Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China

doi:10.17521/cjpe.2020.0153

Abstract

Abstract:

Aims The salinized grassland ecosystem has low level of nutrients, and how increasing nitrogen (N) deposition affects N cycling has not been solved yet.
Methods An experimental platform was set up to simulate increasing N deposition in the saline alkaline grassland ecosystem in Youyu County, Shanxi Province, eight levels of N addition were designed, expressed by 0, 1, 2, 4, 8, 16, 24, 32 g·m^-2·a^-1 (N0, N1, N2, N4, N8, N16, N24, N32). The same amount of ammonium nitrate (NH₄NO₃) was sprayed at the beginning of each month during the growing seasons from May to September every year. From May of 2017, net N mineralization rate was measured once a month using top-cover PVC cylinders. The experiment has been conducted for three years from 2017 to 2019.
Important findings The results showed that, (1) High N addition levels (N16, N24, N32) significantly increased soil inorganic N pool. (2) Soil N mineralization in the salinized grassland ecosystem was dominated by nitrification. After three years of N addition, high levels of N addition (N24, N32) significantly promoted the rate of soil net nitrification, and different N levels showed different effects in different months and years. (3) The sensitivity of soil net N mineralization to different levels of N addition showed significant difference among three years with different precipitation. Lower N addition improved the sensitivity of soil net N mineralization, while higher N addition reduced the sensitivity of soil net N mineralization. (4) There was a positive correlation between soil net N mineralization rate and soil temperature, water content, and a negative correlation between soil net N mineralization rate and soil pH value. The sensitivity of soil N cycle to N addition was higher at low level of N addition than at high level of N addition in the saline-alkaline grassland of north China. The influence of increasing N deposition on soil N cycle should be considered in process-based models in the future.

Key words: nitrogen addition, saline-alkaline grassland, soil net nitrogen mineralization rate, sensitivity, soil nitrification rate

XU Xiao-Hui, DIAO Hua-Jie, QIN Chu-Yi, HAO Jie, SHEN Yan, DONG Kuan-Hu, WANG Chang-Hui. Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China[J]. Chin J Plant Ecol, 2021, 45(1): 85-95.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2020.0153

https://www.plant-ecology.com/EN/Y2021/V45/I1/85

Figures/Tables 9

Table 1 Basic soil properties in the experiment sites in a saline-alkaline grassland sample plot of northern China (mean ± SE)

土壤特性 Soil property	2017年本底值 Original value of 2017
pH	9.82 ± 0.11
土壤全碳含量 TC (g·kg^-1)	7.01 ± 1.02
土壤全氮含量 TN (g·kg^-1)	0.61 ± 0.04
土壤全碳与全氮含量的比 TC:TN	11.57 ± 1.93

Fig. 1 Monthly dynamics of precipitation and air temperature in a saline-alkaline grassland sample plot of northern China.

Fig. 2 Schematic diagram of experimental design in a saline-alkaline grassland of northern China. N0, N1, N2, N4, N8, N16, N24 and N32 indicated that the nitrogen addition levels were 0, 1, 2, 4, 8, 16, 24 and 32 g·m-2, respectively.

Fig. 3 Changes of soil NH4+-N, NO3--N and inorganic nitrogen concentration under different levels of nitrogen addition in 2017-2019 of a saline-alkaline grassland of northern China (mean + SE). Different lowercase letters indicate significant differences (p < 0.05). * means the difference is significant at the level of p < 0.05, and ** means the difference is significant at the level of p < 0.01. Different colors in the small picture correspond to different nitrogen addition levels in the histogram.

Table 2 Two-way ANOVA on the effects of nitrogen addition and year on soil net ammonification, nitrification and net nitrogen mineralization rates of the salinized grassland of northern China

因素 Factor	铵化速率 Ammonification rate		硝化速率 Nitrification rate		净氮矿化速率 Net nitrogen mineralization rate
因素 Factor	F	p	F	p	F	p
年份 Year (Y)	6.167	<0.05	35.338	<0.001	5.275	<0.05
氮添加 Nitrogen addition (N)	7.879	<0.001	12.592	<0.001	9.455	<0.001
年份×氮添加 (Y × N)	3.664	<0.05	14.516	<0.001	2.315	<0.05

Fig. 4 Seasonal dynamics of soil net N mineralization rates under different levels of nitrogen addition in a saline-alkaline grassland of northern China (mean ± SE). ** means the difference is significant at the level of 0.01. N0, N1, N2, N4, N8, N16, N24 and N32 indicated that the nitrogen addition levels were 0, 1, 2, 4, 8, 16, 24 and 32 g·m -2, respectively.

Fig. 5 Changes in soil net nitrogen (N) mineralization and net nitrification rates under different levels of N addition in a saline-alkaline grassland of northern China (mean + SE). Different lowercase letters indicate significant differences (p < 0.05).

Fig. 6 Sensitivity of soil net nitrogen (N) mineralization rate under different levels of N addition in a saline-alkaline grassland of northern China (mean + SE). Different lowercase letters indicate significant differences (p < 0.05) in the absolute value among different treatments.

Fig. 7 Correlation between net nitrogen (N) mineralization rates and soil temperature (ST), soil moisture (SM), pH and below- ground biomass (BGB) under different levels of N addition in a saline-alkaline grassland of northern China. p > 0.05 indicated that there was a significant correlation between the net N mineralization rate and soil indicators; p < 0.05 indicated that there was no significant correlation between the net nitrogen mineralization rate and soil indicators. N0, N1, N2, N4, N8, N16, N24 and N32 indicated that the nitrogen addition levels were 0, 1, 2, 4, 8, 16, 24 and 32 g·m -2, respectively.

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