Josiléia Acordi Zanatta

No-till reduces global warming potential in a subtropical Ferralsol

AimsFor tropical and subtropical soils, information is scarce regarding the global warming potential (GWP) of no-till (NT) agriculture systems. Soil organic carbon (OC) sequestration is promoted by NT agriculture, but this may be offset by increased nitrous oxide (N2O) emissions. We assessed the GWP of a NT as compared to conventional tillage (CT) in a subtropical Brazilian Ferralsol.MethodsFrom September 2008 to September 2009 we used static chambers and chromatographic analyses to assess N2O and methane (CH4) soil fluxes in an area previously used for 3–4xa0years as a field-experiment. The winter cover crop was ryegrass (Lolium multiflorum Lam.) while in summer it was silage maize (Zea mays L.).ResultsThe accumulated N2O emission for NT was about half that of CT (1.26 vs 2.42xa0kgxa0N ha−1xa0year−1, Pu2009=u20090.06). Emission peaks for N2O occurred for a month after CT, presumably induced by mineralization of residual nitrogen. In both systems, the highest N2O flux occurred after sidedressing maize with inorganic nitrogen, although the flux was lower in NT than CT (132 vs 367xa0μgxa0N m−2xa0h−1, Pu2009=u20090.05), possibly because some of the sidedressed nitrogen was immobilized by ryegrass residues on the surface of the NT soil. Neither water-filled pore space (WFPS) nor inorganic nitrogen (NH4+ and NO3−) correlated with N2O fluxes, although at some specific periods relationships were observed with inorganic nitrogen. Soils subjected to CT or NT both acted as CH4 sinks during most of the experiment, although a CH4 peak in May (autumn) led to overall CH4 emissions of 1.15xa0kg CH4-Cxa0ha−1xa0year−1 for CT and 1.08xa0kg CH4-Cxa0ha−1xa0year−1 for NT (Pu2009=u20090.90). The OC stock in the 0–20xa0cm soil layer was slightly higher for NT than for CT (67.20 vs 66.49xa0Mgxa0ha−1, Pu2009=u20090.36). In the 0–100xa0cm layer, the OC stock was significantly higher for NT as compared to CT (234.61 vs 231.95xa0Mgxa0ha−1, Pu2009=u20090.01), indicating that NT resulted in the sequestration of OC at a rate of 0.76xa0Mgxa0ha−1xa0year−1. The CO2 equivalent cost of agronomic practices was similar for CT (1.72xa0Mg CO2eq ha−1xa0year−1) and NT (1.62xa0Mg CO2eq ha−1xa0year−1). However, NT reduced the GWP relative to CT (−0.55 vs 2.90xa0Mg CO2eq ha−1xa0year−1), with the difference of −3.45xa0Mg CO2eq ha−1xa0year−1 (negative value implies mitigation) being driven mainly by OC sequestration. The greenhouse gas intensity (GHGI, equivalent to GWP/silage yield) was lower for NT than CT (−31.7 vs 171.1xa0kg CO2eq Mg−1 for silage maize).ConclusionAs compared to CT, greenhouse gas emissions from a subtropical soil can be mitigated by NT by lowering N2O emissions and, principally, sequestration of CO2-C.

Nitrous oxide and methane fluxes in south Brazilian gleysol as affected by nitrogen fertilizers

