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Understanding of feedbacks between plant N uptake and soil N transformations in different vegetable production systems is crucial to increase vegetable N acquisition and improve N fertilizer management. 15N tracing pot experiments were conducted under greenhouse (GH) and open-field (OF) conditions; plant N uptake and soil gross N transformation rates were simultaneously quantified using the Ntraceplanttool. The feedbacks of plant N uptake on rhizosphere soil gross N transformations differed between GH and OF. Higher plant NH4+and NO3–uptake rates were observed in GH compared to OF, which were associated with higher plant biomass in GH. Rhizosphere soil mineral N production rates (i.e., the sum of mineralization and heterotrophic nitrification rates) in GH ranged from 0.18 to 1.53 mg N kg−1day−1and were significantly lower than in OF (0.60 to 4.56 mg N kg−1day−1). The reduced rhizosphere soil mineral N production rates in GH were attributed to high soil moisture conditions and vegetable activities (especially respiratory activity in rhizosphere). Rhizosphere soil microbial N immobilization rates in GH were 1.6–166.5 times lower compared to OF, which was line with decreased mineral N production rates and enhanced plant N uptake rates. This was also confirmed by the fact that the microbial N immobilization rates showed a negative correlation with the mineral N production rates and the N uptake rates of plants. We showed that the capacity of soil rhizospheric mineral N supply was weaker in GH than OF, which was one of the reasons why more N fertilizer was required to achieve rapid vegetable growth in greenhouse vegetable production. We also highlighted the importance of water management to improve rhizosphere soil N supply in greenhouse vegetable cultivation.

Maize (Zea maysL.) N uptake affects N use efficiency (NUE), which is affected by the N form for uptake. However, maize N preference for NH4+or NO3−is unclear. A field experiment and a 15N tracing study was conducted with various soils differing in pH plant with maize. Maize N (NO3−and NH4+) uptake rate significantly varied with soil pH, but NO3−uptake rate (UNO3) was much higher than NH4+uptake rate (UNH4) irrespectively of the pH, i.e., maize generally preferred NO3−uptake over NH4+at the seedling stage. The generally higher UNH4and UNO3in acidic compared to alkaline soils in the laboratory study were consistent with the maize yield in the field experiment results. The positive relationship between UNO3and UNH4(P< 0.01) indicated a synergistic effect between NH4+and NO3−uptake. The longer NH4+residence time could explain the higher NH4+and total N uptake rates in acidic soil than alkaline soil. With maize planted, the rate of autotrophic nitrification (ONH4) reduced between 30 and 75% compared to unplanted soils supporting N uptake via a prolonged NH4+residence time. The rate of heterotrophic nitrification (ONrec) was stimulated by maize plants, accounting for up to 45% of total NO3−production in acidic soils. The role of ONrecwas illustrated by a positive correlation of ONrec/(ONH4+ ONrec) with UNO3(P< 0.01). Maize seedling preferred NO3−over NH4+irrespectively of pH. Moreover, maize can regulate soil N supply through interactions between plant and soil N transformations to meet their N acquisition.

Dissimilatory nitrate reduction to ammonium (DNRA), the nearly forgotten process in the terrestrial nitrogen (N) cycle, can conserve N by converting the mobile nitrate into non-mobile ammonium avoiding nitrate losses via denitrification, leaching, and runoff. However, global patterns and controlling factors of soil DNRA are still only rudimentarily known. By a meta-analysis of 231 observations from 85 published studies across terrestrial ecosystems, we find a global mean DNRA rate of 0.31 ± 0.05 mg N kg–1day–1, being significantly greater in paddy soils (1.30 ± 0.59) than in forests (0.24 ± 0.03), grasslands (0.52 ± 0.15), and unfertilized croplands (0.18 ± 0.04). Soil DNRA was significantly enhanced at higher altitude and lower latitude. Soil DNRA was positively correlated with precipitation, temperature, pH, soil total carbon, and soil total N. Precipitation was the main stimulator for soil DNRA. Total carbon and pH were also important factors, but their effects were ecosystem-specific as total carbon stimulates DNRA in forest soils, whereas pH stimulates DNRA in unfertilized croplands and paddy soils. Higher temperatures inhibit soil DNRA via decreasing total carbon. Moreover, nitrous oxide (N2O) emissions were negatively related to soil DNRA. Thus, future changes in climate and land-use may interact with management practices that alter soil substrate availability and/or soil pH to enhance soil DNRA with positive effects on N conservation and lower N2O emissions.

