Forty-two years impact of chemical fertilization on soil phosphorus partition and distribution under rice-based cropping systems.

Understanding of soil phosphorus (P) transformation is crucial to minimize its edge-of-field loss associated with ecosystem disservices. A sequential chemical extraction procedure was used to assess the impact (42 years) of organic and chemical fertilizations on soil P partition and distribution under subtropical rice based cropping systems. Experimental treatments were control, N, NP, NK, NS, NZn, NPK, NSZn, NPKSZn, and N+FYM (farmyard manure). Composite soils were collected from 0-5, 20-25 and 40-45 cm depths, extracted, and analyzed for soluble P, NaHCO3-P (inorganic and organic), NaOH-P (inorganic and organic), acid soluble (H2SO4), and residual P fractions. The NPKSZn significantly increased the concentration of soil inorganic P compared to other treatments. When FYM was applied together with N fertilizer, the organic P concentration increased, which was statistically identical to NPKSZn and NPK treatments. While the labile (NaHCO3-Pi, NaOH-Po), residual, and total P concentrations were stratified at 0-5 cm depth, the concentration of NaHCO3-Po, NaOH-Pi, and acidic P fractions increased with soil depth. The soluble, NaHCO3 (Pi and Po), NaOH-Pi and NaOH-Po, acidic, and residual P fractions constituted about 0.4, 6.6, 1.7, 21.3, 37.7, and 8.3%, respectively, of the total P. A higher concentration of the labile P at the surface soil indicated that the impact of chemical fertilization stratified the available P for plant uptake or susceptible to edge-of-field loss. The NPKSZn and N+FYM both had higher NaHCO3-Po and NaOH-Po concentrations within 40-45 cm and 0-25 cm depths, suggesting that N+FYM could promote the transformation of non-labile P into labile P pool, by reducing P fixation by soil and transport them at 20-45 cm depth. It is concluded that long-term fertilization increased the concentration of P pools especially labile P by saturating the soil adsorption sites especially in surface soil.

Biochar and urease inhibitor mitigate NH3 and N2O emissions and improve wheat yield in a urea fertilized alkaline soil.

In this study, we explored the role of biochar (BC) and/or urease inhibitor (UI) in mitigating ammonia (NH3) and nitrous oxide (N2O) discharge from urea fertilized wheat cultivated fields in Pakistan (34.01°N, 71.71°E). The experiment included five treatments [control, urea (150 kg N ha-1), BC (10 Mg ha-1), urea + BC and urea + BC + UI (1 L ton-1)], which were all repeated four times and were carried out in a randomized complete block design. Urea supplementation along with BC and BC + UI reduced soil NH3 emissions by 27% and 69%, respectively, compared to sole urea application. Nitrous oxide emissions from urea fertilized plots were also reduced by 24% and 53% applying BC and BC + UI, respectively, compared to urea alone. Application of BC with urea improved the grain yield, shoot biomass, and total N uptake of wheat by 13%, 24%, and 12%, respectively, compared to urea alone. Moreover, UI further promoted biomass and grain yield, and N assimilation in wheat by 38%, 22% and 27%, respectively, over sole urea application. In conclusion, application of BC and/or UI can mitigate NH3 and N2O emissions from urea fertilized soil, improve N use efficiency (NUE) and overall crop productivity.

Leaching and runoff potential of nutrient and water losses in rice field as affected by alternate wetting and drying irrigation.

