Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N₂O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. −30%) influenced soil N₂O and carbon dioxide (CO₂) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N₂O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N₂O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N₂O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.     

Symbiotic relationships between soil fungi and plants reduce N2O emissions from soil

N₂O is a potent greenhouse gas involved in the destruction of the protective ozone layer in the stratosphere and contributing to global warming. The ecological processes regulating its emissions from soil are still poorly understood. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with the majority of land plants and which influence a range of important ecosystem functions, can induce a reduction in N₂O emissions from soil. To test for a functional relationship between AMF and N₂O emissions, we manipulated the abundance of AMF in two independent greenhouse experiments using two different approaches (sterilized and re-inoculated soil and non-mycorrhizal tomato mutants) and two different soils. N₂O emissions were increased by 42 and 33% in microcosms with reduced AMF abundance compared to microcosms with a well-established AMF community, suggesting that AMF regulate N₂O emissions. This could partly be explained by increased N immobilization into microbial or plant biomass, reduced concentrations of mineral soil N as a substrate for N₂O emission and altered water relations. Moreover, the abundance of key genes responsible for N₂O production (nirK) was negatively and for N₂O consumption (nosZ) positively correlated to AMF abundance, indicating that the regulation of N₂O emissions is transmitted by AMF-induced changes in the soil microbial community. Our results suggest that the disruption of the AMF symbiosis through intensification of agricultural practices may further contribute to increased N₂O emissions.     

Residue incorporation depth is a controlling factor of earthworm‐induced nitrous oxide emissions

Earthworms can increase nitrous oxide (N₂O) emissions, particularly in no‐tillage systems where earthworms are abundant. Here, we study the effect of residue incorporation depth on earthworm‐induced N₂O emissions. We hypothesized that cumulative N₂O emissions decrease with residue incorporation depth, because (i) increased water filled pore space (WFPS) in deeper soil layers leads to higher denitrification rates as well as more complete denitrification; and (ii) the longer upward diffusion path increases N₂O reduction to N₂. Two 84‐day laboratory mesocosm experiments were conducted. First, we manually incorporated maize (Zea mays L.) residue at different soil depths (incorporation experiment). Second, ¹³C‐enriched maize residue was applied to the soil surface and anecic species Lumbricus terrestris (L.) and epigeic species Lumbricus rubellus (Hoffmeister) were confined to different soil depths (earthworm experiment). Residue incorporation depth affected cumulative N₂O emissions in both experiments (P < 0.001). In the incorporation experiment, N₂O emissions decreased from 4.91 mg N₂O–N kg^?1 soil (surface application) to 2.71 mg N₂O–N kg^?1 soil (40–50 cm incorporation). In the earthworm experiment, N₂O emissions from L. terrestris decreased from 3.87 mg N₂O–N kg^?1 soil (confined to 0–10 cm) to 2.01 mg N₂O–N kg^?1 soil (confined to 0–30 cm). Both experimental setups resulted in dissimilar WFPS profiles that affected N₂O dynamics. We also found significant differences in residue C recovery in soil organic matter between L. terrestris (28–41%) and L. rubellus (56%). We conclude that (i) N₂O emissions decrease with residue incorporation depth, although this effect was complicated by dissimilar WFPS profiles; and (ii) larger residue C incorporation by L. rubellus than L. terrestris indicates that earthworm species differ in their C stabilization potential. Our findings underline the importance of studying earthworm diversity in the context of greenhouse gas emissions from agro‐ecosystems.     

