• Energy Research

AbstractThe EU Soil Strategy 2030 aims to increase soil organic carbon (SOC) in agricultural land to enhance soil health and support biodiversity as well as to offset greenhouse gas emissions through soil carbon sequestration. Therefore, the quantification of current SOC stocks and the spatial identification of the main drivers of SOC changes is paramount in the preparation of agricultural policies aimed at enhancing the resilience of agricultural systems in the EU. In this context, changes of SOC stocks (Δ SOCs) for the EU + UK between 2009 and 2018 were estimated by fitting a quantile generalized additive model (qGAM) on data obtained from the revisited points of the Land Use/Land Cover Area Frame Survey (LUCAS) performed in 2009, 2015 and 2018. The analysis of the partial effects derived from the fitted qGAM model shows that land use and land use change observed in the 2009, 2015 and 2018 LUCAS campaigns (i.e. continuous grassland [GGG] or cropland [CCC], conversion grassland to cropland (GGC or GCC) and vice versa [CGG or CCG]) was one of the main drivers of SOC changes. The CCC was the factor that contributed to the lowest negative change on Δ SOC with an estimated partial effect of −0.04 ± 0.01 g C kg−1 year−1, while the GGG the highest positive change with an estimated partial effect of 0.49 ± 0.02 g C kg−1 year−1. This confirms the C sequestration potential of converting cropland to grassland. However, it is important to consider that local soil and environmental conditions may either diminish or enhance the grassland's positive effect on soil C storage. In the EU + UK, the estimated current (2018) topsoil (0–20 cm) SOC stock in agricultural land below 1000 m a.s.l was 9.3 Gt, with a Δ SOC of −0.75% in the period 2009–2018. The highest estimated SOC losses were concentrated in central‐northern countries, while marginal losses were observed in the southeast.

AbstractDenitrification is a key process in the global nitrogen (N) cycle, causing both nitrous oxide (N2O) and dinitrogen (N2) emissions. However, estimates of seasonal denitrification losses (N2O + N2) are scarce, reflecting methodological difficulties in measuring soil‐borne N2 emissions against the high atmospheric N2 background and challenges regarding their spatio‐temporal upscaling. This study investigated N2O + N2 losses in response to N fertilizer rates (0, 100, 150, 200, and 250 kg N ha−1) on two intensively managed tropical sugarcane farms in Australia, by combining automated N2O monitoring, in situ N2 and N2O measurements using the 15N gas flux method and fertilizer 15N recoveries at harvest. Dynamic changes in the N2O/(N2O + N2) ratio (<0.01 to 0.768) were explained by fitting generalized additive mixed models (GAMMs) with soil factors to upscale high temporal‐resolution N2O data to daily N2 emissions over the season. Cumulative N2O + N2 losses ranged from 12 to 87 kg N ha−1, increasing non‐linearly with increasing N fertilizer rates. Emissions of N2O + N2 accounted for 31%–78% of fertilizer 15N losses and were dominated by environmentally benign N2 emissions. The contribution of denitrification to N fertilizer loss decreased with increasing N rates, suggesting increasing significance of other N loss pathways including leaching and runoff at higher N rates. This study delivers a blueprint approach to extrapolate denitrification measurements at both temporal and spatial scales, which can be applied in fertilized agroecosystems. Robust estimates of denitrification losses determined using this method will help to improve cropping system modeling approaches, advancing our understanding of the N cycle across scales.

Abstract Purpose The reduction of the greenhouse gas nitrous oxide (N2O) to dinitrogen (N2) via denitrification and N2O source partitioning between nitrification and denitrification remain major uncertainties in sugarcane systems. We therefore investigated magnitude and product stoichiometry of denitrification and production pathways of N2O from a tropical sugarcane soil in response to increasing soil nitrate (NO3−) availability. Methods Microcosms were established using a tropical sugarcane soil (Qld, Australia) and emissions of N2O and N2 were measured following fertilisation with 15NO3−–N equivalent to 25, 50 and 100 μg N g−1 soil, simulating soil NO3− contents previously observed in situ, and mimicking flood irrigation by wetting the soil close to saturation. Results Cumulative N2O emissions increased exponentially with NO3− availability, while cumulative N2 emissions followed an exponential increase to maximum. Average daily N2 emissions exceeded 5 µg N2–N g soil−1 and accounted for > 99% of denitrification. The response of N2O suggests preferential NO3− reduction with increasing NO3− availability, increasing N2O even when NO3− levels had only a diminishing effect on the overall denitrification rate. The fraction of N2O emitted from denitrification increased with NO3− availability, and was a function of soil water, NO3− and heterotrophic soil respiration. Conclusions Our findings show the exponential increase of N2O driven by excess NO3−, even though the complete reduction to N2 dominated denitrification. The low N2O/(N2O + N2) product ratio questions the use of N2O as proxy for overall denitrification rates, highlighting the need for in-situ N2 measurements to account for denitrification losses from sugarcane systems.

