Effects of Heavy Degradation on Alpine Meadows: Soil N₂O Emission Rates and Meta-Analysis in the Tibetan Plateau

Abstract

Heavy grassland degradation is evident across the Tibetan Plateau. However, atmospheric nitrous oxide (N₂O) emission rates and their underlying driving mechanisms in the southeast regions and across the Tibetan Plateau remain unclear. We analyzed the N₂O emission rates of heavily degraded and undegraded alpine meadow soil incubation using gas chromatography in three river sources and meta-analysis methods across the Tibetan Plateau. The N₂O emission rates of the heavily degraded and control meadows were respectively 4.29 ± 0.64 and 3.27 ± 0.53 g kg⁻¹ h⁻¹ in the southeast Tibetan Plateau (p < 0.01), indicating an increase of 31.16% on the N₂O flux of heavy degradation. Heavy degradation increased N₂O emission rates by 0.55 ± 0.14 (95% confidence interval: 0.27–0.83) through meta-analysis. High degradation increased by approximately 71.6% compared with that of the control. The water-filled pore space (WFPS) significantly influenced the N₂O emission rate based on the moderator test (p < 0.05). The mixed-effect model results revealed that WFPS, soil nitrate, and bulk soil could explain 59.90%, 16.56%, and 15.19% of the variation in the N₂O emission rates between the control and heavily degraded meadows, respectively. In addition, the N₂O emission rates of heavily degraded meadows can be reduced by increasing WFPS and bulk density, and by reducing the soil nitrate content.

1. Introduction

The increase in global temperature compared to the 1850–1900 level reached approximately 1.1 °C in 2021 [1]. Increasing atmospheric nitrous oxide (N₂O) contributed to global warming over the past 150 years [2,3]. N₂O accounts for 7% of the radiative forcing. The global mean abundance of N₂O was 333.2 ppb in 2020, an increase of 123% compared to the 1750 levels and 1.2 ppb higher than in 2019 [1]. Global N₂O emissions were 17.0 teragrams of nitrogen per year [3]. However, more regional observations on N₂O and mitigation strategies are required, especially in under-sampled regions such as the Tibetan Plateau.

Globally, grasslands contribute over 30% of the N₂O fluxes to the atmosphere [4,5]. Meanwhile, the Tibetan Plateau covers approximately 2.5 million km² and is highly vulnerable to environmental changes [6,7]. The degradation of alpine meadows exceeds 80% because of heavy grazing, and the heavily degraded area covers more than a third of the Tibetan Plateau [8,9]. Heavily degraded and control alpine meadows were found to emit 365.1 μg m⁻² h⁻¹ and 118.1 μg m⁻² h⁻¹ of N₂O, respectively [10]. Similar results were reported in this region, with the N₂O emission rates of heavily degraded grassland being approximately 2.66 and 2.29 times those of the control, respectively [11,12]. However, in southwest Tibet, heavy degradation increased grassland N₂O emission rates from 33.2 in the control to 37.5 μg m⁻² h⁻¹, an increase of approximately 12.95% [13]. Furthermore, a decrease was also observed in the N₂O rates in alpine meadows by approximately 17.13% and 26.65% in northeast and southwest Tibet, respectively [14,15].

Heavy degradation accelerates nitrogen transformation rates and increases the available nitrogen content in alpine meadows [16]. The water-filled pore space (WFPS) controls grassland nitrification, denitrification, and N₂O emission rates [17,18]. Soil bulk density affects oxidation reduction conditions and soil nitrification, and N₂O emission rates increase significantly with increasing soil bulk density [19,20]. Furthermore, pH directly influences microbial communities, richness, and soil N₂O emission rates [14,21]. Few studies have measured the N₂O emission rates in the heavily degraded meadows of Maqin county. Thus, this study attempted to reveal the impact of heavy degradation on N₂O emission rates in the southeastern Tibetan Plateau, and the driving factors, using meta-analysis. This would help to mitigate N₂O emission flux across the Tibetan Plateau.

2. Materials and Methods

2.1. Experimental Design and Data Analysis

We separately selected five heavily degraded and undegraded meadows as research objectives from Maqin county, Qinghai Province (34°21′ N, 100°27′ E, 4130 m), on the southeast Tibetan Plateau (Figure 1). There is a large temperature difference between day and night, a low number of sunshine hours throughout the year, and strong solar radiation. It is a plateau continental climate. The average air temperature is −3.9 °C, with average temperatures of −12. 6 and 9.7 °C in January and July, respectively. The annual precipitation is 542.0 mm, mostly concentrated from May to September, accounting for 85%.