Globally, agricultural soils account for approximately one-third of anthropogenic emissions of the potent greenhouse gas and stratospheric ozone-depleting substance nitrous oxide (N2O). Emissions of N2O from agricultural soils are affected by a number of global change factors, such as elevated air temperatures and elevated atmospheric carbon dioxide (CO2). Yet, a mechanistic understanding of how these climatic factors affect N2O emissions in agricultural soils remains largely unresolved. Here, we investigate the soil N2O emission pathway using a 15N tracing approach in a nine-year field experiment using a combined temperature and free air carbon dioxide enrichment (T-FACE). We show that the effect of CO2enrichment completely counteracts warming-induced stimulation of both nitrification- and denitrification-derived N2O emissions. The elevated CO2induced decrease in pH and labile organic nitrogen (N) masked the stimulation of organic carbon and N by warming. Unexpectedly, both elevated CO2and warming had little effect on the abundances of the nitrifying and denitrifying genes. Overall, our study confirms the importance of multifactorial experiments to understand N2O emission pathways from agricultural soils under climate change. This better understanding is a prerequisite for more accurate models and the development of effective options to combat climate change.

Managing reactive nitrogen (Nr) to achieve a sustainable balance between production of food, feed and fiber, and environmental protection is a grand challenge in the context of an increasingly affluent society. Here, we propose a novel framework for national nitrogen (N) assessments enabling a more consistent comparison of the uses, losses and impacts of Nrbetween countries, and improvement of Nrmanagement for sustainable development at national and regional scales. This framework includes four key components: national scale N budgets, validation of N fluxes, cost-benefit analysis and Nrmanagement strategies. We identify four critical factors for Nrmanagement to achieve the sustainable development goals: N use efficiency (NUE), Nrrecycling ratio (e.g., ratio of livestock excretion applied to cropland), human dietary patterns and food waste ratio. This framework was partly adopted from the European Nitrogen Assessment and now is successfully applied to China, where it contributed to trigger policy interventions toward improvements for future sustainable use of Nr. We demonstrate how other countries can also benefit from the application our framework, in order to include sustainable Nrmanagement under future challenges of growing population, hence contributing to the achievement of some key sustainable development goals (SDGs).

Although to date individual gross N transformations could be quantified by 15N tracing method and models, studies are still limited in paddy soil. An incubation experiment was conducted using topsoil (0–20 cm) and subsoil (20–60 cm) of two paddy soils, alkaline and clay (AC) soil and neutral and silt loam (NSL) soil, to investigate gross N transformation rates. Soil samples were labeled with either 15NH4NO3or NH415NO3, and then incubated at 25 °C for 168 h at 60% water–holding capacity. The gross N mineralization (recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil, and the gross N nitrification (autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil. Although gross NO3−consumption (i.e., NO3−immobilization and dissimilatory NO3−reduction to NH4+) rates increased with increasing gross nitrification rates, the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil. These showed that high NO3−production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO3−can increase the risk of NO3−loss through leaching and denitrification.

