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INTRODUCTION
In the terrestrial ecosystems, soil is regarded as the biggest C pool with an estimated 2344 Gt organic C stored in its upper 3 m layers (Stockmann et al., 2013). Soil organic C (SOC) has a key function in soil fertility and ecology due to its significant impact on soil attributes such as its structure (Kong et al., 2020), water holding capacity (Zhu et al., 2018), activities of soil microbes (Zhao et al., 2017; Guan et al., 2018) and nutrient availability (Oelofse et al., 2015). Soils in mountainous terrains in temperate areas are susceptible to temperature changes. Warming may speed up the rate of decomposition of SOC in these areas and affect global climate change (Ayoubi et al., 2012; Guan et al., 2018). The SOC has a significant impact on soil structure and properties and hence affects vegetation productivity. Spatial variability of SOC is complex and is substantially affected by environmental conditions and anthropogenic factors (Qiu et al., 2015; Conforti et al., 2016), which in one way or the other determines changes in the soil C pool. Thus, the SOC spatial distribution reckons on the relative effect of various factors such as vegetation, climate, and soil and their interaction which usually yields overlapping consequences on its spatial distribution pattern (Oueslati et al., 2013). A proper understanding of the quality and chemical composition of SOC will be vital to clarify how it reacts to actual conditions and the implicit mechanisms of prospective climate change.
Soil organic matter (SOM) is a diversified mixture of organic compounds and comprises diverse fractions with differences in stability, decomposition and turnover rate (Smith et al., 2014). The various fractions of SOC present distinct effects on soil and the environment (O’Brien and Jastrow, 2013). The labile fractions of SOC, such as readily oxidizable organic C (ROC) and water-soluble organic C (WSOC) have been regarded as being more responsive indicators for land use changes as compared to SOC (Li et al., 2017). Then again, most SOC exists as recalcitrant humic substances, viz, humic acid C (HAC), fulvic acid C (FAC) and humin C (HUC), which is estimated to be made up of roughly 60%-75% of SOC (Grinhut et al., 2011). Both the labile and chemical resistant fractions are regarded as possible indicators of climate change (Cao et al., 2016; Wang et al., 2016; Hu et al., 2017; Li et al., 2017). In addition, soil aggregate organic C is usually regarded as a vital indicator in measuring SOC sequestration and stability (Kong et al., 2020). Micro-aggregate organic C is usually more stable as compared to macro-aggregate and silt + clay organic C (Yu et al., 2012; Huang et al., 2016). Because of their recalcitrant attributes, humic substances play an essential function in soil C sequestration (Li et al., 2017). Soil aggregate stability is considered a key indicator of soil physical quality (Kong et al., 2020). The establishment and stabilization of soil aggregates in most cases enhance soil C sequestration and renders protection for soil C (Cheng et al., 2015). Therefore, the study of SOC fractions is essential for proper apprehension of the turnover and stabilization techniques of SOC.
The Tibet Plateau is considered the highest and largest plateau in the world with an estimated land area of 2.5 × 106 km2 and an average elevation of 4000 m a.s.l. (Shang et al., 2016). At the same time, literature shows that this area is susceptible to global climate changes (Zhang et al., 2014). Alpine grasslands make up approximately 50% of the whole this area (Chen et al., 2014a; Tang et al., 2015). Studies have shown that the alpine grassland soils contain a substantial quantity of organic C and hence have a key function in the C cycles. Thus, there have been numerous studies on SOC in this area in the past decades (Wang et al., 2013). On high mountain terrains, weather conditions such as climate and vegetation change markedly along an elevation gradient. Thus studies of the variations in features of SOC contents along elevation gradient in this region is vital in order to be able to scientifically evaluate their impact in C sequestration or source under climate change simulations. Notwithstanding, earlier researches mainly concentrated on the storage of SOC among diverse types of alpine grasslands on the Tibet Plateau (Cao et al., 2016; Li et al., 2017). Till present, only a few studies have been carried out to investigate the differences in SOC fractions and soil aggregates stability among the different grasslands in this area, even though such studies are vital. Furthermore, it is yet not clear how the differences in environmental conditions affect the variations in quantitative and qualitative features of SOC storage and soil structural stability at various elevations in this area. In this research along an elevation of 3105-4200 m a.s.l. across the North- eastern slope of the Qilian Mountains on the Tibet Plateau, we investigated the variations in the concentrations of SOC fractions (viz, ROC, WSOC, humic C, and aggregate organic C) in addition to the stability of soil aggregates. In addition, we examined the factors that affect SOC fractions and soil structural stability in the perspective of climate (mean annual temperature and mean annual precipitation), vegetation (aboveground biomass), and soil factors (pH, moisture content, temperature, and particle size distribution). We premised our hypothesis that SOC fractions and soil structural stability varied across elevation gradients owing to changes in conditions.
