Vertical distribution patterns of community biomass, carbon and nitrogen content in grasslands on the eastern Qinghai–Tibet Plateau

The accumulation, transport and distribution of photosynthates within plants not only affect the ratio of shoot-to-root biomass, but also change the distribution pattern of nutrient elements in different organs and tissues. A full understanding of the allocation patterns of biomass and nutrient elements within plants, is helpful to strengthen our understanding of the above- and belowground ecosystem functions. However, at present, we do not know enough about the aboveground–belowground allocation of biomass, carbon (C) and nitrogen (N) contents, and, in particular, the corresponding vertical distribution pattern belowground and its environmental impact. At the community scale, the changes in vegetation community attributes measured in the field can reflect the long-term adaptation and subsequent adaptation strategies of vegetation to the regional environment and environmental changes. In this study, the vertical distribution patterns of community biomass and the carbon and nitrogen contents with soil depth in an alpine grassland on the eastern Qinghai–Tibet Plateau were investigated based on the transect method. Our results show the following: (1) Alpine grasslands can respond to differences in hydrothermal resource availability by changing biomass allocation patterns. In low-temperature and high-humidity environments, aboveground and belowground biomass allocation supports the isometric growth hypothesis. However, in environments with less soil moisture or high temperature and humidity, the aboveground and belowground biomass distributions support the allometric allocation hypothesis. (2) The vertical distribution patterns of the C content, N content and carbon: nitrogen ratio (C:N) within plants are different. From the leaves to the root system, with the increase in root depth, the C content demonstrated a vertical distribution pattern of first decreasing and then increasing, and the N% demonstrated a gradual decreasing distribution pattern, while the C:N ratio demonstrated an exponential increasing distribution pattern. (3) The change rate in C%, N% and C:N related to the soil depth was affected by environmental hydrothermal conditions. C% and C:N ratio changed more rapidly with root depth under low temperature and high humidity. The change rate of N% was mainly affected by temperature and changed more rapidly with root depth at low temperatures. (4) When exploring the relationship between the content of elements within plants, it is necessary to combine both the above- and belowground compartments. There was a nonlinear quadratic relationship between the carbon and nitrogen contents within plants