Semi-arid Mediterranean Environment Research Articles

Fusing Multispectral and LiDAR Data for CNN-Based Semantic Segmentation in Semi-Arid Mediterranean Environments: Land Cover Classification and Analysis

Spectral confusion among land cover classes is quite common, let alone in a complex and heterogenous system like the semi-arid Mediterranean environment; thus, employing new developments in remote sensing, such as multispectral imagery (MSI) captured by unmanned aerial vehicles (UAVs) and airborne light detection and ranging (LiDAR) techniques, with deep learning (DL) algorithms for land cover classification can help to address this problem. Therefore, we propose an image-based land cover classification methodology based on fusing multispectral and airborne LiDAR data by adopting CNN-based semantic segmentation in a semi-arid Mediterranean area of northeastern Aegean, Greece. The methodology consists of three stages: (i) data pre-processing, (ii) semantic segmentation, and (iii) accuracy assessment. The multispectral bands were stacked with the calculated Normalized Difference Vegetation Index (NDVI) and the LiDAR-based attributes height, intensity, and number of returns converted into two-dimensional (2D) images. Then, a hyper-parameter analysis was performed to investigate the impact on the classification accuracy and training time of the U-Net architecture by varying the input tile size and the patch size for prediction, including the learning rate and algorithm optimizer. Finally, comparative experiments were conducted by altering the input data type to test our hypothesis, and the CNN model performance was analyzed by using accuracy assessment metrics and visually comparing the segmentation maps. The findings of this investigation showed that fusing multispectral and LiDAR data improves the classification accuracy of the U-Net, as it yielded the highest overall accuracy of 79.34% and a kappa coefficient of 0.6966, compared to using multispectral (OA: 76.03%; K: 0.6538) or LiDAR (OA: 37.79%; K: 0.0840) data separately. Although some confusion still exists among the seven land cover classes observed, the U-Net delivered a detailed and quite accurate segmentation map.

Mixtures of Forage Species as Pasture for Dairy Ewes in a Mediterranean Environment

In Sicilian forage systems, the introduction of native self-seeding annual legumes can be beneficial in low-input farming. Intercropping would be a valuable strategy for implementing pasture resources in Sicilian forage systems during late spring. The aim of this study was to evaluate the effects on ewes’ milk production of continuous grazing with two different mixtures (i) sulla (Sulla coronarium L.), burr medic (Medicago polymorpha L.), and chicory (Cichorium intybus L.) (SuBuCh); and (ii) barrel medic (Medicago truncatula Gaertn.), snail medic (Medicago scutellata L.), and burr medic (BuSnBa). Twenty lactating ewes were homogeneously divided into four groups of five ewes. Each group was assigned to one of four 1500 m2 grazing sectors consisting of two replicates of SuBuCh and BuSnBa. Ewes fed with the SuBuCh mixture showed higher milk yield, higher protein (5.17 vs. 4.85%, p < 0.001) and casein content (4.02 vs. 3.73%, p < 0.001), lower urea content (37.70 vs. 45.38 mg dL−1, p < 0.001), and better clotting parameters compared to ewes grazing on the BuSnBa mixture. Finally, ewes in the SuBuCh group showed a smaller decrease in live weight at the end of the grazing period compared to BuSnBa ewes (−2.05 vs. −3.55 kg, respectively), although the difference did not reach a significant level. These preliminary one-year results seem to highlight the promising role of SuBuCh intercrop leading to a potential quantitative/qualitative improvement in grazing resources and the productive performance of grazing ewes in a semi-arid Mediterranean environment. However, it is of note that these outcomes might undergo variations when subjected to a prolonged trial extending beyond three years.

