This is the submitted version of the article: Jin, Qiang; Liu, Haitao; Wang, Chun; [et al.]. «Greenhouse gas emissions in a subtropical jasmine plantation managed with straw combined with industrial and agricultural wastes». Experimental agriculture, Vol. 56, Issue 2 (April 2020), p. 280-292. DOI 10.1017/S001447971900036X This version is avaible at https://ddd.uab.cat/record/222640 under the terms of the license 1 Short title: Using wastes to reduce jasmine plantation greenhouse-gas emissions 2 3 EFFECTS OF STRAW COMBINED WITH INDUSTRIAL AND 4 AGRICULTURAL WASTES ON GREENHOUSE-GAS EMISSIONS IN A 5 SUBTROPICAL JASMINE PLANTATION 6 7 By WEIQI WANG1,2*, JORDI SARDANS3,4, QINGWEN MIN5, XIAOHUI LIU1, XU 8 SONG1, XIAOTING HUANG1, JOSEP PEÑUELAS3,4 9 10 1Institute of Geography, Fujian Normal University, Fuzhou 350007, China; 2Key 11 Laboratory of Humid Subtropical Eco-geographical Process, Ministry of Education, 12 Fujian Normal University, Fuzhou 350007, China; 3CSIC, Global Ecology CREAF- 13 CSIC-UAB, Cerdanyola del Valles, 08193 Barcelona, Catalonia, Spain; 4CREAF, 14 Cerdanyola del Valles, 08193 Barcelona, Catalonia, Spain; 5Institute of Geographical 15 Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 16 100101, China; *Corresponding author: wangweiqi15@163.com; Phone: +86-591- 17 83465214; Fax: +86-591-83465397 18 19 SUMMARY 20 21 The effects of straw alone or combined with industrial and agricultural wastes as 22 fertilizers on GHG emissions in cropland areas are still poorly known. Here, we studied 23 the effects of 3.5 Mg ha-1 straw and 3.5 Mg ha-1 straw combined with 8 Mg ha-1 of 24 diverse wastes on GHG emission in a subtropical J. sambac plantation in southeastern 25 China. There were five treatments in a completely randomized block design: control; 26 straw only; straw + biochar; straw + steel slag; and straw + gypsum slag. Emissions of 27 CO2 were generally higher in the treatments with waste than in the control or straw- 28 only treatments, whereas the contrary pattern was observed in CH4 and N2O emission 29 rates. Moreover, the total global-warming potentials (GWPs) were no significantly 30 higher in most of the amended treatments as compared to the control and straw-only 31 treatments. In relation to the treatment with only straw, GWPs was 9.4% lower when 32 steel slag was used. This finding could be a consequence of Fe amount added by steel 33 slag, which would limit and inhibit the emissions of GHGs and their transport from soil 34 to atmosphere. Our results showed that the application of slags did not increase the 35 emission of GHGs and the combination of straw with steel slag or biochar could be 36 more effective than straw alone for controlling GHGs emission and improve soil C and 37 nutrient provision. 38 39 Keywords: CH4 flux; CO2 flux; N2O flux; climate change; straw; biochar; steel slag; 40 gypsum slag; jasmine plantation 41 1 42 INTRODUCTION 43 44 Global warming is caused by emission of large amounts of greenhouse gases (GHGs) 45 to the atmosphere from the combustion of fossil fuels, land-use changes and other 46 human activities (IPCC, 2013). Global GHG emissions from agricultural activities are 47 about 5.1-6.1 Pg CO2-eq y-1 (Smith et al., 2007), which can contribute to approximately 48 20% of the current emissions (Hütsch, 2001). The emissions of carbon dioxide (CO2), 49 methane (CH4) and nitrous oxide (N2O) from agricultural lands are especially relevant 50 (Myhre et al., 2013). Minimizing the emission of GHGs is an important task for 51 mitigating their adverse impacts on climate change. Many methods have been 52 developed to reduce and/or control the emission of GHGs, through water, fertilizers 53 and tillage management (Peyron et al., 2016; Gupta et al., 2016; Malhi et al., 2016). 54 Recycling industrial and agricultural waste is becoming an effective way to solve 55 environmental and resource availability issues, with the application of industrial and 56 agricultural waste on agricultural lands gradually increasing (Wang et al., 2015a,b). The 57 effects of waste application on soil nutrients and properties (Prendergast Miller et al., 58 2014), plant growth (Wang et al., 2015a, b) and GHG emission (Zhang et al., 2012) 59 have been evaluated. Waste materials such as biochar (Zhang et al., 2010) and steel slag 60 (Wang et al., 2012a) have been widely studied for increasing crop yields and mitigating 61 GHG emissions. However, the effect of mixed waste application on the emission of 62 GHGs is more complex and has been rarely and inconclusively studied. Industrial and 63 agricultural wastes contain high concentrations of electron acceptors such as the active 64 and free-oxide forms of iron (Fe), sulfur, nitrogen (N) and phosphorus (P). 