Objective Under the premise of meeting the carbon demand for crop grain filling, promoting more carbon allocation to root is beneficial to root growth, supporting the population growth in the late growing period. Planting density influences the carbon partitioning between shoot and root significantly. We quantitatively analyzed the responses of maize yield and root-carbon allocation to dense planting as affected by different influencing factors, aiming to provide a theoretical basis for guiding rational dense planting of maize.

Method A total of 74 published papers were searched in the CNKI and Web of Science database, and 1272 pairs of effective data were extracted. A meta-analysis was performed to quantify the beneficial effects of dense planting on maize yield and carbon allocation to root under various climatic conditions, soil properties, and management practices.

Results In general, increased planting density had positive effect on maize yield at both individual and population levels. Compared with low-density planting, dense planting increased the leaf area index by 30.4% and yield by 9.0%; The yield gains were greater for cultivars released after 2010 (10.5%) than for those released before 2000. When planting density was < 6×10⁴ plants/hm² and 6−9×10⁴ plants/hm², high planting density increased maize yield by 16.8% and 4.7%, respectively, when the density was >9×10⁴ plants/hm², the yield decrease was declined to 3.1%. At the population level, dense planting increased maize yield by 11.2%−15.0% Under annual mean temperature >10°C and annual precipitation > 600 mm, the yield increment by dense planting was better than under <10°C and drier (< 600 mm) conditions. Dense planting increased absolute population aboveground carbon accumulation by 19.9% at silking and 17.8% at maturity, and the root carbon accumulation by 14.7% and 7.0% at the same two periods, respectively. The root-shoot ratio decreased by 8.4% at silking and by 9.0% at maturity, indicating a reduced proportion of carbon allocated to root under dense planting. When annual sunshine duration exceeded 2200 hours, dense planting enhanced root carbon accumulation by 15.5%. Conversely, under sunshine < 2200 hours, root carbon accumulation decreased by 5.9%, suggesting that radiation limitation constrains root carbon allocation under dense planting. Dense planting showed more effectively on increasing root carbon accumulation at both silking and maturity stages for later released cultivars than for the earlier released ones, with the root carbon accumulation increment at silking stage by 10.3%, 13.1%, and 33.3% for cultivars released before 2000, during 2000−2010, and after 2010, respectively. The magnitude of root carbon increase was depended on the base planting density, the increment by higher planting density was 19.8% at densities < 6×10⁴ plants/hm², and 11.0% at 6−9×10⁴ plants/hm², while little response beyond 9×10⁴ plants/hm². When the density was increased by less than 3×10⁴ plants/hm², between 3-5×10⁴ plants/hm², and greater than 5×10⁴ plants/hm², the root carbon accumulation at silking stage increased significantly by 10.9%, 19.0%, and 16.7%, respectively, with the largest increase in moderate dense planting.

Conclusion Increased planting density lowers the maize root-to-shoot ratio and reduces the proportion of carbon allocated to roots; nevertheless, at the population level, suitable dense planting is capable of coordinating the simultaneous increase of grain yield and below-ground carbon accumulation, and the beneficial effect is more obvious in newly released cultivars. Under current agronomic conditions, the density should not be further increased when the basic density higher than >9×10⁴ plants/hm².