A historical perspective on soil organic carbon in Mediterranean cropland (Spain, 1900-2008)

Joint analysis of land, carbon and nitrogen reveals diverging trends in the sustainability of organic crops in Spain

The world's top ten Organic Farming (OF) countries by converted area include several Mediterranean countries, including Spain. Despite this, little is known about the consequences of OF on crop production and environmental sustainability in this country. In this article, we conduct an agronomic analysis of Spanish considerable conversion rate to OF, which tends to concentrate in certain provinces and crops. Indeed, in the case of various crops and in several provinces, the organic share of total agricultural land exceeds 20-30 %. This concentration makes it possible to compare information obtained from farmers through interviews and provincial statistical information. The study data consisted of information collected from interviews of a representative sample of organic farmers conducted in 2004 and 2020 as well as official statistical information. The results showed that no yield gap between OF and conventional farming was found for vegetables and fruit trees, while it showed an increasing trend in arable crops. Presumably, the reason is that fruit trees and vegetables generate and incorporate high levels of carbon (C) flows into the soil and have a low land cost per unit of incorporated nitrogen (N) (or can be paid for), allowing to meet crop needs and to increase soil organic matter (SOM). Conversely, in the case of rainfed arable crops, the soil C and N inputs are deficient due to the low crop residues and the high land cost of N. Consequently, SOM destruction and N deficit progressively broaden the yield gap, undermining the agroecosystem sustainability. To reverse the situation, among other measures, it is necessary to implement agricultural policies designed to make rotations with high legume ratios viable and to plant varieties presenting higher production of residues and roots, such as traditional varieties.

Soil organic carbon data comparison after 85 years and new 13 C/12 C compositions: The case study of the Ferrara province (Northeastern Italy)

The main causes of soil organic matter (SOM) loss are land use (e.g., conventional agriculture) and land-use change (e.g., conversion of wetlands into croplands). Before World War II and until 1960s, the Ferrara province in the Emilia-Romagna region (Northeast Italy) enlarged its agricultural production area through drainage of wetlands. After that, the newly drained area was put into intensive agricultural production with practices that proved to be unsustainable, and whose negative effects (depletion of soil organic carbon [SOC] and emissions of greenhouse gases [GHGs], e.g., CO2 ) have never been quantified. In this work, we estimated the changes in SOC 85 years after the drainage of the palustrine environment, by comparing 1937 SOC measurements with those made in 2022. Comparison of SOC maps from 1937 and 2022 indicates that most of the area suffered a significant SOC loss (∆OC85 years from 0.05 to 18.57 wt%), except for northern areas in which the peat nature of the soil has been preserved. We also measured the 13 C/12 C on the 2022 soil samples and generated a present-day map of the SOC isotopic ratios, which could be used in future as a benchmark to evaluate changes in soil carbon stocks and fluxes.

A new estimation of carbon emissions from land use and land cover change in China over the past 300 years

Scientific estimation of carbon emissions induced by historical land use and land cover change (LUCC) can improve the accuracy of terrestrial ecosystem carbon budget estimates and deepen understanding of the future carbon-sink potential of terrestrial ecosystems. The present study, using historical-document-based data for provincial cropland, forest, and grassland area in China, and experimental-data-based information for provincial vegetation and soil organic carbon density, re-estimates China's LUCC-induced carbon emissions for 1700-1980 using a bookkeeping model in which we updated tabulated functions for carbon losses and gains. The past 300 years have witnessed a dramatic LUCC in China. The cropland area has increased by 67.11 million ha, while the forest and grassland areas have decreased by 127.96 million ha and 16.72 million ha, respectively. Accordingly, the net carbon emissions for 1700-1980 are 6.17-12.35 Pg C, with 8.55 Pg C in the moderate scenario. Among the contributing factors, deforestation was the largest carbon source, accounting for over 90 % of the total carbon emissions. According to our estimates, over 70 % of carbon emissions were caused by harvesting wood, while <30 % were from converting forest and grassland to cropland. Spatially, for the whole period, carbon emissions in southwestern China (Chuan-Yu, Yunnan, and Guangxi), northeastern China (Liaoning, Jilin, and Heilongjiang), and parts of northwestern China (Gan-Ning, Qinghai, and Xinjiang) were as high as 6.03 Pg C, accounting for 70 % of the total carbon emissions. Extending previous studies, we updated the historical LUCC data, carbon density data, and tabulated functions for carbon losses and gains. The estimation results objectively reveal the historical spatiotemporal changes in LUCC-induced emissions.

