Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Zinc fertilizer placement affects zinc content in maize plant

Background and aims

Adequate zinc (Zn) in maize (Zea mays L.) is required for obtaining Zn-enriched grain and optimum yield. This study investigated the impact of varying Zn fertilizer placements on Zn accumulation in maize plant.

Methods

Two pot experiments with same design were conducted to investigate the effect of soil Zn heterogeneity by mixing ZnSO₄·7H₂O (10 mg Zn kg^?1 soil on an average) in 10–15, 0–15, 25–30, 0–30, 30–60 and 0–60 cm soil layers on maize root growth and shoot Zn content at flowering stage in experiment-1, and assessing effects on grain Zn accumulation at mature stage in experiment-2.

Results

In experiment-1, Zn placements created a large variation in soil DTPA-Zn concentration (0.3–29.0 mg kg^?1), which induced a systemic and positive response of root growth within soil layers of 0–30 cm; and shoot Zn content was increased by 102 %–305 % depending on Zn placements. Supply capacity of Zn in soil, defined as sum of product of soil DTPA-Zn concentration and root surface area at different soil layers, was most related to shoot Zn content (r?=?0.82, P?<?0.001) via direct and indirect effects according to path analysis. In experiment-2, Zn placements increased grain Zn concentration by up to 51 %, but significantly reduced the grain Zn harvest index from 50 % by control to about 30 % in average.

Conclusion

Matching the distribution of soil applied Zn with root by Zn placement was helpful to maximize shoot Zn content and grain Zn concentration in maize.     

Ozone pollution influences soil carbon and nitrogen sequestration and aggregate composition in paddy soils

Background and aims

Much attention has focused on the effects of tropospheric ozone (O₃) on terrestrial ecosystems and plant growth. Since O₃ pollution is currently an issue in China and many parts of the world, understanding the effects of elevated O₃ on soil carbon (C) and nitrogen (N) sequestration is essential for efforts to predict C and N cycles in terrestrial ecosystems under predicted increases in O₃. Thus the main objective of this study was to determine whether an increases in atmospheric O₃ concentration influenced soil organic C (SOC) and N sequestration.

Methods

A free-air O₃ enrichment (O₃-FACE) experiment was started in 2007 and used continuous O₃ exposure from March to November each year during crop growth stage in a rice (Oryza sativa L.)—wheat (Triticum aestivum L.) rotation field in the Jiangsu Province, China. We investigated differences in SOC and N and soil aggregate composition in both elevated and ambient O₃ conditions.

Results

Elevated atmospheric O₃ (18–80 nmol mol^?1 or 50 % above the ambient) decreased the SOC and N concentration in the 0–20 cm soil layer after 5 years. Elevated O₃ significantly decreased the SOC concentration by 17 % and 5.6 % in the 0–3 cm and the 10–20 cm layers, respectively. Elevated O₃ significantly decreased the N concentration by 8.2–27.8 % in three layers at the 20 cm depth. In addition, elevated O₃ influenced the formation and transformation of soil aggregates and the distribution of SOC and N in the aggregates across soil layer classes. Elevated O₃ significantly decreased the macro-sized aggregate fraction (16.8 %) and associated C and N (0.5 g kg^?1 and 0.32 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (61 %) and associated C (1.7 g kg^?1) in the 0–3 cm layer. Elevated O₃ significantly decreased the macro-sized aggregate fraction (9.6 %) and associated C and N (1.4 g kg^?1 and 0.35 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (41.8 %) and decreased the corresponding associated N (0.14 g kg^?1) in the 3–10 cm layer. Elevated O₃ did not significantly effect the formation and transformation of aggregates in the 10–20 cm layer, yet it did significantly increase the C concentration in the macro-sized fraction (1 g kg^?1) and decrease the N concentration in the macro- and micro-sized fractions (0.24 g kg^?1 and 0.16 g kg^?1, respectively).

Conclusion

Long-term exposure to elevated atmospheric O₃ negatively affected the physical structure of the soil and impaired soil C and N sequestration.     

Interactions between leaf litter and soil organic matter on carbon and nitrogen mineralization in six forest litter-soil systems

Background and aims

Leaf litter decomposes on the surface of soil in natural systems and element transfers between litter and soil are commonly found. However, how litter and soil organic matter (SOM) interact to influence decomposition rate and nitrogen (N) release remains unclear.