Carbonate morphological features of vineyard soils in a semiarid Mediterranean environment

AbstractMany vineyard soils in semiarid Mediterranean environments develop from carbonated materials and often show very diverse forms of pedogenic calcium carbonate accumulations. Based on 275 vineyard soil profiles in the Castilla La Mancha (CLM) territory (Spain), this paper presents an in‐depth comprehensive study of a large number of morphological expressions of these carbonate distributions, while exploring various pedogenesis formation hypotheses. The interactions among the calcareous nature of parent material, surface topography and climate are the factors that most affected their morphology and pedogenesis. The occurrence and distribution of carbonate features in CLM vineyard soils can be more complex than in other areas. Many accumulations are relatively ancient, which is the case of petrocalcic horizons, and in‐keeping with conservative means, which reveals the occurrence of successive carbonate accumulation cycles. More recently dissolved and precipitated seasonally pedogenic carbonates (sometimes as coatings) have appeared, which are nothing more than a simple recent redistribution of carbonates. A structural imprint is observed: pedogenic carbonates appeared much more abundantly on the lowermost horizons, which can be related to the repeated redistribution phases that occurred in different pedogenesis stages. The data provided herein help to improve the understanding of the nature of viticultural calcareous soils in semiarid Mediterranean regions.

Response of young fig (Ficus carica cv. Sarılop) trees to different irrigation levels in a semi-arid mediterranean environment

Response of young fig (Ficus carica cv. Sarılop) trees to different irrigation levels in a semi-arid mediterranean environment

Species, Cultivar and Seasonal Effects on Nodulation and Nitrogen Utilization of Spring Mediterranean Grain Legumes

Information on nitrogen economy of grain legumes during seed filling is critical for their sustainable cultivation in rainfed agroecosystems, but a comparable assessment of different legumes under the same growth conditions is lacking. The purpose of this study was to examine species, cultivar and seasonal effects on nodulation and nitrogen accumulation, remobilization, and utilization. Five grain legumes [common vetch (Vicia sativa L.), red pea (Lathyrus cicera L.), lentil (Lens culinaris Medik), chickpea (Cicer arietinum L.), and field pea (Pisum sativum L.)], each represented by two cultivars, were grown for two years in a semiarid Mediterranean environment. There were considerable environmentally induced differences among species in nodulation, with chickpea maintaining a sufficient number of nodules (16 nodules plant‒1) and nodule dry weight (57 mg plant‒1) even in the drier year (2015). Pre-podding nitrogen accumulation varied among species, particularly in the wetter year (2014). Nitrogen translocation from vegetative parts to seeds was correlated with pre-podding nitrogen accumulation (r = 0.99, P < 0.01 in 2014 and r = 0.98, P < 0.01 in 2015). Interspecific differences in nitrogen translocation efficiency were enlarged under drought, with red pea exhibiting the highest (84%) and lentil the lowest value (72%). Chickpea produced the highest seed nitrogen accumulation in both years and had the highest nitrogen utilization efficiency (21 kg seeds per kg N) the drier year. Nitrogen utilization efficiency was positively correlated with nitrogen harvest index and negatively with nitrogen concentration in vegetative parts at maturity. Most trait differences were less evident within species than between species. Overall, chickpea was found to outperform other legumes in semiarid Mediterranean conditions in terms of nitrogen accumulation in seeds and nitrogen utilization efficiency.

Forage mixture productivity and silage quality from a grass/legume intercrop in a semiarid Mediterranean environment

AbstractIn semiarid environments of the Mediterranean region, intercropping is a sustainable agricultural system of long standing. In this area, the pea (Pisum sativum L.) is one of the most commonly grown legume crops. Little information is available on the quality of silages to be obtained from forage mixtures of pea intercropped with cereals or annual grasses. In this study, two experiments were conducted over the course of two growing seasons in Sicily (Italy) with the aim to determine the biomass production of forage crop mixtures and assess, only in the second experiment, the silage quality of grass and legumes. Four cereals and one annual grass species were grown in pure stand and in mixture with pea, and their main agronomic traits were determined. The land equivalent ratio (LER), competitive ratio, and aggressivity index were also calculated. A number of parameters were considered to assess the quality of silage obtained from fermented biomasses derived from pea–ryegrass (Lolium multiflorum Lam. var. Westerwoldicum) intercropping. In the first experiment, the best performance between the intercrops was recorded for the pea–wheat mixture. The total LER calculated for fodder yields was always greater than 1, indicating crop yield advantages ranging from 2.0% to 47.0%. In the second experiment, the pea–ryegrass mixture appeared to respond well, depending on plant arrangement and seeding ratio factors: the ratios 50:50 and 100:50 showed the greatest crop yield advantages, of 12.0% and 11.0%, respectively. All silages revealed a very good suitability of a pea–ryegrass intercropping system with high‐quality silage production in the Southern Mediterranean region.

