Effects of the natural restoration time of abandoned farmland in a semiarid region on the soil denitrification rates and abundance and community structure of denitrifying bacteria

Abstract

Denitrification accounts for the production of mobile forms of nitrogen (N) for plant uptake, N leaching, and gaseous losses. However, few studies have investigated the potential effects of the natural restoration age on denitrification rates and denitrifying microorganisms, especially in fragile ecosystems in semiarid regions. The potential N gas (N₂O and N₂) emissions and denitrification rates significantly decreased after abandonment (< 9 years) compared to those of active farmland and then steadily increased as the restoration proceeded, leading to an enhanced soil N loss. The total bacterial and napA gene abundances significantly decreased after abandonment (< 9 years) compared to that of farmland and then significantly increased as the restoration proceeded. The abundances of the narG, nirK, nirS, qnorB, and nosZ genes steadily increased with the restoration age of abandoned farmland. The community compositions of denitrifying bacteria exhibited different fluctuating patterns, suggesting different response patterns of community traits of N gas emission–related functional guilds to the restoration age of abandoned farmland. Changes in N gas emissions and in the abundance and diversity of denitrifying microorganisms exhibited similar patterns, suggesting an increased population and diversity of denitrifying bacteria are responsible for the enhanced N gas emissions. We observed clear patterns of plant coverage and denitrifying microorganisms that were associated with increases in the organic C, NH₄⁺-N, and NO₃⁻-N contents and decreases in the soil bulk density as well as increases in the abundance and diversity of denitrifiers with the restoration age of abandoned farmland that were linked to an increase in N gas emissions. It is therefore recommended that effective measures (i.e., modest levels of grazing) may be able to be undertaken to assist with decreasing greenhouse gas nitrous oxide (N₂O) and N loss after 32 years of farmland abandonment.

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