Research Article: Water use efficiency and evapotranspiration in maize-soybean relay strip intercrop systems as affected by planting geometries

Date Published: June 9, 2017

Publisher: Public Library of Science

Author(s): Tanzeelur Rahman, Xin Liu, Sajad Hussain, Shoaib Ahmed, Guopeng Chen, Feng Yang, Lilian Chen, Junbo Du, Weiguo Liu, Wenyu Yang, Guoping Zhang.

http://doi.org/10.1371/journal.pone.0178332

Abstract

Optimum planting geometries have been shown to increase crop yields in maize-soybean intercrop systems. However, little is known about whether changes in planting geometry improve the seasonal water use of maize and soybean intercrops. We conducted two different field experiments in 2013 and 2014 to investigate the effects of changes in planting geometry on water use efficiency (WUE) and evapotranspiration (ETc) of maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] relay strip intercrop systems. Our results showed that the leaf area index of maize for both years where intercropping occurred was notably greater compared to sole maize, thus the soil water content (SWC), soil evaporation (E), and throughfall followed a decreasing trend in the following order: central row of maize strip (CRM) < adjacent row between maize and soybean strip (AR) < central row of soybean strip (CRS). When intercropped, the highest grain yield for maize and total yields were recorded for the 40:120 cm and 40:160 cm planting geometries using 160 cm and 200 cm bandwidth, respectively. By contrast, the highest grain yield of intercropped soybean was appeared for the 20:140 cm and 20:180 cm planting geometries. The largest land equivalent ratios were 1.62 for the 40:120 cm planting geometry and 1.79 for the 40:160 cm planting geometry, indicating that both intercropping strategies were advantageous. Changes in planting geometries did not show any significant effect on the ETc of the maize and soybean intercrops. WUEs in the different planting geometries of intercrop systems were lower compared to sole cropping. However, the highest group WUEs of 23.06 and 26.21 kg ha-1 mm-1 for the 40:120 cm and 40:160 cm planting geometries, respectively, were 39% and 23% higher than those for sole cropping. Moreover, the highest water equivalent ratio values of 1.66 and 1.76 also appeared for the 40:120 cm and 40:160 cm planting geometries. We therefore suggest that an optimum planting geometry of 40:160 cm and bandwidth of 200 cm could be a viable planting pattern management method for attaining high group WUE in maize-soybean intercrop systems.

Partial Text

Scarce water resources is one of the crucial factors that contributes to the decline in agricultural productivity [1]. The current challenge in agriculture is to produce more yields by utilizing less water, especially in regions with limited land and water resources [2]. Maximizing crop water productivity by the most effective utilization of rain water resources is particularly important in arid land agriculture [3]. How scarce water resources can be better absorbed by crops, is the core objective of arid land agriculture [4].