Contribution of G.I.S and Remote Sensing for the Risk Mapping of Soil Water Erosion at Saida Province (Western of Algeria)

Abstract Soil water erosion is a major aspect of landscape degradation in sub-wet to semi-arid Mediterranean environments. This problem has not been dealt with in detail in Algeria although it affects large areas. The objective of this study is to develop a methodology using remote sensing, the digital terrain model (DTM) and geographic information systems (GIS) to map areas with a risk of water erosion within the territory of Saida province and to produce a risk map that can be used as a reference document by the local authority. The presented methodology consists to incorporate three factors controlling the erosion: slope, material friability and land use. The resulting erosion risk map shows five areas of vulnerability to water erosion: very low, low, moderate, high and very high. Areas with high and very high risk of erosion cover more than 20% of the mountainous area and are consistent with our field observations. The methodology presented in this study can be generalized to all areas of the foothills of the Tellian Atlas, where erosion risks pose a threat to urban areas. This model is a tool to help implement a soil conservation plan in order to organise the measures to be applied and ensure sustainable soil management.

Variety and Sowing Date Affect Seed Yield and Chemical Composition of Linseed Grown under Organic Production System in a Semiarid Mediterranean Environment

The use of suitable species and varieties in organic cropping systems is essential for improving resource use efficiency, biodiversity, and agroecosystem resilience. Within the SIC-OLEAT project, a 2-year field trial was carried out in two contrasting environments of Central Italy, with the aim to hypothesize a production path for linseed inclusion within organic farming. The effects of location, genotype and sowing date on crop phenology, agronomic performances, and qualitative traits were evaluated. Generally, linseed showed good agronomic traits that make it suitable to be introduced in organic systems. Autumn sowing coupled with milder and wetter conditions seemed to be more favorable for linseed cultivation, allowing a higher seed yield (2.1 vs. 1.3 Mg ha−1) and oil content (47.2 vs. 45.2%). From multivariate analysis, the superior genotypes were Kaolin &gt; Szafir &gt; Galaad, and among these Kaolin had the highest production stability. On the contrary, Libra was the lowest performing one and the most unstable. These findings underline the importance of a site-specific approach for choosing the most suitable variety, since both sowing date and location are meteoclimatic-related factors. Definitively, our results demonstrated that linseed might be a valuable autumn alternative for organic cropping system diversification, contributing to the local production of vegetable oils and proteins.

Resilience of hop (Humulus lupulus L.) to salinity, heat and drought stresses: A mini-review.

Over recent years, the cultivation of hops (Humulus lupulus L.) has spread widely in the Mediterranean, also affecting the southern regions of Spain and Italy with a typical semi-arid climate. Several and recent studies have investigated the responses of this species to the main abiotic stresses, which is an aspect of absolute relevance to the knowledge of the adaptive capacity of hops to the growing conditions of a new cultivation environment. Moreover, given the fact that hops' phytochemical composition is determined primarily by genetic and environmental factors, and that the species is perennial, the lack of knowledge on the effects of abiotic stress could be reflected in subsequent years, which means multi-year economic risks. This review work therefore aims to showcase, based on an in-depth investigation of the available literature, the response of hop to the main abiotic stresses, and the effect of these on productive and qualitative crop performances. The data presented will be useful to the understanding of constraints and to the identification of useful coping strategies to the cultivation of hops in semi-arid Mediterranean environments.

