Published online February 8, 2018
Soil Biology & Biochemistry
Improved Nitrogen Management as a Key Mitigation to Net Global Warming Potential and Greenhouse Gas Intensity on the North China Plain Tao Huang
School of Geography Sci., and Jiangsu Collab. Innov. Center of Geographical Info. Resour. Dev. & Appl., and Jiangsu Provincial Key Lab. of Materials Cycl. & Pollut. Control Nanjing Normal Univ. Nanjing 210023, China and College of Resour. & Environ. Sci. China Agricultural Univ. Beijing 100193, China
Xiaokang Hu
School of Agric. & Life Sci. Dali Univ. Yunnan 671003, China
Bing Gao
Inst. of Urban Environ. Chinese Acad. of Sciences Xiamen 361000, China
Hao Yang Changchun Huang
School of Geography Sci. Nanjing Normal Univ. Nanjing 210023, China
Xiaotang Ju*
College of Resour. & Environ. Sci. China Agricultural Univ. Beijing 100193, China
Core Ideas • Effects of improved N management on net GWP and GHG intensity were investigated. • Fertilizer production and irrigation were the main contributors to GWP. • Optimized chemical N and manure N can decrease net GWP and GHG intensity relative to conventional N. • Improved N management could realize food security and net GWP mitigation concomitantly.
Agricultural production is one of the main sources of greenhouse gas (GHG) emissions globally. Net global warming potential (net GWP) and greenhouse gas intensity (GHGI) were investigated in a maize–wheat rotation from 2011 to 2014 on the North China Plain by summarizing the net exchange of CO2 equivalent (CO2–eq) from direct and indirect GHG emissions. The experiment included four managements: no N input (N0), conventional chemical-N management (Ncon), optimized chemical N management (Nopt), and balanced chemical-N management incorporated with dairy manure (MNbal). Cumulative N2O emissions were significantly increased by N input rates. Soil organic carbon (SOC) content increased linearly in all treatments with 0.11 to 0.46 g C kg–1 yr–1 at 0 to 20 cm. The energy consumption associated with chemical N-fertilizer production and irrigation was the main contributor to GWP. Compared with Ncon, a 45% decrease in chemical-N input by Nopt brought about a 33% decrease in net GWP, but a 10% decrease in grain yield, and thus only reduced 25% of GHGI. A 40% decrease in total N application in the MNbal treatment saved chemical N by 66% and increased soil organic carbon sequestration by 45%, increased grain yield by 18%, and finally, decreased net GWP and GHGI by 48 and 56%, respectively, relative to the Ncon treatment. We conclude that the incorporation of dairy manure with chemical N is a promising strategy for reducing net GWP and ensuring food security concomitantly in these intensive maize–wheat cropping systems on the North China Plain. Abbreviations: CO2–eq, CO2 equivalent; GHG, greenhouse gas; GHGI, greenhouse gas intensity; GWP, global warming potential; MNbal, balanced chemical-N mangement incorporated with dairy manure; Ncon, conventional chemical-N managment; Nopt, Optimized chemical N management; SOC, soil organic carbon.
A
gricultural soil is responsible for about 20% of total global carbon dioxide (CO2), 12% of methane (CH4), and 60% of anthropogenic nitrous oxide (N2O) emissions (IPCC, 2007). On the other hand, the pressure of food production is increasing yearly due to rapid population growth and diet shifts (Kearney, 2010; Foley et al., 2011). Therefore, agricultural best management practices are urgently needed to reduce greenhouse gas (GHG) emissions and concomitantly achieve relatively high grain yields. Previous experiments have revealed that agricultural management practices such as fertilization, tillage, and irrigation, which change soil conditions to mitigate one component of GHG emissions, may bring about favorable conditions to stimulate other components and thus change the overall balance of GHGs (West and Marland, 2002; Mosier et al., 2006; Shang et al., 2011; Gao et al., 2014; Zhang et al., 2016a). For instance, reasonable fertilization would increase soil organic carbon (SOC) due to biomass return, but N2O emissions may be stimulated by fertilizer application (Mosier et al., 2006; Shang et al., 2011). Therefore, the con-
This article contains supplemental material. Soil Sci. Soc. Am. J. 82:136–146 doi:10.2136/sssaj2017.06.0199 Received 21 June 2017. Accepted 10 Oct. 2017. *Corresponding author (
[email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved.
