Sources, fate and influencing factors of nitrate in farmland drainage ditches of the irrigation area.

Global irrigation areas face the contradictory challenges of controlling nitrate inputs and ensuring food-safe production. To prevent and control nitrate pollution in irrigation areas, the study using the Yellow River basin (Ningxia section) of China as a case study, employed nitrogen and oxygen dual isotope tracing and extensive field investigations to analyze the sources, fate, and influencing factors of nitrate in agricultural drainage ditches. The results of source tracing of nitrate showed that annual proportions of nitrate sources entering the Yellow River in the ditches are as follows: for manure & sewage, fertilizer, and natural sources, the ratios are 33%, 35%, and 32% overall. The results of nitrate fate showed that nitrates derived from nitrate fertilizer exhibit a lower residual rate in drainage ditches (ecological ditches) compared to ammonium fertilizer, which can undergo self-ecological restoration within one year. The results of influencing factors showed that crops with high water and nutrient requirements, such as vegetables, the nitrate pollution and environmental harm resulting from "exploitative cultivation" are five times more than normal cultivation practices in dryland and paddy fields, especially winter irrigation without crop interception exacerbates the leaching of nitrate from the soil. Therefore, nitrate management in irrigation areas should focus on preventing and controlling "exploitative cultivation" and losses during winter irrigation, while appropriately adjusting the application ratio of ammonium nitrogen fertilizers. The results of the study can guide strategies to mitigate nitrate pollution in irrigated areas such as livestock farming, fertilizer application, irrigation management, ditch optimization, and crop cultivation.

Decline in carbon emission intensity of global agriculture has stagnated recently.

Using global data for around 180 countries and territories and 170 food/feed types primarily derived from FAOSTAT, we have systematically analyzed the changes in greenhouse gas (GHG) emission intensity (GHGi) (kg CO2eq per kg protein production) over the past six decades. We found that, with large spatial heterogeneity, emission intensity decreased by nearly two-thirds from 1961 to 2019, predominantly in the earlier years due to agronomic improvement in productivity. However, in the most recent decade, emission intensity has become stagnant, and in a few countries even showed an increase, due to the rapid increase in livestock production and land use changes. The trade of final produced protein between countries has potentially reduced the global GHGi, especially for countries that are net importers with high GHGi, such as many in Africa and South Asia. Overall, a continuous decline of emission intensity in the future relies on countries with higher emission intensity to increase agricultural productivity and minimize land use changes. Countries with lower emission intensity should reduce livestock production and increase the free trade of agricultural products and improve the trade optimality.

Optimizing application of dairy effluent with synthetic N fertilizer reduced nitrogen leaching in clay loam soil.

High application rates of dairy effluent and manure are often associated with nitrogen (N) leaching, which can affect groundwater quality. Here, we used a lysimeter to examine N leaching losses and biomass yield following application of dairy effluent and manure under wheat-maize cropping. The field experiment included seven treatments: no N fertilizer (Control); 200/300 kg N ha-1 synthetic N fertilizer only (wheat/maize) (CN); 100/150 kg N ha-1 synthetic N fertilizer plus 100/150 (DE1), 150/200 (DE2) and 250/350 (DE3) kg N ha-1 dairy effluent; 100/150 kg N ha-1 synthetic fertilizer plus 100/150 kg N ha-1 dairy manure (SM1); and 150/225 kg N ha-1 synthetic fertilizer plus 50/75 kg N ha-1 dairy manure (SM2). Compared with CN, DE1 treatment increased maize yield by 10.0 %, wheat N use efficiency (NUE) by 26.5 %, and wheat and maize N uptake by 7.7-16.3 %, while reduced N leaching by 22.4 % in wheat season and by 40.4 % in the maize season. In contrast, DE2 and DE3 treatment increased N leaching by 27.2-241 % and reduced NUE by 26.2-55.2 %. SM2 treatment increased yield and NUE by 8.8 % and 7.8 %, respectively, and reduced N leaching by 42.9 % during the wheat but not the maize season. Annual N leaching losses were 37.6 kg N ha-1 under CN treatment, but decreased to 27.4 kg N ha-1 under DE1. In contrast, N leaching increased to 52.8 and 84.1 kg N ha-1 under DE2 and DE3 treatment, respectively (P < 0.05). Meanwhile, under SM1 and SM2 treatment, N leaching decreased by 71.2 % and 32.0 %, respectively, compared with CN. These results suggest that replacing 50 % and 25 % synthetic N fertilizer with dairy farm effluent and manure could reduce N leaching losses but had varied effects on crop productivity under wheat-maize cropping.

