N fertilization decreases soil organic matter decomposition in the rhizosphere

- Increasing N fertilization facilitates C sequestration in the rhizosphere.
- Negative priming effect induced by N and decreased exponentially with increasing N fertilization.
- Nitrogen fertilization decreased MBC, but depended on N amount and sampling time.
- Microbial carbon use efficiency in the rhizosphere increased with N fertilization.

Agricultural soils have experienced large anthropogenic nitrogen (N) inputs in recent decades. Our mechanistic understanding of the effects of added N on the carbon (C) cycle in agricultural soils, especially in the rhizosphere (C excess and N limitation), remains incomplete. The effects of increasing N fertilization on soil CO2 emissions and microbial biomass in a wheat rhizosphere were investigated in a 56-day incubation experiment. The rhizosphere soil was amended with increasing NH4+ rates of 0 (Control), 52 (Low N), 104 (Medium N), and 208 μg N g−1 soil (High N). N fertilization exponentially decreased soil CO2 emissions by 27-42% compared to the control. Microbial biomass was decreased by N fertilization, but depended on the amount of added N and the timing of measurements. N additions caused pronounced negative priming effects ranging from −72 to −113 μg C g−1 over 56 days, corresponding to a decrease in basal respiration of 27%, 35% and 42% for Low, Medium and High N, respectively. The CO2 fluxes per unit of microbial biomass decreased exponentially with N addition (R2 = 0.84), indicating increased microbial carbon use efficiency under higher N availability. A literature review and own results showed that negative PEs occurred in the most cases and getting more negative exponentially with increasing N fertilization (n = 158, P < 0.001). In conclusion, increasing N fertilization facilitates C sequestration in soil not only by higher root biomass production, but also by reducing the SOM decomposition in the rhizosphere because of decreased N limitation.

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