Forest floor-mineral soil interactions in the internal nitrogen cycle of an old-growth forest

Abstract

Abstract. Seasonal patterns and annual rates of N inputs, outputs, and internal cycling were determined for an old-growth mixed-conifer forest floor in the Sierra Nevada Mountains of California. Rates of net N mineralization within the forest floor, and plant N-uptake and leaching of inorganic N from the forest floor were 13, 10, and 9 kg-N ha-' yr-I, respectively. The Mediterranean-type climate appeared to have a significant effect on N cycling within this forest, such that all N-process and flow rates showed distrinct seasonal patterns. We estimated the forest floor supplies less than one-third of the total aboveground plant N-uptake in this forest. The rate of net nitrification within the forest floor was always low ( < 1 kg-NO;-N ha-' 30d-I). Mean residence times for organic matter and N in the forest floor were 13 and 34 years, respectively, suggesting that this forest floor layer is a site of net N immobilization within this ecosystem. We examined the influence of the forest floor on mineral soil N dynamics by injecting small amounts of I5N-enriched (NH4),S04 solutions into the surface mineral soil with the forest floor present (+FF) or removed (-FF). K,SO,-extractable NOT-N, total inorganic-N, and total-N pool sizes in the mineral soil were initially increased after forest floor removal (after 4 months), but NO;-N and total inorganic-N were not significantly different thereafter. Microbial biomass-N and K,SO,-extractable total-N pool sizes were also found to be larger in mineral soils without a forest floor after 1 and 1.3 years, respectively. Total 'SN-recovery was greater in the +FF treatment compared to the -FF treatment after 1-year (about 50% and 35%, respectively) but did not differ after 1.3 years (both about 35%), suggesting that the forest floor delays but does not prevent the N-loss from the surface mineral soil of this forest. We estimated using our I5N data that fungal translocation from the mineral soil to the forest floor may be as large as 9 kg-N ha-' yr-' (similar in magnitude to other N flows in this forest), and may account for all of the observed absolute increase of N in litter during the early stages of decomposition at this site. Our results suggest that the forest floor acts both as a source and sink for N in the mineral soil.

Seasonal and spatial variations in soil nitrogen and phosphorus supply rates in a boreal aspen forest

“…Intact soil cores have been incubated in sealed plastic bags (Nadelhoffer et al 1984), in covered PVC tubes (Adams and Attiwill 1986), or in uncovered thin metal cans (Raison et al 1987) in the field. Ion exchange resin bags have also been placed in soil cores (inside and outside PVC tubes) in the field (Hart and Firestone 1991). The ion exchange resin method using resin both in bead and membrane forms has been used to assess nutrient supply rates in soils (Binkley 1984;Gibson et al 1985;Gibson 1986;Casals et al 1995;Qian and Schoenau 1995).…”

“…All the 1995 field research was carried out at the lower slope position of transect E. Nutrient supply rate with and without plant root competition was estimated in 1995 using IEM probes inserted inside and outside of soil cores bounded by PVC cylinders (Nadelhoffer et al 1984(Nadelhoffer et al , 1985Hart and Firestone 1991). Sampling was carried out at the study area, in four randomly selected replicates.…”

“…It was assumed that installation of cylinders did not significantly alter the rate of nutrient release and supply to the resin membranes from the soil inside the cylinders (Hart and Firestone 1991). The impact of root uptake in reducing nutrient supply to the resin membrane was calculated as ion adsorbed by the resin inside the cylinders (roots excluded) minus the ion adsorbed by resin placed outside the cylinders where roots compete with the resin for nutrients.…”

Effects of the clear-cutting of a Douglas-fir plantation (Pseudotsuga menziesii F.) on the chemical composition of soil solutions and on the leaching of DOC and ions in drainage waters

“…Situations with noticeable losses [2,4,6,7,12,13,20,27,28,44,54] or more limited losses of nutrients [16,43,57,58] have been reported. The case of the large losses observed the after clear-cutting of the Hubbard Brook experimental forest represents a very specific situation whose results cannot be directly generalized.…”

On the Nature of Environmental Gradients: Temporal and Spatial Variability of Soils and Vegetation in the New Jersey Pinelands

“…Similarly, although seasonal patterns in N availability in both horizons are well documented (e.g. Fahey et al 1985;Poovarodom et al 1988;Hart & Firestone 1991), it is not known whether the two horizons vary in parallel through time. For a plant whose roots penetrate a series of soil horizons, the degree of concordance in temporal and spatial patterning between these horizons could be expected to affect the plant's response to the environment.…”