General patterns of responses
Three main processes produce nitrous oxide in soils: 1) nitrification, the stepwise oxidation of NH3 to nitrite (NO2−) and to nitrate (NO3−) (Kowalchuk and Stephen, 2001); 2) denitrification, the stepwise reduction of NO3− to NO2−, NO, N2O and ultimately N2, where facultative anaerobe bacteria use NO3− as an electron acceptor in the respiration of organic material under low oxygen (O2) conditions (Knowles, 1982); and 3) nitrifier denitrification, which is carried out by autotrophic NH3-oxidizing bacteria and the pathway whereby NH3 is oxidized to nitrite NO2−, followed by the reduction of NO2− to nitric oxide NO, N2O and molecular nitrogen (N2) (Wrage et al., 2001).
Field studies have observed increased soil N2O flux following wetting in cropland (Barton et al., 2008), grazing pasture (Kim et al., 2010a), forest (Butterbach-Bahl et al., 2004), grassland (Hao et al., 1988), savannah (Martin et al., 2003) and fen (Goldberg et al., 2010a). Laboratory incubation experiments with cropland soil (Beare et al., 2009), forest soil (Dick et al., 2001), grassland soil (Yao et al., 2010) and peatland soil (Dinsmore et al., 2009) have yielded similar results of increased N2O flux after rewetting. In tropical soils in Costa Rica, N2O flux pulses began within 30 min, peaking no later than 8 h after rewetting and 25 g N2O−N ha-1 was emitted for three simulated rain events over a 22-day period (control emitted 14 g N2O−N ha-1) and one episodic N2O production event driven by one moderate rain accounted for less than 15% to more than 90% of the total weekly production (Nobre et al., 2001). These studies have observed a short-term (ca., 12 h) up to 15 d N2O response following rewetting (Table 2), and an increase of N2O flux up to 80,000% with respect to the background conditions (Table 2, Fig. 2). Importantly, even a single wetting event can affect annual N2O flux (from 2% up to 50%) (Nobre et al., 2001; Barton et al., 2008; Goldberg et al., 2010a).
Mechanisms and drivers
The mechanisms responsible for increased N2O flux following rewetting have been commonly hypothesised as belonging to two categories: 1) enhanced microbial metabolism by substance supply, and 2) the physical mechanisms described above (§3.1). The relatively importance of processes responsible for N2O fluxes changes (i.e., nitrification, denitrification and nitrifier denitrification) is poorly understood, however, although several studies have found denitrification to be the most important (Groffman and Tiedje, 1988; Priemé and Christensen, 2001).
Magnitudes of increased N2O flux caused by wetting of dry soils vary depending on the labile N soil pool (Van Gestel et al., 1993; Schaeffer et al., 2003), soil texture (Appel, 1998; Austin et al., 2004), soil water content (Appel, 1998), size of the rewetting pulse (Ruser et al., 2006; Yanai et al., 2007), length of drought (van Haren et al., 2005), and soil compaction (Uchida et al., 2008; Beare et al., 2009). A significant relationship between the organic nitrogen extracted from dried soil samples and the magnitude of N2O flushes following soil drying-rewetting has been observed (Appel, 1998). Field and laboratory studies with arid and semiarid soils, fine-textured soils having higher water-holding capacity and labile C and N pools compared to coarse-textured soils showed greater flush of N2O flux following rewetting (Austin et al., 2004). In an incubation experiment with soils from potato field, the amount of increase in N2O flux following rewetting enhanced with the amount of water added (Ruser et al., 2006). Furthermore, in another experiment with soils from a field compaction trail, the production of N2O during the first 24 h following rewetting of dry soil was nearly 20 times higher in compacted than in uncompacted soil (Beare et al., 2009).
Xu, X., Luo, X., 2012. Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season. Geoderma 170, 118-126
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The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ, rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO2 pulses increased from 0.84 to 2.08 g CO2–C m− 2 d− 1 with the intensity of wetting up to ca. 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH4–C m− 2 d− 1 with the intensity of wetting. Wetting dry forest floor increased N2O fluxes from 6.2 to 25.9 μg N2O–N m− 2 d− 1, but there was no significant difference between all experimental wetted plots. The rainfall-induced N2O pulses with increasing wetting intensity were opposite to that of the CO2 pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO2 flux and CH4 uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q10 value of the temperature response of CO2 fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO2 pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.
