Response of carbon exchange between shrub and grass patches to increased seasonal precipitation: a comparative analysis based on aboveground net primary productivity and leaf area index standardization

doi:10.17521/cjpe.2023.0359

Abstract

Abstract:

Aims With the intensification of climate change and human activities, the phenomenon of shrub encroachment in arid and semi-arid grasslands is widespread, significantly impacting the carbon sequestration function of grassland ecosystems. Water availability is the primary limiting factor in the semiarid grassland of Nei Mongol, and future changes in precipitation patterns have important implications for carbon exchange in grassland ecosystems. However, there is limited research on the effects of precipitation changes on shrub-encroached grassland ecosystems, particularly on the carbon exchange processes within heterogenous patches. The underlying mechanisms remain unclear.

Methods In this study, we conducted a seasonal precipitation manipulation experiment by increasing snowfall in winter and rainfall in summer in shrub-encroached grassland dominated by Caragana microphylla in Nei Mongol. Carbon exchange parameters, such as net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) of shrub patches and grass patches, were measured and compared using standardized parameters based on aboveground net primary productivity (ANPP) and leaf area index (LAI). The study investigated the impact of increased seasonal precipitation on carbon exchange in shrub-encroached grassland and the differential responses of heterogeneous patches.

Important findings 1) Increased summer rainfall significantly enhanced |NEE|, GEP and ER of the grass patches, while increased winter snowfall significantly reduced |NEE|_ANPP, GEP_ANPP and ER_ANPP of the grass patches. Increased summer rainfall significantly enhanced the GEP and ER of the shrub patches, while the effect on |NEE| was not significant. Additionally, increased winter snowfall had a positive impact on carbon exchange processes in the shrub patches. Overall, |NEE|, GEP and ER of the shrub patches were significantly higher than those of the grass patches. In comparison to a wet year (2021), the carbon exchange in a dry year (2020) was more sensitive to increased precipitation. 2) Carbon exchange in the shrub patches (|NEE|, GEP and ER) was positively correlated with soil water content and leaf biomass. Increased summer rainfall mainly promoted carbon exchange by enhancing deep soil water content (40-80 cm) and lowering soil temperature. Carbon exchange in the grass patches was positively correlated with shallow soil water content (0-20 cm) and ANPP, and negatively correlated with soil temperature and root-to-shoot ratio. Increased summer rainfall primarily enhanced carbon exchange in grass patches by raising shallow soil water content and lowering soil temperature, while increased winter snowfall hindered carbon exchange by increasing deep soil water content and stimulating belowground biomass. 3) Standardized carbon exchange parameters based on ANPP better revealed the differential responses of the shrub patches and grass patches to changes in precipitation. These research findings provide an important scientific basis for accurately assessing the carbon sink function and carbon sequestration potential of arid and semi-arid grasslands ecosystems under climate change.

Key words: net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), shrub patches, grass patches, increased winter snowfall, increased summer rainfall, shrub encroachment

ZHANG Meng-Di, XIANG Guan-Hai, WEN Yi-Yao, WANG Huan, Hugejile , BAI Yong-Fei, WANG Zhong-Wu, ZHENG Shu-Xia. Response of carbon exchange between shrub and grass patches to increased seasonal precipitation: a comparative analysis based on aboveground net primary productivity and leaf area index standardization[J]. Chin J Plant Ecol, 2024, 48(8): 1035-1049.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2023.0359

https://www.plant-ecology.com/EN/Y2024/V48/I8/1035

Figures/Tables 7

Fig. 1 Experimental platform for water and nutrient addition in Caragana microphylla shrub-encroached grassland of Nei Mongol. C, control; N, nitrogen addition; P, phosphorus addition; S, increased winter snowfall; W, increased summer rainfall. Block 1-5 are replicated for five blocks. The empty spaces in the sample plot layout diagram represent plots with fewer than two shrubs, which are not suitable for long-term experimental plots.

