390 Agronomy Journa l • Volume106 , I s sue2 • 2014

Crop Economics, Production & Management

UnderstandingDryMatterandNitrogenAccumulation forHigh-YieldingFilm-MulchedMaize

JianliangLiu,AiZhan,LingduoBu,LinZhu,ShashaLuo,XinpingChen,ZhenlingCui, ShiqingLi,*RobertLeeHill,andYingZhao

Published in Agron. J. 106:390–396 (2014)doi:10.2134/agronj2013.0404Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

ABSTRACTFilm mulching is a very common technique in agriculture worldwide, but few studies have focused on the dry matter (DM) and N accumulation of mulched crops. Understanding the grain yield (GY) associated with DM and N accumulation is essential for improving crop production. We conducted a 3-yr field experiment with six N fertilizer rates (0, 100, 200, 250, 300, and 400 kg ha–1) in the semiarid climate of northwest China to determine the GY and DM and N accumulation of film-mulched maize (Zea mays L.). The results showed that relatively high GYs (13.1–15.1 Mg ha–1) were obtained using N fertilizer rates of 200 to 400 kg ha–1 in the 3 yr despite large year-to-year differences in rainfall. When the N rate was below 250 kg ha–1, the GY, DM, and N accumulation increased significantly as the N fertilizer rate increased. A linear-plateau model best described the relationship between the GY and the DM and N accumulation during the pre-silking stage and a linear model the relationship during the post-silking stage. We conclude that optimizing N management to improve DM and N accumulation (especially post-silking) is the key to ensuring a high yield of film-mulched maize.

J. Liu, A. Zhan, L. Bu, L. Zhu, S. Luo, S. Li, and Y. Zhao, State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A and F Univ., Yangling 712100, China; X. Chen, and Z. Cui, Center for Resources, Environment and Food Security, China Agricultural Univ., Beijing 100193, China; and R.L. Hill, Dep. of Environmental Science and Technology, Univ. of Maryland, College Park, MD 20742. Received 22 Aug. 2013. *Corresponding author (sqli@ms.iswc.ac.cn).

Abbreviations: DM, dry matter; EN, ear numbers per 10 m2; GY, grain yield; KN, kernel numbers per ear; KW, 1000-kernel weight; N0–N400, 0 to 400 kg nitrogen fertilizer applied per hectare; Nc, nitrogen concentration.

Drylandscompriseapproximately45% of world’s land area and are indispensable for global food production (Schimel, 2010). Water shortage is the major limiting factor for crop produc-tion in dryland areas (Rockström et al., 2007). Due to the uneven distribution of interannual precipitation, crop yields fluctuate greatly over multiple years (Berzsenyi et al., 2000; Fan et al., 2005). To obtain high yields, large quantities of N fertilizers are usually applied in dryland areas (Richter and Roelcke, 2000; Liu et al., 2011), which tends to result in low N use efficiency (Cassman et al., 2002), high greenhouse gas emissions (Burney et al., 2010), and soil mineral N accumulation (Guo et al., 2010). Optimizing soil and crop management to improve the crop productivity and yield stability of dryland agriculture is crucial to ensuring future food security (Lele, 2010).

Numerous studies have indicated that plastic film mulching can markedly increase crop yields, especially in arid and semiarid areas (Raeini-Sarjaz and Barthakur, 1997; Zhou et al., 2009; Liu et al., 2010; Sharma et al., 2011). For example, Sharma et al. (2011) reported that maize GY was significantly higher (58%)

in the film-mulched treatment compared to the non-mulched treatment in a rain-fed area in India. In the semiarid climate of northwest China, film mulching has already been extensively applied to maize production, because it can dramatically increase GY and benefit farmers economically (Liu et al., 2009; Zhou et al., 2009). In particular, a recently developed technique of ridge-furrow coupled with film mulching has been widely adopted by farmers. For instance, the application areas using this technique had reached 193 by 103 ha in 2008 in Gansu Province, which occupied about 7% of farmland and contributed 20% to the total grain production (Wang, 2009). Moreover, high yield levels of maize (>14 Mg ha–1) could be obtained grown with this technique (Chen et al., 2009). The increased yield was mainly due to the higher soil temperature and water content induced by film mulching, especially during the early growth season, which promoted crop growth and DM accumulation (Liu et al., 2010). This mechanism implies that the N accumulation also increased. However, few studies have focused on the N accumulation of high-yielding film-mulched maize (Xu et al., 2010). Understanding the accumulation of N is essential for optimizing N management to further improve yield.