Protist Diversity Responses to Experimental N Deposition in Biological Crusts of a Semiarid Mediterranean Ecosystem

Biological soil crusts (BSC) are associations of different macro and microorganisms and aggregated soil particles located on the surface of soils in many different habitats. BSC harbour a diverse and complex community of ciliates and testate amoebae. These phagotrophic protists play an important role in C and N recycling in soil ecosystems but have not been frequently studied in BSC. In this context, the effects of three increasing N inputs on ciliates and testate amoebae in crusts from a semi-arid Mediterranean ecosystem were evaluated. A field experiment with artificial N-deposition was designed to mimic the effects caused by anthropogenic N depositions. The results have shown that the protist populations of these semi-arid Mediterranean environments have lower species richness than other soil environments. The increase in N produces a net loss of diversity in the populations studied and shifts in the community structure. It has also been shown that some ciliates and testate amoebae, due to their population responses to increased N concentrations, could potentially be used as bio-indicators of N contamination in these BSCs.

Hop (Humulus lupulus L.): Suitability of Traditional Cultivars to a Low-Trellis Farming System in a Semiarid Environment

Humulus lupulus is a dioecious twining herb, with an outstanding vertical development capacity. Hop plants are usually grown on trellises up to 4.5 to 6.0 m high, whose management requires intense use of water, fertilizers, pesticides, and labor. In semiarid Mediterranean areas, where native resources are often scarce, the adoption of low-trellis farming systems could be a sustainable option for hop cultivation. With the aim of evaluating hop suitability to low-trellis cultivation in a Mediterranean environment, in 2018 and 2019 three traditional hop genotypes (‘Cascade’, ‘Chinook’, and ‘Nugget’) were grown, and their development rate was evaluated and put in relation with the plants’ cone, root, and biomass yield. Moreover, organic (fragmented pine bark) and synthetic [black polyethylene (PE) plastic film] mulches were applied on the same cultivars, and both epigeal and hypogeal development were evaluated. The results showed that the faster the growth in the first two phases of plant elongation (up to 50% of the height of the upper wire), the lower the yield in both cones and total epigeal biomass. A fast growth rate was instead associated with a higher hypogeal biomass production. Mulching was able to significantly affect the hypogeal biomass, specifically for cv. Cascade, where the use of synthetic mulching allowed significant root biomass increases. The analysis of the results obtained showed that hop’s suitability to a low-trellis farming system is highly variable among varieties in the semiarid Mediterranean environment.

Soil mineral nitrogen dynamics in fallow periods in a rainfed semiarid Mediterranean agricultural system

Rainfed agricultural systems in semiarid Mediterranean environments are subject to erratic but often heavy rainfall events. As an agronomic practice, fallow periods can be included even within the existing EU common policy for crop diversification. This research aimed to quantify the effects of a previous mineral fertilization on the soil mineral nitrogen (N min ) content and on potential nitrate leaching during no-tilled fallow periods of a crop rotation. Water and N min soil measurements obtained during three fallow seasons were used. Data were also used to check if the Leaching Estimation and Chemistry Model (LEACHM) can be used for the soil N min prediction after fallow. During these periods, the N min measured in the soil profile increased on average by 125 kg N ha -1 , while the model averaged an increase of 95 kg N ha -1 . The estimate of N leached ranged from 11 to 38 kg N ha -1 . The N balance simulated using LEACHM might differ from the actual processes. After a drought period followed by a soil water replenishment, the calibrated LEACHM underestimated soil N min probably due to the “Birch effect”. After occasional rainy spells when soil quickly became saturated, LEACHM overestimated soil N min probably due to occasional N 2 O emissions not being fully accounted by the model and to specific preferential water flow, which might produce a greater nitrate leaching than that simulated by LEACHM using the convection-dispersion equation . The results show that soil N min measurements by sampling after the fallow period cannot be replaced by LEACHM simulations. These samplings are of interest in fallow established inside a rotation and to avoid over-fertilization in the following cropping season, while reducing N environmental impacts.

