Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its delta ¹³C signature. Leaching of biogenic DIC was 8.3 +/- 4.9 g m-2 yr-1 for forests, 24.1 +/- 7.2 g m-2 yr-1 for grasslands, and 14.6 +/- 4.8 g m-2 yr-1 for croplands. DOC leaching equalled 3.5 +/- 1.3 g m-2 yr-1 for forests, 5.3 +/- 2.0 g m-2 yr-1 for grasslands, and 4.1 +/- 1.3 g m-2 yr-1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4 +/- 4.0 g C m-2 yr-1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5-98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24-105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems

The understanding of microbial interactions and trophic networks is a prerequisite for the elucidation of the turnover and transformation of organic materials in soils. To elucidate the incorporation of biomass carbon into a soil microbial food web, we added 13 C-labeled Escherichia coli biomass to an agricultural soil and identified those indigenous microbes that were specifically active in its mineralization and carbon sequestration. rRNA stable isotope probing (SIP) revealed that uncultivated relatives of distinct groups of gliding bacterial micropredators (Lysobacter spp., Myxococcales, and the Bacteroidetes) lead carbon sequestration and mineralization from the added biomass. In addition, fungal populations within the Microascaceae were shown to respond to the added biomass after only 1 h of incubation and were thus surprisingly reactive to degradable labile carbon. This RNA-SIP study identifies indigenous microbes specifically active in the transformation of a nondefined complex carbon source, bacterial biomass, directly in a soil ecosystem.

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Although soils are generally known to be a net source of CO 2 due to microbial respiration, CO 2 fixation may also be an important process. The non-phototrophic fixation of CO 2 was investigated in a tracer experiment with 14 CO 2 in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from 14 CO 2 to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO 2 fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20-40 days). The fixation amounted to 3-5% of the net respiration. After this phase, the CO 2 fixation decreased to 1-2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO 2 was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation of CO 2 . However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO 2 fixation represents a significant factor of microbial activity in soils.Abbreviations: FYM -farmyard manure; LSC -liquid scintillation counting; PCR -polymerase chain reaction; RuBisCo -ribulosebisphosphate carboxylase/oxygenase.

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4 +/- 0.7 Mg C ha(-1) y(-1) but represented a net source of carbon with a net biome production (NBP) of -1.3 +/- 1.1 Mg C ha(-1) y(-1). The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (-24 +/- 28 kg N ha(-1) y(-1)). The main nitrogen losses were nitrate leaching (-11.7 +/- 1.0 kg N ha(-1) y(-1)) and ammonia volatilization (-9 kg N ha(-1) y(-1)). Dry and wet depositions were 6.7 +/- 3.0 and 5.9 +/- 0.1 kg N ha(-1) y(-1), respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8 +/- 0.04; NO: -0.7 +/- 0.04 kg N ha(-1) y(-1)) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems

Th e environmental risks caused by the use of fl uoroquinolone antibiotics in human therapeutics and animal husbandry are associated with their persistence and (bio)accessibility in soil. To assess these aspects, we administered difl oxacin to pigs and applied the contaminated manure to soil. We then evaluated the dissipation and sequestration of difl oxacin in soil in the absence and presence of plants within a laboratory trial, a mesocosm trial, and a fi eld trial. A sequential extraction yielded antibiotic fractions of diff ering binding strength. We also assessed the antibiotic's eff ects on nitrogen turnover in soil (potential nitrifi cation and denitrifi cation). Difl oxacin was hardly (bio)accessible and was very persistent under all conditions studied (dissipation half-life in bulk soil, >217 d), rapidly forming nonextractable residues. Although varying environmental conditions did not aff ect persistence, dissipation was accelerated in soil surrounding plant roots. Eff ects on nitrogen turnover were limited due to the compound's strong binding and small (bio)accessibility despite its persistence.