Changes in soil C dynamics and N2O emissions under minimum tillage cereal crop production in boreal agroecosystems : Implications to climate change mitigation

Soils comprise more carbon (C) than any other terrestrial source and hence even a small change in the C content can be significant in regards to atmospheric CO2 concentration. Cultivated soils have lost soil organic carbon (SOC) during the latest decades in Finland. New cereal crop management practices, like no-till (NT) and reduced tillage (RT), can affect not only soil organic carbon (SOC) stocks and stabilization, but also nitrous oxide (N2O) emissions. The aim of this study was to gain better understanding about the changes in soil C dynamics and N2O emissions as a result of management practice changes in the boreal region, and the implications of these changes to climate change mitigation. Changes in SOC stocks and stabilization rates under different tillage (NT, RT, CT (conventional tillage with a moldboard plow)) and straw management (straw retention, straw removal, straw burning) practices, were studied at different sites with clayey and coarse textured soil across southern Finland. This was done by soil fractionation method (wet sieving and microaggregate isolation) to elucidate the composition of different soil fractions, namely large and small macroaggregates, microaggregates, silt and clay and macroaggregate-occluded soil fractions, and where the C is stored within them. The effects of Lumbricus terrestris on SOC were studied using the same method. Nitrous oxide fluxes were monitored biweekly for 2 years under CT, NT and RT practices using closed chambers. Measurements of several environmental and soil parameters were taken to study the underlying factors controlling the observed changes in soil C stocks and N2O emissions under the different management practices. Climate change mitigation potential through the studied cereal crop management practices seems small in the humid boreal region based on the results of this study. The minimum tillage treatments did not sequester SOC at any of the study sites which had been under NT or RT for a decade and the total C stocks were lower in the 0 15 cm topsoil layer at one clayey site under RT compared to CT after implementing RT for 30 years. All clayey sites had a fairly high SOC content originally and all sites had higher decomposition rate of crop residues in NT compared to CT, possibly hindering C sequestration. However, the aggregate stability was enhanced in NT cropping systems compared to CT, and NT increased the amount of SOC in large macroaggregates at several sites and in microaggregates within macroaggregates in the coarse textured site. L. terrestris mediated the formation of soil aggregates and the increase of SOC in the topsoil but possibly enhanced the decomposition rate in the soils. Cumulative N2O emissions were higher under NT compared to both CT and RT at the clayey sites and lower at the coarse textured site. However, the coarse textured site under NT received slightly less N fertilizer compared to CT. Increased N2O emissions under NT on clayey soils were likely due to denser soil structure with consistently higher soil moisture content and poor aeration. Therefore, mitigating N2O emissions requires special attention to soil structure and drainage. This study suggests that RT is a notable option to control N2O emissions. In the future, climate change could increase precipitation and freeze-thaw cycles in boreal agroecosystems possibly enhancing N2O fluxes and C losses of cultivated soils which puts pressure on finding new mitigation measures.Maaperä on hiilen suurin terrestinen varasto, joten pienelläkin muutoksella maaperän kokonaishiilipitoisuudessa voi olla merkittävä vaikutus ilmakehän CO2-pitoisuuteen. Orgaanisen hiilen määrä on vähentynyt Suomen maatalousmailla viimeisten vuosikymmenien aikana. Uusilla viljelytekniikoilla voidaan vaikuttaa paitsi maaperän hiilipitoisuuteen, myös dityppioksidin (N2O) päästöihin. Tämän tutkimuksen tavoite oli ymmärtää paremmin boreaalisen vyöhykkeen maatalousmaiden hiilen dynamiikassa ja N2O-päästöissä tapahtuvia muutoksia eri viljelymenetelmissä sekä pohtia näiden muutosten vaikutuksia ilmastonmuutoksen hillinnässä. Maaperän orgaanisen hiilen varastoja ja pysyvyyttä tutkittiin eri muokkausmenetelmin (suorakylvö, kevennetty muokkaus ja kyntö) käsitellyillä ja kasvintähteen määrän suhteen eroavilla (kasvintähteen jättäminen, poisto ja poltto) savisilla ja karkeaa maalajia edustavilla pelloilla Lounais- ja Etelä-Suomessa. Fraktiointimenetelmää (märkäseulonta ja mikroaggregaattien eristys) käyttämällä voitiin tarkastella eri maapartikkeleiden määriä ja niiden sisältämän hiilen määrää maaperässä. Tässä tutkimuksessa eriteltiin isot ja pienet makroaggregaatit, mikroaggregaatit, savi ja hiesu sekä edelleen makroaggregaattien sisältämät maapartikkelit. Kastelieron (Lumbricus terrestris) vaikutuksia maaperän orgaanisen hiilen pysyvyyteen tutkittiin samalla menetelmällä. N2O-päästöjä mitattiin joka toinen viikko kahden vuoden ajan. Maaperän orgaanisen hiilen pitoisuudessa ja pysyvyydessä sekä N2O-päästöissä tapahtuvia luonnollisia vaihteluita pyrittiin selittämään useiden mitattujen taustamuuttujien avulla. Mahdollisuudet hillitä ilmastonmuutosta tutkittujen viljelymenetelmien avulla vaikuttavat tämän tutkimuksen valossa heikoilta kostealla, boreaalisella ilmastovyöhykkeellä. Maaperän kokonaishiilipitoisuus laski kevennetyssä muokkauksessa 30 vuoden jälkeen tai pysyi samana kymmenen vuoden aikana sekä suorakylvössä että kevennetyssä muokkauksessa verrattuna kyntöön. Hiilen kertymistä ehkäisi todennäköisesti suorakylvössä orgaanisen aineen nopeampi hajotus ja savimailla peltojen jo alunalkaen suhteellisen korkea hiilipitoisuus. Toisaalta, koska aggregaattien pysyvyys parani ja joillakin pelloilla niiden sisältämän hiilen määrä kasvoi suorakylvössä verrattuna kyntöön, on mahdollista, että pitkäkestoinen suorakylvö parantaisi peltojen hiilitasetta. Kasteliero lisäsi välillisesti maaperän aggregaattien muodostusta sekä hiilen kertymistä isojen makroaggregaattien sisällä oleviin maapartikkeleihin, mutta mahdollisesti nopeutti karikkeen hajotusta maaperässä. Suorakylvön N2O-päästöt olivat korkeampia kuin kynnön tai kevennetyn muokkauksen savipelloilla, mutta alhaisempia karkealla maalajilla. Karkean maalajin pellolla typpilannoituksen määrä oli kuitenkin alhaisempi suorakylvetyillä tutkimusaloilla verrattuna kynnettyihin tutkimusaloihin. Kohonneet N2O-päästöt suorakylvössä savimailla johtuivat todennäköisesti tiiviimmästä maaperän rakenteesta, joka puolestaan kasvattaa maaperän kosteustasoa ja vähentää maan ilmavuutta. N2O-päästöjen hillitseminen vaatiikin huomion kiinnittämistä erityisesti maaperän rakenteeseen ja salaojituksen toimivuuteen. Tämän tutkimuksen perusteella kevennetty muokkaus on huomionarvoinen vaihtoehto N2O-päästöjen hillitsemiseksi. Tulevaisuudessa ilmastonmuutos tulee todennäköisesti voimistamaan sadannan ja jäätymis-sulamissyklien vaikutuksia agroekosysteemeissä, mikä voi mahdollisesti kasvattaa N2O-päästöjä ja maaperän hiilen hävikkiä. Uusien vaihtoehtojen löytäminen ilmastonmuutoksen hillitsemiseksi onkin entistä tärkeämpää