Temperate grasslands in arid and semiarid regions cover about 40% of the total land area in China. So far, only a few studies have studied the N transformations in these important ecosystems. In the present study, soil gross N transformation rates in Inner Mongolia temperate grasslands in China were determined using a 15N tracing experiment and combined with a literature synthesis to identify the soil N transformation characteristics and their controlling factors in a global perspective. Our results showed that the rates of gross N mineralization and immobilization NH4+were significantly lower, while autotrophic nitrification rates were significantly higher in Chinese temperate grassland soils compared to other regions in the world. In particular, the primary mineral N consumption processes, i.e., immobilization of NO3−and NH4+, and dissimilatory nitrate reduction to ammonium, were on average much lower in temperate grassland soils in China, compared to other temperate grassland regions. The reduced heterotrophic activity and microbial growth associated with lower soil organic carbon and arid climate (e.g., mean annual precipitation) were identified as the main factors regulating soil N cycling in the studied regions in China. To restrict NO3−accumulation and associated high risks of N losses in these arid and semiarid ecosystems in China, it is important to develop the regimes of soil organic C and water management that promote the retention of N in these grassland ecosystems. Immobilization NH4+rate was significantly lower in Chinese temperate grassland soils than other regions in the worldAutotrophic nitrification rates were significantly higher in Chinese temperate grassland soils than other regions in the worldLow SOC and arid climate were the main limiting factors regulating soil N cycling in Chinese temperate grassland soils

In March 2013, 40 leading experts from across the world gathered at a workshop, hosted by the European Commission, Directorate General Joint Research Centre, Italy, to discuss the multiple benefits of soil carbon as part of a Rapid Assessment Process (RAP) project commissioned by Scientific Committee on Problems of the Environment (SCOPE). This collaboration led to the publication of the SCOPE Series Volume 71 “Soil Carbon: Science, Management and Policy for Multiple Benefits”; which brings together the essential scientific evidence and policy opportunities regarding the global importance of soil carbon. This short communication summarises the key messages of the assessment including research and policy implications.

Abstract: Terrestrial ecosystems may act as a source or a sink for the atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), depending on land use and management. This paper reviews the literature on carbon, CH4, and N2O fluxes from terrestrial ecosystems in China, and analyzes its national greenhouse gas budget. Carbon storage in biomass and soils in Chinese terrestrial ecosystems decreased in the past 300 years, due to deforestation and expansion of cultivated land, and reached a minimum in the late 1970s. Since then, carbon storage has increased at an estimated rate of 0.19 to 0.26 Pg C yr−1, mainly owing to reforestation and afforestation. CH4 emission from natural wetlands decreased from 6.65 Tg CH4 yr−1 in 1990 to 5.71 Tg CH4 yr−1 in 2000 owing to the decrease in wetland area. CH4 emission from flooded rice fields was 7.41 Tg CH4 yr−1. At the same time, aerobic soils took up atmospheric CH4 at a rate of 2.56 Tg CH4 yr−1. Nitrous oxide emission from forestlands, grasslands, and farmlands was positively correlated with precipitation at a national scale, and the emission rate was positively correlated with the CH4 uptake rate of forestlands and grasslands (P<0.01). Natural N2O sources were estimated to be 419 Gg N yr−1 and anthropogenic sources (from farmlands) to be 292 to 476.3 Gg N yr−1, with a mean of 372.6 Gg N yr−1. The integrated budget of greenhouse gasses indicates that Chinese terrestrial ecosystems act as a small net sink for global warming potential (GWP), ranging from 0.04 to 0.32 Pg CO2-eq yr−1, in a striking contrast to terrestrial ecosystems globally, which are a source of 2.75 to 6.78 Pg CO2-eq yr−1. The ratios of anthropogenic to natural sources of CH4 and N2O are much larger in Chinese terrestrial ecosystems than they are in global averages, reflecting greater human disturbance of terrestrial ecosystems in China.