Abstract Acid-sulphate sugarcane soils in the subtropics are known hot-spots for nitrous oxide (N2O) emissions, yet the reduction of reactive N2O to non-reactive dinitrogen (N2) via specific pathways remains a major uncertainty for nitrogen (N) cycling and loss from these soils. This study investigated the magnitude and the N2O:N2 partitioning of N2O and N2 losses from a subtropical acid-sulphate soil under sugarcane production using the 15N gas flux method, establishing the contribution of hybrid (co- and chemo-denitrification) and heterotrophic denitrification to N2O and N2 losses. Soils were fertilised with potassium nitrate, equivalent to 25 and 50 kg N ha−1, watered close to saturation then incubated over 30 days. An innovative, fully automated incubation system coupled to an isotope-ratio mass-spectrometer enabled real time analysis of 15N2O and 15N2 at sub-diel resolution. Peak losses of N2O and N2 reached 6.5 kg N ha−1 day−1, totalling > 50 kg of N2O+N2-N ha−1. Emissions were dominated by N2, accounting for more than 57% of N2O+N2 losses, demonstrating that the reduction of N2O to N2 proceeded even under highly acidic conditions. Over 40% of N2O, but only 2% of N2 emissions, were produced via hybrid pathways. These findings demonstrate hybrid pathways are generally limited to N2O production, likely driven by high organic matter content and low soil pH, promoting both biotic, and abiotic nitrosation. Regardless of the underlying process, the magnitude of the N2O emissions demonstrates the environmental, but also the potential agronomic significance, of hybrid pathways of N2O formation for N loss from fertilised acid-sulphate soils.

Abstract Rainfall and irrigation trigger large pulses of the powerful greenhouse gas N2O from intensively managed pastures, produced via multiple, simultaneously occurring pathways. These N2O pulses can account for a large fraction of total N2O losses, demonstrating the importance to determine magnitude and source partitioning of N2O under these conditions. This study investigated the response of different pathways of N2O production to wetting across three different textured pasture soils. Soil microcosms were fertilised with an ammonium nitrate (NH4NO3) solution which was either single or double 15N labelled, wetted to four different water-filled pore space (WFPS) levels, and incubated over two days. The use of a 15N pool mixing model together with soil N gross transformations enabled the attribution of N2O to specific pathways, and to express N2O emissions as a fraction of the underlying N transformation. Denitrification and nitrification mediated pathways contributed to the production of N2O in all soils, regardless of WFPS. Denitrification was the main pathway of N2O production accounting for >50% of cumulative N2O emissions even at low WFPS. The contribution of autotrophic nitrification to N2O emissions decreased with the amount of wetting, while the contribution of heterotrophic nitrification remained stable or increased. Following the hole-in-the-pipe model, 0.1%–4% of nitrified N was lost as N2O, increasing exponentially with WFPS, while the percentage of denitrified N emitted as N2O decreased, providing critical information for the representation of N2O/WFPS relationships in simulation models. Our findings demonstrate that the wetting of pasture soils promotes N2O production via denitrification and via the oxidation of organic N substrates driven by high carbon and N availability upon wetting. The large contribution of heterotrophic nitrification to N2O emissions should be considered when developing N2O abatement strategies, seeking to reduce N2O emissions in response to rainfall and irrigation from intensively managed pastures.

Intensive vegetable production is characterised by high nitrogen (N) application rates and frequent irrigations, promoting elevated nitrous oxide (N2O) emissions, a powerful greenhouse gas indicative for the low N use efficiency (NUE) in these systems. The use of nitrification inhibitors (NI) has been promoted as an effective strategy to increase NUE and decrease N2O emissions in N-intensive agricultural systems. This study investigated the effect of two NIs, 3,4-dimethylpyrazole phosphate (DMPP) and 3-methylpyrazole 1,2,4-triazole (Piadin), on N2O emissions and 15N fertiliser recovery in a field experiment in sweet corn. The trial compared the conventional fertiliser N rate to a 20% reduced rate combined with either DMPP or Piadin. The use of NI-coated urea at a 20% reduced application rate decreased cumulative N2O emissions by 51% without yield penalty. More than 25% of applied N was lost from the conventional treatment, while a reduced N rate in combination with the use of a NI significantly decreased N fertiliser losses (by up to 98%). Across treatments, between 30 and 50% of applied N fertiliser remained in the soil, highlighting the need to account for residual N to optimise fertilisation in the following crop. The reduction of overall N losses without yield penalties suggests that the extra cost of using NIs can be compensated by reduced fertiliser application rates, making the use of NIs an economically viable management strategy for growers while minimising environmentally harmful N losses from vegetable growing systems.

Abstract Changes in amplitude and frequency of wetting and drying cycles in pasture systems due to climatic variation and irrigation management are likely to affect magnitude and partitioning of N2 and N2O emissions. This study investigated the effects of irrigation frequency on N2 and N2O emissions from an intensively managed pasture in the subtropics. Irrigation volumes were estimated to replace evapotranspiration and were applied either once (Low-Frequency) or split into four applications (High-Frequency). To test for legacy effects, a large rainfall event was simulated at the end of the experiment. Over 15 days, 7.9 ± 2.7 kg N2+N2O-N ha−1 was emitted on average regardless of irrigation frequency, with N2O accounting for 25% of overall N2+N2O. Repeated, small amounts of irrigation produced an equal amount of N2+N2O losses as a single, large irrigation event. The increase of N2O emissions after the large rainfall event was smaller in the High-Frequency treatment, shifting the N2O/(N2O+N2) ratio towards N2. Our findings suggest that reduced soil-gas diffusivity predominantly drives N2O and N2 emissions following large wetting events, while microbial O2 consumption largely drives N2O and N2 emissions after small, repeated wetting events in high N turnover pasture soils. The abundance of marker genes for N cycling did not differ between treatments, indicating that N and C availability, not gene abundance determined magnitude and N2O:N2 partitioning after rainfall. The observed legacy effect suggests that increased irrigation frequency can reduce the environmental impact (N2O), but not overall magnitude of N2O and N2 emissions from intensively managed pastures.