65 Recently, biochar application has been found to significantly increase soil CO2 66 fluxes by 22%, while decreasing N2O fluxes by 31% without any effect in CH4 fluxes 67 (He et al., 2017). Such effects were dependent on latitude, soil type and soil use (Fan 68 et al., 2017; He et al., 2017; Wang et al., 2017a). The use of steel and gypsum slag 69 application to reduce GHGs emissions has been less studied, despite some positive 70 effects (reduction in GHGs emissions) in rice fields (Susilawati et al., 2015; Wang et 71 al., 2017b). Few studies have provided an overall evaluation of the total global- 72 warming potential (GWP) of CO2, CH4 and N2O, the three main GHGs (Wang et al., 73 2015a,b). The combined application of biochar and steel slag has been associated with 74 decreases in GHG emissions by rice crops (Wang et al., 2016; 2017). 75 At the best of our knowledge no information is available on the effects of industrial 76 and agricultural wastes on GHG emissions from subtropical Jasmine sambac 77 plantations. J. sambac tea is unique and more than half of the J. sambac tea in China is 78 produced in Fuzhou Province (Yang et al., 2008). Developing effective strategies to 79 enhance the yield of J. sambac flowers without increasing GHG emissions is considered 80 as an important policy for minimizing future adverse conditions induced by climate 81 change. On the other hand, use of waste materials for the management of J. sambac 82 plantations and GHG emission would be an alternative way to recycle waste without 83 environmental risks. 84 Here, we evaluated the effects of various waste materials (straw, and straw 2 85 combined with steel slag, biochar or gypsum slag) as fertilizers on the GHG emissions 86 under field conditions, taking into account soil traits and the relationships between CO2, 87 CH4 and N2O emissions and soil traits. 88 89 MATERIALS AND METHODS 90 91 Study site and experimental design 92 A field experiment was conducted in a Jasminum sambac (L.) field in Fujian Province, China (Suppl. material Suppl. Material Figure S1Figure S1, 25°59'10"N, 119°20'7"E, Formatat: Tipus de lletra: 12 pt 93 94 XXX 220 m a.s.l), during J. sambac growing season from April to October 2015. This 95 region has a subtropical monsoonal climate, with a mean air temperature of about 25 96 °C during the study period and a mean annual precipitation of approximately 1400 mm. 97 About 80% of the total rainfall was concentrated in the rainy season between May and 98 October (Figure S1). The soil contained 25%, 59% and 16% sand, silt and clay, 99 respectively. Bulk density was 1.2 g cm-3, pH 4.4, salinity of 0.15 mS cm-1 and 100 concentrations of total carbon, total N, total P and total potassium were 11.7, 1.1, 0.5 101 and 13.3 g kg-1, respectively. The study site has been cultivated with J. sambac since 102 2008. 103 J. sambac was cultivated using a ridge and ditch system, with a ridge height of 20 104 cm, ridge width of 100 cm and ditch width of 30 cm. Double-valve J. sambac branches Formatat: Tipus de lletra: No Cursiva 105 (10 cm long) were transplanted by hand at a distance among them of 3 cm. along the 106 ridges and 20 cm from the nearest plant in the side ridges in April 2008Double-valve J. 107 sambac branches (10 cm long) were transplanted by hand in the ridges at a spacing of 108 3×20 cm in April 2008 . The J. sambac was cut at the end of March or early April each 109 year when the air temperature was about 20 °C. About 3.5 Mg ha-1 of branches and 110 leaves were returned to the field, which was ridged but not plowed each year. J. sambac 111 branches and leaves began to grow from early April to early May. Growth period last 112 from early May to the end of MayGrowth period last from early May to the end of 113 May. Flowering occurred from early June to late September, when the final growth 114 period began. A complete fertilizer (N:P2O5:K2O, 16:16:16) was applied in two 115 applications: the first application was 130 kg ha-1 one day after J. sambac was 116 prunedwas applied in two splits: the first application was 130kg ha-1 one day after J. 117 sambac was pruned; and the second one was 100 kg ha-1 one day after the first J. sambac 118 flowers were collectedand the second one was 100 kg ha-1 one day after the first J. 119 sambac flowers were collected. 