Energy budget and carbon footprint in ridge–furrow with plastic film mulch 1 strategy under a wheat–maize system in dry semi–humid areas

: The well–irrigated planting strategy (WI) consumes a large amount of energy and 13 exacerbates greenhouse gas emissions, endangering the sustainable agricultural production. 14 This work aims to estimate the economic benefit, energy budget and carbon footprint of a 15 wheat–maize double cropping system under conventional rain–fed flat planting (control), 16 ridge–furrows with plastic film mulching on the ridge (RP), and the WI in dry semi-humid areas 17 of China. Significantly higher wheat and maize grain yields and net returns were achieved under 18 RP than those under the control, while a visible reduction was only found for wheat grain yields 19 when compared with the WI. The ratio of benefit: cost under RP was also higher by 10.5% than 20 that under the control in the first rotation cycle, but did not differ with those under WI. The net 21 energy output and carbon output followed the same trends with net returns, but the RP had the 22 largest energy use efficiency, energy productivity carbon efficiency and carbon sustainability 23 among treatments. Therefore, the ridge-furrow planting with plastic film mulch over the ridge 24 was an effective substitution for well–irrigated planting strategy for achieving sustained 25 agricultural development in dry semi-humid areas. 26

Carbon footprint. The annual average CF under RP was obviously higher by 30.9% and 23.8% 175 than those under the control for wheat and maize production, respectively (Table 3). However, 176 there existed no significant difference between WI and RP for maize production, and a 15.4% 177 reduce was found under WI for wheat production (  Energy productivity (kg MJ -1 ) (d) C WI RP increased by 27.2% relative to the control, while reduced by 6.8% relative to the WI in the entire rotation cycle (Table 3). The 165 and 1,908 kg CO2-eq ha -1 was more from uses of farm 180 machinery and plastic film under RP than those under both the control and WI, while 2,785 kg 181 CO2-eq ha -1 was less from uses of electricity for irrigation under RP than that under WI. Over 182 2 rotation cycles, the use of fertilizers and electricity for irrigation occupied 36.6% and 33.4% 183 of the total emissions, followed by N2O emissions based on estimation (20.8%).     What's more, more solar radiation for improving maize photosynthesis and growth, 218 because the rainy days after silking in 2013 were lower than that in 2014.

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The total cost of wheat production ranged from 6,266 Yuan ha -1 under the control 220 to 10,466 Yuan ha -1 under WI (Table 1)  The study has showed that the annual energy inputs of wheat production were 253 ranged from 28,395 to 60,255 (Table 2). However, the total energy inputs of wheat 254 production varied from 10,800 MJ ha −1 to 57,800 MJ ha −1 in other studies 8, 31, 32 . The 255 values has exceeded the reported total energy inputs of wheat production due to the 256 energy inputs from irrigation under WI (Table 2). In previous studies, the energy inputs 257 of irrigation, nitrogen fertilizers, and farm machinery accounted for 23.5~32.1%, 258 24.0~38.3%, and 30.8~60.2% of the total energy inputs for raising wheat 32-34 . But the 259 highest energy inputs under WI, control and RP were irrigation, fertilizer and fertilizer, 260 respectively, which occupied over 40% of total energy inputs of wheat production. In 261 addition, the use of plastic film contributed more than 10% to the total energy inputs 262 under RP. The apparent discrepancy may result from different irrigation strategies and 263 other field managements as well as edaphic and climatic conditions. The total energy 264 inputs of maize production in the study were fairly high compared to other studies of et al. 36 . Similar to wheat production, irrigation, fertilizers, and farm machinery were 267 also the main contributors of the energy inputs. In the entire rotation cycle, the total 268 energy inputs showed: WI>RP>control (Table 2), which revealed that the total energy 269 inputs of crop production under RP increased by 18.5% relative to that under the control, 270 while reduced by 32.6% relative to that under the WI. Furthermore, the energy input 271 derived from the irrigation is on the increase due to the decline of groundwater level 37 .

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This condition approved that adopting energy-save irrigation strategies, such as the  to 9.31 Mg ha −1 for wheat and from 7.6 to 11.6 Mg ha −1 for maize, respectively, with 317 the lowest yields under the control, followed by RP and WI. The gross return and net 318 return had the same trends as those of grain yields, but the benefit: cost ratio was close 319 between the WI and RP. The RP increased the net energy output, energy use efficiency, 320 and energy productivity, but reduced the specific energy relative to the control. The 321 annual average CF under RP increased by 27.2% relative to the control, while reduced 322 by 6.8% relative to the WI. The carbon output under RP was significantly higher by 323 44.8% and 43.9% than those under the control, while slightly lower by 12.3% and 11.5% Olsen P with a pH of 8.45 (soil/water=1:1) and a bulk density of 1.20 g cm −3 .  where, EP is economic profit (×10 3 Yuan ha −1 ).

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where, RIC is the ratio of income to cost.  (Table S1) and outputs (grain yields).

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The inputs and outputs were computed from physical units to energy units through 417 multiplication with the conversion coefficients (Table S2). The energy input (EI) and  where, EP is the energy productivity. WEP is the system productivity.