Methods

Leaf litter and mineral soil of top 0–5 cm from six forests were incubated separately, or together with litter on soil surface at 25 °C for 346 days. Litter N remaining and soil respiration rate were repeatedly measured during incubation. Litter carbon (C) and mass losses and mineral N concentrations in litter and soil were measured at the end of incubation.

Results

Net N transfer from soil to litter was found in all litters when incubated with soil. Litter incubated with soil lost more C than litter incubated alone after 346 days. For litters with initial C: N ratios lower than 52, net N_min after 346 days was 100 % higher when incubated with soil than when incubated alone. Litter net N_min rate was negatively related to initial C: N ratio when incubated with soil but not when incubated alone. Soil respiration rate and net N_min rate did not differ between soil incubated with litter and soil incubated alone.

Conclusions

We conclude that soils may enhance litter decomposition rate by net N transfer from soil to litter. Our results together with studies on litter mixture decomposition suggest that net N transfer between decomposing organic matter with different N status may be common and may significantly influence decomposition and N release. The low net N_min rate during litter decomposition along with the small size of litter N pool compared to soil N pool suggest that SOM rather than decomposing litter is the major contributor to plant mineral N supply.     

Effects of nitrate concentration on the denitrification potential of a calcic cambisol and its fractions of N2, N2O and NO

Background and aims

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO ₃ ^? ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N₂O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands.

Methods

Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N₂), N₂O and NO during denitrification under the conditions of seven levels of NO ₃ ^? concentrations (ranging from 10 to 250 mg N kg^?1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg^?1 dry soil).

Results

Almost all the soil NO ₃ ^? was consumed during anaerobic incubation, with 80–88 % of the consumed NO ₃ ^? recovered by measuring nitrogenous gases. The results showed that the increases in initial NO ₃ ^? concentrations significantly enhanced the denitrification potential and the emissions of N₂ and N₂O as products of this process. Despite the wide range of initial NO ₃ ^? concentrations, the ratios of N₂, N₂O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N₂, 14–36 % for N₂O and 5–22 % for NO.

Conclusions

These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO ₃ ^? concentrations on denitrification process as needed for biogeochemical models.     

Phenotypic seedling responses of a metal-tolerant mutant line of sunflower growing on a Cu-contaminated soil series: potential uses for biomonitoring of Cu exposure and phytoremediation

Background and aims

The potential use of a metal-tolerant sunflower mutant line for both biomonitoring and phytoremediating a Cu-contaminated soil series was investigated.

Methods

The soil series (21–1,170 mg Cu kg^?1) was sampled in field plots at control and wood preservation sites. Sunflowers were cultivated 1 month in potted soils under controlled conditions.

Results

pH and dissolved organic matter influenced Cu concentration in the soil pore water. Leaf chlorophyll content and root growth decreased as Cu exposure rose. Their EC₁₀ values corresponded to 104 and 118 μg Cu L^?1 in the soil pore water, 138 and 155 mg Cu kg^?1 for total soil Cu, and 16–18 mg Cu kg^?1 DW shoot. Biomass of plant organs as well as leaf area, length and asymmetry were well correlated with Cu exposure, contrary to the maximum stem height and leaf water content.

Conclusions

Physiological parameters were more sensitive to soil Cu exposure than the morphological ones. Bioconcentration and translocation factors and distribution of mineral masses for Cu highlighted this mutant as a secondary Cu accumulator. Free Cu²⁺ concentration in soil pore water best predicted Cu phytoavailability. The usefulness of this sunflower mutant line for biomonitoring and Cu phytoextraction was discussed.     

Contribution of 15N-labelled leaf litter to N turnover, nitrous oxide emissions and N sequestration in a beech forest during eleven years

Aims

Decomposition of leaf litterfall plays a major role for nitrogen (N) dynamics in soils. However, little is known as to which extent beech leaf litter contributes to N turnover and nitrous oxide (N₂O) emissions within one decade after litterfall.

Methods

In 1997, we exchanged recently fallen leaf litter by ¹⁵N-labelled litter in a beech stand (Fagus sylvatica) at the Solling, Germany. Measurements were conducted 2–3 and 10–11 years after litter exchange.

