Nitrogen fertilization degrades soil aggregation by increasing ammonium ions and decreasing biological binding agents on a Vertisol after 12 years
Section snippets
INTRODUCTION
Nitrogen (N) fertilizers are routinely applied to agricultural soil, and are essential in promoting soil fertility and stimulating crop growth. Therefore, N fertilization can increase crop residues in terms of the above- and belowground biomass of crops (as C input into soil) (Zhao et al., 2018). It is expected that an increase in C input can result in an increase in soil organic carbon (SOC) and further improve soil aggregation (Blanco-Canqui et al., 2014; Mustafa et al., 2020). However,
Site description and experimental design
A long-term field experiment (2008–2019) was conducted at the Agricultural Science and Technology Demonstration Center in Mengcheng County (33°09′ N, 116°33′ E), Anhui Academy of Agricultural Sciences, Anhui Province, China. The Vertisol is derived from fluvio-lacustrine sediment and has a clay loam texture (31.5% sand, 38.0% silt, and 30.5% clay) (Li et al., 2011). Before the experiment was conducted in 2008, the soil (0–20 cm) contained 8.22 g kg-1 organic carbon, 0.99 g kg-1 total N, 0.67 g
Crop yields
The mean annual yields of wheat and maize (2014–2018) were affected differently by the N application rates and straw management (Fig. 1). When the N rates increased from 360 to 630 kg ha-1, the wheat and maize yields under straw removal conditions gradually increased by 9.7%–22.7% and 1.63%–2.9%, respectively. Relative to N fertilization at a rate of 630 kg ha-1 when straw was not added, an N fertilization rate of 720 kg ha-1 slightly reduced wheat and maize yields by 2.13% and 1.9%,
DISCUSSION
Under straw removal conditions, N fertilization substantially reduced the soil aggregate stability (P < 0.05) compared to no-N treatment (Fig. 3). However, the SOC content also considerably increased (P < 0.001) (Table II), which is inconsistent with many reports in the literature, showing that N fertilization not only increased SOC content, but also improved soil aggregation and MWD (Guo et al., 2018, 2019a; Mustafa et al., 2020). This discrepancy may be related to the changes in soil
CONCLUSIONS
Our study provides direct evidence that long-term N fertilization degrades soil aggregation because it creates more monovalent ions (H+ and NH4+) in acidified soil despite the increasing SOC, especially when the applied N rate exceeds 360 kg ha-1 in a wheat-maize cropping system. In contrast with sole N fertilization, straw incorporation remarkably increased the aggregating agents but did not affect the dispersing agents, thus improving soil aggregation. Thus, optimized N input and straw
ACKNOWLEDGEMENT
This study was supported by the National Natural Science Foundation of China (Nos. 41725004, 42007007, and 41930753) and the Natural Science Foundation of Jiangsu Province, China (No. BK20201104).
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