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cept of net global warming potential (net GWP) has been proposed based on the radiative properties of all GHGs and carbon fixation. Net GWP is expressed as CO2 equivalents (CO2–eq) to give an integrated evaluation of whether certain agricultural practices are positive or negative to the whole system in terms of CO2–eq (West and Marland, 2002). In addition, greenhouse gas intensity (GHGI) has also been introduced to evaluate the trade-off between grain yield and net GWP (Grassini and Cassman, 2012), which could provide full-scale information for decision making. Nitrogen (N) application is considered to be one of the most important agricultural practices to increase cereal yields, but it also significantly influences the net GWP of agricultural systems (Snyder et al., 2009). However, chemical N application might stimulate soil N2O emissions by increasing electron acceptors and soil microorganism activity (Meng et al., 2005; Malhi et al., 2011), apart from GHG emissions from synthetic N compounds and transportation (Huang et al., 2013b). On the other hand, reasonable N fertilizer application could increase the SOC content owing to subsequent increase in the returned aboveground biomass (Galantini and Rosell, 2006; Triberti et al., 2008; Yang et al., 2015), and this cooling effect could partly offset or even outweigh the warming effect of soil N2O emissions (West and Marland, 2002; Shang et al., 2011; Grassini and Cassman, 2012). In addition, the effect of organic N fertilizer application and storage on net GWP in cereal production should not be ignored because an increasing number of farmers have been advised to apply organic fertilizer for higher grain yield (Pan et al., 2004). A life cycle assessment of GHG emissions from manure management gave a value of 1.18 to 1.87 kg CO2–eq per kg of fat- and protein-corrected milk in the Guanzhong Plain, China (Wang et al., 2016b). Moreover, ammonia (NH3) emissions, an important indirect N2O emission source (1 kg NH3–N could indirectly cause 0.01 kg of N2O emissions) (IPCC, 2006), could account for 50 to 83% of N losses in the process of organic fertilizer storage (Wang et al., 2016a). The North China Plain, dominated by maize–wheat cropping systems, is one of the most important cereal production areas in China. It has been suggested that chemical N fertilizer application rates in conventional farming practices are 30 to 60% above the agronomic level in this region, with rates of 263 and 325 kg N ha–1 for maize and wheat, respectively ( Ju et al., 2009). The overuse of N fertilizer can stimulate direct soil N2O emissions and indirect N2O emissions (through routes such as NH3 volatilization and nitrate leaching) (IPCC, 2006). Previous study has shown that the North China Plain produced the highest N2O emissions per unit of arable land in China due to excessive N fertilization from 1980 to 2007 (Gao et al., 2011). In addition, topsoil SOC has also increased due to synthetic N fertilizer application, straw return, and organic fertilizer input over the last two decades (Huang and Sun, 2006). Applying organic manure is an effective management practice to increase SOC sequestration because of the associated large carbon input (Khan et al., 2007; Zhang et al., 2016b). Nevertheless, previous www.soils.org/publications/sssaj
studies have suggested that manure application could increase or decrease N2O emissions simultaneously in this region (Meng et al., 2005; Cai et al., 2013; Hu et al., 2013). However, the extent to which net GWP and GHGI could be reduced following improved N management (reduced chemical N or chemical N incorporated with organic fertilizer) in this cropping system remains unclear, especially compared with conventional farming practices on the North China Plain. Therefore, the objective of our study was to investigate the long-term effects of conventional and improved N management practices, respectively, on net GWP and GHGI in an intensively managed field, thereby proposing more effective suggestions for decision makers to mitigate agricultural GHG emissions.
Materials and methods Site Description
This study was conducted at the experimental station of China Agricultural University in northwest of suburban Beijing, China (39°48¢ N, 116°28¢ E), which was started in October 2006. The site is about 40 m above sea level with a mean annual air temperature of 10 to 12°C and precipitation of 500 to 700 mm, respectively. The air temperature, soil temperature at 10-cm depth, precipitation, and irrigation during the study period are given in the supplemental material (Supplemental Fig. S1). The studied soil is a calcareous fluvo-aquic soil with a bulk density of 1.31 g cm–3 in the upper 20 cm. Other soil properties for the uppermost 20 cm were: pH 8.1 (1:2.5, soil/water), organic carbon content 7.1 g kg–1, total N 0.8 g kg–1, NO3–N 24.5 mg kg–1, NH4–N 1.20 mg kg–1, Olsen-P 7.8 mg kg–1, and available K 76.2 mg kg–1. Detailed information about the study site was presented in a previous paper (Huang et al., 2013b).
Experimental Design A randomized block design with four treatments and three replicates was adopted in the present study. The size of each plot was 8 m by 8 m, and within each plot two specific planted areas (2 m by 2 m) were used for measuring N2O emissions and nitrate leaching, respectively. Winter wheat was cultivated from early October to middle June in the next year and that of summer maize from middle June to late September (Table 1). The four treatments were: 1. Control (N0), in which there was no N input. 2. Conventional N management (Ncon), in which urea applied at rates of 300 and 260 kg N ha–1 for wheat and maize, respectively. 3. Optimized N management (Nopt), in which chemical N input was based on the synchronization of crop N demand and soil N supply (Zhao et al., 2006). Detailed rates are given in Table 2. 4. Balanced N incorporated with dairy manure (MNbal), in which chemical N fertilizer was based on N-balanced calculations (Qiu et al., 2012), namely N output (260 and 280 kg N ha–1 from summing total N uptake by
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Table 1. Schedules for tillage, sowing, fertilization, irrigation and harvest in double-cropping systems from June 2011 to June 2014. Management Tillage
2011 Maize –
2011–2012 Wheat 2 Oct. 2011
2012 Maize –
2012–2013 Wheat 3 Oct. 2012
2013 Maize –
2013–2014 Wheat 2 Oct. 2013
Sowing
19 June 2011
3 Oct. 2011
17 June 2012
4 Oct. 2012
22 June 2013
3 Oct. 2013
Basal fertilization†
–
2 Oct. 2011
–
3 Oct. 2012
–
2 Oct. 2013
Top-dressing
–
15 Apr. 2012
–
21 Apr. 2013
–
10 Apr. 2014
Four-leaf stage dressing
12 July 2011
–
14 July 2012
–
20 July 2013
–
Ten-leaf stage dressing
17 Aug. 2011
–
11 Aug. 2012
–
11 Aug. 2013
–
Irrigation‡
–
9 Oct. 2011 (40), 25 Nov. 2011 (60), 15 Apr. 2012 (60), 18 May 2012 (50)
–
17 Nov. 2012 (60), 21 Apr. 2013 (60), 12 May 2013 (60)
–
21 Nov. 2013 (60), 10 Apr. 2014 (60), 13 May 2014 (60)
Harvest¶ 1 Oct. 2011 16 June 2012 2 Oct. 2012 21 June 2013 30 Sept. 2013 20 June 2014 † Chemical fertilizers (N, P2O5, K2O) and dairy manure were applied together as basal fertilizer. ‡ Parenthetical values are irrigation (mm), which depended on soil water content. ¶Maize cob was harvested manually, and straw was returned by machine before tillage; Wheat grain and straw were harvested and mulched by machine, respectively.
