Effects of Different Nitrogen Application Rates on Carbon and Nitrogen Content of Soil Aggregates and Wheat Yield

The influence mechanism of varying nitrogen (N) rates on the carbon (C) and N content of soil aggregate, as well as wheat yield were investigated in this study, so as to provide a scientific basis for the rational application of N fertilizer.

The 11-year experiment was conducted in Zhangpan Town, Xuchang City, Henan Province, with four different N levels, including 0 (N0), 180 kg·hm^-2 (N1), 240 kg·hm^-2 (N2), and 300 kg·hm^-2 (N3). The study systematically analyzed changes in soil carbon and nitrogen content, cluster distribution and their carbon and nitrogen content in different soil layers as a result of long-term N application, and investigated the regulatory pathways of long-term N application on wheat yield and its composition.

There was a transformation in the composition of soil aggregates in every soil layer, specifically from larger macroaggregates (>0.25 mm) to microaggregates (0.25-0.053 mm) and silt and clay particles (<0.053 mm), as well as an increase in N rate. Additionally, the application of N resulted in a significant decrease in the mean weight diameter (MWD). As N application rates increase, the C and N content of the soil increased in the 0-20 cm layer, the C and N content of the soil in the 20-40 cm soil layer showed the trend to increase at first and then decrease. Compared with the N0 treatment, N application increased soil organic carbon (SOC) and soil total nitrogen (STN) content by 13.1%-37.2% and 19.4%-29.4% in the 0-20 cm layer and by 15.3%-32.2% and 6.1%-29.3% in the 20-40 cm layer, respectively. The N treatment significantly increased the SOC content of each particle size aggregates compared with N0 treatment, with the SOC content of macroaggregates increasing by 31.6%-62.0%, the SOC content of microaggregates increasing by 8.7%-61.2% and the SOC content of silt and clay increasing by 14.0%-81.7%. As N application rates increased, the STN content of the soil increased in the 0-20 cm layer. With the STN content of macroaggregates increasing by 32.6%-51.0%, the STN content of microaggregates increased by 25.7%-35.9% and the STN content of silt and clay increased by 3.2%-9.7%, the N3 treatment had the highest STN content of all particle size aggregates. In the 20-40 cm soil layer, the STN content of all particle size aggregates tended to increase at first and then decrease. With the STN content of macroaggregates increasing by 17.6%-35.2%, the STN content of microaggregates increased by 11.7%-24.0% and the STN content of silt and clay increased by 1.1%-12.9%, and the N1 treatment had the highest STN content of all particle size aggregates. The study results indicated that long-term nitrogen application had a significant impact on the spike number and grain number per spike in wheat, resulting in increased yield. Compared with the N0 treatment, the application of N1, N2, and N3 treatments resulted in a significant increase in wheat yield, with improvements of 188.1%, 177.3%, and 173.2%, respectively. The correlation and structural equation modelling analyses revealed a significant and positive correlation between wheat yield and soil carbon and nitrogen content, as well as carbon and nitrogen content in aggregates. Additionally, the long-term application of nitrogen was found to influence wheat yield formation by affecting carbon and nitrogen content in microaggregates.

In summary, the application of nitrogen over a long period of time raised the content of carbon and nitrogen in both soil and aggregates, enhanced soil fertility, ultimately promoting wheat yield. The optimal nitrogen application rate was 180 kg·hm^-2 under the condition of this experiment.