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Net Greenhouse Gas Balance in China’s Croplands over the Last Three Decades and Its Mitigation Potential Wen Zhang,† Yongqiang Yu,† Tingting Li,† Wenjuan Sun,‡ and Yao Huang*,‡ †

LAPC, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China LVEC, Institute of Botany, Chinese Academy of Sciences, Beijing, 100093, China

‡

S Supporting Information *

ABSTRACT: Cropland soils have been shown to emit nitrous oxide (N2O) and methane (CH4) into the atmosphere and to sequester carbon when field management is improved, yet the spatiotemporal changes in the N2O and CH4 emissions and the soil organic carbon (SOC) in China’s croplands are unclear with regard to an integrated global warming potential (GWP). This limits our overall evaluation of anthropogenic greenhouse gas (GHG) emissions and impairs effective decision making. On the basis of model simulations primarily from 1980 to 2009, we estimated a 69% increase in the gross GWP of CH4 and N2O emissions, from 244 Tg CO2-equiv yr−1 in the early 1980s to 413 Tg CO2-equiv yr−1 in the late 2000s. The SOC was estimated to have increased from 54 Tg CO2-equiv yr−1 to 117 Tg CO2-equiv yr−1 during the same period. A reduction in the carbon input during the rice season, along with an improvement of synthetic nitrogen use efficiency in crops to 40%, would mitigate GHG emissions by 111 Tg CO2-equiv yr−1 and keep SOC sequestration at 82 Tg CO2 yr−1. Together, this would amount to a reduction of 193 Tg CO2equiv yr−1, representing ∼47% of the gross GWP in the late 2000s. The mitigation of GHG emissions in Henan, Shandong, Hunan, Jiangsu, Hubei, Sichuan, Anhui, Jiangxi, Guangdong and Hebei Provinces could lead to a ∼66% national improvement and should be given priority.

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INTRODUCTION

and N fertilization lead inevitably to GHG emissions. Nevertheless, appropriate management can reduce GHG emissions. Agricultural soils in China have been cultivated over a long period of time and may have lost 30 to 50% or more of the antecedent soil organic carbon (SOC) pool.10 The adoption of recommended management practices, such as crop residue retention and no-till cultivation, could resequester some of the depleted SOC pool.11,12 These practices could offset climate radiative effects from CH4 and N2O emissions. At a national scale in China, great efforts have been made in the past decade to estimate CH4 emissions from rice paddies,13,14 N2O emissions from croplands,8,15 and C sequestration in agricultural soils.16,17 Researchers have also been searching for mitigation options for GHG emissions, primarily at the site level.18,19 We previously estimated the changes in CH4 emissions from irrigated rice cultivation,14 the direct N2O emissions induced by synthetic N fertilizers,8 and the SOC change in the croplands of China,20 yet the spatiotemporal changes in the N2O and CH4 emissions and soil carbon sequestration are unclear with regard to an integrated global warming potential (GWP) in China’s croplands. This shortcoming limits our overall evaluation

Along with carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the atmosphere are two critical greenhouse gases (GHG) because of their potent impact on global warming. Of global anthropogenic emissions in 2005, agriculture accounts for about 60% of N2O and about 50% of CH4.1 Irrigated rice cultivation is a major source of anthropogenic terrestrial CH4.2 The primary source of N2O emissions from agricultural soils is nitrogen (N) fertilizer.3 Agricultural soils sequester carbon (C) when field management is improved,4 thus offsetting global warming potential (GWP) from the CH4 and N2O emissions. Agriculture in China utilizes only 7% of the world’s arable land area while feeding 22% of the global population. Rice harvest areas in China occupy 29 M ha (1 M = 106), making up 19% of the world’s total.5 Substantial growth in the use of synthetic N fertilizer since the 1970s has contributed to the national food security. China, the largest consumer of synthetic N in the world, used 32% of the world’s yearly total during the period of 2006−2008,6 The overuse of synthetic N fertilizers has become widespread across China,7 resulting in a rapid increase in N2O emissions8 that has outpaced increases in crop production.9 An improvement in crop productivity is essential to meet the food demands of an increasing population in China, whereas agricultural practices such as irrigated rice cultivation © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2589

September 30, 2013 January 25, 2014 February 10, 2014 February 10, 2014 dx.doi.org/10.1021/es404352h | Environ. Sci. Technol. 2014, 48, 2589−2597

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Figure 1. Harvest area (a), irrigation area and water consumption (b), nitrogen application (c), and organic matter input (d).

and validation of the CH4MOD and Agro-C models is given in the Supporting Information (SI). Calculation of GWP and Assessment of Uncertainty. We calculated the net GWP from the CH4 and N2O emissions and the change in SOC by

of anthropogenic GHG emissions from China’s croplands and thus impairs effective decision regarding mitigation. The objectives of this paper are to evaluate the spatiotemporal changes in GWP across China’s croplands and to assess the options for mitigation so that policymakers can make sensible region-oriented mitigation decisions.

