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This study aimed to explore the soil metabolic response to long-term fertiliser application and the effect of this response on the microbial community by taking advantage of the Woburn Organic Manuring Experiment (UK; operational since 1964). Untargeted metabolomes detected by gas chromatography-time of flight mass spectrometer/mass spectrometry (GC-TOFMS/MS) and ultra-high-pressure liquid chromatography-quadrupole time of flight mass-spectrometer/mass spectrometry (UHPLC-QTOFMS/MS) were used to explore which method better reflected soil microbe-accessible metabolites. Microbial community abundance was detected by high-throughput sequencing. We found that long-term farmyard manure application enhanced the soil’s total and dissolved C and N contents. The metabolite content detected by GC-TOFMS/MS (TOF detector with a cold injection unit) had a negative linear correlation with soil organic matter, extractable organic nitrogen (N), and microbial carbon (C). Conversely, the metabolite content detected by UHPLC-QTOFMS/MS was positively correlated with soil organic matter, indicating that metabolites detected by UHPLC-QTOFMS/MS were the main components of soluble soil organic matter. More positive than negative correlations were observed between metabolites and bacterial (69.5%) and fungal (67.9%) taxa in the co-occurrence network. Among the bacterial taxa in the network, the family Planococcaceae and genus Paenibacillus showed the most correlations with metabolites. The choice of extraction and detection method affects the identity and number of metabolites detected. Therefore, careful consideration is needed when selecting which methods to use. We demonstrated a strong correlation between soil metabolites and microbial community abundance. However, a deeper understanding of soil microbial function and metabolite formation, content, and decomposition is still needed.

Before spreading to land, farmyard manures (FYM) from pigs (Sus scrofa) and beef cattle (Bos taurus) were stored, for c. 120–150 days, either uncompacted or compacted. Compaction was carried out as the manures were put into store and the compacted manures were covered with plastic sheeting. Both compacted and uncompacted FYM were either incorporated by ploughing immediately after spreading, within 4 h, within 24 h or left on the surface until the soil was cultivated prior to planting. Despite greater amounts of total ammoniacal nitrogen (TAN) remaining in the compacted pig FYM, ammonia (NH3) emissions following spreading were not significantly greater than from the uncompacted FYM, even when left on the soil surface. Incorporation of pig FYM reduced NH3 emissions by c. 90, 60 and 30% for immediate, within 4 h and within 24 h incorporation, respectively. There were no effects of compaction during storage on nitrous oxide (N2O) emissions following spreading for the measurement period of 60 days. Incorporation had no effect on N2O emissions from pig FYM following spreading in the first experiment, but reduced emissions following spreading in the second year and reduced N2O emissions following the spreading of cattle FYM in both experiments. These results indicate that rapid incorporation of FYM after spreading to land is an effective means of reducing NH3 emissions and need not lead to increases in N2O emissions.

An automated laboratory soil incubation system enabled the effects on gaseous emissions from a soil to be quantified accurately, when amended with slurry plus a nitrification inhibitor: dicyandiamide (DCD), or 3,4-dimethylpyrazole phosphate (DMPP). Nitrification inhibitors applied with slurry under simulated Portuguese conditions were very efficient in reducing N2O emission, and did not increase CH4 emissions significantly, when the soil was predominantly aerobic. The inhibitors were also indirectly effective in reducing N2O emissions due to denitrification during a subsequent anaerobic phase. All gaseous emissions followed strong diurnal patterns that were positively correlated with soil temperature and obeyed a Q10=2 relationship. The widespread use of DCD and DMPP inhibitors with slurry applied to Portuguese soils could have the potential to reduce N2O emissions from this source by ten- to 20-fold.

This position paper summarizes the current understanding of biological nitrification inhibition (BNI) to identify research needs for accelerating the development of BNI as a N2O mitigation strategy for grazed livestock systems. We propose that the initial research focus should be on the systematic screening of agronomically desirable plants for their BNI potency and N2O reduction potential. This requires the development of in situ screening methods that can be combined with reliable N2O emission measurements and microbial and metabolomic analyses to confirm the selective inhibition of nitrification. As BNI-induced reductions in N2O emissions can occur by directly inhibiting nitrification, or via indirect effects on other N transformations, it is also important to measure gross N transformation rates to disentangle these direct and indirect effects. However, an equally important challenge will be to discern the apparent influence of soil N fertility status on the release of BNIs, particularly for more intensively managed grazing systems.