Soil Aggregates in Wheat Fields and Their Carbon and Nitrogen Distribution Characteristics in Response to Green Manure Application Combined with Reduced Nitrogen Fertilizer Application

Based on a long-term field experiment, this study investigated the response of soil aggregate distribution characteristics, along with their associated organic carbon and total nitrogen contents, to green manure incorporation combined with nitrogen reduction. The aim was to provide a theoretical basis for developing rational soil fertility management practices in wheat production in this region.

Based on seven treatments from a long-term field experiment initiated in 2018: N100 (no green manure incorporation with 180 kg·hm^-2 nitrogen), G1N85 (15 000 kg·hm^-2 green manure+153 kg·hm^-2 nitrogen), G2N85 (22 500 kg·hm^-2 green manure+153 kg·hm^-2 nitrogen), G3N85 (30 000 kg·hm^-2 green manure+153 kg·hm^-2 nitrogen), G1N70 (15 000 kg·hm^-2 green manure+127 kg·hm^-2 nitrogen), G2N70 (22 500 kg·hm^-2 green manure+127 kg·hm^-2 nitrogen), and G3N70 (30 000 kg·hm^-2 green manure+127 kg·hm^-2 nitrogen). Using data from this field experiment collected in 2024 and 2025, we analyzed the effects of these treatments on soil aggregate composition and carbon-nitrogen nutrient contents.

Compared to N100, the combination of green manure incorporation and reduced nitrogen fertilizer application not only significantly increased the content of large soil aggregates (> 0.25 mm) in the 0-20 cm and 20-40 cm soil layers of wheat fields but also enhanced the mean weight diameter (MWD) and geometric mean diameter (GMD), with the G3N85 treatment yielding the optimal effect. Compared to other treatments, G3N85 increased the organic carbon and total nitrogen content of all particle size agglomerates. with increases in organic carbon content of 0.7%-12.9%, 1.3%-12.4%, 1.2%-15.0%, and 3.2%-13.4% for aggregates > 2 mm, 2-0.25 mm, 0.25-0.053 mm, and < 0.053 mm, respectively. while aggregate total nitrogen increased by 3.1%-11.4%, 3.6%-11.1%, 2.1%-13.1%, and 4.9%-10.9%, respectively. Moreover, the contribution rates of organic carbon and total nitrogen in large aggregates to soil organic carbon and total nitrogen significantly increased under this treatment, with increases of 19.7%, 23.3%, 19.6%, and 24.2%, respectively. Conversely, the contribution rates of organic carbon and total nitrogen from micro-aggregates and silt-clay particles to soil organic carbon and total nitrogen significantly decreased.

The combination of green manure incorporation and reduced nitrogen fertilizer application not only increased the content and stability of large soil aggregates in the 0-40 cm layer but also enhanced the contribution rates of organic carbon and total nitrogen in these aggregates. The optimal treatment was applying 30 000 kg·hm^-2 of green manure combined with 153 kg·hm^-2 of nitrogen fertilizer. This regimen can serve as a rational fertilization system for maintaining soil structure stability and promoting carbon-nitrogen accumulation in wheat fields in the Hexi Oasis irrigation district.