Inter-specific root interactions and water use efficiency of maize/soybean relay strip intercropping

Crop diversity results in inters-specific interactions which defines the productivity and resource utilization efficiency of intercropping systems. However, little attention was paid to the below ground interactions of the maize (Zea mays L.) and soybean (Glycine max (Linn.) Merr.) in relay strip intercropping, and how these two crop species compete for resources by stimulating root development and water uptake, still remains unclear. Therefore, in our three-year field experiment, in addition to the continuous planting of maize/soybean relay strip intercropping(MS1), traditional maize/soybean intercropping (MS5), sole maize (M) and sole soybean (S), we swapped the maize and soybean rows (MS2) and also used nylon net (MS3) and plastic sheet (MS4) barriers between swapped rows to study the root distribution and water use efficiency, and the significance of inter-specific root interactions for corresponding yield responses of maize and soybean in relay strip intercropping. Our results showed the advantage of maize/soybean relay strip intercropping (LER1.46–2.00) over the sole cropping, with row swapping treatment (MS2) showing the highest yield (5360.35 kg ha−1) and land equivalent ratio (LER 1.98). In addition, intercropping promoted the root growth and distribution of the both crops, with lateral expansion to the middle inter-row spaces during the co-growth period, resulting in 72.15% and 15.72% greater root length density of maize and soybean, respectively. Importantly, compared to monocrops, the unseparated planting patterns (MS1, MS2, MS5) showed lower decrease in WUE, with MS2 treatment showing the lowest decrease of WUE in maize (0.71%) and soybean (2.69%). In contrast, the highest decrease in water use efficiency of maize (32.30% and 27.79%) and soybean (33.65% and 29.23%) was always found in separated planting pattern MS4 and MS4 respectively, signifying the importance of inter-specific root interactions for high productivity and resource use efficiency of intercropping. Furthermore, swapping of rows also reduced the maize dominance (Ams 0.09 and Kms 1.46) and competitive pressure between maize and soybean, which caused highest yield increase in maize and lowest yield decrease in soybean in MS2. Altogether, these results showed that intercropping allows the absorption of more soil water through better root proliferation and inter-specific root interaction while the swapping of 2 rows of maize and 2 rows of soybean (MS2) enhances it even more, resulting in higher WUE and productivity of maize-soybean relay strip intercropping. Therefore, in southwest China and other regions with same attributes, the findings of this study could be served as a cleaner and environment friendly production practice to get more return from the maize/soybean relay strip intercropping.