Inefficient water management in rice paddy is responsible for a large quantity of water and nutrient loss, which causes tremendous economic and environmental costs. Yet, quantified data on the water and nutrient losses are limited. A study was conducted during 2018-2019 with an Aman (wet)-Boro (dry)-Aman (wet) rice rotation to evaluate the effect of water management on water and nutrient losses through different pathways. The treatments in 2018 Aman season were: (i) rainfed, (ii) I6D (irrigation after six days of ponded water disappearance), and (iii) I3D. In 2019, the Boro season had (i) I6D and (ii) I3D, and the Aman season had (i) rainfed, (ii) I9D, and (iii) I1D treatments. The water input and output from the studied lysimeters were measured daily, and samples from the leachates, ponded water, and topsoil were routinely analyzed for nutrient content. In both Aman seasons, the rainfed cultivation had lower percolation losses (38-44 % of total input) than other treatments (45-70 %). Evapotranspiration in the Boro season (5.4-5.9 mm/day) was higher than that in the Aman seasons (4.2-4.6 mm/day) because of the drier Boro season. Ammonium (NH4⁺-N) leached at 0.6-6.7 mg/L and nitrate (NO3⁻-N) 0.6-5.6 mg/L in these rice seasons. Phosphorus concentration ranged 0.04-0.37 mg/L in the leachates and 0.04-0.51 mg/L in the ponded water. The rainfed and I9D exerted higher nutrient leaching concentration in some events and less so for the I6D treatment than the I3D and I1D, possibly because of the better nitrification and preferential flow paths induced by the prolonged drying processes. However, the rainfed, I9D, and I6D had less leaching load than the I3D and I1D because the latter had larger percolation volume. For example, the I6D treatment in the Boro season reduced the N leaching load by 44 % and P load by 39 % compared with the I3D, and the I9D in 2019 Aman season had 42 and 13 % less N and P leaching load, respectively, than the I1D treatment. The findings will contribute to the effort of developing a sustainable and climate-resilient rice production system.

Ammonia fluxes and emission factors under an intensively managed wetland rice ecosystem.

Nitrogen (N) loss from rice production systems in the form of ammonia (NH3) can be a significant N loss pathway causing significant economic and environmental costs. Yet, data on NH3 fluxes in wetland rice ecosystems are still very scarce which limits the accuracy of national and global NH3 budgets. We measured the NH3 fluxes in situ in a wetland rice field and estimated emission factors (EF) under two soil management systems (i.e. conventional tillage, CT and strip tillage, ST); two residue retention levels (i.e. 15%, LR and 40% crop residue by height, HR); and three N fertilization rates (i.e. 108, 144 and 180 kg N ha-1) in two consecutive years (2019 and 2020). The highest NH3 peaks were observed within the first 3 days after urea application. The mean and cumulative NH3 fluxes significantly increased with the increases in N fertilization rates and were 18.5% and 18.6% higher in ST than in CT in 2020 but not in 2019. Overall, the highest mean NH3 fluxes were in 180 kg N ha-1 coupled with either HR or LR and ST or CT. In 2019, the NH3 EF was unchanged by any treatments. In 2020, the lower EF was in CT coupled with LR (15%) than all other treatment combinations, where ST with HR showed the highest EF (20%). Likewise, the lowest N rate (108 kg N ha-1) in ST had the highest NH3 EF (20%) that was similar to higher N rates (144 and 180 kg N ha-1) in the same tillage treatment and to 180 kg N ha-1 in CT. Our results highlight that NH3 fluxes in rice field particularly the effects of ST correlated with higher soil pH and NH4+ content and lower redox potential. Our results highlight that NH3 fluxes are a potentially large N loss pathway in wetland rice under conventional and decreased soil disturbance regimes.

Application of 15N tracing for estimating nitrogen cycle processes in soils of a constructed wetland.

