Role of amendments on N cycling in Mediterranean abandoned semiarid soils

Soils found in semiarid areas of the Mediterranean Basin are particularly prone to degradation due to adverse climatic conditions with annual rainfall <300 mm and high temperatures being responsible for the scant vegetal growth and the consequent lack of organic matter. A three-year field experiment was conducted to test the potential of two organic amendments (sludge and compost) to improve soil quality and plant growth in a semiarid degraded Mediterranean ecosystem. Since little is known about N dynamics in such assisted ecosystems, we investigated the effects of this practice on key processes of the global N cycle. Besides soil chemical and biological parameters and vegetation cover, we measured absolute and specific potential nitrification and denitrification rates and quantified the size of the ammonia oxidising and denitrifying bacterial populations via quantitative PCR (amoA and nirS genes). At the end of the experiment soil fertility, microbial activity and plant growth had improved in treated plots. Amendments increased the amount of ammonia oxidisers and denitrifiers in soil, but the relative proportion of these groups varied in relation to the total microbial community, being higher in the case of ammonia oxidisers but not in the case of denitrifiers. As a consequence, significantly higher potential nitrification and denitrification rates were measured on a global basis in amended soils. Yet specific activities (potential rate/gene copy numbers) were lower for ammonia oxidisers in amended soils and for denitrifiers in sludge treated soils than those observed in control plots. Organic amendments influenced resource availability, the size and the activity patterns of microbial populations involved in long-term N dynamics. Therefore N cycling processes may play a key role to assist sustainable restoration practices in semiarid degraded areas.