Soil mineral nitrogen and yield-scaled soil N2O emissions lowered by reducing nitrogen application and intercropping with soybean for sweet maize production in southern China

Journal of Integrative Agriculture 2017, 16(11): 2586–2596 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Soil mineral nitrogen and yield-scaled soil N2O emissions lowered by reducing nitrogen application and intercropping with soybean for sweet maize production in southern China TANG Yi-ling*, YU Ling-ling*, GUAN Ao-mei , ZHOU Xian-yu, WANG Zhi-guo, GOU Yong-gang , WANG Jian-wu Institute of Tropical and Subtropical Ecology/Key Laboratory of Agro-environment in the Tropics, Ministry of Agriculture/Key Laboratory of Agroecology and Rural Environment of Guangdong Regular Higher Education Institutions, South China Agricultural University, Guangzhou 510642, P.R.China

Abstract The increasing demand for fresh sweet maize (Zea mays L. saccharata) in southern China has prioritized the need to find solutions to the environmental pollution caused by its continuous production and high inputs of chemical nitrogen fertilizers. A promising method for improving crop production and environmental conditions is to intercrop sweet maize with legumes. Here, a three-year field experiment was conducted to assess the influence of four different cropping systems (sole sweet maize (SS), sole soybean (SB), two rows sweet maize-three rows soybean (S2B3) intercropping, and two rows sweet maize-four rows soybean (S2B4) intercropping), together with two rates of N fertilizer application (300 and 360 kg N ha–1) on grain yield, residual soil mineral N, and soil N2O emissions in southern China. Results showed that in most case, intercropping achieved yield advantages (total land equivalent ratio (TLER=0.87–1.25) was above one). Moreover, intercropping resulted in 39.8% less soil mineral N than SS at the time of crop harvest, averaged over six seasons (spring and autumn in each of the three years of the field experiment). Generally, intercropping and reduced-N application (300 kg N ha–1) produced lower cumulative soil N2O and yield-scaled soil N2O emissions than SS and conventional-N application (360 kg N ha–1), respectively. S2B4 intercropping with reduced-N rate (300 kg N ha–1) showed the lowest cumulative soil N2O (mean value=0.61 kg ha–1) and yield-scaled soil N2O (mean value=0.04 kg t–1) emissions. Overall, intercropping with reduced-N rate maintained sweet maize production, while also reducing environmental impacts. The system of S2B4 intercropping with reduced-N rate may be the most sustainable and environmentally friendly cropping system. Keywords: sweet maize-soybean intercrop, cropping system, N fertilizer rate, grain yield, soil mineral N, soil N2O emissions

Received 18 January, 2017 Accepted 6 May, 2017 TANG Yi-ling, E-mail: [email protected]; YU Ling-ling, E-mail: [email protected]; Correspondence WANG Jianwu, Tel/Fax: +86-20-38604886, E-mail: [email protected] * These authors contributed equally to this study. © 2017 CAAS. Publishing services by Elsevier B.V. All rights reserved. doi: 10.1016/S2095-3119(17)61672-1

1. Introduction There is a worldwide increase in the demand for sweet maize (Zea mays L. saccharata) for both the fresh production and processing markets (Williams 2014), with a global annual cultivation of approximately 1 million ha (USDA 2010). Sweet maize is also becoming increasingly popular in China

TANG Yi-ling et al. Journal of Integrative Agriculture 2017, 16(11): 2586–2596

(Singh et al. 2014), and Guangdong Province is the most important region for sweet maize production in South China, ranking the highest in terms of planting area (220 000 ha, which is 66.1% of the total planting area in China), and total yield (3 102 000 t, or 62.1% of sweet maize production in China) in 2014 (Liu et al. 2016). However, some current sweet maize management practices are inappropriate in South China, because continuous sole sweet maize cropping combined with high chemical nitrogen (N) fertilizer application not only causes soil degradation but also leads to the retention of a large amount of mineral N in the soil instead of being absorbed by crops (Chen et al. 2008). The high-N-input practices trigger environmental problems such as groundwater pollution by nitrate and greenhouse gas emissions to the atmosphere (Chen et al. 2010; Bouwman et al. 2013). Those problems become even more severe when cropping two sweet maize seasons per year, because the N fertilizer application is doubled (Chen et al. 2010). Many previous studies have demonstrated that cereallegume intercropping systems as compared to sole crop systems can increase crop productivity, while at the same time they improve land use efficiency (crop productivity per unit area) and reduce environmental problems (Pelzer et al. 2012; Li et al. 2013; Bedoussac et al. 2015; Brooker et al. 2015). In addition, the roots of intercropped maize was deeper than sole-crop maize (Xiao et al. 2013), and maize roots can regenerate after soybean harvesting in the intercrop system (Li et al. 2011), thus roots from intercropped sweet maize can absorb more nitrogen from the soil and reduce soil nitrogen accumulation and the intercropping system can lower the potential risk of N pollution of ground water (Li et al. 2011). In comparison with the sole-crop system, a decrease in nitrate leaching from cereal-legume intercropping has been demonstrated for barley-field bean intercropping (Mariotti et al. 2015), and for barley-pea intercropping (Pappa et al. 2011). Besides that, studies on N2O emissions from cereal-legume intercropping such as barley-pea and maize-soybean systems have shown that intercropping can significantly reduce N2O emissions as compared to barley or maize sole crop systems (Pappa et al. 2011; Huang et al. 2014). Similarly, a study on sugarcane-soybean intercropping in South China revealed a decrease of N2O emissions and N fertilizer requirements while sugarcane yield was maintained (Luo et al. 2016). Maize-legume intercropping is successfully and widely adopted in North China, where it provides benefits such as increased yields (Li et al. 2011), reduced residual soil N concentrations (Li et al. 2011) and N2O emissions (Huang et al. 2014). However, to our best knowledge, the potential of sweet maize-soybean intercropping to allow reduction of nitrogen fertilization under tropical and subtropical conditions has not been assessed. In this study we performed a long-term field

