Subsoil tillage loosens compacted soil for better plant growth, but promotes water loss, which is a concern in areas that are commonly irrigated. Therefore, our objective was to determine the physiological responses of high yield spring maize (Zea mays L.) to subsoil tillage depth when grown in the Western plain irrigation area of Inner Mongolia, China. Our experiment during 2014 and 2015 used Zhengdan958 (Hybrid of Zheng58 × Chang7-2, produced by Henan academy of agricultural sciences of China, with the characteristics of tight plant type and high yield) and Xianyu335 (Hybrid of PH6WC × PH4CV, produced by Pioneer Corp of USA, with the characteristic of high yield and suitable of machine-harvesting) with three differing subsoil tillage depths (30, 40, or 50 cm) as the trial factor and shallow rotary tillage as a control. The results indicated that subsoil tillage increased shoot dry matter accumulation, leading to a greater shoot/root ratio. Subsoil tillage helped retain a greater leaf area index in each growth stage, increased the leaf area duration, net assimilation rate, and relative growth rate, and effectively delayed the aging of the blade. On average, compared with shallow rotary, the grain yields and water use efficiency increased by 0.7–8.9% and 1.93–18.49% in subsoil tillage treatment, respectively, resulting in the net income being increased by 2.24% to 6.97%. Additionally, the grain yield, water use efficiency, and net income were the highest under the treatment of a subsoil tillage depth of 50 cm. The results provided a theoretical basis for determining the suitable chiseling depth for high-yielding spring corn in the Western irrigation plains of Inner Mongolia.

This study examined the planting density tolerance, grain yield improvement potential, and mechanisms of high-yielding spring maize varieties under increasing planting density and subsoiling. We planted two high-yielding spring maize varieties with a high or low tolerance to high planting densities (LM33 and XD20, respectively) at five different densities (D1: 60,000 plants ha⁻¹, D2: 75,000 plants ha⁻¹, D3: 90,000 plants ha⁻¹, D4: 105,000 plants ha⁻¹, and D5: 120,000 plants ha⁻¹) using two tillage methods (35-cm subsoiling and 15-cm traditional rotary tillage). The response characteristics used to compare the performance of the two maize varieties under different planting densities and tillage methods included root characteristics, canopy physiology, yield, and yield components. The results show that: (1) Under rotary tillage, with the increase of planting density from 75,000 plants ha⁻¹ to 90,000 plants ha⁻¹, yields of high-yielding spring maize varieties improved. However, when the planting densities were beyond 90,000 plants ha⁻¹, the yields stopped increase, or even decrease. Subsoiling increased the planting density by 15,000 plants ha⁻¹, enhanced the highest yield by 1080 kg ha⁻¹–1940 kg ha⁻¹, and raised the yield gain by 11.17–30.72%. (2) Under rotary tillage, the functional indexes of the roots and canopy of high-yielding spring maize decreased as planting density increased, and the largest reductions of root dry weight, leaf area index (LAI) of post-anthesis, light transmission percentage (LTP) of ear leaves, bottom leaves LTP, and dry matter accumulation all occurred between D2 and D4. The largest decline of high tolerance variety emerged between D3 and D5, and the extent was smaller than the low tolerance variety. (3) Compared with rotary tillage, subsoiling reduced the extent declines in root dry weight, root length, and root surface area; delayed the attenuation of LAI and the relative chlorophyll content (SPAD) determined in leaves; and improved the LTP of ear layers and bottom layer during the late growth stage. The post-anthesis populations dry matter accumulation of XD20 and LM33 increased by 7.07% and 13.18%, respectively. In addition, subsoiling significantly increased the number of kernels/spike and 1000-grain weight as the planting density increased. Meanwhile, the planting densities at which dry root weight, population LAI, ear leaf LTP, bottom leaf LTP, and dry matter accumulation arose the largest reductions was raised to 15,000 plants ha⁻¹. The effects of subsoiling in the high density-tolerant variety were more pronounced than the low density-tolerant variety.