aboveground and target residual NO3–N at 0- to 100cm depth for maize and wheat, respectively) minus N input (20 and 40% of Kjeldahl N from manure for maize and wheat, respectively) before 2011. After that, to alleviate disturbances from preferential flow in subsurface soil as affected by frequent soil core samples, the N-fertilizer application rates were changed based on summarized the N-fertilizer application rates from 2006 to 2011, which as follows: 180 kg N ha–1 (N uptake by the maize aboveground) minus N input (20% of Kjeldahl N from manure) in the maize season, and 170 kg N ha–1 (N uptake by the wheat aboveground) minus N input (40% of Kjeldahl N from manure) in the wheat season. Detailed input rates of chemical and organic N used in the present study are given in Table 2. Straw was returned in all treatments. The maize residue was chopped into pieces of 5 to 8 cm after harvest and was mechanically plowed into the soil before seeding. The wheat straw was mulched on the soil surface by a combined harvester. Phosphorus (as triple superphosphate), potassium (as potassium sulfate) and organic fertilizer (as dairy manure) were applied only as basal fertilizers for wheat at the rates of 160 kg P2O5 ha–1 yr–1, 90 kg K2O ha–1 yr–1, and 30 Mg ha–1 yr–1 (fresh weight), respectively. Moisture content of the organic fertilizer was determined by weighing a subsample before and after oven drying at 60°C. Kjeldahl N concentrations of dry solid dairy manure were 24.8,
20.9, 13.4, and 11.5 g kg–1 (a mixture of matured forage and dairy manure) for 2010, 2011, 2012, and 2013, respectively.
Nitrous Oxide Emission Measurements Nitrous oxide emissions were measured using static chambers and a gas chromatograph, as described by Zheng et al. (2008) and Wang et al. (2010). There were two types of static chambers because of the height of maize, which consisted of a base frame and a removable top chamber. Bigger chambers (length × width × height = 60 × 50 × 50 cm) were used to monitor N2O emissions from the wheat season and early stages of maize (before 50 cm height), which sealed by a groove filled with water. Smaller chambers (length × width × height = 50 × 30 × 20 cm) were used when the maize height higher than 50 cm. This kind of chambers was separated vertically into two parts and with a hole (diameter 11 cm) drilled in the center of the top of the chamber. These chambers allowed the cornstalks to pass through the chamber tops and so that only the maize roots were covered. The gaps between the chambers and cornstalks were sealed using a preservative film (1.2-µm-thick polyvinylidene chloride) when the chambers were closed. The two parts of chambers were sealed with rubber. The detailed information by these two types of chamber was showed in Liu et al. (2012). Emissions of N2O were measured between 08:00 and 11:00 AM on every sampling day. Four 50-mL plastic injectors were collected the gas samples at intervals of 15 min us-
Table 2. Inorganic and organic N rates used in the double-cropping systems from June 2011 to June 2014. Maize Treatment†
2011
2012
2013
Average
2011– 2012
Wheat 2012– 2013– 2013 2014
Average
2011– 2012
Annual 2012– 2013– 2013 2014
Average
–––––––––––––––––––––––––––––––––––––––––––––––––––––––– kg N ha–1––––––––––––––––––––––––––––––––––––––––––––––––––––––– N0 0 0 0 0 0 0 0 0 0 0 0 0 Ncon 260 260 260 260 300 300 300 300 560 560 560 560 Nopt 159 130 130 140 150 150 150 150 309 280 280 290 MNbal 82(55)‡ 118(64) 144(36) 115(52) 46(128) 98(72) 90(80) 78(93) 128(183) 216(136) 234(116) 193(145) † N0, control with no chemical N input; Ncon, conventional chemical N management; Nopt, optimized chemical N management; MNbal, balanced chemical N incorporated with dairy manure. ‡P arenthetical values are N input rates associated with manure application, which were estimated with conversion factors of 0.4 and 0.2 for total Kjeldahl N for the wheat and maize seasons, respectively. 138
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ing through a three-way stopcock and a Teflon tube connected to the chamber after the chambers were enclosed. Gas samples were analyzed by a gas chromatograph (model 6820, Agilent Technologies, Inc., Santa Clara, CA) equipped with a 63Ni-electron capture detector (ECD) running at 350°C. High-purity dinitrogen (N2, 99.999%) was used as the carrier gas and 10% CO2 in pure N2 as a buffering gas for the ECD (Zheng et al., 2008). The flow rate of the carrier gas was 30 mL min-1. The detection limits of the emission measurements were 2 µg N2O-N m-2 h-1. The gas samples were calibrated using known concentrations of compressed air (333 µL m-3). Based on the previous studies by Livingston et al. (2005) and Kroon et al. (2008), the initial N2O emission rate at t = 0 was calculated by linear regression or nonlinear methods according to the changing pattern of gas concentration in the headspace of the closed chamber and detailed information was showed by Huang et al. (2013b). Daily N2O measurements were performed for about 10 and 5 d after fertilizer application and rainfall or irrigation, respectively; for the remaining periods emissions were measured twice per week and once a week when the soil was frozen (Hu et al., 2013). The seasonal or annual cumulative N2O emissions were estimated from the sum of measurement and nomeasurement days which was estimated by linear interpolation (Mosier et al., 2006).