Long‐term field fertilization experiments may provide useful insight into the relative contributions of NH3–oxidizing bacteria (AOB) and NH3–oxidizing archaea (AOA) to soil nitrification. In this study, the abundance and composition of AOB and AOA were investigated in bulk soil from paddy field plots (4 by 5 m) that received no fertilization (CK), 180 kg urea N ha−1yr−1(NPK), or 180 kg urea N plus 4500 kg rice (Oryza sativaL.) straw ha−1yr−1(NPK/OM) since 1988 (each with three replicated plots). Our results clearly indicate that long‐term fertilization (22 yr) significantly altered the community structure of AOB rather than AOA in the soil within fertilized plots compared with the CK plot. Quantitative polymerase chain reaction analysis of the amoAgenes that encode the ammonia monooxygenase subunit A revealed that the AOB in the fertilized plots were 50 times more abundant than in the CK plot. Denaturing gradient gel electrophoresis fingerprinting of the amoAgenes demonstrated a drastic shift in the AOB community structure, resulting in highly enriched AOB similar to Nitrosospiracluster 3 in the fertilized plots. In contrast, the population size and composition of the AOA community remained largely unchanged in all plots. Furthermore, a positive correlation was observed between the bacterial rather than archaeal amoAgene copy numbers and potential nitrification activity among the soils. The distinct responses of AOA and AOB to long‐term field fertilization were probably related to the availability of NH3. Our results suggest that AOB might play major roles in the NH3oxidation that occurs in bulk soil under the in situ fertilized paddy field conditions tested in this study.

It has been reported that 15N‐NO3−added to soils cannot be completely recovered in the remaining NO3−and its measurable products under anaerobic conditions. Abiotic immobilization of NO3−into dissolved organic N (DON) has been hypothesized to explain this phenomenon. Six forest soil samples were collected from temperate, subtropical, and tropical regions in East China, amended with K15NO3, and incubated at 25°C under anaerobic conditions for 264 h. The results showed that only 36 to 56% of the added N was recovered in the temperate soils, while up to 90% of the added N was recovered in the subtropical and tropical soils 1.5 h after incubation commenced. Compared with the initial concentration, the DON concentration dramatically increased when measured in temperate soils at 1.5 h, but increased only slightly in the subtropical and tropical soils. In all the soils, there was a strong positive correlation between recovery of added N at 1.5 h and soil redox potential (Eh) (R2= 0.87, P< 0.01), and a strong negative correlation between added N recovery at 1.5 h and dissolved organic C (measured at the beginning of incubation) (R2= 0.72, P< 0.05). There was a significant decrease in recovery of added N along with an increase in DON as soil Eh fell in the subtropical and tropical soils during long‐term incubation, suggesting that a majority of unrecovered N is incorporated into dissolved organic matter. All these results indicate added N was rapidly converted to DON via abiotic immobilization and was an important component of the N cycle in temperate forest soils but not in subtropical and tropical soils with high soil Eh, which was the controlling factor affecting this process.

Concerns regarding the detrimental effects of burning crop residues on human health and the environment have increased interest in alternative uses of crop residues. We examined the in situ use of crop residue as a source of supplemental N for succeeding crops in rice (Oryza sativaL.)‐based cropping systems at three sites during 3 yr. The experiments included a rice–wheat (Triticum aestivumL.) rotation at Yixing, Jiangsu Province, China; a rice–wheat rotation at Ludhiana, Punjab, India; and double‐rice cropping at Taojiang, Hunan Province, China. The supply of N from crop residues was assessed in the absence of fertilizer N as the difference in total plant N between plots with and without residue. At Yixing, incorporation of wheat residue before rice significantly increased the N supply to the rice by 14 kg N ha−1averaged across 3 yr. At Ludhiana, incorporation of rice residue before wheat reduced the N supply by 3 kg N ha−1to the wheat, but increased the N supply by 5 kg N ha−1to the rice crop following the wheat. In all cases, the return of crop residues had no net benefit on crop yield when fertilizer N was supplied at rates sufficient to eliminate N deficiency. The incorporation of crop residues did not increase the N supply to the succeeding crop during its vegetative growth phase, but the N supply to the crop at later growth stages was often increased. Adjustments in the timing and rate of fertilizer N are probably necessary to optimally supply N to crops receiving residues.