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16 44 × 25 + E N2O − N × × 298 12 28 44 − ΔSOC × 12

GWP = ECH4 − C ×

MATERIALS AND METHODS Changes in N2O and CH4 emissions and in the SOC between 1980 and 2009 were estimated. N2O emissions were calculated on an annual basis. CH4 emissions and the changes in SOC were computed using the CH4MOD model21,22 and the Agro-C model23 with a daily step. Estimation of N2O Emissions from Croplands. We focused on the fertilizer N-induced direct N2O emissions from croplands. Following Lu et al.,24 the direct N2O emissions from upland crop seasons were calculated by E N2O − N = 0.0186( ± 0.0027) × P × Ninput

(2)

where ECH4−C, EN2O−N and ΔSOC are the annual rates of CH4 emissions, N2O emissions and the change in SOC respectively; the fractions 16/12, 44/28, and 44/12 are used to convert the mass of CH4−C to CH4, the mass of N2O−N to N2O and the mass of ΔSOC to CO2, respectively; the constants 25 and 298 are the radiative forcing constants of CH4 and N2O, respectively, relative to CO2 at a 100-year time horizon;25 ΔSOC > 0 signifies SOC sequestration and the resulting offset of GWP from CH4 and N2O; and ΔSOC < 0 signifies SOC loss and the addition of GWP to the gross GWP. Uncertainties in the estimates of GHG emissions and SOC change were assessed. Detailed description of the uncertainty assessment is provided in the Supporting Information (SI). Mitigation Options. With the assumption that no additional OM input occurred except for the retention of crop stubbles and roots in the rice season but there was OM input in the off-rice upland crop seasons as normal, we simulated CH4 emission and SOC change in the rice-based cropping system between 1980 and 2009, using the integrated CH4MOD/Agro-C model. The overuse of synthetic N fertilizer is a serious environmental problem across China,7 resulting in high N2O emissions and the loss of N into the environment.3 The synthetic N use efficiency (NUE) in the late 1990s/early 2000s was approximately 26−29%,26,27 much lower than the average of 42−49% in developed countries.28 Using Huang and Tang,27 we estimated the reduction of synthetic N-induced direct N2O emissions in the area where the NUE was lower than 40%.

(1)

where EN2ON (kg N2O−N ha−1) is the direct N2O−N emissions induced by N fertilizers; Ninput (kg N ha−1) is the application rate of N fertilizers, including synthetic fertilizers, farm manure (FM) and crop residues; and P is the annual precipitation (in meters) in the rain-fed uplands or the annual precipitation plus irrigation when irrigation was used in the upland crop seasons. The term “0.0186 × P” is referred to as the N-induced emission factor as in IPCC.2 The value “±0.0027” is the 95% confidence interval (CI) of the emission factor. For irrigated rice fields, the default emission factor of 0.003 (uncertainty range 0.000− 0.006) was adopted from the IPCC2 to estimate the direct N2O emissions from N fertilizers. Estimation of CH4 Emissions from Rice Paddies and the SOC Change in Croplands. Methane emissions from rice paddies and the SOC change in croplands were estimated using the CH4MOD and Agro-C models, respectively. We integrated CH4MOD and Agro-C (CH4MOD/Agro-C) in a practical manner when the simulations of CH4 emissions and SOC change were implemented. A more detailed description about the structure 2590

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a

2005−2009

2000−2004

1995−1999

1990−1994

1985−1989

138.1 (73.2−205.4)d 144.0 (75.7−212.4) 150.7 (81.4−228.3) 159.2 (88.1−247.2) 152.2 (88.7−248.7) 161.0 (95.5−267.8) 1980−1984

Calculated using eq 2. bSummation of the GWP from CH4 and N2O emissions. cSummation of the net GWP_R and net GWP_U. d95% confidence interval.

