Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N$_2$O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N$_2$O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N$_2$O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N$_2$O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N$_2$O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N$_2$O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N$_2$O emissions

Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2O emissions

Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2O emissions

Karbon og biologisk aktivitet i jord med eng- og potetdyrking (K-BEP)

Eng- og potetdyrking påvirker biologisk aktivitet og karbon i jord som viktige deler av jordhelsa. I prosjektet «Karbon og biologisk aktivitet i jord med eng- og potetdyrking (K-BEP)» undersøkte vi flere ting. Mellom annet hvordan engdyrking i 30 år og en nylig «karbonboosting» med ulike typer organisk materiale (OM) i potetdyrking, påvirket jordbiologi, mengde og typer karbonfraksjoner og jordhelse. Åtte ulike praksisnære tester ble brukt for å vurdere hvor sensitive og egnet de var til å gi informasjon om jordhelse og som jordhelse-indikatorer. Disse ble testet i jord under eng, beite og potet, i behandlinger der vi forventet variasjon i jordhelsa. Det var en gradient i «jordarbeiding» og ulik tilførsel av organisk materiale. Undersøkelsene ble gjennomført i 2021 og testene dekket ulike aspekter av jordhelse, inkludert innhold av OM, karbonfraksjoner, jordbiologi og biologisk aktivitet. To dataverktøy og egne målinger over 30 år ble brukt til å estimere karbonmengder i jord og endringer i jordkarbon over tid. De fleste jordhelsetestene viste forskjeller mellom engsystemet og potetsystemet, og da med høyere verdier i engsystemet. Det var imidlertid få forskjeller innad mellom behandlinger og arealer innad i systemene. Beite viste tendens til bedre jordhelse enn eng av ulik alder, mens det i potetfeltet ikke ble funnet noen forskjeller knyttet til ettereffekt av ulike typer tilført OM. Det anbefales derfor å bruke flere tester samlet. Ved å samle data i et jordhelsediagram basert på normalverdier av hver test, kan man visualisere resultatene fra hvert skifte i forhold til hverandre. Da får man en indikasjon på om endringene går til det bedre eller ikke mht. jordhelse, før og etter et eventuelt tiltak i drifta, eller mellom ulike behandlinger eller mellom skifter med samme behandling. Det anbefales i tillegg å grave i jorda og vurdere forholdene visuelt og sensorisk, og se testresultatene i sammenheng med jordanalyser. Det trengs flere norske data fra ulike jordhelse-tester for å tolke resultatene bedre, for å opparbeide norske referansenivåer og som grunnlag for å anbefale et utvalg tester egnet til gardsbasert vurdering av jordhelse under norske forhold. I snitt var det 4-8 % høyere innhold av OM (GT%) der det to år tidligere ble tilført fast biorest (BR), hestegjødsel (HG) og biokull med flytende del av biorest (BK+). Det var også noen små forskjeller mellom behandlingene om karbonet ble funnet i som POM (partikulært OM) eller MAOM (bundet til jordmineraler) i jorda. BK+ ble estimert som mer stabilt over tid i jord basert på laboratorieforsøk, enn de andre typene OM. Ved å bruke skiftevis målte verdier for glødetap (OM) fra jordanalyser, ble karbonbeholdningen i et 20 cm tykt jordlag estimert. De fleste bønder forvalter mye karbon i matjordlaget. Innholdet var lavest i potetfeltet, med estimater på 2,9 tonn-3,3 tonn C/daa, middels i engene, med 4,7-9,9 tonn C/daa og høyest i permanente beiter med 11,9-14,4 tonn C/daa. Det var i tillegg betydelige mengder karbon i jordprøvene tatt fra 20-40 cm dyp, bare målt i engarealene. Vi sammenliknet målte og simulerte jorddata for å si noe om endringer i jordkarbon på en melkeproduksjonsgard over tid. Jorddata fra ulike skifter på garden viser generelt et høyt innhold av OM i jorda, i snitt 7 % i 2021. Videre viste resultatene at beholdningen av OM og karbon i matjordlaget (0-20 cm) avtok over en 30 årsperiode på de fleste skiftene. Simuleringer gjort i C-Tools modellen for karbonendring i mineraljord over en 100 års periode, stemte bra overens med de målte verdiene for skifter der utgangsmengden OM i jorda var 12 % (GT%) og lavere. For skifter med over 12 % OM ved prøvetakingsstart, var det ikke så bra samsvar. De målte verdiene viser da en raskere nedgang i innholdet av OM og karbon enn de simulerte endringene i C-Tools. Dette indikerer at utgangsnivået er viktig for skjebnen til OM og karbon i jordbruksjord. Videre at tross i kjente fordeler av flerårig gras og tilførsel av husdyrgjødsel, virker det krevende over tid å opprettholde eller øke innholdet av jordkarbon i norsk jordbruksjord

Assessing soil health indicators in an organically managed grassland

Overall, the permanent pasture had somewhat better soil health, based on higher values for a higher numbers of the selected soil health indicators, than the grass-clover leys (Fig.3). To visualize the results from several indicators together in a participatory manner, we normalised the data for each soil indicator. By doing so, we obtained an overall “soil health diagram” that farmers and advisors can interpret and use in the discussion of soil health. Conclusion: Assessment and graphic visualization of several indicators and fields altogether seems useful in the discussion about soil health on farm level. Depending on the purpose of the assessment, a visual evaluation of the topsoil by digging and discussing in field is recommended to complement the selected indicators. More field data is needed to obtain “reference levels” for interpretation of soil health, and for a better understanding of optimal or low values of soil health indicators between cropping systems and management