The higher soil temperature during the early growth season under film-mulching conditions is beneficial for promoting crop growth and development, but the higher soil temperature during the later growth season may accelerate crop senescence and reduce DM accumulation, consequently decreasing crop yields. This negative effect has already been observed in maize production (Liu et al., 2010). Thus, we speculate that the high yield of film-mulched

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maize is mainly due to the increased DM accumulation during the early growth season. However, this speculation is inconsistent with previous studies (Tollenaar and Lee, 2006, 2011; Ciampitti and Vyn, 2012), which have reported that the maize GY increase is largely dependent on the improvement of DM accumulation after the silking stage. Therefore, an understanding of the relationships between GY and DM accumulation before and after the silking stage is essential for improving management practices to further increase film-mulched maize yield.

We conducted a 3-yr field study of high-yielding film-mulched maize to determine the GY and DM and N accumulation under different N fertilizer application rates, and the response of GY to DM and N accumulation before and after the silking stage.

MATERIALS AND METHODSSite Description

The study was conducted from 2009 to 2011 at the Changwu Agricultural and Ecological Experiment Station (35.28°N, 107.88°E; 1200 m elevation) located on the Loess Plateau of China. The field site has a typical semiarid climate with an average annual temperature of 9.2°C, and an annual evaporation from a free water surface of 1565 mm. The average annual rainfall was 555 mm, with 73% falling during the maize growth season. The rainfall was 481, 588, and 644 mm in 2009, 2010, and 2011, respectively, with 372, 496, and 477 mm falling during the maize growth season, respectively (Table 1). The area is rain-fed, and the primary cropping system consists of one maize or wheat crop harvested per year. The soils at this site are Cumuli-Ustic Isohumosols (Gong et al., 2007). The top 20 cm of the soil had a pH of 8.4, a bulk density of 1.3 g cm–3, contained 15.5 g kg–1 soil organic matter, 0.95 g kg–1 total N, 15.9 mg kg–1 available P (Olsen-P), 136.7 mg kg–1 available K (NH4OAc-K), and 22.4 mg kg–1 mineral N.

Experimental Design and Treatments

In this study, N fertilizer rates of 0, 100, 200, 250, 300, and 400 kg ha–1 were examined and designated as N0, N100, N200, N250, N300, and N400, respectively. Nitrogen fertilizer in the form of urea (N 46%) was applied three times. A basal dressing of 40% of the urea was broadcast over the soil before sowing and then plowed into the subsurface. Thirty percent was applied at the 10-leaf stage (V10), and the remainder was applied at the silking stage (R1) using a hole-sowing machine. The basal N fertilizer was applied together with 40 kg P ha–1 in the form of calcium superphosphate (P2O5 12%) and 80 kg K ha–1 in the form of potassium sulfate (K2O 45%). The experiment was arranged in a randomized complete block design with four replicates for each treatment using a plot size of 56 m2 (7 by 8 m). In 2010 and 2011, each plot was located at the same site as that in 2009.

A mulching pattern consisting of the repetition of a broad and a narrow ridge and a furrow covered by a sheet of plastic film was used (Fig. 1). The broad ridges were 60 cm wide and 10 cm high; the narrow ridges were 40 cm wide and 15 cm high. Both ridges were manually mulched with the plastic film after being constructed. The joint of the two pieces of film was in the midline of the broad ridge, and the joint was secured by placing soil on top of the film.

A high-yielding maize hybrid (Pioneer 335) was selected and planted at a density of 85,000 plants ha–1 in monoculture. The maize seeds were sown in the furrows to a depth of 5 cm using a hand-powered hole-drilling machine on 23, 27, and 28 April in 2009, 2010, and 2011, respectively, and the cobs were manually harvested on 18, 15, and 23 September in 2009, 2010, and 2011, respectively.