Integrated Constructed Wetlands (ICW) area technology for the attenuation of contaminants such as organic carbon (C), nitrogen (N), phosphorous (P) and sulphur (S) in water coming from point or diffuse sources. Currently there is a lack of knowledge on the rates of gross N transformations in soils of the ICW bed leading to losses of reactive N to the environment. In addition, the kinetics of these processes need to be studied thoroughly for the sustainable use of ICW for removal of excessive N in the treatment of waste waters. Gross N transformation processes were quantified at two soil depths (0-15 and 30-45 cm) in the bed of a surface flow ICW using a 15N tracing approach. The ICW, located in Dunhill village at Waterford in Southeastern Ireland, receives 500 person equivalent waste waters containing large quantities of organic pollutants (ca. mean annual C, N, P and S contents of 240, 60, 5 and 73 mg L-1). Soil was removed from these depths in December 2014 and incubated anaerobically in the laboratory, with either 15N labeled ammonium (NH4+) or nitrate (NO3-), differentially labeled with 14NH415NO3 and 15NH414NO3 in parallel setups, enriched to 50 atm% 15N. Results showed that at both soil depths, NO3- production rates were small, which may have resulted in lower NO3- reduction by either denitrification or dissimilatory NO3- reduction to ammonium (DNRA). However, despite being low, the DNRA rates were greater than denitrification rates. Direct transformation of organic N to NO3-, without mineralization to NH4+, was a prevalent pathway of NO3- production accounting for 28-33% of the total NO3- production. Relative contribution of this process to the total N mineralization was negligible at depth 1 (0.01%) but dominant at depth 2 (99.7%). Total NO3-production to total immobilization of NH4+ and NO3- was very small (<0.50%) suggesting that ICW soils are not a source of NO3-. Despite a large potential of N immobilization existed at both the layers, relative N immobilization to the total N conversion was higher at depth 2 (ca. 2.2) than at depth 1 (ca. 1.5). The NH4+ desorption rate at 30-45 cm was high. However, immobilization in the recalcitrant and labile organic N pools was higher. Mineralization and immobilization of NH4+ processes showed that recalcitrant organic N was the predominant source in ICW soils whereas the labile organic N was comparatively small. Source apportionment of N2O production showed that the majority of the N2O produced through denitrification (ca. 92.5%) followed by heterotrophic nitrification (ca. 5.5%), co-denitrification (ca. 1.90%) and nitrification (0.20%). These results revealed that application of a detailed 15N tracing method can provide insights on the underlying processes of ecosystem based abundances of reactive N. A key finding of this study was that both investigated ICW layers were characterised by large N immobilization which restricts production of NO3- and further gaseous N losses.

Groundwater nitrate reduction versus dissolved gas production: A tale of two catchments.

At the catchment scale, a complex mosaic of environmental, hydrogeological and physicochemical characteristics combine to regulate the distribution of groundwater and stream nitrate (NO3-). The efficiency of NO3- removal (via denitrification) versus the ratio of accumulated reaction products, dinitrogen (excess N2) & nitrous oxide (N2O), remains poorly understood. Groundwater was investigated in two well drained agricultural catchments (10km2) in Ireland with contrasting subsurface lithologies (sandstone vs. slate) and landuse. Denitrification capacity was assessed by measuring concentration and distribution patterns of nitrogen (N) species, aquifer hydrogeochemistry, stable isotope signatures and aquifer hydraulic properties. A hierarchy of scale whereby physical factors including agronomy, water table elevation and permeability determined the hydrogeochemical signature of the aquifers was observed. This hydrogeochemical signature acted as the dominant control on denitrification reaction progress. High permeability, aerobic conditions and a lack of bacterial energy sources in the slate catchment resulted in low denitrification reaction progress (0-32%), high NO3- and comparatively low N2O emission factors (EF5g1). In the sandstone catchment denitrification progress ranged from 4 to 94% and was highly dependent on permeability, water table elevation, dissolved oxygen concentration solid phase bacterial energy sources. Denitrification of NO3- to N2 occurred in anaerobic conditions, while at intermediate dissolved oxygen; N2O was the dominant reaction product. EF5g1 (mean: 0.0018) in the denitrifying sandstone catchment was 32% less than the IPCC default. The denitrification observations across catchments were supported by stable isotope signatures. Stream NO3- occurrence was 32% lower in the sandstone catchment even though N loading was substantially higher than the slate catchment.

In situ denitrification and DNRA rates in groundwater beneath an integrated constructed wetland.

