Organic Matter and Nutrient Dynamics in Forest Floors of Young and Mature Abies amabilis Stands in Western Washington, as Affected by Fine‐Root Input

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Abstract
Seasonal patterns of decomposition and nutrient release from the major litterfall components were determined using litterbags in young (23—yr—old) and mature (180—yr—old) Abies amabilis stands in western Washington, USA. The time required for each litterfall component to decompose completely was estimated in both stands. Long—term organic matter and nutrient residence times in the forest floor were estimated using forest floor and aboveground litterfall masses. In addition, the effect of root turnover in the forest floor on estimates of organic matter and nutrient residence time was determined. Similar litter substrates lost mass at a significantly faster rate in the young than in the mature stand. An initial rapid mass loss (9—67%) for litter substrates during the winter months coincided with increased immobilization of nitrogen and calcium and mineralization of phosphorus, potassium, and magnesium. However, herb species and epiphytic lichen decomposition in both stands showed no immobilization of N, P, K, Ca, or Mg or increased mass in litterbags at all sampling dates. The remaining litter substrates showed no loss or gain in mass during summer and autumn, with continued immobilization of N and P, while K, Ca, and Mg levels remained the same or decreased. From 36 to 77% of the total 2—yr mass loss of all litter substrates occurred during the initial 4 mo. The various litter components required °6—15 and 2—24 yr for 99% decomposition to occur in the young and mature stands, respectively. The addition of roots into the estimation of organic matter residence time (MRT) in the forest floor reduced the estimate by °75% in both stands. The predicted turnover time of 1 yr's litterfall (11—12 yr) was very similar to the estimated residence time of total forest floor when roots were included in the calculations. The inclusion of roots in the estimation of forest floor MRT decreased the nutrient residence time for N by 74%, for P by 75%—86%, for K by 81—90%, for Ca by 52—55%, and for Mg by 66—85% in both stands. Addition of root nutrient input into estimates of forest floor nutrient MRT increased the annual turnover of forest floor nutrients from 1—2% to 4—16% in the young stand and from 1—4% to 4—8% in the mature stand. The element mobility series resulting from litterbag studies did not give the same pattern of mobility as obtained from the total forest floor with or without root input.