Predicting in situ soil N2O emission using NOE algorithm and soil database

This paper presents a new algorithm, Nitrous Oxide Emission (NOE) for simulating the emission of the greenhouse gas N₂O from agricultural soils. N₂O fluxes are calculated as the result of production through denitrification and nitrification and reduction through the last step of denitrification. Actual denitrification and nitrification rates are calculated from biological parameters and soil water‐filled pore space, temperature and mineral nitrogen contents. New suggestions in NOE consisted in introducing (1) biological site‐specific parameters of soil N₂O reduction and (2) reduction of the N₂O produced through nitrification to N₂ through denitrification. This paper includes a database of 64 N₂O fluxes measured on the field scale with corresponding environmental parameters collected from five agricultural situations in France. This database was used to test the validity of this algorithm. Site per site comparison of simulated N₂O fluxes against observed data leads to mixed results. For 80% of the tested points, measured and simulated fluxes are in accordance whereas the others resulted in an important discrepancy. The origin of this discrepancy is discussed. On the other hand, mean annual fluxes measured on each site were strongly correlated to mean simulated annual fluxes. The biological site‐specific parameter of soil N₂O reduction introduced into NOE appeared particularly useful to discriminate the general level of N₂O emissions from site to site. Furthermore, the relevance of NOE was confirmed by comparing measured and simulated N₂O fluxes using some data from the US TRAGNET database. We suggest the use of NOE on a regional scale in order to predict mean annual N₂O emissions.     

Greenhouse gas budget (CO2, CH4 and N2O) of intensively managed grassland following restoration

The first full greenhouse gas (GHG) flux budget of an intensively managed grassland in Switzerland (Chamau) is presented. The three major trace gases, carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) were measured with the eddy covariance (EC) technique. For CO₂ concentrations, an open‐path infrared gas analyzer was used, while N₂O and CH₄ concentrations were measured with a recently developed continuous‐wave quantum cascade laser absorption spectrometer (QCLAS). We investigated the magnitude of these trace gas emissions after grassland restoration, including ploughing, harrowing, sowing, and fertilization with inorganic and organic fertilizers in 2012. Large peaks of N₂O fluxes (20–50 nmol m^?2 s^?1 compared with a <5 nmol m^?2 s^?1 background) were observed during thawing of the soil after the winter period and after mineral fertilizer application followed by re‐sowing in the beginning of the summer season. Nitrous oxide (N₂O) fluxes were controlled by nitrogen input, plant productivity, soil water content and temperature. Management activities led to increased variations of N₂O fluxes up to 14 days after the management event as compared with background fluxes measured during periods without management (<5 nmol m^?2 s^?1). Fluxes of CO₂ remained small until full plant development in early summer 2012. In contrast, methane emissions showed only minor variations over time. The annual GHG flux budget was dominated by N₂O (48% contribution) and CO₂ emissions (44%). CH₄ flux contribution to the annual budget was only minor (8%). We conclude that recently developed multi‐species QCLAS in an EC system open new opportunities to determine the temporal variation of N₂O and CH₄ fluxes, which further allow to quantify annual emissions. With respect to grassland restoration, our study emphasizes the key role of N₂O and CO₂ losses after ploughing, changing a permanent grassland from a carbon sink to a significant carbon source.     

What plant functional traits can reduce nitrous oxide emissions from intensively managed grasslands?

Plant species exert a dominant control over the nitrogen (N) cycle of natural and managed grasslands. Although in intensively managed systems that receive large external N inputs the emission of the potent greenhouse gas nitrous oxide (N₂O) is a crucial component of this cycle, a mechanistic relationship between plant species and N₂O emissions has not yet been established. Here we use a plant functional trait approach to study the relation between plant species strategies and N₂O emissions from soils. Compared to species with conservative strategies, species with acquisitive strategies have higher N uptake when there is ample N in the soil, but also trigger N mineralization when soil N is limiting. Therefore, we hypothesized that (1) compared to conservative species, species with acquisitive traits reduce N₂O emissions after a high N addition; and (2) species with conservative traits have lower N₂O emissions than acquisitive plants if there is no high N addition. This was tested in a greenhouse experiment using monocultures of six grass species with differing above‐ and below‐ground traits, growing across a gradient of soil N availability. We found that acquisitive species reduced N₂O emissions at all levels of N availability, produced higher biomass and showed larger N uptake. As such, acquisitive species had 87% lower N₂O emissions per unit of N uptake than conservative species (p < .05). Structural equation modelling revealed that specific leaf area and root length density were key traits regulating the effects of plants on N₂O emission and biomass productivity. These results provide the first framework to understand the mechanisms through which plants modulate N₂O emissions, pointing the way to develop productive grasslands that contribute optimally to climate change mitigation.     

Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO2 (Swiss FACE)

Emissions of N₂O were measured during the growth season over a year from grass swards under ambient (360 μL L^?1) and elevated (600 μL L^?1) CO₂ partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m^?2 yr^?1) and low (14 g N m^?2 yr^?1) rates of fertilizer application, split over 5 re‐growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO₂ increased annual emissions of N₂O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N₂O emissions) of 179 and 111 g CO₂ equivalents m^?2, respectively, compared with the GWP of ambient pCO₂ swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO₂Lolium swards were attributed to greater below‐ground C allocation under elevated pCO₂ providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N₂O‐N m^?2 yr^?1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO₂. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO₂Lolium swards measured during the first two re‐growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re‐growths (April–June 2001) and the third, fourth and fifth re‐growths (late June–October 2000), with available soil NO₃^? and rainfall explaining 70%, and soil water content explaining 72% of the variability in N₂O in these periods, respectively. Caution is therefore required when extrapolating from short‐term measurements to predict long‐term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO₂ may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N₂O, thereby exacerbating the forcing effect of elevated CO₂ on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO₂ concentrations on global warming.     

Spatial Variation in Denitrification and N2O Emission in Relation to Nitrate Removal Efficiency in a N-stressed Riparian Buffer Zone

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N₂O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N₂O were measured using flux chambers and incubation experiments. In winter, N₂O emissions were significantly higher (12.4 mg N m⁻² d⁻¹) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m⁻² d⁻¹). In summer a reverse pattern was observed, with higher N₂O emissions (13.6 mg N m⁻² d⁻¹) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N₂O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N₂O emission from riparian zones. Furthermore, we conclude that high variability in N₂O:N₂ ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.     

京郊典型设施蔬菜地土壤N_2O排放特征

利用静态暗箱-气相色谱法对北京郊区设施蔬菜地典型种植模式(番茄-白菜-生菜)下土壤N2O排放特征进行了周年(2012年2月22日—2013年2月23日)观测,探讨了不同处理下(即不施氮肥处理(CK)、农民习惯施肥处理(FP)、减氮优化施肥处理(OPT)和减氮优化施肥+硝化抑制剂处理(OPT+DCD))N2O排放特征及土壤温度、土壤湿度、土壤无机氮含量对土壤N2O排放的影响。结果表明:每次施肥+灌溉之后设施蔬菜地会出现明显的N2O排放高峰,持续时间一般为3—5 d。不同处理N2O排放通量变化范围在-0.21—14.26 mg N2O m-2h-1,平均排放通量0.03—0.36 mg N2O m-2h-1。整个蔬菜生长季各处理N2O排放与土壤孔隙含水率(WFPS)均表现出极显著的正相关关系(P0.01);不施氮处理5 cm深度土壤温度与N2O排放通量呈现显著的正相关关系(P0.05);各处理N2O排放与土壤表层硝态氮含量具有较一致变化趋势。不同处理下N2O年度排放总量差异显著,依次顺序为FP((20.66±0.91)kg N/hm2)OPT((12.79±1.33)kg N/hm2)OPT+DCD((8.03±0.37)kg N/hm2)。与FP处理相比,OPT处理和OPT+DCD处理N2O年排放总量分别减少了38.09%和61.13%。各处理N2O排放系数介于0.36%—0.77%,低于IPCC 1.0%的推荐值。在目前的管理措施下,合理减少施氮量和添加硝化抑制剂是减少设施蔬菜地N2O排放量的有效途径。     