Soil Organic C and Plant Measurements Soil samples, mixed with five randomly subsamples, were determined for SOC content of 0 to 20 cm from each plot. The SOC content at the deeper depth did not change significantly from a previous study (Huang et al., 2013b). Each soil sample was sieved by a 2-mm mesh, air-dried, removed inorganic carbon by soaking in a 0.3 mol L-1 HCl solution for 24 h, oven dried at 65°C, and determined the soil organic carbon using a CN analyzer (Vario Max CN, Elementar, Hanau, Germany). We also measured the topsoil (0–20 cm) bulk density by the cutting ring method annually after the maize harvest, but this did not change significantly (data not shown). The SOC stocks (SOCstock, Mg ha-1) of 0- to 20-cm soil was calculated by:
SOCstock = c × BD × 20/10
[1]
where c and BD are the SOC content and bulk density of the topsoil (0–20 cm) in different years, respectively. The numbers 20 and 10 in the equation are the topsoil depth and the area conversion coefficients, respectively. The annual SOC sequestration rate (SOCSR , Mg C ha-1 yr-1) was estimated using the following equation:
SOC SR = SOCstock_i − SOCstock_pre-soil
[2]
where SOCstock_i and SOCstock_pre-soil are the SOC stocks in the year i and in 2006, respectively. The grain yield was measured in the specific area of 14.4 m2 (six rows 4 m in length) and 9 m2 (3 m by 3 m) from maize and wheat, respectively. The maize and wheat was separated into grains, cobs, stover and grains, straw to determine the oven-dried www.soils.org/publications/sssaj
weight at 65°C, respectively. Detailed information has been described in Huang et al. (2013b).
Calculation of Net GWP and GHGI The net GWP calculation included chemical fertilizer production, indirect N2O emissions from organic management and nitrate leaching, electricity consumption from irrigation, diesel fuel for tillage, harvest and pesticides application, soil N2O emissions, CH4 uptake and SOC sequestration rate (West and Marland, 2002; Mosier et al., 2006; Shang et al., 2011; Gao et al., 2014). In the present study, we measured N2O emissions over a 3-yr period. We adopted CH4 uptake by 1 kg CH4–C ha-1 yr-1 based on previous multi-year and site studies with the same cropping systems and climatic zone (Hu et al., 2013; Gao et al., 2014). The nitrate leaching rate was from a previous study (Huang et al. 2017). The ammonia volatilization from fertilization is almost negligible in present study because we were ditching, fertilizing and covering. The annual consumption of diesel oil by farm machinery operations was 64 and 70 L ha-1 from the straw removal and return treatments, respectively. The annual consumption of pesticide was 6.2 kg ha-1. The completed equations including coefficients for each term were: Net GWP = 8.3´(chemical N rate, kg N ha-1) + 1.51 ´ (P2O5 rate, kg P2O5 ha-1) + 0.98 ´ (K2O rate, kg K2O ha-1) + 15.8% ´ 1% ´ 265 ´ (organic N rate, kg N ha-1) + 1% ´ 265 ´ (organic N rate, kg N ha-1) + 160 ´ (organic fertilizer rate, dry, Mg ha-1) + 0.75% ´ 265 ´ (nitrate leaching rate, kg N ha-1) + 1.30 ´ (electricity, kW) ´ (time, h) + 3.94 ´ (diesel oil, L) + 18.0 ´ (pesticide, kg) + (1/28) ´ 44 ´ 265 ´ (N2O, kg N2O-N ha-1) − (1/12) ´ 16 ´ 28 ´ (CH4, kg CH4–C ha-1) − (1/12) ´ 44 ´ (SOCSR , kg C ha-1) [3] and
GHGI = (net GWP)/(grain yield)
[4]
In Eq. [3], the values of 8.3, 1.51, 0.98, 160, 1.30, 3.94, and 18.0 are the GHG emissions (kg CO2–eq kg-1) that are associated with the manufacture and transportation of fertilizer N (Zhang et al., 2013), P and K fertilizers (Huang et al., 2013a), transport and handling of manure (IPCC, 2007), production and utilization of electricity by coal combustion (Zhang et al., 2013), diesel oil combustion (Huang et al., 2013a), and pesticide production (Huang et al., 2013a), respectively. The values of 15.8%, 1% (first occurrence), 1% (second occurrence), and 0.75% are the NH3 emission factors from the process of fresh dairy manure storage in intensive systems (Huang et al., 2012), N2O emission rate induced by volatile NH3 (IPCC, 2006), direct N2O emission factor from the process of fresh dairy manure storage in intensive system (Huang et al., 2012), and indirect N2O emission factor from the leached nitrate (IPCC, 2006), respectively. The nitrate 139
leaching rate was from our previous study (Huang et al., 2017). The factors of 265 and 28 (second occurrence) were used to convert N2O and CH4 to CO2–eq based on 100 yr (IPCC, 2013). The values of 28 (second occurrence), 44 (first occurrence), 12, 16, and 44 (second occurrence) are the molecular weights of N in N2O, N2O, C in CO2, CH4, and CO2, respectively.