Quantifying the relationships of maize yield and nitrogen use efficiency (NUE) to inherent soil productivity (ISP) could provide a theoretical basis for implementing strategies that concurrently narrow the yield gap, increase NUE, and improve soil quality. A field study under irrigation conditions was conducted at five locations with large differences in ISP (estimated by maize grain yield without using fertilizers) across various ecological regions in Inner Mongolia, China. Our results showed that the changes in maize yield and nitrogen partial factor productivity (PFP_N) to ISP followed a linear-plateau model, with a common inflection point. When ISP was below 8.0 t ha^–1, maize yield and PFP_N were stagnating at a low level, due to a trade-off between the contribution of soil and chemical fertilizers to yield. When ISP exceeded 8.0 t ha^–1, the contribution rate of soil to yield stabilized at 80%, resulting in a simultaneous increase in maize yield by 1.2 t ha^–1 and PFP_N by 4.6 kg kg^–1, for increasing ISP at each t ha^–1. Our results indicated that while keeping other management strategies unchanged, to increase maize yield and PFP_N by 15% or further 30%, it is necessary to increase ISP to 9.3 and 10.4 t ha^–1, respectively. In this scenario, N input will be reduced by 33.5 and 47.5%, and apparent N losses will be reduced by 11.7 and 21.5%, respectively. Therefore, enhancing ISP could lead to a simultaneous improvement in yield and NUE as well as strongly support the green development of maize production.

In China, there is an increasing need for greater genetic diversity in maize (Zea mays L.) germplasm and hybrids appropriate for mechanical harvesting. In order to test and distinguish American maize inbred lines with exceptional combining ability, four Chinese maize inbred lines (Chang7-2, Zheng 58, four-144 and four-287) were used to judge the combining ability and heterosis of 16 USA inbred lines by a NCII genetic mating method. The results showed that among the American inbred lines, 6M502A, LH208, NL001, LH212Ht, PHW51, FBLA and LH181 expressed good GCA for yield characteristics; while RS710, PHP76, FBLA, and PHJ89 showed excellent GCA for machine harvesting characteristics. Five hybrids (NL001 × Chang7-2, LH212Ht × Chang7-2, FBLA × four-144, LH181 × four-287, PHK93 × four-287) had better SCA values for yield characteristics, at 1.69, 1.07, 1.48, 1.84 and 1.05, respectively; while NL001 × Chang 7-2, 6M502A × Chang7-2, LH212Ht × Chang7-2, LH181 × four-287, PHW51 × Chang7-2 had better TCA values for yield characteristics, at 3.03, 2.80, 2.41, 2.19 and 1.91, respectively; NL001 × Chang7-2, 6M502A × Chang7-2, LH212Ht × Chang7-2, LH181 × four-287, PHW51 × Chang7-2 showed excellent Control Heterosis values, with 21.48%, 19.64%, 15.93%, 14.05% and 11.60% increases, respectively, compared to the check and potential for future utilization in Inner Mongolian corn production.

The nitrogen use efficiency (NUE) of modern maize hybrids has been greatly improved, but for decades, little was known on whether the NUE heterosis of maize hybrids had increased. A two-year field study was conducted on eight maize hybrids and their parental inbred lines from the 1970s to 2000s under two N levels (0, 150 kg N ha⁻¹). These were the most popular hybrids in China’s main maize-growing areas at the time. The results showed that the yield of the maize hybrids increased significantly at an average rate of 37.5% every 10 years. The evolution of NUE heterosis was mainly related to the increased kernel number per ear. The absolute NUE heterosis (AH_NUE) and the mid-parent NUE heterosis (MPH_NUE) of maize hybrids increased by 151.4% and 76.4% in the past four decades, or an average rate of 2.11 kg kg⁻¹ and 19.1% every 10 years. Based on the coefficient of determination, the contribution of the mid-parent nitrogen internal efficiency heterosis (MPH_NIE) to MPH_NUE (43–57%) was significantly higher than that of the mid-parent nitrogen recovery efficiency heterosis (MPH_NRE) (19–32%), indicating that the evolution of maize NUE heterosis was mainly derived from its NIE heterosis evolution. The increase of NIE heterosis in the past 40 years was closely related to the increased heterosis of kernel number per ear, the pre-silking N accumulation and the post-silking N remobilization. Therefore, the enhancement of maize NUE heterosis can be attributed to (i) heterosis improvement in post-silking N remobilization, which results mainly from greater heterosis in pre-silking N accumulation; (ii) heterosis improvement in carbon and nitrogen sink capacity, which exhibit as heterosis enhancement in grain yield and grain nitrogen concentration. To further improve yield and NUE, the pre-silking N and carbon accumulation and post-silking N remobilization should both continue to increase in maize breeding. Our results will provide new insights into NUE and help breeders select genotypes with both higher yields and higher NUE for the future.