Resumo em Inglês:

ABSTRACT No-tillage (NT) has been largely adopted in Brazil as a strategy for soil conservation, but for the last decade, there have been governmental incentives for its adoption arising from its potential for soil C accumulation. Notwithstanding, the soil mulch formed from crop residues favors the maintenance of soil moisture and nutrients in the upper soil layers, which stimulates soil microbial activity and may increase the potential for nitrous oxide (N2O) emissions. In addition, double-cropping systems in the same year are typical in Brazil and the impact on the fraction of fertilizer N lost as N2O needs to be evaluated. This study aimed to assess the influence of soil tillage and N fertilization on N2O emissions in a wheat-soybean succession system as commonly practiced in southern Brazil. The experiment was carried out at Embrapa Soybean research station located in Southern Brazil. Treatments were conventional tillage (CT) and no-tillage (NT), with and without nitrogen fertilization for the wheat and no N fertilizer for the soybean. Closed-static chambers were used to monitor N2O fluxes for two consecutive years. Together with gas monitoring, soil samples were also taken and analyzed for mineral N, soil moisture and labile carbon. Soybean yields were higher under NT, which seemed to be the result of a higher soil water availability that helped to overcome extended periods without rainfall. Soil N2O emissions were similar between CT and NT, with just a tendency for higher emissions under NT. The highest emissions occurred from the soybean crop. In the second year under NT, the emissions from the soybean crop were higher when preceded by N-fertilized wheat, but the converse was true under CT. None of the soil variables consistently correlated with N2O emissions, with mineral-N as the best predictor in the second wheat cycle and soil moisture in the first soybean cycle. Calculated emission factors were not statistically different between CT and NT and consistently lower than the IPCC default of 1 %. The calculated N2O emission intensity by relating N2O emission to grain yield showed an environmental advantage of NT compared to CT by presenting a 44 % reduction in soybean and similar values for fertilized wheat.

Resumo em Inglês:

ABSTRACT Adopting land-uses that contribute with a considerable litter input can affect the accumulation, protection, and bioavailability of organic carbon in the edaphic environment, compromising the different compartments of soil organic matter (SOM) and the associated benefits. Moreover, changes in seasons can influence the dynamic of SOM. Notably, the mechanisms involved in SOM stabilization and storage, particularly in agroforestry production areas, are still poorly explored. This study aimed to verify if the contents of soil organic carbon (SOC) and the physical fractions of the SOM are modified as a function of agroforestry systems implemented in the short term, and verify if seasonality can affect the compartmentalization of SOM in agrifood systems. Also, we tested if the carbon management index (CMI) is sensitive to detecting management practices quality across the unmanaged pasture, different agroforestry systems, and a reference area (forest). We measured soil physical properties, SOC content in bulk soil, particular organic carbon (POC), and mineral-associated organic carbon (MAOC) fractions at three different depths (0.00-0.05, 0.05-0.10, and 0.10-0.20 m) in response to the adoption of agroforestry systems. Our results show that after a short period of implementation of agroforestry systems, significant changes were observed in SOC contents and the physical fractions of SOM in the most superficial layers (0.00-0.05 and 0.05-0.10 m), with emphasis on the particulate fraction of SOM. We verified that the seasonality affected the SOC, POC, and MAOC contents. We also found that the CMI index was more sensitive and efficient in detecting changes arising from seasonality and the management practices involved. According to this index, it was possible to verify that the agroforestry system with the highest density of species for biomass production (AS3) has been accumulating more carbon in the soil. Therefore, this study provides relevant information regarding soil carbon management in agroforestry systems.

Resumo em Inglês:

ABSTRACT Climate-smart agriculture (CSA) practices, mainly no-tillage (NT), cover cropping (CC), soil fertilization with organic amendments (OA), and crop-livestock (CL) and crop-livestock-forestry (CLF) systems, has been widely adopted in areas from Brazilian Cerrado. The CSA may partly offset former soil C losses and contribute to climate change mitigation. However, contradictory findings brought uncertainties about the effect of CSA on soil C. Here, by a systematic review of 87 papers and using 621 data pairs, we provided a pervasive biome-scale analysis of soil C stock changes associated with the adoption of CSA across Brazilian Cerrado. All CSA practices evaluated showed average positive rates of C stock change, indicating a general tendency of soil C accretion after its adoption. In areas under NT, CC and CLF, greater rates were estimated for the deeper soil profile evaluated (0.00-1.00 m) (1.24 ± 0.85, 0.54 ± 0.54 and 1.00 ± 1.47 Mg ha–1 yr–1, respectively), while OA and CL showed more soil C accretion when the assessment was limited down to 0.10 m depth (0.82 ± 0.60 and 0.59 ± 0.66 Mg ha–1 yr–1, respectively). Unfortunately, the lack of basic information precluded any attempt to statically compare our estimations. In this sense, we must be cautious in stating that soil C sequestration occurs at those rates after the adoption of CSA practices. Despite these limitations, the results clearly show that the diversification and intensification of agricultural areas in the Cerrado by the adoption of CSA is a promising pathway to increase soil C stocks, and consequently, contribute to climate change mitigation and adaptation. Finally, our findings emphasize the importance of efforts that stimulate farmers to adopt these practices on large scale, such as Brazil’s Low-Carbon Agriculture Plan, besides providing sound empirical evidence about the role of soil C sequestration in Brazil achieving its Nationally Determined Contributions commitments.