Evaluation of the environmental benefits of constructed wetlands (CWs) requires an understanding of their impacts on the groundwater quality under the wetlands. Empirical mass-balance (nitrogen in/nitrogen out) approaches for estimating nitrogen (N) removal in CWs do not characterise the final fate of N; where nitrate (NO3--N) could be reduced to either ammonium (NH4+-N) or N2 with the potential for significant production of N2O. Herein, in situ denitrification and DNRA (dissimilatory nitrate reduction to ammonium) rates were measured in groundwater beneath cells of an earthen lined integrated constructed wetland (ICW, used to remove the nutrients from municipal wastewater) using the 15N-enriched NO3--N push-pull method. Experiments were conducted utilising replicated (n = 3) shallow (1 m depth) and deep (4 m depth) piezometers installed along two control planes. These control planes allowed for the assessment of groundwater underlying high (Cell 2, septic tank waste) and low (Cell 3) load cells of the ICW. Background piezometers were also installed off-site. Results showed that denitrification (N2O-N + N2-N) and DNRA were major NO3--N consumption processes accounting together for 54-79% of the total biochemical consumption of the applied NO3--N. Of which 14-16% and 40-63% were consumed by denitrification and DNRA, respectively. Both processes differed significantly across ICW cells indicating that N transformation depends on nutrient loading rates and were significantly higher in shallow compared to the deep groundwater. In such a reduced environment (low dissolved oxygen and low redox potential), higher DNRA over the denitrification rate can be attributed to the high C concentration and high TC/NO3--N ratio. Low pH (6.5-7.1) in this system might have limited denitrification to some extent to an incomplete state, evidenced by a high N2O-N/(N2O-N+N2-N) ratio (0.35 ± 0.17, SE). A relatively higher N2O-N/(N2O-N+N2-N) ratio and higher DNRA rate over denitrification, suggest that the end products of N transformations are reactive. This N2O can be consumed to N2 and/or emitted to the atmosphere. The DNRA rate and accumulation of NH4+-N indicated that the ICW created a suitable groundwater biogeochemical environment that enhanced NO3--N reduction to NH4+-N. This study showed that CWs significantly influence NO3--N attenuation to reactive forms of N in the groundwater beneath them and that solely focusing on within wetland NO3--N attenuation can underestimate the environmental benefits of wetlands.

Carbon amendment and soil depth affect the distribution and abundance of denitrifiers in agricultural soils.

The nitrite reductase (nirS and nirK) and nitrous oxide reductase-encoding (nosZ) genes of denitrifying populations present in an agricultural grassland soil were quantified using real-time polymerase chain reaction (PCR) assays. Samples from three separate pedological depths at the chosen site were investigated: horizon A (0-10 cm), horizon B (45-55 cm), and horizon C (120-130 cm). The effect of carbon addition (treatment 1, control; treatment 2, glucose-C; treatment 3, dissolved organic carbon (DOC)) on denitrifier gene abundance and N2O and N2 fluxes was determined. In general, denitrifier abundance correlated well with flux measurements; nirS was positively correlated with N2O, and nosZ was positively correlated with N2 (P < 0.03). Denitrifier gene copy concentrations per gram of soil (GCC) varied in response to carbon type amendment (P < 0.01). Denitrifier GCCs were high (ca. 10(7)) and the bac:nirK, bac:nirS, bac:nir (T) , and bac:nosZ ratios were low (ca. 10(-1)/10) in horizon A in all three respective treatments. Glucose-C amendment favored partial denitrification, resulting in higher nir abundance and higher N2O fluxes compared to the control. DOC amendment, by contrast, resulted in relatively higher nosZ abundance and N2 emissions, thus favoring complete denitrification. We also noted soil depth directly affected bacterial, archaeal, and denitrifier abundance, possibly due to changes in soil carbon availability with depth.

Mustard catch crop enhances denitrification in shallow groundwater beneath a spring barley field.

