Nitrogen uptake by Carex aquatilis during spring thaw in a sub-Arctic wet sedge meadow

Background/Question/Methods

   Plant productivity in Arctic ecosystems is typically limited by the availability of nitrogen and sometimes phosphorus, however the seasonal timing of nutrient availability and plant nutrient uptake is not well understood for these systems.  Soil nutrient properties and microbial biomass levels have been monitored over several years at sub-Arctic wet sedge-meadow sites near Churchill, Manitoba, and both nutrient and microbial biomass levels have been found to peak annually at the end of winter. These levels then decline rapidly and concurrently during the early part of spring thaw. It is not known when the dominant graminoid in this system (Carex aquatilis) begins to take up nutrients in spring and how the early growth phenology of this plant relates to below-ground nutrient dynamics. A study was conducted to determine whether Carex aquatilis has the capacity to take up available nitrogen early in spring when levels are still moderately high but these inorganic resources are rapidly dwindling as soils thaw. 15NH4Cl was used as a tracer and was injected in soil cores containing living C. aquatilis shoots that were sampled at three times during the nutrient decline phase between winter and spring. On each occasion soils were incubated in the field for 8-day intervals, following which plant tissues (roots, shoots, rhizomes, mosses) were sorted and soils were processed to obtain microbial and soil 15N pools.
Results/Conclusions   Analyses of plant tissues revealed that plants took up a large proportion of the injected 15N (approximately 50%) into their roots during all three incubation periods, but it was not until the third interval (when soils were completely thawed) that nitrogen was reallocated from roots to shoots.  This study demonstrates that C. aquatilis is able to capitalize on sources of nitrogen that become available early in spring, despite the partially-frozen condition of the soils. This has implications for plant growth phenology under changing climatic conditions, particularly as winters become warmer. For example, as soils thaw earlier in the year, nutrients may become available when high-latitude systems are still experiencing winter-time darkness, potentially resulting in a mis-match between nutrient and light availablility for these plants.