244 (158−333) 273 (178−368) 322 (219−434) 369 (257−497) 379 (273−517) 413 (302−565) 57.8 (30.3−86.7) 72.7 (41.5−105.4) 89.3 (48.8−131.9) 97.1 (45.3−151.3) 103.4 (45.2−164.6) 119.7 (54.0−189.1) 24.1 (7.1−39.7) 26.4 (8.1−43.2) 42.9 (19.5−64.2) 62.9 (31.9−91.4) 72.1 (36.4−104.8) 78.4 (37.8−115.4) 81.9 (70.0−93.8) 99.1 (84.7−113.5) 132.2 (113.0−151.4) 160.0 (136.7−183.2) 175.5 (150.0−201.0) 198.1 (169.4−226.9) 132.1 (37.8−230.5) 142.9 (42.2−245.5) 152.6 (49.4−265.9) 168.8 (62.9−293.2) 164.1 (66.3−296.4) 176.1 (75.8−319.6)

ΔSOC N2O CH4 period

N2O

ΔSOC

net GWP_R

a

rice-based cropping systems

Table 1. Estimated GWPs (Tg CO2‑equiv yr−1) from China’s Croplands during Different Periods

upland cropping systems

net GWP_Ua

RESULTS Trends in the Cropping System, Nitrogen and Carbon Input. Whereas the arable area decreased from 135 Mha in the early 1980s to 122 Mha in the late 2000s, the harvested area increased from 145 Mha to 155 Mha during this time period (SI, Table S1). This difference was due to the intensification of crop cultivation, particularly the expansion of vegetable crops and orchards (Figure 1a; SI, Table S2). The irrigation area expanded from 45 Mha to 57 Mha during this time, though the consumption of irrigation water decreased slightly (Figure 1b) as a result of an improvement in water use efficiency29,30 and a reduction in the rice harvest area. The application of N fertilizers increased remarkably, from 19 Tg N yr−1 in the early 1980s to 41 Tg N yr−1 in the late 2000s (Figure 1c) with an annual growth rate of ∼0.9 Tg yr−1, of which the N from manure and crop residues accounted for 30−45% (SI, Table S3). The increase in the use of synthetic N fertilizers was pronounced, with an approximately 2.6-fold increase between the early 1980s and the late 2000s (Figure 1c). Largely due to an increase in crop residue retention (SI, Table S3), the total amount of OM input increased from 196 to 333 Tg C yr−1 (Figure 1d), with an annual growth rate of ∼5.3 Tg C yr−1. Temporal Characteristics of GHG Emissions and SOC. Methane emissions from irrigated rice cultivation increased from 138 Tg CO2-equiv yr−1 (1 Tg = 1012 g) in the early 1980s to 161 Tg CO2-equiv yr−1 in the late 2000s (Table 1), which was mainly due to improved rice production and OM incorporation. Accompanying the increasing application of N, the fertilizer N-induced direct N2O emissions increased greatly from 106 Tg CO2-equiv yr−1 in the early 1980s to 252 Tg CO2equiv yr−1 in the late 2000s (Table 1) with an annual growth rate of ∼6 Tg CO2-equiv yr−1. The gross GWP of the CH4 and N2O emissions was estimated to be 244 (95% CI, 158−333) Tg CO2-equiv yr−1 in the early 1980s and 413 (302−565) Tg CO2-equiv yr−1 in the late 2000s; overall, this measure increased by 69%. The incremental increase in N2O emissions is responsible for ∼86% of the increase in gross GWP. This increase was more pronounced from the 1980s to the 1990s than in the 2000s due to a remarkable growth in N2O emissions (Table 1). Rice-based cropping systems emitted more than half of the gross GWP (Table 1), although these areas accounted for only 19−23% of the total cropland. This disproportionate contribution is due not only to CH4 emissions during the rice growing season but also to N2O emissions in both rice and offrice upland crop seasons (e.g., double-cropping systems with an annual rotation of rice and upland crops). N2O emissions accounted for 43% of the gross GWP in the early 1980s, increasing to 61% in the late 2000s. It is noteworthy that the upland cropping systems were responsible for the majority of N2O emissions, accounting for 77−79% of the total fertilizer N-induced N2O emissions over the same period (Table 1). Furthermore, the N2O emissions from upland

30.3 (9.0−49.9) 31.0 (9.5−50.7) 37.6 (17.0−56.4) 39.7 (20.1−57.6) 39.1 (19.7−56.9) 38.5 (18.5−56.7)

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24.3 (14.5−34.1) 29.9 (17.2−42.5) 39.5 (24.4−54.6) 49.3 (32.4−66.1) 51.0 (34.5−67.4) 53.6 (37.0−70.3)

gross GWPb

net GWPc

Compilation of Input Data. The main input data used to run the integrated Agro-C/CH4MOD model and to estimate N2O emissions included climate, soil, and farming management. Up-scaling was completed by rasterizing the inputs with 10 km × 10 km gridded data sets across China. Each grid included a set of model inputs, and the model ran grid by grid across the area of interest. A more detailed description about the compilation of input data can be found in the SI.