MATERIALS AND METHODS
Site description
In this study, field experiments were conducted in the alpine grasslands of the Qilian Mountains in Zhuaxixiulong Township in the Tianzhu Tibetan Autonomous County of Gansu Province of China. The area has a typical alpine climate and is usually cold and wet for most parts of the year. It also has weather conditions such as thin air with low oxygen concentrations, high solar, and high ultraviolet radiation. The area has an average annual temperature of 0.13 °C, and average annual rainfall of 414.98 mm, which usually falls from July to September. The planting season is about 120 d, ranging from May to September. The soil is a typical alpine Chernozem with bulk density of about 0.73 g cm-3, organic C of about 138.45 g kg-1, total N of about 4.31 g kg-1, and total P of about 0.65 g kg-1. Figure 1 presents a map showing the vegetation types and altitude of the study area. Typical alpine plant species include Kobresia humilis, Elymus nutans, Koeleria pers, Blysmus sinocompressus. The study was carried out across a north-eastern slope of the Qilian Mountains. The elevation ranges from 3105 to 4200 m a.s.l., with the apex at approximately 4200 m a.s.l. The elevation was covered by pasture, except the apex which had few scattered grasses with bare land. In late August 2020, six sampling sites were set up along the elevation gradient, i.e., at 3105, 3350, 3480, 3710, 3925, and 4200 m, Table 1 shows the basic characteristics of these sampling sites.
Field sampling
Five 30 × 30 m2 plots were randomly located in the alpine grassland on a north-eastern slope, for a total of 30 sample plots. Five samples were collected using a core sampler at depth 0-20 cm. In total, 150 samples were collected at six elevations. Quadrats 1 m × 1 m were laid at approximately 5 m intervals and above ground plants were harvested, oven dried, and used for estimating aboveground biomass (AGB). Soil samples were sieved, homogenized, and air-dried, and used for analyzing soil aggregates, SOC, and other properties.
Table 1 Basic characteristics of the sampling sites across an elevation gradient.
Mean values ± standard error. Values in the same column with different letters show significant difference according to Tukey’s test (P ≤ 0.05). AGB: Above ground biomass; MC: soil moisture content; TEMP: soil temperature; MAT: mean annual temperature; MAP: mean annual precipitation.
Soil analysis
Organic C concentrations were estimated using sulphuric acid-potassium dichromate oxidation method. For oxidizable organic C (ROC) analysis, bulk soil samples were oxidized with 333 mM KMnO4 solution for 1 h, and the quantity of ROC was estimated by spectrophotometry. For water-soluble organic C (WSOC) and humic C analysis, water soluble and humic fractions were estimated by extracting soil samples with distilled water and 0.1 M NaOH + 0.1 M Na4P2O7 solution, respectively. Humic acid (HA) was separated from fulvic acid (FA) by acidifying the alkaline supernatants to pH 1.0 and the alkaline insoluble residue was humin (HUC). The concentrations of WSOC, humic acid C (HAC) and fulvic acid C (FAC) were determined via the use of a total organic C (TOC) analyzer (Shimadzu, Kyoto, Japan), while that of HUC was estimated as the difference of the sum of WSOC, HAC, and FAC from SOC.
