2 emission flux in N fertilizer had a higher value than in no fertilizer (Fig. 2), and the slow-release fertilizers that had the lowest cumulative emissions among fertilizer treatments were USF (28.35 t·ha−1) and CSF (29.05 t·ha−1) (Table 5). The cumulative emissions during the maize growing season are higher than it during the wheat growing season. That result may contribute to temperature, the higher temperature could product a higher gas emission flux. The

Fig. 2. Seasonal variations of precipitation and temperature at the 5 cm depth during the wheat and maize growing seasons, and the CO2 flux emission in 2014 (left) and 2015 (right) from different fertilizer treatments. Data points represent the mean values (n = 3). indicates applied N fertilizer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 5 Mean and cumulative CO2 emissions during the maize and wheat growing seasons. Treatment

Total average 2014

U0 UUF UAF USF C0 CUF CAF CSF

537.77b 633.49a 639.72a 585.78a 538.12b 645.65a 635.60a 600.11a

Wheat cumulative flux/(t·ha−1)

Maize cumulative flux/(t·ha−1)

2015

2014

2015

2014

2015

425.40ab 636.04a 633.60a 594.55a 526.9b 681.32a 673.45a 660.83a

12.74 ± 4.87b 14.32 ± 5.23a 14.62 ± 3.27a 13.75 ± 3.12a 12.64 ± 4.63b 14.61 ± 5.21a 15.29 ± 5.64a 14.14 ± 6.43a

10.95 ± 2.33ab 14.01 ± 7.04a 14.52 ± 3.76a 13.58 ± 4.21a 12.43 ± 5.08b 15.32 ± 3.12a 15.22 ± 4.03a 14.41 ± 5.97a

13.28 ± 3.23b 16.37 ± 7.12a 16.35 ± 2.25a 14.60 ± 3.87a 13.41 ± 6.65b 16.66 ± 5.87a 17.46 ± 7.65a 14.91 ± 2.89a

11.73 ± 4.75ab 16.94 ± 9.62a 17.02 ± 7.67a 15.37 ± 5.23a 12.33 ± 6.24b 17.82 ± 7.32a 18.64 ± 8.21b 15.06 ± 4.45a

Total average was the entire cumulative emissions (wheat + maize) divided by the sampling days. Values are mean ± standard deviation (n = 3). Means followed by the same letter within a column are not significantly different according to t-test (P b 0.05).

variation of CO2 emissions in 2015 had similar emission pattern. From 2014 to 2015, the cumulative soil CO2 emission had decreased during the wheat experimental period, and U0, which had decreased most among the other kinds of treatments, had changed by 14.12% (Table 5). In contrast, there was a difference between the wheat and maize crops; each of the treatments in the maize crop increased separately from the previous year by 3%–11% (Table 5). Two-by-two comparative analysis found that CSF had increased least (1.01%) among the other kinds of fertilizer treatments, and CUF had increased most (6.96%) (Table 5). 3.3. The variables of soil properties During the entire sampling period, reclaimed water treatments had no significant impacts on soil pH or soil electrical conductivity. The surface soil pH appeared to be slightly alkaline, with the mean value ranging from 7.5 to 8.3. The results showed that the pH of U0 and C0 were slightly higher compared to the fertilizer treatments, but there was no significant difference of soil pH between 2014 and 2015 (Fig. 3). In 2014, most of the soil electrical conductivity values for all treatments was no more than 800 μs/cm, although the urea and amine had a higher value than the other treatments. In 2015, the soil electrical conductivity value decreased in each unit; the value was no more than 400 μs/cm, aside from CFS. There was no significant difference between reclaimed water and underground water during two years (P N 0.05). The type of N fertilizer can impact soil N significantly during the sampling period (Fig. 3), meanwhile the urea and amine fertilizer had a higher content of soil N in soil. In 2014 and 2015, UUF and UAF had the highest average soil N values, which were 1.53 and 1.39 g/kg, 1.46 and 1.31 g/kg, respectively. In contrast, between 2014 and 2015, U0 decreased the most (10.7%) in comparison to the other treatments. The difference between reclaimed water and underground water was −9%–6% in 2014, and −16.39%–0.55% in 2015; From Fig. 3, our study found that this difference increased gradually with time. A negative correlation between CO2 flux and soil N was reported in our study. The UF and AF had the maximum values, and the value of SF was similar to the no-fertilizer treatment, which could be a result of the fertilization methods. During the sampling period, the quality of water had no significant effect on the soil organic C; the variety of content in soil was mostly affected by fertilization, with the amine fertilizer being the most influential factor and UAF having the highest value (14.24 g/kg). The content of soil organic C was different than soil N, the content of soil organic C improved slightly between 2014 and 2015. Our study showed that fertilizer can increase the content of soil organic C. The difference between reclaimed water and underground water was 9%–12% in 2014, and 4%–11% in 2015; it increased gradually with time. 3.4. Yield During the wheat growing season in 2014, the yield for the different treatments was 9692.5 (U0), 11,613.14 (UUF), 11,338.167 (UAF),