Interactive impact of landscape composition and configuration on river water quality under different spatial and seasonal scales.

Currently, the comprehensive effect of the landscape pattern on river water quality has been widely studied. However, the interactive influences of landscape type, namely composition (COM) and configuration (CON) on water quality variations, as well as the specific landscape driving types affecting water quality variations under different spatial and seasonal scales remain unclear. To further improve the effectiveness of landscape planning and water quality protection, this study collected monthly water samples from the Fengyu River Watershed in southwestern China from 2018 to 2021, the Biota-Environment Matching Analysis (Bioenv) was used to identify key metrics representing landscape COM and CON, respectively. Then, the multiple regression (MLR) and redundancy analysis (RDA) were used to explore the relationship between these landscape metrics and water quality. In addition, this study used a variation partitioning analysis (VPA) to quantify the interactive and independent influence of landscape COM and CON on water quality. Results revealed that construction land and the Shannon's diversity index (SHDI) were the key metrics of landscape COM and CON, respectively, for predicting water pollution concentrations. The interactive contribution was particularly sensitive to seasonal changes in riparian buffer areas (27.66 % to 48.73 %), while it remained relatively stable at the sub-watershed scale (38.22 % to 40.51 %). Moreover, landscape CON had a higher independent contribution to variations on water quality across most spatio-temporal scales. Overall, identifying and managing key landscape type and consequential metrics, matching with the spatio-temporal scale, holds promise for enhancing water quality conservation. Furthermore, this study provides valuable insights into the identification and selection of core landscape metrics.

Nitrification inhibitor chlorate and nitrogen substrates differentially affect comammox Nitrospira in a grassland soil.

Introduction: Through the combined use of two nitrification inhibitors, Dicyandiamide (DCD) and chlorate with nitrogen amendment, this study aimed to investigate the contribution of comammox Nitrospira clade B, ammonia oxidizing bacteria (AOB) and archaea (AOA) to nitrification in a high fertility grassland soil, in a 90-day incubation study. Methods: The soil was treated with nitrogen (N) at three levels: 0 mg-N kg-1 soil, 50 mg-N kg-1 soil, and 700 mg-N kg-1 soil, with or without the two nitrification inhibitors. The abundance of comammox Nitrospira, AOA, AOB, and nitrite oxidising bacteria (NOB) was measured using qPCR. The comammox Nitrospira community structure was assessed using Illumina sequencing. Results and Discussion: The results showed that the application of chlorate inhibited the oxidation of both NH4+ and NO2- in all three nitrogen treatments. The application of chlorate significantly reduced the abundance of comammox Nitrospira amoA and nxrB genes across the 90-day experimental period. Chlorate also had a significant effect on the beta diversity (Bray-Curtis dissimilarity) of the comammox Nitrospira clade B community. Whilst AOB grew in response to the N substrate additions and were inhibited by both inhibitors, AOA showed litle or no response to either the N substrate or inhibitor treatments. In contrast, comammox Nitrospira clade B were inhibited by the high ammonium concentrations released from the urine substrates. These results demonstrate the differential and niche responses of the three ammonia oxidising communities to N substrate additions and nitrification inhibitor treatments. Further research is needed to investigate the specificity of the two inhibitors on the different ammonia oxidising communities.

Foliar N2 O emissions constitute a significant source to atmosphere.