Intensive measurement of nitrous oxide emissions from a corn–soybean–wheat rotation under two contrasting management systems over 5 years

No‐tillage (NT), a practice that has been shown to increase carbon sequestration in soils, has resulted in contradictory effects on nitrous oxide (N₂O) emissions. Moreover, it is not clear how mitigation practices for N₂O emission reduction, such as applying nitrogen (N) fertilizer according to soil N reserves and matching the time of application to crop uptake, interact with NT practices. N₂O fluxes from two management systems [conventional (CP), and best management practices: NT + reduced fertilizer (BMP)] applied to a corn (Zea mays L.), soybean (Glycine max L.), winter‐wheat (Triticum aestivum L.) rotation in Ontario, Canada, were measured from January 2000 to April 2005, using a micrometeorological method. The superimposition of interannual variability of weather and management resulted in mean monthly N₂O fluxes ranging from − 1.9 to 61.3 g N ha⁻¹ day⁻¹. Mean annual N₂O emissions over the 5‐year period decreased significantly by 0.79 from 2.19 kg N ha⁻¹ for CP to 1.41 kg N ha⁻¹ for BMP. Growing season (May–October) N₂O emissions were reduced on average by 0.16 kg N ha⁻¹ (20% of total reduction), and this decrease only occurred in the corn year of the rotation. Nongrowing season (November–April) emissions, comprised between 30% and 90% of the annual emissions, mostly due to increased N₂O fluxes during soil thawing. These emissions were well correlated (r²= 0.90) to the accumulated degree‐hours below 0 °C at 5 cm depth, a measure of duration and intensity of soil freezing. Soil management in BMP (NT) significantly reduced N₂O emissions during thaw (80% of total reduction) by reducing soil freezing due to the insulating effects of the larger snow cover plus corn and wheat residue during winter. In conclusion, significant reductions in net greenhouse gas emissions can be obtained when NT is combined with a strategy that matches N application rate and timing to crop needs.     

Impacts of monoculture cropland to alley cropping agroforestry conversion on soil N2O emissions

Monoculture croplands are a major source of global anthropogenic emissions of nitrous oxide (N₂O), a potent greenhouse gas that contributes to ozone depletion. Agroforestry has the potential to reduce N₂O emissions. Presently, there is no systematic comparison of soil N₂O emissions between cropland agroforestry and monoculture systems in Central Europe. We investigated the effects of converting the monoculture cropland system into the alley cropping agroforestry system on soil N₂O fluxes at three sites (each site has paired agroforestry and monoculture) in Germany, where agroforestry combined crop rows and poplar short-rotation coppice (SRC). We measured soil N₂O fluxes monthly over 2 years (March 2018–January 2020) using static vented chambers. Annual soil N₂O emissions from agroforestry ranged from 0.21 to 2.73 kg N ha⁻¹ year⁻¹, whereas monoculture N₂O emissions ranged from 0.34 to 3.00 kg N ha⁻¹ year⁻¹. During the rotation of corn crop, with high fertilization rates, agroforestry reduced soil N₂O emissions by 9% to 56% compared to monocultures. This was mainly caused by low soil N₂O emissions from the unfertilized agroforestry tree rows. Soil N₂O fluxes were predominantly controlled by soil mineral N in both agroforestry and monoculture systems. Our findings suggest that optimized fertilizer input will further enhance the potential of agroforestry for mitigating N₂O emissions.     