190 (68−317) 216 (84−351) 242 (98−398) 266 (108−445) 267 (112−461) 296 (130−509)

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Figure 2. Spatial distribution of GWP (Gg CO2-equiv yr−1 per 10 km × 10 km grid) averaged over 30 years: N2O (a), CH4 (b), −ΔSOC (c), and net GWP (d) as calculated by eq 2.

cropping systems increased rapidly from 82 Tg CO2-equiv yr−1 in the early 1980s to 198 Tg CO2-equiv yr−1 in the late 2000s, representing an increase of approximately 2.4-fold. The organic carbon in the cropland soils was estimated to have increased from 54 (16−90) Tg CO2-equiv yr−1 in the early 1980s to 117 (56−172) Tg CO2-equiv yr−1 in the late 2000s (Table 1). Averaging for each decade, the amount of carbon sequestered was estimated to be 56, 92, and 114 Tg CO2-equiv yr−1 in the 1980s, 1990s, and 2000s, respectively. Upland cropping systems contributed about two-thirds to the total amount of carbon sequestered in the late 2000s (Table 1). On a national scale, the carbon sequestered in the cropland soils offset 22−28% of the gross GWP. In upland cropping systems, 27−41% of the gross GWP was offset by SOC sequestration. In contrast, the SOC sequestration offset 18−20% of the gross GWP in rice-based systems. Considering SOC sequestration, the net GWP in the early 1980s was estimated to be 190 (68−317) Tg CO2-equiv yr−1, and it increased by ∼55% by the late 2000s to 296 (130−509) Tg CO2−equiv yr−1 (Table 1). Spatiotemporal Characteristics of the GWP. Higher CH4 and N2O emissions occurred in the regions east of 110° E, with a maximum of ∼73 Gg CO2-equiv yr−1 grid−1 (1 Gg = 109 g) for CH4 and ∼27 Gg CO2-equiv yr−1 grid−1 for N2O, while showing latitudinal differences. The region between 30° N and 40° N exhibited higher N2O emissions (Figure 2a), and the region between 25° N and 33° N showed higher CH4 emissions (Figure 2b). Both CH4 and N2O emissions were higher in the region between 30° N and 33° N, where ricebased cropping systems with a rice/upland crops rotation dominated.

The SOC in ∼81% of China’s croplands increased between 1980 and 2009 (Figure 2c). The most significant increase occurred in eastern China, whereas cropland soils in northeast China lost carbon. Figure 2d outlines the spatial distribution of the net GWP. Similar to the spatial characteristics of N2O and CH4 emissions (Figure 2a,b), a higher net GWP was found in the regions between latitudes 25−40° N and longitudes east of 110° E (Figure 2d), although the SOC sequestration offset some of the GHG emissions. There were fragmented areas in northern China where the SOC sequestration exceeded the GHG emissions (Figure 2d). The gross GWP of the N2O and CH4 emissions in the late 2000s increased by 169 Tg CO2−equiv yr−1 compared with that in the early 1980s. The increase in the N2O and CH4 emissions were more pronounced in eight provinces (Heilongjiang, Hebei, Shandong, Henan, Hubei, Hunan, Anhui and Jiangsu) than in other provinces (Figure 3). These eight provinces together accounted for 61% of the total increment. Six provinces (Hebei, Shandong, Henan, Hubei, Anhui and Jiangsu) contributed approximately half of the total increase in N2O emissions. The increase in CH4 emissions was significant in Heilongjiang Province, where the area of irrigated rice cultivation jumped from 0.2 Mha in the early 1980s to 2.3 Mha in the late 2000s.31 In addition, cropland soils in Heilongjiang Province lost C at a substantial rate (21.7 Tg CO2 yr−1) in the early 1980s, and this loss decreased to 0.7 Tg CO2 yr−1 in the late 2000s (Figure 3). Mitigation Potential. Model simulations suggested that CH4 emissions could be reduced by 62−69 Tg CO2-equiv yr−1 without a decrease in SOC when no additional OM input during the rice growing season, although SOC sequestration 2592