Nitrous oxide (N2 O) is a potent greenhouse gas and causes stratospheric ozone depletion. While the emissions of N2 O from soil are widely recognized, recent research has shown that terrestrial plants may also emit N2 O from their leaves under controlled laboratory conditions. However, it is unclear whether foliar N2 O emissions are universal across varying plant taxa, what the global significance of foliar N2 O emissions is, and how the foliage produces N2 O in situ. Here we investigated the abilities of 25 common plant taxa, including trees, shrubs and herbs, to emit N2 O under in situ conditions. Using 15 N isotopic labeling, we demonstrated that the foliage-emitted N2 O was predominantly derived from nitrate. Moreover, by selectively injecting biocide in conjunction with the isolating and back-inoculating of endophytes, we demonstrated that the foliar N2 O emissions were driven by endophytic bacteria. The seasonal N2 O emission rates ranged from 3.2 to 9.2 ng N2 O-N g-1 dried foliage h-1 . Extrapolating these emission rates to global foliar biomass and plant N uptake, we estimated global foliar N2 O emission to be 1.21 and 1.01 Tg N2 O-N year-1 , respectively. These estimates account for 6%-7% of the current global annual N2 O emission of 17 Tg N2 O-N year-1 , indicating that in situ foliar N2 O emission is a universal process for terrestrial plants and contributes significantly to the global N2 O inventory. This finding highlights the importance of measuring foliar N2 O emissions in future studies to enable the accurate assigning of mechanisms and the development of effective mitigation.

Modeling denitrification nitrogen losses in China's rice fields based on multiscale field-experiment constraints.

Denitrification plays a critical role in soil nitrogen (N) cycling, affecting N availability in agroecosystems. However, the challenges in direct measurement of denitrification products (NO, N2 O, and N2 ) hinder our understanding of denitrification N losses patterns across the spatial scale. To address this gap, we constructed a data-model fusion method to map the county-scale denitrification N losses from China's rice fields over the past decade. The estimated denitrification N losses as a percentage of N application from 2009 to 2018 were 11.8 ± 4.0% for single rice, 12.4 ± 3.7% for early rice, and 11.6 ± 3.1% for late rice. The model results showed that the spatial heterogeneity of denitrification N losses is primarily driven by edaphic and climatic factors rather than by management practices. In particular, diffusion and production rates emerged as key contributors to the variation of denitrification N losses. These findings humanize a 38.9 ± 4.8 kg N ha-1 N loss by denitrification and challenge the common hypothesis that substrate availability drives the pattern of N losses by denitrification in rice fields.

Balancing potato yield, soil nutrient supply, and nitrous oxide emissions: An analysis of nitrogen application trade-offs.

Potato has been promoted as a national key staple food to alleviate pressure on food security in China. Appropriate nitrogen (N) application rate is prerequisite and is crucial for increasing yield, improving fertilizer efficiency, and reducing N losses. In the present study, we determined the optimum N application rates by analyzing field trial data from the main potato producing areas of China between 2004 and 2020. We considered the equilibrium relationships between potato yield, N uptake, partial N balance (PNB), and N2O emission under different soil indigenous N supply (INS) scenarios. The results showed that N rate, INS, and their interactions all significantly affect potato yield and nutrient uptake increment. On average, N application increased potato yield and N uptake by 29.5 % and 56.7 %, respectively. The relationship between N rate and yield increment was linear-plateau, while the relationship between N rate and N uptake increment was linear-linear. Soil INS accounted for 63.5 % of total potato N requirement. Potato yield increment and nutrient uptake increment were exponentially negatively correlated with INS and had a significant parabolic-nonlinear relationship with the interaction of N fertilizer application rate and INS. PNB was negatively correlated with fertilizer N supply intensity as a power function. Based on our analysis, a N application rate of 166 kg N ha-1 was found to be sufficient when the target yield was <34 t ha-1. However, when the target yield reached 40, 50 and 60 t ha-1, the recommended N application rate increased to 182, 211, and 254 kg N ha-1, respectively, while ensuring N2O emissions low with an emission factor of 0.2 %. Our findings will help guide potato farming toward cleaner production without compromising environmental benefit.

Changes in soil fertility under partial organic substitution of chemical fertilizer: a 33-year trial.

