Yield Advantage of a Maize-peanut Intercropping System

7 Maize-peanut intercropping is an important element of China’s agricultural planting model, 8 as it confers ecological benefits, promotes species diversity, and increases economic efficiency and 9 yield. The aim of this study was to explore the yield differences between intercropping and 10 monoculture, and to determine the mechanism underlying the high yield efficiency of the 11 intercropping system using the 13 C isotope tracer labelling method. The early maturing corn 12 hybrid Denghai 618 and the early maturing and high-yielding peanut variety Huayu 22 were used 13 as test materials. Three kinds of planting methods were employed, i.e. the sole maize (SM), the 14 sole peanut (SP) and maize – peanut intercropping (intercropped maize, IM; intercropped peanut, 15 IP), for two consecutive years. IM increased yield by 59.7% and 62.3% comparing with SM in 16 2015 and 2016, respectively. IP reduced yield by 31.3% and 32.3% comparing with SP in 2015 17 and 2016, respectively. IM significantly increased the photosynthetic rate, leaf area, 13 C 18 assimilation distribution, and dry matter accumulation of summer maize, which led to an increase 19 in kernel number, resulting in an increased yield. The decrease in intercropped peanut yield was 20 mainly caused by a decrease in the percent of plump pod and number of pods per plant. The 21 decrease in peanut yield did not affect the production of intercropping, because of the large intercropping advantage and land equivalence ratio. Maize-peanut intercropping provided greater 23 economic benefits than monoculture. These results showed the utility of the peanut-maize intercropping model.


Introductions 27
The rapid industrialization of the agricultural sector is conducive to increased labor 28 productivity and crop yields, meanwhile it also brings many ecological problems, including loss of 29 biodiversity, reduced soil fertility, and increased pollution caused by the intensive use of chemical 30 fertilizers and pesticides (Jacobsen et al., 2013). In recent years, the Chinese government has paid 31 more and more attention to the ecological benefits of agriculture, requiring the reduction in the use 32 of fertilizer and pesticide in agricultural production and prohibition of straw burning. To ensure 33 both food security and ecological benefits, it is essential to seek best management practices, which 34 include appropriate cropping systems that can efficiently utilize solar and soil resources with 35 minimum nutrient inputs. 36 Intercropping is a farming practice involving two or more crop species, or genotypes, 37 growing together and coexisting for a time with a definite row arrangement. (Carrubba.et al., 2008;38 Bedoussac et al., 2015). Compared with its component monocrops, it is reported to deliver pest 39 [键入文字] to August. The changes in the climate observed during the maize growing season were shown in 89 Soil physical and chemical parameters were measured in the 0-20 cm soil layer before 91 sowing in 2015: pH (6.3), organic matter (11.2 g kg -1 ), total nitrogen (0.92 g kg -1 ), available 92 phosphorus (47.1 mg kg -1 ), and available potassium (84.2 mg kg -1 ). The early maturing corn 93 hybrid Denghai 618 (DH618), and the early maturing and high-yielding peanut cultivar Huayu 22 94 (HY22), were the test crops in this study. 95 Three kinds of planting methods were used, i.e. the sole maize (SM), sole peanut (SP) and 96 maize-peanut intercropping (intercropped maize, IM; intercropped peanut, IP), which were applied 97 in two consecutive years. SP was planted in equally spaced rows with a density of 180,000 holes 98 ha -1 ; there were two seeds per hole, with row spacing of 35 cm and plant spacing of 16 cm. The 99 SM planting density was 105,000 plants ha -1 , with row spacing of 60 cm and plant spacing of 15.9 100 cm. For the intercropping system, the planting ratio was 4:6, i.e. four rows of maize to six rows of 101 peanut. With this row-ratio design, the two crops cover a similar area, which facilitates rotation of 102 the two crops to eliminate continuous peanut obstacles. The maize and peanut were planted at the 103 same density in the intercropping system as used in their respective monocultures. The spacing 104 between crops in the intercropping system was 50 cm, and the bandwidth was 455 cm (Fig. 2). 105 The plot size was 6 m in width by 30 m in length in SM, 7 m in width by 30 m in length in SP, and 106 13.65 m in width by 30 m in length in the maize-peanut intercropping system. The row orientation 107 was north-south, the experimental design was a completely randomized design with 108 three times repetition. Furthermore, the spacing between the single crop system was 300 cm. In 109 the pure crop plots, we discard the external two rows of both corn and peanut due to potential 110 [键入文字] border effects, to eliminate differences among duplications, six rows of peanut and then several 111 rows of maize were planted at the left of the maize/peanut intercropping system, and four rows of 112 maize and then several rows of peanut were planted at the right of the intercropping system. In 113 both growing seasons, maize and peanut were supplied with 200 and 45 kg N ha -1 in resin-coated 114 urea (43% N), respectively, and both crops were supplied with 100 kg P2O5 ha -1 in calcium 115 superphosphate (12% P2O5) and 120 kg K2O ha -1 in potassium chloride (62% K2O). All N, P and 116 K fertilizers for both crops were applied as basal fertilizers. Maize and peanut were sown and 117 harvested at the same time; they were sown on 10 June 2015 and 2016, and harvested on 5 118 October 2015 and 2016. 119 Weeds were controlled chemically (Refined iso-alachlor, a pre-emergence herbicide can 120 be used for maize and peanuts) before maize and peanut emergence. Diseases and insect pests of 121 maize and peanut were controlled by conventional techniques using an isolation belt spraying 122 machine. Supplementary water was applied during the growing season according to the estimation 123 of weekly plant water demand (evapotranspiration) and precipitation. 124

