. 2021 Feb 12;9:e10767. doi: 10.7717/peerj.10767

Table 1. Summary of nitrogen dynamics during dry-wet cycles (cited literature): Settings, N-cycling pathways, nitrogen species and N₂O emissions.

Ref	Settings	N dynamics			Observations
		N processes	Mineral N	N₂O emissions
Lentic systems
Kern et al. (1996)	Seasonal floodplain lake (Brazil) Aquatic/terrestrial interface: exposed sediments (in situ incubation) vs. flooded sediments (laboratory incubation) N₂O⁽¹⁾ (in situ field measurements, glass domes inserted into the sediment; laboratory incubation measurements, flasks)	Denitrification: - Higher rates in exposed sediments (increase with distance to the water line) - Flooded sediments: detected only during low water	NO $_{3}^{-}$ No accumulation in exposed sediments (pore water) NH $_{4}^{+}$ High in exposed sediments (pore water)	Exposed sediments (increase with distance to the water line; average rate; MC >50%/waterlogged): - 1 m from the water line: 256.2 µg N m⁻² h⁻¹ - 5 m from the water line: 396.2 µg N m⁻² h⁻¹ Flooded sediments: not detected	Denitrification rates in flooded sediments 1 –2 orders of magnitude smaller than in temperate regions. Nitrate removal of exposed sediments higher than in undisturbed wetland soils on temperate regions. Strong coupling of nitrification and denitrification in exposed sediments. High N release during transition from the aquatic to the terrestrial ecotope (high impact on gaseous N turnover)
Merbach et al. (2002)	Kettle holes (NE, Germany) Sampling along a depositional sequence with different hydrological conditions: periodically flooded (terrestrial zone), drying-out periodically (lower bank), and drying-out episodically (shallow water zone) N₂O (in situ field measurements, static flux chambers)	_	_	Decreased with higher water level and increased with lowering of the water table Generally higher in the terrestrial zone	Environmental factors (water level) superimposed relations between available N compounds and trace gas emissions
Huttunen et al. (2003)	Boreal lake (Finland) Sampling sites: temporarily flooded littoral zone (A), upper littoral infralittoral (B), continuously flooded littoral zone (C) Year 1997 (extremely dry): water table <0 (site A) and <0 (site B and C); Year 1998 (extremely wet): water table >0 (all sites) N₂O⁽²⁾ (in situ field measurements, static flux chambers)	_	_	Site A and B (1997): N₂O emissions in the driest part of the littoral zone ranged from 11 ± 7 to 22 ± 7 µg N m⁻² h⁻¹; Peaks = 140 µg N m⁻² h⁻¹ (site A) and 59 µg N m⁻² h⁻¹ (site B) Site C: N₂O flux near zero Higher emissions during the dry vs. wet summer 15 (32) ± 8 µg N m⁻² h⁻¹ and6 (15) ± 2 µg N m⁻² h⁻¹, respectively (mean, median, standard deviation)	The littoral zone occupied 26% of the lake area but was estimated to account for most of the N₂O emissions from the lake.
Koschorreck (2005)	Seasonal floodplain lake (Brazil) Sampling: shallow part which dries out for ≈ 3 months Dry period: 55 d N₂O (in situ field measurements, glass domes inserted into the sediment)	Denitrification: - Lag phase [0 –2 d] - Increase [2 –5* d] - Peak: 574 µg N m⁻²h⁻¹) DEA (surface sediments): - Increase [0 –5* d] - Peak: 406 µg N h⁻¹ g DW⁻¹	NO $_{3}^{-}$ Low content, little change with depth and time NH $_{4}^{+}$ - Decrease [0 –40 d] - Constant low [40 –55 d]	Lag phase [0 –2 d] Increase [2 –5* d]; Peak: ≈ 245 µg N h⁻¹ m⁻²	Highest inorganic N loss occurred during the first 10 days of the drying period. Inorganic N loss was higher in the deeper (1 –5 cm) vs. surface layer (0 –1 cm). Cell numbers of nitrate reducers increased in the deeper layer. Coupled nitrification-denitrification was the main mechanism of N removal in the sediments, with peak activity shortly after drying.