The soil types include alpine meadow soil, which is rich in total nitrogen and organic carbon. The dominant plants are Kobresia humilis, Potentilla saudersiana, Saussurea superba, Koeleria cristata, Leontopodium nanum and Lancea tibetica in alpine meadow regions.

The plant coverage, biomass, and richness were investigated in the heavily degraded and native alpine meadows using 50 cm × 50 cm quadrats with five replications [6,8] in late August 2021. Meanwhile, soils were collected after vegetation community investigation at a depth of 0–20 cm. The soil samples were air-dried and passed through a mesh sieve of 0.25 mm. The soil organic carbon content was measured using the dry oxidation method with the TOC-5000A analyzer (Shimadzu Corporation, Kyoto, Japan). The available nitrogen was measured with an ultraviolet spectrophotometer (752, Shanghai, China,

Table 1. Plant and soil physical characteristic in heavily grazed and native alpine meadows.

Item	Undegraded Meadows (CK)	Heavily Degraded Meadows
Coverage %	84.7 ± 4.9 a	35.4 ± 5.3 b
Biomass g m−2	287.2 ± 23.6 a	254.4 ± 15.8 b
Richness	24 ± 1.7 a	17 ± 2.5 b
pH	6.4 ± 0.5 b	7.7 ± 0.3 a
Bulk density g cm−3	0.86 ± 0.12 b	1.12 ± 0.15 a
Soil organic carbon %	9.6 ± 1.1 a	3.8 ± 0.7 b
Available nitrogen g kg−1	7.8 ± 1.2 b	12.5 ± 1.1 a

Note: data presented as mean ± standard error at 0–20 cm (n = 5). Different letters meant significant difference in the same row.

). The soil pH was measured using a pH meter equipped with a water mass ratio of 1:2.5, and the bulk density was measured using the cylinder method.

A total of 50 g dry soil was sampled and put into a bottle with 500 volumes. The soils from heavily degraded and control meadows were incubated for nine weeks in air at an indoor temperature of 25 °C and a soil field water capacity of 53%. We collected the gas with a 30 mL syringe with a three-way valve, and determined the N₂O emission rate by gas chromatography (GC2014, Shimadzu). The column box and detector temperatures were 70 °C and 300 °C, respectively, and the minimum factor detection limit was ±5 × 10⁻⁹ L L⁻¹.

The soil N₂O emission rate (F) is calculated as follows:

$$F=\frac{M\times \rho \times \left(V_1-V_2\right)\times {10}^6}{S\times 24\mathrm{h}}$$

where F and M are the soil emission rates (g kg⁻¹ h⁻¹), and the gas concentration of N₂O, ρ, is the gas density. V₁ and V₂ are the volume of the culture chamber, and soil, S, is in the bottom area.

2.2. Data Compilation and Selection Criteria

We collected published papers using the keywords “nitrous oxide or N₂O”, “grazing” and “Tibet*” in Web of Science (https://www.webofscience.com/wos/alldb/basic-search, accessed on 2 June 2022) with a publication date of up to May 2022. A total of 49 original published articles were retrieved. The full articles were perused and screened based on the following criteria: (1) N₂O were measured using static chamber–chromatographic concentration analysis; (2) all studies included heavy grazing activity and controls. The data were extracted from the paper figures using the WebPlotDigitizer software. In this study, we selected 13 published studies, including 13 field experiment results (Figure 1), soil physical characteristics, and plant communities.

The log response ratios were used as a measure of effect size. We utilized a random effects model meta-analysis method.

$$\ln R=Ln\frac{x_e}{x_c}=\ln \left(x_e\right)- \ln \left(x_c\right)$$

where x_c and x_e are the mean values of each individual trait in the control and the treatment groups, respectively.

Heavy grazing on N₂O emission rates and CI was calculated using the random effects model.

The weight of an individual effect of heavy grazing on N₂O emission rates was calculated using:

$$w_i^{*}=1/(v_i+{\tau }^2)$$

where v_i and ${\tau }^2$ represent the intra-study variance and inter-study variance.