BACKGROUND: This study examined the changes in soil fertility in a maize cropping area when chemical fertilizer was partially replaced with straw or livestock manure over a 33-year period. Four treatments were included: (i) CK (no fertilizer application); (ii) NPK (only chemical fertilizer application); (iii) NPKM (chemical fertilizer partly replaced with livestock manure); (iv) NPKS (chemical fertilizer partly replaced with straw). RESULTS: Soil organic carbon increased by 41.7% and 95.5% in the NPKS and NPKM treatments, respectively, over the 33-year trial compared with the initial concentration. However, soil organic carbon in NPK was significantly reduced by 9.8%. Soil total N, P and K increased in both NPKM and NPKS treatments compared to the original soil. Soil pH was significantly acidified from 7.6 to 5.97 in the NPK treatment during the experimental period. The NPKM and NPKS treatments buffered the acidification compared to NPK. Meta-analysis results showed that, compared with NPK, NPKM significantly raised soil bacteria and fungi populations by 38.7% and 58.6%; enhanced microbial biomass carbon and nitrogen by 66.3% and 63%, respectively; and increased sucrase, urease and catalase activities by 34.2%, 48.2% and 21.5%. NPKS significantly increased soil fungi and actinomycetes populations by 24.3% and 41.2%, respectively; enhanced microbial biomass carbon and nitrogen by 27.1% and 45%; and strengthened sucrase and urease activities by 36% and 20.3%, respectively. CONCLUSION: Long-term chemical fertilizer application led to the deterioration of soil fertility and environment. Partial replacement of chemical fertilizers with organic materials could significantly amend and buffer such negative effects. © 2023 Society of Chemical Industry.

Optimization of the N footprint model and analysis of nitrogen pollution in irrigation areas: A case study of Ningxia Hui Autonomous Region, China.

Water diverted from rivers for irrigation areas often contains large amounts of nitrogen (N), which is frequently overlooked and its role in contributing to N pollution is unknown. To investigate the influence of water diversion on N in different systems within irrigation areas, we developed and optimized the N footprint model, taking into account the N carried by irrigation water diversion and drainage in irrigated areas. This optimized model can serve as a reference for evaluating N pollution in other irrigated areas. By analyzing 29 years (1991-2019) of statistical data from a diverted irrigation area in Ningxia Hui Autonomous Region (Ningxia), China, the study assessed the contribution of water diversion to N in agriculture, animal husbandry, and human domestic activities. The results demonstrated that water diversion and drainage accounted for 10.3% and 13.8% in whole system, of the total N input and output in Ningxia, highlighting the potential N pollution risks associated with these activities. Additionally, the use of fertilizers in the plant subsystem, feed in the animal subsystem, and sanitary sewage in the human subsystem represented the main sources of N pollution in each subsystem. On a temporal scale, the study found that N loss increased year by year before reaching a stable level, indicating that N loss had reached its peak in Ningxia. The correlation analysis suggested that rainfall could regulate N input and output in irrigated areas by showing a negative correlation with water diversion, agricultural water consumption, and N from irrigated areas. Moreover, the study revealed that the amount of N brought by water diverted from rivers for irrigation should be taken into account when calculating the amount of fertilizer N required in the irrigation area.

Nitrate as an alternative electron acceptor destabilizes the mineral associated organic carbon in moisturized deep soil depths.

Numerous studies have investigated the effects of nitrogen (N) addition on soil organic carbon (SOC) decomposition. However, most studies have focused on the shallow top soils <0.2 m (surface soil), with a few studies also examining the deeper soil depths of 0.5-1.0 m (subsoil). Studies investigating the effects of N addition on SOC decomposition in soil >1.0 m deep (deep soil) are rare. Here, we investigated the effects and the underlying mechanisms of nitrate addition on SOC stability in soil depths deeper than 1.0 m. The results showed that nitrate addition promoted deep soil respiration if the stoichiometric mole ratio of nitrate to O2 exceeded the threshold of 6:1, at which nitrate can be used as an alternative acceptor to O2 for microbial respiration. In addition, the mole ratio of the produced CO2 to N2O was 2.57:1, which is close to the theoretical ratio of 2:1 expected when nitrate is used as an electron acceptor for microbial respiration. These results demonstrated that nitrate, as an alternative acceptor to O2, promoted microbial carbon decomposition in deep soil. Furthermore, our results showed that nitrate addition increased the abundance of SOC decomposers and the expressions of their functional genes, and concurrently decreased MAOC, and the ratio of MAOC/SOC decreased from 20% before incubation to 4% at the end of incubation. Thus, nitrate can destabilize the MAOC in deep soils by stimulating microbial utilization of MAOC. Our results imply a new mechanism on how above-ground anthropogenic N inputs affect MAOC stability in deep soil. Mitigation of nitrate leaching is expected to benefit the conservation of MAOC in deep soil depths.