Results

Two years after litter exchange, 92 % of added ¹⁵N was recovered in the surface 10 cm of the soil. The labelled N was primarily found in the upper part of the F layer of the moder type humus. Eleven years after litter exchange, 73 % of the added ¹⁵N was lost and the remaining 27 % was mainly recovered in the lower part of the F layer indicating N sequestration. The remaining leaf litter N was subject to measurable N mineralisation (2–3 % of litter N) and N₂O production (0.02 %). Between 0.3 % (eleventh year) and 0.6 % (second year) of total annual N₂O emissions were attributed to beech leaf litter of a single year.

Conclusions

Most of the annual N₂O emissions (1.33–1.54 kg N ha^?1 yr^?1) were probably derived from older soil N pools.     

The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivation

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N₂O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N₂O production from the intensive vegetable fields and to identify the affecting factors, a ¹⁵N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with ¹⁵NH₄NO₃ or NH ₄ ¹⁵ NO₃. The average N₂O emission rate was 24.2 ng N?kg^?1 h^?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg^?1 h^?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N₂O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N₂O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N₂O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N₂O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N₂O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO ₃ ^- or NH ₄ ⁺ ) could also significantly affect the sources and rates of N₂O emission.     

Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.     

Liebig’s law of the minimum applied to a greenhouse gas: alleviation of P-limitation reduces soil N2O emission

Background and aims

Emission of the greenhouse gas (GHG) nitrous oxide (N₂O) are strongly affected by nitrogen (N) fertilizer application rates. However, the role of other nutrients through stoichiometric relations with N has hardly been studied. We tested whether phosphorus (P) availability affects N₂O emission. We hypothesized that alleviation of plant P-limitation reduces N₂O emission through lowering soil mineral N concentrations.

Methods

We tested our hypothesis in a pot experiment with maize (Zea mays L.) growing on a P-limiting soil/sand mixture. Treatment factors included P and N fertilization and inoculation with Arbuscular Mycorrhizal Fungi (AMF; which can increase P uptake).

Results

Both N and P fertilization, as well as their interaction significantly (P?<?0.01) affected N₂O emission. Highest N₂O emissions (2.38 kg N₂O-N ha^?1) were measured at highest N application rates without P fertilization or AMF. At the highest N application rate, N₂O fluxes were lowest (0.71 kg N₂O-N ha^?1) with both P fertilization and AMF. The N₂O emission factors decreased with 50 % when P fertilization was applied.

Conclusions

Our results illustrate the importance of the judicious use of all nutrients to minimize N₂O emission, and thereby further underline the intimate link between sound agronomic practice and prudent soil GHG management.     

Nitrogen contributions from faba bean (Vicia faba L.) reliant on soil rhizobia or inoculation

Background and Aims

Understanding the impact of soil rhizobial populations and inoculant rhizobia in supplying sufficient nodulation is crucial to optimising N₂ fixation by legume crops. This study explored the impact of different rates of inoculant rhizobia and contrasting soil rhizobia on nodulation and N₂ fixation in faba bean (Vicia faba L.).

Methods

Faba beans were inoculated with one of seven rates of rhizobial inoculation, from no inoculant to 100 times the normal rate of inoculation, sown at two field sites, with or without soil rhizobia present, and their nodulation and N₂ fixation assessed.

Results

At the site without soil rhizobia, inoculation increased nodule number and increased N₂ fixation from 21 to 129 kg shoot N ha^?1, while N₂ fixation increased from 132 to 218 kg shoot N ha^?1 at the site with high background soil rhizobia. At the site without soil rhizobia, inoculation increased concentrations of shoot N from 14 to 24 mg g^?1, grain N from 32 to 45 mg g^?1, and grain yields by 1.0 Mg (metric tonne) ha^?1. Differences in nodulation influenced the contributions of fixed N to the system, which varied from the net removal of 20 kg N ha^?1 from the system in the absence of rhizobia, to a net maximum input of 199 kg N ha^?1 from legume shoot and root residues, after accounting for removal of N in grain harvest.

Conclusions

The impact of inoculation and soil rhizobia strongly influenced grain yield, grain N concentration and the potential contributions of legume cropping to soil N fertility. In soil with resident rhizobia, N₂ fixation was improved only with the highest inoculation rate.     

Effect of biochar amendment on maize yield and greenhouse gas emissions from a soil organic carbon poor calcareous loamy soil from Central China Plain

Aims

A field experiment was conducted to investigate the effect of biochar on maize yield and greenhouse gases (GHGs) in a calcareous loamy soil poor in organic carbon from Henan, central great plain, China.