Influence of the Washing Process and the Time of Fruit Harvesting throughout the Day on Quality and Chemosensory Profile of Organic Extra Virgin Olive Oils

In recent years, there has been a growing demand for organic extra virgin olive oils (OEVOOs) as quality products with greater added value. The aim of the present work was to determine whether the washing process and time of harvesting (morning or afternoon) plays an important role in the quality of Picual OEVOOs by studying quality parameters (degree of acidity, peroxide value, K232, K270, oxidative stability), and volatile and sensory profiles. Olive fruits were harvested at three maturity stages and within each phase at two times of the day (morning and afternoon). Then, samples were divided into two blocks (washed and unwashed). Volatile profile was obtained by headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography-mass spectrometry (GC-MS). Regarding the physicochemical quality parameters, sensory and volatile profiles, there were no differences between the oils produced under the two treatments applied (washed/non-washed). However, the time of harvesting (morning or afternoon) did influence the volatile and sensory profile, with higher values in the oils obtained from fruits harvested in the morning, being statistically significant for the families of aldehydes, hydrocarbures and lactones. Besides, the olives harvested during the mornings gave rise to oils with higher values in the green and apple fruit attributes.

Climate change and industrialization as the main drivers of Spanish agriculture water stress

In recent decades, there has been growing concern regarding worsening global climate conditions for agricultural production, especially in Mediterranean climate and other semi-arid areas where the scarcity of water is a major problem. The objective of the study was to estimate the long-term evolution (in Spain, over 1922-2016) of the decoupling between crop's water requirements and the green water available, and to analyze the causes of the changes. The FA0-56 methodology was followed to estimate the green water, blue water and water stress of crops, via an explicit territorial distribution, using 0.5^o grid cells. We propose a new indicator called Violet Water. This indicator groups together the water stress of the rainfed croplands and the blue water of the irrigated croplands, representing the components of this decoupling. Our results show that crops´ water requirements increased by 21% and violet water by 30% over the study period. The decomposition analysis used to study the drivers of these changes suggests that increasing reference evapotranspiration, caused by global warming, was the major driver triggering the growth of crop water requirements. Meanwhile, the changes in violet water values were mainly driven by the coupling of the cropland's spatiotemporal structure and climate change. The sensitivity analysis performed showed that the increasing trends in water stress, blue water and violet water were consistent for almost all the tested scenarios, but significant differences on the scale aroused depending on the climate data used (climate normal or each year climate) and specially on the geographical and historical Kc adjustment (crop-fixed Kc versus adjusted Kc). This highlights the importance of considering simultaneously the effect of the different factors, but also the need for refining this type of adjustment in future research.

Detailed global modelling of soil organic carbon in cropland, grassland and forest soils

Assessments of the global carbon (C) cycle typically rely on simplified models which consider large areas as homogeneous in terms of the response of soils to land use or consider very broad land classes. For example, "cropland" is typically modelled as an aggregation of distinct practices and individual crops over large regions. Here, we use the process-based Rothamsted soil Carbon Model (RothC model), which has a history of being successfully applied at a global scale, to calculate attainable SOC stocks and C mineralization rates for each of c. 17,000 regions (combination of soil type and texture, climate type, initial land use and country) in the World, under near-past climate conditions. We considered 28 individual crops and, for each, multiple production practices, plus 16 forest types and 1 grassland class (total of 80 classes). We find that conversion to cropland can result in SOC increases, particularly when the soil remains covered with crop residues (an average gain of 12 t C/ha) or using irrigation (4 t C/ha), which are mutually reinforcing effects. Attainable SOC stocks vary significantly depending on the land use class, particularly for cropland. Common aggregations in global modelling of a single agricultural class would be inaccurate representations of these results. Attainable SOC stocks obtained here were compared to long-term experiment data and are well aligned with the literature. Our results provide a regional and detailed understanding of C sequestration that will also enable better greenhouse gas reporting at national level as alternatives to IPCC tier 2 defaults.