Anthropogenic N input increases global warming potential by awakening the "sleeping" ancient C in deep critical zones.

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO2 concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12 m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO2 released and the SOC had a similar 14C age (6000 to 10,000 years before the present). Our results indicate that N fertilization of crops may enhance CO2 emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils.

Comammox Nitrospira Clade B is the most abundant complete ammonia oxidizer in a dairy pasture soil and inhibited by dicyandiamide and high ammonium concentrations.

The recent discovery of comammox Nitrospira, a complete ammonia oxidizer, capable of completing the nitrification on their own has presented tremendous challenges to our understanding of the nitrification process. There are two divergent clades of comammox Nitrospira, Clade A and B. However, their population abundance, community structure and role in ammonia and nitrite oxidation are poorly understood. We conducted a 94-day microcosm study using a grazed dairy pasture soil amended with urea fertilizers, synthetic cow urine, and the nitrification inhibitor, dicyandiamide (DCD), to investigate the growth and community structure of comammox Nitrospira spp. We discovered that comammox Nitrospira Clade B was two orders of magnitude more abundant than Clade A in this fertile dairy pasture soil and the most abundant subcluster was a distinctive phylogenetic uncultured subcluster Clade B2. We found that comammox Nitrospira Clade B might not play a major role in nitrite oxidation compared to the role of canonical Nitrospira nitrite-oxidizers, however, comammox Nitrospira Clade B is active in nitrification and the growth of comammox Nitrospira Clade B was inhibited by a high ammonium concentration (700 kg synthetic urine-N ha-1) and the nitrification inhibitor DCD. We concluded that comammox Nitrospira Clade B: (1) was the most abundant comammox in the dairy pasture soil; (2) had a low tolerance to ammonium and can be inhibited by DCD; and (3) was not the dominant nitrite-oxidizer in the soil. This is the first study discovering a new subcluster of comammox Nitrospira Clade B2 from an agricultural soil.

Differential immediate and long-term effects of nitrogen input on denitrification N2O/(N2O+N2) ratio along a 0‒5.2 m soil profile.

High nitrogen (N) input to soil can cause higher nitrous oxide (N2O) emissions, that is, a higher N2O/(N2O+N2) ratio, through an inhibition of N2O reductase activity and/or a decrease in soil pH. We assumed that there were two mechanisms for the effects of N input on N2O emissions, immediate and long-term effect. The immediate effect (field applied fertilizer N) can be eliminated by decreasing the N input, but not the long-term effect (soil accumulated N caused by long-term fertilization). Therefore, it is important to separate these effects to mitigate N2O emissions. To this end, soil samples along a 0‒5.2 m profile were collected from a long-term N fertilization experiment field with two N application rates, that is, 600 kg N ha-1 year-1 (N600) and no fertilizer N input (N0). External N addition was conducted for each subsample in the laboratory incubation study to produce two additional treatments, which were denoted as N600+N and N0+N treatments. The results showed that the combined immediate and long-term effects led to an increase in the N2O/(N2O+N2) ratio by 6.8%. Approximately 32.6% and 67.4% of increase could be explained by the immediate and long-term effects of N input, respectively. Meanwhile, the long-term effects were significantly positively correlated to soil organic carbon (SOC). These results indicate that excessive N fertilizer input to the soil can lead to increased N2O emissions if the soil has a high SOC content. The long-term effect of N input on the N2O/(N2O+N2) ratio should be considered when predicting soil N2O emissions under global environmental change scenarios.

Spatio-temporal variation of net anthropogenic nitrogen inputs (NANI) from 1991 to 2019 and its impacts analysis from parameters in Northwest China.

