Old wheat varieties. An opportunity to improve the sustainability of mediterranean drylands and organic farming

Resumen

Wheat is grown in more than 220 million ha in the world, being the most widely cultivated crop on Earth. Additionally, it is considered the most important crop for global food security, accounting for 20% of total protein intake worldwide, while in countries of the Mediterranean basin this share amounts to 31%. Global wheat production has increased 3.5-fold since the 1960s and it has been a key element in the development of the so-called Green Revolution. Modern varieties were designed to respond to the increase of industrial inputs associated with the intensification of agriculture (e.g. chemical fertilizers, pesticides, irrigation), which involved the abandonment of old wheat varieties, or their displacement to marginal agricultural lands. However, under less optimal conditions for agriculture such as rainfed Mediterranean conditions, or under management schemes that exclude the use of chemical inputs (such as organic agriculture), modern cereal varieties may not constitute an advantage over those grown before modern genetic breeding. Investigations comparing modern and traditional varieties are usually conducted under conditions that clearly favor modern ones, introducing an important bias in the widely extended idea that old varieties are less productive. In addition, most studies tend to focus on grain yield when comparing traditional and modern varieties. Yet, in the face of global change, the goal of crop cultivation should not only be grain production but also the provision of ecosystem services and the generation of sustainable and resilient agroecosystems. This is especially important in Mediterranean agriculture, which is particularly vulnerable to the expected impacts of climate change and global change. In this sense, it is necessary to investigate the role played by old varieties in these agroecosystems, and the consequences of their replacement by modern ones. Against this background, the main goal of this PhD is to compare the production and supply of ecosystem services of modern and traditional wheat varieties under rainfed Mediterranean conditions, both in the present and in the past. To this purpose, a field experiment comparing both types of cultivars under organic, traditional and conventional managements, along with a broad bibliographic research from historical sources, has been carried out with the objective of identifying traits of traditional varieties that should be considered for the development of sustainable rainfed agroecosystems. In this PhD dissertation, some of the main ecosystem services derived from the cultivation of old wheat cultivars are analyzed. In Study 1, we evaluated the effects of soil incorporation of the straw of old and modern wheat cultivars under laboratory-controlled conditions. Results show that ecosystems services such as soil nutrient conservation and carbon accumulation can be enhanced after the incorporation of the straw of old varieties. In Study 2, a life cycle assessment is carried out to compare the carbon footprint of old and modern varieties grown under rainfed organic and conventional farming systems, in order to elucidate the climate change mitigation potential of old varieties related to their carbon sequestration potential. Finally, the main consequences of wheat varietal replacement in the last century in Spanish cereal drylands are investigated in Study 3, considering the impacts on ecosystem services found in Studies 1 and 2. The results show that the cultivation of old wheat varieties under Mediterranean rainfed conditions is advisable in many aspects. Results of Study 1 show that the soil incorporation of the straw residues of old varieties is advantageous for soil carbon accumulation and the reduction of nitrogen losses from the soil. Accordingly, Study 2 shows that the higher straw and root biomass production of old cultivars can result in higher carbon sequestration rates, which is responsible for a lower carbon footprint of these varieties with respect to modern ones. The central role of carbon sequestration in the reduction of the environmental impact of cereal cultivation highlights the necessity to include it in carbon footprint accountings. Additionally, synthetic fertilizer in conventional farming systems and the use of machinery in organic ones were the main hotspots of the GHG emissions profiles, indicating that climate change mitigation efforts in Mediterranean rainfed systems should focus on these steps. Last, Study 3 shows that the reduction in crop residues due to varietal replacement entailed the degradation of the fund elements soil and biodiversity during the last century, threatening the agroecosystem sustainability. In conclusion, the cultivation of old wheat varieties in Mediterranean drylands can involve environmental advantages and climate change adaptation and mitigation synergies, without diminishing grain yields, especially under organic farming conditions.