Indirect emissions of nitrous oxide in a cropland watershed with contrasting hydrology in central France.

Nitrous oxide (N2O) is an important greenhouse gas. Its atmospheric concentration have increased with the industrialisation and the use of N fertilizer. The contribution of freshwater systems to N2O emissions is still very uncertain, while regional transfer of nitrogen depends on soil and hydrology. Riverine and spring N2O dissolved in water was therefore measured over two years in the 3453 km2 Haut-Loir watershed (France). This temperate cropland watershed is characterized by two different hydrological systems east and west of the Loir River. The eastern rivers, fed by the emergence of the deep Beauce aquifer, exhibited significantly higher dissolved N2O concentrations (Beauce region, mean: 2.93 µg-N L-1) than the western rivers (Perche region, mean: 0.87 µg-N L-1), which were largely influenced by runoff during winter flooding. The eastern rivers had large nitrate concentrations all over the year; in the Perche, nitrate underwent a seasonal cycle with large loads during winter floods, but there were no consistent seasonal patterns in N2O. The ratios of N2O in excess of equilibrium on nitrate, often used as a proxy of emission factor (EF), were much smaller than the default IPCC values, both for rivers (0.014% versus 0.25% for IPCC EF5r) and the Loir spring (0.085% versus 0.6% for the IPCC EF5g for groundwater and springs). EF5r were significantly different between the two parts of the watershed only in winter, because of the seasonal variability of NO3-. Moreover dissolved N2O is controlled not only by NO3-, as it is considered in the calculation of the EF5, but also by water pH and dissolved organic carbon. A good prediction of dissolved N2O was obtained using these physicochemical variables and hydrological regions. Thus, these results suggest that the spatial variability of riverine N2O depends on local hydrology, while further research is needed to understand the seasonal variability.

Can N2 O emissions offset the benefits from soil organic carbon storage?

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5°C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large-scale deployment of other climate mitigation strategies is also necessary. Among these, increasing soil organic carbon (SOC) stocks is an important lever because carbon in soils can be stored for long periods and land management options to achieve this already exist and have been widely tested. However, agricultural soils are also an important source of nitrous oxide (N2 O), a powerful greenhouse gas, and increasing SOC may influence N2 O emissions, likely causing an increase in many cases, thus tending to offset the climate change benefit from increased SOC storage. Here we review the main agricultural management options for increasing SOC stocks. We evaluate the amount of SOC that can be stored as well as resulting changes in N2 O emissions to better estimate the climate benefits of these management options. Based on quantitative data obtained from published meta-analyses and from our current level of understanding, we conclude that the climate mitigation induced by increased SOC storage is generally overestimated if associated N2 O emissions are not considered but, with the exception of reduced tillage, is never fully offset. Some options (e.g. biochar or non-pyrogenic C amendment application) may even decrease N2 O emissions.

Management of soil pH promotes nitrous oxide reduction and thus mitigates soil emissions of this greenhouse gas.

While concerns about human-induced effects on the Earth's climate have mainly concentrated on carbon dioxide (CO2) and methane (CH4), reducing anthropogenic nitrous oxide (N2O) flux, mainly of agricultural origin, also represents an opportunity for substantial mitigation. To develop a solution that induces neither the transfer of nitrogen pollution nor decreases agricultural production, we specifically investigated the last step of the denitrification pathway, the N2O reduction path, in soils. We first observed that this path is mainly driven by soil pH and is progressively inhibited when pH is lower than 6.8. During field experiments, we observed that liming acidic soils to neutrality made N2O reduction more efficient and decreased soil N2O emissions. As we estimated acidic fertilized soils to represent 37% [27-50%] of French soils, we calculated that liming could potentially decrease France's total N2O emissions by 15.7% [8.3-21.2%]. Nevertheless, due to the different possible other impacts of liming, we currently recommend that the deployment of this solution to mitigate N2O emission should be based on local studies that take into account agronomic, environmental and economic aspects.