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Figure 3. Spatiotemporal distribution of GWP (Tg CO2−equiv yr−1) in China’s croplands. The first and second columns show the GWP in the early 1980s (1980−1984) and in the late 2000s (2005−2009), respectively. All columns have the same scale. Different colors signify the GWP of N2O (red) and CH4 (blue) emissions, and the offset effect of ΔSOC (dark brown) that shows negative values. Province abbreviations: AH, Anhui; BJ, Beijing; CQ, Chongqing; FJ, Fujian; GD, Guangdong; GS, Gansu; GX, Guangxi; GZ, Guizhou; HEB, Hebei; HEN, Henan; HLJ, Heilongjiang; HN, Hainan; HUB, Hubei; HUN, Hunan; IM, Inner Mongolia; JL, Jilin; JS, Jiangsu; JX, Jiangxi; LN, Liaoning; NX, Ningxia; QH, Qinghai; SAX, Shaanxi; SC, Sichuan; SD, Shandong; SH, Shanghai; SX, Shanxi; TB, Tibet; TJ, Tianjin; TW, Taiwan; XJ, Xinjiang; YN, Yunnan; ZJ, Zhejiang.

would drop from 30−40 Tg CO2-equiv yr−1 to 2−4 Tg CO2equiv yr−1 (SI, Table S4). The five provinces with the greatest reductions in CH4 emissions are Hunan, Jiangxi, Hubei, Heilongjiang, and Sichuan, together contributing ∼52% to the national total amount of CH4 reduction (Table 2). The N2O emission could be reduced by 44 (37−51) Tg CO2−equiv yr−1 when the NUE is improved to 40% (Table 2). The five provinces with the greatest reduction in N2O emissions are Henan, Shandong, Hebei, Jiangsu, and Hubei. Together, these provinces would account for ∼48% the total reduction in N2O (Table 2). Relative to the mean values of GWP in the late 2000s, the gross GWP of N2O and CH4 emissions could be reduced by 111 Tg CO2−equiv yr−1, and cropland soils could sequester C at a rate of 82 Tg CO2 yr−1. Together, this amounts to a reduction of 193 Tg CO2-equiv yr−1 (Table 2) when the mitigation options (i.e., no additional OM input during the rice growing season and improving NUE to 40%) are put into practice. This amount is the equivalent of ∼47% of the gross GWP in the late 2000s, or 2.7% of China’s total GHG emissions in 2005. The 10 provinces with the highest gross GWP of N2O and CH4 emissions in the late 2000s were Henan, Shandong, Hunan, Jiangsu, Hubei, Sichuan, Anhui, Jiangxi, Guangdong, and Hebei (Figure 3), together being responsible for ∼63% of the national gross GWP from cropland. The mitigation of GHG emissions in these provinces could lead to a ∼66% national improvement (Table 2) and should be given priority.

time period (Table 1). In rice-based cropping systems, the amount of OM entered into soils was 79 Tg C yr−1 in the early 1980s and increased to 109 Tg C yr−1 in the late 2000s. Approximately 48% of the OM input occurred during the rice season. The increase in OM amendments promoted CH4 emissions.32,33 Water management in rice season can have a significant influence on CH4 emissions. When field drainage or intermittent flooding schedule was adopted, the methane emissions were greatly reduced.18,34 Irrigated rice cultivation in China is distributed across a vast area spanning wide ranges of temperate, subtropical, and tropical climates. The water management varies widely from place to place. Although we cataloged five patterns of water management to simulate the CH4 emissions (SI, section SI-3), the spatial variation of field water status may introduce errors into the simulation. Nevertheless, the simulated CH4 emissions (Table 1) were comparable to other estimates whose values ranged from 1.7 to 7.8 Tg CH4−C yr−1,35−37 the equivalent of 57 to 260 Tg CO2-equiv yr−1. It is expected that the errors in the estimates of CH4 emission derived from imperfect input of water management can be reduced when the spatial pattern of water status in rice season can be identified by remote sensing.38 N2O Emissions. It is well recognized that soil moisture regulates the processes of denitrification and nitrification and thus N2O emissions.39 Irrigation during the upland crop season promotes N2O emissions,40 particularly irrigation with fertilization.41 Our estimate of the fertilizer N-induced direct N2O emission factor is 1.29%, which is higher than the IPCC default value of 1%.2 This discrepancy occurs because irrigation is considered in the upland crop seasons (eq 1). The emission factor would be 1.1% if irrigation in the upland crop seasons was not taken into account.

