Nitrogen fertilization degrades soil aggregation by increasing ammonium ions and decreasing biological binding agents on a Vertisol after 12 years

• Published in: Pedosphere 2022-08, Vol.32 (4), p.629-636
• Main Authors: GUO, Zichun, LI, Wei, UL ISLAM, Mahbub, WANG, Yuekai, ZHANG, Zhongbin, PENG, Xinhua
• Format: Article
• Language: English
• Subjects: Acidic soils; Acidification; Agglomeration; aggregate stability; aggregating agents; Ammonium; Binders (materials); Binding; Carbon; Crop production; Cropping systems; Dispersion; Fertilization; Ions; long-term nitrogen fertilization; Microorganisms; Nitrogen; Organic carbon; Organic soils; Soil acidification; Soil conditions; Soil degradation; Soil fertility; Soil improvement; Soil stability; Soil structure; Straw; straw incorporation; Triticum aestivum; Wheat; Zea mays
• ISSN: 1002-0160; 2210-5107
• DOI: 10.1016/S1002-0160(21)60091-7

Degraded soil aggregation arising from nitrogen (N) fertilization has been reported in many studies; however, the mechanisms have not yet been clarified. Elucidating the impact of N fertilization on soil aggregation would help to improve soil structure and sustain high crop production. The objective of this study was to determine the impact of long-term N fertilization on soil aggregation and its association with binding and dispersing agents. A 12-year (2008–2019) N fertilization field experiment on a Vertisol was performed, covering a wide range of N application rates (0, 360, 450, 540, 630, and 720 kg ha-1 year-1) and including straw management (straw return and straw removal) in a wheat (Triticum aestivum L.)-maize (Zea mays L.) cropping system. Soil samples of 0–20 cm depth were collected from 12 field treatments with 3 replications in 2019. Soil aggregate stability (mean weight diameter (MWD)) and contents of soil organic carbon (SOC), glomalin-related soil protein (GRSP), microbial biomass carbon (MBC), and mineral N (NH4+ and NO3-) were determined. Long-term N fertilization under straw removal conditions reduced soil MWD by 12%–18% at N rates from 0 to 720 kg ha-1 compared to that under straw return (P < 0.05). Soil MWD was positively associated with pH (P < 0.05) and MBC (P < 0.05), but negatively correlated with NH4+ (P < 0.05) and NO3- (P < 0.05). Compared with the straw removal treatment, the straw incorporation treatment significantly improved the contents of aggregating agents (SOC, GRSP, and MBC) (P < 0.001), but did not affect that of the dispersing agent (NH4+) (P > 0.05); consequently, it improved soil aggregation. Overall, our results indicate that long-term N fertilization may degrade soil aggregation because of the increases in monovalent ions (H+ and NH4+) and the decrease in MBC during soil acidification, especially when the applied N dose exceeded 360 kg ha-1 year-1. Our finding can minimize the negative structural impacts on Vertisol.