Seasonal variations of soil inorganic nitrogen contents and their responses to changing grazing intensity in grasslands of an agro-pastoral ecotone in northern Shanxi, China

doi:10.17521/cjpe.2023.0222

Abstract

Abstract:

Aims Soil inorganic nitrogen, including ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N), is one of essential nutrients for plant growth. Grazing affects soil physical and chemical properties and microbial activities through livestock feeding, trampling and manure return, changing soil inorganic nitrogen content, and thus affecting plant productivity. However, under different grazing intensities, the seasonal dynamics and interannual differences of soil inorganic nitrogen are not clear.

Methods This study takes the grassland in an agro-pastoral ecotone of northern Shanxi as the research object, using the grazing intensity manipulation experiment established in August 2016 (no grazing (UG), light grazing (LG, 2.35 sheep unit·hm^-2 per growing season), moderate grazing (MG, 4.80 sheep unit·hm^-2 per growing season), heavy grazing (HG, 7.85 sheep unit·hm^-2 per growing season)). The seasonal variations (May to September) of soil inorganic nitrogen contents from 2017 to 2021 were measured, and the seasonal dynamics and interannual differences of soil inorganic nitrogen contents under different grazing intensity were studied.

Important findings Results showed that: (1) different grazing intensity had no significant effect on the seasonal means of soil inorganic nitrogen contents, but significantly decreased plant biomass, which was related to the changes in the grazing effect in different periods of the growing season. (2) The soil inorganic nitrogen content increased at first and then decreased in the whole growing season. (3) The soil inorganic nitrogen content showed significant interannual difference, which was related to the changes in the interannual precipitation. Results showed that the responses of soil inorganic nitrogen content in northern agro-pastoral ecotone to short-term grazing intensity was not significant, and the changes in seasonal precipitation was the main reason for the interannual variations of soil inorganic nitrogen content. In the future, grassland management should pay more attention to the effect of precipitation changes on soil inorganic nitrogen.

Key words: grazing intensity, ammonium nitrogen, nitrate nitrogen, interannual variation, plant biomass

MA Teng-Fei, HAO Jie, DIAO Hua-Jie, NING Ya-Nan, $\boxed{\hbox{WANG Chang-Hui}}$ , DONG Kuan-Hu. Seasonal variations of soil inorganic nitrogen contents and their responses to changing grazing intensity in grasslands of an agro-pastoral ecotone in northern Shanxi, China[J]. Chin J Plant Ecol, 2025, 49(6): 965-974.

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https://www.plant-ecology.com/EN/Y2025/V49/I6/965

Figures/Tables 9

Fig. 1 Air temperature and precipitation in the growing season from 2017 to 2021. Each data point in A is the average of the dayʼs observations.

Fig. 2 Soil temperature and moisture content during the growing season from 2017 to 2021. Each data point is the average of the monthʼs observations. HG, heavy grazing; LG, light grazing; MG, moderate grazing; UG, ungrazing.

Fig. 3 Dynamic changes of soil ammonium nitrogen (NH4+-N) (A, D), nitrate nitrogen (NO3--N) (B, E) and inorganic nitrogen (C, F) contents (mean ± SE) in the growing season under different grazing intensities from 2017 to 2021. HG, heavy grazing; LG, light grazing; MG, moderate grazing; UG, ungrazing. * and ** indicated significant differences among treatments at the levels of p < 0.05 and p < 0.01, respectively.

Fig. 4 Inorganic nitrogen contents at different grazing intensities from 2017 and 2021 (mean ± SE). HGNH4+, ammonium nitrogen content under heavy grazing treatment; HGNO3-, nitrate nitrogen content under heavy grazing treatment; LGNH4+, ammonium nitrogen content under light grazing treatment; LGNO3-, nitrate nitrogen content under light grazing treatment; MGNH4+, ammonium nitrogen content under moderate grazing treatment; MGNO3-, nitrate nitrogen content under moderate grazing treatment; UGNH4+, ammonium nitrogen content under non-grazing treatment; UGNO3-, nitrate nitrogen content under non-grazing treatment. Different lowercase letters indicated significant differences among different grazing intensity treatments in the same year (p < 0.05); different uppercase letters indicate significant differences in the inorganic nitrogen content among different years (p < 0.05).

Table 1 Two way ANOVA of the effects of year, grazing intensity and their interactions on soil ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) and inorganic nitrogen (IN) contents (p value)

	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N	无机氮 IN
年际 Year (Y)	<0.001	<0.001	<0.001
放牧强度 Grazing intensity (G)	0.342	0.232	0.417
Y × G	0.068	0.050	0.066

Table 2 Effects of sampling date, grazing intensity and their interactions on soil ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) contents (p value)

	2017		2018		2019		2020		2021
	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N	铵态氮 NH₄⁺-N	硝态氮 NO₃^--N
日期 Date (D)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
放牧强度 Grazing intensity (G)	0.256	0.144	0.283	0.426	0.141	0.870	0.075	<0.050	<0.050	0.347
D × G	0.104	<0.050	0.311	0.632	0.725	0.709	0.152	0.238	0.086	0.919

Fig. 5 Box plots of the relationships between aboveground biomass (A) and belowground biomass (B) of plants under different grazing intensities from 2017 to 2021 (mean ± SE). HG, heavy grazing; LG, light grazing; MG, moderate grazing; UG, ungrazing. Different lowercase letters indicate significant differences among different grazing intensity treatments in the same year (p < 0.05).

Fig. 6 Linear correlation analysis of the relationships between soil inorganic nitrogen content and aboveground biomass (A, B, C) and between soil inorganic nitrogen content and belowground biomass (D, E, F) under different grazing intensities from 2017 to 2021. NH4+-N, ammonium nitrogen; NO3--N, nitrate nitrogen.

Fig. 7 Redundancy analysis of soil ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) contents and air temperature and soil moisture. MAP, mean annual precipitation; MAT, mean annual air temperature; ST, soil temperature; SW, soil moisture.

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