11,218.44 (USF), 9777.89 (C0), 11,950.06 (CUF), 11,708.44 (CAF), and 11,358.56 (CSF) kg·ha−1. In contrast, the amount of reclaimed water was higher than underground water although the difference was b2% (Fig. 4). Focusing on fertilization, the urea treatments resulted in the maximum yields, with UUF being 2.42% and 3.52% higher than UAF and USF, respectively, and CUF being 2.06% and 5.21% higher than CAF and CSF, respectively. The findings of the maize yield were similar to those of wheat described previously; the reclaimed water can slightly increase the yield, and the amine fertilizer had the highest yield (Fig. 4). UAF was found to be 0.01% and 8.23% higher than UUF and USF, respectively, and CAF was found to be 0.01% and 4.58% higher than CUF and CSF, respectively. The UF and AF had nearly equal outputs in maize and wheat (Fig. 4). During the 2015 growing season, the findings were similar to those of 2014 described previously (Fig. 4). Comparing yields between 2014 and 2015, our study found that the yield decreased slightly, and the no-fertilizer treatment decreased the most (Fig. 4) because the soil nutrients were lower than in the other treatments (Fig. 3), and thus there were not enough nutrients provided to grow crops. As seen in Fig. 3, the soil N and C content of these two treatments (U0 and C0) was lower than for the fertilizer treatments; the soil N content in 2015 was 7.8% (C0) and 3.9% (U0) lower than it was in 2014. In contrast, the slowrelease fertilizer had the lowest yield among the fertilizers. Between 2014 and 2015, the ratios CO2/Y (the ratio of cumulative CO2 emission and yield) increased 3.95% (U0), 0.53% (UUF), 1.65% (UAF), 0.09% (USF), 5.69% (C0), 8.19% (CUF), 2.86% (CAF), and 4.94% (CSF) during the wheat growth season and 6.69% (U0), 5.21% (UUF), 7.29% (UAF), 6.26% (USF), 7.22% (C0), 7.66% (CUF), 7.89% (CAF), and 3.69% (CSF) during the maize growth season. The mean value CO2/Y during the maize growth season was 1.6–1.9 times that of winter wheat. Among the fertilizers, the slow-release fertilizer had the lowest value, and the amine fertilizer had the highest CO2/Y value. As seen in the Fig. 4, the CO2/Y in the reclaimed water treatment had a higher value. During the wheat growth season, the reclaimed water was higher −0.65% (0F), −0.85% (UF), 1.28% (AF), 1.57% (SF) in 2014 and1.05% (0F),6.70% (UF), 2.48% (AF), 6.49% (SF) in 2015 than underground water; During the maize growth season, the reclaimed water was higher 0.86% (0F), 6.00% (UF), 10.19% (AF), 1.77% (SF) in 2014 and1.36% (0F),8.56% (UF), 10.81% (AF), −0.69% (SF) in 2015 than underground water. So we could consider the SF treatments had a stable variation, the UF and AF can increase strongly the value. 4. Discussion 4.1. Effect of fertilization on soil CO2 flux Throughout the trial period, the high and short-term CO2 emission peak values were observed after fertilization; this result mirrored most studies (Iqbal et al., 2009; Ward et al., 2017). Among all N treatments, the cumulative emissions were different and, as shown in Table 5, the AF and UF treatments had slightly higher cumulative emissions than SF. These results illustrate that fertilizer could clearly

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Fig. 3. Variations of soil properties among fertilization treatments in 2014 (left panels) and 2015 (right panels).

improve the CO2 emission, and SF could reduce the CO2 emission. In comparison to the fertilizer treatments (FT), no fertilizer (NFT) had a CO2 emission flux pulse with a peak value that was slightly lower than that of the FT. It appears the peak value mainly contributed to soil WFPS that can impact CO2 emission significantly (Dossou-Yovo et al., 2016). Ruser et al. (2006) thought the higher CO2 fluxes that occurred after rewetting indicate the release of easily available organic matter during drying-rewetting. The result could be attributed to the release

of available carbon by the disruption of soil aggregates. The increased C turnover following rewetting is associated with an enhanced O2 consumption which stimulates denitrification (Flessa and Drsch, 2007). That illustrated the peak accompany irrigation, the fertilizer could improve the value. As we know, nitrogen availability can be utilized by soil respiration, so there exists a process between applied nitrogen and CO2 emission from soil, and the CO2 emission from soil that is derived from both autotrophic and heterotrophic respiration can be

