Soil profile at the control site (Panel a) and the charcoal hearth (Panel b). The letters indicate different pedologic horizons. In the charcoal hearth the dark anthropogenic layer (Acoal; 0–10 cm) can be easily identified. doi:10.1371/journal.pone.0091114.g001 

Contexts in source publication

Context 1

... data in the text and in the tables are reported as mean 6 standard error (n = 3) if not differently indicated. Gaussian error propagation technique (GEP) was used in error analysis to analytically determine the error or uncertainty produced by multiple and interacting measurements or variables. For this, the uncertainty associated to each measurement was calculated as standard error (se) of the mean (se = standard deviation/root square of the number of samples) and the classical error propagation theory and equations were used [41]. Error sensitivity analysis was also made by constructing an error budget [42], thus enabling further understanding of the error structure, i.e. the relative contribution of the errors associated with each parameter to the overall error estimate. Such sensitivity indices provide in this way a measure of the percentage rate of change in an output variable produced per unit percentage change in its input variable, an information that may be used critically to identify where error reduction of estimates may lead to lower uncertainty. The anthropogenic layer of the charcoal hearth soils has a lower bulk density than control soils (0.60 6 0.08 vs. 0.87 6 0.12 Mg m 2 3 ; p , 0.01). Soil bulk density is an important indicator of physical soil quality, being linked to air capacity, resistance to root growth and capacity of storing and transmitting water [43]. Such a decrease in bulk density is associated with a 97.3% 6 1.6 decrease in hydrophobicity. This is in line with the water infiltration data from charcoal production sites in Ghana [44], but in contrast with short term observations following biochar applications to soil [45] [46] that showed small but consistent increases in soil hydrophobicity which is largely controlled by the surface chemistry of fresh biochar particles [47]. We speculate that a prolonged residence time of charcoal caused substantial leaching or degradation of hydrophobic compounds [48], a shift in soil texture ([49], [44]), and a microbially-driven creation of functional groups [50]. Decreased hydrophobicity is known to increase water availability for plants and is also important for nutrient cycling, as it favors water infiltration into the soil and reduces runoff, thus preventing lateral nutients losses. Nutrient content (total and plant available fractions) is higher in the hearths than in the control soils (Table 2; Table 3). In particular, the total P-stock is 107% larger in hearths than in the control (95 6 6 vs. 46 6 3 g m 2 2 , p = 0.003), while the plant available P is 24% higher (1.2 6 0.04 g m 2 2 vs. 0.9 6 0.3 g m 2 2 ). The higher P content is not surprising as charcoal contains at least 20% of P originally contained in the wood (Table 2). If we assume that charcoal made in the middle of the XIX century had a P content comparable to that of modern charcoal (3.0 6 0.05 g P kg 2 1 ; Table 3), we may estimate that carbonization events led to the addition of 117 6 21 g P m 2 2 (Table 2). In the absence of grass mowing, a virtually closed P-cycle can be hypothesized for these soils as P-leaching does not usually occur if the overall concentrations are low so that P can be considered ‘‘virtually immobile’’ [51] [52]. Nevertheless, large herbivores are known to be net P-exporters in alpine grasslands as they may preferentially graze in P-enriched areas and then release P as dung elsewhere [53]. This export largely depends on the grazing pressure, but it is unlikely to exceed the maximum value of 0.07 g P m 2 2 y 2 1 that was assessed experimentally in the Swiss Alps [53]. When scaled to the time that charcoal was added to the soil the amount of P exported would not have exceeded 11.5 g m . Atmospheric deposition may have also contributed to the P balance as a result of long-range desert dust transport and as a consequence of atmospheric pollution, including biomass combustion [54]. Although the latter is known to be variable in time and space, this input is not large in the Eastern Alps, with less than 0.01 g P m 2 2 y 2 1 [55]. When measured today, the total amount of P contained in the anthropogenic layer of the hearth’s soil is only 19% less than what was initially added during the carbonization events (95 6 6 vs. 117 6 21 g m 2 2 ; Table 2). This highlights the fact that charcoal production was indeed a long-lasting source of P in an otherwise P-limited environment. P-fertilization persisted on a centennial time-scale and it is also interesting to observe that both organic and inorganic (extractable) fractions of P that were added to the soil are now mostly contained in the non-pyrogenic fraction of the SOM, as the P contained in old charcoal fragments is only 12% of that of modern laboratory-produced larch charcoal (Table 3). Other nutrients, such as potassium (K) and calcium (Ca) are also more abundant in the charcoal hearth soils even though there are no significant differences with control soils due to the large spatial variability (Table 2). Furthermore K and Ca are 155% and 61% higher than what was added to the soil with charcoal (Table 2). This excess may be attributed to an higher retention of atmospheric K and Ca depositions which have been previously reported to be relevant in the Alpine region [56] and can be related to the higher cation exchange capacity (CEC) of charcoal [57]. A strong correlation between current atmospheric deposition rates and element excesses found in the hearth soil seems to confirm such hypotheses (Figure 2). Total N content is not significantly different between hearth and control soils (p = 0.42; Table 2) and no significant difference was 2 + found in the concentrations of mineral N (NO 3 : p = 0.31; NH 4 : p = 0.92; Table 3). Total C content of the anthropogenic layer at the charcoal hearths is three times higher than that of the adjacent control soils (p = 0.03; Table 1). The amount of C that is now contained in the anthropogenic soil layer (C TOT , kg C m 2 2 ) is the sum of pyrogenic (C CHAR , kg C m 2 2 ) and non-pyrogenic components (C SOM , kg C m 2 2 ) as carbonates are absent due to the low pH (4.2 6 0.3 and 4.6 6 0.3 in charcoal hearths and control areas, respectively). Charcoal fragments larger than 2 mm represent approximately 4.1 6 1.7% (by weight) of the entire mass of the anthropogenic layer within the deeper soil horizons where no charcoal debris can be identified (Figure 1). Any meaningful evaluation of net C sequestration achieved in charcoal hearths firstly requires a precise separation of C fractions contained in the charcoal (C CHAR ) and in the rest of the soil, followed by a accurate estimation of the C input at the time of the carbonization event. Assessing the exact ratio between C CHAR and C TOT using sable C isotopes is problematic ...