Highlights
► Wetting intensities were manipulated by subsequently maintained soil moisture levels. ► In situ GHG fluxes from forest floor differently responded to wetting intensity. ► Wetting increased soil microbial biomass and metabolic quotient differently. ► Wetting could significantly affect the sizes of soil active C and N pools.
Harrison-Kirk, T., Beare, M.H., Meenken, E.D., Condron, L.M., 2012. Soil organic matter and texture affect responses to dry/wet cycles: Effects on carbon dioxide and nitrous oxide emissions. Soil Biology and Biochemistry, http://dx.doi.org/10.1016/j.soilbio.2012.1010.1008
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Soil organic matter (SOM) content and texture are important factors affecting carbon (C) and nitrogen (N) mineralisation under constant soil moisture but their effects on organic matter mineralisation and associated biogenic gas (carbon dioxide (CO2) and nitrous oxide (N2O)) production during dry/wet cycles is poorly understood. A laboratory incubation study was conducted to quantify CO2 and N2O production during sequential dry/wet cycles and under constant soil moisture conditions along a gradient of SOM contents in two soil types representing different texture classes (silt loam vs. clay loam). Three soil moisture treatments were established: wet (WW; field capacity), moderately dry (MD; 120% of soil moisture content (SMC) at wilting point (WP)) and very dry (VD; 80% of SMC at WP). To each of the two ‘dry’ treatments two different dry/wet treatments were applied where the soils were either maintained continuously dry (MD & VD) or subjected to three sequential 20-day long dry/wet cycles (MDW & VDW) during the treatment phase of the experiment. At field capacity soil moisture content, the rate of C mineralisation increased with increases in SOC content and the increase per unit of C was twice as high in silt loam (0.30 mg CO2-C g−1 SOC d−1) as in clay loam (0.13 mg CO2-C g−1 SOC d−1) soils. N2O-N emissions also increased with increasing in SOC content. However, in contrast to C mineralisation, the effect was four-fold greater for clay loam (1.38 μg N2O-N g−1 SOC d−1) than silt loam (0.32 μg N2O-N g−1 SOC d−1) soils. Following rewetting, the VDW and MDW soils produced a short-term C mineralisation flush that was, on average, 30% and 15% greater, respectively, than in WW soils. However, the flush of C mineralisation was not sufficient to compensate for the reduction in mineralisation during the drying phase of each cycle, resulting in a lower total C mineralisation from MDW and VDW soils, on average, compared with WW soils over the three sequential dry/wet cycles. The C mineralisation flush also remained a relatively constant proportion of the total C mineralised from both silt loam (23%) and clay loam soils (22%), irrespective of their SOC content. In contrast, the short-term flush of N2O that followed rewetting of dry soil accounted for 62% and 68% of the total N2O emissions from silt loam and clay loam soils, respectively. On average, the total N2O emissions from dry/wet treatments imposed on silt loam and clay loam soils were 33% and 270% greater, respectively, than from the WW treatments, though the effect varied greatly and depended on SOC content. Overall, N2O emissions were highest where we had a combination of fine texture, an adequate supply of available C (i.e. high SOM content), and a water-filled pore space (WFPS) > 0.60 cm cm−3 at field capacity. Prediction of C mineralisation over dry/wet cycles using mineralisation data from soils at constant moisture content is possible, but knowledge of the stress history for the soil would be required to improve accuracy. The prediction of N2O-N emissions during dry/wet cycles using emission data from soils at constant moisture was very inaccurate, due to the inherent spatial variability of N2O emissions.
Highlights
► C mineralisation increased with increasing SOC, silt loam twice clay loam. ► N2O emissions increased with increasing SOC, clay loam four times silt loam. ► CO2 and N2O response to dry/wet cycles depended on SOC content. ► CO2 flush following rewetting was 22–23% of the total C mineralised. ► N2O flush following rewetting was 62–68% of total N2O emissions.
Hartmann, A., Niklaus, P., 2012. Effects of simulated drought and nitrogen fertilizer on plant productivity and nitrous oxide (N2O) emissions of two pastures. Plant and Soil 361, 411-426
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Aims
As a consequence of global climate change, increases in the frequencies and severities of drought are anticipated for many parts of the world. Soil moisture and nitrogen (N) are among the major factors limiting grassland productivity. In pastures, N fertilizer returns by grazing animals are spatially and temporally heterogenous, and we therefore hypothesized that responses of plants and soil processes to drought may differ at the patch level.
