Variation of soil organic carbon, stable isotopes and soil quality indicators across an eroding-deposition catena in an historical Spanish olive orchard

This study compares the distribution of bulk soil organic carbon (SOC also reported as Corg), its fractions (unprotected, physical, chemical and biochemically protected), available P (Pavail), organic nitrogen (Norg) and stable isotopes 15 (δ N and δ C) signatures at four soil depths (0–10, 10–20, 20–30, 30–40 cm) between a nearby forested reference area and an historical olive orchard (established in 1856) located in Southern Spain. In addition, these soil properties, as well as water stable aggregates (Wsagg) were contrasted at eroding and deposition areas within the olive orchard, previously determined using Cs. Results highlight a significant depletion of SOC stock in the olive orchard as compared to the forested area, approximately 120 vs. 55 t C ha at the top 40 cm of soil respectively, being severe in the case of unprotected carbon 20 fraction. Erosion and deposition within the old olive orchard created large differences in soil properties along a catena, resulting in higher Corg, Pavail and Norg contents and δ N at the deposition area and therefore defining two areas with a different soil quality status (degraded vs. non-degraded). Differences in δ N at such different catena locations suggest that this isotopic signature has the potential for being used as an indicator of soil degradation magnitude, although additional studies would be required to confirm this finding. These overall results indicate that proper understanding of Corg content and 25 soil quality in olive orchards require the consideration of the spatial variability induced by erosion/deposition processes for a convenient appraisal at farm scale.

3 representative area of the slope, which is a common assumption in many soil quality studies (e.g. Andrews and Carroll, 2001). Although there is a limited number of experiments on the spatial variability of soil properties in olive orchards, they suggest the existence of significant in-field variability (e.g. Gargouri et al., 2013;Huang et al. 2017). Moreover, Gómez et al. (2012) suggested that part of this on-site variability of soil properties, regarding organic carbon, might be related to erosion/deposition processes. 70 In-field variability associated with erosion/deposition processes is relatively well documented for organic carbon content in field crops (e.g., De Gryze et al. 2008, Mabit and Bernard, 1998Van Oost et al., 2005).While the human-induced acceleration of soil erosion has depleted the SOC stock of agroecosystems, the fate of SOC transported across the landscape and that deposited in depressional sites are not fully understood, despite the fact that these transfers might explain a high proportion of the on-site variability of soil properties. 75 Most of the erosion rates recorded or established in olive orchards come from runoff plots or small catchment experiments (e.g. Gómez et al., 2014). The use of the 137 Cs approach has demonstrated its potential in establishing long-term soil erosion rates in this specific land use. An example of these studies is that of Mabit et al. (2012) in which erosion as well as deposition rates since the 1950´s were determined in one ancient olive orchard in the municipality of Montefrío, showing an average annual rate in the eroding part of the slope of 12.3 t ha -1 yr -1 , and an average deposition rate in the lower section of 80 the hillslope, much shorter than the eroding section, of 13.1 t ha -1 yr -1 . This study involved a reference area for establishing precisely the initial 137 Cs inventory, a natural undisturbed area located at 200 m from the orchard. As reported by Mabit et al. (2012), based on 13 investigated soil profiles, the local reference 137 Cs inventory in this undisturbed area was evaluated at 1925 ± 250 Bq m -2 (mean ± 2 standard error) with a CV of 23%.
To complement and/or to circumvent some limitation associated with the use of this anthropogenic radioisotope (see Mabit 85 et al., 2008) and to maintain the capacity to determinate erosion and deposition rates without the need to use direct measurements, other natural radioisotopes such as 210 Pb (e.g. Mabit et al., 2014;Matisoff et al., 2014) or stable isotopes such as δ 15 N or δ 13 C (e.g. Meusburger et al., 2013) have been proposed.
In this study, we hypothesized that the contribution of the long-term erosion-deposition processes on the in-field variability of soil properties in olive orchards (or other woody crops) under medium-high slope is relevant and should be taken into 90 account when analysing the effects of specific strategies on SOC sequestration or on other soil properties. In addition, we exploited the advantage provided by the unique location of an ancient olive orchard near an undisturbed reference area and the previous information on this site from studies on historical erosion rates (Vanwalleghem et al., 2011;Mabit et al. 2012), to fulfil the following objectives: 1-To quantify the long-term variability in soil total organic carbon and in their different fractions, and soil quality indicators 95 in relation to erosion and deposition areas in an historical olive orchard; 2-To evaluate these differences in relation to the reference values found in an undisturbed natural area; 3-To evaluate differences in stable isotope signatures (δ 13 C and δ 15 N) and explore their potential for identifying degraded areas within the olive orchard.