Plant Sampling and Analysis

Plant samples were collected at the 6-leaf stage (V6), 10-leaf stage (V10, only in 2011), 12-leaf stage (V12, in 2009 and 2010), silking stage (R1), milk stage (R3, in 2010 and 2011), dough stage (R4, only in 2009), dent stage (R5, in 2010 and 2011), and physiological maturity (R6). On each sampling occasion, three adjacent plants in the same row were selected randomly and cut at ground level. The plant samples were dried initially at 105°C for 30 min and then oven-dried to constant weight at 80°C before being ground for chemical analysis. At the R6 stage, plant samples were divided into grain and straw. All of the plant samples were analyzed for nitrogen concentration (Nc) (micro-Kjeldahl), and the N content was calculated by multiplying the Nc by the oven-dry weight. At harvest, 10 m2 (four rows each 2.5 m long) in the middle of each plot were manually harvested to determine the GY and yield components (ear numbers per 10 m2, kernel numbers per ear, and 1000-kernel weight). The GY was expressed at 15.5% moisture.

Data Analysis

The effects of the treatments and years on the measured parameters were evaluated using the GLM procedure of SAS (SAS Institute, 1998). Mean differences between the treatments were compared by least significant difference (LSD) at P < 0.05. The regression analysis was generated by using the NLIN procedure of SAS (SAS Institute, 1998).

RESULTSGrain Yield and Yield Components

Analysis of variance indicated that N application rate, year, and N × year had significant effects on GY (Table 2). The application of N fertilizer significantly increased the GY. The average GYs of the N-fertilized treatments were higher than those of the N0 treatment by 20.7, 153.1, and 97.4% in 2009, 2010, and

Table1.Annualandmonthlyrainfallatthestudysite:long-term(previ-ous20yr)andduring2009,2010,and2011.

Timeperiod Long-term 2009 2010 2011———————-mm———————-

Annual 555 481 588 644April 34 16 38 7May 47 59 32 69June 72 14 29 30July 98 142 165 112August 105 101 191 78September 87 53 79 198

Fig.1.Schematicdiagramillustratingthetworidgesandfurrowarrangementwithplasticfilmmulching.

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2011, respectively. The highest GYs were recorded in the N250 treatment: 14.0, 15.1, and 14.2 Mg ha–1 in 2009, 2010, and 2011, respectively, although no significant difference was observed between the N250, N300, and N400 treatments. In 2010 and 2011, the GYs in the N0 and N100 treatments were significantly lower than that in 2009, but no significant difference was observed for the other treatments in the three experimental years except for the N300 treatment, which was significantly higher in 2010.

The increase in GY with N fertilizer rates was mainly attributed to the kernel numbers per ear (KN) and 1000-kernel weight (KW) but not the ear numbers per 10 m2 (EN) (Table 2). Both the KN and KW increased with N fertilizer application. The KN showed no significant difference between the N200, N250, N300, and N400 treatments, but these values were markedly higher than those for the N0 and N100 treatments except for the N100 treatment in 2009. Over the 3 yr, the mean values of KN were 341, 448, 512, 538, 527, and 520 for the N0, N100, N200, N250, N300, and N400 treatments, respectively. The KW showed a pattern similar to that of the KN with changes in the N levels and averaged 235, 257, 273, 286, 281, and 278 g for the N0, N100, N200, N250, N300, and N400 treatments, respectively, in the three experimental years. The EN and KN varied among years, and N × year interaction had a significant effect on KN (Table 2). For the N0 and N100 treatments, the KN in 2010 and 2011 were significantly lower than that in 2009, but no significant difference was observed for the other treatments among the 3 yr.

Dry Matter Accumulation and the Grain Yield Response

Overall, the N applications increased the DM accumulation over the entire growth season (Fig. 2). In 2009, however, no significant difference in DM accumulation in each sampling stage was observed between the N200, N250, N300, and N400 treatments, but the productions were significantly higher than that in the N0 treatment during the post-silking stage. In 2010 and 2011, the amounts of DM accumulated were markedly lower in the N0 and N100 treatments than in the other treatments after the third sampling (R1 stage), but no significant difference was observed between the N250, N300, and N400 treatments.

