The nitrogen, carbon and greenhouse gas budget of a grazed, cut and fertilised temperate grassland

Intensively managed grazed grasslands in temperate climates are globally important environments for the exchange of the greenhouse gases (GHGs) carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). We assessed the N and C budget of a mostly grazed and occasionally cut and fertilised grassland in SE Scotland by measuring or modelling all relevant imports and exports to the field as well as changes in soil C and N stocks over time. The N budget was dominated by import from inorganic and organic fertilisers (21.9 g N m−2 a−1) and losses from leaching (5.3 g N m−2 a−1), N2 emissions (2.9 g N m−2 a−1), and NOx and NH3 volatilisation (3.9 g N m−2 a−1), while N2O emission was only 0.6 g N m−2 a−1. The efficiency of N use by animal products (meat and wool) averaged 9.9 % of total N input over only-grazed years (2004–2010). On average over 9 years (2002–2010), the balance of N fluxes suggested that 6.0 ± 5.9 g N m−2 a−1 (mean ± confidence interval at p > 0.95) were stored in the soil. The largest component of the C budget was the net ecosystem exchange of CO2 (NEE), at an average uptake rate of 218 ± 155 g C m−2 a−1 over the 9 years. This sink strength was offset by carbon export from the field mainly as grass offtake for silage (48.9 g C m−2 a−1) and leaching (16.4 g C m−2 a−1). The other export terms, CH4 emissions from the soil, manure applications and enteric fermentation, were negligible and only contributed to 0.02–4.2 % of the total C losses. Only a small fraction of C was incorporated into the body of the grazing animals. Inclusion of these C losses in the budget resulted in a C sink strength of 163 ± 140 g C m−2 a−1. By contrast, soil stock measurements taken in May 2004 and May 2011 indicated that the grassland sequestered N in the 0–60 cm soil layer at 4.51 ± 2.64 g N m−2 a−1 and lost C at a rate of 29.08 ± 38.19 g C m−2 a−1. Potential reasons for the discrepancy between these estimates are probably an underestimation of C losses, especially from leaching fluxes as well as from animal respiration. The average greenhouse gas (GHG) balance of the grassland was −366 ± 601 g CO2 eq. m−2 yr−1 and was strongly affected by CH4 and N2O emissions. The GHG sink strength of the NEE was reduced by 54 % by CH4 and N2O emissions. Estimated enteric fermentation from ruminating sheep proved to be an important CH4 source, exceeding the contribution of N2O to the GHG budget in some years

Identification of Bacterial Micropredators Distinctively Active in a Soil Microbial Food Web

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

Fate of microbial biomass-derived amino acids in soil and their contribution to soil organic matter

Soil organic matter (SOM) is important for soil fertility and for the global C cycle. Previous studies have shown that during SOM formation no new compound classes are formed and that it consists basically of plant- and microorganism-derived materials. However, little data on the contribution from microbial sources are available. Therefore, we investigated previously in a model study the fate of C from 13C-labelled Gram-negative bacteria in soil (Kindler, R., Miltner, A. Richnow, H.H., Kästner, M., 2006. Fate of gram negative bacterial biomass in soil – mineralization and contribution to SOM. Soil Biology and Biochemistry 38, 2860–2870) and showed that 44% of the bulk 13C remained in the soil. Here we present the corresponding data on the fate of amino acids hydrolysed from proteins, which are the most abundant components of microbial biomass. After 224 days incubation, the label in the total amino acids in the soil amended with 13C-labelled cells decreased only to >95%. The total amino acids therefore clearly showed a lower turnover than the bulk 13C and a surprisingly stable concentration. Proteins therefore have to be considered as being stabilised in soil in dead, non-extractable biomass or cell fragments by known general stabilisation mechanisms. The label in the amino acids in a fraction highly enriched in living microbial biomass decreased to a greater extent, i.e. to 25% of the initially added amount. The amino acids removed from this fraction were redistributed via the microbial food web to non-living SOM. All amino acids in the microbial biomass were degraded at similar rates without a change in isotopic signature. The nuclear magnetic resonance (NMR) spectra of the soils were very similar and indicate that the residues of the degraded microbial biomass were very similar to those of the SOM and are a significant source for the formation of the SOM.Peer reviewe

Effect of Sulfadiazine-Contaminated Pig Manure on the Abundances of Genes and Transcripts Involved in Nitrogen Transformation in the Root-Rhizosphere Complexes of Maize and Clover▿ †

The antibiotic sulfadiazine (SDZ) can enter the environment by application of manure from antibiotic-treated animals to arable soil. Because antibiotics are explicitly designed to target microorganisms, they likely affect microbes in the soil ecosystem, compromising important soil functions and disturbing processes in nutrient cycles. In a greenhouse experiment, we investigated the impact of sulfadiazine-contaminated pig manure on functional microbial communities involved in key processes of the nitrogen cycle in the root-rhizosphere complexes (RRCs) of maize (Zea mays) and clover (Trifolium alexandrinum). At both the gene and transcript level, we performed real-time PCR using nifH, amoA (in both ammonia-oxidizing bacteria and archaea), nirK, nirS, and nosZ as molecular markers for nitrogen fixation, nitrification, and denitrification. Sampling was performed 10, 20, and 30 days after the application. SDZ affected the abundance pattern of all investigated genes in the RRCs of both plant species (with stronger effects in the RRC of clover) 20 and 30 days after the addition. Surprisingly, effects on the transcript level were less pronounced, which might indicate that parts of the investigated functional groups were tolerant or resistant against SDZ or, as in the case of nifH and clover, have been protected by the nodules

Carbon, nitrogen and Greenhouse gases budgets over a four years crop rotation in northern France

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