Hernandez & Mitsch (2006)	Artificial marsh (Ohio, USA) Sampling along a transect with different hydrological conditions: dry (edge zone), alternate dry-wet (high marsh zone) and permanently flooded (low marsh zone) sediments N₂O⁽²⁾ (in situ field measurements, static flux chambers)	_	NO $_{3}^{-}$ Low marsh <open water <high marsh <edge zones NH $_{4}^{+}$ Low marsh >open water >high marsh >edge zones Total N Similar between low marsh, high marsh and open water and significantly higher than in the edge zone	Edge zone (dry): - Before flood pulse 4.1 ± 1.8 µg N m⁻² h⁻¹(significantly lower) - During flood pulse: 11.3 ± 3.1 µg N m⁻² h⁻¹ - After the flood pulse: 7.3 ± 3.3 µg N m⁻² h⁻¹ High marsh (dry-wet): - Before flood pulse: 2.4 ± 6.5 µg N m⁻² h⁻¹ [WL = -0.22 m] - During flood pulse: 6.9 ± 2.2 µg-N m⁻² h⁻¹ [WL = 0.16 m] - After flood pulse: 25.9 ± 13.8 µg-N m⁻² h⁻¹ (significantly higher) [WL = -0.09 cm, just below the surface] Low marsh zone (permanently flooded): low flux rates, regardless of the flood pulse condition	Surface flooding was infrequent at the edge zone (dry) but ground-water levels changes affected emissions. Low and high marsh: the total nitrogen content was significantly lower in deeper (8–16 cm) vs. top (0–8 cm) layer.
Fromin et al. (2010)	Mediterranean seasonal pond (SE France) Sampling at different periods of drying and rewetting: drainage, dry, rainfall (after several weeks of drying) Dry period: 8 w Wet period: rain event; 48 h	NEA (dry period): - Null or very low (Surface sediments); 0 –0.4 µg nitrified-N g⁻¹ DW h⁻¹ - Increase (Subsurface sediments); 0.6 –1.1 µg nitrified-N g⁻¹ DW h⁻¹ NEA (rain event): - Increase (Surface sediments) - ns (Subsurface sediments) DEA (dry period): - Low in flooded sediments (0.2 –0.6 µg N₂O-N g⁻¹ DW h⁻¹) - Increase [0 –5 w]; 0.8 –2.6 µg N₂O-N g⁻¹ DW h⁻¹(Surface sediments) - Decrease [6 –8 w]; mean value: 1 µg N₂O-N g⁻¹ DW h⁻¹ (Surface sediments) DEA (rain event): - Increase (Surface sediments) - ns (Subsurface sediments)	Total N Increase during drying period	_	Proportion of N potentially denitrified as N₂O was positively correlated to the duration of dry period. Rain triggered surface DEA yet values were not significantly higher than the maximum rates observed during mid-drought period. DEA rates after the rain event were higher in pond margins sites (hot spots; McClain et al., 2003). Subsurface sediments (2-10cm) were less affected by MC variations than surface (0-2cm) sediments. Bacterial density was significantly lower in the surface vs. subsurface layer.
Akatsuka & Mitamura (2011)	Lake (Japan) Sampling sites: littoral wetland sediments in the flooded and exposed region; sediments in the exposed region selected at various distances from the water line (varying degrees of dryness) Sediment cores from the flooded region were exposed to the atmosphere (10 days; drying treatment) and sediment cores from the exposed region were flooded (2 weeks; wetting treatment) N₂O measured from the denitrification assay under no addition of acetylene (units of volume)	Potential denitrification rate (with nitrate addition) higher than the denitrification rate (no nitrate addition) Potential denitrification rates and denitrification rates higher in the exposed vs. flooded region	NO $_{3}^{-}$ - Undetectable in the flooded region - Concentration increased with the degree of sediment dryness in the exposed region NH $_{4}^{+}$ Concentration increased with wetting and decreased with drying	With nitrate application: - Before and after wetting treatment: 0.74 and 0.32 µg N cm⁻³h⁻¹, respectively - Before and after drying treatment: 0.031 and 0.38 µg N cm⁻³h⁻¹, respectively Negligible under no nitrate application	Nitrous oxide emission/denitrification ratios decreased in the wetting treatment (from 55 to 23%) and increased in the drying treatment (from 18 to 70%)
Wang et al. (2012)	Lake (N China) Sampling along a littoral gradient: deep sediment (A), near-transition sediment (B), transition site (C), near-transition land (D) and land soil (E) N₂O measured from the denitrification assay under no addition of acetylene	Potential denitrification rate in sediments (A and B) 25 times higher than soils (D and E)	NO $_{3}^{-}$ Content significantly lower in sediments than soils NH $_{4}^{+}$ Deep sediment and near-transition sediment: 150 times higher content than soils	Potential N₂O production rate in sediments 3.5 times higher than soils	The N₂O/(N₂O+N₂) ratio of sediments was seven times lower than in land soils (higher proportion of N₂O transformed into N₂ in sediments)
Jin et al. (2016)	Hydroelectric reservoir (S Korea) Sampling sites with different hydrological conditions: flooded sediments, recently expose (moist) sediments, drier sediments N₂O (laboratory incubation)	_	_	Significantly higher in recently exposed sediments Peak (mean): ≈ 11.46 µg N m⁻²h⁻¹ Flooded sediments (mean): <≈ 0.63 µg N m⁻²d⁻¹	N₂O production was suggested to derive from both aerobic and anaerobic processes.