Grain yield 126
Grain yield (14% moisture) of summer maize at the physiologically mature stage (R6, Ritchie 127 et al., 1993) was estimated based on 30 consecutive plants in each row. All grain was air-dried to 128 investigate yield as follows: 129 Grain yield (kg ha -1 ) = ear number (ears ha -1 ) × kernel number per ear × 1000 kernel weight 130 (g)/(1-moisture content)/10 6 /(1-14%) 131 During the peanut harvest period, all plants under a given treatment were harvested in a 3-m 132 [键入文字] strip and air-dried; the yield, 100-kernel weight, pods per plant, and the percent of plump pod were 133 then determined. 134 The land equivalence ratio (LER) and Intercropping advantage 135 Maize and peanut intercropping performance was assessed according to the LER: 136 Where and indicate the actual yield of intercropped maize and intercropped peanut, 138 respectively. and are the yield of SM and SP, respectively. An LER value > 1 indicates 139 that intercropping is advantageous and LER < 1 indicates that intercropping is disadvantageous. 140 indicates the yield of the intercropping system, and = + ; and represent 142 the proportion of area between the intercropped maize and peanut, respectively. 143

Economic benefit 144
Economic benefit (USD ha -1 ) was calculated according to: 145 where Y is yield (kg ha -1 ), P is grain price (USD ha -1 ), LF is labor fees (USD ha -1 ), FF is 147 fertilizer fees (USD ha -1 ), SF is seed costs (USD ha -1 ) and BF is pesticide expenses (USD ha -1 ). 148 USD is U.S. dollar. The leaf area of peanut was determined by the specific leaf weight method. 168 Where S1 is green blades of a fixed area; M1 is the dry weight of green blades with a fixed 170 area; M2 is the dry weight of the remaining leaf area; and S is the total leaf area. 171

Distribution and accumulation of assimilation products 172
Ten representative plants were selected during the 2015 and 2016 maize silking periods. 173 Their ear leaves were sealed and 13 CO2 feeding was carried out immediately to maintain 174 photosynthesis for 60 min. Five plants were obtained after 24 h and during R6, and the organs 175 were dried in the oven and weighed with an electronic balance. The 13 C abundance was 176 [键入文字] determined by a stable isotope mass spectrometer (ISOPRIME100) and the accumulation and 177 distribution of 13 C assimilates in the aboveground organs were calculated. 178

Statistical analysis 179
Data were analysed using Microsoft Excel (Microsoft Corp., Redmond, WA, USA), 180 SigmaPlot (ver. 11.0; Systat Software, San Jose, CA, USA) and SPSS (ver. 16.0; SPSS Inc., 181 Chicago, IL, USA) software. All measured and calculated features were analyzed as dependent 182 variable; cropping treatments was analyzed as fixed factors. Significant differences among means 183 were determined by Duncan's multiple range test at 5% level. 184

Results 185
Yield and economy advantage of intercropping system 186 Compared to monoculture, maize-peanut intercropping system had intercropping advantages 187 in yield and economy (i.e., promoted crop yield and farmers' income) (Table 1) cropping systems with 385.5 USD ha -1 greater than SM and 585.5 USD ha -1 greater than SP over 192 the two growing seasons. In addition, the intercropping system had a LER value greater than that 193 in both growing seasons. 194

Yield composition of maize and peanut 195
Intercropping significantly affected the yield components of maize and peanut (Table 2). Ear 196 number (expressed per unit area of the whole system) was significantly lower in intercrops than in 197 SM, while kernel number and 1000-grain weight were significantly greater in intercrops than in 198 SM, which indicated that the advantage of intercropping was due mainly to the increase in maize 199 yield per plant. Averaged the two growing seasons, the kernel number and 1000-grain weight of 200 [键入文字] IM were increased by 20.0% and 8.0% compared to SM, respectively. The pod number per plant, 201 100-pod weight and percent of plump pod of the peanut grown in intercrops treatment were lower 202 than that in monoculture, with decreased by 2%, 12.9% and 28.6% over the two years. decreased significantly by 13.8% with IP during 2016 compared with SP (Fig. 4). 216