Analysis of variance indicated that N application rate, year, and N × year had significant effects on DM accumulation during the pre- and post-silking stage (Table 3). The DM accumulation during both the pre- and post-silking stages increased with increasing N application rate. During the pre-silking stage, no significant difference in DM accumulation was observed between the N250, N300, and N400 treatments but these values were significantly higher than that in the N0 and N100 treatments. During the post-silking stage, the lowest DM accumulation was observed in the N0 treatment. And no significant difference in DM accumulation was observed among the N200, N250, N300, and N400 treatments except for the N400 treatment in 2010. In general, more DM was accumulated during the post-silking stage than that for the pre-silking stage for the N-fertilized treatments.

Table2.Maizegrainyield(GY),earnumbersper10m2(EN),kernelnumbersperear(KN),and1000-kernelweight(KW)fordifferentNtreatments(N0-N400represent0–400kgNfertilizerappliedperhectare)in2009,2010,and2011.

TreatmentGY EN KN KW

2009 2010 2011 2009 2010 2011 2009 2010 2011 2009 2010 2011—————Mgha-1————– —————g—————-

N0 11.0c† 5.3d 6.6c 80a 80a 82a 480b 252c 290c 242b 232d 231cN100 12.6b 10.0c 9.9b 82a 81a 82a 510ab 427b 406b 262ab 257c 253bN200 13.1ab 13.3b 13.4a 81a 79a 81a 505ab 513a 518a 272a 272b 274aN250 14.0a 15.1a 14.2a 81a 78a 81a 520a 561a 532a 283a 295a 281aN300 13.3ab 14.7a 13.9a 80a 78a 81a 518a 540a 523a 271a 293a 278aN400 13.2ab 14.0ab 13.9a 82a 80a 83a 507ab 528a 524a 271a 287a 275aSourceofvariationN *** ns‡ *** ***Year *** ** *** nsN×Year *** ns *** ns**SignificantatP<0.01.***SignificantatP<0.001.†MeanvalueswithincolumnsfollowedbydifferentlettersaresignificantlydifferentatP<0.05bytheleastsignificancedifferencetest.‡ns,notsignificant.

Fig.2.DrymatteraccumulationcurvesfordifferentNleveltreatments(N0-N400represent0–400kgNfertilizerappliedperhectare)in2009,2010,and2011.Errorbarsarestandarddeviationsofthemean(n=4).Solidarrowsdenotetimesofsilking.

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Except for the N100 treatment in 2011, the DM accumulations post-silking accounted for 54 to 62% of the total accumulation at harvest (Table 3). The DM accumulation was significantly affected by year. For the N0 and N100 treatments, the DM accumulations during the post-silking stage in 2010 and 2011 were significantly lower than that in 2009.

The GY response to the DM accumulation during the pre-silking stage exhibited a linear-plateau shape, and the minimum DM accumulation required to obtain the highest GY was 8.8 Mg ha–1. In contrast, a positive linear relationship was observed between the GY and the DM accumulation during the post-silking stage (Fig. 3).

Nitrogen Concentration, Nitrogen Accumulation, and the Grain Yield Response

The plant Nc at silking, and the straw and grain Nc at maturity varied among N application rates and years, with a significant N × year interaction for these parameters (Table 4). Nitrogen fertilization significantly increased the plant Nc at silking, and the straw and grain Nc at maturity. At silking, the plant Nc in the N400 treatment was significantly higher than that in the other treatments in the first 2 yr except for the N300 treatment in 2010. However, in 2011, the plant Nc did not differ significantly among the N200, N250, N300, and N400 treatments but was markedly higher than that in the N0 and N100 treatments. At maturity, the grain Nc showed no significant difference between the N200, N250, N300, and N400 treatments except for the N200

treatment in 2010, but these values were markedly higher than that for the N0 treatment. The straw Nc at maturity increased with the N fertilizer rate, and the highest values were found in the N200, N250, N300, and N400 treatments in 2009, and the N300 and N400 treatments in 2010 and 2011. The plant Nc at silking and straw Nc at maturity were significantly affected by year, but the year did not affect grain Nc.

Table3.Drymatteraccumulationduringthepre-andpost-silkingstagesfordifferentNtreatmentsin2009,2010,and2011.

