Overall patterns of leaf ecological stoichiometry and soil physicochemical properties of S. chamaejasme across northern China
The patterns of N and P status in plant biomass, and especially in leaves, have been intensely studied (Koerselman and Meuleman 1996; Reich and Oleksyn 2004; Han et al. 2005; He et al. 2006, 2008; Song et al. 2014; Fan et al. 2016), but few studies have attempted to document intraspecific leaf stoichiometry, especially for poisonous weeds in grasslands. This study presents, to our knowledge, the first analysis of leaf element concentrations (C, N, P, K) and ratios (C:N, C:P, N:P) of S. chamaejasme across degraded grassland in northern China. Our results show that leaf C (498.60 g kg-1), N (19.95 g kg-1), and P (2.15 g kg-1) of S. chamaejasme were higher than the mean value of all species in the China Grassland Transect (Fan et al. 2016), and there was no obvious difference between two habitats of S. chamaejasme. N and P are the most important limiting nutrients for primary productivity in terrestrial ecosystems (Elser et al. 2007), and a high concentration of N and P in S. chamaejasme leaves means high nutrient uptake efficiency of S. chamaejasme in degraded grasslands, which could facilitate S. chamaejasme outcompete other species in nutrient-poor environments. Moreover, K is one of the essential macronutrients that play critical roles in various metabolic processes, but it has been partly overshadowed in ecological stoichiometry by nitrogen and phosphorus (Shin 2014; Adams and Shin 2014). Although most studies did not involve the K content in plant leaves, it is worth noting that K concentrations of S. chamaejasme were significantly greater in QT than that in IM. The reason may be that the content of nutrients in plants can be constrained by nutrient supply in the soil, and the content of soil K is significantly higher in QT, therefore generating this difference. The leaf C:N ratio of S. chamaejasme was 25.46, C:P ratio 246.22, N:P ratio 9.84. Generally, it is not uncommon that using N:P ratios of plant biomass as indicators of N or P limitation in various studies (Koerselman and Meuleman 1996; Tessier and Raynal 2003). The low N/P ratio in S. chamaejasme might imply that its growth is restricted by N, which was consistent with the results reported by Guo et al. (2019).
We found that S. chamaejasme could survive in a soil environment with considerable variation, which is consistent with the fact that S. chamaejasme is wide-ranging species with a wide geographic range in China grassland (Zhang et al. 2015). The soil condition for S. chamaejasme growth varies considerably from site to site. Soil physicochemical properties varied with a difference of more than 10 times between the maximum and the minimum included C (14.43 times), N (30.94 times), K (32.27 times), NN (26.66 times), WC (10.60 times), Ec (21.86 times). Moreover, one sampling site (Haiyan in QT) showed the greatest soil physicochemical properties. For example, the content of soil P was lowest (0.20g kg-1), and soil WC was the minimum (0.03), but pH reached a maximum (8.81), indicating S. chamaejasme could tolerate an extreme environment. This may provide a competitive advantage for S. chamaejasme against other plant species and help explain its rapidly expansion in degraded grasslands. Additionally, we also found that the CV of NN was 100.57%, greater than for AN (41.16 %), indicating that S. chamaejasme was less sensitive to the NN variation.
Generally, Tibetan alpine grasslands and Inner Mongolian temperate grasslands, which have different limiting factors, are both zonal grassland types in China (Li et al. 2020). Alpine grasslands are mainly limited by low temperatures in the growing season, while temperate grasslands are affected by drought (Fan et al. 2016). Accordingly, our analysis indicated that some soil physicochemical properties of S. chamaejasme for the regions were significantly different. Soil WC and pH for Qinghai-Tibet were significantly higher, and Ec lower than those for Inner Mongolia. However, apart from SP, SK and SAP, soil C and N concentrations and other soil available nutrients (AN, NN, AK) for the regions were insignificantly different. These findings suggest that climate imposes important controls on soil nutrients.