Methods

Biochar was applied at rates of 0, 20 and 40?t?ha^?1 with or without N fertilization. With N fertilization, urea was applied at 300?kg?N ha^?1, of which 60% was applied as basal fertilizer and 40% as supplementary fertilizer during crop growth. Soil emissions of CO₂, CH₄ and N₂O were monitored using closed chambers at 7?days intervals throughout the whole maize growing season (WMGS).

Results

Biochar amendments significantly increased maize production but decreased GHGs. Maize yield was increased by 15.8% and 7.3% without N fertilization, and by 8.8% and 12.1% with N fertilization under biochar amendment at 20?t?ha^?1 and 40?t?ha^?1, respectively. Total N₂O emission was decreased by 10.7% and by 41.8% under biochar amendment at 20?t?ha^?1 and 40?t?ha^?1 compared to no biochar amendment with N fertilization. The high rate of biochar (40?t?ha^?1) increased the total CO₂ emission by 12% without N fertilization. Overall, biochar amendments of 20?t?ha^?1 and 40?t?ha^?1 decreased the total global warming potential (GWP) of CH₄ and N₂O by 9.8% and by 41.5% without N fertilization, and by 23.8% and 47.6% with N fertilization, respectively. Biochar amendments also decreased soil bulk density and increased soil total N contents but had no effect on soil mineral N.

Conclusions

These results suggest that application of biochar to calcareous and infertile dry croplands poor in soil organic carbon will enhance crop productivity and reduce GHGs emissions.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

Annual nitrous oxide emissions from open-air and greenhouse vegetable cropping systems in China

Aims

A field experiment was conducted to quantify annual nitrous oxide (N₂O) fluxes from control and fertilized plots under open-air and greenhouse vegetable cropping systems in southeast China. We compiled the reported global field annual N₂O flux measurements to estimate the emission factor of N fertilizer for N₂O and its background emissions from vegetable fields.

Methods

Fluxes of N₂O were measured using static chamber-GC techniques over the 2010–2011 annual cycle with multiple cropping seasons.

Results

The mean annual N₂O fluxes from the controls were 46.1?±?2.3 μg N₂O-N m^?2 hr^?1 and 68.3?±?4.1 μg N₂O-N m^?2 hr^?1 in the open-air and greenhouse vegetable systems, respectively. For the plots receiving 900 kg?N?ha^?1, annual N₂O emissions averaged 90.6?±?8.9 μg N₂O-N m^?2 hr^?1 and 106.4?±?6.6 μg N₂O-N?m^?2 hr^?1 in the open-air and greenhouse vegetable systems, respectively. By pooling published field N₂O flux measurements taken over or close to a full year, the N₂O emission factor for N fertilizer averaged 0.63?±?0.09 %, with a background emission of 2.67?±?0.80 kg N₂O-N ha^?1 in Chinese vegetable fields. Annual N₂O emissions from Chinese vegetable systems were estimated to be 84.7 Gg N₂O-N yr^?1, consisting of 72.5 Gg N₂O-N yr^?1 and 12.2 Gg N₂O-N yr^?1 in the open-air and greenhouse vegetable systems, respectively.

Conclusions

While N₂O emissions from the greenhouse vegetable cropping system tended to be slightly higher compared to the open-air system in our experiment, the synthesis of literature data suggests that N₂O emissions would be greater at low N-rates but smaller at high N-rates in greenhouse systems than in open-air vegetable cropping systems. The estimates of this study suggest that vegetable cropping systems covering 11.4 % in national total cropping area, contributed 21–25 % to the total N₂O emission from Chinese croplands.     

Long-term retention of post-fire soil mineral nitrogen pools in Mediterranean shrubland and grassland

Background and Aims

The post-fire mineral N pool is relevant for plant regrowth. Depending on the plant regeneration strategies, this pool can be readily used or lost from the plant–soil system. Here we studied the retention of the post-fire mineral N pool in the system over a period of 12 years in three contrasted Mediterranean plant communities.

Methods

Three types of vegetation (grassland, mixed shrub-grassland and shrubland) were subjected to experimental fires. We then monitored the fate of ¹⁵?N-tracer applied to the mineral N pool in soils and in plants over 12 years.

