Rol del biochar en el ciclatge del nitrogen en un agroecosistema mediterranipossibles beneficis i trade-offs

Resumen

Anthropogenic inputs of reactive nitrogen (N) via the Haber-Bosch process (synthetic N2 fixation for fertiliser production) and legume cultivation are responsible for over half of the input of N to terrestrial ecosystems. These excessive inputs from agroecosystems have resulted in enhanced fluxes of nitrous oxide (N2O), ammonia (NH3), and nitrate (NO3−), which induce far-reaching adverse effects, including the acceleration of global warming, promoting eutrophication, and depleting stratospheric ozone, among others. Soil-application of biochar, the carbon(C)-rich solid produced by thermochemical conversion of biomass under oxygen-limited conditions, could be a valuable tool to cope with elevated N fluxes. However, broad-scale biochar implementation is hindered by the uncertainties concerning possible unintended consequences as well as its long-term effects on soil quality, given its persistence in soil. In order to fill in some of these knowledge gaps, a pine gasification biochar that was found to reduce nitrate contents in soil solution in a previous study was assessed both under greenhouse conditions (Chapter 1) and under field conditions (Chapter 2). Chapter 1 was aimed at evaluating whether this particular biochar could induce a nitrate leaching mitigation response both in the short and long-term and at two contrasted application rates (12 and 50 t ha-1). To do so, soil columns filled with either fresh biochar or 6-yr field-aged biochar were monitored for 8 months. At the end of the trial, a significant reduction of NO3− in soil solution as well as other ions (including chloride, magnesium, sodium, and calcium) was found for both application rates in the fresh biochar-amended columns but not in the aged biochar scenario. The formation of an organo-mineral coating that entrapped nutrient-enriched water into biochar pores was ascribed as the most plausible explanation. Despite the alleviation of NO3− in soil solution, its leaching was unaffected by biochar treatment, casting doubt about possible environmental effects. In Chapter 2, by monitoring a wide range of soil properties in 6-yr old field mesocosms (from where the soil-aged biochar mixtures of Chapter 1 were taken), a soil quality assessment was conducted. The 50 t ha-1 application rate was the most effective in sequestering C and presented enhanced water contents at some sampling dates. However, an important trade-off emerged, since it exerted negative effects towards soil fauna communities (nematodes and collembolans), and also it boosted methane (CH4) soil emissions. Conversely, the 12 t ha-1 rate did not pose serious risks to soil faunal communities and soils acted as a CH4 sink. On the other hand, this same treatment showed signs of promotion of recalcitrant carbon metabolism, which, if maintained over time, could affect biochar’s C sequestration potential and reduce its expected persistence. Taking all into consideration, in our agroecosystem, the 12 t ha-1 rate would be safer to apply. Finally, Chapter 3 addressed the potential agronomical benefits of including biochar into fertiliser formulation. Particularly, a slow pyrolysis biochar was included in three organo-mineral fertiliser formulations (NPK, NP, and K). Nutrient leaching was slowed down in the formulation combining biochar and NPK (NPK+B). This effect was plausibly attributed to the labile-C added with the slow-pyrolysis biochar and the concomitant provision of NPK, which could have promoted microbial growth and caused a temporary storage of nutrients in microbial biomass. While the NPK+B fertiliser only significantly enhanced barley straw biomass and not grain, all biochar-based fertilisers presented improved plant nutrient content and export (regarding potassium, sulphur, calcium, and manganese). Both the improved nutrient release pattern in NPK+B and the enhanced crop nutrient status found for all biochar-based fertilisers indicate that the investigated formulations hold promise for further research and development of new generation fertilisers.