Loss in microbial diversity affects nitrogen cycling in soil.

Microbial communities have a central role in ecosystem processes by driving the Earth's biogeochemical cycles. However, the importance of microbial diversity for ecosystem functioning is still debated. Here, we experimentally manipulated the soil microbial community using a dilution approach to analyze the functional consequences of diversity loss. A trait-centered approach was embraced using the denitrifiers as model guild due to their role in nitrogen cycling, a major ecosystem service. How various diversity metrics related to richness, eveness and phylogenetic diversity of the soil denitrifier community were affected by the removal experiment was assessed by 454 sequencing. As expected, the diversity metrics indicated a decrease in diversity in the 1/10(3) and 1/10(5) dilution treatments compared with the undiluted one. However, the extent of dilution and the corresponding reduction in diversity were not commensurate, as a dilution of five orders of magnitude resulted in a 75% decrease in estimated richness. This reduction in denitrifier diversity resulted in a significantly lower potential denitrification activity in soil of up to 4-5 folds. Addition of wheat residues significantly increased differences in potential denitrification between diversity levels, indicating that the resource level can influence the shape of the microbial diversity-functioning relationship. This study shows that microbial diversity loss can alter terrestrial ecosystem processes, which suggests that the importance of functional redundancy in soil microbial communities has been overstated.

Effect of topography on nitrous oxide emissions from winter wheat fields in Central France.

We assessed nitrous oxide (N(2)O) emissions at shoulder and foot-slope positions along three sloping sites (1.6-2.1%) to identify the factors controlling the spatial variations in emissions. The three sites received same amounts of total nitrogen (N) input at 170kgNha(-1). Results showed that landscape positions had a significant, but not consistent effect on N(2)O fluxes with larger emission in the foot-slope at only one of the three sites. The effect of soil inorganic N (NH(4)(+)+NO(3)(-)) contents on N(2)O fluxes (r(2)=0.55, p<0.001) was influenced by water-filled pore space (WFPS). Soil N(2)O fluxes were related to inorganic N at WFPS>60% (r(2)=0.81, p<0.001), and NH(4)(+) contents at WFPS<60% (r(2)=0.40, p<0.01), respectively. Differences in WFPS between shoulder and foot-slope correlated linearly with differences in N(2)O fluxes (r(2)=0.45, p<0.001). We conclude that spatial variations in N(2)O emission were regulated by the influence of hydrological processes on soil aeration intensity.

In situ dynamics of microbial communities during decomposition of wheat, rape, and alfalfa residues.

Microbial communities are of major importance in the decomposition of soil organic matter. However, the identities and dynamics of the populations involved are still poorly documented. We investigated, in an 11-month field experiment, how the initial biochemical quality of crop residues could lead to specific decomposition patterns, linking biochemical changes undergone by the crop residues to the respiration, biomass, and genetic structure of the soil microbial communities. Wheat, alfalfa, and rape residues were incorporated into the 0-15 cm layer of the soil of field plots by tilling. Biochemical changes in the residues occurring during degradation were assessed by near-infrared spectroscopy. Qualitative modifications in the genetic structure of the bacterial communities were determined by bacterial-automated ribosomal intergenic spacer analysis. Bacterial diversity in the three crop residues at early and late stages of decomposition process was further analyzed from a molecular inventory of the 16S rDNA. The decomposition of plant residues in croplands was shown to involve specific biochemical characteristics and microbial community dynamics which were clearly related to the quality of the organic inputs. Decay stage and seasonal shifts occurred by replacement of copiotrophic bacterial groups such as proteobacteria successful on younger residues with those successful on more extensively decayed material such as Actinobacteria. However, relative abundance of proteobacteria depended greatly on the composition of the residues, with a gradient observed from alfalfa to wheat, suggesting that this bacterial group may represent a good indicator of crop residues degradability and modifications during the decomposition process.

Influence of 15N enrichment on the net isotopic fractionation factor during the reduction of nitrate to nitrous oxide in soil.

