Douglas-fir reforestation in North Apennine (Italy): Performance on soil carbon sequestration, nutrients stock and microbial activity

The aim of the study was to determine the effect of mature Douglas fir reforestation (DR) on nutrients and C stocks in mineral soils compared with beech forests (BF) at two altitudes in North Apennine, Italy, at around (1) 1028 and (2) 1281m a.s.l. To explore the effects of Douglas fir reforestation with respect to natural beech forest, the microbial biomass and its activity were also taken in account as a driving force of organic C mineralization. For each fixed-depth mineral layer (0–5, 5–10 and 10–30cm) the bulk density values were determined, also estimated using common pedotransfer functions (PTF). Total organic C (TOC) stock in the 0–30cm layer was determined using both the measured and the PTF-estimated bulk density. The nutrients stocks (C pools, N, P, S, Ca) as well as potential microbial respiration and enzyme activities per centimeter were also calculated for the whole depth. TOC and C pools were positively affected by Douglas fir at 1028m a.s.l. with respect to the other altitude and trees cover. In DR1 with respect to DR2, BF1 and BF2, double contents of TOC (344±13 vs. 157±1, 122±6, 162±1gm−2cm−1) and residual C pool (220±8 vs. 65±1, 63±3, 116±1gm−2cm−1) were observed. Conversely, P stocks were significantly higher under beech at 1300m a.s.l. because highest P litter content were found in BF2. Moreover, the activity of enzyme involved in C cycle (SEIc) suggest that to establish the effects on soil organic matter biochemical degradation an interaction between plant cover and altitudes may occur. The fast biochemical degradation of organic matter produced by beech forest and Douglas fir plantation occurred at 1281 and 1028m a.s.l., respectively (BF1 vs. BF2=1.40 vs. 4.35 DF1 vs. DF2=2.54 vs. 2.14mmol MUF m−2h−1cm−1). In conclusion, in the North Apennine (Italy) Douglas fir old reforestation with respect to natural beech forest caused substantial improvement on soil C sequestration, N stock and microbial activity at lower altitudes (1028m vs. 1281m a.s.l.). Moreover, to assess soil nutrient stocks, the applicability of PTFs might be improved by the use of soil depth in BD estimation.