Statistical Analysis Grain yield, seasonal N2O emissions, and annual N2O emissions of the different treatments were tested by analysis of variance, and mean values were compared by least significant difference (LSD) at the 5% level using the SAS statistical software package (Version 8.2; SAS Inst., 2001). Log transformations were applied as necessary to meet the assumptions of ANOVA.
Results
N Input Rate and Grain Yield Compared with Ncon, the practice of Nopt reduced chemical N-fertilizer input by 46, 50, and 48% in maize, in wheat, and annually, respectively, and MNbal reduced chemical N input by 56, 70, and 66% in maize, in wheat, and annually, respectively. The total N input rate (inorganic N plus organic N) in MNbal
outweighed that in Nopt by 19, 21, and 17% in maize, in wheat, and annually, respectively (Table 2). Compared with N0, Ncon significantly increased maize, wheat, and annual grain yield by 42 to 62, 84 to 142, and 41 to 67%, respectively (P < 0.05). The mean annual grain yield from Nopt was decreased by 10% relative to Ncon (P < 0.05). The mean annual grain yield from MNbal was 18 and 31% higher than that from Ncon and Nopt, respectively (P < 0.05) (Fig. 1).
Soil Organic C Content and Stock During the 7 yr of investigation, SOC content increased linearly in all treatments (P < 0.05), with an annual increasing rate of 0.11, 0.32, 0.21, and 0.46 g C kg-1 soil for N0, Ncon, Nopt, and MNbal, respectively (Fig. 2). After 7 yr of amendments, the 0 to 20 cm soil profile accrued 22.4 Mg C ha-1 (N0), 26.3 Mg C ha-1 (Ncon), 24.1 Mg C ha-1 (Nopt), and 27.9 Mg C ha-1 (MNbal), and the magnitude of SOC accrual in MNbal was the largest of the four treatments (P < 0.05).
Nitrous Oxide Emissions Nitrous oxide emission rates in N0 were consistently low over the entire experimental period (Fig. 3A). Episodes of
Fig. 1. Grain yield and N2O emissions throughout the experimental period: (A, E) 2011 to 2012, (B, F) 2012 to 2013, (C, G) 2013 to 2014, and (D, H) annual average. Treatments are a control with no chemical N input (N0), conventional chemical N management (Ncon), optimized chemical N management (Nopt), and balanced chemical N incorporated with dairy manure (MNbal). Vertical bars are the standard deviation of three replicates. Different lowercase letters indicate significant differences (P < 0.05) among the four treatments within each of three categories: maize season, wheat season and annual. 140
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high N2O emission events were observed following the input of N in Ncon (Fig. 3B), Nopt (Fig. 3C), and MNbal (Fig. 3D), which lasted for approximately 10 d. When fertilization was coupled with rainfall or irrigation, massive pulses of N2O emission were observed (Fig. 3). The 3-yr mean-annual cumulative N2O fluxes were 0.60 kg N2O-N ha-1 (ranging from 0.43 to 0.83 kg N2O-N ha-1), 3.67 kg N2O-N ha-1 (ranging from 2.13 to 4.54 kg N2O-N ha-1), 2.00 kg N2O-N ha-1 (ranging from 1.20 to 2.65 kg N2O-N ha-1), and 3.40 kg N2O-N ha-1 (ranging from 2.18 to 4.56 kg N2O-N ha-1) for N0, Ncon, Nopt, and MNbal, respectively. The mean-annual cumulative N2O emissions for Ncon and MNbal were significantly higher than those of Nopt (P < 0.05), but there was no significant difference between Ncon and MNbal (P > 0.05) (Fig. 1). Cumulative N2O emissions increased linearly with N input rate, which explained the 60.3, 67.6, and 68.2% of the variation in N2O emissions in the maize season, in the wheat season, and in annual N2O emissions, respectively (P < 0.01). Nitrous oxide emission factors of N fertilizer were 0.6, 0.5, and 0.5% in the maize season, in the wheat season, and annually, respectively (Fig. 4).