Fertilizantes nitrogenados incrementam os fluxos de oxido nitroso (N2O) e podem deprimir a oxidacao de metano (CH4) em solos agricolas. Entretanto, nao existem resultados da magnitude desses efeitos nas condicoes edafoclimaticas do Sul do Brasil, tampouco do potencial de algumas fontes de N em mitigar esses efeitos. O presente estudo objetivou avaliar o impacto da aplicacao de fertilizantes nitrogenados (ureia, sulfato de amonio, nitrato de calcio, nitrato de amonio, Uran, N de liberacao lenta e ureia com inibidor de urease) nos fluxos de N2O e CH4 em um Gleissolo no Sul do Brasil (Porto Alegre, RS), em comparacao a um tratamento controle sem aplicacao de N. O experimento seguiu um delineamento de blocos ao acaso, com tres repeticoes, e os fertilizantes foram aplicados, em cobertura, numa dose unica de 150xa0kgxa0ha-1 N, no estadio V5 da cultura do milho. A avaliacao dos gases foi feita utilizando-se o metodo da câmara estatica, nos 15xa0dias que sucederam a aplicacao de N, e a analise das concentracoes de N2O e CH4 nas amostras de ar foi realizada por meio de cromatografia gasosa. O pico de emissao de N2O ocorreu no terceiro dia apos a aplicacao dos fertilizantes nitrogenados e a sua intensidade variou de 187,8 a 8.587,4xa0µgxa0m-2xa0h-1 N, destacando-se as fontes nitricas com as maiores emissoes, as fontes amoniacais e amidicas com emissoes intermediarias e os fertilizantes de liberacao lenta e com inibidor de urease com as menores emissoes. As emissoes no terceiro dia tiveram relacao direta com os teores de N-NO3- do solo (R2 = 0,56, pxa0<xa00,08) e ocorreram quando este apresentava valores de porosidade preenchida por agua (PPA) maiores que 70xa0%, o que indica que a desnitrificacao foi o processo predominante na producao de N2O. Os fluxos de CH4 do solo variaram de -30,1xa0µgxa0m-2xa0h-1 C (absorcao) a +32,5xa0µgxa0m-2xa0h-1 C (emissao), e a emissao acumulada desse gas teve relacao direta com os teores de NH4+ no solo (R2 = 0,82, pxa0<xa00,001), possivelmente pela competicao enzimatica entre os processos de nitrificacao e de metanotrofia. Apesar de os fluxos de ambos os gases terem sido alterados pela aplicacao dos fertilizantes nitrogenados, na media dos tratamentos, o impacto das emissoes de CH4 (0,2xa0kgxa0ha-1C-CO2xa0 equivalente) foi centenas de vezes menor que o verificado para as emissoes de N2O (132,8xa0kgxa0ha-1xa0C-CO2 xa0equivalente). Considerando as emissoes desses gases no solo fertilizado e o custo medio de 1,3xa0kgxa0C-CO2xa0kg-1 N referente a producao, transporte e aplicacao do fertilizante, o impacto ambiental dos fertilizantes nitrogenados variou de 220,4 a 664,5xa0kgxa0ha-1 C-CO2, o qual pode ser apenas parcialmente contrabalanceado pelo acumulo de C na materia orgânica do solo, pois nenhum estudo realizado no Sul do Brasil evidenciou taxa anual de acumulo de C no solo, decorrente da adubacao nitrogenada, maior que 160xa0kgxa0ha-1 C. A reducao das emissoes de N2O do solo e, portanto, do impacto ambiental pode ser obtida pelo uso de fontes amoniacais e amidicas em detrimento de fontes nitricas. Os fertilizantes de liberacao lenta e com inibidores de urease tambem sao alternativas potenciais visando a mitigacao das emissoes de N2O para atmosfera, e esforcos deverao ser empreendidos numa avaliacao sistematica desse potencial em agroecossistemas brasileiros.

Mitigation of methane and nitrous oxide emissions from flood-irrigated rice by no incorporation of winter crop residues into the soil