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DISCUSSION CH4 Emissions. Although the harvest area of rice decreased from 33.3 Mha in the early 1980s to 29.3 Mha in the late 2000s (Figure 1a), the CH4 emissions increased by ∼17% during this 2593

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Table 2. Estimated Greenhouse Gas Emissions, Soil Organic Carbon Changes, and Mitigation Potential in Each Province mean of 2005−2009 (Tg CO2_equiv yr−1) province Beijing Tianjin Hebei Shanxi Inner Mongolia Liaoning Jilin Heilongjiang Shanghai Jiangsu Zhejiang Anhui Fujian Jiangxi Shandong Henan Hubei Hunan Guangdong Guangxi Hainan Chongqing Sichuan Guizhou Yunnan Tibet Shaanxi Gansu Qinghai Ningxia Xinjiang national total

crop land area (Mha)

CH4

N2O

with mitigation (Tg CO2_equiv yr−1)

ΔSOC

CH4

N2O

ΔSOC

net GWP (Tg CO2_equiv yr−1) mean of 2005−2009

with mitigation

reduction (%)

contribution to national total (%)

0.24 0.44 6.36 4.10 7.24

0.01 0.10 0.38 0.01 0.38

1.15 1.61 18.83 5.04 4.98

0.10 0.32 9.39 3.33 0.51

0.01 0.05 0.16 0.00 0.21

1.01 1.46 15.65 4.44 4.89

0.10 0.29 8.82 3.16 0.42

1.07 1.39 9.82 1.72 4.85

0.93 1.22 6.98 1.29 4.68

13.1 11.9 28.9 25.2 3.5

0.2 0.2 3.8 0.6 0.2

4.09 5.54 11.83 0.25 4.79 1.94 5.75 1.34 2.84 7.53 7.94 4.69 3.80 2.87 4.23 0.73 2.25 6.02 4.52 6.10 0.36 4.14 4.69 0.55 1.12 4.11 122.4

3.57 3.72 11.04 0.61 13.14 4.80 10.84 4.25 18.48 0.66 2.87 13.62 25.70 10.40 10.02 1.44 3.75 11.81 3.30 4.84 0.00 0.51 0.03 0.00 0.49 0.26 161

6.89 6.83 5.53 0.76 13.70 4.68 13.12 4.07 1.79 29.95 34.96 13.18 3.16 9.66 8.75 1.76 5.28 14.94 6.80 10.84 0.46 9.11 2.88 0.52 0.88 9.57 252

4.01 3.74 −0.71 0.22 6.72 1.88 6.17 1.05 4.79 14.83 14.47 7.06 7.09 3.97 3.81 0.77 1.60 7.73 2.53 3.10 0.18 3.36 2.09 −0.02 0.78 2.04 117

2.72 2.79 5.46 0.38 8.09 2.71 6.44 2.41 10.65 0.31 1.20 7.85 14.94 6.24 6.01 0.87 2.25 6.67 1.92 2.67 0.00 0.24 0.02 0.00 0.29 0.11 94

5.01 6.50 4.20 0.45 9.33 3.19 11.00 2.50 1.73 25.57 29.17 10.43 2.74 7.00 8.58 1.37 3.99 12.93 5.54 8.79 0.42 7.06 2.65 0.51 0.66 9.39 208

2.98 2.81 0a 0.05 3.83 0.75 4.04 0.04 0.50 13.92 13.15 4.72 1.34 1.53 1.56 0.39 0.86 4.70 1.75 2.07 0.17 3.08 1.98 −0.02 0.67 1.90 82

6.45 6.81 17.27 1.15 20.12 7.59 17.79 7.27 15.48 15.78 23.35 19.74 21.76 16.09 14.95 2.43 7.43 19.02 7.57 12.58 0.28 6.27 0.83 0.54 0.59 7.78 296

4.75 6.48 9.66 0.77 13.58 5.15 13.40 4.87 11.88 11.96 17.23 13.56 16.33 11.71 13.04 1.85 5.38 14.90 5.71 9.39 0.25 4.22 0.69 0.53 0.28 7.60 220

26.4 4.8 44.1 32.8 32.5 32.1 24.7 33.0 23.3 24.2 26.2 31.3 25.0 27.2 12.8 24.1 27.6 21.6 24.5 25.4 10.3 32.7 16.2 1.7 52.8 2.3 25.5

2.3 0.4 10.1 0.5 8.7 3.2 5.8 3.2 4.8 5.1 8.1 8.2 7.2 5.8 2.5 0.8 2.7 5.5 2.5 4.2 0.0 2.7 0.2 0.0 0.4 0.2 100

a With the assumption that the minimum rate of OM input in the upland crops and in the off-rice season could be reached. See section SI-4 in the SI for details.