Description of the area
The study area is located in the municipality of Montefrío, southwestern Spain ( Figure 1). The municipality extension is around 220 km 2 , of which 81 % is cultivated, mostly with olive trees. The climate in the region is continental Mediterranean with a long-term  average annual precipitation of 630 mm, a mean annual evapotranspiration of 750 mm, and a yearly average temperature of 15.2 ºC. It is a mountainous area, with elevation ranging between 800-1600 m a.s.l. at the 105 highest point (Sierra de Parapanda). Soil sampling took place in two areas around the archaeological site "Peña de los Gitanos", where the soil is classified as Calcic Cambisol according to the FAO classification. The reference undisturbed area was inside an archaeological site (Figure 1). This undisturbed area is covered by open Mediterranean forest interspersed with shrubs and annual grasses on limestone material (calcarenites). The status of this protected site guarantees that no anthropogenic activities have impacted it for a long period of time, approximately since the end of 16 th century. This area is 110 covered by natural vegetation typical of the Mediterranean region, mainly bushes like Pistacia lentiscus and Retama sphaerocarpa and herbaceous species such as Anthemis arvensis, Calendula arvensis, Borago officinalis, Bracchypodium spp., Bromus spp., and Medicago spp. The area studied was an olive orchard located within tens of meters of the reference area ( Figure 1) which had been established in 1856. Both areas were described in detail in previous studies on historical erosion rates in the region (Vanwalleghem et al., 2011, Mabit et al., 2012. This olive orchard is rainfed, and soil 115 management in the decades before the sampling was based on bare soil with pruning residues (trees pruned every 2 years) being chopped and left on the soil surface. Olive trees are fertilized annually with 5 kg of 15 N-P-K, spread below the tree canopy area.

Soil sampling
The reference site, adjacent to the olive orchard, belongs to an archaeologically protected site and therefore it is a non-120 cropped area where neither erosion nor sedimentation processes take place ( Figure 1). This site was sampled at thirteen points across a transect, spaced at an average distance of 6 m, with only four of these sampling points used in this study ( Figure 1). We use only four soil samples because not enough soil for the analysis was collected in the other samples. At each sampling point, the excavation method was used. Soil samples were collected at 5 cm depth intervals till bedrock was reached (between 20-60 cm). The four samples used in this study reached 40 cm depth or more. Composite soil samples at 125 10 cm intervals were prepared at the laboratory to perform the chemical analysis of the reference area as described below.
In the olive orchard a hydraulic mechanical core sampler was used. It gently rotates and pushes an 8 cm diameter core to sample 8 points along a 452 m long catena (Figure 1). To minimize soil disturbance, soil sampling was made when soil water content was between 40 to 80 % of water holding capacity. Precautions were taken to assure that four range of soil depth (0-10, 10-20, 20-30 and 30-40 cm) was collected at each sampling point in the orchard soil.

5
In a previous study, soil erosion and deposition rates were determined at each sampling point, comparing the 137 Cs inventory among these points and that of the undisturbed reference area (Mabit et al., 2012). The positions of all sampling points were recorded by RTK-GPS at submeter resolution (Table 1). Overall, 12 points were sampled, 4 in the reference area and 8 along the catena across the olive orchard, with all of them reaching the bedrock below 40 cm depth.