The conventional wet-sieving method was used to separate soil aggregate fractions (Cambardella and Elliott, 1993). Briefly, a 100 g air-dried soil sample was wet-sieved with mesh sizes of 2, 1, 0.25 and 0.053 mm to separate it into four size fractions of water-stable aggregate, i.e., macro-aggregate (> 2 mm, 1-2 mm, 0.25-1.00 mm), and micro-aggregate (0.053-0.250 mm). The sample was pre-soaked in distilled water on top of 2 mm sieve for 10 min before vertical oscillation in water for 10 min with 30 oscillations per minute and amplitude of 4 cm. Organic material floating on water after sieving was discarded because it is not considered to be soil organic matter (Han et al., 2021). Finally, each fraction was dried at 50 °C for 24 h, and weighed for calculating the proportions of each aggregate fraction in bulk soils and for further chemical analysis. Each aggregate fraction was calculated as a percentage relative to the total dry sample. The index of stability was expressed as the percentage of aggregates larger than 0.25 mm diameter following wet sieving and drying. The aggregate-size classes and the concentration of soil aggregate organic C were expressed on a sand-free basis (Chen et al., 2014b). Nonetheless, SOC concentration in the macro-aggregates at the apex of the elevation (i.e., 4200 m) was not corrected for sand because it contained very high sand quantities. The mean weight diameter (MWD), geometric mean diameter (GMD), percentage of > 0.25 mm water-stable aggregates (W0.25), structure deterioration rate (SDR), and stability ratio of water-stable aggregate (WSAR) of water-stable aggregates were estimated according to Choudhury et al. (2014) and Liu et al. (2014).
Water-stable macro- and micro-aggregates: The macro-aggregates were determined by adding the aggregates retained over 0.25-2.0 mm sieves while the micro-aggregates referred to aggregates retained on 0.05-0.25-mm sieves.
where i denotes the size of the sieve. The percentage of water-stable macro-aggregates and water-stable micro-aggregates is the summation of soil aggregate size fractions of > 0.25 mm and < 0.25 mm, respectively. These two were summed up to estimate the total water stable aggregates.
The mean weight diameter (MWD) and geometric mean diameter (GMD) of aggregates were calculated as:
where n is the number of fractions (0.1-0.25, 0.25-0.5, 0.5-1.0, 1.0-2.0, > -2.0 mm), Xi is the mean diameter (mm) of the sieve size class (0.175, 0.375, 0.75, 1.5 and 2.0 mm) and Wi is the weight of soil (g) retained on each sieve.
Statistical analysis
We used SPSS 16.0 software for the analysis (SPSS, Chicago, Illinois, USA). One-way ANOVA with least significant difference (LSD) test was used to examine the differences between SOC contents, soil structural stability, and humic C fractions among the various elevations. We used Pearson correlation to determine the connections between SOC and its fractions and aggregate stability indices with environmental conditions, i.e., elevation, mean annual temperature (MAT), mean annual precipitation (MAP), and AGB and other soil properties, i.e., particle size distribution, moisture content (MC), temperature, and pH. The P < 0.001 level was considered to be significant. Principal component analysis was carried out using PAST software version 2.17c (Hammer et al., 2001).
RESULTS
Distribution of SOC, ROC, WSOC, and humic C
The SOC concentrations had an increasing trend with elevation from 76.04 g kg-1 at 3105 m up to 110.42 g kg-1 at 3710 m and 103.87 g kg-1 and then decreased to 71.25 g kg-1 at the peak of the slope at 4200 m (Figure 2). The elevations 3710 and 3925 m recorded significantly higher amounts of SOC compared to the others. There was a significant difference in SOC quantity at the lowest elevation (3105 m) and that of the highest elevation (4200 m). The concentrations of ROC, WSOC (Figure 2), and HUC (Figure 3) in the soils followed the same trends as SOC along the elevation. Both HAC and FAC recorded their highest concentrations at 3925 m while HAC/FAC had its highest concentration at 3480 m (Figure 3). The middle portions of the elevation recorded the largest concentrations of the SOC fractions and were significantly higher compared to the foot and apex of the slope.
Soil aggregates organic C and aggregate stability
There were variations in aggregates size distributions across the elevation (Figure 4). Along the elevations, the ratio of micro-aggregates was greater than macro-aggregates and silt + clay fractions except at the 3925 m where macro- aggregates ratio was higher than micro-aggregates and silt + clay fractions. Across the elevations, the amount of micro- aggregate organic C (MicAOC) was higher compared to macro-aggregate and silt + clay organic C (Figure 4). In a similar trend to SOC, the amount of macro- and micro-aggregates and silt + clay organic C were also significantly higher at 3710 and 3925 m compared to the rest. Furthermore, the ratio of macro-aggregates organic C (MacAOC) to MicAOC was significantly higher at 3710 and 3925 m elevations and significantly lowest at 4200 m in relation to the other elevations (Figure 5). Ratio MacAOC/MicAOC along the elevation gradient equally followed the same trend as organic C and its fractions by increasing with increasing elevation and attaining a peak at 3480 and 3710 m and then decreased afterward.