To evaluate the response of soil respiration to soil moisture, temperature, and N fertilization, and estimate the contribution of soil and rhizosphere respiration to total soil CO2emissions, a field experiment was conducted in the Fengqiu State Key Agro‐Ecological Experimental Station, Henan, China. The experiment included four treatments: bare soil fertilized with 150 kg N ha−1(CK), and maize (Zea maysL.)‐cropped soils amended with 0 (N0), 150 (N150), and 250 (N250) kg N ha−1Mean seasonal soil CO2emissions in the CK, N0, N150, and N250 treatments were estimated to be 294, 598, 541, and 539 g C m−2, respectively. The seasonal soil CO2fluxes were significantly affected by soil temperature, with the change in the rate of flux for each 10°C increase in temperature (Q10) of 1.90 to 2.88, but not by soil moisture. Nitrogen fertilization resulted in a 10.5% reduction in soil CO2flux; however, it did not significantly increase the maize aboveground biomass but did increase maize yield. Soil respiration measurement using the root‐exclusion technique indicated that soils fertilized with 150 kg N ha−1contributed 54% of the total soil CO2emission, or 8% of soil organic C down to a depth of 40 cm. An amount of C equivalent to 26% of the net assimilated C in harvested above‐ and belowground plant biomass was returned to the atmosphere by rhizosphere respiration.

To understand the underlying factors affecting the seasonal variation of the methane concentration in a cool temperate freshwater marsh vegetated with Carex lasiocarpa in the Sanjiang Plain of northeast China, we measured methane emission from, and the concentrations of methane, dissolved organic carbon (DOC) and acetate in, water samples taken from the standing water surface to the top of the gley soil layer in the C. laisocarpa marsh, before and after plants were covered with a black cloth at the three growing stages of June, July and August 2002. The methane oxidation rate was also measured in situ by applying acetylene, a methane oxidation inhibitor, to whole plants, and the methane production rate in water sampled in June and July was measured via the anaerobic incubation in the laboratory. The methane production rate in water samples was significantly correlated with acetate concentration rather than DOC concentration, whereas the mean acetate concentration in water samples was higher in June than in July and August. Hence, the low methane concentration in June did not result from a lack of acetate for methane production. The mean methane and DOC concentrations in water samples were enhanced by 22.3 and 31.1% in June, 2.1 and 5.0% in July, and 3.4 and 15.2% in August, respectively, after plants were covered with a black cloth. The methane oxidation rate and redox potential in the freshwater marsh decreased from June to July or August. These results suggest that there was more oxygen in the rhizome and rhizosphere in June than in July and August, which not only accentuated methane oxidation but also reduced methane production. Therefore, the high methane concentration in water in July and August could be ascribed to both an increase in temperature and a decrease in redox potential or oxygen concentration in the rhizosphere.

Rice fields are either continuously flooded or drained in China in the winter (non-rice growth season). Due to great spatial variation of precipitation and temperature, there is a spatial variation of soil moisture in the fields under drained conditions during the winter season. The effect of water regime in winter on CH4emissions during the following rice growing period and their regional variation were investigated. Soil moisture in the winter was simulated by DNDC model with daily precipitation and temperature as model inputs. Under the same management during the rice growing period, CH4emissions was higher from rice fields flooded, compared to those from fields drained during winter. CH4emission from rice fields correlated significantly with simulated soil moisture and with mean precipitation of the preceding winter season. Spatial variation of precipitation in winter and corresponding variations of soil moisture regimes control the regional and annual variation of CH4emissions from rice fields in China. Keeping soils drained as much as possible during winter seems to be a feasible option to reduce CH4emissions during the following rice growing seasons.

