There are two important allocation hypotheses in plant biomass allocation: allometric and isometric. We tested these two hypotheses in an alpine steppe using plant biomass allocation under nitrogen (N) addition and precipitation (Precip) changes at a community level. An in situ field manipulation experiment was conducted to examine the two hypotheses and the responses of the biomass to N addition (10 g N m−2 y−1) and altered Precip (±50% precipitation) in an alpine steppe on the Qinghai–Tibetan Plateau from 2013 to 2016. We found that the plant community biomass differed in its response to N addition and reduced Precip such that N addition significantly increased aboveground biomass (AGB), while reduced Precip significantly decreased AGB from 2014 to 2016. Moreover, reduced Precip enhanced deep soil belowground biomass (BGB). In the natural alpine steppe, the allocation between AGB and BGB was consistent with the isometric hypotheses. In contrast, N addition or altered Precip enhanced biomass allocation to aboveground, thus leading to allometric growth. More importantly, reduced Precip enhanced biomass allocation into deep soil. Our study provides insight into the responses of alpine steppes to global climate change by linking AGB and BGB allocation.
Soil respiration (Rs) is an important source of atmospheric CO2 flux and is sensitive to changes in soil nutrient and water contents. Despite extensive studies on the effects of enhanced atmospheric nitrogen (N) deposition and changes in precipitation (P) on Rs, few studies have taken into account the effects of interactions between these factors on Rs of alpine grasslands. To address these questions, we investigated the effects of N addition (10 g N m−2 yr−1), changes in precipitation (±50% precipitation), and their interaction on soil respiration and its components, including heterotrophic respiration (Rh) and autotrophic respiration (Ra),in a Tibetan alpine steppe during three consecutive growing seasons. We found that Rs differed in its response to N addition and precipitation regimes. Specifically, decreased precipitation led to a significant reduction in Rs during the last two years, whereas N addition minimally impacted Rs. Another important finding was that soil respiration components differed in their response to N addition and precipitation regimes. Nitrogen addition significantly enhanced Ra, whereas Rh was not altered in response to N addition. By contrast, the precipitation regime led to marked changes in Rh, but exhibited marginally significant effects on Ra. Therefore, our findings highlighted that soil respiration differed in its response to N addition and precipitation regimes mainly due to the different responses of soil respiration components to these factors. Therefore, carbon dynamics should take soil respiration components into account under global change scenarios.
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