Bars with different letters show significance at p < 0.001. SOC: Soil organic C; ROC: readily oxidizable organic C; WSOC: water-soluble organic C.
Figure 2 Concentration of soil organic C (A) and its labile fractions (B).
The MWD, GMD, W0.25, and water-stable aggregates ratio (WASAR) of the soil aggregates increased with increasing elevation and got to a maximum at 3925 m and then decreased (Table 2). The 3925 m elevation recorded significantly larger values than the others. The SDR of the soils along the elevation gradient was lowest at 3925 m and highest at the peak of 4200 m.
Bars with different letters show significant difference at p < 0.001. HAC: Humic acid C; FAC: fulvic acid C; HUC: humin C; HA/FA: degree of humification.
Figure 3 Concentrations of resistant organic C fractions (A) and degree of humification (B).
Bars with different letters show significant difference at p < 0.001.
Figure 4 Aggregate size distribution (A) and aggregate organic C concentrations (B).
Bars with different letters show significant difference at p < 0.001.
Figure 5 Ratio of soil macro-aggregate to micro-aggregate organic C (MacAOC/MicAOC).
Table 2 Spatial distribution of soil aggregate stability indices along elevation gradient.
Mean values ± standard errors. Values with different letters show significant difference show significant difference according to Tukey’s test (P ≤ 0.05).
MWD: Mean weight diameter; GMD: geometric mean diameter; W0.25: percentage of > 0.25 mm water-stable aggregates; SDR: structure deterioration rate; WASAR: stability ratio of water-stable aggregates.
Relation between SOC content, aggregate stability, and environmental factors
The correlations between SOC fractions, aggregate stability with environmental factors are presented in Table 3, SOC and its fractions had significantly positive correlations with AGB. However, HAC/FAC, silt + clay-associated C had negative correlations with AGB. In addition, there were positive correlations between ROC, WASAR, FAC, HAC, and W0.25 with MAP. Nonetheless, SOC and its fractions had nonsignificant correlations with MAT, except micro-aggregates which had a positive correlation with MAT. On the whole, the aggregate stability indices had significant positive correlations with AGB except SDR, which was significantly and negatively correlated with AGB. The correlations between SOC fractions and aggregate stability indices are presented in Tables 4 and 5. There were significant positive correlations between SOC and its fractions except for HAC and HAC/FAC. In addition, there were strong positive correlations between SOC and its fraction with the aggregate stability indices. Principal component analysis (PCA) was carried out to evaluate the effects of elevation gradient on soil organic C fractions, aggregate stability, and environmental factors. Principal component 1 (PC1) explained 61.17%, principal component 2 (PC2) explained 19.91% while principal component 3 (PC3) explained 12.73% of the total variations (Figure 6). The elevations 3710 and 3925 m showed significant separations compared to the others; 3925 m elevation had the highest score, and 3710 m elevation followed. The lowest score was recorded at 4200 m elevation along PC1. However, 3480 m elevation had the highest score along PC2 and that of 3925 m recorded the highest score along PC3. Four of the elevations (3105, 3710, 3925, and 4200 m) were far from the origin, indicating that elevation strongly affects SOC fractions, aggregate stability, and environmental factors.
Table 3 Pearson correlation coefficients between soil organic C (SOC) and its fractions and aggregate stability indices with environmental factors along an elevation gradient.
AGB: above ground biomass; MAT: mean annual temperature; MAP: mean annual precipitation: HAC: humic acid C; FAC: fulvic acid C; HUC: humin C; HA/FA: humification degree; MWD: mean weight diameter; GMD: geometric mean diameter; W0.25: percentage of > 0.25 mm water-stable aggregates; SDR: structure deterioration rate; WASAR: stability ratio of water stable aggregate; ROC: readily oxidizable organic C; WSOC: water-soluble organic C; MacA: Macro-aggregate; MicA: micro-aggregate; MacAOC: macro-aggregate-associated organic C; MicAOC: micro-aggregate-associated organic C; Bolded values are significant according to Tukey’s test (P ≤ 0.05).