At present, excessive nutrient inputs caused by human activities have resulted in environmental problems such as agricultural non-point source pollution and water eutrophication. The Net Anthropogenic Nitrogen Inputs (NANI) model can be used to estimate the nitrogen (N) inputs to a region that are related to human activities. To explore the net nitrogen input of human activities in the main grain-producing areas of Northwestern China, the county-level statistical data for the Ningxia province and NANI model parameters were collected, the spatio-temporal distribution characteristics of NANI were analyzed and the uncertainty and sensitivity of the parameters for each component of NANI were quantitatively studied. The results showed that: (1) The average value of NANI in Ningxia from 1991 to 2019 was 7752 kg N km-2 yr-1. Over the study period, the inputs first showed an overall increase, followed by a decrease, and then tended to stabilize. Fertilizer N application was the main contributing factor, accounting for 55.6%. The high value of NANI in Ningxia was mainly concentrated in the Yellow River Diversion Irrigation Area. (2) The 95% confidence interval of NANI obtained by the Monte Carlo approach was compared with the results from common parameters in existing literature. The simulation results varied from -6.4% to 27.4% under the influence of the changing parameters. Net food and animal feed imports were the most uncertain input components affected by parameters, the variation range was -20.7%-77%. (3) The parameters of inputs that accounted for higher proportions of the NANI were more sensitive than the inputs with lower contributions. The sensitivity indexes of the parameters contained in the fertilizer N applications were higher than those of net food and animal feed imports and agricultural N-fixation. This study quantified the uncertainty and sensitivity of parameters in the process of NANI simulation and provides a reference for global peers in the application and selection of parameters to obtain more accurate simulation results.

In situ nitrous oxide and dinitrogen fluxes from a grazed pasture soil following cow urine application at two nitrogen rates.

Cattle grazing of pastures deposits urine onto the pasture soil at high nitrogen (N) rates that exceed the pasture's immediate N demands, increasing the risk of N loss. Nitrous oxide (N2O), a greenhouse gas, and dinitrogen (N2) are lost from the cattle urine patches. There is limited information on the in situ loss of N2 from grazed-pasture systems which is needed for understanding pasture soil N dynamics and balances. The 15N flux method was used to determine N2 and N2O fluxes over time following synthetic urine-15N application at either 400 or 800 kg N ha-1 to a grazed perennial pasture soil. Results showed that daily N2O fluxes were higher under 800 kg N ha-1 than under 400 kg N ha-1, but there was no significant difference in N2 fluxes. Cumulative N2O emissions from soil with 400 kg N ha-1 and 800 kg N ha-1 applied represented 0.16 ± 0.08% and 0.43 ± 0.08% of deposited N, respectively, while emitted N2 accounted for 32.1 ± 4.1% and 14.4 ± 1.7%, respectively, over 95 days after urine application. Codenitrification and denitrification co-occurred, with denitrification accounting for 97.9 to 98.5% of total N2 production. Recovery of urine-15N in pasture decreased with increasing N rate with 14.7 ± 0.5% and 9.9 ± 0.8% recovered at 400 and 800 kg N ha-1, respectively after 95 days. The N2O/(N2 + N2O) product ratio was generally higher during periods of nitrification of urine-N (the first month after urine application) but with no clear relationship to other measured variables. Contrary to our hypothesis, an elevated urine-N rate did not enhance N2 loss. This is speculated to be due to enhanced ammonia volatilisation and transfer of N as nitrate, to deeper soil layers. Soil relative gas diffusivity indicated that high N2 fluxes resulted from entrapped N2 diffusing from the draining soil.

Improving the accuracy of nitrous oxide emission factors estimated for hotspots within dairy-grazed farms.

Nitrous oxide (N2O) emissions from dairy-grazing pastures can be dominated by large emissions from small areas ('hotspots') frequently used by grazing dairy cattle (i.e., water troughs and gateways). N2O emissions from these hotspots are quantified by investigating whether N2O emissions and emission factors (% of applied N emitted as N2O, EF3) from potential hotspots are different from non-hotspots. To better characterise N2O emissions from hotspots and non-hotspots of farms to understand their contributions to national agricultural greenhouse gas inventory calculations, a series of measurements were conducted during winter and spring on two NZ typical dairy farms with contrasting soil drainage (poorly versus well drained). Before measurements were taken, the soils either received a cow urine application or remained untreated. The results showed that changes in water-filled pore space (WFPS) and mineral N around water troughs and gateways, due to additional stock movements and disproportionate excreta-N deposition during previous grazing events, affected both background and total N2O emissions. But there was little impact on EF3 values (calculated using IPCC guidelines) from deposited urine between hotspot and pasture areas. These results suggest the same EF3 values can be used for both to calculate emissions from urine deposited on grazed pastures. However, these results raise concerns about higher background emission in hotspots subtracted from measured emissions from urine-N deposition in calculating EF3 values and discounting the effects of disproportionate N inputs in intensive agriculture on increased background emissions (legacy effect). This IPCC inventory method does not account for the legacy effect of N loading prior to the measurements which may underestimate the emissions. Thus, an allowance for higher hotspot background emissions could be included in the Inventory to accurately estimate total emissions from agriculture.

