Inhibition of Elevated Atmospheric Carbon Dioxide to Soil Gross Nitrogen Mineralization Aggravated by Warming in an Agroecosystem

Abstract

The response of soil gross nitrogen (N) cycling to elevated carbon dioxide (CO 2 ) concentration and temperature has been extensively studied in natural and semi-natural ecosystems. However, how these factors and their interaction affect soil gross N dynamics in agroecosystems, strongly disturbed by human activity, remains largely unknown. Here, a 15 N tracer study under aerobic incubation was conducted to quantify soil gross N transformation rates in a paddy field exposed to elevated CO 2 and/or temperature for 9 years in a warming and free air CO 2 enrichment experiment. Results show that long-term exposure to elevated CO 2 significantly inhibited or tended to inhibit gross N mineralization at elevated and ambient temperatures, respectively. The inhibition of soil gross N mineralization by elevating CO 2 was aggravated by warming in this paddy field. The inhibition of gross N mineralization under elevated CO 2 could be due to decreased soil pH. Long-term exposure to elevated CO 2 also significantly reduced gross autotrophic nitrification at ambient temperature, probably due to decreased soil pH and gross N mineralization. In contrast, none of the gross N transformation rates were affected by longterm exposure to warming alone. Our study provides strong evidence that long-term dual exposure to elevated CO 2 and temperature has a greater negative effect on gross N mineralization rate than the single exposure, potentially resulting in progressive N limitation in this agroecosystem and ultimately increasing demand for N fertilizer.

Warming-Induced Stimulation of Soil N2O Emissions Counteracted by Elevated CO2 from Nine-Year Agroecosystem Temperature and Free Air Carbon Dioxide Enrichment

“…Similarly, significant increases in SOC and NH 4 + immobilization coupled with a significant decrease in gross nitrification under elevated CO 2 have been reported in the same field after 3 years of field exposure . However, we did not detect increased N immobilization under elevated CO 2 after 9 years of exposure . Moreover, the reduced nitrification and denitrification rates, resulting from the elevated CO 2 and intensified competition for available N between plants and microorganisms, could potentially account for the decrease in soil N 2 O emissions .…”

“…In agreement with our second hypothesis, the combined treatment reduced the effects on both nitrification- and denitrification-derived N 2 O emissions compared with the single warming treatment (Figures and ). This may be explained by elevated CO 2 having an inhibitory effect on gross autotrophic nitrification due to decreasing soil pH and N mineralization (Figure a,i), which prevents the stimulation of nitrification and denitrification by warming. Soil pH and mineralization play important roles in controlling gross autotrophic nitrification by regulating the nitrifier activity and the substrate NH 4 + supply, respectively. − The decrease of about half a pH unit could be one of the contributing factors to the observed reduction in autotrophic nitrification through inhibiting nitrifiers .…”

“…19 However, we did not detect increased N immobilization under elevated CO 2 after 9 years of exposure. 55 Moreover, the reduced nitrification and denitrification rates, resulting from the elevated CO 2 and intensified competition for available N between plants and microorganisms, could potentially account for the decrease in soil N 2 O emissions. 10 Nevertheless, the anticipated reductions in NH 4 + and NO 3 − contents under the CO 2 treatment were not observed in this study (Figure 4f,g).…”

Fe(II) Oxidation Shaped Functional Genes and Bacteria Involved in Denitrification and Dissimilatory Nitrate Reduction to Ammonium from Different Paddy Soils

“…A paddy soil with ∼40 mmol/kg Fe(II) showed faster nitrate reduction and emitted less N 2 O than did the other paddy soil with one-half of the Fe(II) concentration, 19 and chemodenitrification, the abiotic reaction between Fe(II) and NO 2 − , accounted for 6.8% ∼ 67.6% of the total N 2 O emissions in paddy soils and estuarine/coastal sediments. 20,21 The addition of 5 mM Fe 2+ to 15 N-NO 3 − experiments with estuarine sediments results in a reduction in denitrification, accounting for only 6% of nitrate reduction, and increases in DNRA and Fe 2+ removal rates. 22,23 On the other hand, both denitrification and DNRA rates in lake sediment microcosms are found to be stimulated by Fe(II) addition.…”

“…Microbial nitrate reduction is prevalent in soil and aquatic environments under anoxic conditions, which is one of the important processes driving nitrogen biogeochemical cycling and the associated transformation of other elements such as carbon. − Generally, nitrate-reducing bacteria require organic carbon to serve as energy source/electron donors and utilize NO 3 – as an electron acceptor in two different respiratory processes: denitrification and dissimilatory nitrate reduction to ammonium (DNRA) . While denitrification results in gaseous losses in the form of nitrogen gas (N 2 ) and the greenhouse gas nitrous oxide (N 2 O), DNRA can lead to conservation of nitrogen via NH 4 + production. , Therefore, microbial nitrate reduction strongly influences the carbon transformation, greenhouse gas emission, and nutrient availability to microorganisms and plants. , …”