Physicochemical analysis 135
Soil samples were passed through a 2 mm sieve and homogenized. The fraction >2mm was determined and expressed as % in dry mass basis. Separation of the four soil organic carbon (C org ) pools was performed by a combination of physical and chemical fractionation techniques through a three-step process developed by Six et al. (2002) and modified by Stewart et al. (2009), summarized here. This three-step process isolates a total of 12 fractions and it is based on the assumed link between the isolated fractions and the protection mechanisms involved in the stabilization of organic C. First a partial dispersion and 140 physical fractionation of the soil is performed to obtain three size fractions: >250 mm (coarse non-protected particulate organic matter, POM), 53-250 mm (microaggregate fraction), and <53 mm (easily dispersed silt and clay). This physical fractionation is done on air-dried 2-mm soil sieved over a 250-mm sieve. Material greater than 250 mm remained on the sieve. Microaggregates were collected on a 53-mm sieve that was subsequently wet-sieved to separate the easily dispersed silt-and clay-sized fractions from the water-stable microaggregates. The suspension was centrifuged at 127 x g for 7 min to 145 separate the silt-sized fraction. This supernatant was subsequently separated, flocculated and centrifuged at 1730 x g for 15 min to separate the clay-sized fraction. All fractions were dried in a 60 ºC oven and weighed. Afterwards there was a second step involving a further fractionation of the microaggregate fraction isolated in the first step. A density flotation with sodium polytungstate was used to isolate fine non-protected POM (LF): After removing the fine non-protected POM, the heavy fraction was dispersed overnight by shaking and passed through a 53 mm sieve to separate the microaggregate-protected 150 POM (>53 mm in size, iPOM) from the microaggregate-derived silt and clay-sized fraction. The resulting suspension was centrifuged to separate the microaggregate-derived silt-versus clay-sized fraction as described above. A final third step involved the acid hydrolysis of each of the isolated silt-and clay-sized fractions. The silt-and clay-sized fractions from both the density flotation and the initial dispersion and physical fractionation were subjected to acid hydrolysis. The unprotected pool includes the POM and LF fractions, isolated in the first and second fractionation steps, respectively. The physically 155 protected SOC consists of the SOC measured in the microaggregates. It includes not only the iPOM but also the hydrolysable and non-hydrolysable SOC of the intermediate fraction (53-250 μm). The chemically and biochemically protected pools correspond to the hydrolysable and non-hydrolysable SOC in the fine fraction (< 53 μm), respectively. In all cases, SOC fractions and in the bulk soil, organic carbon concentrations were determined by using the wet oxidation sulfuric acid and potassium dichromate method of Anderson and Ingram (1993). 160 Inorganic carbon was removed prior to stable isotope analysis by acid fumigation following the method of Harris et al. (2001). Moistened subsamples were exposed to the exhalation of HCl in a desiccator overnight. Afterwards, the samples were dried at 40º C before measuring the stable isotope ratio. The N measurements were done with unacidified samples and the stable N isotope ratios and the C and N concentrations were measured by isotope ratio mass spectrometry (Isoprime 100 coupled with an Elementar Vario Isotope Select elemental analyser; both instruments supplied by Elementar, Langenselbold, 165 Germany). The instrumental standard deviation for δ 15 N is 0.16% and 0.11% for δ 13 C. Stable isotopes are reported as delta values (º/ oo ) which are the relative differences between the isotope ratios of the samples and the isotope ratio of a reference standard.
In addition, available phosphorus (P avail ) was determined by the Olsen method (Olsen and Summers 1982) and organic nitrogen (N org ) was determined by the Kjeldahl method (Stevenson, 1982). Water stable aggregates (W sagg ) were measured 170 using the method of Barthes and Roose (2002). Soil particle size distribution was determined using the hydrometer method (Bouyoucos, 1962) for the topsoil (0-10 cm) of the reference area and the olive orchard. Two bulk density values were calculated for the whole profile: a) one considering the mass of soil finer than 2 mm, and b) considering the mass of soil finer than 2 mm and also the stone contents, which were determined from the excavation and core sampling described above.
Additionally, the bulk density of the top soil (0-10 cm depth) soil bulk density was measured using a manual soil core 175 sample with a volume of 100 cm 3 . Soil carbon stocks were calculated for the fine soil fraction after discounting rock or stone fragments larger than 2 mm, and considering bulk density, and presented on equivalent soil mass as described by Wendt and Hauser (2013). As proposed by Hassink and Whitmore (1997), theoretical values of carbon saturation were calculated from the soil particle analysis. Finally, the soil degradation index developed by Gómez et al. (2009a) was calculated from the C org , P avail and W sagg . 180