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experiment (3 years, 6 cropping seasons) to investigate the effects of sweet maize-soybean intercropping combined with reduced N fertilization on crop yield, residual soil mineral N and N2O emission to provide a strategy for maintaining sweet maize production and reducing environmental problems in the tropical and subtropical regions of South China. The hypotheses of the present study were that the sweet maize-soybean intercropping system combined with reduced N fertilizer could achieve yield advantage, reduce residual soil mineral N, and mitigate soil N2O emissions in South China. The objectives of the current study were: 1) to determine the yield advantage; 2) to measure residual soil mineral N concentrations; and 3) to examine the cumulative soil N2O and yield-scaled soil N2O emissions under single crop and soybean sweet maize intercropping and different fertilizer rates.

2. Materials and methods 2.1. Experimental site A three-year field experiment was carried out from August 2013 to June 2016, at the Experiment Center of South China Agricultural University, Guangzhou, China (23°08´N, 113°15´E). This area has a tropical ocean monsoon climate. The annual average temperature is 21.9°C, and annual rainfall and evaporation are 1 696 and 1 591 mm, respectively. The soil is classified as a latosolic red soil. The conventional N fertilizer application (360 kg N ha–1) by local farmers was applied for the previous three years to a sole sweet maize field crop. At the start of the experiment, the soil in the upper 20 cm had an average of 17.48 g kg–1 organic matter, 0.77 g kg–1 total N, 71.45 mg kg−1 available N, 85.05 mg kg−1 Olsen P, 200.07 mg kg−1 exchangeable K, and a pH of 6.63.

2.2. Experimental design The experimental design was a two-factor randomized complete block. Factor A was cropping system and factor B was N fertilization rate. The cropping systems were sweet maize (SS) and soybean (SB) in monoculture, and sweet maize-soybean intercropping with crop line ratios of 2:3 (S2B3) and 2:4 (S2B4). S2B3 indicated two rows of sweet maize intercropping with three rows of soybean, and S2B4 indicated two rows of sweet maize with four rows of soybean. The N fertilizer was applied to sweet maize at a conventional rate (360 kg N ha–1, N2), and a reduced rate (300 kg N ha–1, N1). Sole soybean was grown without N fertilization. There were seven treatments (SS-N1, SS-N2, S2B3-N1, S2B3-N2, S2B4-N1, S2B4-N2, and SB) and each treatment repeated three times (n=3) and therefore generating 21 plots in total (17.76 m2 for each plot). There

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Rusinamhodzi et al. (2012).

were eight rows of sweet maize in SS, with row distance and spacing within rows of 60 and 30 cm, respectively (Fig. 1-A). The density of SS was 54 054 plants ha–1. In SB there were 16 rows of soybean with row distance and spacing within rows of 30 and 20 cm, respectively (Fig. 1-B), giving a density of SB of 486 486 plants ha–1. In both S2B3 and S2B4, row distance was 50 and 30 cm for sweet maize and soybean, respectively, and a 30-cm gap was maintained between sweet maize and associated soybean. Spacing with row of sweet maize and soybean were 30 and 20 cm, respectively (Fig. 1-C and D). In S2B3, both sweet maize and soybean were comprised of six rows, and 59% of each intercropped area was occupied by sweet maize and 41% by soybean. The densities of sweet maize and soybean were 39 865 and 199 459 plant ha–1, respectively. In S2B4, sweet maize and soybean comprised six rows and eight rows, respectively, and 51% of each intercropped area was occupied by sweet maize and 49% by soybean. The densities of intercropped sweet maize and soybean were 34 460 and 238 378 plant ha–1, respectively. S2B3 and S2B4 were substitutive intercropping designs where a number of rows of cereal are replaced by legumes according to

2.3. Crop management All of the treatments were double cropping systems, consisting of a spring season, from March to June, and an autumn season, from August to November. The sweet maize cultivar Huazhen was used in this study as it adapted to both spring and autumn seasons. The soybean cultivar Maodou 3 adapted to the spring planting season, is generally high-yielding, and was used in the spring seasons. The soybean cultivar Shanghaiqing is adapted to the autumn planting season, has high protein content, and was used in the autumn seasons. These varieties were representative and widely used by local farmers in South China. Seeds of sweet maize and soybean were planted simultaneously, and soybean was harvested before sweet maize, except in spring 2015 (Appendix A). Sweet maize was supplied with N fertilizer as urea, and soybean was grown without N fertilization. The N fertilizer was injected by hand to 5-cm soil depth. Both sweet maize and soybean were supplied with 150 kg P ha–1 (as calcium superphosphate) and 300 kg