120 Triplicate plots (20 m2 each) were established for five treatments in a completely 121 randomized block design: 1) no straw or waste (control); 2) straw only (straw); 3) straw 122 combined with steel slag (straw+steel slag); 4) straw combined with biochar 123 (straw+biochar); and 5) straw combined with gypsum slag (straw+gypsum slag). Straw 124 was added at 3.5 Mg ha-1, and the biochar and steel and gypsum slags were added at 8 125 Mg ha-1 as granules (2 mm in diameter). The steel slag was collected from the Jinxing 126 Iron & Steel Co., Ltd in Fujian (China). The biochar was collected from the Qinfeng 127 Straw Technology Co., Ltd in Jiangsu Province. The gypsum slag was collected from 128 building waste (from the indoor decoration of buildings). The industrial and agricultural Formatat: Tipus de lletra: No Cursiva 129 wastes used in this study were rich in Si, Ca and K, which are essential nutrients for 130 plant growth (Prendergast‐Miller et al., 2014) (Table 1). All treatments received the 3 131 same water managementThe industrial and agricultural wastes used in this study were 132 rich in Si, Ca and K, which are essential nutrients for plant growth (Prendergast‐Miller 133 et al., 2014) (Table 1). All treatments received the same water management. 134 135 Measurement of CO2, CH4 and N2O emissions 136 The experimental period was from April 2015 to March 2016. Static closed chambers 137 were used to measure CO2, CH4 and N2O emissions, as described by Wang et al. 138 (2015a). The chambers were made of rigid PVC and consisted of two parts, an upper 139 transparent compartment (100 cm height, 30 cm width, 30 cm length) placed on a 140 permanently installed bottom collar (10 cm height, 30 cm width, 30 cm length). Each 141 chamber had two battery-operated fans to mix the air inside the chamber headspace, an 142 internal thermometer to monitor temperature changes during gas sampling and a gas- 143 sampling port with a neoprene rubber septum at the top of the chamber for collecting 144 gas samples. Three replicate chambers in each treatment were used. A wooden 145 boardwalk was built for accessing the plots to minimize disturbance of the soil during 146 gas sampling. The chambers had a vent to avoid pressure buildup. 147 Gas flux was measured for all chambers twice weekly during the growing season 148 and four times a week during the other seasons. The temperature in chamber was not 149 significantly changed during sampling process (30 min). Gas samples were collected 150 from the chamber headspace using a 100-mL plastic syringe with a three-way stopcock 151 0, 15 and 30 min after chamber deployment. The samples were immediately transferred 152 to 100-mL air-evacuated aluminum-foil bags (Delin Gas Packaging Co., Ltd., Dalian, 153 China) sealed with butyl rubber septa and transported immediately to the laboratory for 154 evaluation of CO2, CH4 and N2O concentrations. 155 CO2, CH4 and N2O concentrations were determined by gas chromatography 156 (Shimadzu GC-2010 and Shimadzu GC-2014, Kyoto, Japan) using a stainless steel 157 Porapak Q column (2 m length, 4 mm OD, 80/100 mesh). A methane-conversion 158 furnace, flame ionization detector (FID) and electron-capture detector (ECD) were used 159 for determining CO2, CH4 and N2O concentrations, respectively. The operating 160 temperatures of the column, injector and detector for quantifying CO2, CH4 and N2O 161 were adjusted to 45, 100 and 280°C; to 70, 200 and 200°C and to 70, 200, and 320°C, 162 respectively. Helium (99.999% purity) was used as a carrier gas (30 mL min-1), and a 163 make-up gas (95% argon and 5% CH4) was used for the ECD. The gas chromatograph 164 was calibrated before and after each set of measurements using 503, 1030 and 2980 µL 165 CO2 L-1 in He; 1.01, 7.99 and 50.5 µL CH4 L-1 in He and 0.2, 0.6 and 1.0 µL N2O L-1 166 in He (CRM/RM Information Center of China) as standards. CO2, CH4 and N2O fluxes 167 were then calculated as the rate of change in the mass of CO2, CH4 and N2O per unit of 168 surface area and per unit of time. Three injections were used for each analysis. One 169 sample was injected to the GC for each analysis. The detection limits of the instrument 170 for CO2, CH4 and N2O were 1, 0.1 and 0.05 ppm, respectively. 