Statistical analysis
The overall effect of depth and area (reference site vs. olive orchard or eroded vs. deposition area within the olive orchard) were evaluated using a two factor ANOVA (p<0.05). Additionally, for some comparison at similar soil depths, values of soil properties between two different areas were assessed using a one-way ANOVA test (p<0.05). In both situations, data were log-transformed when necessary to fulfil ANOVA requirements. Exploratory analysis using Principal Component Analysis 185 (PCA) was performed in the olive orchard area using the variables and sampling depths showing significant differences in the ANOVA analysis. This PCA analysis was complemented by determining the linear correlation coefficient variables showing the highest load on the PCA analysis and erosion/deposition rates using the Pearson correlation test. The statistical software package Stata SE14.1 was used for these analyses. Table 2 shows the significance of the differences in bulk soil C org and the various C org fractions between reference and olive orchard plots, between soil depths and due to the interaction between both factors (Table 2A), and the effects of erosion/deposition ratio (Table 2B). Overall, bulk soil C org was always significantly higher in the reference area as compared to the olive orchard (0.0001<p<0.0013, depending on depth) (Table 2A and Figure 2), and this was independent of the soil 195 sampling depth. C org values on the reference site were between 2 to 5 times higher than that of the olive orchard for a given depth, with the greater differences in the top 10 cm of the soil. Soil depth has a significant effect on bulk C org and C org fractions, with values typically decreasing with depth in both areas. C org concentrations in the unprotected, and physically, chemically and biochemically protected fractions were significantly higher (p<0.0001) (Table 2A), in the reference site as compared to the olive orchard and across the different depths (0.0001<p<0.0007) ( Figure 3). C org values were between 2 to 6 200 times higher for the unprotected and chemically protected fractions, and between 2 to 3.5 times for the physically and biochemically protected fractions, with differences tending to decrease with the soil depth.

Organic carbon concentration and distribution among fractions
Within the olive orchard, there were statistically significant differences between the erosion and deposition areas for bulk C org values (p=0.0198) (Table 2B). Higher C org values (1.1 to 0.6%) were observed in the deposition area located downslope, whereas lower values (0.85 to 0.55 %) were measured in the areas with net erosion in the upper and mid sections of the 205 catena. It is worth noting that these differences between erosion and deposition areas are detected for the overall analysis using a two-way ANOVA (Table 2B), although an individual analysis at each depth ( Figure 4) does not detect statistically significant differences, probably due to the moderate number of replications. Significant differences between the deposition and eroding area were also found for the unprotected (p=0.04) and the physically (p=0.0077) and chemically protected fractions (p=0.0299) (Table 2B). However, differences for the biochemically protected (Table 2B, Figure 4) were not 210 significant.
The percentage distribution of SOC among fractions was also significantly different between both areas (reference vs. olive orchard), except for the biochemically protected fraction (p<0.0001) (Table 3A) and depths (P<0.011) ( Figure 5). In the reference area most of the C org was unprotected (between 50 and 65% approximately) with no significant trend with depth (Table 3A, Figure 5), followed in relative importance by the chemically and physically protected fractions which contributed 215 between 18 to 30 % and 10 to 20 % of the bulk soil C org , respectively. The biochemically protected fraction represents a very low percentage (between 4 to 6 %). In the olive orchard, C org is stored predominantly in the physically and chemically protected fractions which accounted for about 38 to 27 and 34 to 28 % respectively, followed by the pool of unprotected fraction (between 22 to 32%) ( Figure 5). The biochemically protected fraction represents from 4 to 11 % of the organic carbon stored in the olive orchard. There are no clear differences in the organic carbon distribution among the different 220 fractions between the erosion and deposition areas in the olive orchard, with the exception of the physically protected fractions at 10-20 cm depth (p=0.024) ( Figure 6).