The average effect size was analyzed:

$$\overline{y}=\frac{{\displaystyle {\sum }_{i=1}^k{w}_i{}^{*}{y}_i}}{{\displaystyle {\sum }_{i=1}^k{w}_i{}^{*}}}$$

The heterogeneity test of the effect size was measured:

$$Qt={{\displaystyle \sum _{i=1}^{k}w{i}^{*}(yi-\overline{y})}}^2$$

The influence of the explanatory variables on effect size was also calculated:

$$Qm={{\displaystyle \sum _{j=1}^p{\displaystyle \sum _{i=1}^{ni}w{i}^{*}}(yij-\overline{y})}}^2$$

where n_i and p are the sample sizes of the heterogeneity test value of the control and moderator variable, and i and j are the effect sizes of the control and single treatment.

2.3. Statistical Analysis

The repeat measurement variance of the grazing and sample data on N₂O emission rates was statistical analyzed using the “aov” package. This was used to analyzed the effect of incubation days, grazing, and their interaction on N₂O emission flux. A random effects model of this meta-analysis was performed in the metafor1.9-8 package using R 3.6.2 [22]. It was conducted to reveal the comprehensive effect of heavy degradation on alpine meadows’ N₂O emission flux. Meanwhile, the residual heterogeneity was tested with categorical and continuous variables using mixed-effect models (mods).

3. Results

3.1. Heavy Degradation Significantly Increased Alpine Meadow N₂O Rates through Incubation Experiment

The N₂O emission rates exhibited a pattern similar to those of heavily degraded and control meadow soils during 63 days of incubation (Figure 2). The averaged N₂O emission rates were 4.29 ± 0.64 and 3.27 ± 0.53 g kg⁻¹ h⁻¹ in heavily degraded and control meadows, respectively (p < 0.01). Heavy degradation increased the N₂O emission rates by 31.16% in alpine meadows in the southeastern Tibetan Plateau. Furthermore, repeated measurement variance analysis revealed that there was a significant difference during the incubation period and its interaction with grazing (p < 0.05).

3.2. Effects of Heavy Degradation on Grassland N₂O Rates through Meta-Analysis across Tibetan Plateau

Heavy degradation significantly increased the nitrous oxide emission rates by 0.55 ± 0.14 (95% 0.27–0.83,

Table 2. Effect size of high degradation on alpine meadow N₂O rates on the Tibetan Plateau.

Items	Effect Sizes	Increase Range (%)	95% Confidence Interval	p	Qt
Heavy degradation	0.55 ± 0.14	71.60	0.27–0.83	<0.001 ***	2597.36

Note: effect sizes presented as average ± stand error. *** meant significant differences with p < 0.001. Q_t is the heterogeneity test of effect size.

) on the Tibetan Plateau (p < 0.001). This indicated that high degradation increased by approximately 71.6% compared with the control plots. Furthermore, a significant residual heterogeneity was also observed (p < 0.001). We can explain this with the categorical variables in the next section.

3.3. Effects of Climate and Soil Physical Factors on Alpine Meadow N₂O Emission Rates

The WFPS significantly influenced grassland N₂O emission rates through the moderator test analysis (p < 0.05). The WFPS could explain 59.90% of this difference in the N₂O emission rates between the control and heavily degraded meadow based on the mixed-effects model (

Table 3. Effect of air temperature, altitude, and other factors on effect size.

Moderators	Qm	p Value	Models	R2 (%)
WFPS	3.83	0.05 *	Y = 3.2483 − 0.0835 x	59.90
Nitrate	2.53	0.11	Y = 0.2751 + 0.0175 x	16.56
Bulk	1.92	0.17	Y= 2.9311 − 2.6431 x	15.19
Temperature	1.02	0.31	Y = 0.5987 − 0.0499 x	0.28
Precipitation	0.88	0.35	Y= 1.2636 − 0.0011 x	0.00
SOC	0.36	0.55	Y = 0.7876 − 0.0147 x	0.00
pH	0.34	0.56	Y= −0.5554 + 0.1395 x	0.00
Altitude	0.05	0.83	Y = −0.1684 + 0.0002 x	0.00
Ammonia	0.01	0.94	Y = 0.5125 + 0.0007 x	0.00

Note: Y refers to effect sizes. WFPS: water-filled pore space; SOC: soil organic carbon. * meant significant differences with p < 0.05

). In addition, the soil nitrate and bulk density explained 16.56% and 15.19% of the variation, respectively. However, air temperature, precipitation, soil organic carbon, pH, altitude, and ammonia nitrogen had minimal influence on the variation. Furthermore, increasing the WFPS and bulk density reduced the N₂O emission rates from heavy degradation. The reduction of soil nitrate content could reduce the emission rates of heavily degraded meadows.