Resumo em Inglês:

ABSTRACT Soil quality can be understood as its capacity to provide several essential services within the ecosystem and has been used to understand the impact of different managements, providing information that proves the benefits and your maintenance of the agroecosystem. To understand the impact of different managements, this study aimed to compare the chemical, physical, and biological soil properties in pasture areas managed with different recovery times in an integrated crop-livestock system about perennial pastures and secondary forest. The following management systems were evaluated: Secondary Forest (SF), Perennial Pasture (PP), pasture recovered to five years through the intercropping corn + Brachiaria brizantha (Palisade grass) (ICL5), and pasture recovered to eight years through the intercropping corn + Brachiaria brizantha (Palisade grass) (ICL8). Different soil properties were evaluated, namely: Chemical: pH, H+Al, Al 3+ , P, K + , Ca 2+ , Mg 2+ , TOC, SB, CEC, V, and m; Physical: soil bulk density (Bd), total porosity (Tp), macroporosity (Ma), microporosity (Mi), soil resistance to penetration (Pr), and gravimetric soil water content (GWc); and biological: soil microbial biomass carbon (SMB-C), basal soil respiration (BSR), metabolic quotient (qCO 2 ), and microbial quotient (qMic). Perennial pasture and ICL8 areas were the ones that most contributed to the increase in nutrients (Ca 2+ , Mg 2+ , and K + ), TOC and sorption complex. The ICL8 area showed the best results in soil physical variables Ma, Tp, Pr, and GWc were the best results for the ICL8 area. Secondary forest and ICL8 areas presented the best results from SMB-C and qMic. Between periods of pasture recovery through the integration of crops and livestock, the longer the recovery time, the greater its beneficial effects on the different chemical, physical and biological soil properties, overcoming secondary forest and perennial pasture.

Resumo em Inglês:

ABSTRACT Paddy rice production based on traditional soil management emits large amounts of methane (CH4) into the atmosphere. This study assessed the potential of no-tillage (NT) and winter cover crops (WCC) to mitigate net greenhouse gas (GHG) emissions in a subtropical paddy rice ecosystem. A long-term (20-yrs) experiment was evaluated regarding the effect of NT combined with winter fallow or three WCC (ryegrass, white oat, and birdsfoot trefoil) on seasonal CH4 -C and nitrous oxide (N2O-N) emissions and on soil organic carbon (SOC) stocks in comparison to conventional tillage (CT) under winter fallow in a Gleysol of Southern Brazil. The changes in SOC were used as a proxy for annual net carbon dioxide (CO2) exchanges in the soil-atmosphere, taking the CT treatment as a reference. The GHG balance (summation of CH4 , N2O and CO2 emissions multiplied by their global warming potential of 34, 298, and 1, respectively) and emissions intensity of GHG emissions were calculated. Across winter managements, NT decreased 25 % of GHG emissions in comparison to CT system. This effect was mainly related to the decrease of seasonal CH4 -C emissions (31-113 kg ha-1) and by promoting SOC accumulation (0.45-0.65 Mg ha-1yr-1) in comparison to CT system, since soil N2 O-N emission was not affected by management practices. Increased soil CH4 -C emissions offset the positive effect of WCC on SOC accumulation compared with winter fallow. Based on our findings, NT mitigates net GHG emissions in subtropical paddy rice ecosystems, but no additional effect is observed combining NT with WCC.

Resumo em Inglês:

ABSTRACT Global warming potential (GWP) of rice paddies depends on straw management. This study evaluated methane (CH4) and nitrous oxide (N2O) emissions and soil C stocks to determine GWP and yield-scaled GWP under different strategies and intensities of rice straw management in a subtropical climate. We hypothesized that decreasing soil management intensity and straw incorporation in the soil would reduce GWP. Methane fluxes were substantially higher during the rice growing season than in the off-season, while the opposite was observed for N2O fluxes. The cumulative emissions of CH4 during the growing season among the straw management strategies evaluated ranged from 165.8 to 586.0 kg ha-1. Annual CH4 emissions were lower when soil and straw received some type of management compared to no-tillage. Daily N2O fluxes ranged from -2.8 to 201.7 g ha-1 day-1; cumulative N2O emissions during the off-season ranged from 455.2 to 2816.5 g ha-1. During the off-season, strategies to reduce N2O emissions include post-harvest straw incorporation using a disc harrow, winter straw removal, and ryegrass cropping. Soil organic C stocks ranged from 35.96 to 38.36 Mg ha-1. Straw management using a disc harrow reduced soil organic C stocks, with more adverse effects than straw removal. Soil and rice straw management did not affect rice grain yield, with an average of 10.4 Mg ha-1. Methane emissions were the main component of GWP in all straw management systems. The contribution of N2O emissions to GWP was small and mostly (>85 %) determined by off-season emissions. Yield-scaled GWP ranged from 0.64 to 1.06 Mg CO2eq Mg-1 yield and was lower when soil and straw management systems occurred shortly after the rice harvest. Our results indicate that soil and straw management immediately after rice harvest reduces CH4 emissions, GWP, and yield-scaled GWP.