Relationships between leaf C,N, P and K and environmental variables
Plant nutrient concentrations and their correlations with soil nutrient conditions are considered effective tools for exploring plant adaptation and resource utilization strategies in a severe environment (Ordoñez et al. 2009; Hong et al. 2015; Geng et al. 2011). Stoichiometric homeostasis (H) is the ability of plants to maintain their element composition relatively stable regardless of changes in nutrient availability via various physiological mechanisms (Sterner & Elser 2002; Persson et al., 2010). The degree of stoichiometric homeostasis can be indicated by the homeostatic coefficient (H), which reflects the ability of plants to maintain a stable nutrient composition regardless of changes in environmental nutrients (Sterner and Elser 2002; Hessen et al. 2004; Giordano 2013; Wang et al. 2019). It is well known that stoichiometric homeostasis had been reported in dominant palatable species (Li et al. 2016; Yu et al. 2015; Peng et al. 2016) in grasslands. However, this has not been established in unpalatable species. Since poisonous plants represent the majority of the plant species detected after grasslands have been degraded, reveal the eco-physiology characteristics of poisonous weeds will help us better understand why poisonous weeds can spread widely on degraded grasslands. We found HC and HN of S. chamaejasme were 0, indicating that S. chamaejasme could not maintain carbon and nitrogen internally. Compared with other grassland species, S. chamaejasme had no HN, which was different from previously reported results for other species (Wu and Wang 2019; Yu et al. 2011). Our data clearly show that the stoichiometric homeostasis coefficient of P (HP) of S. chamaejasme was higher than N (HN), indicating S. chamaejasme was relatively less sensitive responses to soil P than soil N. However, HN was consistently higher than HP at the levels of community, family, and species in China reported by previous studies (Peng et al. 2016; Yu et al. 2015). Like many other terrestrial ecosystems, grassland ecosystems face an ongoing increase of atmospheric nitrogen deposition in recent decades (Menge & Field, 2007). The increase of N availability in the soil leads to ecosystems limited by N have gradually transformed to P-limitation or other elements (Galloway et al. 2008; Lebauer and Treseder 2008). Thus S. chamaejasme with HP could survive outstandingly than species without HP in the future P-limited surroundings. Moreover, although globally N and P are considered of paramount importance to plant function, it is widely known that many other elements are also important in specific contexts or regions (Han et al. 2011). We also found that HK was greater than HP, implying K could be used as another important element that indicates the degree to which an organism maintains homeostasis.
Our results indicate that in the macro scale, leaf C and N do not directly correlate with meteorological factors (MAT and MAP), which is in agreement with previous studies conducted in the grassland biomes of China (He et al. 2006). The weak relationships observed between leaf C, N, and climatic variables may result from plant growth, development, metabolism, phenological and life-history traits rather than from the specific geographic environment. On the contrary, there were close relationships between leaf P and K and climatic factors (Fig. 4). The relationship between leaf P and climate factors was significant only in IM, and the K content of S. chamaejasme leaves was significantly related to climate factors only in QT. We noticed that the correlation of leaf P and MAP (R2=0.5523) was greater than the relationship between P and MAT (R2=0.4886) in IM, and the relationship between K and MAT (R2=0.3338) was greater than that with MAP (R2=0.2920) in QT. These again reflect the different limiting factors of plant growth in different regions (Fan et al. 2016). It is a reasonable assumption that precipitation is a more important limiting factor than the temperature for vegetation growth in arid and semi-arid regions like Inner Mongolian Plateau temperate grasslands. In contrast, the temperature is more likely to have a greater effect on leaf element concentrations than precipitation in Qinghai Tibet Plateau alpine grasslands with high-altitude and low temperature. We also found that only LK was negative related with MAP in QT. In fact, K leaches more easily from leaves than N and P (Sardans and Peñuelas 2015); hence it is easy to ascertain the increasement of MAP in the studies leading more leaf leaching of K in S. chamaejasme.
To explore complex relationships between soil and climatic factors on leaf P and K contents of S. chamaejasme, we conducted a PLS-PM analysis. We found soil exerted a significant effect on leaf P and climate affected leaf P and K both directly and indirectly in QT, while LP appeared to be limited mainly by climatic factors and via direct ways and LK was not affected significantly by any environmental factors in IM. More specifically, climatic factors had an significant influence on soil in IM. Meanwhile, climatic variables had stronger direct effects on LP than the indirect effects in IM. This was contradicting our knowledge that climate factors often affect leaf elements through their influence on soil nutrient status (Luo et al. 2021). However, the arid conditions of the Inner Mongolia Plateau (Arid and Semi-Arid Areas) have no doubt restricted grassland plants growth by insufficient water, and Zhang et al found that drought has become the primary natural disaster threatening to Inner Mongolia grassland ecosystem (Zhang et al. 2009). In addition, climatic factors affected soil state insignificantly and directly or indirectly influenced leaf P or K in QT. Soil factors had significant influence over LP and the influence of climatic variables on LK was significant, indicating the effects of climatic factors and soil factors on LP or LK occurred separately. Our model suggests that underlying mechanisms behind the LP or LK content were different in the two habitats.
In addition, species natural habitats will be subject to more disturbances in the future due to climate change and habitat degradation caused by intensive anthropogenic activities (Wei et al. 2019; Chen et al. 2020). In future works, continuing wide-scale sampling and considering the influence of human activities are required to further develop a deeper understanding of the geographic patterns in S. chamaejasme.