4. Discussion

4.1. Heavy Degradation Increased Alpine Meadows N₂O Emission Rates on the Tibetan Plateau

Heavily degraded meadows were identified according to the following criteria: the coverage, biomass, and richness were lower than 35%, 254 g m⁻², and 17, respectively. The results of our soil incubation experiments demonstrated that heavy grassland degradation increased N₂O emission rates by 31.16% in the southeastern Tibetan Plateau. This is supported by some previous case studies in which the N₂O emission rates of heavily degraded and control alpine meadows were 43.4 and 39.7 μg m⁻² h⁻¹, respectively, in the northeast Tibet Plateau [18]. Heavy degradation increased by 55.04% and 76.55% in terms of the average N₂O emissions rates in the southeastern Tibetan Plateau [16,23]. We further indicated that high grassland degradation significantly increased N₂O emission rates by approximately 71.6% in this region.

The two main reasons for these grassland degradation effects are as follows. First, heavy degradation exerts powerful effects on the plant community and soil physical characteristics [24]. The total carbon and available nitrogen were higher than in more lightly degraded grassland soils [16]. Organic carbon and inorganic nitrogen triggered substantial N₂O emissions in grasslands [2,25]. Second, grazing activity enhanced ammonia-oxidizing archaea gene abundance by 74.5% and 95.2% in moderately and highly degraded grasslands, respectively [14]. Nitrification is taken as the dominant procedure when over 68.8% for the N₂O emission of alpine meadows [18,26]. Thus, high degradation could increase alpine meadow N₂O emission rates.

4.2. Driving Factors of Soil and Climate Characteristics on N₂O Emission Rates in Heavily Degraded Meadow

The grassland N₂O emission rates were influenced by multiple decisive factors, including the available soil nitrogen and precipitation [14,17]. Our study indicated that WFPS, soil nitrate, and bulk density explained 59.90%, 16.56%, and 15.19% of the residual heterogeneity, respectively, in the N₂O emission rates between the control and heavily degraded meadows. Moreover, the increased effect size of the N₂O emission rates from heavy degradation was negative with WFPS and bulk density, and positive with soil nitrate. Increasing both the WFPS and bulk density reduced the N₂O emission rates. The precipitation and soil moisture have increased during the past 20 years on the Tibetan Plateau [10,16]. Thus, the N₂O emission rates from heavily degraded meadows would be mitigated. This is because the increased moisture enhances the denitrification process, and more N₂O leads to a reduction in nitrogen (N₂). Meanwhile, the increased bulk density means that more soils fill into alpine meadows replicated with roots, and more anaerobic environments are formed to increase the reduction process of N₂O.

Similar findings reported that the effects of soil moisture on N₂O fluxes were significantly simulated using negative linear regression [27]. Reduced soil moisture decreases N₂O efflux due to limitations in soil nitrification [28,29]. Both the soil water content and temperature are the principal drivers of grassland N₂O emission rates [5]. Precipitation events are vital for N₂O emissions from semiarid grasslands [25]. As soil moisture increases, more N₂O is consumed and transformed into N₂ in an anaerobic environment [30,31].

Soil nitrification and denitrification are limited by low available nitrogen content [2]. The N₂O emission rates are driven by the available soil nitrogen on the Tibetan Plateau [14], as soil nitrate is a fundamental substrate for denitrification and N₂O flux [29]. Furthermore, soil bulk density is an important factor in determining grassland N₂O emissions, and there is a considerable negative relationship between bulk density and N₂O flux [18,32]. Soil bulk density affects nitrogen transformation rates and oxidation and reduction states [8].

5. Conclusions

The N₂O emission rates exhibited a similar pattern to that of heavily degraded and control meadow soils. Heavy degradation significantly increased the N₂O emission rates. A significant difference was observed during the incubation period and its interaction with grazing. Heavy degradation increased the N₂O emission rates by approximately 71.6% compared to the control. WFPS explained 59.90% of the variation in the N₂O emission rates between the control and heavily degraded meadows. The soil nitrate and bulk density explained 16.56% and 15.19% of the variation, respectively. In addition, increasing the WFPS and bulk density can mitigate the N₂O emission rates from heavy degradation.