It is noteworthy that N from manure and crop residues contributed ∼42% in the early 1980s and ∼31% in the late 2000s to the total N input (SI, Table S5), suggesting that manure and crop residues are important sources of N2O emissions. The amount of N from manure and crop residues averaged ∼13 Tg yr−1 in the late 2000s. Accordingly, the organic N-induced direct N2O emissions were estimated to be 0.18 Tg N2O−N yr−1, the equivalent of 84 Tg CO2−equiv yr−1 (SI, Table S5). We also estimated the background N2O emissions under zero fertilizer N addition, although special attention has been given to anthropogenic GHG emissions.1 Our estimates suggested that the annual rate of background N2O emissions remained relatively stable from 0.09 to 0.12 Tg N2O−N yr−1, approximately 20% of the fertilizer N-induced direct N2O emissions in the late 2000s (SI, Table S5). Large uncertainties may exist in the estimates of organic N-induced N2O emissions due to the imperfect data concerning manure and crop residues. We estimated a total organic N input of 12.9 Tg yr−1 in the late 2000s (SI, Table S3); this estimate is similar to that of Li and Jin (11.9 Tg yr−1 in 2008),42 but greater than that of Zhang et al. (9.3 Tg N yr−1 in 2005).43

Depending on the plant species, the animal species and the nutritional value of the feed, the chemical composition of manure varies remarkably. The N2O emission factors among different forms of manure, for example, range from 0.5% to 13.9%.44 It is currently difficult to accurately estimate the organic N input and manure-specific N2O emissions across croplands in China, although we recognized their importance when quantifying N2O emissions. To reduce this uncertainty, future efforts should be made to ensure more accurate manureor region-specific estimates when sufficient measurements are available. Trade-off between CH4 and N2O Emissions in Rice Paddies. Field drainage during the rice-growing season can significantly reduce CH4 emissions, but this practice stimulates N2O emissions.18,35 Our estimates indicated that 28%−35% of the N2O emissions in the rice-based cropping systems occurred in the rice-growing season, accounting for ∼5% of the gross GWP of CH4 and N2O emissions from the croplands. In this case, the trade-off between CH4 and N2O emissions due to water management in the rice-growing season (SI section SI-3) may not be of great significance as far as the mitigation potential 2594

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Figure 4. Trade-off between SOC sequestration and N2O emissions from the addition of OM in upland cropping systems. Changes in N2O emissions and SOC (0−30 cm) sequestration with OM input (a), and trade-off between SOC sequestration and N2O emissions vs the rates of OM input (b).

amount of synthetic N is substituted for this organic N, 1.14 ± 0.15 Tg CO2-equiv yr−1 would be released as N2O in the rice season. It has been well recognized that soils rich in organic C require a high amount of OM input to compensate for the loss in C. Similar to the Heilongjiang case (SI, Figure S1), we simulated SOC changes for rice-based cropping systems and obtained the minimum rates of the OM input under various SOC densities (0−30 cm depth). Plotting the minimum rate of the OM input against the SOC density, a linear correlation for the rice-upland cropping system and a nonlinear correlation for the double rice cropping system are observed (Figure 5).