Winter cover crops are sources of C and N in flooded rice production systems, but very little is known about the effect of crop residue management and quality on soil methane (CH4) and nitrous oxide (N2O) emissions. This study was conducted in pots in a greenhouse to evaluate the influence of crop residue management (incorporated into the soil or left on the soil surface) and the type of cover-crop residues (ryegrass and serradella) on CH4 and N2O emissions from a flooded Albaqualf soil cultivated with rice (Oryza sativa L.). The closed chamber technique was used for air sampling and the CH4 and N2O concentrations were analyzed by gas chromatography. Soil solution was sampled at two soil depths (2 and 20 cm), simultaneously to air sampling, and the contents of dissolved organic C (DOC), NO3-, NH4+, Mn2+, and Fe2+ were analyzed. Methane and N2O emissions from the soil where crop residues had been left on the surface were lower than from soil with incorporated residues. The type of crop residue had no effect on the CH4 emissions, while higher N2O emissions were observed from serradella (leguminous) than from ryegrass, but only when the residues were left on the soil surface. The more intense soil reduction verified in the deeper soil layer (20 cm), as evidenced by higher contents of reduced metal species (Mn2+ and Fe2+), and the close relationship between CH4 emission and the DOC contents in the deeper layer indicated that the sub-surface layer was the main CH4 source of the flooded soil with incorporated crop residues. The adoption of management strategies in which crop residues are left on the soil surface is crucial to minimize soil CH4 and N2O emissions from irrigated rice fields. In these production systems, CH4 accounts for more than 90 % of the partial global warming potential (CH4+N2O) and, thus, should be the main focus of research.

Reforestation with loblolly pine can restore the initial soil carbon stock relative to a subtropical natural forest after 30 years

We hypothesized that long-term loblolly pine (Pinus taeda L.) land-use restores SOC stock and lability of a subtropical Cambisol to the original levels of the natural forest (NF). Additionally, we hypothesized that roots are the major contributor to SOC and that soil stores most of the ecosystem total carbon (ETC). We investigated a chronosequence of loblolly pine land-use of 17 (first rotation) and 32xa0years (second rotation, unthinned or thinned) following clearing of the NF. The original SOC stock to 100xa0cm of NF (209u2009±u20099.4xa0Mgxa0Cxa0ha−1) was depleted by 22% after 17xa0years of pine, possibly because of intense soil disturbance and low quantity and quality of the residue inputted during the pine stand implementation. However, the SOC stock was restored to the original stock of NF after 32xa0years of pine, with the input of above and belowground biomass at harvest of the first rotation possibly playing a role in this recovery. Thinning did not affect SOC stocks after 1xa0year. The POM-C reduced after 17xa0years and was not recovered after 32xa0years. We could not ascertain in 5-year evaluation whether root or litter was the major contributor to SOC. Soil held 72% of the ETC in NF and 48–59% in pine plantations, confirming that it stores most of the ETC. Overall, long-term loblolly pine land-use seems to restore the original soil carbon stock in this subtropical site, regardless of some lability losses.

Methane fluxes from waterlogged and drained Histosols of highland areas

Soil can be either source or sink of methane (CH4), depending on the balance between methanogenesis and methanotrophy, which are determined by pedological, climatic and management factors. The objective of this study was to assess the impact of drainage of a highland Haplic Histosol on CH4 fluxes. Field research was carried out in Ponta Grossa (Parana, Brazil) based on the measurement of CH4 fluxes by the static chamber method in natural and drained Histosol, over one year (17 sampling events). The natural Histosol showed net CH4 eflux, with rates varying from 238 µg m-2 h-1 CH4, in cool/cold periods, to 2,850 µg m-2 h-1 CH4, in warm/hot periods, resulting a cumulative emission of 116 kg ha-1 yr-1 CH4. In the opposite, the drained Histosol showed net influx of CH4 (-39 to -146 µg m-2 h-1), which resulted in a net consumption of 9 kg ha-1 yr-1 CH4. The main driving factors of CH4 consumption in the drained soil were the lowering of the water-table (on average -57 cm, vs -7 cm in natural soil) and the lower water content in the 0-10 cm layer (average of 5.5 kg kg-1, vs 9.9 kg kg-1 in natural soil). Although waterlogged Histosols of highland areas are regarded as CH4 sources, they fulfill fundamental functions in the ecosystem, such as the accumulation of organic carbon (581 Mg ha-1 C to a depth of 1 m) and water (8.6 million L ha-1 = 860 mm to a depth of 1 m). For this reason, these soils must not be drained as an alternative to mitigate CH4 emission, but effectively preserved.