171 172 Global warming potential (GWP) 173 GWP is typically estimated using CO2 as the reference gas, and changes in emission of 174 CH4 or N2O are converted to “CO2-equivalents” (Hou et al., 2012). The GWP of the 4 175 combined emission of CH4 and N2O was calculated as (Ahmad et al., 2009): GWP = 176 (cumulative CO2 emission × 1) + (cumulative CH4 emission × 34) + (cumulative N2O 177 emission × 298) 178 179 Measurement of soil properties 180 Three replicates of soil samples were collected from each treatment. The samples were 181 transported to the laboratory and stored at 4°C until analysis. The temperature, pH, 182 salinity and water content of the top 15 cm of soil were measured in situ at each plot on 183 each sampling day. Soil pH was measured with a pH/temperature meter (IQ Scientific 184 Instruments, Carlsbad, USA), while salinity was measured using a 2265FS EC meter 185 (Spectrum Technologies Inc., Paxinos, USA) and water content measured using a TDR 186 300 meter (Spectrum Field Scout Inc., Aurora, USA). Soil samples were collected from 187 the 0-15 cm layer from each plot for quantifying ferric, ferrous and total Fe 188 concentrations. Total Fe concentration was determined by digesting fresh soil samples 189 with 1 M HCl. Ferrous ions were extracted using 1,10-phenanthroline and measured 190 spectrometrically (Lu, 1999). Ferric concentration was calculated by subtracting the 191 ferrous concentration from the total Fe concentration. After 60 DAT total soil C, N and 192 P were analyzed. Total soil C and N were determined by Elementar Vario MAX CN 193 Analyzer (Elementar Scientific Instruments, Hanau, Germany). Total soil P 194 concentration was quantified by perchloric-acid digestion followed by ammonium- 195 molybdate colorimetry, using a UV-2450 spectrophotometer (Shimadzu Scientific 196 Instruments, Kyoto, Japan). 197 198 Statistical analysis 199 Differences in soil properties and CO2, CH4 and N2O emissions among the treatments 200 were tested for statistical significance by repeated-measures analyses of variance. The 201 relationships between mean GHG emissions and soil properties were evaluated by 202 Pearson correlation analysis. These statistical analyses were performed using SPSS 203 Statistics 18.0 (SPSS Inc., Chicago, USA). The effects of treatments on total soil C, N 204 and P concentrations were analyzed by one-way ANOVA, with Tukey post hoc test 205 using Statistica 8.0 (StatSoft, Inc. Tule, Oklahoma, USA). 206 207 RESULTS 208 209 CO2, CH4 and N2O emissions and plant growth 210 CO2 emission varied significantly among sampling dates but not among treatments ( 211 Tables S1, S2). In general, CO2 flux remained low (<1040 mg m-2 h-1) during the first 212 16 days after treatment (DAT) and then increased to a seasonal peak (>1610 mg m-2 h- 213 1) between 16 and 91 DAT (Figure 2A1A). J. sambac was nearly mature by 106 DAT, 214 with a corresponding decrease in CO2 emission. 215 CH4 emission did not vary among sampling dates and treatments N2O emission 216 varied significantly among sampling dates but not among treatments (Tables S1, S2). 217 Maximum fluxes were earliest in the straw+gypsum treatment at 16 DAT (2.11 mg m- 218 2 h-1). The N O flux peaked at 46 DAT in the control (2.56 mg m-2 h-1), straw (2.01 mg 2 5 219 m-2 h-1), straw+biochar (1.78 mg m-2 h-1) and straw+steel slag (1.57 mg m-2 h-1) 220 treatments (Figure 2C1C). N2O emissions from the J. sambac plantation presented a 221 decreasing trend until 316 DAT. 222 The flowering period lasts from 91 DAT to the sampling date. During this period Formatat: Tipus de lletra: No Cursiva 223 224 plants had reached their maximum height and did not grow more (Figure 2D1D). The 225 little changes in plant height from 91 DAT to the harvest (286 DAT) observed in Figure 226 2D 21D were not significant. 227 228 Cumulative emissions 229 The cumulative CO2 emissions during the study period were higher for straw + gypsum Formatat: Tipus de lletra: No Cursiva 230 231 slag than the control. Cumulative CO2 emission were not statistically different in steel 232 slag, the straw+biochar and straw+gypsum slag treatments than in the straw treatment (Figure 3A2A). The cumulative CH4 emissions were not statistically different in straw, Formatat: Tipus de lletra: No Cursiva 233 234 steel slag, the straw+biochar and straw+gypsum slag treatments than in the control (Figure 3B2B). Cumulative N2O emissions were not statistically different in straw, steel Formatat: Tipus de lletra: No Cursiva 235 236 slag, the straw+biochar and straw+gypsum slag treatments than in the control (Figure 237 2C) (Tables S1 and S2). 