TreatmentPre-silking Post-silking

2009 2010 2011 2009 2010 2011——————————–Mgha–1——————————–

N0 7.6c† 6.6c 6.3c 10.5b 3.3d 5.7bN100 8.3c 8.0b 8.3b 12.2a 10.1c 7.7bN200 8.7b 8.5a 8.8ab 12.3a 13.5ab 12.3aN250 9.2ab 8.8a 9.7a 12.8a 14.7a 12.7aN300 9.4ab 9.2a 9.5a 12.3a 14.3ab 12.0aN400 9.8a 9.3a 10.0a 11.4ab 13.0b 11.7aSourceofvariationN *** ***Year ** ***N×Year * ****SignificantatP<0.05.**SignificantatP<0.01.***SignificantatP<0.001.†Meanvalueswithincolumnsfollowedbydifferentlettersaresignificantlydiffer-entatP<0.05bytheleastsignificancedifferencetest.

Fig.3.Grainyieldresponsetodrymatteraccumulationduringpre-andpost-silkingstages.Relationshipswerefittedtodatafromindividualreplicates.Asterisks(**)indicateasignificantdifferenceatP<0.01.

Table4.NitrogenconcentrationatsilkingandmaturityfordifferentNtreatmentsin2009,2010,and2011.

Treatment

Silking MaturityGrain Straw

2009 2010 2011 2009 2010 2011 2009 2010 2011———————————————————————————–%—————————————————–————————

N0 1.37c† 1.02e 0.70c 1.24b 1.08c 0.99c 0.72b 0.49d 0.41dN100 1.46b 1.27d 1.20b 1.33a 1.18bc 1.23b 0.73b 0.53d 0.56cN200 1.50b 1.39cd 1.67a 1.39a 1.28b 1.38a 0.81a 0.63c 0.70bN250 1.50b 1.48bc 1.70a 1.39a 1.43a 1.42a 0.85a 0.77b 0.69bN300 1.48b 1.58ab 1.77a 1.37a 1.48a 1.50a 0.88a 0.81ab 0.78abN400 1.56a 1.69a 1.79a 1.40a 1.53a 1.45a 0.88a 0.85a 0.81aSourceofvariationN *** *** ***Year ** ns‡ ***N×Year *** *** *****SignificantatP<0.01.***SignificantatP<0.001.†MeanvalueswithincolumnsfollowedbydifferentlettersaresignificantlydifferentatP<0.05bytheleastsignificancedifferencetest.‡ns,notsignificant.

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Similar to the DM accumulation, the N accumulation increased with the N application over the whole growth season (Fig. 4). In 2010 and 2011, the N accumulations in the N0 and N100 treatments were significantly less than those in the other treatments after the second sampling (V12 in 2010 and V10 in 2011). In general, the N accumulations showed no significant difference between the N250, N300, and N400 treatments in most of the growing stages.

Analysis of variance indicated that N application rate, year, and N × year had significant effects on N accumulations during both the pre- and post-silking stages (Table 5). The N fertilizer applications markedly increased the N accumulation during the pre-silking stage, and the highest values were found in

the N400 treatment in 2009 and 2010 (152 and 157 kg ha–1, respectively), and in the N250, N300, and N400 treatments in 2011 (165–179 kg ha–1). During the post-silking stage, no significant difference in N accumulation was observed among the N200, N250, N300, and N400 treatments except for the N200 treatment in 2010, which ranged from 94 to 113, 113 to 133, and 72 to 84 kg ha–1 in 2009, 2010, and 2011, respectively. For the N0 and N100 treatments, the N accumulations during both the pre- and post-silking stages clearly decreased with years.

The GY response to the accumulation of N during the pre-silking stage also fit a linear-plateau model, and the minimum N accumulation required to obtain the highest GY was 132 kg ha–1. However, the GY increased linearly as the N accumulation increased during the post-silking stage (Fig. 5).