× Sweet maize A

B

60 cm ×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

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C

50 cm

30 cm

Soybean

×

×

×

×

×

×

20 cm

30 cm

D

30 cm 30 cm

×

30 cm

50 cm

30 cm 30 cm

×

×

×

×

×

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×

×

×

×

×

×

×

×

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×

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×

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20 cm 30 cm

20 cm

Fig. 1 Layout of sweet maize and soybean in different cropping systems. A, sole sweet maize. B, sole soybean. C, sweet maizesoybean intercropping with crop line ratios of 2:3. D, sweet maize-soybean intercropping with crop line ratios of 2:4.

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K ha–1 (as muriate of potash). The N and K fertilizers were divided into three proportions: 30, 30 and 40% of N fertilizer, and 15, 40 and 45% of K fertilizer, and were applied as seed dressing, jointing dressing, and head dressing, respectively. P fertilizer was applied as a basal dressing.

2.4. Measurements and statistical analysis Grain yield Crop samples were harvested from all plots when sweet maize and soybean were ripe. For intercrops, two interior rows of sweet maize were collected to measure grain yield of sweet maize, and two interior rows of soybean were collected to measure grain yield of soybean. The area harvested in the single species cropping systems was identical to that harvested in the intercropping systems. The harvested dates of sweet maize and soybean are presented in Appendix A. Total soil mineral N at the time of crop harvest Soil samples were collected by auger (8 cm in diameter) at a soil depth of 0–20 cm when the crops were ripe. In accordance with Li et al. (2011), four soil cores were collected from each plot. In the intercrop plots, two cores were collected from sweet maize rows and mixed thoroughly, and two from soybean rows and again mixed thoroughly. In the sole crops, four soil cores were collected from each plot and mixed thoroughly. Nitrate-N and ammonium-N were determined colorimetrically with an autoanalyzer (TrAAcs 2000; Bran Luebbe, Germany) according to the methods described in Eaton et al. (1999). Nitrate-N was determined after cadmium reduction to nitrite-N followed by sulphanilamide-NAD reaction. Ammonium-N was determined before and after Kjeldahl digestion by the nitroprusside catalysed indophenol reaction. Total soil mineral N in the 0–20 cm soil layer was calculated according to the equation of Emteryd (1989): 20×BD×Cmineral N (1) Total soil mineral N (kg ha–1)= 10 Where, 20 is the thickness of soil layer in cm, BD is the soil bulk density in g cm–3, and Cmineral N is the soil mineral N concentration in mg kg–1. Except in spring 2015, when the BD was 1.25 g cm–3, the bulk density was 1.30 g cm–3 in every season. Soil bulk densities were calculated as mass of oven dry soil core divided by volume of the core (Rusinamhodzi et al. 2012), and the equation was: G×100 (2) BD (g cm–3)= V×(100+w) Where, G is the mass of the oven dry soil core in g, V is the volume of the core in cm3, and w is the water content of soil. The soil sample for bulk density was taken at the same time as the soil sample for mineral N. Soil N2O emissions Soil N2O emissions were measured by

using a static chamber-gas chromatography technique over the entire crop growing season as described by Dyer et al. (2012). Polyvinyl chloride (PVC) chambers (25 cm in height and 15 cm in inner diameter) were permanently inserted into the soil to a depth of 10 cm throughout the experiment. During gas collection, the chambers were sealed with PVC caps (15 cm in inner diameter). Each PVC cap had a rubber septa (2 cm in diameter) used as a sampling port for the extraction of the gases, and was vented using a 10-cm-length of tubing (6 mm in diameter). Two chambers per replicate for each plot were placed between crop rows, and between sweet maize and soybean rows in the intercrop plots. Soil N2O was collected using 60 mL plastic syringes at regular intervals of 15 min (three gas samples per flux measurement: 0, 15, 30 min after closure) between 9:00 and 11:00 a.m. local time every two weeks. Concentrations of N2O were analyzed by gas chromatography on an Agilent 7890B (Agilent Technologies, Santa Clara, California), using high purity standards (0.5 mg L–1) for the calibration and calculation. Soil N2O emission flux was calculated according to Jantalia et al. (2008) as: Soil N2O emissions flux (g m−2 d−1)=(V/A)×(∆c/∆t)×(m/Vm) (3) Where, V is the volume of the chamber (m3), A is the area from which N2O emits into the chamber (m2); ∆c/∆t is the rate of accumulation of N2O gas concentration in the chamber (mg m−3 h−1); and m is the molecular weight of N2O and Vm is the gas molar volume of N2O. Seasonal cumulative N2O emission was calculated according to Singh et al. (1999) as: Cumulative soil N2O emissions=∑ i (Fi×Di) n

(4)