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Fig. 4. The yield of wheat (left panels) and maize (right panels) in 2014 and 2015. CO2/Y means the ratio of cumulative CO2 emission and yield.

impacted by different factors, such as WFPS, temperature, soil texture, organic carton, etc. (Buragienė et al., 2019; de Oliveira Silva et al., 2019; Puastian et al., 2000; Ward et al., 2017). So we could reasonably assume the N fertilizer can affect the CO2 emission by impacting these factors strongly and weakly. As Fig. 3 indicates, we found that the FTs had a little variation of pH and conductivity, however there was no difference significantly between FT and (NFT), so the properties might be weakly affected by N fertilizer. In Fig. 3, the amount of soil N and C could be impacted significantly by NFT, these variations could explain the difference between FT and NFT. Fertilizers can indirectly impact the respiration of crops and soil microorganisms and directly impact others, such as soil C:N (Ward et al., 2017), soil pH (Barak et al., 1997), etc. These illustrated that the process which FT impact the CO2 emission was complicated. In our study, the major factors were soil C and N comparing with other factors. Complex interactions between applied N and other nutrients have important effects on composition (Fynn et al., 2003; Kowalenko et al., 1978). According to the effects of soil N and organic C on CO2 fluxes, the essential factor affected by this action was the soil N and soil organic C concentration, aside from the top dressing, as there was a better correlation between them (Fig. 3). Our study found that there was a positive correlation between soil organic C and CO2 fluxes, with the fluxes increasing with the content of soil organic C. In comparison to the correlation between soil N and CO2 flux, organic C and CO2 flux had a stronger correlation; In our study, the CO2 emission flux increased with decreasing soil N levels; a previous study also had similar findings (Ramirez et al., 2010). In 2017, Ward found that the exhaustion of nutrients present in the decrease in soil inorganic N content may be the cause of the reduction of the CO2 fluxes to background levels (Ward et al., 2017). The relationship between soil N and CO2 flux might be differnet under different environment. Fig. 5 shows that the C: N in soil was similar among all treatments, the NFT and SF had a higher C:N value in soil; this ordering is different than what was found for cumulative CO2 emission (AF ≥ UF N SF N 0F). The phenomenon showed that the lower C: N value had a higher CO2 emission, it had a similar result in N2O emission (Ward et al., 2017). Iqbal also found that nutrients supplied via fertilizers would be

expected to affect the CO2 flux in the soil by changing the C:N in soil (Iqbal et al., 2009). The SF had a lower CO2 emission, the content of soil N and C had also different significantly with UF and AF. Analyzing the different FT, the differences could have been produced by two ways. The first way was because the different NFTs had different ways of releasing the N element: the AF treatments contain NH+ 4 that can be absorbed by crop roots and soil microorganisms; urea can be absorbed and utilized by crops only after it is hydrolyzed into ammonium carbonate or ammonium bicarbonate by urease in soil; and SF can regulate the timing of N release from fertilizer. The second way contributed to the result of fertilization methods; The slow-release fertilizer was applied all at once; the urea and amine fertilizer were applied in three phases. The SF was the basal application and released the N slowly, and the soil N in the SF treatment was significantly lower than in the UF treatment as seen in Fig. 3. Our study showed that the slow-release fertilizer had a minimum cumulative value during the sampling seasons (Table 5). The results showed very clearly that there is scope for reducing CO2 emission fluxes from N-fertilized wheat-maize rotations by selecting the form of N applied. However we could not establish a clear relationship between fertilization and CO2 fluxes due to the lack of different levels in fertilization. Small to moderate application rates would allow producers to potentially build soil organic matter and utilize nutrients most efficiently, assuming supplemental needs for inorganic fertilization could be accurately assessed. Therefore, a further long-term study from several cropping cycles should be conducted to fulfill the quest of reclaimed water. 4.2. Effect of water quality on soil CO2 fluxes Reclaimed water (RW) has been considered as one kind of unconventional water source that is transformed from industrial and domestic wastewater. Most studies attested reclaimed water can influence the soil physical and chemical properties (Lyu and Chen, 2015; Pedrero and Alarcón, 2009; Wang et al., 2007); Hence, our research considered that different types of water quality can impact CO2 emissions by directly affecting the soil environment (pH, WFPS, etc.). Although our

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Fig. 5. The mechanism in N fertilizer and reclaimed water irrigation. The solid line is the direct influence; the dotted line is the indirect influence.