Net GWP and GHGI The results show that all treatments induced positive net GWP (Table 3), and N management had a great influence on net GWP. The chemical N-fertilizer application (including direct N2O emissions and indirect GHG emissions from chemical N-fertilizer production) and irrigation (indirect GHG emissions from generated energy) were the two main sources of the GHG effects in the fertilized treatment, at 41.1 to 68.3% and 21.9 to 32.8%, respectively. The net GWP of N0 was 1583 kg CO2–eq ha-1, of which the positive GWP was 2682 kg CO2–eq ha-1, including a 6% contribution from N2O emissions, 12% from fertilizer production (0% from N), 67% from power, and 15% from nitrate leaching, fuel and pesticides, whereas the negative GWP was 1098 kg CO2–eq ha-1, including 2 and 98% from CH4 uptake and SOC sequestration, respectively (Table 3). Compared with N0, the net GWP of Ncon increased by 3565 kg CO2–eq ha-1, of which the positive GWP increased by 5544 kg CO2–eq ha-1 (increased chemical-N production accounted for 84% of the increased positive GWP, while increased N2O emissions and nitrate leaching accounted for 15 and 1%, respectively). On the other hand, the negative GWP increased by 1979 kg CO2–eq ha-1 due to increased SOC sequestration. Compared with Ncon, the net GWP of Nopt decreased by 1681 kg CO2–eq ha-1, with the positive GWP decreased by 2739 kg CO2–eq ha-1 (chemical N production, N2O emissions, and nitrate leaching accounted for 82%, 16%, and 2% of the decreased positive GWP, respectively), the negative GWP decreased by 1058 kg CO2–eq ha-1 due to SOC sequestration. The MNbal approach reduced the net GWP by 3548 kg CO2–eq ha-1, relative to Ncon, of which the reduced positive GWP consisted of 72 kg CO2–eq ha-1 from N2O emissions, 3046 kg CO2–eq ha-1 from N fertilizer production, and 40 kg CO2–eq ha-1 from nitrate leaching, and www.soils.org/publications/sssaj
Fig. 2. Changes in soil organic carbon (SOC) at 0- to 20-cm depth in the double-cropping systems from 2006 to 2013. Treatments are a control with no chemical N input (N0), conventional chemical N management (Ncon), optimized chemical N management (Nopt), and balanced chemical N incorporated with dairy manure (MNbal). Vertical bars are the standard deviation of three replicates. Data for N0, Ncon, and Nopt from 2006 to 2012 were retrieved from a previous paper (Huang et al., 2013b).
the increased negative GWP was 390 kg CO2–eq ha-1, which consisted of 1374 kg CO2–eq ha-1 from increased SOC sequestration, while it was offset by 1970 kg CO2–eq ha-1 from manure management. The GHGI of N0, Ncon, Nopt, and MNbal was 220, 429, 321, and 188 kg CO2–eq Mg-1 grain, respectively (Table 3). The GHGI of Ncon was 1.95 times as high as that of N0, which was mainly caused by increased N2O emission and N fertilizer production.
Discussion
Effect of N Management on Soil Emissions Synthetic N fertilizer application in agriculture was considered as the primary reason for atmospheric N2O escalation, which accounts for ~50% of total anthropogenic N2O emissions (IPCC, 2007). The availability of soil N for soil microbes increased sharply after applying chemical N fertilizer, which stimulated the growth and activity of soil microbes, and, consequently, N2O emissions increased rapidly (Meng et al., 2005; Huang et al., 2014). A meta-analysis indicated that the effect of N input rates on N2O emissions could divide into three steps: linear, exponential, and steady state (Kim et al., 2013). Our findings agree with the first step and show that N2O emissions usually increase linearly with increased chemical N-fertilizer rate (Fig. 4), and thus reducing the chemical N rate to an appropriate level could substantially decrease N2O emissions in cropping rotations (Liu et al., 2015; Zhang et al., 2016a). Annual N2O emissions decreased by 31 to 44% when the chemical N input rates were decreased from conventional N (560 kg N ha-1 yr-1) to optimized N (290 kg N ha-1 yr-1) and balanced N (338 kg N ha-1 yr-1) based on the linear equation from six N rates in the same study region (Liu et al., 2012), which is similar with the 37 to 56% reduction in our study (Fig. 1). Therefore, soil N2O emission rates could be linearly reduced by modified N management with 141
Fig. 3. Nitrous oxide (N2O) fluxes throughout the experimental period of June 2011 to June 2014 in the double-cropping system. Treatments are a control with no chemical N input (N0), conventional chemical N management (Ncon), optimized chemical N management (Nopt), and balanced chemical N incorporated with dairy manure (MNbal). Dotted arrows, thin arrows, and thick arrows denote the timing irrigation, fertilizer N application, and tillage, respectively. Vertical bars are the standard deviation of three replicates. Data for N0, Ncon, and Nopt from June 2011 to June 2013 were retrieved from a previous paper (Huang et al., 2013b).
reduced, synthetic N rates and increased N use efficiency in our studied region. Currently, there is no consensus with respect to the effects of organic fertilizer incorporated with chemical N fertilizer on soil N2O emissions. A study in Tsukuba, in the Japanese uplands, indicated that annual N2O emissions increased by 84 and 615% because of poultry manure and pelleted poultry manure input, 142
respectively, compared with inorganic fertilization (Hayakawa et al., 2009). Li et al. (2013) also showed that, compared with pure chemical fertilization, the cumulative N2O emissions induced by a combination of pig manure and chemical fertilizer were increased by 78% in northeast China. However, some studies also revealed that combined organic and chemical fertilizer did not increase N2O emissions (Webb et al., 2014), and what’s more, it Soil Science Society of America Journal
Fig. 4. Relationships between chemical N input rate and cumulative N2O emissions in the (A) maize season, (B) in the wheat season, and (C) annually throughout the experimental period of June 2011 to June 2014 in the double-cropping system. Treatments are a control with no chemical N input (N0), conventional chemical N management (Ncon), optimized chemical N management (Nopt), and balanced chemical N incorporated with dairy manure (MNbal). Vertical bars are the standard deviation of three replicates.
could decrease N2O emissions (Cai et al., 2013). The results of the present study are consistent with Webb et al. (2014) showing that chemical N incorporated with manure did not significantly increase annual N2O emissions (Fig. 1 and 3). This was mainly attributed to the annual total N input rates from MNbal (338 kg N ha-1, Table 2) was much lower than the crop aboveground uptake (420 kg N ha-1, Huang et al., 2017) in the present study. In addition, organic fertilizer input always stimulates the denitrification process for N2O production, but nitrification and nitrifier denitrification were the main N2O processes in this kind of soil (Cui et al., 2012; Huang et al., 2014).