Impact of six lignocellulosic biochars on C and N dynamics of two contrasting soils

Both soil and biochar properties are known to influence greenhouse gas emissions from biochar‐amended soils, but poor understanding of underlying mechanisms challenges prediction and modeling. Here, we examine the effect of six lignocellulosic biochars produced from the pyrolysis of corn stover and wood feedstocks on CO₂ and N₂O emissions from soils collected from two bioenergy cropping systems. Effects of biochar on total accumulated CO₂‐C emissions were minimal (<0.45 mg C g^?1 soil; <10% of biochar C), consistent with mineralization and hydrolysis of small labile organic and inorganic C fractions in the studied biochars. Comparisons of soil CO₂ emissions with emissions from microbially inoculated quartz–biochar mixtures (‘quartz controls’) provide evidence of soil and biochar‐specific negative priming. Five of six biochar amendments suppressed N₂O emissions from at least one soil, and the magnitude of N₂O emissions suppression varied with respect to both biochar and soil types. Biochar amendments consistently decreased final soil NO₃^? concentrations, while contrasting effects on pH, NH₄⁺, and DOC highlighted the potential for formation of anaerobic microsites in biochar‐amended soils and consequential shifts in the soil redox environment. Thus, results implicated both reduced substrate availability and redox shifts as potential factors contributing to N₂O emission suppression. More research is needed to confirm these mechanisms, but overall our results suggest that soil biochar amendments commonly reduce N₂O emissions and have little effect on CO₂ emissions beyond the mineralization and/or hydrolysis of labile biochar C fractions. Considering the large C credit for the biochar C, we conclude that biochar amendments can reduce greenhouse gas emissions and enhance the climate change mitigation potential of bioenergy cropping systems.     

UV-B增强下施硅对稻田CH4和N2O排放及其增温潜势的影响

大气平流层臭氧损耗导致的地表紫外辐射增强作为全球变化重要问题之一,受到广泛关注。硅是水稻生长有益元素,但施硅是否影响稻田CH_4和 N_2O排放,迄今相关报道尚不多见。通过大田试验,研究UV-B增强下施硅对水稻生长、稻田甲烷(CH_4)和氧化亚氮( N_2O)排放及其增温潜势的影响。UV-B辐照设2水平,即对照(A,自然光)和增强20%(E);施硅量设2水平,即对照(Si0,0 kg SiO_2/hm2)和施硅(Si1,200 kg SiO_2/hm2)。结果表明,UV-B增强降低了成熟期水稻地上部和地下部生物量,而施硅能缓解UV-B增强对水稻生长的抑制作用,使水稻地上部和地下部生物量增加。UV-B增强可显著提高稻田CH_4和 N_2O排放通量和累积排放量,增加稻田CH_4和 N_2O排放的综合增温潜势。施硅能明显降低稻田CH_4排放,促进 N_2O排放,降低稻田CH_4和 N_2O排放的综合增温潜势。研究表明,施硅显著降低稻田CH_4和 N_2O的全球增温潜势,缓解UV-B增强对稻田CH_4和 N_2O的全球增温潜势的促进作用。     

The effect of different N substrates on biological N2O production from forest and agricultural light textured soils

N₂O emissions from two slightly alkaline sandy soils, from arable land and a woodland, were determined in a laboratory experiment in which the soils were incubated with different sources of nitrogen, with or without glucose, and with 0, 1 and 100 mL C₂H₂ L^-1. Large differences in the rate of N₂O production were observed between the two soils and between the different N treatments. The arable soil showed very low N₂O emissions derived from reduced forms of N as compared with the N₂O which was produced when the soil was provided with NO ₂ ^- or NO ₃ ^- and a C source, suggesting a very active denitrifier population. In contrast, the woodland soil showed a very low denitrification activity and a much higher N₂O production derived from the oxidation of NH ₄ ⁺ and reduction of NO ₂ ^- by some processes probably mediated by autotrophic or heterotrophic nitrifiers or dissimilatory NO ₂ ^- reducers. In both soils, the highest N₂O emissions were induced by NO ₂ ^- addition. Those emissions were demonstrated to have a biological origin, as no significant N₂O emissions were measured when the soil was autoclaved.     