Net photosynthetic rate (Pn) 217
Intercropping increased the Pn of maize significantly in different growing stages in both 218 seasons (Fig. 5). With the development of the growing stage, the Pn of ear leaves of SM and IM 219 showed a single-peak trend, reaching its maximum at R2, and then decreasing. Averaged over the 220 two seasons, the Pn of IM at R2 significantly increased by 6.5% compared with SM. The 221 photosynthetic rate of peanut tended to decrease as growth proceeded. IP significantly reduced the 222 [键入文字] photosynthetic rate of peanut during the entire growth period, which was due to the shading effect 223 of maize, as a high-stalk crop, on the photosynthetic rate of peanut. 224

Distribution and accumulation of assimilated matter 225
Intercropping treatment altered the pattern of distribution of 13 C-photosynthates among 226 different organs (Table 3). At the silking and after 24 h of isotope tracer labelling, the maximum 227 ratio of distribution was recorded in the stem followed by leaves. At R6, the distribution of 228 13 C-photosynthates was mainly concentrated in the grains. Relative to the SM, 13 C-photosynthates 229 distribution in grain increased by 3.6% in IM, averaged the two growing seasons. 230

Discussion 231
Intercropping improved resource acquisition and productivityrelative comparied to 232 monoculture (Giles et al., 2017). Complementarity is likely as intercropped maize uses N from 233 the soil for growth whilst the legume canrely more on atmospheric N2 fixation for growth. These outputs (Li et al., 2010). In this study, we found that land use efficiency, measured by the LER, 244 [键入文字] varied from 1.15 to 1.16 over the two growing seasons (Table 1); intercropping greatly increased 245 land use efficiency, which indicated that maize and peanut intercropping is a compound system 246 with high quality, high yield and high efficiency. It can not only increase grain yield, but also 247 increase farmers' income, alleviate the contradiction between grain and oil. increased, while the pod yield of IP decreased over the 2-yr study period, which was consistent 265 with previous studies (Jiao et al., 2008). Intercropped legume probably facilitated growth of grass 266 [键入文字] by transferring the N fixed (Li et al., 2009;Seran and Brintha, 2010;Altieri et al., 2012), which 267 may be another reason for the increased maize yield in the intercropping system. 268 An efficient way to assess the contribution of each part of a maize plant to grain yield is by 269 assessing the amount of 13 CO2 fixed in the plant and transferred to each of its parts, although total 270 13 C fixed was underestimated in a previous study because the amount of 13 C lost to the soil was 271 not measured (Wu et al., 2010). The results of a 13 C tracer study showed that assimilates were 272 mainly concentrated in grains during the mature period of maize, and that stem and leaf 273 assimilation products were transferred to the grains (Table 3). Under the intercropping system, 274 maize was more assimilated to grain than under the SP system, which laid the foundation for an 275 increase in the intercropped maize yield and had a positive effect on grain filling. 276 Intercropping improves crop colony structure, enhances the land utilization ratio and 277 enhances resistance at the group level; it also reduces fertilizer and has remarkable economic 278 (Table 1), environmental and social benefits. Intercropping is thus valuable for food security at the 279 national level and helps to improve the market competitiveness of agricultural products. Our next 280 step will be to conduct detailed studies on the effects of the intercropping system on soil 281 microenvironment and community structure. 282

Conclusion 283
Maize played an important role in determining the yields in the intercropping system; it was 284 the dominant and superior crop and had a stronger ability to obtain resources than peanut when 285 intercropped. Compared to conventional monoculture of maize, the maize-peanut intercropping 286 had significant advantage in yield and land utilization ratio, due to the improved canopy structure 287 of crop population, which make the leaves of maize on different levels enjoy appropriate sun 288 [键入文字] radiation, and the optimized distribution and utilization of assimilation during the later stage of 289 crop production. In contrast, the IP method significantly decreased yield compared with SP, 290 primarily because the long-term exposure to maize shading. Maize-peanut intercropping system 291 yielded greater economic benefits and land use efficiency than monoculture, which provides 292 strong justification for widespread adoption of this cultivation method in the Huang-Huai-Hai 293 Plains.      Effects of planting pattern on the LAI of maize and peanut Effects of planting pattern on the net photosynthetic rate (Pn) of maize and peanut