Where, Fi is the rate of N2O emissions flux (g m−2 d−1) in the ith sampling interval, Di is the number of days in the ith sampling interval, and n is the number of sampling intervals. Yield-scaled soil N2O emissions were used to express the cumulative soil N2O emissions per unit grain yield according to Zhou et al. (2014): Yield-scaled N2O emissions (kg t–1)=Cumulative N2O emissions/Grain yield (5) Land equivalent ratio Total land equivalent ratio (TLER) is defined as the relative land area required when growing sole crops to produce the grain yield achieved with intercropping (Willey 1979), and it is the sum of the partial land equivalent ratio (LER) values of sweet maize (LERS) and soybean (LERB) according to Li et al. (2011): Yis LERs= (6) Yss Yib (7) LERB= Ysb TLER=LERs+LERB

(8) Where, Yis and Yss are the grain yields of sweet maize in intercrop and sole crop plots, respectively; Yib and Ysb

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are the grain yields of soybean in intercrop and sole crop plots, respectively. If TLER is greater than 1.00, there is an advantage of intercropping; if TLER is less than 1.00, there is no interspecies advantage. Data analysis An analysis of variance (ANOVA) was conducted using the SPSS19.0 Software package. Comparisons between treatments were measured by using evaluation statements in a one-way ANOVA. Mean values (n=3) were compared using a Duncan’s multiple range test at the 5% level. Graphical representation of the data was made using OriginPro 8.0.

3. Results 3.1. Grain yield and land equivalent ratio Cropping system had a significant effect on crop grain yield (Table 1), which increased with planting density. Averaged over the six seasons, grain yields of sweet maize were (14.73±0.313), (14.86±0.125), (11.00±0.124), (11.23±0.075), (9.41±0.053), and (9.81±0.067) t ha–1 in SS-N1, SS-N2, S2B3-N1, S2B3-N2, S2B4-N1, and S2B4-N2, respectively, and were the highest and the lowest in the SS and S2B4 treatments, respectively (Table 1). Similarly, the highest and the lowest soybean grain yields in this study were in treatments SB and S2B3, respectively (Table 1). Averaged over the six seasons, soybean grain yields were (6.38±0.217), (6.12±0.392),

(8.21±0.152), (7.77±0.200), and (19.42±0.566) t ha –1 in treatments S2B3-N1, S2B3-N2, S2B4-N1, S2B4-N2, and SB, respectively (Table 1). In addition, there was no significant difference in crop yields between the two N fertilizer rates, and crop yields varied markedly among the different growing seasons (Table 2). The TLER for grain yield was above 1, except in spring 2015 and in S2B4 in autumn 2015 (Fig. 2). On average, over the six seasons, the values of TLER did not differ significantly (P=0.17) and were 1.08±0.008, 1.06±0.018, 1.06±0.012, and 1.05±0.003 for S2B3-N1, S2B3-N2, S2B4-N1, and S2B4-N2, respectively. The results indicated that the two intercropping systems could increase land use efficiency. Moreover, N fertilizer rates and intercropping patterns had no significant effect on TLER (Table 2). However, intercropping patterns had a significant effect on partial LER (Table 2). On average for the six seasons, the values of LERS were 0.75±0.007, 0.75±0.003, 0.64±0.002, and 0.66±0.005 for S2B3-N1, S2B3-N2, S2B4-N1, and S2B4-N2, respectively (Fig. 2). Due to higher grain yield of sweet maize in S2B3, LERS was significantly greater (P<0.05) in S2B3 than that in S2B4, which indicated that sweet maize made a greater contribution to yield advantage in S2B3. Similarly, the values of LERB were 0.33±0.009, 0.30±0.019, 0.41±0.009, and 0.39±0.006 for S2B3-N1, S2B3-N2, S2B4-N1, and S2B4-N2, respectively (Fig. 2). Soybean in S2B4 had higher grain yield and obviously greater LERB as compared to S2B3,

Table 1 Sweet maize grain yield, soybean grain yield, and total grain yield in seven treatments and six seasons1) Autumn 2013 Sweet maize grain yield (t ha–1) SS-N1 17.84±0.107 a SS-N2 17.62±0.498 a S2B3-N1 13.06±0.111 b S2B3-N2 13.70±0.580 b S2B4-N1 10.77±0.100 c S2B4-N2 11.74±0.572 c Soybean grain yield (t ha–1) S2B3-N1 7.44±0.694 b S2B3-N2 7.38±0.956 b S2B4-N1 10.23±0.527 b S2B4-N2 9.54±2.520 b SB 23.96±3.056 a Total grain yield (t ha–1) SS-N1 17.84±0.109 bc SS-N2 17.62±0.498 c S2B3-N1 20.49±0.292 abc S2B3-N2 21.08±1.030 abc S2B4-N1 21.01±0.208 abc S2B4-N2 21.28±1.823 ab SB 23.96±3.056 a 1)

Spring 2014

Autumn 2014

Spring 2015

Autumn 2015

Spring 2016

11.43±0.310 a 11.68±0.184 a 9.10±0.093 b 8.69±0.253 b 7.78±0.081 c 7.77±0.302 c

17.16±0.183 a 17.63±0.409 a 13.26±0.823 b 13.39±0.791 b 11.24±0.269 c 11.22±0.578 c