Nanoplastics alter ecosystem multifunctionality and may increase global warming potential

Although the presence of nanoplastics in aquatic and terrestrial ecosystems has received increasing attention, little is known about its potential effect on ecosystem processes and functions. Here, we evaluated if differentially charged polystyrene (PS) nanoplastics (PS-NH₂ and PS-SO₃H) exhibit distinct influences on microbial community structure, nitrogen removal processes (denitrification and anammox), emissions of greenhouse gases (CO₂, CH₄, and N₂O), and ecosystem multifunctionality in soils with and without earthworms through a 42-day microcosm experiment. Our results indicated that nanoplastics significantly altered soil microbial community structure and potential functions, with more pronounced effects for positively charged PS-NH₂ than for negatively charged PS-SO₃H. Ecologically relevant concentration (3 g kg⁻¹) of nanoplastics inhibited both soil denitrification and anammox rates, while environmentally realistic concentration (0.3 g kg⁻¹) of nanoplastics decreased the denitrification rate and enhanced the anammox rate. The soil N₂O flux was always inhibited 6%–51% by both types of nanoplastics, whereas emissions of CO₂ and CH₄ were enhanced by nanoplastics in most cases. Significantly, although N₂O emissions were decreased by nanoplastics, the global warming potential of total greenhouse gases was increased 21%–75% by nanoplastics in soils without earthworms. Moreover, ecosystem multifunctionality was increased 4%–12% by 0.3 g kg⁻¹ of nanoplastics but decreased 4%–11% by 3 g kg⁻¹ of nanoplastics. Our findings provide the only evidence to date that the rapid increase in nanoplastics is altering not only ecosystem structure and processes but also ecosystem multifunctionality, and it may increase the emission of CO₂ and CH₄ and their global warming potential to some extent.     

Soil CN ratio as a scalar parameter to predict nitrous oxide emissions

Forested histosols have been found in some cases to be major, and in other cases minor, sources of the greenhouse gas nitrous oxide (N₂O). In order to estimate the total national or global emissions of N₂O from histosols, scaling or mapping parameters that can separate low‐ and high‐emitting sites are needed, and should be included in soil databases. Based on interannual measurements of N₂O emissions from drained forested histosols in Sweden, we found a strong negative relationship between N₂O emissions and soil CN ratios (r²_adj=0.96, mean annual N₂O emission=ae^{(?b CN ratio)}). The same equation could be used to estimate the N₂O emissions from Finnish and German sites based on CN ratios in published data. We envisage that the correlation between N₂O emissions and CN ratios could be used to scale N₂O emissions from histosols determined at sampled sites to national levels. However, at low CN ratios (i.e. below 15–20) other parameters such as climate, pH and groundwater tables increase in importance as regulating factors affecting N₂O emissions.     

Arbuscular mycorrhizas and their role in plant growth, nitrogen interception and soil gas efflux in an organic production system

Background and aims

Roots and mycorrhizas play an important role in not only plant nutrient acquisition, but also ecosystem nutrient cycling.

Methods

A field experiment was undertaken in which the role of arbuscular mycorrhizas (AM) in the growth and nutrient acquisition of tomato plants was investigated. A mycorrhiza defective mutant of tomato (Solanum lycopersicum L.) (named rmc) and its mycorrhizal wild type progenitor (named 76R) were used to control for the formation of AM. The role of roots and AM in soil N cycling was studied by injecting a ¹⁵N-labelled nitrate solution into surface soil at different distances from the 76R and rmc genotypes of tomato, or in plant free soil. The impacts of mycorrhizal and non-mycorrhizal root systems on soil greenhouse gas (CO₂ and ¹⁴⁺¹⁵N₂O and ¹⁵N₂O) emissions, relative to root free soils, were also studied.

Results

The formation of AM significantly enhanced plant growth and nutrient acquisition, including interception of recently applied NO ₃ ^? . Whereas roots caused a small but significant decrease in ¹⁵N₂O emissions from soils at 23?h after labeling, compared to the root-free treatment, arbuscular mycorrhizal fungi (AMF) had little effect on N₂O emissions. In contrast soil CO₂ emissions were higher in plots containing mycorrhizal root systems, where root biomass was also greater.

