Restoration of nitrogen cycling community in grapevine soil by a decade of organic fertilization

Traditional intensive agricultural practices negatively impact soil quality, leading to increased erosion in the Mediterranean region. In contrast, sustainable management practices that have utilized organic fertilization over a decade improved soil quality in the Spanish "Pago Casa Gran" vineyard, Valencia. Grape prunings, combined with sheep manure or leguminous cover, have improved soil chemical and biological parameters, such as organic carbon, nitrogen content, respiration, PLFA, total DNA and aggregation, however, there has been a lack of information on their effect on specific functional microbial groups. The nitrogen cycling community of the soil was investigated by quantification of genes involved in key pathways, in particular nitrogen fixation, denitrification and nitrification. The abundances of bacterial nifH, nosZ, nirS and nirK genes significantly increased under a decade of organic fertilization when compared to inorganic fertilization, and was linked to increased soil organic carbon. The abundance of nifH was lower where fertilizers rich in ammonia and nitrate were used and its increase under organic fertilization was more related to the availability of organic carbon than to the nature of the organic amendment. Archaeal amoA abundance did not correlate with the type of fertilization and the bacterial amoA abundance was more dependent on the availability of soluble nitrogen than on the type of management. An increase in nosZ under organic fertilization may suggest a greater abundance of denitrifiers with the ability to reduce nitrous oxide to nitrogen gas, reducing greenhouse gas emission. Our results indicate that soil microbial communities involved in biological nitrogen cycling, in particular nitrogen fixation and de nitrification, are more abundant under management practices that include organic fertilization when compared to traditional agricultural practices. It may therefore be recommended that organic fertilization be utilized in agricultural systems to assist with sustaining healthy soils.