Forest floor and mineral soil development in Casuarina equisetifolia plantations on the coastal sand dunes of Senegal

Abstract

Forest floor and mineral soil samples were collected from the interdune depressions of 6-, 8-, 17-, and 34-year old Casuarina equisetifolia Forst. & Forst. plantations growing on the coastal sand dunes of Senegal. The accumulation of organic matter as well as all mineral elements in the forest floor except sodium followed a linear pattern. The thickness of the litter layer increased from 4.2 to 8.0 cm. Forest floor biomass increased up to 120.3 Mg ha −1; an average increase of 3.3 Mg ha −1 year −1. Nitrogen in the forest floor accumulated up to 1567 kg ha −1; an average increase of 45 kg ha −1 year −1. Calcium, aluminum, magnesium, iron, potassium and phosphorus in the forest floor accumulated at 6.8, 3.7, 1.8, 1.3, 0.49 and 0.25 kg ha −1 year −1, respectively. In the mineral soil, total phosphorus and exchangeable calcium decreased with plantation age. Nitrogen in the mineral soil increased by 30 kg ha −1 year −1 while organic matter in the mineral soil increased by 61.5 kg ha −1 year −1. The pH of the mineral soil decreased from 7.3 to 6.5. As a result of a particular combination of species characteristics and ecological conditions, C. equisetifolia plantations on these coastal sand dunes appear to accumulate forest floor organic matter at rates comparable to those found in boreal and subalpine forests. A nitrogen to phosphorus ratio in the litter layer of 164:1 indicates that decomposition processes are strongly limited by phosphorus. This suggests that introducing strains of nitrogen-fixing bacteria capable of fixing even greater amounts of nitrogen would do little to improve soil fertility. Since there was no sign that accumulation of either organic matter or mineral nutrients had reached steady-state, a minimum of a 34 year rotation would seem justified if soil improvement is the primary management objective. Since the overwhelming majority of the organic matter and nutrient capital is located in the forest floor and upper 2 cm of mineral soil, proper management of this fragile layer following harvest will be extremely important if the benefits realized in the first rotation are to be capitalized on during subsequent rotations.