Methods
Using rain-exclusion roofs, we simulated severe summer drought in a three-year field experiment replicated at two grassland sites contrasting in climate and management intensity. The study included a factorial N application treatment encompassing the application of cattle urine and mineral nitrogen. Responses of plants, soil microbes, and soil organic matter were assessed (carbon and nitrogen pools). N2O emissions were measured on 72 dates, and soil N2O concentration profiles on 44 dates.
Results
Plant productivity responded negatively to drought and positively to N application. Interestingly, no or only small drought-effect were found on plant productivity when cumulated over the entire experimental duration, despite large effects during and shortly after the period when rain-exclusion roofs were installed. We further did not find evidence for compensatory growth after drought, and drought-effects did not differ between fertilizer hot spots and unaffected areas. In the short-term, soil microbial biomass responded positively to drought, but no long-term effects were detected. Nitrous oxide (N2O) emissions originated primarily from fertilizer hot spots, and these emissions were massively reduced under drought, with effects lasting throughout most of the growing season. On a growing season basis, N2O emissions were estimated to be 1 to 2 orders of magnitude lower under drought.
Conclusions
Overall, our data suggest that even severe summer drought may have relatively little effect on plant productivity in the type of grassland and climate investigated, at least when considered on an annual basis. In contrast, drought may result in a large and sustained reduction of N2O emissions.
Liu, J., Hou, H., Sheng, R., Chen, Z., Zhu, Y., Qin, H., Wei, W., 2012. Denitrifying communities differentially respond to flooding drying cycles in paddy soils. Applied Soil Ecology 62, 155-162
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Paddy soils are an important source of nitrous oxide (N2O) emission, especially during frequent flooding and drying cycles. The N2O flux from paddy soils is mainly driven by denitrifying microorganisms, but the response of denitrifying communities to flooding drying cycles has been little studied. N2O emission was monitored under laboratory conditions in two paddy soils. Quantitative PCR (qPCR) and terminal restriction fragment length polymorphism (T-RFLP) were used to determine the abundance and community composition of narG- and nosZ-genes in denitrifiers. The N2O emission was more significantly related to soil Eh than soil water content during the drying process. Significant increases in the copy number and obvious alterations in the community composition of both narG- and nosZ-containing denitrifiers were detected after only one day of drying. Among the two denitrifying communities, narG gene abundance was significantly correlated to both Eh and water content, whereas nosZ was only significantly correlated to water content during the flooding drying process, indicating that different responses among various denitrifiers occurred in flooding and drying cycles. Furthermore, narG copy number varied following the flooding drying cycles, where drying caused an obvious increase and flooding caused a decrease in numbers. However, despite the first drying phase resulting in a significant increase in nosZ copy number, further flooding and drying cycles did not cause any remarkable changes compared to the first drying. The narG-containing denitrifiers were much more closely correlated with the N2O flux than nosZ-containing communities in the flooding drying cycles in the paddy soils studied here.
Highlights
► Eh is more significantly related to N2O flux from paddy soil than water content. ► narG- and nosZ-containing denitrifiers respond quickly to drainage. ► narG and nosZ gene abundance are significantly related to water content. ► narG gene abundance is significantly correlated to Eh changes. ► narG gene is more directly linked to the N2O flux in drainage process than nosZ.
Decock, C., Six, J., 2013. An assessment of N-cycling and sources of N2O during a simulated rain event using natural abundance 15N. Agriculture, Ecosystems & Environment 165, 141-150.
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In order to accurately predict N2O emissions from agricultural soils and to develop effective management strategies, it is important to understand mechanisms underlying N2O emissions under field conditions. This involves identification of sources of N2O, which is currently methodologically challenging, especially under field conditions. We assessed the suitability of 15N tracers and natural abundance 15N to study N cycling and sources of N2O after a rainfall simulation in an annual cropping system in the Central Valley of California. Our natural abundance 15N approach differed from other studies due to a combination of emphasizing a per-event (e.g. rainfall simulation in this study) assessment of N2O emissions, applying high temporal sampling frequency during this event, determination of 15N of NH4+ and NO3− in addition to N2O, and data analysis using isotope models. In our study, the suitability of 15N tracers to assess N cycling and sources of N2O emissions was limited, likely due to a combination of a fine soil texture, the use of undisturbed soil cores, and a low 15N application rate. Based on natural abundance 15N, we were able to calculate gross NH4+ mineralization, NH4+ immobilization, nitrification and NO3− immobilization rates of 5.37 ± 1.72, 2.70 ± 1.72, 3.01 ± 1.13 and 0.15 ± 0.29 μg N g−1 soil d−1, respectively. Natural abundance 15N was, however, a rather poor predictor of the contribution of nitrification versus denitrification to N2O production. Nevertheless, important trends in N2O reduction rates could be observed, showing a sharp increase from 48% to 78% in reduction of produced N2O between 2 hours and 24 hours after rainfall simulation, followed by a gradual decrease to 46% of reduction by the fifth day after rainfall simulation. We conclude that the natural abundance 15N approach is very promising to elucidate mechanisms driving N-cycling and N2O emissions during agricultural management or weather events, especially if isotope dynamics are incorporated in site-specific biogeochemical process models.