Results

The plant community with legumes (mixed shrub-grasslands) showed the lowest soil retention of ¹⁵?N-tracer during the first 9 months after fire. Between years 6 and 12 post-fire, a drought promoted plant and litter deposition. Coinciding with this period, ¹⁵?N-recovery in the first 15 cm of the soil increased in all cases, except in mixed shrub-grassland. This lack of increase may be attributable to the input of impoverished ¹⁵?N plant residues and enhanced leaching and denitrification, possibly by N₂-fixing shrubs. After the drought, the deepest soil layer showed large decreases in total N and ¹⁵?N-recovery, which were possibly caused by N mineralization.

Conclusions

Twelve years after the fires, plant communities without N₂-fixing shrubs recycled a significant part of the N derived from the post-fire mineral N and this pool continued to interact in the plant–soil system.     

Reductive soil disinfestation (RSD) alters gross N transformation rates and reduces NO and N2O emissions in degraded vegetable soils

Background and aims

Continuous vegetable cultivation in greenhouses can easily induce soil degradation, which considerably affects the development of sustainable vegetable production. Recently, the reductive soil disinfestation (RSD) is widely used as an alternative to chemical soil disinfestations to improve degraded greenhouse vegetable soils. Considering the importance of nitrogen (N) for plant growth and environment effect, the internal N transformation processes and rates should be well investigated in degraded vegetable soils treated by RSD, but few works have been undertaken.

Methods

Three RSD-treated and three untreated degraded vegetable soils were chosen and a ¹⁵?N tracing incubation experiment differentially labeled with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ was conducted at 25 °C under 50 % water holding capacity (WHC) for 96 h. Soil gross N transformation rates were calculated using a ¹⁵?N tracing model combined with Markov Chain Monte Carlo Metropolis algorithm (Müller et al. 2007), while the emissions of N₂O and NO were also measured.

Results

RSD could significantly enhance the soil microbial NH₄ ⁺ immobilization rate, the heterotrophic and autotrophic nitrification rates, and the NO₃ ^? turnover time. The ratio of heterotrophic nitrification to total inorganic N supply rate (mineralization + heterotrophic nitrification) increased greatly from 5.4 % in untreated vegetable soil to 56.1 % in treated vegetable soil. In addition, low release potential of NO and N₂O was observed in RSD-treated vegetable soil, due to the decrease in the NO and N₂O product ratios from heterotrophic and autotrophic nitrifications. These significant differences in gross N transformation rates, the supply processes and capacity of inorganic N, and the NO and N₂O emissions between untreated and treated vegetable soils could be explained by the elimination of accumulated NO₃ ^?, increased pH, and decreased electrical conductivity (EC) caused by RSD. Noticeably, the NO₃ ^? consumption rates were still significantly lower than the NO₃ ^? production rates in RSD-treated vegetable soil.

Conclusions

Except for improving soil chemical properties, RSD could significantly alter the supply processes of inorganic N and reduce the release potential of N₂O and NO in RSD-treated degraded vegetable soil. In order to retard the re-occurrence of NO₃ ^? accumulation, acidification and salinization and to promote the long-term productivity of greenhouse vegetable fields, the rational use of N fertilizer should be paid great attention to farmers in vegetable cultivation.     

Selenium accumulation in durum wheat and spring canola as a function of amending soils with selenite, selenate and or sulphate

Aims

A comparison was performed between plant species to determine if extractable, rather than total soil Se, is more effective at predicting plant Se accumulation over a full growing season.

Methods

Durum wheat (Triticum turgidum L.) and spring canola (Brassica napus L.) were sown in potted soil amended with 0, 0.1, 1.0, or 5.0 mg kg^?1 Se as SeO₄ ^2? or SeO₃ ^2?. In addition, SeO₄ ^2?-amended soils were amended with 0 or 50 mg kg^?1 S as SO₄ ^2?. Soils were analyzed for extractable and total concentration of Se ([Se]). Twice during the growing season plants were harvested and tissue [Se] was determined.

Results

Plants exposed to SeO₃ ^2? accumulated the least Se. Fitted predictive models for whole plant accumulation based on extractable soil [Se] were similar to models based on total [Se] in soil (R²?=?0.73 or 0.74, respectively) and selenium speciation and soil [S] were important soil parameters to consider. As well, soil S amendments limited Se toxicity.

Conclusions

Soil quality guidelines (SQGs) based on extractable Se should be considered for risk assessment, particularly when Se speciation is unknown. Predictive models to estimate plant Se uptake should include soil S, a modifier of Se accumulation.     