Intercropping involves the growing of two or more crop species different in their growth habit, phenological attributes, resource use, and productivity [34]. Maize-soybean intercrops have shown to use resources in different spatial and temporal sequences owing to their root length density and canopy structure [35]. Optimum planting geometry could increase the total yields for maize-soybean relay strip intercrop systems [13]. In this study, we investigated the effects of changes in planting geometry on WUE and ETc of maize and soybean intercrops. During the two-year period, the LAI of maize in the different planting geometries of intercropping using 160 and 200 cm bandwidth was considerably higher compared to that of sole maize (Fig 3A and 3B), which could be due to the border row effect [22, 36]. At 60–65 DAS, the LAI of intercropped maize reached a maximum for the 20:140 cm and 20:180 cm planting geometries. Later LAI of intercropped maize gradually decreased with increasing distance between maize narrow-row spacing. We believe this is a result of decrease in crop coverage and dying of lower leaves [29]. Besides, the negative relationship between average maize LAI and maize narrow-row spacing, implying a reduction in crop coverage with increasing distance between maize narrow-row spacing (Fig 4A and 4B). This result is supported by a previous study, where LAI of maize and LAI affecting photosynthesis (LAIp) were markedly decreased by 4.7–31.2% when the distance between maize narrow-row spacing increased from 35 to 100 cm [37]. In general, the average SWC in different planting geometries of intercropping was greater compared to sole cropping (Table 1). This result is synonymously related with those from previously reported studies, demonstrating that when roots are in drying soil, substantial amount of abscisic acid (ABA) can be produced and transported through the xylem to the shoots, thereby regulating stomatal opening and thus helping in soil water saving [38, 39, 40]. Furthermore, the imbalanced but slightly higher SWC within the three strips of maize-soybean relay strip intercrop systems may be due to the difference in root mass density and crop coverage of maize and soybean, as each component crop in strip intercrop systems more likely taken up the water from its strip first and intermixed zone later [29, 41]. Similarly, changes in planting geometries had decreased the canopy coverage of maize plants, thereby altering the microclimate of the field by increasing air transport, decreasing air water vapor content and ultimately resulting in greater soil E [42]. In addition, a decreasing trend of soil E within the three strips of intercropping was more likely due to the difference in canopy coverage and root mass density of maize and soybean (Table 2). This result is in agreement with those from previously published reports [29, 41]. Greater soil E in different planting geometries of intercropping compared to sole maize was closely related with poor covering of the surface soil by the wide soybean rows, whereas on the other side, sole maize provided a uniform covering to the surface soil and resulting in lower soil E [43]. Throughfall had shown remarkable variations within the three strips of intercropping, however, throughfall of maize strip showed a notable increment with increasing distance between maize narrow-row spacing (Table 3). This was rather because of the changes in canopy structure of maize plants related to planting geometries [29, 44]. Despite of changes in canopy structure, higher average rainfall in 2014 than that in 2013 could probably also accounted for the higher throughfall of the 200 cm bandwidth (Fig 1). Moreover, the lower grain yields of maize-soybean intercrops compared to corresponding sole crop are consistent with those from previously published reports [16, 45]. The narrow maize row spacing had a dominant effect on the grain yields of intercropped maize. The lowest grain yields of intercropped maize for the 20:140 cm and 20:180 cm planting geometries could be associated with the severe intraspecific competition between maize plants. However, the intraspecific competition between maize plants weakened and the grain yields of intercropped maize reached a maximum with increasing distance between maize narrow-row spacing. Contrasting a decline was seen for the grain yields of intercropped soybean with increasing distance between maize narrow-row spacing (Table 4). This was more likely due to the shading effect caused by maize plants on soybeans in the relay strip intercrop systems [13, 46]. The highest total yields for the 40:120 cm planting geometry using 160 cm bandwidth and 40:160 cm planting geometry using 200 cm bandwidth could be due to improvement in the light environment of the soybean canopy and lower intraspecific competition [13]. The variations in LERs ranged from 1.41–1.62 in 2013 and 1.61–1.79 in 2014, indicating that planting geometries directly affect LERs (Table 4). Moreover, the lower WUEs of maize and soybean intercrops than those of sole crops could be due to the lower grain yields and longer growth period of relay strip intercrop systems (Table 5). This result is in line with those from previously published reports [47–48]. However, it is noteworthy that the highest group WUEs of 23.06 and 26.21 kg ha-1 mm-1 for the 40:120 and 40:160 cm planting geometries, respectively, were 39% and 23% higher than those for sole cropping (S1 Table). In addition, the highest WERs 1.66 for the 40:120 cm planting geometry and 1.76 for the 40:160 cm planting geometry could possibly due to the complimentary root distribution of maize and soybean intercrops, which led to a significant water use advantage [49].

The opportunities for increasing the effective water use through intercropping are limited. Our results confirm that increasing the distance between maize narrow-row spacing reduced the canopy coverage and LAI for intercropped maize, thereby decreasing SWC, soil E, and throughfall in the following order: central row of maize strip (CRM) < adjacent row between maize and soybean strip (AR) < central row of soybean strip (CRS). Compared to sole soybean, the soil evaporative water loss in maize-soybean relay strip intercrop systems was greatly minimized by the 40:160 cm planting geometry and 200 cm bandwidth. Moreover, the highest total yields and LERs for the 40:160 cm planting geometry using 200 cm bandwidth, indicating the yield and land use advantage, respectively. The lower WUEs of intercrops were mainly due to their lower grain yields compared to corresponding sole crops. However, the highest group WUEs and WERs for the 40:160 cm planting geometry using 200 cm bandwidth, suggesting that an optimum planting geometry of 40:160 cm and bandwidth of 200 cm could be a viable planting pattern management method for attaining high group WUE in maize-soybean relay strip intercrop systems. Further studies under limited water conditions may be useful to explore greater gains in the group WUE of maize-soybean relay strip intercrop systems.   Source: http://doi.org/10.1371/journal.pone.0178332