Improving rice production sustainability by reducing water demand and greenhouse gas emissions with biodegradable films

In China, rice production is facing unprecedented challenges, including the increasing demand, looming water crisis and on-going climate change. Thus, producing more rice at lower environmental cost is required for future development, i.e., the use of less water and the production of fewer greenhouse gas (GHG) per unit of rice. Ground cover rice production systems (GCRPSs) could potentially address these concerns, although no studies have systematically and simultaneously evaluated the benefits of GCRPS regarding yields and considering water use and GHG emissions. This study reports the results of a 2-year study comparing conventional paddy and various GCRPS practices. Relative to conventional paddy, GCRPSs had greater rice yields and nitrogen use efficiencies (8.5% and 70%, respectively), required less irrigation (−64%) and resulted in less total CH4 and N2O emissions (−54%). On average, annual emission factors of N2O were 1.67% and 2.00% for conventional paddy and GCRPS, respectively. A cost-benefit analysis considering yields, GHG emissions, water demand and labor and mulching costs indicated GCRPSs are an environmentally and economically profitable technology. Furthermore, substituting the polyethylene film with a biodegradable film resulted in comparable benefits of yield and climate. Overall, GCRPSs, particularly with biodegradable films, provide a promising solution for farmers to secure or even increase yields while reducing the environmental footprint.

final drainage ( Figure S2c-d). For the GCRPS plots, the soil water content, which was expressed as WFPS (water-filled pore space), ranged from 42% to 96% across the two rice-growing seasons, with mean values of 85% and 82% for GCRPS sat , 83% and 80% for GCRPS bio and 73% and 70% for GCRPS low in the 2012 and 2013 growing seasons, respectively. As expected, the GCRPS sat and GCRPS bio treatments had comparable WFPS values that were higher than the WFPS values observed for GCRPS low (Figures S3c-d, S4c-d and S5c-d). During the fallow periods, soil moisture variations were driven by rainfall events, with WFPS values ranging from 43% to 80%. Similar to soil temperature, no significant treatment effects on soil WFPS were observed during the fallow periods, indicating that legacy management effects on both parameters do not exist.
During the rice-growing seasons, the amplitude of soil Eh under each rice cultivation practice (fluctuating between -101 to 400 mV) was primarily affected by soil drying and wetting conditions, which were regulated by water regimes and rainfall events ( Figures S2e-f, S3e-f, S4e-f and S5e-f). Averaged across the two rice-growing seasons, the soil Eh was significantly higher in the GCRPS (88-210 mV) treatments than in the CP (27 mV). When compared with GCRPS low (210 mV), the GCRPS sat (88 mV) and GCRPS bio (92 mV) treatments had lower average soil Eh values.
Across the rice-growing seasons, soil NH 4 + was the dominant mineral form of N in all treatments, with substantial peaks following fertilizer application (Figures S2g-j, S3g-j, S4g-j and S5g-j). However, soil NH 4 + in the urea-fertilized GCRPS plots were on average 42% higher (mean: 18.0-24.5 mg N kg -1 SDW (soil dry weight)) than those of the urea-fertilized CP plots (14.6 mg N kg -1 SDW). Additionally, the soil NO 3 concentrations in the GCRPS treatments were 77% higher, on average, than those in the CP, although their concentrations were generally < 5.0 mg N kg - year or season, which was calculated as the difference in emissions between the fertilized and unfertilized plots. In the present study, the irrigation water use efficiency (IWUE) was defined as the grain yield divided by the amount of irrigation water supplied for each rice cultivation practice. The fertilizer N-use efficiency (NUE) was computed based on the percentage of the differences in the amount of aboveground N uptake between fertilized and unfertilized plots compared to the fertilizer N input.