Organic carbon stock
SOC stock in the reference area is approximately 160 t ha -1 being significantly higher, at p<0.05, than that of the olive orchard for an equivalent mass (Figure 7), which stores between 38 and 41 t ha -1 in the eroded and deposition areas 225 respectively. There were no significant differences between these two orchard areas. Similar results were obtained across the top 40 cm soil profile. Clay, silt and sand contents of the topsoil (0-10 cm) along the catena in the olive orchard averaged 41, (average coefficient of variation of 17%) and there were no significant changes between the erosion and deposition areas. In the reference area, the soil has an average clay, silt and sand content of 30, 31 and 39% respectively, also with a 230 homogeneous distribution across the sampling area (coefficient of variation of 10%). According to the Hassink and Whitmore (1997) model, the percentages of organic carbon of maximum soil stable C org are of 3.240.11 and 3.630.19 % in the reference site and olive orchard, respectively. So, protected C org in the reference and olive orchard areas account for 49.811.5 % and 20.495.2 % of the maximum soil stable C org , respectively, in the topsoil.
3.3. δ 15 N and δ 13 C isotopic signal 235 Figure 8 and Table 4A compare stable isotope delta values between the reference site and the overall olive orchard according to depth. There are significant statistical differences in δ 15 N, δ 13 C, and δ 13 C:δ 15 N ratio between the two areas (p<0.002; Table 4A), although in the case of δ 15 N the differences at individual depths were not significant. Soil depth had also significant effect, (0.0026<p< 0.029, Table 4A). When comparing differences between the erosion and deposition areas within the olive orchard, we detected statistical significant differences only in δ 15 N and δ 13 C:δ 15 N ratio, (p=0.01; Table  240 4B), and they were most marked in the upper 20 cm of the soil (Figure 8). Figure 9 depicts the comparison between the P avail , N org , W sagg as well as the soil degradation index (SDI, Gómez et al. 2009a) in the top 10 cm of the soil between the erosion and deposition areas of the olive orchard and Table 5 shows a similar comparison for N org , P avail and bulk density at the different soil depths. P avail in the deposition area is significantly higher than 245 that of the erosion area in the top soil (p=0.005; Figure 9) and for the whole profile (p=0.01, Table 5), whereas no significant differences in individual soil layers were found for N org and W sagg . SDI, which combines these three variables, in the eroded area was about 3 times higher than that in the deposition area. Table 6 shows the loads in the three first principal components (PC) of the principal component analysis (PCA) for the variables used in this analysis. More than 70% of the variance was explained by the first two PC. Soils of the eroded and 250 deposition area were clearly separated in the space defined by the two PC ( Figure 10). The variables with higher contributions in PC 1 and 2 were related to P avail concentration in the 0-10 cm and 0-40 cm soil depths, to δ 15 N, δ 13 C in the 10 to 30 cm soil depths, and to C org concentration and distribution in some fractions also in soil depths between 10 to 30 cm (Table 6). Deposition area tended to show higher values in the PC1 and PC2, and there was no clear tendency in the erosion area along the catena. The linear correlation coefficient between variables and the erosion/deposition rate was rather 255 significant (r>±0.731) for most of the variables (Table 7). Interestingly, P avail in the whole soil layer was highly positively correlated. Among the two most robust correlations with erosion/deposition rates were that with P avail concentration across the 40cm soil depth, y=0.3975x+9.8364 (r 2 =0.907), and δ 13 C at 10-20 cm depth, y=-0.0094x-25.318 (r 2 =0.632).

Organic carbon concentration and distribution among fractions 260
After approximately 175 years of contrasted land use between the undisturbed reference site and the olive orchard, bulk soil organic carbon concentration and its fractions have been dramatically reduced in the olive orchard. Current levels of C org concentration in the soil profile are approximately 20 to 25% of that found in the reference area covered by the natural vegetation in the area adjacent to the orchard. This ratio is similar, albeit in the lower range, of the comparison of C org in topsoil among olive orchards with different managements and natural areas reported for the region (Millgroom et al., 2007). 265 The increased soil disturbance, the lower annual rate of biomass returned to the soil and the higher erosion rate in the olive orchard explain this difference. In both areas, the C org is clearly stratified, indicating that despite the different mechanisms involved there is a periodic input of biomass from the olive trees (e.g. fall of senescence leaves and tree pruning residues) plus the annual ground vegetation. Vicente-Vicente et al. (2017) estimated this biomass contribution in the range of 1.48 to 0.56 t ha -1 yr -1 . It is worth noticing that the decrease in C org as compared to the natural area is much higher than the reported 270 rates of increase in C org in olive orchards using conservation agriculture (CA) techniques, such as cover crops and incorporation of organic residues from different sources. In a meta-analysis Vicente-Vicente et al. (2016) found a response ratio (the ratio of C org under CA management as compared to bare soil managed orchard) from 1.1 to 1.9 suggesting that under CA management, which combines cover crops and organic residues, C org doubled as a maximum.
Combining all C org data of the olive orchard, the variability was about 35% which is similar to what has been reported so far 275 in the few studies found on soil C org variability in olive orchards. For instance, Gargouri et al. (2013) indicated a 24% coefficient of variation (CV) in a 34 ha olive orchard in Tunisia, while Huang et al. (2017) reported an average CV of 41% in a 6.2 ha olive orchard in Southern Spain. Neither of these two studies reported clear trends in the distribution of C org depending on topography. Huang et al. (2017) pointed out the additional difficulties in the determination of C org. due to the topography heterogeneity, although this was compounded by the fact that within the orchards there were two areas with 280 different planting dates for the trees. Gómez et al. (2012) reported a CV of 49% with higher C org in areas where there was a change in the slope gradient from the hillslope to a draining central channel into the catchment, although they could not find a simple relationship between the increase in content and the topographic indices. Despite the fact that a lot of work has been done on the correlation between erosion-deposition and the redistribution of soil C org , (e.g. Van Oost et al. 2005), our study is, to our knowledge, the first attempt to quantify this in detail under olive orchard agro-environmental conditions. The 285 variability induced by the combined effects of water and tillage erosion in this olive orchard was similar to that described in

