Increasing the planting density can exacerbate crop competition for water, nutrients and space which results in a decline in the crop yields. However, the effect of increasing planting density on crop growth and soil biological characteristics in barren sandy land in the semi-arid regions are still unclear. In this study, we investigated the effects of six planting densities (5.4×10⁴, 6.45×10⁴, 7.95×10⁴, 9.5×10⁴, 9.75×10⁴ and 10.5×10⁴ plants/hm²) on maize growth, photosynthesis characteristics, yield and soil biological characteristics in barren sandy soil in the semi-arid region of Ningxia, China. The results indicated that the stem diameter and spike length decreased linearly with increasing planting density. The plant height, spike weight, grain weight and 100-grain weight decreased with increasing plating density. Moreover, the root length increased with increasing planting density. The diameter, volume and activity increased and then decreased with increasing planting density. There was no significant difference (p>0.05) in the effect of planting density on transpiration rate intercellular CO₂ concentration. As well, the soil microbial biomass carbon and microbial biomass nitrogen decreased with increasing planting density. The soil catalase activities increased and then decreased with increasing planting density. The alkaline phosphatase activity, the amounts of soil bacteria and actinomycetes increased with increasing planting density. Generally, a moderately increasing planting density can improve maize yield when water and nutrients are sufficient. The optimal planting density was 8.29×10⁴ plants/hm² and the highest yield was 15.84 t/hm² in barren sandy soil in semi-arid region of Ningxia, China. This study provides a theoretical basis for high yield and high efficiency of maize.

To understand the combined effect of organic and chemical fertilizers on soil carbon emissions and carbon balance of a farmland ecosystem, this study investigated the organic fertilizer nitrogen replacing different proportions of chemical fertilizer nitrogen. The results showed that, compared to F₁₀₀, the O₁₅F₈₅ treatment increased the yield and net ecosystem productivity carbon sequestration of silage maize under mild, moderate, and severe salinization levels, as well as the contents of soil organic carbon, microbial carbon, and humin carbon, while reducing plant carbon emissions. The O₁₅F₈₅ treatment did not significantly increase soil carbon emissions (CEC), but O₃₀F₇₀, O₄₅F₅₅ and O₁₀₀ treatments significantly increased CEC. The soil carbon balance analysis showed that the farmland ecosystem was a “sink” for atmospheric CO₂ under each treatment. The O₁₅F₈₅ treatment produced an “excitation effect” to enhance the carbon sink effect of silage maize farmland under mild, moderate and severe salinization levels while maintaining stable production and emissions. Although the O₁₀₀ treatment increased the carbon sink of farmland under different salinization levels, the yield was significantly reduced and did not represent practical production levels. Correlation analysis showed that soil organic carbon components and ecosystem carbon balance were closely related to soil total salt, pH and bulk density, while soil dissolved organic carbon, humus carbon components and carbon emissions were closely related to soil moisture and temperature. Therefore, the purpose of improving the carbon sink of saline-alkali land can be achieved through soil salt inhibition, soil structure remodeling and water supplement and warming regulation, which provides technical and theoretical support for reducing carbon emissions, achieving carbon neutrality and alleviating global warming.