238 239 Soil properties 240 Soil pH, temperature, water content and ferrous, ferric and total Fe concentrations 241 varied among the treatments (Figure 43, Tables S3, S4). Soil pH was higher in the 242 straw+biochar, straw+steel slag and straw+gypsum slag treatments than in the control 243 and straw treatments (P<0.05). Soil salinity did not differ significantly among the Formatat: Tipus de lletra: No Cursiva 244 different experimental treatments but all the treatments were significantly higher than 245 in the control (P<0.05), and straw+biochar and straw gypsum slag treatments had more 246 effect on the soil salinity than that of straw treatment (P<0.05). Soil-water content was 247 also higher in the straw+biochar and straw+gypsum slag treatments than the in control 248 and was higher in the straw+biochar treatment than in the straw treatment (P<0.05). 249 Soil Fe2+ concentration was significantly higher in all amended treatments than the 250 control (P<0.05), while Fe3+ concentration was significantly lower in the straw 251 treatment than in the control, straw+biochar, straw+steel slag and straw+gypsum slag 252 treatments (P<0.05). Total Fe concentration was significantly higher in the straw+steel 253 slag treatment than in the control (P<0.05). 254 255 Relationships between CO2, CH4 and N2O emissions and soil properties 256 Seasonal CO2 and N2O emissions were correlated positively (P<0.01) with soil 257 temperature (Table S5) and negatively with soil Fe3+ and total Fe concentrations 258 (P<0.01) in all treatments. Soil salinity and water content were also positively 259 correlated with CO2 emission in all treatments (P<0.01). Seasonal CH4 emission was 260 negatively correlated with soil Fe2+ concentration (P<0.05) in the straw and straw+steel 261 slag treatments. Salinity and water content were also positively correlated and soil pH 262 was negatively correlated with N2O emission (P<0.05) in all treatments. 263 6 264 Global warming potential (GWP) 265 GWP was significantly higher for CO2 than for CH4 and N2O emissions. However, total 266 GWP was similar in among treatments (Table 2). 267 268 Total soil C, N and P concentrations 269 Soil total C, N and P concentrations after 60 days (growing period) of treatments 270 application were higher in treatment plots than in control plots (Figure 54). Total soil 271 C and N concentrations followed the order of straw+biochar > straw+steel slag = 272 straw+gypsum slag > straw > control (Figure 4A34A,B). Total soil P concentration 273 values followed the order straw = straw+steel slag > straw+biochar = straw+gypsum 274 slag > control (Figure 5C4C). 275 DISCUSSION 276 Effects of the treatments on soil properties 277 Soil pH was higher in the straw+biochar, straw+steel slag and straw+gypsum slag 278 treatments than the control and straw treatments, likely due to the high alkaline contents 279 of the amendment material. Many cations are released due to the increase of pH (Wang 280 et al., 2015a). Soil salinity was significantly higher in the amended treatments than the 281 control (P<0.05). The straw, biochar, steel and gypsum amendment materials used in 282 this study contained some elements such as K and Ca that can easily been released into 283 the soil solution increasing the salinity (Wang et al., 2015a). Soil-water content was 284 also higher in the straw+biochar treatment than in the control and straw treatments. 285 Biochar can absorb water, because it has high total porosity, and can thus increase soil- 286 water content (Asai et al., 2009). 287 Soil Fe2+ concentration was significantly higher in all amended treatments than the 288 control. The wastes are rich in minerals containing Fe3+ and straw decomposition would 289 add more carbon substrates, such as short-chain fatty acids that can act as electron 290 donors, favoring altogether the conditions for Fe3+ reduction to Fe2+ (Li et al., 2011). 291 At this regard it is important to highlight the important inputs of iron that the treatments 292 suppose (207, 16 and 17 kg ha-1, for steel slag, biochar and gypsum slag, respectively). 