Table 4 Pearson correlation coefficients between soil organic C (SOC) and is fractions and soil aggregate stability.
SOC: Soil organic C; HAC: humic acid C; FAC: fulvic acid C; HUC: humin C; HA/FA: humification degree; MWD: mean weight diameter; GMD: geometric mean diameter; W0.25: percentage of > 0.25 mm water-stable aggregates; SDR: structure deterioration rate; WASAR: stability ratio of water-stable aggregate; ROC: readily oxidizable organic C; WSOC: water-soluble organic C; MacA: macro-aggregate; MicA: micro- aggregate; MacAOC: macro-aggregate organic C; MicAOC: micro-aggregate organic C.
Bolded values are significant according to Tukey’s test (P ≤ 0.05).
Table 5 Pearson correlation coefficients between soil organic C (SOC) and is fractions and soil aggregate stability
WASAR: Stability ratio of water-stable aggregate; ROC: readily oxidizable organic C; WSOC: water-soluble organic C; MacA: macro-aggregate; MicA: micro-aggregate; MacAOC: macro-aggregate organic C; MicAOC: micro-aggregate organic C; MacOC/MicOC; SOC: soil organic C; HAC: humic acid C; FAC: fulvic acid C; HUC: humin C; HA/FA: humification degree; MWD: mean weight diameter; GMD: geometric mean diameter; W0.25: percentage of > 0.25 mm water-stable aggregates; SDR: structure deterioration rate.
Bolded values are significant according to Tukey’s test (P ≤ 0.05).
DISCUSSION
Variability of SOC and its fractions across the selected elevation gradient
The study revealed that there was significantly larger SOC, including labile SOC fractions, recalcitrant SOC fractions, and stable aggregate-associated organic C fractions at higher elevations of 3710 and 3925 m compared to the lower and apex of the elevation (Figures 2 and 3). This is an indication that the apportioning of SOC fractions across the elevation gradient had a unimodal pattern. These results are in line with those of Yuan et al. (2014) and Li et al. (2017). For instance, Li et al. (2017) recorded significantly higher values of SOC storage in the middle of the elevation as compared to the lower and peak of the elevation. In addition, Fu et al. (2014) found that SOC and WSOC increased with increasing elevation. The ratio HAC/FAC is regarded as an indicator of the degree of humification (Putra et al., 2016). The differences in HAC/FAC ratio (Figure 3) indicated that the degree of humification is also affected by elevation gradient, with the highest value recorded at 3480 m. A large HAC/FAC shows a high degree of humification (Seran, 2011; Reddy et al., 2012). Therefore, these results indicated that the degree of humification of SOC was higher in the middle portion of the elevation gradient as compared to the lower and the top. The SOC and its fractions had a positive correlation with AGB (Table 3) indicating that vegetation productivity was a key factor affecting SOC distribution along elevation gradient. This could be attributable to the fact that SOC is formed primarily via the decomposition and turnover of vegetative residue and therefore higher plant biomass could lead to higher SOC concentrations. Hence elevations below 3710 m with low plant biomass recorded low SOC and humification values. The effect of altitude on SOC content is principally due to the combination of moisture and temperature. The temperature in the Qilian Mountains is reported to decrease at a rate of 0.58 °C/100 m with increasing altitude, and precipitation increases at a rate of 18.6 mm/100 m with increasing altitude (Wan et al., 2019). The findings showed that SOC concentration increased with increasing altitude primarily due to high precipitation and low temperature. High moisture and low temperature decrease microbial activity and hence slows decomposition and mineralization rate of SOC. Soil pH decreases with elevation, and this beyond doubt affects soil microbial activity. Soil pH is a driver of soil microbial activity (Fierer, 2017; Tripathi et al., 2018). Low pH and high moisture leads to slower microbial growth rates that limits decomposition of organic matter causing accumulation of SOC (Tripathi et al., 2018). Our results revealed that the labile, recalcitrant, and soil aggregate organic C fractions were strongly positively correlated with SOC concentration (Tables 4 and 5). In addition, the labile, recalcitrant, and soil aggregate organic C were significantly correlated with each other, showing that they are closely interconnected.
