High precipitation rates increase potassium density in plant communities in the Tibetan Plateau

Potassium (K) is a crucial nutrient for plant growth, impacting various physiological processes. Despite its importance, significant global soil K deficiency exists, affecting agricultural productivity. The Tibetan Plateau (TP), with its diverse vegetation and minimal anthropogenic influence, provides a unique setting to study vegetation-environment interactions, particularly concerning K. Previous research has largely focused on nitrogen (N) and phosphorus (P), neglecting K's role. This study addresses this gap by examining K density at the community level – a fundamental ecological unit offering insights beyond species-level studies. The study also explores plant nutrient partitioning strategies, contrasting the optimal partitioning hypothesis (prioritizing K allocation to organs with limited resource access) with the isometric partitioning hypothesis (allocation determined solely by intrinsic plant characteristics). Global warming significantly impacts the TP, leading to vegetation heterogeneity and uncertainty regarding element distribution. Existing hypotheses, like the temperature-biogeochemistry hypothesis and the temperature-plant physiology hypothesis, provide frameworks for understanding element distribution; however, their application to K remains largely unexplored. This research aims to determine K density in TP plant communities, identify key drivers of its spatial distribution, and assess K storage, thereby contributing to understanding plant K utilization strategies and the effects of climate change on nutrient cycling.

The critical role of potassium in plant growth and development has been acknowledged for over a century, with studies emphasizing its importance in various physiological processes such as stomatal regulation, enzyme activation, and protein synthesis. However, substantial global soil potassium deficiency affects a significant portion of agricultural lands, highlighting the conflict between K deficiency and sustainable plant productivity. Research on terrestrial ecosystems has predominantly focused on nitrogen and phosphorus, neglecting the role of potassium. While species-specific, small-scale investigations exist, comprehensive analyses of potassium at larger scales, particularly at the community level, are lacking. The Tibetan Plateau, with its diverse ecosystems and environmental gradients, offers a unique opportunity to investigate plant-environment interactions regarding potassium. Existing hypotheses, including the optimal partitioning and isometric partitioning hypotheses, provide differing frameworks for understanding nutrient allocation strategies. The temperature-biogeochemistry and temperature-plant physiology hypotheses propose competing explanations for the spatial variation in nutrient content, yet their applicability to potassium remains to be investigated.

This study involved consistent measurement of K content and density in plant organs across 2040 natural communities on the Tibetan Plateau, using a systematic gridding approach with a 0.5° × 0.5° grid. Field sampling occurred from July to August 2019-2021, focusing on four major vegetation groups: forests, shrublands, grasslands, and deserts. At each site, at least three plots were established, and dominant plant communities were identified. In forests, three quadrats (tree, shrub, and herb) were used, measuring tree height and diameter at breast height (DBH). Healthy, mature specimens were selected for organ collection (leaves, branches, trunk, and roots). Similar sampling methods were applied to shrublands, grasslands, and deserts. Aboveground parts and roots were weighed, and soil samples (0-10 cm) were collected to determine soil physical and chemical properties (SOC, soil N, soil K, and pH). Laboratory analyses employed ICP-OES to determine K content, an elemental analyzer for SOC and soil N, and a pH meter for soil pH. Environmental variables (temperature, precipitation, radiation, wind speed, water vapor pressure) were obtained from WorldClim and the Science Data Bank. Oxygen partial pressure (PO2) was calculated from altitude using a specified formula. Biomass for trees and shrubs was calculated using a growth equation based on DBH and height. K content (mg g⁻¹) was calculated, and K density (g m⁻²) was determined by multiplying K content by biomass. K storage (Tg) was calculated using a specific formula. Statistical analysis included one-way ANOVA, correlation analysis (R software), structural equation modeling (SEM), and an allometric model to explore K density distribution. Random forest (RF) modeling in MATLAB predicted the spatial distribution of K content and density. The allometric model was applied to the obtained data using the function ‘lmodel2’ in the ‘lmodel2’ package in R to quantify the scaling relationship between K density in different plant organs. This approach allows to test the optimal partitioning hypothesis and the isometric partitioning hypothesis.

The study revealed average K contents of 16.05 mg g⁻¹ (leaves), 3.91 mg g⁻¹ (branches), 1.46 mg g⁻¹ (trunks), and 4.03 mg g⁻¹ (roots). K density varied significantly among organs, with the highest density found in branches (12.84 g m⁻²). Allometric analysis showed an allometric relationship among organs in all vegetation types except deserts, which exhibited an isometric relationship. SEM indicated that precipitation was the primary driver of total vegetation K density, explaining 29% of the variation in leaf K density. Precipitation and temperature positively correlated with leaf K density. Spatial distribution analysis showed a decreasing trend in K density from southeast to northwest across the Tibetan Plateau, with forests having the highest K density (44.01 g m⁻²) and deserts the lowest (3.41 g m⁻²). Total K storage in TP vegetation was estimated at 19.92 Tg.

The findings highlight the significant role of precipitation in driving potassium density in Tibetan Plateau vegetation, challenging the traditional emphasis on soil nutrients as the primary regulators. The contrasting K allocation strategies between desert and other vegetation types emphasize the adaptive responses of plants to environmental constraints. The study's findings provide valuable data for understanding the biogeochemical cycle of potassium and its response to climate change. The high accuracy achieved in predicting spatial K density using the random forest model suggests the potential of this approach for large-scale ecological studies.

This study provides the first comprehensive assessment of potassium content, density, and storage in the Tibetan Plateau's diverse vegetation. The results demonstrate the strong influence of precipitation on K distribution and the adaptive K allocation strategies employed by different plant communities. The application of random forest modeling successfully captured the spatial patterns of K density. Future research should investigate the underlying mechanisms of K uptake and allocation in response to environmental changes and explore the long-term impacts of climate change on K cycling in the TP ecosystem. The datasets generated in this study provide essential input for refining biogeochemical models and improving our understanding of nutrient dynamics in high-altitude ecosystems.

While this study offers a comprehensive overview of potassium distribution in the Tibetan Plateau, certain limitations exist. The study's reliance on environmental data from publicly available databases introduces potential uncertainties related to data quality and spatial resolution. The estimation of total K storage is based on area data extracted from previous research and might not perfectly reflect the actual area of different vegetation types. Further studies could refine these estimates through direct field measurements. The focus on community-level analyses may mask some of the specific variations in K accumulation among individual species or genotypes.