Table 1 Linear mixed model analysis of the effects of increased seasonal precipitation and year (Y) on carbon exchange parameters in shrub and grass patches

Fig. 2 Effects of increased winter snowfall (S) and increased summer rainfall (W) on carbon exchange parameters (A) in shrub and grass patches, and their impact on carbon exchange parameters normalized by aboveground net primary productivity (ANPP) (B) and leaf area index (LAI) (C) (mean ± SE). Control, without water addition; ER, ecosystem respiration; ERANPP, ecosystem respiration standardized by aboveground net primary productivity; ERLAI, ecosystem respiration standardized by leaf area index; GEP, gross ecosystem productivity; GEPANPP, gross ecosystem productivity standardized by aboveground net primary productivity; GEPLAI, gross ecosystem productivity standardized by leaf area index; Grass, grass patch; NEE, net ecosystem CO2 exchange; NEEANPP, net ecosystem CO2 exchange standardized by aboveground net primary productivity; NEELAI, net ecosystem CO2 exchange standardized by leaf area index; Shrub, shrub patch; SW, increased winter snowfall and summer rainfall. * indicates that the treatment effect is significant in a given year; *, p < 0.05; **, p < 0.01; ***, p < 0.001. Different uppercase letters indicate significant differences between shrub and grass patches in the same year (p < 0.05).

Fig. 3 Carbon exchange parameters (NEE, GEP, ER) of shrub and grass patches responses to increased seasonal precipitation (S, W, SW) in dry (A) and wet (B) years, and the corresponding response values are based on standardization of aboveground net primary productivity (ANPP) and leaf area index (LAI) (mean ± SE). ER, ecosystem respiration; ERANPP, ecosystem respiration standardized by aboveground net primary productivity; ERLAI, ecosystem respiration standardized by leaf area index; GEP, gross ecosystem productivity; GEPANPP, gross ecosystem productivity standardized by aboveground net primary productivity; GEPLAI, gross ecosystem productivity standardized by leaf area index; Grass, grass patch; NEE, net ecosystem CO2 exchange; NEEANPP, net ecosystem CO2 exchange standardized by aboveground net primary productivity; NEELAI, net ecosystem CO2 exchange standardized by leaf area index; S, increased winter snowfall; Shrub, shrub patch; SW, increased winter snowfall and summer rainfall; W, increased summer rainfall. Response value = (treatment - control)/control. * indicates whether the response value of the carbon exchange parameter is significant when compared with 0; *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Fig. 4 Effects of increased winter snowfall (S) and increased summer rainfall (W) on abiotic and biological factors in shrub (A) and grass (B) patches (mean ± SE). ANPP, aboveground net primary productivity; BGB, belowground biomass; Control, without water addition; LB, leaf biomass; RSR, root shoot ratio; SLR, stem leaf ratio; SWC0-20, soil water content (0-20 cm depth); SWC40-80, soil water content (40-80 cm depth); ST0-10, soil temperature (0-10 cm depth); SW, increased winter snowfall and summer rainfall. Abiotic factors include soil moisture content in shallow layer (0-20 cm) and deep layer (40-80 cm), surface soil temperature (0-10 cm), while biological factors include ANPP, BGB, LB, SLR and RSR. * indicates that the treatment effect is significant in a given year; *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Fig. 5 Correlation heatmaps of carbon exchange parameters with soil moisture content in shallow (SWC0-20) and deep (SWC40-80) layers, and soil temperature (ST0-10) in shrub and grass patches. ER, ecosystem respiration; ERANPP, ecosystem respiration standardized by aboveground net primary productivity; ERLAI, ecosystem respiration standardized by leaf area index; GEP, gross ecosystem productivity; GEPANPP, gross ecosystem productivity standardized by aboveground net primary productivity; GEPLAI, gross ecosystem productivity standardized by leaf area index; NEE, net ecosystem CO2 exchange; NEEANPP, net ecosystem CO2 exchange standardized by aboveground net primary productivity; NEELAI, net ecosystem CO2 exchange standardized by leaf area index; SWC0-20, soil water content (0-20 cm depth); SWC40-80, soil water content (40-80 cm depth); ST0-10, soil temperature (0-10 cm depth). Blue indicates a negative correlation, while orange indicates a positive correlation. The more prolate the ellipse, the darker the color, and the stronger the correlation between the two variables. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Fig. 6 Relationships between carbon exchange parameters of shrub and grass patches and aboveground net primary productivity and below-ground biomass, leaf biomass, and allocation (root-shoot ratio (RSR), stem-leaf ratio (SLR)). ER, ecosystem respiration; GEP, gross ecosystem productivity; NEE, net ecosystem CO2 exchange. Hollow symbols represent 2020 shrub or grass patches and solid symbols represent 2021 shrub or grass patches. Shadows represent the 95% confidence intervals of the fitted models. Because it is difficult to measure the belowground biomass of Caragana microphylla, only the belowground biomass and root-shoot ratio of grass patches are shown.

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