Methane Emissions from Artificial Waterbodies Dominate the Carbon Footprint of Irrigation: A Study of Transitions in the Food-Energy-Water-Climate Nexus (Spain, 1900-2014)

Irrigation in the Mediterranean region has been used for millennia and has greatly expanded with industrialization. Irrigation is critical for climate change adaptation, but it is also an important source of greenhouse gas emissions. This study analyzes the carbon (C) footprint of irrigation in Spain, covering the complete historical process of mechanization. A 21-fold total, 6-fold area-based, and 4-fold product-based increase in the carbon footprint was observed during the 20th century, despite an increase in water use efficiency. CH₄ emissions from waterbodies, which had not previously been considered in the C footprint of irrigation systems, dominated the emission budget during most of the analyzed period. Technologies to save water and tap new water resources greatly increased energy and infrastructure demand, while improvements in power generation efficiency had a limited influence on irrigation emissions. Electricity production from irrigation dams may contribute to climate change mitigation, but the amount produced in relation to that consumed in irrigation has greatly declined. High uncertainty in CH₄ emission estimates from waterbodies stresses a need for more spatially resolved data and an improved empirical knowledge of the links between water quality, water level fluctuations, and emissions at the regional scale.

Evaluation of global historical land use scenarios based on regional datasets on the Qinghai-Tibet Area

Global historical land use scenarios are widely used to model human-induced climate change from the regional to global scales. It is necessary to conduct regional scale assessments of these global scenarios, identifying their uncertainties and pointing out directions for improvement. Based on the regional reconstruction Li-dataset, remotely sensed dataset, and grazing intensity dataset, the uncertainties of land use area and geographical distribution in HYDE3.1, HYDE3.2, and SAGE (a global land dataset from the Center for Sustainability and the Global Environment) scenarios for the Qinghai-Tibet Area (QTA) are evaluated. The comparisons show that the cropland areas on the QTA in HYDE3.2 for 1900-2000 are close to those of the Li-dataset, whereas HYDE3.1 underestimated and SAGE overestimated the cropland areas significantly. Spatially, HYDE3.1, HYDE3.2, and SAGE have large uncertainties, which cannot reflect the distribution of cropland on the QTA and its changes for 1900-2000 well, and too much cropland is allocated to southeastern Tibet. HYDE3.1 and HYDE3.2 overestimated the pasture area and its distribution on the QTA significantly. The distribution of pasture in SAGE showed overall an agreement with the spatial pattern for grazing intensity, but changes in grazing intensity for 2000-2010 was not reflected in SAGE. The FAO pasture definition and estimates and the method of using population as a proxy for pasture area are not appropriate for the QTA. Methodology which uses the pasture inventory data to calibrate satellite-based grassland maps to obtain the current pasture maps may also not be appropriate because of the lacking differentiation between natural and anthropogenic grasslands in remotely sensed data. More regional level land use estimates with concise definitions, define the land use more clearly, and stratification reconstruction based on differences in agro-climatic conditions and resource endowments may be used to improve global maps.

Evaluation and simulation of the impact of land use change on ecosystem services based on a carbon flow model: A case study of the Manas River Basin of Xinjiang, China

Land use change affects ecosystem services by changing the structure and function of ecosystems. Carbon flows throughout natural and socioeconomic systems can effectively reveal this process. The Manas River Basin has experienced rapid oasis expansion for decades, and land use change in the basin is very typical. Oasis expansion has caused a large amount of cropland to invade natural vegetation, thus affecting ecosystem services. This study used a biomass-based ecosystem service estimation model to assess changes in ecosystem services in the Manas River Basin. The carbon flow model was constructed using energy systems language, and the future development of ecosystem services was simulated based on different land use scenarios. The results show the following: (1) From 1980 to 2015, the river basin provisioning service was continuously optimized, while the regulating, supporting and cultural services were reduced. (2) If the expansion of cropland continues, then carbon will be transferred from the natural ecosystem to the cropland. Regulation, support and cultural services in the basin continue to decrease. Due to the shortage of water resources in the basin, the growth of provisioning services is limited. (3) If the project of returning cropland to grassland is implemented, then the carbon in the natural ecosystem will gradually recover. The regulating, supporting and cultural services of the river basin are rising, but provisioning services are gradually decreasing. In general, the model based on energy systems language can reflect the ecological process within the system and effectively reveal the carbon flow process between ecosystems. The use of carbon to quantify ecosystem services can harmonize dimensions, facilitate comparisons, and mitigate errors in outcomes due to different evaluation criteria or subjective factors. Therefore, this study combines energy systems language with carbon flow, which helps to more rationally explore the impact of land use change on ecosystem services, thereby providing valuable information for river basin management.