DISCUSSIONIn the present study, the GYs for the N0 and N100 treatments

were significantly lower in 2010 and 2011 than that in 2009 (Table 2), which was mainly due to the N inputs being lower than the maize N requirement, resulting in soil N depletion (Berenguer et al., 2009). As a result, the GYs were markedly lower in the following 2 yr because the N deficit limited DM and N accumulation, especially during the post-silking stage (Tables 3 and 5). Regardless of the N0 and N100 treatments, the GYs in the three experimental years were 13.1 to 15.1 Mg ha–1 (Table 2), which were much higher than the maize GYs of local farmers (5.3 Mg ha–1 on average) (Meng et al., 2013). Moreover, the GYs obtained in the present study were 2.3 to 2.7 times the mean maize GY in China (5.7 Mg ha–1) and 2.5 to 2.9 times the mean maize GY globally (5.2 Mg ha–1) in 2011 (FAO, 2013), and they were also higher than the average county-level yields (2.4–8.1 Mg ha–1)

Fig.4.NitrogenaccumulationcurvesfordifferentNleveltreatmentsin2009,2010,and2011.Errorbarsarestandarddeviationsofthemean(n=4).Solidarrowsdenotetimesofsilking.

Fig.5.GrainyieldresponsetoNaccumulationduringthepre-andpost-silkingstages.Relationshipswerefittedtodatafromindividualreplicates.Asterisks(**)indicateasignificantdifferenceatP<0.01.

Table5.Nitrogenaccumulationduringthepre-andpost-silkingstagesfordifferentNtreatmentsin2009,2010,and2011.

TreatmentPre-silking Post-silking

2009 2010 2011 2009 2010 2011—————————-kgha–1———————-——

N0 104e† 67f 45d 75b 8d 38cN100 121d 101e 99c 94ab 50c 45bcN200 131c 118d 146b 104a 94b 79aN250 138b 131c 165a 113a 133a 77aN300 140b 144b 168a 107a 128a 84aN400 152a 157a 179a 94ab 113ab 72abSourceofvariationN *** ***Year *** ***N×Year *** ******SignificantatP<0.001.†Meanvalueswithincolumnsfollowedbydifferentlettersaresignificantlydiffer-entatP<0.05bytheleastsignificancedifferencetest.

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but comparable to the upper level of highest reported yields (9–16 Mg ha–1) for rain-fed maize in the western U.S. Corn Belt (Grassini et al., 2009).

Previous studies have indicated that optimized soil and crop management is essential for obtaining high yields (Chen et al., 2011; Mueller et al., 2012). We speculate that three reasons were responsible for the high yields in the present study: (i) increased water availability, (ii) N was no longer limiting, and (iii) higher yielding varieties were used. The mulched film effectively captured the limited rainfall and retained it in the soil, reduced soil evaporation, and increased soil water availability, thus promoting maize growth and development (Liu et al., 2010; Sharma et al., 2011). Liu et al. (2010) reported that the film-mulched maize GY was significantly higher (20–25%) than the non-mulched maize GY in the same area, while the total water consumption was similar for the two treatments. In a drier area, the yields increased 5.6 to 10.4 times with the film mulching (Zhou et al., 2009). The rainfall in 2009 was lower than the long-term average (Table 1), but the GYs obtained under N rates of 200 to 400 kg ha–1 were not significantly different than those in the wet years of 2010 and 2011 (Table 2), suggesting that yield stability was increased and that high yield could be obtained in most years under film-mulching conditions.

In this study, the N fertilizer application rates and times in the N200, N250, N300, and N400 treatments nearly satisfied the maize N requirements. Studies have indicated that synchronizing the N supply with crop N demand is crucial to improving crop yields (Cassman et al., 2002; Chen et al., 2011). However, the GY did not increase further when the N fertilizer rate increased from 250 to 300 or 400 kg ha–1, indicating that 200 to 250 kg N ha–1 is rational for high-yielding film-mulched maize. These application rates are comparable to the rates (237 kg N ha–1 on average) recommended for high-yielding maize in China (Chen et al., 2011), and the rates (183 kg N ha–1 on average) used for high-yielding maize in Nebraska (Grassini et al., 2011).

A high-yielding maize hybrid (Pioneer 335) was selected and planted at a high density determined by the Hybrid-Maize model (Yang et al., 2006). Numerous studies have reported that the increase in maize yield is mainly dependent on the breeding of high-yield varieties and the increase in plant density (Boomsma et al., 2009; Hammer et al., 2009; Tollenaar and Lee, 2011). The plant density in this study was 85,000 plants ha–1, which was higher than the densities in most farmers’ fields (<60,000 plants ha–1, Chen et al., 2009) and comparable to the densities used in commercial maize production in North America (75,000–80,000 plants ha–1) (Lee and Tollenaar, 2007).