Organic carbon stock
The differences in soil organic carbon stock between the reference site and the olive orchard are similar to those described previously when comparing cropland and forested areas, with the latter presenting a higher concentration of C org , and most of it in the unprotected fraction, while the cropland presented a higher fraction of the carbon in the physically and chemically protected fractions (e.g. Poeplau and Don, 2013). This is likely due to the fact that under soil degradation processes, such as 300 water erosion, and low annual organic carbon inputs, as is the case under olive orchard land use, most of the unprotected C org decomposes relatively quickly and a great proportion of the remained low SOC is protected. In addition, the mobilisation of the unprotected C org is expected to be reduced in the protected forested area because of the canopy and the existing vegetation on the ground that protects the soil against runoff and splash erosion processes. In fact, the protected C org concentration in the topsoil of the olive orchard in the eroded area accounted for the 18.63.9 % of the upper limit of 305 protected C org (3.640.23 %) according to the model of Hassink and Whitmore (1997). Therefore, the low unprotected SOC concentration found in the olive orchard is an issue in the increase of SOC stocks. This is because protected fractions are fuelled from recently derived, partially decomposed plant residues together with microbial and micro, meso and macrofaunal debris (unprotected organic carbon) through processes like SOC aggregation into macro-and/or microaggregates (physically protected SOC) and complex SOC associations with clay and silt particles (chemically protected SOC) which are disrupted 310 in the cropland area in comparison with the reference area. The distribution among soil C org fractions in the orchard of this study was similar to the result obtained by Vicente-Vicente et al. (2017) who measured C org fractions distribution in olive oil orchards with temporary cover crops, with the exception of the unprotected SOC, which was much higher in soils under cover crops than that of our study under bare soil. Also interesting is the difficulty obtaining statistically significant differences in SOC stock between the eroding and deposition areas (Figure 7) despite the apparent clear differences between 315 the two areas in some other soil properties, such as P avail or δ 15 N and δ 13 C isotopic signatures in the subsoil (see below).

δ 15 N and δ 13 C isotopic signal
Changes in vegetation types induced differences in δ 13 C between the olive orchard and the reference area but, as expected, no differences in δ 13 C were detected between the erosion and deposition areas in the olive orchard given the same origin of 320 vegetation derived organic matter, i.e. C3-plants. Interestingly, within the olive orchard, significant differences in δ 15 N were detected between the erosion and deposition areas, especially for the top 20 cm of soils (Figure 9). This suggests the potential of using δ 15 N as a variable for identifying degraded area in olive growing fields, as has been proposed for other eroding regions in the world (e.g. Meusburger et al. 2013), which might provide an alternative when other conventional or isotopic techniques are not available. Nevertheless, further studies exploring this potential are necessary in order to also 325 consider the influence of the N-P fertilizer modifying the δ 15 N in relation to the reference area (e.g. Bateman and Kelly, 2007). The source of N in soil is multifarious and subject to a wide range of transformations that affect δ 15 N signature and therefore we can only speculate on the reasons for this difference in δ 15 N in a relatively homogeneous area. Bulk soil δ 15 N tended to be more positive (e.g. more enriched in δ 15 N) as N cycling rate increases soil microbial processes (e.g. N mineralization, nitrification and denitrification) resulting in products (e.g. nitrate, N 2 O, N 2 , NH 3 ) depleted in 15 N while the 330 substrate from which they were formed becomes slightly enriched (Robison, 2001). The higher δ 15 N signature of the soil at deposition location suggests that rates of processes involved in the N cycling are higher than in the erosion area and that it is in accordance with the higher bulk C org and P avail contents and lower SDI of the deposition site. The relatively lower soil δ 15 N signature at the reference site could be partially due to the input of litter N from the natural legumes and to the closed N cycling which characterize natural forest ecosystems. The trend in 15 N enrichment with soil depth, as found in the reference 335 site, is a common observation in forest and grassland sites, which has been related to different mechanisms, including 15 N isotope discrimination during microbial N transformations, differential preservation of 15 N-enriched soil organic matter components during N decomposition, and more recently, to the build-up of microbial 15 N-enriched microbial necro mass (Huygens et al., 2008). However, there still remains the need for a careful calibration against an undisturbed reference site and a better understanding of the influence of different vegetation between the reference and the studied area in the change 340 of the δ 15 N signal for its further use as an additional tool to determine soil degradation.