study was conducted over a shorter time period than previous studies, the soil pH, electrolyte, soil N and organic C in our two-year RW treatments showed no significant changes in comparison to underground water (P N 0.05). Pedrero and Alarcón considered that wasted water could improve soil fertility and increase the content of soil DOC (Pedrero and Alarcón, 2009). Reviewing the results of CO2 emissions with reclaimed water in our study, the RW CO2 emissions were higher than those of underground water. As seen in Table 5, our study found that the cumulative CO2 emissions between 2014 and 2015 in the C0 treatment were small, but the U0 treatment had a significant decreasing trend. This may be a result of changes in soil nutrients (N, C). In contrast, between reclaimed water and underground water during the entire sampling seasons, the average content of soil C among fertilization treatments (0F, UF, AF, SF) had fallen 3.11% 7.43%,1.05% and 1.64%, respectively, and the average content of soil N among fertilization treatments (0F, UF, AF, SF) had increased 5.94%, 11.18%, 10.62% and 9.37%, respectively. 4.3. Effect of seasonal changes on soil CO2 fluxes There was a clear seasonal variation in soil CO2 fluxes depending on the soil growth stage of the wheat or maize, but our study showed that a correlation between soil temperature and CO2 emissions was not found (Fig. 2). (Dossou-Yovo et al., 2016; Peregrina, 2016) also did not find any correlation between these parameters even though most research showed there was a strong correlation between the CO2 emissions and soil temperature (Buragienė et al., 2019; Price et al., 2015). They thought CO2 emissions should increase with increasing temperature. Despite the fact that there was not any correlation between these parameters in our study, the CO2 emissions were higher with certain growth stages. The results from the no fertilizer and slow-release fertilizer treatments showed that the CO2 emissions were higher during the main growing stages (jointing stage and milking stage) than any other stage; the maturity and seedling stages had lower values of CO2 emissions (Fig. 2). Fig. 2 shows that the CO2 emission flux had a lower value from May 20 to July 20, which corresponds to the maturity period of wheat and the maize seedling stage during which the crop root has weak activity. Javed I et al. (2009) found CO2 flux variation can be attributed to the soil temperature/growth stage of rice; maximum CO2 flux from row and interrow was observed during the flowering stage of rice. In general, the seasonal trend observed in our study was similar to the results of Zou et al. (2004) and Saito et al. (2005). Through correlation analysis, we found a weak correlation (R2 b 0.3) between soil temperature and CO2 emissions, but the cumulative CO2 emission of summer maize was 4%–15% higher than winter wheat.

This relationship could be contributed to the higher fertilizer and temperature in summer. 4.4. The yield and CO2/yield It is noted from the results shown in Fig. 4 that the yield in the plots irrigated with reclaimed water was similar to those that used underground water suggesting that the treated sewage was safe for irrigation (Wang et al., 2007). Combined with yield and CO2 emission, we found the micronutrients in RW impact the soil environment year by year (Chen et al., 2013) and improve the overall nutrition of plants (Pereira et al., 2011); the nutrients in RW, aside from the additional N fertilizer input, was not enough to provide the crop growth (Wang et al., 2007). We only performed a short-term study regarding the fertilization effect on the CO2 flux from a wheat-maize rotation. However, we also attempted to estimate the threshold value of the CO2 emission flux for the treatment that had a higher yield and lower CO2 emission. Fig. 4 shows that with an increased number of cultivation years, a large number of topsoil nutrients of the cultivated land is lost, and this results in a land productivity decline, but the CO2/yield ratio improved when the reclaimed water treatment had a maximum value. Despite the finding that the reclaimed water treatment had little impact on soil properties, CO2 flux emission or yield, and focused on the U0 and C0 treatments, the CO2/yield ratio was affected significantly by reclaimed water. If there was no fertilizer added all year, nutrient elements in the reclaimed water played an important role in the crop growth and CO2 flux emissions. Considering the yield and CO2/yield, the slow-release fertilizer had a minimum value of CO2/yield and good output when compared to the urea and amine fertilizers. In our study, during the entire sampling seasons, the content of soil N and C in the slow-release fertilizer treatment could control the CO2 emission flux effectively. This contributed to the process that available N was gradually released from the slow-release fertilizer during decomposition, which was regulated by a membrane of polymer (Liang and Liu, 2006). 5. Conclusion (1) Depending on the soil growth stage of wheat and maize, there is a clear seasonal variation in soil CO2 fluxes. The jointing and milk stages are peak periods of CO2 emission. Irrigation can also significantly affect CO2 emission aside from fertilization treatments. (2) Reclaimed water did not play an important role in explaining the variations in soil CO2 emissions, which were mainly influenced by the fertilization-management. Therefore, taking into

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consideration the assessments of the variation in CO2 from agricultural soils, the reclaimed water could be used for irrigation in the wheat-maize rotation. (3) The variation in soil CO2 emissions is not influenced by fertilizer types. Regardless of irrigation water, the addition of a nitrogen fertilizer improves soil respiration. The SF that had a lower CO2 emission and better yield is the best fertilizer under this management.

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