Compost application in an olive grove influences nitrogen dynamics under Mediterranean conditions

Abstract

Agro-industrial composts represent both a sustainable alternative to inorganic fertilizers and a waste recycling management tool. Albeit, there is still a great deal of uncertainty as to what extent their use can contribute to the global N2O budget. Here, two different composts [olive waste “alperujo” (AC) and biosolid (BC)] were tested during a three-year period to evaluate changes in N pools (NH4+ and NO3−), N2O emissions and key N-cycle associated genes [ammonia monooxygenase (amoA) and N2O reductase (nosZ)] in comparison with a control, treated only with mineral fertilization. To that end, a low dose (equivalent to 17.8 kg per tree of AC or BC) and high dose (equivalent to 26.7 kg per tree of AC+ or BC+) was applied to each individual plot composed of nine olive trees without irrigation in a completely randomized design. N2O emissions rates showed a high spatial and temporal variability and no significant changes were detected in N pools when comparing with the control. However, the fact that the soil treated with BC+ presented 20% more NO3− compared to the control after the second compost addition and that the low doses of both composts displayed about 30% less NH4+ than the control, suggests different pathways in N mineralization. Moreover, the N balance showed that soils treated with AC used all N provided and in turn promoted native N consumption while soil amended with BC only consumed part of the exogenous N provided. The assessment of the bacterial amoA and nosZ genes reflected that it was tillage practices rather than fertilization which had the most significant impact on their abundance, decreasing this effect towards the end of the experiment. Our results showed that under Mediterranean conditions the use of agro-industrial byproducts do not contribute to N2O emissions and therefore it could be potentially used as a replacement of inorganic fertilizers favouring a circular economy of zero waste.