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Abstract The dynamics of soil water-stable aggregation (WSA) following organic matter (OM) addition are controlled by microbial activity, which in turn is influenced by carbon substrate quality and mineral N availability. However, the role of microbial communities in determining WSA at different stages of OM decomposition remains largely unknown. This study aimed at evaluating the role of microbial communities in WSA during OM decomposition as affected by mineral N. In a 35-day incubation experiment, we studied the decomposition of two high-C/N crop residues (miscanthus, C/N = 311.3; and wheat, C/N = 125.6) applied at 4 g C kg−1 dry soil with or without mineral N addition (120 mg N kg−1 dry soil). Microbial characteristics were measured at day 0, 7, and 35 of the experiment, and related to previous results of WSA. Early increase in WSA (at 7 days) was related to an overall increase of the microbial biomass (MBC) with wheat residues showing higher values in MBC and WSA than miscanthus. In the intermediate stage of decomposition (from day 7 to 35), the dynamics of WSA were more associated with the dynamics of microbial polysaccharides and greatly influenced by mineral N addition. Mineral N addition resulted in a decrease or leveling off of WSA whereas it increased in its absence. We suggest that opportunistic bacterial populations stimulated by N addition may have consumed binding agents which decreased WSA or prevented its increase. To the contrary, microbial polysaccharide production was high when no mineral N was added which led to the higher WSA in the late stage of decomposition in this treatment. The late stage of decomposition was associated with a particular fungal community also influenced by the mineral N treatment. We suggest that WSA dynamics in the late stage of decomposition can be considered as a « narrow process³ where the structure of the microbial community plays a greater role than during the initial stages.

Standard 13 C-labelled plant material was exposed over 2‐3 yr at 8 sites in a north‐south climatic gradient of coniferous forest soils, developed on acid and calcareous parent materials in Western Europe. In addition to soils exposed in their sites of origin, replicate units containing labelled material were translocated in a cascade sequence southwards along the transect, to simulate the eAects of climate warming on decomposition processes. The current Atlantic climate represented the most favourable soil temperature and moisture conditions for decomposition. Northward this climatic zone, where the soil processes are essentially temperature-limited, the prediction for a temperature increase of 38C estimated a probable increase of C mineralisation by 20‐ 25% for the boreal zone and 10% for the cool temperate zone. Southward the cool Atlantic climate zone, (the Mediterranean climate), where the processes are seasonally moisture-limited, the predicted increase of temperature by 1‐28C little aAected the soil organic matter dynamics, because of the higher water deficit. A significant decrease of C mineralisation rates was observed only in the superficial layers recognised in Mediterranean forest soils as ‘xeromoder’ and subject to frequent dry conditions. In the deeper Mediterranean soil organic horizons (the mull humus types), representing the major C storage in this zone, C mineralisation was not aAected by a simulated 28C temperature increase. The temperature eAect is probably counteracted by a higher water deficit. 7 2000 Elsevier Science Ltd. All rights reserved.

Abstract Nitrous oxide (N2O) is an important greenhouse gas and fundamental questions about the capacity of soil microbial communities to act not only as sources, but also as sinks for N2O remains unanswered. We evaluated the capacity of non-denitrifying N2O-reducers to mitigate the production of this greenhouse gas in soil. We showed experimentally that the addition of the non-denitrifying strain Dyadobacter fermentans, which possesses the previously unaccounted N2O reductase NosZII, to 11 different soils significantly reduced N2O production of up to 189% in more than 1/3 of the soils. The magnitude of this effect was significantly influenced by the soil pH and C/N ratio. Overall, our results provide unambiguous evidence that the overlooked non-denitrifying NosZII-type bacteria can contribute to N2O consumption in soil.

Because earthworms sustain soil functioning and fertility, there is a need to advance the knowledge of their adaptation potential to chemicals in anthropogenic landscapes. Our hypothesis is that there is acclimation to organic chemicals (pesticides) in earthworms that durably persist under conventional farming in anthropogenic landscapes. The adaptation capability of two populations of earthworms (Aporectodea caliginosa) having a different chemical exposure history, – one originating from 20 years of organic farming (naive population) and another from 20 years of conventional farming (pre-exposed population) – to cope with soil organic pollutant (Opus®, epoxiconazole a worldwide used fungicide) were investigated. Several complementary metabolic and energetic endpoints were followed, and cast production was assessed as a behavioural biomarker related to earthworms ecological role for the soil. Basal metabolism reflected by respiration rate was increased in both fungicide-exposed worms compared to controls. Glycogen resources were decreased in the same proportion in the two populations but more rapidly for the naive (7 days) than for the pre-exposed population (28 days). Soluble protein and most amino-acids contents increased in the pre-exposed population only, suggesting a detoxification mechanism. Metabolomic profiles showed a cut-off between fungicide-exposed and control groups in the pre-exposed earthworms only, with an increase in most of the metabolites. Exposure to a low dose of epoxiconazole increased cast production of pre-exposed earthworms, and this resulted in an increase in pesticide disappearance. As far as we know, this is the first study which evidenced there is an acclimation to an agricultural chemical in earthworms derived from conventional farming that also relates to a change in their burrowing behaviour, and for which larger consequences for the soil ecosystem need to be addressed. This original finding is of major interest in the frame of ecosystem resilience to global changes. Whether this physiological adaptation is a general pattern of response against fungicides or other pesticides would need to be confirmed with other molecules and agricultural contexts.

It remains unclear how soil microbes respond to labile organic carbon (LOC) inputs and how temperature sensitivity ($Q_{10}$) of soil organic matter (SOM) decomposition is affected by LOC inputs in a short-term. In this study, $^{13}$C-labeled glucose was added to a pristine grassland soil at four temperatures (10, 15, 20, and 25 ◦C), and the immediate utilization of LOC and native SOM by microbes was measured minutely in a short-term. We found that the LOC addition stimulated the native SOM decomposition, and elevated temperature enhanced the intensity of microbial response to LOC addition. The ratio between microbial respiration derived from LOC and native SOM increased with higher temperature, and more LOC for respiration. Additionally, LOC addition increased the $Q_{10}$ of SOM decomposition, and the $Q_{10}$ of LOC decomposition is higher than that of native SOM. Overall, these findings emphasize the important role of temperature and LOC inputs in soil C cycles.

<p>Global warming is leading to increased temperatures, accentuated evaporation of terrestrial water and increased the atmosphere moisture content, resulting in frequent droughts and heavy precipitation events. It necessary to assess the sensitivity of soil organic carbon (SOC) under storing practices in response to increasing soil moisture, temperature and frequent dry-wet cycles in order to anticipate future soil carbon losses. We evaluated the impact of these climatic events through an incubation experiment on temperate luvisols from conservation agriculture, organic agriculture, organic waste products applications, i.e. biowaste, residual municipal solid waste and farmyard manure composts compared with conventionally managemed soils. The alternative management options all have led to increased SOC stocks. Soil samples were incubated in the lab under different temperatures (20, 28 and 35°C), different moisture conditions (pF1.5; 2.5 and 4.2) and under dry(pF4.2)-wet (pF1.5) cycles. Dry-wet cycles caused CO2 flushes but overall did not stimulate soil carbon mineralization relative to wet controls (pF1.5 and pF2.5). Overall the additional SOC stored under alternative management options was not more sensitive to climate change (temperature, moisture, dry-wet cycles) than the existing SOC.</p>