A pot experiment was carried out to investigate the effect of soil water content in the non-rice growth season (winter season) on CH4emission during the following rice-growing period. The results showed that CH4fluxes increased significantly with the increase of soil water content in the winter season, except air-dry water condition. The mean CH4fluxes of treatments with soil water contents in the winter of 3.89–5.37% (air-dry), 25–35%, 50–60%, 75–85% and 107% (flooded) of field water capacity (FWC) were 13.04, 4.04, 8.61, 13.26 and 20.47 mg m−2h−1, respectively. Antecedent soil water contents also markedly affected temporal variation patterns of CH4fluxes and soil redox potential (Eh) during the rice-growing period. The higher soil water contents in the winter season were, the quicker soil Ehdecreased, and the earlier CH4emission occurred after rice transplanting, except air-dry water condition. Though the seasonal mean CH4flux was significantly correlated with the seasonal mean soil Eh, the seasonal variation of CH4fluxes was not always significantly correlated with soil Eh. For the treatment flooded in the fallow season, there was no significant correlation between CH4flux and soil Eh, but there was significant correlation between CH4flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil Eh, not soil temperature that significantly affected the temporal variation of CH4emissions. Soil water contents in the fallow season significantly influenced concentrations of soil labile organic carbon (including undecomposed plant debris), active Fe and Mn immediately before rice transplanting. The mean CH4fluxes during rice-growing period were significantly correlated with soil labile organic carbon contents (positively) and contents of soil active Fe and Mn (negatively).

Coupled nitrification and denitrification is considered as one of the main pathways of nitrogen losses in paddy soils. The effect of NO3−on NH4+transformation was investigated by using the 15N technique. The paddy soils were collected from Wuxi (soil pH 5.84) and Yingtan (soil pH 5.02), China. The soils were added with either urea (18.57 μmol urea-N enriched with 60 atom% 15N excess) plus 2.14 μmol KNO3-N (natural abundance) per gram soil (U+NO3) or urea alone (U). The KNO3was added 6 days after urea addition. The incubation was carried out under flooded condition in either air or N2gas headspace at 25°C. The results showed that in air headspace, 15NH4+oxidization was so fast that about 10% and 8% of added 15N in the treatment U could be oxidized during the incubation period of 73 hours after KNO3addition in Wuxi and Yingtan soil, respectively. The addition of KNO3significantly inhibited 15NH4+oxidation (p<0.01) in air headspace, while it stimulated 15NH4+oxidation in N2gas headspace, although the oxidation was depressed by the N2gas headspace itself. Therefore, the accumulation of NO3−would inhibit further nitrification of NH4+at micro-aerobic sites in paddy soils, especially in paddy soils with a low denitrification rate. On the other hand, NO3−would lead to oxidation of NH4+in anaerobic bulk soils.

In order to make more efficient use of mineral nitrogen fertilizers, the mineralization of organic N has to be fully understood and taken into account when meeting the nitrogen demand of crops. Aerobic long-term incubation experiments (147 days) based on the method by Stanford and Smith (1972, Soil Sci Soc Am Proc 36: 465), modified by Nordmeyer and Richter (1985, Plant Soil 39: 433), were carried out with paddy soils (Typic Epiaquepts) from two different locations in the Taihu Region in Eastern China, each in three depth increments (0–30, 30–60, 60–90 cm). The two soils had pH (H2O) values between 6 and 7, silty clay loam texture, Corgcontents (0–30 cm) of 8.9 and 16.6 g kg−1and Ntotcontents of 1030 and 1650 mg kg−1, respectively. Mineralization parameters were estimated from the cumulative amounts of leached nitrate-N via non-linear regression, using a two-pool approach based on first-order kinetics reactions. The results were compared to those from incubation experiments carried out with topsoils from the southern edge of the Chinese Loess Plateau (Roelcke et al., 1996, Trans. 9th Nitrogen Workshop, Braunschweig, p, 313). The optimized parameters were used to simulate the N regime in paddy soils with a deterministic model (Han et al., 2001, Proc. Int. Conf. China, p. 411). A winter wheat growing period (November 1995 to June 1996) in Wuxi County was simulated. A total of 220 kg ha−1mineral N fertilizer were applied as NPK and urea, split into four doses. Simulated results were compared to the measured mineral nitrogen (Nmin) contents over the growing season. Cumulative aerobic N mineralization (0–1.0 m profile) yielded 84 kg N ha−1. The modelled total cumulative N losses amounted to 76 kg N ha−1or 35% of the N applied.