Peralta, A.L., Ludmer, S., Kent, A.D., 2013. Hydrologic history influences microbial community composition and nitrogen cycling under experimental drying/wetting treatments. Soil Biol. Biochem. 66, 29-37.
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Understanding how historical and contemporary environmental conditions interact to affect the relationship between community structure and function is necessary to predict the response of microbial community function to a changing environment. We focused on nitrogen transformations – denitrification and nitrification – which directly impact nitrate concentration in wetland ecosystems. Nitrate removal can occur via denitrification, while nitrate can be generated through nitrification; these two microbial transformations are strongly influenced by hydrology and redox conditions. We carried out a manipulative mesocosm experiment to determine how microbial communities adapted to different hydrologic conditions (upland vs. wetland) respond to experimental soil moisture treatments (dry, wet-dry, saturated). We assessed local soil characteristics (e.g., soil moisture, inorganic nitrogen), and microbial community composition and activity of denitrifiers and ammonia oxidizers (targeted by the nosZ and amoA genes, respectively) before and after moisture treatments. Wetland soils were characterized by higher soil fertility compared to upland soils. In response to the drying/flooding treatments, we observed a small but significant change in community composition of denitrifier assemblages, but no change in the ammonia oxidizer communities. In addition, potential denitrification rates significantly increased under wetter conditions (upland: 62–118% increase; wetland: 78–96% increase), whereas potential nitrification rates generally showed no significant change following experimental drying/flooding treatments, regardless of the hydrologic history. Based on these results, fluctuations in soil moisture are expected to influence denitrification rates to a greater extent than nitrification rates, ultimately influencing nitrate pools in this wetland. This imbalance in microbial functional response may result in a shift in dominant nitrogen cycling transformations within a wetland as a consequence of the different responses of these nitrogen-cycling functional guilds to changes in soil moisture. A shift in nitrogen transformations can be most noticeable under fluctuating hydrologic conditions, and there is potential for the wetland to be resilient to hydrologic change if soil microbes are exposed to dynamic hydrology over the long-term.
Chen, C., Whalen, J.K., Guo, X., 2013. Earthworms reduce soil nitrous oxide emissions during drying and rewetting cycles. Soil Biol. Biochem.http://dx.doi.org/10.1016/j.soilbio.2013.09.020
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Nitrous oxide (N2O) is a greenhouse gas that is released from both nitrification and denitrification processes. Soil moisture content is a key controller of the biochemical pathways leading to N2O emission, causing a switch between nitrification and denitrification processes. Earthworms are reported to increase N2O emissions from soil under aerobic and anaerobic conditions, but how earthworm-induced N2O emissions are affected by soil drying and rewetting cycles is unknown. The objectives of this study were to (1) evaluate earthworm-induced N2O emissions from soils with aerobic, anaerobic, and fluctuating soil moisture conditions; and (2) determine the earthworm effects on soil denitrifiers responsible for N2O fluxes. Soils were kept in mesocosms (polyvinyl chloride plastic tubes, 10 cm diameter, filled with soil to 15 cm depth) at constant 33% water-filled pore space (WFPS), constant 97% WFPS or underwent three wetting-drying cycles (WD). Each soil moisture treatment had 2 earthworm treatments, including (1) a mixture of endogeic Aporrectodea turgida and anecic Lumbricus terrestris and (2) no earthworm treatment. These gave a total of 6 treatments in this study, with 5 replicates for each treatment. The N2O fluxes were quantified every one to three days, and the soil denitrifier activities were measured after 69 days, when the experiment ended. Soil moisture significantly affected N2O emissions and the WD treatment had the highest cumulative N2O emissions. Earthworms increased N2O emissions by 50% in the 33% WFPS treatment but decreased N2O emissions by 34% in the 97% WFPS treatment, probably due to more complete reduction of N2O to N2. Earthworms strongly reduced N2O emission rate in WD treatment, and they significantly reduced cumulative N2O emissions by 82%. Soil denitrification enzyme activity (DEA) increased significantly when earthworms were present. Abundance of 16S rRNA, nirS, and nosZ genes was affected significantly by the earthworm × soil moisture interaction, with the highest 16S rRNA and nosZ abundance in soil from the WD treatments. We conclude that the decrease in cumulative N2O emissions from soil at 97% WFPS and the WD treatment by earthworms was due to an alteration of the denitrifying bacterial community composition.