Nitrous oxide, a greenhouse gas, is mainly emitted from soils during the denitrification process. Nitrogen stable-isotope investigations can help to characterise the N(2)O source and N(2)O production mechanisms. The stable-isotope approach is increasingly used with (15)N natural abundance or relatively low (15)N enrichment levels and requires a good knowledge of the isotopic fractionation effect inherent to this biological mechanism. This paper reports the measurement of the net and instantaneous isotopic fractionation factor (alpha(s/p) (i)) during the denitrification of NO(3) (-) to N(2)O over a range of (15)N substrate enrichments (0.37 to 1.00 atom% (15)N). At natural abundance level, the isotopic fractionation effect reported falls well within the range of data previously observed. For (15)N-enriched substrate, the value of alpha(s/p) (i) was not constant and decreased from 1.024 to 1.013, as a direct function of the isotopic enrichment of the labelled nitrate added. However, for enrichment greater than 0.6 atom% (15)N, the value of alpha(s/p) (i) seems to be independent of substrate isotopic enrichment. These results suggest that for isotopic experiments applied to N(2)O emissions, the use of low (15)N-enriched tracers around 1.00 atom% (15)N is valid. At this enrichment level, the isotopic effect appears negligible in comparison with the enrichment of the substrate.

Dynamics and identification of soil microbial populations actively assimilating carbon from 13C-labelled wheat residue as estimated by DNA- and RNA-SIP techniques.

This work is the first report on the use of DNA-, RNA-SIP approaches to elucidate the dynamics and the diversity of bacterial populations actively assimilating C derived from plant residues labelled at more than 90% (13)C. Wheat-residues, were incorporated and incubated into soil microcosms for 28 days. At the end of the incubation time, no more than 55% of the total CO(2) released was (13)C-labelled, suggesting the occurrence of an important priming effect process. After 7 days, more than 30% of the whole DNA extracted were labelled, allowing an efficient separation of labelled from unlabelled DNA using density gradient centrifugation. The genetic structure of bacterial community, assessed by Automated Ribosomal Intergenic Spacer Analysis technique, was deduced from the (13)C- and (12)C-fractions of control and enriched conditions, over the time course of the experiment. Dynamics showed that wheat residues directly induced a rapid and durable stimulation of fresh organic matter (FOM) degrading populations ((13)C), while specific soil organic matter (SOM) degrading populations ((12)C) seemed to be indirectly stimulated only at the early time point (t7d). After 14 days of incubations, 16S rRNA clone libraries were elaborated on (12)C- and (13)C-RNA extracted from enriched microcosms, as well as (12)C-RNA extracted from control condition. Stimulation of the beta- and gamma-subgroups of proteobacteria, where numerous populations were previously described as r-strategists or copiotrophic organisms, was recorded in the (13)C-fraction. In the mean time, several phyla like Actinobacteria, Cyanobacteria, Candidate, Gemmatimonadetes and Planctomycetes were only present in (12)C fractions. Surprisingly, several sequences affiliated to species characterized as oligotrophic organisms were retrieved in both types of fraction. Trophic relationships between soil bacteria involved in FOM and SOM degradation were discussed on the basis of different hypotheses of Fontaine and colleagues (2003) concerning the mechanisms of the priming effect induction.

Microbial community structure in soils with decomposing residues from plants with genetic modifications to lignin biosynthesis.

Lignin is a major determinant of the decomposition of plant materials in soils. Advances in transgenic technology have led to the possibility of modifying lignin to improve the pulping properties of plant materials for papermaking. Previous studies have shown that lignin modifications also affect the rate of plant material decay in soil. The aim of this work was to investigate short-term changes in soil microbial community structures when tobacco residues with reduced activity of enzymes in the monolignol pathway decompose. The residues from lignin-modified plants all decomposed faster than unmodified plant materials. The relative proportions of some of the structural groups of microbial phospholipid fatty acids were affected by genetic modifications, especially the proportion of double unsaturated chain fatty acids, indicative of fungi.