293 Higher soil concentrations of organic matter and CO2 production and Fe2+ under 294 treatments should also favor methane production (da Silva et al., 2010), mostly in the 295 soil water contents between 40 and 60% observed in the studied period. At these levels 296 of soil moisture, several soil meso- and micro-pores should be flooded favoring 297 anaerobic conditions and methane production (Wagner, 2017).In our study, the Fe3+ 298 concentration was lower in the straw treatment than the control, supporting the premise 299 that straw or straw combined with waste could increase the reduction of Fe3+ to Fe2+ 300 and increase the Fe2+ concentration. 301 All treatments increased total soil C, N and P concentrations respect to control 302 after a year of application, which suppose a general increase of soil nutritional capacity, 303 especially in the soils receiving the application of straw + biochar that presented the 304 highest total soil C and N concentrations. The increases of soil N concentrations would 7 305 be especially important considering that those wetlands have been proved to be N- 306 limited (Wang et al., 2015a). 307 308 Effects of the treatments on CO2, CH4 and N2O emissions 309 CO2 emission changed significantly during the J. sambac growing season, and seasonal 310 CO2 emission was positively correlated with soil temperature in all treatments. CO2 311 emission was negatively correlated with soil Fe3+ and total Fe concentrations. Iron 312 plaques form around J. sambac roots at high Fe concentrations, which would limit the 313 transport of CO2 from the soil to the atmosphere by the internal system of 314 interconnected gas lacunae of the plants. Soil-water content was also positively 315 correlated with CO2 emission. When soil is not water saturated, soil respiration can 316 increase during wetter periods, so soil CO2 emissions are then positively related to soil 317 water content (McElligott et al., 2017). 318 Straw application can increase CO2 emissions by several mechanisms. First, the 319 decomposition of straw directly releases CO2 (Curtin et al., 1998). Second, the 320 decomposition of straw adds carbon substrates to the soil, which will increase the 321 amount and activity of soil microbes and then increasing emission of CO2 (Curtin et al., 322 1998). Despite not statistically significantly, mean CH4 emissions were generally higher 323 in the straw, straw+biochar and straw+gypsum slag treatments than the control, most 324 likely due to the input of carbon substrates from the straw. The decomposition of straw 325 would increase the carbon concentration of the soil and promote the emission of CH4 326 (Wang et al., 2015b). Contrarily mean CH4 emissions tend to be lower in the straw+steel 327 slag treatment than the control. The steel slag is rich in Fe3+ and would increase the 328 concentration of soil Fe3+, which is an alternative electron acceptor that can use C 329 substrates before methanogens (Jiang et al., 2013) and thereby decrease the amount of 330 CH4 production and emission (Gauci et al., 2008) by competing with methanogens for 331 C substrates (Jiang et al., 2013). 332 Mean N2O emissions tended to be lower in the straw+biochar and straw+steel slag 333 treatments than the straw treatment. Biochar can decrease N2O emission (Wang et al., 334 2011) and is rich in alkaline material, so it can increase soil pH, stimulate N2O reductase 335 activity and thereby induce the reduction of N2O to N2 (Yanai et al., 2007). Biochar 336 may also improve soil aeration and impede the function and diversity of denitrifying 337 bacteria, thereby decreasing N2O emission (Cavigelli and Robertson, 2001). Steel slag 338 is also rich in Fe3+, and an increase in soil Fe3+ concentration could suppress microbial 339 activity, including N2O production (Noubactep, 2011). 340 341 CONCLUSIONS 342 The total GWPs for all emissions were not different in the amended treatments than the 343 control. 