is concerned on a national scale, but special attention should be given to this cropping system in the future. Trade-off between SOC Sequestration and GHG Emissions. The application of OM not only contributes C and nutrients to the soil but also enhances CH4 and N2O emissions. In upland cropping systems, the N2O emissions induced by organic N were estimated to be 36−67 Tg CO2equiv yr−1 between the early 1980s and the late 2000s. The corresponding SOC sequestration was estimated to be 24−78 Tg CO2-equiv yr−1. Clearly, the SOC sequestration could fully offset N2O emissions only if a certain amount of OM is incorporated into the soil (Figure 4a). We calculated the yearly trade-off between SOC sequestration and N2O emissions from the addition of OM (farm manure and crop residue) as the N2O minus the ΔSOC in terms of nationwide CO2-equiv from 1980 to 2009. A linear correlation can be observed when plotting (N2O−ΔSOC) against the OM input (Figure 4b). This correlation suggests that 1.9 Mg C yr−1 (1 Mg = 106 g) per hectare of arable area is a minimum rate of OM input to fully offset the N2O emissions. The SOC sequestration would exceed N2O emissions if the OM application rates were greater than 1.9 Mg C yr−1 per hectare of arable area. With applications of OM during both the rice season and the off-rice season, SOC sequestration could offset 22−26% of CH4 emissions (Table 1). In contrast, the trade-off between CH4 emissions and SOC sequestration would result in an overall mitigation of between 24−33% of the baseline scenario if no OM were to be added during the rice season, though the SOC sequestration would decrease in this scenario (SI, Table S4). Feasibility of Mitigation Options in OM Management. Before the 1980s, complete residue removal for fodder and fuel was the norm in China, but this practice has greatly decreased.45 Recent trends in agricultural practices throughout China give evidence for the decreasing removal of crop residues,46 which has contributed to an increase in SOC (Table 1). Completely ceasing the application of OM during the rice season could significantly mitigate CH4 emissions, while the risks of crop N deficiency and SOC loss could increase. To meet crop N demand in the areas where the application rates of N are low, additional application of synthetic N fertilizers would be needed when completely ceasing the application of OM is adopted during the rice season. The organic N from manure application and crop straw retention in the rice season was estimated to be 0.81 ± 0.11 Tg N yr−1 in the late 2000s, corresponding to 27 ± 4 kg N ha−1 yr−1. When an equivalent

Figure 5. Minimum rate of OM input for the sequestration of carbon by soils with different carbon densities.

The amount of stubble and roots was estimated to be 1.2−1.6 Mg C ha−1 yr−1 for the rice-upland system and 0.9−1.3 Mg C ha−1 yr−1 for the double rice system. Using the functions in Figure 5, we estimated that additional OM should be applied during the off-rice season, with a minimum rate of 0.1−1.9 Mg C ha−1 yr−1 for the rice-upland system and 0.3−1.2 Mg C ha−1 yr−1 for the double rice system, depending on the SOC density. In the late 2000s, the application of OM during the off-rice season was generally higher than the minimum in China’s rice-based cropping systems, with the exception of Heilongjiang Province. Feasibility of Mitigation Options in N Management. The excessive use of N (Figure 1c) has greatly contributed to 2595

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N2O emissions (Table 1). Evidently, the mitigation of N2O emissions from Chinese croplands depends principally on the management of N. There is substantial evidence that the effective management of fertilizers in cropping systems not only improves NUE but also improves crop yields. By putting knowledge-based optimum N management into practice, the current N application rates in southern China (rice/wheat system) and northern China (maize/wheat system) could be reduced by 30−60% with no yield loss.7 The practice of in-season root-zone N management at 121 on-farm experimental sites reduced the N application rate by 61% compared to the farmers’ N practice, with no loss in wheat yield.47 A two-year field measurement with an annual wheat/maize rotation in northern China indicated a decrease in N2O emissions as the NUE improved.48 The application of 50% composted N and 50% synthetic N in the maize/wheat system reduced N2O emissions by 26−51% relative to the use of synthetic fertilizer alone.49 China has implemented a soil testing and fertilizer recommendation program to reduce the overuse of synthetic N on cereal crops since the late 1990s. The balanced application of N, phosphate (P), and potassium (K) fertilizers may have reduced the rates of synthetic N use.50 Taken together, these findings suggest the feasibility of mitigating N2O emissions from China’s croplands by improving NUE (Table 2). In light of current N usage (SI, Table S3) and NUE,26,27 integrated N management strategies that take into consideration the knowledge-based applications of optimum N, P, and K are feasible options for improving NUE and therefore mitigating N2O emissions from China’s croplands.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the description of the models CH4MOD and Agro-C, the assessment of uncertainties in the estimates of GHG emissions and SOC change, the compilation of input data, and the minimum rates of OM application to compensate for SOC loss in Heilongjiang Province are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 6283 6597; fax: +86 10 8259 6146; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was jointly supported by the National Natural Science Foundation of China (Grant Nos. 41175132, 41075107) and the CAS Strategic Priority Research Program (Grant Nos. XDA05020200, XDA05050507). We would also thank the Resources and Environmental Scientific Data Center of the CAS and the National Meteorological Information Center of the CMA for their support in providing data.

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