Digital Theses Archive


Tesi etd-03302021-190534

Type of thesis
A field study for Mediterranean lowlands: assessment of a silvoarable alley-cropping system for feed and fuel production
Scientific disciplinary sector
Istituto di Scienze della Vita - AGROBIOSCIENCES
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Exam session start date
Agroforestry has been defined as “the deliberate integration of woody vegetation (trees and/or shrubs) as an upper storey on land, with pasture (consumed by animals) or an agricultural crop in the lower storey”.<br>Despite the multitude of positive benefits, many traditional Italian and European agroforestry systems disappeared throughout the 20th century, because tree removal from the agricultural landscape had been associated with an empowering of agricultural intensification. <br>With the increasing awareness of the potential consequences of Climate Change, the ongoing decrease of fossil energy resources and the environmental problems (e.g. land degradation by erosion and overexploitation), the concept of agroforestry has recently received renewed attention as one of the mitigation strategies for its potential to increase the carbon storage of agroecosystems and deliver other ecosystem services. <br>If the socioeconomical opportunities and the potentially deliverable ecosystem services of agroforestry are numerous, many challenges underly the implementation and study of the agroforestry systems. <br>The major problems posed are about the effectiveness of the C sequestration, the trade-offs between the beneficial or detrimental effects on crops yield, the higher complexity in agronomic management on the field, the economic rentability. These aspects need to be studied and quantified to make available important information for all the stakeholders involved in agroforestry systems implementation.<br>This thesis was focused on a field trial called AGROFORCES (AGROFORestry for Carbon sequestration and Ecosystem Services)started in 2017 as a joint research together with the Centre for Agro-Environmental Research (CiRAA) “Enrico Avanzi” and the University of Pisa, in the Pisa plain in Tuscany, Central Italy. The field trial was established on a former field trial for poplar SRC clones cultivation testing established in 2009. The project aimed to transform this former poplar SRC stand in a new alley cropping system (ACS) inspired by the experiences in Central Europe and by the idea of transition to new forms of agroforestry farming systems. Poplar SRC was intercropped with a two-year warm season rotation with sorghum and soybean, investigated for three consecutive seasons from 2018 to 2020.<br>The first objective of the thesis was to investigate the effect of tree presence upon sorghum and soybean yield components in ACS for two consecutive years, according to the position of the crop plants in the alley (West, Mid-West, Center, Mid-East, East) and (ii) to evaluate the effect of light availability at the herbaceous layer and of soil characteristics on soybean grain yield and composition in an ACS. (Chapter 2). The second objective of the thesis was to implement a methodology for measuring all relevant C fluxes at a small scale in order to estimate and compare the annual Net Biome Production (NBP) of three cropping systems over two years: (i) a sorghum-poplar Alley Cropping System (ACS), (ii) a sorghum Open Field cultivation (OF) and (ii) a poplar Short Rotation Coppice system (SRC) (Chapter 3).<br>The last objective considered, at a more comprehensive scale, the economic rentability of a rainfed ACS after the transition from a poplar SRC mature pure stand compared over a three-years rotation with sorghum and soybean (Chapter 4). <br>From the experiments conducted, we could derive that light availability in the ACS is the most important factor impacting on herbaceous crops yield in the understory, especially in the tree-crop interface (closer to the tree rows), more important than other factors (e.g. soil properties). The interesting fact, given from the choice of a C3 (soybean) and a C4 species (sorghum), is that not all the crops react in the same way to the reduced light availability, saying that in temperate Mediterranean environments a ‘one fit for all’ approach is generally reductive. Sorghum produced better when the poplar tree height is reduced, while soybean in different light availability conditions showed the same behaviour. <br>Regarding the C sequestration potential of the ACS, our results showed us that the ACS is less performing than the former control poplar SRC, proving to be a net C source (as the open field cultivation) while the SRC proved to be a net C sink. However, the situation could change according to the poplar management, since in the poplar first year of growth after harvest the C sink effect was not evident in the poplar SRC and all the three systems were a net C source. The interesting insight in the C budget calculation came out from the soil CO2 efflux measurements. The poplar SRC showed a steady state with similar cumulated CO2 emission values in the two years, while in the ACS this value was lower in the second year of measurements. This could be probably due to the faster respiration activity induced from a soil brought back to cultivation after years of low disturbance in the first year, with a slowdown in the year after (second year).<br>The economic assessment of the field trial showed that all the investigated systems showed a negative trend, even with some differences. The SRC showed a negative gross income owing to the high annual reintegration and its short-scheduled lifespan after 10 years. The sorghum-soybean rotation in the ACS and OF systems showed a better performance than the soybean-sorghum rotation due to the detrimental effect on sorghum biomass accumulation of light reduction under the poplar canopy during its second year of growth. Considering the single crop succession, soybean have had a better economic performance compared to sorghum (in both ACS and OF systems). The ACS systems have shown low or negative gross income, considering the ACS with 2-years rotation as well as the ACS with single crop rotation. The OF crop rotation showed the best economic performances among the three analysed cropping systems; nevertheless, its gross income is quite always under the ideal financial threshold of the annual CAP payment. <br>All this considered, from this experience we draw results which explored in detail for the first time and on a field scale the effect of the transition from an SRC stand to an agroforestry layout through an ACS. For the light availability studies, few experiences were available on alley cropping systems, and the majority of studies and approaches were related to forestry and more rarely on trees outside forests as in our experiment. Moreover, the impact of light reduction was rarely quantified on crops in the understory. There is still to do in measuring the adaptation to light availability, not only in studying the adaptability of different species and varieties but also to give information to push crop breeding to find solutions for the adaptability to shading, which is important in an scenario of increasing temperatures and higher frequency of drought events which is severely impacting Mediterranean agriculture. Moreover, we observed that it is not only the presence of trees which could give a higher sequestration potential to agroforestry. In this case, were a transition had to be adopted, we measured that the removal of a high quantity of C-rich biomass in a short time span (two years in our case with poplar SRC) could be detrimental on the C budget and that the respiration processes occurring to soil when it is brought back to cultivation could vanish the beneficial tree presence in an agroecosystem scale budget. The observations made on the economic assessment of over three years stimulated a deeper and broader thinking on agroecosystem design. In particular, the role of the CAP will be crucial to ameliorate the economic sustainability of farming goods of agroforestry systems as well as conventional cropping systems in Mediterranean rainfed agroecosystems. Hence, despite the potentially deliverable ecosystem services, the economic sustainability of the ACS remains uncertain, as observed from the results of this study. Thus, some future challenges must be addressed on alley cropping systems, as genetic selection of shade-tolerant varieties, the assessment of optimal alley width in order to avoid excessive yield reduction, quantification of the ACS potential on the carbon farming market in the framework of carbon farming. <br>On an agroforestry system like ours, there is still room for investigating other ecosystem services which could be delivered. We focused on the C sequestration potential, but further analysis could focus on the poplar capacity of watershed protection and higher nutrient removal, or on the role of tree rows on enhancing floral and faunal biodiversity, an aspect we observed empirically on the field but which is worth of a detailed description with specifically tailored multi-approach studies.<br>