Reverey et al. (2018)	Kettle holes (NE Germany) Sediment cores Sampling sites with different hydrological history: predominantly inundated (zone A), intermediate/occasionally dry (zone B), sediments frequently exposed to dry-wet cycles (Zone C); (sampling during sediment exposure) Exposed vs. inundated experiments	Nitrification NH $_{4}^{+}$ depleted in zones B and C (exposed phase), likely nitrified to NO $_{3}^{-}$ DEA >DNRA in all zones DNRA Higher potential in sediments less exposed to drought (NH $_{4}^{+}$ accumulation coupled to NO₃⁻depletion during exposed phase)	NO $_{3}^{-}$ - Exposed conditions: lowest concentration in zone A - Inundated conditions: decreased significantly after rewetting in zones B and C; similar concentrations between zones NH $_{4}^{+}$ - Exposed conditions: highest concentration in zone A - Inundated conditions: highest concentration in zone A and lowest in zone C; increased significantly in all three zones after rewetting	_	Water content of the sediment did not drop below 80% (zones A and B) and 60% (zone C).
Shi et al. (2020)	Hydroelectric reservoir (China) 10 sampling sites along a transect after the water level receded (0.5, 1.5, 3.5, 6.5, 10.5, 15.5, 20.5, 25.5, 30.5, and 35.5 m) Sediment core system (in situ) Wetting-drying cycles with different periods or water level rising and dropping N₂O (in situ field measurements after the water level receded, static flux chambers)	Denitrification: - Strong positive relationship with the period of water level rising-falling cycle; - Increasing potential from the water edge to the center of the transect (0.5 –10.5 m), followed by a reduction for longer distances (15.5 –35.5 m)	NO $_{3}^{-}$ Below detection limit (0.5 to 25.5 m)	Similar pattern to the observed for denitrification potential rate: - Increase from the water edge to the center of the transect (0.5 –10.5 m): 51 to 62 µg N m⁻² h⁻¹ - Decrease to 0.27 µg N m⁻² h⁻¹ (35.5 m)	Denitrification more active in the surface sediment than in the subsurface sediment.
Lotic systems
Austin and Strauss (2011)	Experimental stream (outdoor; NE Kansas) Dry period: 1, 3, 7, 14, 21, 28 d Wet period: 1, 3, 7, 14, 21, 28 d	Nitrification: - Dry period: decrease (after 1 d) - Wet period: recovery in sediment dried for 1, 3, 7 (after 1 d rewetted) and 21 d (after 3 d rewetted); sediments dried 14 and 28 d increased but failed to recover; rates declined below control rates after recovery DEA - Dry period: decrease (after 3 d) - Wet period: recovery in sediment dried for 3, 7 and 21 d, after 7 d rewetted; sediment dried for 1 d (ns); sediment dried for 14 and 28 d failed to recover	NH $_{4}^{+}$ - Dry period: increase - Wet period: decrease	_	Lag between rewetting and recovery of process rates. Denitrifiers appear to be more diverse and/or drought tolerant than nitrifiers. Water availability appears to regulate N cycling during drying, and O₂ availability was more important during rewetting.