12.95±1.117 a 12.38±0.205 a 8.87±0.573 b 9.13±0.177 b 8.20±0.327 b 8.74±0.177 b

14.92±0.585 a 15.02±0.171 a 11.38±0.344 b 11.88±0.409 b 9.68±0.452 c 9.55±0.180 c

14.08±0.488 a 14.85±0.173 a 10.31±0.345 b 10.60±0.215 b 8.77±0.052 c 9.83±0.170 b

4.86±0.623 b 4.77±0.133 b 6.26±0.330 b 6.66±0.200 b 14.06±2.221 a

11.33±0.629 c 4.04±0.553 bc 3.81±0.176 b 12.51±1.050 bc 3.34±0.460 bc 3.44±0.527 b 15.80±0.989 b 4.80±0.592 b 3.89±0.819 b 15.81±1.683 b 2.47±0.339 c 3.63±0.365 b 33.69±0.398 a 14.71±0.856 a 15.51±1.770 a

6.82±0.166 b 6.38±0. 217 c 5.30±0.973 b 6.12±0.392 c 8.31±0.367 b 8.21±0.152 b 8.53±0.139 b 7.77±0.200 b 14.61±1.901 a 19.42±0.566 a

11.43±0.310 a 11.68±0.183 a 13.96±0.580 a 13.46±0.165 a 14.03±0.257 a 14.43±0.457 a 14.06±2.221 a

17.16±0.181 c 17.63±0.409 c 24.59±0.888 b 25.90±0.763 b 27.04±0.892 b 27.03±3.260 b 33.69±0.398 a

14.08±0.489 b 14.85±0.173 b 17.13±0.506 ab 15.90±1.187 ab 17.08±0.417 b 18.35±0.179 a 14.61±1.901 b

12.95±1.179 ab 12.38±0.203 b 12.91±0.610 ab 12.47±0.636 ab 13.00±0.480 ab 11.21±0.339 b 14.71±0.856 a

14.92±0.585 a 15.02±0.172 a 15.19±0.501 a 15.32±0.811 a 13.58±1.221 a 13.18±0.544 a 15.51±1.770 a

Mean 14.73±0.313 a 14.86±0.125 a 11.00±0.124 b 11.23±0.075 b 9.41±0.053 c 9.81±0.067 c

14.73±0.313 d 14.86±0.125 d 21.25±0.214 a 17.12±0.308 c 19.44±0.080 b 17.18±0.252 c 19.42±0.566 b

SS, sole sweet maize; N1, reduced-N rate; N2, conventional-N rate; S2B3, sweet maize-soybean (2:3) intercropping; S2B4, sweet maize-soybean (2:4) intercropping; SB, sole soybean. Values are means±standard error (n=3). The different lowercase letters indicate the significant difference (P<0.05) among different treatments in the same season.

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Table 2 Three way analysis of variance for crop yield, partial land equivalent ratio, total soil mineral N, cumulative soil N2O emission, and yield-scaled soil N2O emission Item1) Sweet maize yield

Soybean yield

Total yield

LERS

LERB

TLER

Total soil mineral N

Cumulative soil N2O emission

Yield-scaled soil N2O emission

1)

Index Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value Sum square df F-value P-value

Season

N rate

Cropping system

Season× N rate

345.07 5 128.60 <0.01 1 734.96 5 126.43 <0.01 2 455.12 5 188.54 <0.01 0.01 5 1.14 0.35 0.59 5 34.99 <0.01 0.56 5 17.56 <0.01 61 175.37 5 14.89 <0.01 26.93 5 13.63 <0.01 0.18 5 18.76 <0.01

1.76 1 3.28 0.07 2.21 1 0.81 0.37 0.01 1 0.01 0.94 0.00 1 1.57 0.22 0.01 1 3.64 0.06 0.00 1 0.33 0.57 60.56 1 0.07 0.79 5.33 1 13.48 <0.01 0.02 1 11.14 <0.01

512.80 2 477.78 <0.01 54.51 1 19.86 <0.01 174.44 2 33.49 <0.01 0.18 1 79.51 <0.01 0.13 1 38.03 <0.01 0.00 1 0.61 0.44 6 5724.37 2 40.00 <0.01 13.89 2 17.58 <0.01 0.12 2 30.89 <0.01

1.75 5 0.65 0.66 7.77 5 0.57 0.73 5.98 5 0.46 0.81 0.02 5 1.68 0.16 0.03 5 1.80 0.13 0.01 5 0.44 0.82 3 164.25 5 0.77 0.57 5.89 5 2.98 0.02 0.03 5 3.02 0.01

Season× Season× N rate×Cropping N rate×Cropping Cropping system system system 19.36 0.33 3.06 10 2 10 3.61 0.31 0.57 <0.01 0.73 0.83 33.94 0.15 5.65 5 1 5 2.47 0.05 0.41 0.04 0.82 0.84 253.37 0.17 9.56 10 2 10 9.73 0.03 0.37 <0.01 0.97 0.96 0.02 0.00 0.00 5 1 5 1.60 0.36 0.35 0.18 0.55 0.88 0.02 0.00 0.02 5 1 5 1.32 0.00 1.32 0.07 1.00 0.27 0.08 0.00 0.03 5 1 5 2.42 0.06 1.04 0.05 0.80 0.40 8 370.52 518.63 9 674.62 10 2 10 1.02 0.32 1.18 0.44 0.73 0.32 20.96 3.91 2.96 10 2 10 5.31 4.94 0.75 <0.01 0.01 0.68 0.13 0.02 0.02 10 2 10 6.59 4.03 0.89 <0.01 0.02 0.54