Effect of N Management on Soil C Sequestration Our results agree with previous studies indicated that SOC sequestration rates would increase by synthetic N application rate increasing (Ncon vs. Nopt, Fig. 2) because the great aboveground biomass returned (5.4 Mg C ha-1 yr-1 vs. 4.8 Mg C ha-1 yr-1, data from another under viewed paper) (Triberti et al., 2008; Zhang et al., 2010). However, the potential of SOC increasing by long-term higher chemical N application would lower than the reasonable chemical N input because of soil acidification (Guo et al., 2010), especially the higher chemical N rates far more than crop uptake. The highest SOC sequestration was from the chemical N incorporated with organic manure (MNbal) in present study (Fig. 2). Many long-term field experimental studies consistently demonstrated that organic manure application could effectively enhance SOC contents because of directly C input and more aboveground and root biomass across climate re-
gions and cropping systems (Galantini and Rosell, 2006; LópezBellido et al., 2010; Zhang et al., 2010; Malhi et al., 2011; Zhang et al., 2016a). However, we also paid attention to the fact that the accumulation of SOC would slow down when soil was close to its saturation (West and Six, 2007). In the present study, the SOC content of MNbal at 0- to 20-cm soil depth in 2013 was 10.9 g C kg-1 after 7 yr of fertilization, which was much lower than SOC content (26.7–28.9 g C kg-1) at the same depth after 15 yr of N application (200–600 kg N ha-1) in same region (Dong et al., 2016). It indicated that this kind of soils in northern China was not SOC saturated. Furthermore, it is implied that whether or not SOC was sequestrated is mainly dependent on the balance between C inputs and outputs as influenced by agronomic managements (e.g., fertilization, tillage, straw return) (Khan et al., 2007). Therefore, increasing C inputs by improved N management practices could continue to sequestrate SOC in the agricultural soil of this region. In general, the initial SOC sequestration rates were much higher than the later (West and Marland, 2002; Pan et al., 2004). For instance, Fan et al. (2014) showed that the SOC sequestration rate decreased from 0.90 Mg C ha-1 yr-1 in 1989 to 1994 to 0.29 Mg C ha-1 yr-1 in 2004 to 2009 for the compost manure treatment in the same region. The latest model (DNDC, Denitrification–Decomposition) of study also reported that SOC only significantly increased after 20 yr of continuous fertilization with straw (Zhang et al., 2017). On the other hand, the soil sample depth was only 20 cm in the present study, which could overestimated SOC sequestration as deeper soil also plays
Table 3. Carbon dioxide equivalent of agricultural management practices, net global warming potential (GWP), and greenhouse gas intensity (GHGI) in the double-cropping systems from June 2011 to June 2014.
Treatment†
N2O
CH4
SOC
Chemical fertilizer Organic Fuel used production fertilizer Nitrate by farm Pesticide N P2O5 K2O management leaching Irrigation machinery production Net GWP
GHGI
–––––––––––––––––––––––––––––––––––––––––––– kg CO2–eq ha-1 –––––––––––––––––––––––––––––––––––––––––––– kg CO2–eq Mg-1 N0 159 22 1076 0 242 88 0 5 1801 276 112 1583 220 Ncon 973 22 3055 4648 242 88 0 87 1801 276 112 5148 429 530 22 1998 2407 242 88 0 33 1801 276 112 3467 321 Nopt MNbal 901 22 4429 1602 242 88 1970 47 1801 356 112 2666 188 † N0, control with no chemical N input; Ncon, conventional chemical N management; Nopt, optimized chemical N management; MNbal, balanced chemical N incorporated with dairy manure. www.soils.org/publications/sssaj
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a role in the changes of total SOC stocks (Zhang et al., 2010). A 32-yr study on sandy loam (Typic Ustipsament) by Rasool et al. (2008) in India suggested that a significant increase in SOC was only found at 0- to 15- and 15- to 30-cm soil depths, but not at 30- to 45- and 45- to 60-cm soil depths. Moreover, López-Bellido et al. (2010) showed that the effects of tillage and crop rotation on SOC sequestration at 30 to 90 cm were higher than those at 0 to 30 cm over a 20-yr period on Vertisol (Typic Haploxererts) in Spain. Therefore, experimental duration and sampled soil depths must be taken into account and more work is needed to conclude the effect of nitrogen management on SOC sequestration in the intensively managed soil on the North China Plain.
