Influencia de las condiciones climáticas en el banco de semillas del suelo y en la flora arvense emergida en un sistema de rotación cerealista de secano en dos localidades diferentes

Resum

The term "soil seed bank" is used to define the reservoir consisting of all viable seeds, fruits and fruit fragments (with ability to germinate) present in or on the soil, and with the capacity to regenerate the plant community in different time periods depending on whether the seeds are dormant or not. It represents the potential flora of a site, and is particularly important in deeply disturbed habitats such as agroecosystems, being the cause of successive crop re-infestations. The composition of the seed bank and the “emerging flora” are conditioned by soil and climatic factors, as well as by crop and soil management. Knowing their composition and dynamics are key aspects for crop management and for evaluating the possible success or failure of any control techniques implemented. The general objective of this Thesis is to determine the effect of environmental and management conditions on seed banks, and the emerging flora of two Mediterranean cereal agroecosystems located in the centre and south of the Iberian Peninsula. The material to be analysed in this work was obtained from seed bank samples from Madrid and Seville trials, over a period of 3 years. The study of the evolution of the seed banks and the emerging flora was carried out according to the meteorological conditions (reflected by the years), the edaphic conditions (represented by the localities) and 4 different rotations. Quantification of the soil seed bank was carried out using extraction and germination methodologies. At the same time, the emerging flora was quantified and identified and the correlation between both populations (potential and emerging flora) was analysed. In addition, the incidence of the variables year, locality and crop on the evolution of density and biodiversity, in both populations, was evaluated. These studies were completed analysing the similarity of the populations and a network analysis, which provided the global integration, in a graphic way, of all the variables studied, and their time evolution both in the seed bank and in the emerging flora. The results obtained from the quantification of soil seed banks using the extraction and germination methodologies showed significant differences (quantitative in terms of seed density) and not qualitative (in terms of species number). As a result, the germination methodology is recommended for studies related to the emerging flora, or in the short term, and the extraction methodology in resilience studies of populations. Factors that generate these differences include the morphological characteristics of the seeds or fruits, as well as the existence of dormancy mechanisms affected by environmental conditions. The potential and emerging flora at the beginning of the trials were strongly affected by the intensity of management immediately prior to their start point. This management intensity also conditioned the dynamics of both the seed bank and the flora, which were the opposite in the two sites, depending on the time, in one case decreasing (Madrid) and in the other increasing (Seville). In relation to the climatic influence, significant differences were detected in the populations, densities and biodiversity indices (Shannon, Simpson, Pielou and Margalef indices) depending on the year, the locality and the rotation. With regard to the crops, under the conditions in which the trials were carried out, the analysis of the data showed that the crop with the greatest impact on weed control, in both cases, was barley, followed by wheat (Madrid) and rapeseed (Seville), depending on the locality. All data obtained were integrated for similarity analysis (Jaccard and Sorensen indices), observing the changes in the relationships between emerging flora and potential flora over time. Although, a very poor relationship was observed, it increased over time, with significant differences between localities. In Madrid, the similarity between potential and emerging flora was lower than in Seville. To conclude the study, network analysis showed all multifactorial relationships between the species distribution in the seed bank and the emerging flora. Adding to the scientific aspects of this Doctoral Thesis, an equipment was developed to increase the efficiency of the seed extraction process, which is currently being presented as an industrial patent application.