LERS, partial land equivalent ratio of sweet mazie; LERB, partial land equivalent ratio of soybean; TLER, total land equivalent ratio.

inferring that soybean made a greater contribution to yield

decrease nitrate leaching in soil (Fig. 3). Over the six

advantage in S2B4. However, the sum of LERS and LERB

seasons, the average values of soil mineral N in SS-N1,

showed no significant difference between S2B3 and S2B4,

SS-N2, S2B3-N1, S2B3-N2, S2B4-N1, S2B4-N2, and SB

which indicated that those two crops in S2B3 and S2B4

were (135.47±13.279), (131.94±3.880), (79.58±6.755),

contributed equally to the advantages of intercropping. In

(74.66±5.402), (75.42±3.522), (81.08±4.736), and

addition, TLER and partial LER were influenced by seasons

(23.62±1.852) kg ha–1, respectively. S2B3-N1 and S2B4-N1

(Table 2).

decreased residual soil mineral N by 35.54 and 41.17% as compared to SS-N1, respectively. S2B3-N2 and S2N4-N2

3.2. Soil mineral N at the time of crop harvest

decreased residual soil mineral N by 43.17 and 39.30% as compared to SS-N2, respectively. In addition, N fertilizer

Residual soil mineral N was significantly (P<0.05) lower

rate did not significantly influence residual soil mineral N,

in sweet maize-soybean treatments than in sole sweet

but the effect of season did have a significant influence on

maize indicating that intercropping could potentially

residual soil mineral in this study (Fig. 3).

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Sweet maize

TLER and partial LER

1.5

1.0

0.5

A

A

a

a

ab a

0.0

A

A

A

A

A

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Fig. 2 Sweet maize and soybean partial land equivalent ratio (LER) and total land equivalent ratio (TLER) for grain yield in different intercropping systems and seasons. S2B3, sweet maize-soybean (2:3) intercropping; S2B4, sweet maize-soybean (2:4) intercropping; N1, reduced-N rate; N2, conventional-N rate. Values are means±standard error (n=3). Different lowercase letters in the white and light gray bars indicate a significant difference (P<0.05) in partial LER of sweet maize (LERS) and soybean (LERB) among treatments in the same season. Different capital letters above the bars indicate a significant difference (P<0.05) in TLER among treatments in the same season.

Sweet maize row

Soil mineral N at the time of crop harvest (kg N ha–1)

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Fig. 3 Soil mineral N at the time of crop harvest in different treatments and seasons. SS, sole sweet maize; S2B3, sweet maize-soybean (2:3) intercropping; S2B4, sweet maize-soybean (2:4) intercropping; SB, sole soybean; N1, reduced-N rate; N2, conventional-N rate. Values are means±standard error (n=3). Different lowercase letters above the bars indicate a significant difference (P<0.05) in soil mineral N for the whole system among different treatments in the same season.

3.3. Cumulative soil N2O emissions and yield-scaled

by season, cropping system, and N fertilizer rate (Table 2).

soil N2O emissions

Over the six seasons, average cumulative soil N2O emissions were (1.63±0.104), (2.10±0.263), (1.19±0.158), (1.15±0.228),

Cumulative soil N2O emissions were significantly influenced

(0.61±0.103), (1.50±0.147), and (0.14±0.009) kg ha–1

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Table 3 Cumulative soil N2O emission, and yield-scaled soil N2O emission in seven treatments and six seasons1) Autumn 2013 Spring 2014 Cumulative soil N2O emission (kg ha–1) SS-N1 0.49±0.010 abc 1.67±0.310 b SS-N2 0.55±0.052 ab 3.03±0.087 a S2B3-N1 0.44±0.095 bc 1.36±0.296 bc S2B3-N2 0.53±0.049 ab 1.96±0.449 ab S2B4-N1 0.25±0.066 bc 0.38±0.111 c S2B4-N2 0.78±0.215 a 2.37±0.736 ab SB 0.18±0.082 c 0.22±0.030 c Yield-scaled soil N2O emission (kg ha–1) SS-N1 0.03±0.001 ab 0.15±0.031 b SS-N2 0.03±0.003 a 0.26±0.011 a S2B3-N1 0.02±0.004 abc 0.10±0.026 bc S2B3-N2 0.02±0.001 abc 0.15±0.034 b S2B4-N1 0.01±0.003 bc 0.03±0.008 c S2B4-N2 0.04±0.013 a 0.16±0.048 b SB 0.01±0.004 c 0.02±0.001 c

Autumn 2014

Spring 2015

Autumn 2015

Spring 2016

Mean

1.70±0.375 ab 2.55±0.489 a 1.31±0.244 abc 1.73±0.539 ab 1.08±0.180 bc 2.20±0.552 ab 0.17±0.030 c