Effect of N Management on Net GWP and GHGI Compared with the conventional N management (Ncon), the reasonably reduced N management by integrating the N demand of crops and N supply (Nopt) reduced chemical N fertilizer by 270 kg N ha-1 yr-1, significantly decreased annual N2O emissions by 45.5%, energy consumed from fertilizer production by 48.2%, SOC sequestration by 34.6%, and consequently net GWP by 32.6% (Fig. 1; Table 3). Similarly, when the chemical N rate was reduced by 25 and 15%, the net GWP decreased by 13.7 and 20.3% in a rice–wheat rotation in South China, respectively (Ma et al., 2013). Mosier et al. (2006) also indicated that net GWP could decrease by 21.4 to 28.0% if the chemical N input were reduced 33.6 to 40.2% in conventional-till continuous corn in the United States. Relative to the Ncon, though Nopt significantly decreased annual grain yield by 10% (Fig. 1), but the GHGI still reduced by 25.2% because of a 32.6% net GWP reduction (Table 3). Moreover, Liu et al. (2015) indicated that the grain yield could also increase by 11.5% although with a 15% chemical N savings because of optimizing application ratio and increasing plant density in a double rice cropping system in Central China, which decreased GHGI by 29.3%. The positive GWP from fertilization (consist of fertilizer manufacture, N2O emissions and organic manure management) of MNbal was 3817 kg CO2–eq ha-1, which was higher than that of Nopt (3267 kg CO2–eq ha-1), but lower than that of Ncon (5951 kg CO2–eq ha-1) (Table 3). However, the negative GWP from SOC sequestration due to nitrogen management was in the order: MNbal (4429 kg CO2–eq ha-1) > Ncon (3055 kg CO2–eq ha-1) > Nopt (1998 kg CO2–eq ha-1) (Table 3). The annual grain yield was also in the same order (Fig. 1). Therefore, the net GWP and GHGI from MNbal could be reduced by 54 to 69% and 65 to 74%, relative to Nopt and Ncon, respectively (Table 3). Many previous studies on rice– wheat rotation in Eastern China (Yang et al., 2015), double-rice in Central China (Shang et al., 2011), and intensive vegetable cropping systems in southeastern China (Zhang et al., 2016a) have consistently demonstrated that incorporated organic fertilizer with chemical N could simultaneously achieve high grain yields and less GHG emissions, and consequently lower GHGI. All of these findings suggest that MNbal is an effective N man-
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agement practice to get more grain yield with less net GWP in this maize–wheat double-cereal cropping system. Overall, the MNbal seems to be a suitable nitrogen management with the lowest net GWP and GHGI, a highest SOC sequestration and grain yield (Table 3; Fig. 1). However, the storage and management of organic manure also played an important role in determining the effects of organic fertilizer on net GWP and GHGI in the present study. For example, we adopted the emissions factors of 1 and 15.8% for N2O and NH3, respectively, for dairy manure, because our dairy manure was slurry during the storage phase (Huang et al., 2012). However, if the organic fertilizer is stored in solid form, the emissions factors are much higher (8% for N2O and 4.2% for NH3) (Huang et al., 2012), and the net GWP and GHGI for MNbal increase sharply to 8515 kg CO2–eq ha-1 and 600 kg CO2–eq Mg-1 grain, respectively. A Chinese inventory of N2O emissions from 2008 showed that N2O emissions from manure management were 550.8 Gg N2O, which was 26% of national N2O emissions (2150 Gg N2O) and 40% of agricultural emissions (1375.2 Gg N2O) in particular (Zhou et al., 2014). Therefore, paying attention to the management of organic fertilizer could further reduce the net GWP and GHGI from this doublecropping system.
Conclusions The present study indicated that chemical N fertilizer manufacture and energy consumption associated with irrigation were the two main sources of GHG effects in the highly intensive agricultural system with wheat and maize double-cropping rotation on the North China Plain. The GHG effects of N-fertilizer application were derived mainly from the process of N-fertilizer production and transportation, whereas N2O emissions induced by N fertilizer were the secondary contributor. The soils have a large capacity to accrue SOC and thereby sequester CO2. The input of N fertilizer promoted SOC accrual, but the GHG effects associated with N-fertilizer input outnumbered the mitigated GHG effects induced by SOC accrual, and thus the net GWP associated with N-fertilizer input was positive. Organic fertilizer incorporated with chemical N fertilizer, thus reducing the GHG effects dramatically. Moreover, the input of organic fertilizer substantially increased the accrual of SOC and crop yields, reducing net GWP and GHGI substantially. Therefore, integrating the management of chemical N fertilizer and organic fertilizer into this intensive agricultural system is a promising strategy to reduce chemical N-fertilizer input, promote C sequestration in soil and increasing crop yields, thereby reduce net GWP and GHGI. Nonetheless, further investigations are needed to improve the amounts of applied N provided by organic and chemical N fertilizers, thereby optimizing the trade-off between GHG emissions and food security.
Supplemental Material
A supplemental figure is available with the online version of this article. Supplemental Fig. S1 contains meteorological data (air temperature, soil temperature, precipitation, and irrigation) for the study period of June 2011 to June 2014. Soil Science Society of America Journal
Acknowledgments
This work was funded by the National Natural Science Foundation of China (Grant No. 41503075), and the National Program on Key Basic Research Project (973 Program) (Grant No. 2014CB953800), the China Postdoctoral Science Foundation Funded Project (Grant No. 2015M581826), the National Key Research and Development Program of China (2016YFD0800102), and the Special Fund for the Agricultural Profession (201503106), Newton Fund (Grant Ref: BB/ NO13484/1). Two anonymous reviewers are acknowledged for their useful comments that greatly improved the manuscript.
conflict of interest statement The authors declare that there is no conflict of interest.
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