Soil aggregate stability across the selected elevation gradient
The MWD, GMD, W0.25, SDR, and WASAR are widely used as indicators for exploring soil aggregate stability. For an all-round exploration of soil aggregate stability, it is important to employ a combination of these indices. A bigger MWD, GMD, W0.25, and WASAR, and a smaller SDR suggest the structural stability of the soil aggregates was higher (Choudhury et al., 2014; Liu et al., 2014; Li et al., 2017). Therefore, our results (Table 2) indicated that the structural stability of the soil aggregates was highest in the middle part of the elevation (3925 m). This could be attributable to higher SOC which is the chief binding material for the establishment and stability of soil aggregates at higher elevations (Li et al., 2017). There were also significant correlations between SOC concentrations and soil aggregate stability indices in the present study (Tables 4 and 5). Furthermore, the soil aggregate stability indices had strong positive correlations with AGB (Table 3). These results are consistent with Cao et al. (2016) and Li et al. (2017). Aside SOC, labile and recalcitrant C contents also have key roles in soil aggregate stability. Several studies revealed that the quantum of WSOC (Padbhushan et al., 2016) and humic fractions (Kong et al., 2020) in the soil had a significant positive impact on aggregate stability, this was in line with the present study (Tables 4 and 5). The AGB is regarded as a key determinant of SOC concentration and soil aggregate stability across the elevation gradient. However, the low AGB at the lower elevations might be the result of climate warming as found by Du et al. (2012). The negative impact of warming on AGB (Fu et al., 2013) could be the reason for the low SOC and low soil structural stability at the foot of the elevations. Therefore if the tendency of global warming is to increase the Earth’s temperature, then in the future those cooler regions in higher altitude will produce less biomass and thus have lower soil stability. Tables 4 and 5 clearly indicate that soil aggregate stability was closely connected with ROC, WSOC, HAC, FAC, HUC, macro- and micro-aggregate associated C in comparison with silt + clay- associated organic C, implying that both the labile and recalcitrant forms of C were major determinants of soil aggregate stability in the study. The PCA showed that elevations 3710 and 3925 m showed significant separations compared to the others and also recorded the highest scores. This implies that those sites contributed much to variations in SOC fractions and aggregate stability. This could be attributed to the high plants’ biomass at those sites. In general, elevation has a strong influence on soil microclimates and hydrothermal processes by controlling the climatic conditions across the vertical gradients. In turn, these microclimate variations then result in major differences in SOC along altitude gradients (Karst and Landhäusser, 2014; Zhang et al., 2018). Also, temperature and precipitation along the elevation gradients are the major factors affecting C budget by controlling C inputs from plant biomass and decomposition (Conant et al., 2011; Guan et al., 2018) and this may further affect the stability of soil aggregates. Clay also plays a vital function in aggregate stability of SOC because soil mineral particles provide protection for organic matter. This could be the reason why aggregate stability at 4200 m elevation was the least because it contains a significant amount of sand and less clay content.
Protection mechanisms of SOC along elevation gradient
Aggregate formation has the possibility of physically protecting SOC (Li et al., 2017). In the present research, the significant MicAOC along the elevation gradient indicated that the major mechanisms of SOC protection were by micro- aggregates. Yu et al. (2012) and Huang et al. (2016)) reported that micro-aggregate-associated organic C is more stable as compared to macro-aggregates-associated organic C and silt + clay fractions. The significantly lower ratio MacAOC/ MicAOC recorded at 4200 m elevation as compared with the rest (Figure 4) indicates that micro-aggregate-associated organic C could be the main factor contributing to SOC preservation at the highest elevation.
CONCLUSIONS
The content of soil organic C (SOC) and its recalcitrant, labile, and soil aggregate stability increased with increasing elevation, attaining peak concentrations at 3710 or 3925 m. There were positive correlations between SOC, oxidizable organic C, water-soluble organic C, humic acid C, fulvic acid C, humin C, macro- and micro-aggregate organic C, aboveground biomass (AGB), and all the aggregate stability indices except structure deterioration rate. The positive correlations between the fractions of SOC and soil aggregate stability indices imply that both the resistant and labile forms of C were the major determinants of soil structural stability in the study. The findings of this research show that prospective global climate warming could have a telling effect on SOC and soil structural stability by affecting the distribution of AGB. Due to the low structural stability of soils at the foot of the elevations, we proposed that these areas should be barred from continuous grazing to enhance high grassland productivity and ability to cope with prospective climate change in this area.