Land use shapes the resistance of the soil microbial community and the C cycling response to drought in a semi-arid area

The aim of this study was to understand the responses of the microbial community of soil under different land uses to drought in a semi-arid Mediterranean area. In a laboratory incubation, soil samples from different land uses (natural forest, drip-irrigated orchard, rain-fed almond tree cultivation and abandoned area) were maintained at 20% and 60% of the WHC. The microbial biomass and potential enzyme activities were determined after four and fifty days of soil incubation. The diversity and composition of the microbial community were studied after 50 days of incubation. The total mineralisation of soil organic C (SOC), as well as, the mineralisation of fresh organic matter (FOM) and the "priming effect" were analysed after addition of ¹³C-enriched plant tissue. Both land use and drought had significant effects in the soil microbial community, but the effect of land use was stronger than that of drought. The PLFA content (microbial biomass) of the forests soil was greater under drought. After 50 days of soil incubation, the microbial biomass and most of potential enzyme activities of the almond tree and abandoned soil samples were not significantly affected by drought contrary to those in orchard soil. The total and FOM mineralisation were on average lower in soil under drought than under optimal moisture for all land uses. However, the responses of the priming effect to drought were dependent on the land use. Overall, we conclude that the resistance to drought of the soil microbial community from an agroecosystem having a semi-arid climate is strongly influenced by the previous land use.

“BalSim”: A Carbon, Nitrogen and Greenhouse Gas Mass Balance Model for Pastures

Animal production systems are increasingly required to co-produce meat products and other ecosystem services. Sown biodiverse pastures (SBP) were developed in Portugal as an improvement over semi-natural pastures (SNP). SBP increase yields and animal intake during grazing, are substantial carbon sinks, and the abundance of legumes in the mixtures provides plants with a biological source of nitrogen. However, the data available and the data demands of most models make integrated modelling of these effects difficult. Here, we developed “BalSim”, a mass balance approach for the estimation of carbon and nitrogen flows and the direct greenhouse gas (GHG) balance of the two production systems. Results show that, on average, the on-farm GHG balance is −2.6 and 0.8 t CO2e/ha.yr for SBP and SNP, respectively. Ignoring the effects of carbon sequestration, and taking into account only non-CO2 emissions, the systems are responsible for 17.0 and 16.3 kg CO2e/kg live weight.yr. The annual analysis showed that non-CO2 emissions were highest in a drought year due to decreased yield and stocking rate. We also showed through scenario analysis that matching the grazing level to the yield is crucial to minimize emissions and ensure reduced feed supplementation while maintaining high soil carbon stocks.

Modern Wheat Varieties as a Driver of the Degradation of Spanish Rainfed Mediterranean Agroecosystems throughout the 20th Century

The high grain yield of modern varieties (MV) respond to the increase in fossil-based inputs, and the widespread belief that they are more productive than old varieties (OV) is biased. This belief focuses only on marketable biomass, without considering the consequences on agroecosystem sustainability of the reductions in other portions of NPP. Additionally, field comparisons of OV and MV were normally conducted under industrialized farming conditions, which is detrimental for OV performance. Both trials carried out in this study comparing wheat OV and MV show that, under Mediterranean rainfed conditions and traditional organic management, aerial and belowground biomass production of OV is higher than that of MV, without significantly decreasing yield and enabling a better competition against weeds. From the data of our trials, bibliographic review and information from historical sources, we have reconstructed the NPP and destinations of biomass of Spanish wheat fields (1900–2000). Varietal replacement entailed the reduction in residues and unharvested biomass (UhB), which involved soil degradation in rainfed cereal fields and undermining heterotrophic trophic webs. Our results suggest that OV can increase the sustainability of rainfed Mediterranean agroecosystems at present through the improvement of soil quality, the reduction of herbicides use, and the recovery of biodiversity.