Trophic cascades mediated by copepods, not nutrient supply rate, determine the development of picocyanobacteria

Soil nitrogen (N) and phosphorus (P) supply is one of the growth limiting factors in many forest ecosystems. Seasonal patterns in soil N and P supply rate were examined during a 2-yr period (1994–1995) for forest floor (L, F and H) and upper mineral (Ae) horizons in an 80-yr-old aspen forest in Saskatchewan, Canada. Accumulation of plant nutrient ions on ion exchange resins incubated in the field can provide an estimate of nutrient supply rate in soils because ion exchange resins have the potential ability to simulate nutrient flux to plant roots. Nutrient supply rates and the effect of plant uptake on nutrient supply rate was assessed using ion exchange membranes buried inside and outside polyvinyl chloride (PVC) cylinders. The difference between ion flux to the membranes inside (root uptake excluded) versus outside the cylinders was used as an index of plant nutrient uptake. From May to October, nutrient uptake (µg 10 cm−2 2 wk−2) by plants ranged from 1.6 to 31.7 (NO3−-N), from 2.7 to 13.7 (NH4+-N) and from 2.6 to 12.7 (P), with maximum N and P uptake in summer. Nutrient uptake by plants also varied among horizons. In general, plant uptake of NO3−-N, NH4+-N and P was highest in the H horizon, followed by the F and Ae horizons, with lowest uptake apparent in the L horizon. The results are consistent with the distribution of plant fine roots: most were found in the H horizon (68%), followed by the Ae and F horizons (15%), and the L (2%) horizon. Autumn litterfall represented a nutrient return of 28–40 kg N ha–1 and 4–7 kg P ha–1 to the forest floor which coincided with an increase in ion supply rates in the forest floor. During the growing season, atmospheric inputs via bulk deposition and throughfall contributed small amounts of N (1.8 kg NH4+-N ha–1 and 0.23 kg NO3–-N ha–1) and P (1.38 kg ha−1 inorganic P) to the forest floor. Recycling of nutrients by litterfall and subsequent mineralization and re-assimilation by plant roots in the forest floor is a dynamic and important component of nutrient cycling in boreal aspen forest ecosystems. Key words: Forest floor, ion exchange membranes, nutrient supply

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Availability of nitrogen and phosphorus in the forest floors of Rocky Mountain coniferous forests

Canadian Journal of Forest Research ◽

10.1139/x92-079 ◽

1992 ◽

Vol 22 (4) ◽

pp. 593-600 ◽

Cited By ~ 23

Author(s):

Cindy E. Prescott ◽

John P. Corbin ◽

Dennis Parkinson

Keyword(s):

Forest Floor ◽

Lodgepole Pine ◽

Ammonium Phosphate ◽

Nutrient Supply ◽

Nutrient Concentrations ◽

Supply Rate ◽

Nitrogen And Phosphorus ◽

Forest Floors ◽

P Supply ◽

Floor Material

Nutrient supply rate and limitation were measured in forest floors of lodgepole pine, white spruce–lodgepole pine, and Engelmann spruce–subalpine fir (pine, spruce, and fir forests, respectively) forests in the Kananaskis Valley of southwestern Alberta. Earlier analyses of the nutrient content of foliage and litter indicated low N and P supply in the pine forest, high P supply in the spruce forest, and high N–low P supply in the fir forest. Measurements of nutrient supply (insitu rates of net mineralization, extractable P, and uptake of N and P from the forest floor in pot trials) confirmed the differences in N and P supply among the forests and indicated that nutrient concentrations in needle litter were useful as an index of nutrient supply rate. Subtractive tests were useful in identifying the most limiting nutrients in each forest: lodgepole pine seedlings grown in forest floor material from the pine and spruce stands responded with increased growth to the addition of N; those in fir forest floor material responded to P addition. Vector analysis of N and P concentrations and contents in needles from trees fertilized with ammonium phosphate sulphate showed responses to both N and P in the pine site, no response at the spruce site, and response to P at the fir site.

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