Comparison of static chambers to measure CH4 emissions from soils

The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 g CH4m−2 h−1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m3 in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (Rlin), whereas the chamber fluxes calculated by the exponential flux calculation method (Rexp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.Peer reviewe

Mitigating greenhouse gas fluxes from cultivated organic soils with raised water table

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Aggregate stability and aggregate-associated carbon in no-till and reduced tillage practice in Finland

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Fluxes of nitrous oxide in tilled and no-tilled boreal arable soils

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Impact of no-till and reduced tillage on aggregation and aggregate-associated carbon in Northern European agroecosystems

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Earthworm Lumbricus terrestris mediated redistribution of C and N into large macroaggregate-occluded soil fractions in fine-textured no-till soils

By processing large quantities of crop residues, earthworms enhance the mineralization of organic matter but have also been shown to stabilize soil organic carbon (SOC) into soil fractions like microaggregates (53-250 mu m) within macroaggregates (> 250 mu m) especially in no-till soils. Our objective was to find direct evidence on the impact of an anecic, soil surface-feeding earthworm, Lumbricus terrestris L., on the redistribution of SOC and soil nitrogen (N) into macroaggregate-occluded soil fractions of boreal soils. We sampled soil (0-5 cm depth) from the middens of L. terrestris (mounds of collected residue and surface casts at the openings of its permanent burrows) and the adjacent non-midden (bulk) soil at three no-till sites in southern Finland: two clayey sites (sites 1-2) and one coarse textured site (site 3). Compared to bulk soil, the soil in L. terrestris middens featured general increase in aggregate size and content of SOC and N within the large macroaggregates (> 2000 mu m) at the clayey sites. The microaggregates within the large macroaggregates had accumulated more SOC and N in the midden soil especially at site 1 where 99% of the difference in total SOC between midden and bulk soil was associated with this type of SOC stabilization. At site 2, the increase in SOC found in the large macroaggregates was counteracted by a decrease in SOC in microaggregates within the small macroaggregates (250-2000 mu m). No differences in SOC stored in soil fractions were found between midden and non-midden soil at the coarse soil site 3 with higher top soil decomposition rate compared to sites 1 and 2. Across the study sites, the total amount of SOC was 6% higher in midden soil compared to the bulk soil. These results suggest L. terrestris mediates the storage of SOC and N into better protected soil fractions in clay soils under boreal conditions.Peer reviewe

Tillage and crop residue management methods had minor effects on the stock and stabilization of topsoil carbon in a 30-year field experiment

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Thirty years of reduced tillage or straw management had minor effects on soil carbon sequestration

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