3.66±0.905 a 4.19±0.083 a 1.84±0.281 b 0.81±0.189 bc 0.75±0.219 bc 1.38±0.348 bc 0.08±0.022 c

0.46±0.047 ab 0.85±0.402 a 0.63±0.196 ab 0.34±0.056 ab 0.25±0.104 b 0.21±0.072 b 0.10±0.006 b

1.81±0.308 a 1.45±0.144 ab 1.54±0.207 ab 1.54±0.585 ab 0.93±0.283 ab 2.09±0.871 a 0.12±0.009 b

1.63±0.104 ab 2.10±0.263 a 1.19±0.158 b 1.15±0.228 b 0.61±0.103 c 1.51±0.147 b 0.14±0.009 c

0.10±0.022 ab 0.14±0.028 a 0.05±0.009 bc 0.07±0.024 b 0.04±0.007 bc 0.09±0.024 ab 0.01±0.001 c

0.29±0.085 a 0.34±0.067 a 0.14±0.017 b 0.07±0.017 b 0.06±0.018 b 0.12±0.031 b 0.01±0.001 b

0.03±0.002 ab 0.06±0.026 a 0.04±0.012 ab 0.02±0.005 ab 0.02±0.008 ab 0.02±0.001 b 0.01±0.00 b

0.13±0.019 a 0.10±0.010 a 0.09±0.010 a 0.10±0.038 a 0.06±0.017 ab 0.11±0.047 b 0.01±0.001 b

0.12±0.012 ab 0.15±0.018 a 0.08±0.009 c 0.07±0.015 c 0.04±0.009 d 0.09±0.010 bc 0.01±0.000 d

1)

SS, sole sweet maize; N1, reduced-N rate; N2, conventional-N rate; S2B3, sweet maize-soybean (2:3) intercropping; S2B4, sweet maize-soybean (2:4) intercropping; SB, sole soybean. Values are means±standard error (n=3). The different lowercase letters indicate the significant difference (P<0.05) among different treatments in the same season.

for SS-N1, SS-N2, S2B3-N1, S2B3-N2, S2B4-N1, S2B4-N2, and SB, respectively (Table 3). Cumulative soil N2O emissions were, in most cases, lower in the reduced-N rate compared to the conventional-N rate. The average cumulative soil N2O emissions from SS-N1 and S2B4-N1 were lower by 22.41 and 59.53%, respectively, when compared to SSN2 and S2B4-N2. However, S2B3-N1 showed an increase of 3.02% in cumulative soil N2O emissions compared to S2B3-N2. Intercropping produced less cumulative soil N2O emissions than sole sweet maize. On average, S2B3-N1 and S2B4-N1 reduced cumulative soil N2O emissions by 27.23 and 62.69% as compared to SS-N1. S2B3-N2 and S2B4-N2 reduced cumulative soil N2O emissions by 45.19 and 28.48% as compared to SS-N2. Except for the autumn season in 2015, cumulative soil N2O emissions in S2B4-N1 were the lowest among all treatments except SB (no N application). Yield-scaled soil N2O emissions were significantly influenced by season, cropping system, and N fertilizer rate (Table 2). On average over the six seasons, yield-scaled soil N 2O emissions were (0.12±0.012), (0.15±0.018), (0.07±0.009), (0.07±0.015), (0.04±0.009), (0.09±0.010) and (0.01±0.000) kg t–1 for SS-N1, SS-N2, S2B3-N1, S2B3-N2, S2B4-N1, S2B4-N2, and SB respectively (Table 3). As with cumulative soil N2O emissions, yield-scaled soil N2O emissions decreased in response to reduced-N application and intercropping. Among the intercrop systems, S2B4-N1 produced the lowest yield-scaled soil N2O emissions, which were 29% of those produced by SS-N1, when averaged over the six seasons.

4. Discussion 4.1. Grain yield and land equivalent ratio This study showed that grain yield of sweet maize and soybean were greater in autumn than in spring in most seasons (Table 1), likely because of higher effective accumulated temperature in autumn than in spring in southern China (Chen et al. 2010). Moreover, there was no significant difference in sweet maize yield between the two different (conventional vs. reduced) N fertilization rates (Table 1), indicating that the application rate of N fertilizer by local farmers probably exceeded the amount required for crop growth and could be decreased in order to reduce resource waste and environmental pollution. Sweet maize intercropping with soybean achieved a yield advantage as the TLER was greater than 1. However, the TLER was only slightly greater than 1 because the value of LERB was small (Fig. 2), the low value resulting from the considerably lower grain yield of intercropped soybean compared to sole soybean. The likely explanation is that shading and resource competition by the dominant sweet maize suppressed the associated soybean growth (Bedoussac et al. 2015). Moreover, the growing period of the sweet maize and soybean almost completely overlapped, leading to reduced temporal niche differentiation and enhanced interspecific competition for the same resource (Gou et al. 2016). Yu et al. (2015) reported that high TLER was achieved by allowing temporal niche differentiation to mitigate strong competition between cereal and legume. Fur-

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thermore, in the present study, although sweet maize made a greater contribution to TLER than soybean (LERS>LERB), the major factor in determining intercrop advantage was the productivity of the subordinate soybean. The results are consistent with previous studies that have reported the improved competitive ability of legumes in planting mixtures plays a key role in the enhanced yield of intercropping (Bedoussac et al. 2015; Hu et al. 2016).

may enhance soil organic nitrogen immobilization and reduce soil N mineralization as compared to sole-cropping, thereby possibly decreasing N2O emissions (Cong et al. 2015; Regehr et al. 2015).