Effects of harvest time on bioethanol production and bagasse characteristics for combustion and forage in sweet sorghum

Determining the optimal harvest time is very crucial to achieve sustainable production of first-generation bioethanol as well as efficient use of its bagasse for combustion or feed in sweet sorghum. The aim of the current research was to understand the effects of different harvest times, which comprised flowering (FLW), soft dough (SD), hard dough (HD), and physiological maturity (PM) on the bioethanol production and the combustion and forage characteristics of its bagasse in sweet sorghum. In one study, averaged over a period of 2 years, the mean theoretical total ethanol yield (TEY) was between 4802 and 5316 L ha–1. The PM harvest produced the highest TEY, which was followed by the HD and SD harvests, with sligh differences. These results suggested that sweet sorghum should be harvested at any phenological stage after FLW to achieve sustainable total (juice + lignocellulosic) bioethanol production in a semi-arid Mediterranean environment. Averaged over a period of 2 years, the mean dry bagasse yield (DBY) achieved from the harvest times herein ranged between 12.0 and 14.1 t ha–1. As with TEY, the PM harvest exhibited the highest DBY and desirable bagasse combustion caharacteristics due to exhibiting the lowest moisture and ash contents, which indicated that it seemed be the most suitable harvest time if obtaining solid biofuel production from the bagasse is the main target after the production of juice ethanol. On the other hand, the HD harvest provided the highest bagasse forage quality among the harvest times due to exhibiting the significantly lowest neutral detergent fiber and acid detergent fiber contents, but the significantly highest relative feed value. These results suggested that the HD harvest time was the most suitable for sweet sorghum cultivation when producing juice ethanol that is complemented by the production of forage utilizing the bagasse.

Occurrence of some rare earth elements in vineyard soils under semiarid Mediterranean environment

A comprehensive investigation has been carried out into the concentrations of a range of REEs (neodymium Nd, cerium Ce, lanthanum La, yttrium Y, scandium Sc) in soils of vineyards belonging to the protected denomination of origin (PDO) Valdepeñas (Central Spain). The mean concentrations (expressed in mg kg−1) are Ce 70.6, Nd 32.9, La 36.2, Y 21.6, and Sc 13.7 in surface horizons (Ap), while in subsurface horizons (Bt or Bw and some Ck), the values are Ce 67.6, Nd 31.8, La 34.4, Y 19.6, and Sc 13.9. The relative abundance in these soils is Ce > La > Nd > Y > Sc in both the surface and subsurface horizons. These values are close to, or slightly higher than, the regional levels but similar to national and global averages, although relatively high values have been detected at certain sampling points. Another aim was to explain the spatial variations in these elements within the territory under study. It was found that the spatial variations are due to the nature of the parent materials and the pedogenetic processes, although the sparse spatial distribution patterns with prominent anomalies are interpreted arising from anthropogenic sources (fertilization). However, these anomalies did not present any environmental risk in the studied zone.

Modeling Soil Organic Carbon Changes under Alternative Climatic Scenarios and Soil Properties Using DNDC Model at a Semi-Arid Mediterranean Environment

Soil organic carbon (SOC) is one of the central issues in dealing with soil fertility as well as environmental and food safety. Due to the lack of relevant data sources and methodologies, analyzing SOC dynamics has been a challenge in Morocco. During the last two decades, process-based models have been adopted as alternative and powerful tools for modeling SOC dynamics; whereas, information and knowledge on the most sensitive model inputs under different climate, and soil conditions are still very limited. For this purpose, a sensitivity analysis was conducted in the present work, using the DeNitrification-DeComposition (DNDC) model based on the data collected at a semi-arid region (Merchouch station, Morocco). The objective is to identify the most influential factors affecting the DNDC-modeled SOC dynamics in a semi-arid region across different climatic and soil conditions. The results of sensitivity analysis highlighted air temperature as the main determinant of SOC. A decrease in air temperature of 4 °C results in an almost 161 kg C ha−1 yr−1 increase in C sequestration rate. Initial SOC was also confirmed to be one of the most sensitive parameters for SOC. There was a 96 kg C ha−1 yr−1 increase in C sequestration rate under low initial SOC (0.005 kg C ha−1). In the DNDC, air temperature in climatic factors and initial SOC in variable soil properties had the largest impacts on SOC accumulation in Merchouch station. We can conclude that the sensitivity analysis conducted in this study within the DNDC can contribute to provide a scientific evidence of uncertainties of the selected inputs variables who can lead to uncertainties on the SOC in the study site. The information in this paper can be helpful for scientists and policy makers, who are dealing with regions of similar environmental conditions as Merchouch Station, by identifying alternative scenarios of soil carbon sequestration.