Gómez et al. (2012)	Mediterranean intermittent stream (S Spain) Microcosms (outdoor) Dry period: 18 d	Nitrification: - Lag phase [0 –4 d] - Increase [4 –8* d] - Decrease [8 –18 d] Denitrification: - Peak (24 h) - Inhibited [1 –18 d] Mineralization stimulated during the first days	NO $_{3}^{-}$ - Increase [0 –10* d] - Decrease [10 –18 d] NH $_{4}^{+}$ Decrease [4 –18 d]	_	Denitrification pattern differs from (Kern et al., 1996; Koschorreck, 2005) which reported an increase in denitrification with drying. This was attributed to differences in MC (8% vs. >50% and 25–50%, respectively) and better sediment aeration.
Shrestha et al. (2012)	Flash flow river regime (River Thur, NE Switzerland) Sampling sites in different functional process zones, FPZs (flooding frequency, yr⁻¹): frequently flooded gravel bars (>10), bank (4-6), forests (1-2) and embankment (1-2) N₂O (in situ field measurements, static flux chambers)	GN >GM DEA >GN	NO $_{3}^{-}$ - Slower turnover rates (compared to NH $_{4}^{+}$ ) - Higher in gravel bar and forest NH $_{4}^{+}$ Fast turnover rate	The N₂O efflux generally increased as a result of flood disturbance	N₂O efflux did not correlate with denitrification, suggesting that: (i) other processes also contributed to N₂O emissions, namely nitrification, or (ii) production in deeper layers. N turnover governed by mineralization. NO $_{3}^{-}$ quickly denitrified during water saturation, and during drying phases in anoxic microsites. Under predominant aerobic conditions, NH $_{4}^{+}$ not consumed or immobilized is quickly nitrified, except when conditions are too dry. The N₂O efflux generally increased as a result of flood disturbance.
Shrestha et al. (2014)	Flash flow river regime (River Thur, NE Switzerland) Sampling sites in different functional process zones, FPZs (flooding frequency, yr⁻¹): frequently flooded gravel bar (>10), bank (4-6), forest (1–2)	GN stimulated during the drying phase (gravel bar) DEA: - Increased after flood events - Temporary drop a few days after flood (gravel bar) GM: - Strongly increased immediately after flood events (gravel bar) - Temporary dropped during the drying phase after the flood (possible leaching of available organic N)	NO $_{3}^{-}$ Marked temporary drop in gravel bar (negative correlation with DEA) NH $_{4}^{+}$ Increase after flood events	Hot moments of N transformations may lead to temporary high N₂O emissions	Stronger reaction of N pools and transformation rates to flooding, in the gravel bar. High N turnover by coupled nitrification–denitrification during the drying phase after a major flood.
Arce et al. (2014)	Mediterranean temporary stream (intermittent reach) (SE Spain) Sampling during dry and wet periods	Nitrification: - Higher in rewet sediments (incubation with stream water) than during wet period - Quick recovery to pre-dry levels or higher upon rewetting (12h) Denitrification: - Similar between dry and wet period - Quick recovery to pre-dry levels or higher upon rewetting (5h)	NO $_{3}^{-}$ - Increase (dry period) - Decrease (wet period) NH $_{4}^{+}$ ns between wet and dry period	_	Rapid recovery (hours) of processing rates after drying period under natural conditions (4 months). Austin and Strauss (2011) report a variable lag (∼30 d) between rewetting and recovery of processing rates, depending on the pre-drought period (experimental study) in a temperate stream. The lack of a negative effect of duration of the sediment desiccation may be related to field conditions, e.g., occurrence of small water/rainfall pulses (field vs. experimental study).
Gallo et al. (2014)	Semi-arid urban ephemeral waterway (Tucson, Arizona) Wet period: artificial rainfall event (10 mm; after extensive dry period of 3 –4 months) N₂O (in situ field measurements, static flux chambers)	_	_	Pre-wetting: 1.5 ± 0.6 µg N m⁻² h⁻¹ Post-wetting: 207.4 ± 76.3 µg N m⁻² h⁻¹ Instantaneous: 458.6 ± 237.7 µg N m⁻² h⁻¹	N₂O emissions following wetting were among the highest ever published.
Arce, Sánchez-Montoya & Gómez (2015)	Mediterranean temporary stream (intermittent reach) (SE Spain) Microcosms (outdoor) Wet period: 7 d (after 3 months sediment desiccation)	Denitrification: - Peak (24 h) - Decrease [24 h–7 d] DNRA (low Eh)	NO $_{3}^{-}$ - Increase [0 –1 h] - Decrease [1 h –7 d] despite low denitrification NH $_{4}^{+}$ Increase (DNRA)	_	Rapid recovery (hours) of denitrification rates after drying period under controlled conditions (3 months).