Krüger, J.P., Beckedahl, H., Gerold, G., Jungkunst, H.F., Greenhouse gas emission Krüger, J.P., Beckedahl, H., Gerold, G., Jungkunst, H.F., Greenhouse gas emission peaks following natural rewetting of two wetlands in the southern Ukhahlamba-Drakensberg Park, South Africa. South African Geographical Journal, DOI: 10.1080/03736245.2013.847798
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The global importance of wetlands in the carbon and nitrogen cycles is well documented, but the specific greenhouse gas characteristics of South African wetlands are less well known. These wetlands most likely differ from more prominent wetlands from continuously humid climate zone (boreal, temperate and tropics). Particular wetlands in the southern Drakensberg are adapted to the seasonal drying during the winter months. Greenhouse gas emissions were measured during natural rewetting at two wetlands. A rapid reaction and significant positive correlation between greenhouse gas fluxes and ground water level were determined. Methane emissions were observed after two days of rewetting at one of the wetlands, and nitrous oxide emissions started within a day of rewetting at the other wetland. The high nitrous oxide emissions may be caused by the recent winter burning of vegetation, which most likely resulted in a greater availability of nitrogen in the soil. High nitrous oxide emissions following natural rewetting (the annual cyclical process in these wetlands) could contribute significantly to the local greenhouse gas budget. Hence, besides the methane emissions, the nitrous oxide emissions of wetlands in southern Africa should be taken into account.
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ReplyDeleteUrakawa, R., Shibata, H., Kuroiwa, M., Inagaki, Y., Tateno, R., Hishi, T., Fukuzawa, K., Hirai, K., Toda, H., Oyanagi, N., Nakata, M., Nakanishi, A., Fukushima, K., Enoki, T., Suwa, Y., Effects of freeze-thaw cycles resulting from winter climate change on soil nitrogen cycling in ten temperate forest ecosystems throughout the Japanese archipelago. Soil Biol. Biochem. 74, 82-94.
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In temperate forest ecosystems, accelerated freeze–thaw cycles caused by winter climate change are expected to affect nitrogen (N) cycling in soils. Net N mineralization and nitrification rates were investigated via incubations of sieved soils transplanted from ten temperate forest ecosystems to two northern Japan sites with natural snowfall gradients. This was done to address: 1) how freeze–thaw cycles affect N mineralization and nitrification in temperate forest soils; 2) whether freeze–thaw cycles change the soil N transformation rates in the following growing season; and 3) which soil characteristics affect the response of the N transformation rates to freeze–thaw cycles. The effect of freeze–thaw cycles on inorganic N and dissolved organic carbon productions differed among soils, that is, some soils produced more inorganic N and dissolved organic carbon in the conditions imposed by freeze thaw cycles than in the non-frozen treatment but the others did not. The response to the freeze–thaw cycles was explained by soil microbial activity (gross N mineralization and nitrification rate) and soil fertility (inorganic N pools in the early spring and water soluble ions). Freeze–thaw cycles significantly increased N transformation rates in the following growing season, suggesting that winter climate change might also affect nutrient availability for vegetation and soil microbes in the growing season. The magnitude and frequency of freeze–thaw cycles were considered to be important indicators of N transformation rates during the growing season, suggesting that the higher intensity of freeze–thaw cycles in the original locations of soils changed the microbial communities and functions with high tolerance to freeze–thaw cycles; this resulted in greater N transformation rates in the following growing season. Microbial activity, soil fertility and climate patterns in the original locations of soils are believed to have an effect on the response to winter climate change and to cause large variability of soil response of N transformation rates to freeze–thaw cycles in both the dormant and growing seasons.