A morphological approach to evaluating the nature of vineyard soils in semiarid Mediterranean environment

AbstractLa Mancha (Central Spain) is one of the most extensive vineyard regions in the world, and ‘Valdepeñas’ is a representative Protected Denomination of Origin (PDO) in this region. However, what are their main soil types? what kind of horizons are the most common? and what is the role of the geomorphological positions in their pedodiversity? After describing and sampling 90 soil profiles in this area, Alfisols, Inceptisols and Entisols were mainly identified in Soil Taxonomy terms; in other words, Luvisols, Cambisols, Regosols, Leptosols and a highly significant proportion of Calcisols according to FAO‐UNESCO‐ISSS. The accumulation of carbonate, the thickness of which varies from a diffuse or powdery form to crusted forms, appear sometimes like polycyclic. The presence of red soils, with or without a calcic or petrocalcic horizon, indicates the most representative edaphic stages in this region. Consequently, the morphological signature is calcic or petrocalcic, followed by argillic and/or cambic horizons, under ochric horizons. It can be concluded that the nature of soils in Valdepeñas can be considered a differential factor to bear in mind for quality viticultural production.Highlights There are unknown conceptual zones in support production of wine. A comprehensive study in a local case was performed due to its traditional production of vineyards. This study highlights the importance and uniqueness of the calcic and petrocalcic horizons. A disconnect exists between some tradicional viticultural zones with low reputation and the real value of their soils.

Trifolium subterraneum cover cropping enhances soil fertility and weed seedbank dynamics in a Mediterranean apricot orchard

The soils of Mediterranean semiarid environments are commonly characterized by low levels of organic matter and mineral elements, as well as severe weed infestations, which, taken together, cause an intensive use of auxiliary inputs (tillage, fertilizers, herbicides). Although cover crops are recognized to sustainably improve soil health, the impact of Trifolium subterraneum L. cover cropping needs specific attention. This research investigates for the first time the effects over 4 years of T. subterraneum and spontaneous flora cover crops, after either incorporating their dead mulches into the soil or leaving them on the soil surface, on soil organic matter (SOM), macroelements, mineral nitrogen, microelements, and weed seedbank dynamics as indicators of soil quality in an apricot orchard. Compared to a conventional management control, the T. subterraneum cover crop with the burying of dead mulch into the soil increased the amount of SOM (+ 15%), ammoniacal (+ 194%) and nitric (+ 308%) nitrogen, assimilable P2O5 (+ 5%), exchangeable K2O (+ 14%), exchangeable Na (+ 32%), exchangeable K (+ 16%), Fe (+ 15%), Mn (+ 28%), Zn (+ 36%), and Cu (+ 24%), while it decreased the weed seedbank size (‒ 54%) and enhanced weed biodiversity. These findings suggest that T. subterraneum cover cropping may be an environment-friendly tool to enhance soil quality and limit auxiliary input supply in Mediterranean orchards.

Life Cycle Assessment of Biomass Production from Lignocellulosic Perennial Grasses under Changing Soil Nitrogen and Water Content in the Mediterranean Area

Low iLUC risk feedstocks, such as lignocellulosic no-food crops, have been indicated as sustainable crops for the transition to a bio-based economy. Given the high output to input ratio and the environmental benefits that can be obtained from renewable heat production replacing fossil fuels, the present study addressed the biomass yield, CO2-sequestration, and life cycle assessment of giant reed (Arundo donax L.) and miscanthus (Miscanthus × giganteus Greef et Deuter) growing under different soil water availability and nitrogen fertilization for three consecutive growing seasons in a semiarid Mediterranean environment. Giant reed outperformed miscanthus, showed a higher CO2-sequestration and a lower overall environmental impact. In case of both crops, the irrigation effect was significant, while the one of nitrogen fertilization was not apparent. While giant reed responded positively to reduced irrigation, compared to its highest level, as the plantation became older, miscanthus needed high water volume to get most out its potential yield. Nonetheless, the growing season had also a significant effect on both crops, mainly when low yields were achieved following the establishment year. Unlike the environmental benefits in the impact categories “non-renewable energy use” and “global warming potential”, environmental burdens concerning ozone depletion, acidification, and eutrophication were observed, indicating that further improvements of the evaluation of impact assessment associated with bioenergy production might be necessary.