Over-winter green cover crops have been reported to increase dissolved organic carbon (DOC) concentrations in groundwater, which can be used as an energy source for denitrifiers. This study investigates the impact of a mustard catch crop on in situ denitrification and nitrous oxide (N2O) emissions from an aquifer overlain by arable land. Denitrification rates and N2O-N/(N2O-N+N2-N) mole fractions were measured in situ with a push-pull method in shallow groundwater under a spring barley system in experimental plots with and without a mustard cover crop. The results suggest that a mustard cover crop could substantially enhance reduction of groundwater nitrate NO3--N via denitrification without significantly increasing N2O emissions. Mean total denitrification (TDN) rates below mustard cover crop and no cover crop were 7.61 and 0.002 µg kg(-1) d(-1), respectively. Estimated N2O-N/(N2O-N+N2-N) ratios, being 0.001 and 1.0 below mustard cover crop and no cover crop respectively, indicate that denitrification below mustard cover crop reduces N2O to N2, unlike the plot with no cover crop. The observed enhanced denitrification under the mustard cover crop may result from the higher groundwater DOC under mustard cover crop (1.53 mg L(-1)) than no cover crop (0.90 mg L(-1)) being added by the root exudates and root masses of mustard. This study gives insights into the missing piece in agricultural nitrogen (N) balance and groundwater derived N2O emissions under arable land and thus helps minimise the uncertainty in agricultural N and N2O-N balances.

Denitrification and indirect N2O emissions in groundwater: hydrologic and biogeochemical influences.

Identification of specific landscape areas with high and low groundwater denitrification potential is critical for improved management of agricultural nitrogen (N) export to ground and surface waters and indirect nitrous oxide (N2O) emissions. Denitrification products together with concurrent hydrogeochemical properties were analysed over two years at three depths at two low (L) and two high (H) permeability agricultural sites in Ireland. Mean N2O-N at H sites were significantly higher than L sites, and decreased with depth. Conversely, excess N2-N were significantly higher at L sites than H sites and did not vary with depth. Denitrification was a significant pathway of nitrate (NO3⁻-N) reduction at L sites but not at H sites, reducing 46-77% and 4-8% of delivered N with resulting mean NO3⁻-N concentrations of 1-4 and 12-15 mg N L⁻¹ at L and H sites, respectively. Mean N2O-N emission factors (EF5g) were higher than the most recent Intergovernmental Panel on Climate Change (IPCC, 2006) default value and more similar to the older IPCC (1997) values. Recharge during winter increased N2O but decreased excess dinitrogen (excess N2-N) at both sites, probably due to increased dissolved oxygen (DO) coupled with low groundwater temperatures. Denitrifier functional genes were similar at all sites and depths. Data showed that highly favourable conditions prevailed for denitrification to occur--multiple electron donors, low redox potential (Eh<100 mV), low DO (<2 mg L⁻¹), low permeability (k(s)<0.005 m·d⁻¹) and a shallow unsaturated zone (<2 m). Quantification of excess N2-N in groundwater helps to close N balances at the local, regional and global scales.

Evaluation of headspace equilibration methods for quantifying greenhouse gases in groundwater.

The objective of the study was to evaluate the different headspace equilibration methods for the quantification of dissolved greenhouse gases in groundwater. Groundwater samples were collected from wells with contrasting hydrogeochemical properties and degassed using the headspace equilibration method. One hundred samples from each well were randomly selected, treatments were applied and headspace gases analysed by gas chromatography. Headspace equilibration treatments varied helium (He):water ratio, shaking time and standing time. Mean groundwater N(2)O, CO(2) and CH(4) concentrations were 0.024 mg N L(-1), 13.71 mg C L(-1) and 1.63 µg C L(-1), respectively. All treatments were found to significantly influence dissolved gas concentrations. Considerable differences in the optimal He:water ratio and standing time were observed between the three gases. For N(2)O, CO(2) and CH(4) the optimum operating points for He:water ratio was 4.4:1, 3:1 and 3.4:1; shaking time was 13, 12 and 13 min; and standing time was 63, 17 and 108 min, respectively. The headspace equilibration method needs to be harmonised to ensure comparability between studies. The experiment reveals that He:water ratio 3:1 and shaking time 13 min give better estimation of dissolved gases than any lower or higher ratios and shaking times. The standing time 63, 17 and 108 min should be applied for N(2)O, CO(2) and CH(4), respectively.