For the high-yielding film-mulched maize, the contribution of DM accumulation pre-silking to GY may be greater than the accumulation during the post-silking stage, because film mulching promoted DM accumulation during the early growth season but accelerated plants’ senescence and shortened the grain-filling period (Liu et al., 2010). However, the present study showed that the DM accumulations during the post-silking stage accounted for 54 to 62% of the total accumulation for the whole growth period when GY was higher than 10 Mg ha–1, which were higher than the proportion (50%) reported by Tollenaar et al. (2004), most likely because the GY was higher in the present study (10.0–15.1 Mg ha–1 vs. 6.0–6.9 Mg ha–1). Moreover, we found that the GY exhibited a linear-plateau correlation to the DM

accumulation pre-silking, implying that the over-accumulation of DM pre-silking is unnecessary. This is probably because the ear growth rate and kernel number increase were constrained by the limited re-translocation of DM to the developing grains during the critical bracketing silking period (Andrade et al., 1999). However, the GY increased linearly with the increase in DM accumulation post-silking (Fig. 3). This finding is in line with previous studies (Tollenaar and Lee, 2006, 2011; Ciampitti and Vyn, 2012), which reported that the increase in maize yield relies on the consistent improvement of DM accumulation, especially during the post-silking stage, because the promotion of DM accumulation post-silking is significant for increasing the final kernel number and kernel weight (Borrás and Otegui, 2001; D’Andrea et al., 2008).

Dry matter accumulation clearly increased with the N applications over the entire growth season (Fig. 2). This was mainly due to the N application increased plant Nc (Table 4) and N accumulation (Fig. 4), promoted photosynthesis, and thus increased DM. High levels of N accumulation are essential for high yields (Ciampitti and Vyn, 2012). However, although most of the N was accumulated during the pre-silking stage (50–94% of the total accumulation), we found that the GY did not consistently increase with increasing N accumulation pre-silking, but it linearly increased with increasing N accumulation post-silking (Fig. 5). This could be attributed to the high N uptake capacity but low N remobilization efficiency of modern late-senescing maize hybrids (Ciampitti and Vyn, 2012). A high N accumulation post-silking could prevent N remobilization from leaves to grain, maintain the leaves in a “functional stay-green” state to accumulate more DM, and ultimately increase the GY (Lee and Tollenaar, 2007; Tollenaar and Lee, 2011). Our results also showed that excessive N fertilizer application (e.g., 300 and 400 kg ha–1) did not result in higher N accumulation post-silking (Table 5), most likely due to limited root growth and distribution in deep soil layers (Oikeh et al., 1999), resulting in a reduction in N absorption. These root limitations may have been exacerbated due to the lack of additional N fertilizer top-dressed during the later growth season, and increased rainfall during July to September (Table 1) often transported soil mineral N to deep soil layers (data not shown). Studies have reported that splitting or delaying N application can maintain soil N supply, promote N accumulation, and increase GY (Randall et al., 2003; Abbasi et al., 2012). Therefore, optimizing N fertilization to improve N accumulation, especially post-silking, is essential for maintaining high yields of film-mulched maize.

CONCLUSIONSOur study indicated that improved N management is crucial for

obtaining high maize yield in semiarid areas. Under N fertilizer rates of 200 to 250 kg ha–1, high GYs were consistently obtained in the 3 yr. Consistent improvements in DM and N accumulation during the post-silking stage are essential for obtaining high yields of film-mulched maize. Nitrogen fertilization increased DM and N accumulation over the entire growth season, and thereby increased the final GY, but excessive N fertilizer applications did not further increase DM or N accumulation. These findings contribute to our understanding of the DM and N accumulation, and the GY response of high-yielding film-mulched maize. This study provides valuable information for optimizing management practices to improve maize production.

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ACKNOWLEDGMENTS

This research was financially supported by the National Natural Science Foundation of China (31270553), the Ministry of Science and Technology of China (2009CB118604), and the Special Fund for Agricultural Profession (201103003).

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