Plant uptake of cations under nutrient limitation: An environmental tracer study using Ca/Sr and K/Rb ratios
Abstract
Vascular plant growth builds soils and ecosystem nutrient capital by sequestering and partitioning atmospheric CO2 into organic matter and continental runoff and driving terrestrial water and energy balances. Plant root-system functions, e.g. nutrient mobilization and uptake, are altered by environmental stress. However, the stress-response relationships are poorly understood. Chemical tracers have potential for assessing contributions of nutrients from various nutrient pools. Our objective is to quantitatively study how varying degrees of nutrient limitation (and corresponding needs to extract base cations from mineral sources) influence Ca and K uptake functions in a plant-root-mineral system. We are studying plant-driven mineral weathering in column experiments with red pine (Pinus resinosa) seedlings. The columns contain quartz sand amended with anorthite and biotite that constitute the sole mineral sources of Ca and K. These minerals also contain known amounts of Sr and Rb, which exhibit chemical behavior similar to Ca and K, respectively. The solution source of Ca and K was varied by adding 0% (no dissolved Ca and K), 10%, 30%, or 100% of a full strength Ca and K nutrient solution through irrigation water in which both Sr and Rb concentrations were negligible. Selected columns were destructively sampled at 3, 6 and 9 months to harvest biomass and measure plant uptake of cations. We used Ca/Sr and K/Rb ratio results to estimate the contributions of Ca and K from mineral and solution sources. For the 0% nutrient treatment, the Ca/Sr and K/Rb ratios in total biomass at 3 months, compared with those in the mineral phases, suggested preferential uptake of Ca and K over Sr and Rb, respectively, and allowed us to determine uptake discrimination factors for both cations. The K/Rb ratios in total biomass increased with greater K availability in the solution source, as expected, but Ca/Sr ratios did not show any dependence on Ca availability in the solution source. Using a tracer mass balance model, we computed the fraction of K from the mineral source in the total biomass, which decreased with longer growth time and more K supply from the solution source. For Ca, the model results suggested that the mineral was the major source of Ca in the plants throughout the experiment. Comparison of results for above- and below-ground biomass showed significantly higher Ca/Sr and K/Rb ratios above ground, indicating strong fractionation of Ca and K during transport within the tree. Our results highlight the potential importance of species-specific and temporal variations in cation uptake that may require consideration as we work to understand water and nutrient cycles in aggrading forest ecosystems.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H53C1533S
- Keywords:
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- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 1852 HYDROLOGY / Plant uptake;
- 1886 HYDROLOGY / Weathering