Context 2

... study site is located in Val di Pejo (Trentino, Northern Italy; 46 u 20 9 16.18 0 N, 10 u 36 9 07.02 0 E) at an elevation ranging from 2120 to 2170 m a.s.l. The mean annual temperature at the site is 3.5 u C and the mean annual precipitation is 903 mm [22]. The lowest precipitations are registered in January, while the highest are distributed between April and November. Starting from the 16 th century the area was subject to intensive wood resource exploitation for larch charcoal production which was subsequently used in the local iron industry. Production ceased in 1858 when a severe fire event destroyed the major iron foundry in the valley [13] [23]. Charcoal production was based on large forest clear-cuts, wood chopping and downhill transportation to artificial flat terraces (charcoal hearths) with an elliptical shape. Some of these hearths are still identifiable today as terraces where wood piles were prepared and subsequently covered by soil and tree branches [24]. The relatively large size and elliptical shape of the hearths suggest that more than one pile of wood was carbonized at a particular time as it was already well known that only circular piles could ensure a uniform and high quality charcoal production [25]. Wood carbonization required between four and ten days according to the dimension of the wood pile. After a two-day cooling down period, the charcoal was finally transported to the foundry. Three hearths and three adjacent control areas with southerly aspects and unaffected by significant geo-morphological dynamics (erosion or sedimentation) or recent anthropogenic disturbances were selected. These are flat (2% slope) and have an average surface area ( s ) of 94 m 2 . Soils within the control areas are shallow to moderately deep (35–70 cm), sandy-loam brown acid soils and with restricted areas of podzols (Lithic Dystrudept and Entic Haplorthod according to USDA, 2010 [26]) with an approximate 25% slope. Soils in the charcoal hearths show a truncated profile, with a shallow surface organic horizon approximately 2 cm deep covering a thicker (19.3 6 2.8 cm) black anthropogenic layer. This horizon contains a large amount of charcoal fragments and fine particles left after carbonization which have been subsequently incorporated and well mixed with the pre-existing soil in response to bioturbation [27] and freeze-thaw processes [28] (Figure 1). Both control soils and charcoal hearths are, nowadays, covered by the same herbaceous vegetation dominated by Nardus stricta L . while the surrounding forest is dominated by Larix decidua L. and Picea abies L. The exact date of charcoal production was assessed using a dendro-anthracological approach. This method relies on cross- dating tree ring widths in charcoal fragments with known tree chronologies and has been used previously by [29], who showed that the oldest and youngest tree rings identified in charcoal fragments at our study area were dated 1530 and 1858 respectively. This last date corresponds to the year in which a wildfire event down in the valley caused the destruction of industrial plants thus determining the interruption of the local iron industry and charcoal production in the area [13]. The anthropogenic layer within the charcoal hearths was sampled using a manual soil corer at five different sampling points in each hearth. Similarly, the soil at approximately the same depth was sampled at five points in each control area. Soil samples were dried for 72 hours at 35 u C and sieved to 2 mm. In the case of the charcoal hearths, the fraction of soil . 