Merbt et al. (2016)	Mediterranean intermittent stream (NE Spain) Sampling sites with different hydrological regimes, including short-term dry (5 d) and long-term dry (30 d)	Increase (AO)	NO $_{3}^{-}$ Increase NH $_{4}^{+}$ Increase (cell lysis)	_	The increase in AO activity and nitrate content with the degree of stream drying was more evident in surface sediments.
Arce et al. (2018)	Temperate intermittent river (Germany) Microcosms (laboratory) Dry period: 9 w Wet period: rainfall pulses (after 6 and 9 w drying); modest (4 mm) and intense (21 mm) N₂O (laboratory, gas-tight microcosms)	Nitrification: - Decrease (AO; surface sediments) - Stable (AO; deep sediments) COMAMMOX (uncertain)	NO $_{3}^{-}$ Increase with drying NH $_{4}^{+}$ - Decrease [0 –3 w ] - Constant low [3 –9 w]	Increase (-3 –11* d); Peak: 182.5 µg N m⁻²h ⁻¹ Decrease (2 –9 w); range: 0 –5.6 µg N m⁻²h ⁻¹ Immediately after drainage (0 d): ≈ 5 µg N m⁻²h⁻¹ (≈ 0 µg N m⁻²h⁻¹ during the preceding flooded conditions) Immediately after rainfall pulses (0 h) (4 mm and 9 mm rainfall, respectively): - 6 weeks desiccation period: ≈ 6 and 19 µg N m⁻²h⁻¹ (≈ 6 and 17 µg N m⁻²h⁻¹ during the preceding dry conditions, respectively) - 9 weeks desiccation period: ≈ 17 and 0 µg N m⁻² h⁻¹ (≈ 0 µg N m⁻²h⁻¹ during the preceding dry conditions)	Based on MC conditions (wet-saturated), denitrification was assumed to be the dominant source of N₂O emissions during the first 11 days. An increase in AO in the sediments before the first sampling time (3 weeks) was probably overlooked. Rewetting had a negative impact on N₂O emissions as its magnitude increased (physical trapping). NO $_{3}^{-}$ increase was not as high in deeper (3 –15 cm) vs. surface (0 –3 cm) sediments. NH $_{4}^{+}$ decrease more pronounced in deep vs. surface layers (higher initial NH $_{4}^{+}$ concentrations in deeper sediments).
Pinto et al. (2020)	Temperate river (Lower Austria) Sampling sites along an hydrological gradient (parafluvial zone): frequently flooded sediments, rarely flooded sediments, and non-flooded floodplain soil Sampling periods defined as a function of water level fluctuations at the parafluvial zone: intermittent, desiccation and post flood period N₂O (in situ field measurements, static flux chambers)	_	NO $_{3}^{-}$ - Frequently flooded sediments: Intermittent >post flood >desiccation period - Rarely flooded sediments: similar concentrations between periods NH $_{4}^{+}$ - Highest concentrations in frequently flooded sediments, except during the desiccation period (enhanced mineralization) - Strong increase after the flood in rarely flooded sediments	Intermittent period: 37 ± 24 µg N m⁻² h⁻¹ Desiccation period: 13 ± 16 µg N m⁻² h⁻¹ Post flood period: 19 ± 27 µg N m⁻² h⁻¹ Frequently flooded sediments: 32 ± 25 µg N m⁻² h⁻¹ Rarely flooded sediments: 23 ± 19 µg N m⁻² h⁻¹ Non-flooded soil: 27 ± 33 µg N m⁻² h⁻¹ (mean ± SD)	Tight link between C and N cycles, presumably originating from the quality of the DOM pool.

Notes.

(*) indicates peak timing; Periods in hours (h), days (d), weeks (w); ns: not significant; AO – ammonia oxidation; DEA – potential denitrification; NEA – potential nitrification; GN – gross nitrification; GM – gross mineralization; DW – dry weight. N2O emissions from headspace measurements

Table 1. Summary of nitrogen dynamics during dry-wet cycles (cited literature): Settings, N-cycling pathways, nitrogen species and N2O emissions.

Table 1. Summary of nitrogen dynamics during dry-wet cycles (cited literature): Settings, N-cycling pathways, nitrogen species and N₂O emissions.