• agroforestry forage sulla ryegrass N-fixation 15N

Agroforestry is considered an agroecological strategy with the potential to reduce nutrient leaching, conserve soils, increase diversity of the production system, and produce complementary wood for various uses. This practice is gaining pace as a land use strategy to cope with climate change and deliver important ecosystem services with an impact on crop productivity and reduction of agri-environmental risks. To achieve the goal of reducing the global warming potential of the agricultural sector, it is also of pivotal importance to reduce the reliance of agricultural systems on non-renewable external inputs, such as inorganic nitrogen (N) fertilizers. Agroforestry systems (AFS) including legume crops, and especially perennial forage legumes, can be considered a sustainable approach to enhance N cycling and use efficiency, thanks to the high amount of N supplied by biological N2-fixation and stored in stable forms in the soil. Moreover, perennial legumes can supply a permanent soil cover during the whole year, thus reducing soil erosion and nitrate leaching. Nevertheless, to design sustainable and productive agroforestry systems, net responses of crops to trees need to be clarified, especially in regions, like the Mediterranean area, where competitive interactions for water and nutrients usually prevail.
This thesis reports results about the application of agroecological practices such as the (i) use of perennial forage legumes, (ii) forage crop mixtures, (iii) real and simulated agroforestry conditions with the purpose of identifying new key management strategies for farmers to approach the agroecological transition to AFS. In particular, this work focused on forage-based cropping systems under Mediterranean and rainfed conditions. The two field experiments object of this thesis were carried out at the Center for Agri-Environmental Research “Enrico Avanzi” of the University of Pisa, in a typical coastal plain area of Tuscany, Central Italy.
The first field plot trial was carried out to assess the agronomic and qualitative performance of both, cool-season and warm-season perennial forage legumes and a cool-season grass sward, grown under diverse levels of artificial shade (i.e., 30%, moderate shade; 50%, intense shade). Regarding legume crops, specific analyses on biological N2-fixation (BNF) were conducted. Considering cool-season legume and grass species, i.e., sulla (Hedysarum coronarium L.), ryegrass (Lolium multiflorum Lam.) and their mixture, results showed that the mixture was more productive and able to maintain stable yields over time. Moreover, the production of the mixture was still the highest in comparison to the pure standing crops also under artificial shade conditions. We observed a detrimental effect of artificial shade on sulla and mixture especially in spring, when the cool season crop is more productive and light resulted to be a limiting factor. On the other hand, despite its low productivity in comparison to the other crops, ryegrass resulted not affected by shade. Considering the quality of the forage, the nutritive value of the mixture resulted averagely well balanced in comparison to sulla and ryegrass as pure stands. Shading affected the forage quality, showing a slight but not relevant increase in all parameters tested; among the different species evaluated the mixture maintained the best performance also under shading conditions. Despite the variable effects on the performance of the mixture components, these outcomes demonstrated the protective effect of legume-grass intercropping against potential negative effects of light availability reduction on quantitative and qualitative aspects of forage production.
The warm-season crop tested under the same shading treatments was alfalfa (Medicago sativa L.). Although its productivity was sensibly affected by shade at each level of light reduction following a linear trend (more shade, less yield), cumulative data showed anyway an acceptable yield also under shading conditions, especially under moderate shading. Regarding the quality of the forage, we focused on crude protein, the pivotal parameter for alfalfa, and no effect of shade was observed. Overall, these results suggest that, if we consider only the aspect of light reduction, alfalfa can be introduced in real AFS, thanks to its high nutritive value and high yield, but especially under extensive AFS rather than intensive systems.
Regarding the N2-fixation performance of both, sulla and alfalfa, the percentage of N derived from N2-fixation was higher in the shaded plots, while the N yield, which depends mainly on shoot biomass production, showed averagely higher values in not shaded plots. Thus, it is likely legumes in shaded plots might invest less resources in shoot biomass production while spending more energy in accumulating reserves in the roots, and rhizobial activity might benefit from this increased availability of resources. Further studies are needed to better understand the ecophysiological relations between symbiotic N2-fixation and shading in AFS, also considering the microclimate conditions, studying microbiological community, and testing these crops also in real AFS.
The second field plot trial was conducted to assess: (i) the ability of trees during the first year after planting to reduce N leaching, (ii) the N-fixation of legume sward and (iii) its capability to transfer N-fixed to the trees. Additionally, we wanted also to test the agronomic performance of sulla and ryegrass when introduced in AFS with 1-year old poplars trees. Results showed a higher productivity of sulla in AF in comparison to ryegrass in AF and all the forages grown without trees, probably because sulla may be even less affected by competition with young trees thanks to BNF and the subsidiary production of N. N2-fixation in sulla resulted affected slightly by the tree presence, showing no clear trends, even if the amount of N fixed and accumulated in its biomass resulted higher in close proximity of the trees than far from them. For this reason, we supposed that in the tree-crop interface, sulla invests more energy in N2-fixation, likely due to the belowground competition for resources and soil N that are less available for sulla roots. When trees are still young, and light availability isn’t a limiting factor, this effect does not imply a significant yield depletion in the legume. In our case, it was possible to observe a N-transfer effect from sulla towards poplars, but this did not increase poplar growth. Even if non-fixing trees were intercropped with legume crops, the belowground competition for nutrient resources might have caused a delayed growth rate of trees. Finally, despite the very young age of poplars, a reduction of nitrate content from the center of the plots to the ditch was observed, especially in the tree-crop sampling area. We may think that in the deepest soil layers the roots of poplars could be a barrier to reduce the N loss already in the first year after planting.
We can conclude that under Mediterranean rainfed conditions, the presence of the trees and the use of perennial legumes has demonstrated to be pivotal in supplying ecosystems services supporting acceptable and more stable forage productions from both, quantitative and qualitative points of view. Hence, a transition towards agroecology management of forage-based systems seems reasonably possible and could be crucial to increase productivity and land use efficiency, while mitigating environmental risks and soil degradation.