2 mm was further separated into two subsamples, one including charcoal fragments and one including plant debris, roots and stones. All further analysis was completed on the five sampling points separately. Soil pH was measured in a soil/water solution (1:2.5 ratio). Soil C and N contents were determined by dry combustion using a CHN elemental analyzer ( ß Perkin Elmer 2400 series II CHNS/ O elemental analyzer). Total Ca, K, Mg, Na, P, and available Ca, K, Mg, P concentrations were determined for subsamples oven- dried at 105 u C for 24 h according to the EPA method 3052 [30] and the filtered solutions were analyzed using an ICP-OES spectrophotometer (Varian Inc., Vista MPX). A further set of subsamples was used to assess NO 3– N according to the method + proposed by [31] and NH 4 -N according to the method proposed by [32]. Hydrophobicity was measured following the method of [33]. Such term defines the affinity for soils to water controlling infiltration or wetting. It can be caused by coating of long-chained hydrophobic organic molecules on individual soil particles in response to the decay of organic matter but also to the diversity of soil micro-organisms. Increased hydrophobicity is normally observed after wildfires that leaves charcoal fragments at the soil surface. C and N content, and total and available Ca, K, Mg, Na, P concentrations of . 2 mm charcoal individual fragments were determined using the same methodology described above. Micrographs of those charcoal fragments were made using a Scanning Electron Microscope, XL 20 FEI SEM, with CRYO- GATAN ALTO 2100 technology on samples dried under vacuum, following standard procedures [34]. To enable a comparison between old and fresh charcoal, fragments of larch wood were carbonized in a muffle furnace at 400 u , 500 u , 600 u , 860 u C. The time needed to complete the carbonization corresponds to the time needed for the sample to stabilize its weight loss. C and N contents were determined on samples using the methodology described above. The relative contribution of charcoal-C (C CHAR : C TOT ) to total soil carbon (C TOT ) was estimated using a mass balance method [35] based on the d 13 C values of charcoal fragments excavated from the anthropogenic soil layer ( d 13 C CHAR ), the mean d 13 C of the entire layer ( d 13 C TOT ) and the d 13 C of the SOM contained in the adjacent control soils ( d 13 C ) (Table 1): Stable C isotope ratio ( d C) measurements were made on the fine fraction ( , 2 mm) of representative subsamples of control and hearth soils using an Isotope Ratio Mass Spectrometer ( ß Thermo Fischer Scientific, Delta V Plus) following total combustion in an elemental analyser ( ß EA Flash 1112 ThermoFinnigan). The d 13 C signature of respired CO 2 from incubated charcoal hearth and control soils was measured using the Picarro G2131- i d 13 C High-precision Isotopic CO 2 Cavity Ring Down Spectrometer (CRDS) and Keeling plot method [36]. Representative subsamples ( , 250 cm 3 ; n = 3) were incubated in Erlenmeyer flasks at 40 u C for 15 minutes. Air was continuously circulated from the flask to a pump and then back into the flask at a rate of 0.8– 1.0 l min 2 1 . The CRDS was connected to the pump inlet tube and the air sub-sampled at 0.015 l min 2 1 for measurements of CO 2 concentration and d 13 C. Sampling frequency was 0.5 Hz. To determine the d 13 C signature of the respired CO 2 , the Keeling method was applied [36]. The intercept of the linear regression with the y-axis represents the isotopic signature of the source of the flux. Regression coefficients were calculated on averaged data at each 50 ppm interval of CO 2 concentration, starting from 450 to 800 ppm to establish a steady mixing within the flask. Finally, mean and standard deviation values of d 13 C were computed for both charcoal hearth and control soils (n = 3). We assumed that any difference in the d 13 C of SOM in control and charcoal hearths would also be reflected by a difference in the d 13 C of the respired CO . Ancillary information that is required to estimate the net C sequestration in hearths’ soils was gathered from different ...