Conclusions

Taken together, these data indicate that roots and AMF have an important role to play in plant nutrient acquisition and ecosystem N cycling.     

Greenhouse gas fluxes respond to different N fertilizer types due to altered plant-soil-microbe interactions

The application of inorganic nitrogen (N) fertilizers strongly influences the contribution of agriculture to the greenhouse effect, especially by potentially increasing emissions of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) from soils. The present microcosm-study investigates the effect of different forms of inorganic N fertilizers on greenhouse gas (GHG) emissions from two different agricultural soils. The relationship between greenhouse gas emissions and soil microbial communities, N transformation rates and plant (Hordeum vulgare L. cv. Morex) growth were investigated. Repeated N fertilization led to increased N₂O emissions. In a parallel survey of functional microbial population dynamics we observed a stimulation of bacterial and archaeal ammonia oxidisers accompanied with these N₂O emissions. The ratio of archaeal to bacterial ammonium monooxygenase subunit A (amoA) gene copies (data obtained from Inselsbacher et al., 2010) correlated positively with N₂O fluxes, which suggests a direct or indirect involvement of archaea in N₂O fluxes. Repeated N fertilization also stimulated methane oxidation, which may also be related to a stimulation of ammonia oxidizers. The fertilizer effects differed between soil types: In the more organic Niederschleinz soil N-turnover rates increased more strongly after fertilization, while in the sandy Purkersdorf soil plant growth and soil respiration were accelerated depending on fertilizer N type. Compared to addition of NH ₄ ⁺ and NO ₃ ^? , addition of NH₄NO₃ fertilizer resulted in the largest increase in global warming potential as a summary indicator of all GHG related effects. This effect resulted from the strongest increase of both N₂O and CO₂ emission while plant growth was not equally stimulated, compared to e.g. KNO₃ fertilization. In order to decrease N losses from agricultural ecosystems and in order to minimize soil derived global warming potential, this study points to the need for interdisciplinary investigations of the highly complex interactions within plant-soil-microbe-atmosphere systems. By understanding the microbial processes underlying fertilizer effects on GHG emissions the N use efficiency of crops could be refined.     

Excessive use of nitrogen in Chinese agriculture results in high N2O/(N2O+N2) product ratio of denitrification,primarily due to acidification of the soils

China is the world's largest producer and consumer of fertilizer N, and decades of overuse has caused nitrate leaching and possibly soil acidification. We hypothesized that this would enhance the soils' propensity to emit N₂O from denitrification by reducing the expression of the enzyme N₂O reductase. We investigated this by standardized oxic/anoxic incubations of soils from five long‐term fertilization experiments in different regions of China. After adjusting the nitrate concentration to 2 mM, we measured oxic respiration (R), potential denitrification (D), substrate‐induced denitrification, and the denitrification product stoichiometry (NO, N₂O, N₂). Soils with a history of high fertilizer N levels had high N₂O/(N₂O+N₂) ratios, but only in those field experiments where soil pH had been lowered by N fertilization. By comparing all soils, we found a strong negative correlation between pH and the N₂O/(N₂O+N₂) product ratio (r² = 0.759, P < 0.001). In contrast, the potential denitrification (D) was found to be a linear function of oxic respiration (R), and the ratio D/R was largely unaffected by soil pH. The immediate effect of liming acidified soils was lowered N₂O/(N₂O+N₂) ratios. The results provide evidence that soil pH has a marginal direct effect on potential denitrification, but that it is the master variable controlling the percentage of denitrified N emitted as N₂O. It has been known for long that low pH may result in high N₂O/(N₂O+N₂) product ratios of denitrification, but our documentation of a pervasive pH‐control of this ratio across soil types and management practices is new. The results are in good agreement with new understanding of how pH may interfere with the expression of N₂O reductase. We argue that the management of soil pH should be high on the agenda for mitigating N₂O emissions in the future, particularly for countries where ongoing intensification of plant production is likely to acidify the soils.