Soil quality of topsoil across the catena. Variability between eroded and deposition areas in the orchard
This horizontal distribution due to tillage and water erosion also simultaneously affected other soil properties and has been described previously in other cultivated areas. For instance, De Gryze et al. (2008) described, in a field crop area under 345 conventional tillage in Belgium, how P avail almost doubled (22.9 vs. 12.2 mg kg -1 ) in the depositional area as compared to the eroding upper part. They also reported that in half of the field under conservation tillage, these differences in P avail between the upper and lower areas of the field disappeared. We have observed in our sampled orchard a pronounced increase in topsoil P avail in the deposition area of around 400% as compared to the eroding part of the orchard, which can be attributed to the deposition of enriched sediment coming from the upslope area. The cumulative effects of the differences in C org , P avail , 350 and the trend towards higher, although non-significant, W sagg in the deposition area, resulted in two areas within the orchard with marked differences in soil quality: the eroded part which is within the range considered as degraded in the region (Gómez et al., 2009a) and the depositional area, representing 20% of the orchard transect length (Table 1), which is within the range of non-degraded according to the same index. Topography and sediment redistribution by erosive processes introduce a gradient of spatial variability that questions the concept of representative area when it comes to describing a 355 whole field. In fact, several studies (e.g. Dell and Sharpley, 2006), have suggested that the verification of compliance of environmental programs such as those related to C org sequestration should be based preferentially, at least partially, on modelling approaches. Our results raise the need for a careful delineation of sub-areas when analysing soil quality indicators and/or SOC carbon stock within the same field unit. It also warns about the difficulty of drawing hypotheses for quantifying the differences between these delineated areas in relation to specific soil properties. For instance, in our study case 360 erosion/deposition processes had a major impact on P avail and soil quality, but the impact on C org concentration and stock was moderate and extremely difficult to detect statistically using a moderate number of samples.
Our PCA and regression analyses confirmed the relatively high variability of C org and stock in relation to other soil quality indicators related to erosion/deposition processes, such as P avail , as discussed in the previous section. While in the catena 365 studied in the current research P cycle seems to be mostly driven by sediment mobilization, the C org and N cycle seem to be much more complex. The moderate differences in C org and the homogeneity in δ 15 N and δ 13 C isotopic signals between the eroding and deposition areas can be due to several processes, some of them were discussed above, such as spatial variability of carbon inputs due to biomass in the plot, surface soil operations in the orchard and fertilization. We found both interesting and worth exploring in future studies that significant correlations between erosion/deposition rates and C org , δ 15 N and δ 13 C 370 related variables were found for samples from the 10-20 and 20-30 cm layers, indicating that short term disturbance by surface processes can mask experimental determination of the impact of erosion deposition processes in this olive orchard for these variables.

Conclusions 375
1-The results indicate that erosion and deposition within the investigated old olive orchard have created a significant difference in soil properties along the catena, which is translated into different soil C org , P avail and N org contents and δ 15 N, and thus contrasting soil quality status.
2-This variability was lower than that of the natural area, which indicated a severe depletion of SOC as compared to the natural area and a redistribution of available organic carbon among the different SOC fractions. 380 3-The results suggest that δ 15 N has the potential for being used as an indicator of soil degradation, although more investigation under different agroecosystems would be required for confirming this statement at a larger scale.
4-This research highlights that proper understanding and management of soil quality and C org content in olive orchards require considering the on-site spatial variability induced by soil erosion/deposition processes.

Acknowledgements 385
The authors would like to thank professor Manuel González de Molina for providing valuable insight into the management of the olive orchard in the decades before sampling. Furthermore, the authors are grateful to the three anonymous reviewers of the manuscript version in Soil Discussion, for their care and attention to the manuscript. All their comments and