4.4. Yield-scaled soil N2O emissions

The probability of nitrate leaching increases with increasing soil mineral N accumulation (Li et al. 2005, 2011). The intercropping treatments (S2B3-N1, S2B3-N2, S2B4-N1, and S2B4-N2) reduced residual soil mineral N and therefore potentially decreased soil nitrate leaching compared to sweet maize alone (Fig. 3). As soybean was grown without N fertilizer input, residual soil mineral N in soybean rows was considerably less than that in sweet maize rows (Fig. 3). Because of the replacement of some rows of sweet maize with soybean in the intercropping systems, the intercropping systems had a lower residual soil mineral N than sole sweet maize. However, for both the intercrop and sole cropping systems, there was no significant difference in total soil mineral N between the two N fertilizer rates, possibly because higher N input led to greater total N loss in the conventional as compared to the reduced-N treatment (Luo et al. 2016).

To address the complex linkages among crop productivity, N fertilization practice and N2O emissions, the term “yieldscaled N2O emissions” was used to determine soil N2O emission per unit of crop grain yield (Zhou et al. 2014). In the present study, due to lower planting density, grain yields of intercropped crops were lower than those in sole crops. However, total crop grain yield increased in the intercropping system when compared to sole sweet maize (Table 1). Higher total crop grain yield, combined with reduced soil N2O emissions, resulted in reduced yield-scaled soil N2O emissions in intercropped systems in comparison with sole sweet maize. These results suggest that sweet maize-soybean intercropping could produce less soil N2O emissions per unit of yield, showing a potential advantage for sustainable and environmentally friendly crop production. Among the intercropping treatments, S2B4-N1 not only maintained sweet maize productivity and increased total grain yield but also reduced the need for N fertilizer application and mitigated soil N2O emission. It is therefore suggested that S2B4-N1 is the optimal system to achieve the lowest yield-scaled soil N2O emissions.

4.3. Cumulative soil N2O emissions

5. Conclusion

Mineral N is the necessary substrate for microbial nitrification and denitrification processes and high N applications generally lead to greater cumulative N2O emissions in agricultural fields (Ma et al. 2010; Hoben et al. 2011). The results of the present study demonstrated that less N fertilizer input in reduced-N treatments (N1) could result in less soil N2O emissions when compared to the conventional-N rate (N2) in most cases (Table 3). Furthermore, this study also indicated that sweet maize-soybean intercropping could reduce cumulative soil N2O emissions as compared to sole sweet maize (Table 3). A potential explanation is that available N in soil was lower in the intercrop treatment as compared to sole sweet maize. Firstly, in the substitutive intercrops, soybeans replaced some of the rows of sweet maize and therefore decreased the total amount of N fertilizer input since soybean was grown without N fertilizer application. Secondly, almost all of the biologically fixed-N may have been acquired by the legume per se, leaving less N in the soil for nitrification and denitrification (Jenson et al. 2012; Migliorati et al. 2015). Further, cereal-legume intercropping

The results from the present study should be helpful in optimizing the sustainable and environmentally friendly production of sweet maize in South China. Sweet maize-soybean intercropping combined with reduced-N application could maintain sweet maize productivity, achieve yield advantage, and decrease N fertilizer waste as demonstrated in this study. It also showed that intercropping with soybean was an environmentally friendly system due to decreased residual soil mineral N and soil N2O emissions by reducing N fertilizer input. Among all of the intercropping treatments, the S2B4-N1 treatment emitted the lowest soil N2O per ton of grain yield (the average of the yield-scaled soil N2O emission was 0.04 kg t–1), combined with the lower soil mineral N (decreased by 41.17%) compared to sole sweet maize, indicating it may be the most sustainable and environmentally friendly cropping system. It will be important for future studies to explore the mechanisms of soil microbial communities involved in nitrification and denitrification that produce different levels of N2O emissions in intercropping and sole-cropping systems.

4.2. Soil mineral N at the time of crop harvest

TANG Yi-ling et al. Journal of Integrative Agriculture 2017, 16(11): 2586–2596

Acknowledgements We would like to thank Dr. Juerg Enkerli at Agroscope in Zurich, Switzerland for proofreading the manuscript and Mr. Chen Xingyuan, Mr. Chen Peishou and Mr. Xie Zhengsheng from South China Agricultural University for helping us to manage sweet maize cultivation for six seasons in the field. The study was supported by the Key Technologies R&D Program of China during the 12th Five-year Plan period (2012BAD14B16-04) and the Science and Technology Development Program of Guangdong, China (2012A020100003 and 2015B090903077). Appendix associated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm

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