Combined biochar and double inhibitor application offsets NH3 and N2O emissions and mitigates N leaching in paddy fields.

The effects of combined biochar and double inhibitor application on gaseous nitrogen (N; nitrous oxide [N2O] and ammonia [NH3]) emissions and N leaching in paddy soils remain unclear. We investigated the effects of biochar application at different rates and double inhibitor application (hydroquinone [HQ] and dicyandiamide [DCD]) on NH3 and N2O emissions, N leaching, as well as rice yield in a paddy field, with eight treatments, including conventional urea N application at 280 kg N ha-1 (CN); reduced N application at 240 kg N ha-1 (RN); RN + 7.5 t ha-1 biochar (RNB1); RN + 15 t ha-1 biochar (RNB2); RN + HQ + DCD (RNI); RNB1 + HQ + DCD (RNIB1); RNB2 + HQ + DCD (RNIB2); and a control without N fertilizer. When compared with N leaching under RN, biochar application reduced total N leaching by 26.9-34.8% but stimulated NH3 emissions by 13.2-27.1%, mainly because of enhanced floodwater and soil NH4+-N concentrations and pH, and increased N2O emission by 7.7-21.2%, potentially due to increased soil NO3--N concentrations. Urease and nitrification inhibitor addition decreased NH3 and N2O emissions, and total N leaching by 20.1%, 21.5%, and 22.1%, respectively. Compared with RN, combined biochar (7.5 t ha-1) and double inhibitor application decreased NH3 and N2O emissions, with reductions of 24.3% and 14.6%, respectively, and reduced total N leaching by up to 45.4%. Biochar application alone or combined with double inhibitors enhanced N use efficiency from 26.2% (RN) to 44.7% (RNIB2). Conversely, double inhibitor application alone or combined with biochar enhanced rice yield and reduced yield-scaled N2O emissions. Our results suggest that double inhibitor application alone or combined with 7.5 t ha-1 biochar is an effective practice to mitigate NH3 and N2O emission and N leaching in paddy fields.

Plants are a natural source of nitrous oxide even in field conditions as explained by 15N site preference.

Plants are either recognized to produce nitrous oxide (N2O) or considered as a medium to transport soil-produced N2O. To date, it is not clear whether in their habitat plants conduit N2O produced in soil or are a natural source. We aimed to understand role of plants in N2O emissions in field conditions. Therefore, rubber plants (Ficus elastica) were planted in the field; then plant and soil chambers were deployed simultaneously to collect gas samples, and 15N site preference (SP) of N2O was evaluated. The mean SP values of plant and soil emitted N2O were -20.85 ± 2.8‰ and -8.85 ± 1.08‰, respectively, and were significantly different (p < 0.0001); while bulk 15N of plant and soil emitted N2O were -10.83 ± 3.33‰ and -22.56 ± 3.37‰, respectively and were similar (p = 0.06). In the current study, soil always acted as a source of N2O, while plants were both source and sink. Plant and soil N2O fluxes had significant positive exponential relationship with both soil and air temperature. Soil water-filled pore space (WFPS) had significant negative linear relationship with only soil N2O fluxes. Plant N2O fluxes had significant positive linear relationship with plant respiration rates and negative linear relationship with plant surface areas. Based on the relationship between plant respiration rates and N2O fluxes, we suggest that mitochondria are the possible sites of N2O formation in plant cells while the relationship between plant surface areas and N2O fluxes suggests that roots are the parts of its formation in natural and field conditions. Our results suggest that plants are a natural source of N2O even at field conditions and challenge a view that plants are a medium to transport soil-produced N2O into the atmosphere.