Long-term effects of contrasting tillage systems on soil C and N pools and on main microbial groups differ by crop sequence

Determining the best conservation agriculture practices for increasing soil organic carbon (C) and hence soil quality is of paramount importance in the semi-arid Mediterranean environment, where soils are experiencing a continuous decline in organic matter. Therefore, the aim of this long-term study was to assess the combined effects of tillage system and crop sequence on soil organic C and biochemical properties of soil generally used as indicators of soil quality. After 23 years of continuous application of contrasting tillage systems (conventional tillage [CT], vs. no tillage [NT]) and crop sequences (wheat monoculture vs. wheat-faba bean rotation), soil samples were collected from topsoil (0–15 cm) and subsoil (15–30 cm) at three different times during a cropping year. Soil samples were analyzed for total and labile organic C pools, microbial biomass C (MBC) and microbial biomass N, basal respiration, and the abundance of main microbial groups by phospholipid fatty acids. Long-term NT increased total organic C (TOC) at a yearly rate of 0.17 g kg−1. This in turn stimulated microbial biomass, in particular Gram-negative bacteria. This suggests a higher soil quality in NT, as was confirmed by the increase in MBC/TOC and the decrease in stress indices. In contrast, no differences were observed with regard to fungal biomass. These findings suggest the need to reconsider the role of specific bacterial groups in organic C accumulation in soils of semiarid environments. It is interesting that the effects of long-term NT varied widely by crop sequence, whereas in CT changes in biochemical characteristics and in the main microbial groups due to crop sequence were modest. Thus, the interaction among various aspects of agronomic management modulates the effects of substrate quality on chemical and biological properties of soil.

Relief and calcium from gypsum as key factors for net inorganic carbon accumulation in soils of a semiarid Mediterranean environment

In semiarid environments, the total inorganic carbon (TIC) in soil may contribute to the total carbon (C) pool more than the total organic C pool (TOC), thus playing a key role in storing atmospheric CO2. However, due to the different origin pathways of soil carbonates, not all of the TIC pool can be accounted for CO2 sequestration. Indeed, the inorganic C can be accounted for a net sink of CO2 only when calcium (Ca2+) forming carbonates originate from non-carbonate minerals (atmogenic inorganic C, AIC). The aim of this study carried out in a gypsiferous area is to investigate the dissolution of Ca2+ that comes from gypsum (CaSO4·2H2O) in the formation of soil atmogenic carbonates and to quantify the different forms of inorganic C pools to understand their role in storing CO2 in comparison to the TOC pool. To this end, five soil profiles were studied along a hillslope in a gypsiferous afforested area. Soil samples were analysed to determine their main chemical properties, as well as Ca2+, Sr2+, and the 87/86Sr isotopes ratios in different soil fractions. Ca2+ that comes from gypsum, which contributes to the formation of atmogenic carbonates, ranged from 0% to 63%; the remaining percentage resulted from the parent material or other sources (e.g., aeolian dust). The distribution of Ca2+ in soils depended on the relief and the distance of the soils from gypsum outcrops. The accumulation of AIC in soil developed on Holocene deposits reached a maximum accumulation of 16 kg m−3 in the first meter of depth. The average Sr2+/Ca2+ ratio in primary carbonates was 0.333, whereas it was 0.192 in secondary carbonates, thus suggesting that different sources contribute to Ca2+ in both carbonate types. Consequently, the Sr2+/Ca2+ ratio of soil carbonates could be a useful indicator of the presence of secondary carbonates. Overall, results suggest that gypsum plays a key role in the net accumulation of inorganic C in the soil, contributing to store atmospheric CO2. Finally, considering that the AIC pool was lower than the organic C pool, the latter was by far the most important element in fixing atmospheric CO2 in the soils of the semiarid Mediterranean environment that were surveyed.