Cost-benefit analysis
To further evaluate whether GCRPSs are an economically feasible approach for S5 reducing environmental impacts and increasing rice yields, we performed a cost-benefit analysis based on ecosystem services following the study of Compton et al. (2) The overall impacts of GCRPSs on ecosystem services were quantified as monetary values, which were determined by summarizing the costs/benefits associated with the impacts of GCRPSs on GHG emissions, irrigation water demands and crop productivity, as well as the expenses of purchasing plastic film and hiring labor for preparing and mulching the fields.
The cost-benefit analysis indicated that the conversion from CP to GCRPS resulted in an average net environmental benefit of $55.2-75.7 ha -1 yr -1 across the 2-year study, mainly due to the mitigating effects of the GCRPS treatments on CH 4 emissions (Table S3). In comparison with the CP, the increases in rice yield and decreased demands for irrigation water in the GCRPS resulted in a revenue increase of $164.0-297.8 ha -1 yr -1 (Table S3). Although the GCRPS practices require more labor-time than the CP and require the manufacture and purchase of mulching materials, the GCRPS sat and GCRPS low practices have net monetary benefits of $203.9 and $251.5 ha -1 yr -1 , respectively. The benefit for GCRPS bio reaches up to $39.8 ha -1 yr -1 , which is lower than the benefits of the GCRPS sat or GCRPS low treatments because the price of the GCRPS bio mulching material is approximately twice the price of the polyethylene plastic film.  .46 ± 0.23 aB † The data shown are means ± standard errors (n=3); GCRPS sat , the ground cover rice production system with polyethylene films, where soil water content was kept nearly saturated; GCRPS bio , the ground cover rice production system with biodegradable films, where water was managed the same as in the GCRPS sat treatment; GCRPS low , the ground cover rice production system with the same covering film as the GCRPS sat and with near saturation until the rice-regreening stage and at approximately 80% of the GCRPS sat management for the reminder of the season; -N, no synthetic nitrogen fertilizer application; +N, a local common application rate of 150 kg N ha -1 . * The area weighted CH 4 and N 2 O emissions were calculated based on the areal extent of the raised bed (87%) and furrow (13%); these emissions within each row followed S9 by the same lowercase letter are not significantly different among the rice cultivation practices under each N application rate at the P < 0.05 level, and those followed by the same capital letter are not significantly different between unfertilized and fertilized treatments under each rice cultivation practice at the P < 0.05 level.   The area-scaled CO 2 equivalents of CH 4 , N 2 O and CH 4 +N 2 O emissions for each N application rate followed by same letter are not significant at P <0.05. CP, the conventional paddy rice production system with an initial flooding-midseason drainage-reflooding irrigation mode; GCRPS sat , the ground cover rice production system with polyethylene films when the soil water content was kept nearly saturated; GCRPS bio , the ground cover rice production system with biodegradable films, where water is managed the same as in the GCRPS sat treatment; GCRPS low , the ground cover rice production system with the same covering film as GCRPS sat and the soil water content maintained near saturation until the rice-regreening stage and at approximately 80% of the GCRPS sat management for the reminder of the season. S19 Figure S7. Yield-scaled carbon dioxide (CO 2 ) equivalents of methane (CH 4  The yield-scaled CO 2 equivalents of CH 4 , N 2 O and CH 4 +N 2 O emissions for each N application rate followed by same letter are not significant at P <0.05. CP, the conventional paddy rice production system with an initial flooding-midseason drainage-reflooding irrigation mode; GCRPS sat , the ground cover rice production system with polyethylene films when the soil water content was kept nearly saturated; GCRPS bio , the ground cover rice production system with biodegradable films, where water is managed the same as in the GCRPS sat treatment; GCRPS low , the ground cover a a (i) S20 rice production system with the same covering film as GCRPS sat and the soil water content maintained near saturation until the rice-regreening stage and at approximately 80% of the GCRPS sat management for the reminder of the season. Figure S8. Seasonal cumulative emissions of soil respiration (CO 2 ) from the different rice cultivation practices fertilized with urea application at a common rate of 150 kg N ha -1 (+N) during the fallow season of 2012-2013 (a) and 2013-2014 (b). CP, the conventional paddy rice production system with an initial flooding-midseason drainage-reflooding irrigation mode; GCRPS sat , the ground cover rice production system with polyethylene films when the soil water content was kept nearly saturated; GCRPS bio , the ground cover rice production system with biodegradable films, where water is managed the same as in the GCRPS sat treatment; GCRPS low , the ground cover rice production system with the same covering film as GCRPS sat and the soil water content maintained near saturation until the rice-regreening stage and at approximately 80% of the GCRPS sat management for the reminder of the season. Mar.10 Jan.10 Nov.10 Sep.10