CAPTURE- Assessment of cover cropping as climate action in cereal production in Norway

Recently, the role of cover crops in agriculture has broadened due to their ecosystem services and their potential as a climate mitigation tool via carbon sequestration. However, their net climate effect from carbon sequestration may be offset by nitrate leaching and nitrous oxide (N2O) emissions when plant residues decompose in the soil. In CAPTURE (a 3-year project funded by the Agriculture and Food Industry Research Funds), we evaluate the net climate effect of different cover crops in the main cereal production areas in South and Mid-Norway. To determine the soil carbon sequestration potential from different cover crops, four species (Italian ryegrass, phacelia, oilseed radish and summer vetch) were planted in monoculture (in 4 replicates) in spring 2021 and pulse labelled (5-6 pulses) with 13CO2 during the growing season in 1 m3 transparent chambers. Shortly after the last pulse, biomass (above- and belowground) and soil samples were collected and brought to the laboratory for processing. These samples are now under analysis and together with consecutive samplings in 2022 we will determine the input of cover crop-derived carbon (13C) to the soil. These data will also be used to feed a model to evaluate the potential for long-term carbon storage of different cover-crops. Moreover, we will investigate the fate and persistence of this carbon input, by assessing how much of the 13C was retained as particulate organic matter (POM) and as mineral-associated organic matter (MAOM). The fate of soil organic matter -13C in each fraction will be monitored in the following years. To study the contribution of shoot- versus root biomass to carbon sequestration, we established microplots in which aboveground biomass was exchanged and incorporated reciprocally between the labelled microplot and an equivalent unlabelled microplot with the same cover crop. In this way, each field replicate includes one microplot with labelled shoots and unlabelled roots and another microplot with unlabelled shoots and labelled roots, which will be sampled and analysed in 2022. Cover crop-specific carbon sequestration will be scaled to barley plots in the same experiment, to which cover crops had been undersown in spring or summer 2021. In these plots, N2O emissions have been quantified weekly throughout the growing period and more frequently during autumn and winter, which should allow us to estimate greenhouse gas trade-offs of cover crops in Norwegian cereal production and contribute to improve the national greenhouse gas inventory

Molecular characterization of biochar from five Brazilian agricultural residues obtained at different charring temperatures

Important parameters that influence biochar properties include charring temperature and biomass type. We characterized the molecular properties of biochars from five agricultural residues with pyrolysis gas chromatography mass/spectrometry (pyrolysis-GC/MS) in comparison with atomic H/C and N/C ratios. Feedstocks included chicken manure, eucalyptus sawdust, coffee husk, sugarcane bagasse and pine bark. Biochars produced at three different temperatures (Tchar) were analyzed, including 350, 450 and 750 °C, as well as the uncharred materials. The optimum temperature during analysis with pyrolysis-GC/MS (Tpy) was examined. Tpy 600 °C gave the best results for all Tchar by showing a larger diversity of pyrolysis products compared to Tpy 700 °C and 800 °C; Tpy 600 °C was therefore used for qualitative and quantitative comparison of the samples. Charring temperature was the dominant factor that determined the chemical composition of the biochar pyrolysates. Uncharred feedstocks had the largest contribution from carbohydrates, lignin phenols and long chain n-alkanes, all of which rapidly decreased with charring; biochars produced at Tchar 350 and 450 °C showed the largest contribution from phenols, mid-chain n-alkanes, benzofurans, indenes, biphenyls and PAHs, from which the benzofurans, indenes, biphenyls and PAHs were particular abundant in samples produced at Tchar 450 °C; pyrolysates of biochars produced at Tchar 750 °C were characterized by branched aliphatics, short chain n-alkanes/n-alkenes and low molecular weight (LMW) benzenes. Factor analysis showed that the variation of products differed largely within some chemical groups. For the N-containing compounds, caffeine, C16 alkylnitrile and diketopiperazines were associated with uncharred materials, benzonitriles and quinolines were associated with Tchar 350–450 °C. Another part of the variation of N-containing compounds was associated with chicken manure, and to a lesser extend also coffee husk, independently of Tchar. For all five agricultural residues, the highest chemical diversity was found for biochar produced at Tchar 350 °C. As the charring temperature increased, the diversity of pyrolysis products diminished

Predicting field N2O emissions from crop residues based on their biochemical composition: A meta-analytical approach

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N2O). Previous meta-analyses have linked various biochemical properties of crop residues to N2O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N2O emissions from specific residues. Here we combine comprehensive databases for N2O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N2O emissions. On average, crop residue incorporation increased soil N2O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N2O emissions. Crop residue effects on N2O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N2O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N2O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N2O emissions, whereas mature residues with opposite characteristics had marginal effects on N2O. The most important crop types belonging to the immature residue group – cover crops, grasslands and vegetables – are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N2O emissions

Predicting field N2O emissions from crop residues based on their biochemical composition: A meta-analytical approach

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N2O). Previous meta-analyses have linked various biochemical properties of crop residues to N2O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N2O emissions from specific residues. Here we combine comprehensive databases for N2O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N2O emissions. On average, crop residue incorporation increased soil N2O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N2O emissions. Crop residue effects on N2O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N2O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N2O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N2O emissions, whereas mature residues with opposite characteristics had marginal effects on N2O. The most important crop types belonging to the immature residue group – cover crops, grasslands and vegetables – are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N2O emissions