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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 CO 2 emissions and microbial biomass in a wheat rhizosphere were investigated in a 56-day incubation experiment. The rhizosphere soil was amended with increasing NH 4 + rates of 0 (Control), 52 (Low N), 104 (Medium N), and 208 μg N g −1 soil (High N). N fertilization exponentially decreased soil CO 2 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 CO 2 fluxes per unit of microbial biomass decreased exponentially with N addition (R 2 = 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. [ABSTRACT FROM AUTHOR]

The response of stable and labile C pools to global warming is uncertain, especially in paddy soils with very low oxygen availability and the dominance of electron acceptors with low efficiency. To clarify the response of organic matter decomposition to warming, flooded paddy soil was incubated at four temperatures (5, 15, 25, and 35 °C) for 75 days. The 13C-labelled Na-acetate was used as an analogue for root exudates and as a methane (CH 4) source. Soil with acetate had higher C availability to microorganisms leading to 2–2.7 times and 2–153 times higher emission of carbon dioxide (CO 2) and CH 4 on day 75 than from soil without acetate, respectively. Incubation temperature explained >40% of the variance of CO 2 and CH 4 effluxes. Acetate stimulated microbial activities and turnover and so, increased soil organic matter (SOM) mineralisation in the first week, especially at low temperatures (<15 °C) with slow acetate consumption and longer oxygen (O 2) availability. The priming effects measured as CH 4 emissions were especially sensitive to temperatures from 5 to 15 °C. The high Q 10 value of primed CH 4 (Q 10 > 10) at low temperature indicates that flooded paddy fields will contribute greatly to the greenhouse effect in warm winters, which have become common from 1970s. Caution is necessary for interpretations of previous estimates of the temperature sensitivity of SOM decomposition because the priming effect was ignored, especially that of CH 4 under the condition of limited O 2 availability in paddy and other wetland soils. [ABSTRACT FROM AUTHOR]