344 8 345 All treatments increased total soil C, N and P concentrations after one-year of their 346 application. The high increase of total soil N concentration under straw + biochar was 347 especially important due to the limiting role of N in these wetlands. Moreover, biochar 348 and steel slag are rich in nutrients, such as Si, Ca and magnesium, which can all improve 349 soil fertility 350 351 The alkalinity of the biochar and the steel and gypsum slags also improved the soil 352 quality in this J. sambac producing area with high levels of acid deposition. The soil- 353 water content also increased. Our results also suggested that the application of straw 354 combined with biochar, steel slag or gypsum slag effectively reduced the adverse 355 impacts of J. sambac agriculture on plant growth and soil properties, even increasing 356 the soil N and P stocks without increasing significantly the GWPs. 357 358 Another positive aspect is that the wastes used as fertilizer materials in this study are in 359 abundant supply for application to agriculture. They also have a low cost and contribute 360 to the recycling of wastes. 361 362 Summarizing, our results thus provide strong evidence for several benefits from the 363 application of these fertilizers: recycling of several waste materials and improved J. 364 sambac production without affecting GHG emissions. Thus, the application of straw 365 combined with steel slag, and especially with biochar is a potentially positive and 366 sustainable agricultural practice that moreover is largely available and have a low 367 economical cost.” 368 369 Acknowledgments. The authors would like to thank Guixiang Chen, Qiuli Zhu and Weijie Lin for 370 their assistance with field sampling. Funding was provided by the National Science Foundation of 371 China (41571287), the Program for Fuzhou Jasmine and Tea Culture Global Important Heritage 372 System Monitor, Science and Technology Program of Fuzhou City (2014-G-66), the Program for 373 Innovative Research Team at Fujian Normal University (IRTL1205), the European Research 374 Council Synergy grant ERC-SyG-2013-610028 IMBALANCE-P, the Spanish Government grant 375 CGL2016-79835-P and the Catalan Government grant SGR 2014-274. 376 377 378 9 379 REFERENCES 380 Ahmad, S., Li, C., Dai, G., Zhan, M., Wang, J., Pan, S. and Cao, C. (2009). 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Effects of biochar amendment on soil quality, 517 crop yield and greenhouse gas emission in a Chinese rice paddy: A field study of 518 2 consecutive rice growing cycles. Field Crops Research 127: 153–160. 519 13 Tables Table 1. Chemical properties of different waste amendments in this study. Treatments Chemical properties Fe2O3 Fe SO3 S SiO2 C N P K Mg Ca (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Steel slag 4.8 - - - 40.7 0.79 0.01 0.01 0.5 0.36 24.9 Biochar - 0.2 - 0.6 - 56.6 1.4 1.0 1.8 1.0 0.5 Gypsum slag 0.4 - 54.4 - 0.7 0.7 0.01 0.01 0.1 0.3 30.6 Table 2. Effect of the amended treatments on the global-warming potential. Global warming potential (kg CO2-eq ha-1) Global-warming potential Treatment CO2 CH4 N2O (kg CO2-eq ha-1) Control 52572.44±1226.13a 2.65±2.89a 41.34±13.66a 52616.43±1321.25a Straw 66162.44±11791.43a 4.11±4.40a 31.56±4.90a 66198.12±11790.94a Straw+biochar 66714.37±8762.01a 25.14±22.09a 28.99±2.45a 66768.50±8753.62a Straw+steel slag 59953.90±7961.67a 1.71±2.66a 20.41±7.89a 59976.01±7971.45a Straw+gypsum slag 71004.78±5828.47a 15.20±10.37a 32.21±7.10a 71052.19±5822.38a Different letters within a column indicate significant differences between the amended and control treatments (P<0.05) obtained by Bonferroni’s post hoc test. 14 1 Figure legends 2 Figure 1. CO2 (A), CH4 (B), N2O (C) emissions and Plant height (D) in the control and 3 treatment. Error bars indicate one standard error of the mean of triplicate measurements. 4 Different letters indicate significant differences (P<0.05) between treatments. 5 Figure 2. Cumulative emissions of CO2 (A), CH4 (B) and N2O (C) in the amended and 6 control treatments during the study period. Error bars indicate one standard error of the 7 mean of triplicate measurements. Different letters indicate significant differences 8 (P<0.05) between treatments. 9 Figure 3. Soil pH (A), temperature (B), salinity (C), water content (D), Fe2+ 10 concentration (E), Fe3+ concentration (F) and total Fe concentration (G) in the control 11 and treatment. Error bars indicate one standard error of the mean of triplicate 12 measurements. Different letters indicate significant differences (P<0.05) between 13 treatments. 14 Figure 4. Total soil C (A), N (B) and P (C) concentrations observed in each treatment 15 plots after one-year of treatments application. Error bars indicate one standard error of 16 the mean of triplicate measurements. Different letters indicate significant differences 17 (P<0.05). 18 15