Role of maize stover incorporation on nitrogen oxide emissions in a non-irrigated Mediterranean barley field

Aims

Agricultural soils in semiarid Mediterranean areas are characterized by low organic matter contents and low fertility levels. Application of crop residues and/or manures as amendments is a cost-effective and sustainable alternative to overcome this problem. However, these management practices may induce important changes in the nitrogen oxide emissions from these agroecosystems, with additional impacts on carbon dioxide emissions. In this context, a field experiment was carried out with a barley (Hordeum vulgare L.) crop under Mediterranean conditions to evaluate the effect of combining maize (Zea mays L.) residues and N fertilizer inputs (organic and/or mineral) on these emissions.

Methods

Crop yield and N uptake, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification capacity, N₂O, NO and CO₂ fluxes were measured during the growing season.

Results

The incorporation of maize stover increased N₂O emissions during the experimental period by c. 105 %. Conversely, NO emissions were significantly reduced in the plots amended with crop residues. The partial substitution of urea by pig slurry reduced net N₂O emissions by 46 and 39 %, with and without the incorporation of crop residues respectively. Net emissions of NO were reduced 38 and 17 % for the same treatments. Molar DOC:NO ₃ ^? ratio was found to be a robust predictor of N₂O and NO fluxes.

Conclusions

The main effect of the interaction between crop residue and N fertilizer application occurred in the medium term (4–6 month after application), enhancing N₂O emissions and decreasing NO emissions as consequence of residue incorporation. The substitution of urea by pig slurry can be considered a good management strategy since N₂O and NO emissions were reduced by the use of the organic residue.     

Assessment of nitrate leaching loss on a yield-scaled basis from maize and wheat cropping systems

Background and aims

It is so far a gap in knowledge to assess nitrate (NO₃ ^?) leaching loss linking with crop yield for a given cereal cropping system.

Methods

We conducted a meta-analysis on 32 published studies reporting both NO₃ ^? leaching losses and crop yields in the maize (N?=?20) and wheat (N?=?12) systems.

Results

On average, 22 % and 15 % of applied fertilizer N to wheat and maize systems worldwide are leached in the form of NO₃ ^?, respectively. The average area-scaled NO₃ ^- leaching loss for maize (57.4 kg N ha^?1) was approx. two times higher than for wheat (29.0 kg N ha^?1). While, if scaled to crop yields, the average yield-scaled NO₃ ^? losses were comparable between maize (5.40 kg N Mg^?1) and wheat (5.41 kg N Mg^?1) systems. Across all sites, the lowest yield-scaled NO₃ ^? leaching losses were observed at slightly suboptimal fertilization rates, corresponding to 90 % and 96 % of maximum maize or wheat yields, respectively.

Conclusions

Our findings suggest that small adjustments of agricultural N management practices can effectively reduce yield-scaled NO₃ ^? leaching losses. However, further targeted field experiments are still needed to identify at regional scale best agricultural management practices for reducing yield-scaled NO₃ ^? leaching losses in maize and wheat systems.     

Energy-dispersive X-ray fluorescence spectrometry as a tool for zinc, iron and selenium analysis in whole grain wheat

Background and aims

Crop biofortification programs require fast, accurate and inexpensive methods of identifying nutrient dense genotypes. This study investigated energy-dispersive X-ray fluorescence spectrometry (EDXRF) for the measurement of zinc (Zn), iron (Fe) and selenium (Se) concentrations in whole grain wheat.

Methods

Grain samples were obtained from existing biofortification programs. Reference Zn, Fe and Se concentrations were obtained using inductively coupled plasma optical emission spectrometry (ICP-OES) and/or inductively coupled plasma mass spectrometry (ICP-MS). One set of 25 samples was used to calibrate for Zn (19–60?mg?kg^–1) and Fe (26–41?mg?kg^–1), with 25 further samples used to calibrate for Se (2–31?mg?kg^–1 ). Calibrations were validated using an additional 40–50 wheat samples.

Results

EDXRF limits of quantification (LOQ) were estimated as 7, 3 and 2?mg?kg^–1 for Zn, Fe, and Se, respectively. EDXRF results were highly correlated with ICP-OES or -MS values. Standard errors of EDXRF predictions were ±2.2?mg Zn kg^–1, ±2.6?mg Fe kg^–1, and ±1.5?mg Se kg^–1.

Conclusion

EDXRF offers a fast and economical method for the assessment of Zn, Fe and Se concentration in wheat biofortification programs.