Biochar, hydrochar and uncarbonized feedstock application to permanent grassland—Effects on greenhouse gas emissions and plant growth
Introduction
Biochar, an organic carbon soil amendment, has great potential to alleviate the CO2 accumulated in the atmosphere by sequestration of recalcitrant carbon into the soil (Lehmann, 2007, Glaser et al., 2009). Such a biological sequestration of CO2 would be cost-effective (Blair et al., 2006) and serve as a fast action strategy for climate change mitigation (Molina et al., 2009). Positive effects of biochar amendments on crop yields (Jeffery et al., 2011, Biederman and Harpole, 2013) would provide an additional incentive for its agricultural use. However, before using biochar as a carbon sink and environmental management tool, it must be proven that it remains stable after soil application and that such application does not create adverse effects, e.g. increased greenhouse gas emissions (GHG). Greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4), increase the radiative forcing of the Earthś atmosphere (Houghton et al., 1997) by contributing to ozone depletion (N2O) (Ravishankara et al., 2009) and by interaction with aerosols (CH4) (Shindell et al., 2009). Possible positive feedback effects of biochar or biochar–slurry mixtures on GHG emissions would be detrimental for the field use of biochar as a carbon sequestration tool. To date, the effects of biochar on GHG emissions are rather diverse. They depend on the biochar production process parameters, the feedstock used, the ecosystem and soil properties to which biochar is applied, and the strategy of application and (agricultural) management.
Biochar could be beneficial as a soil conditioner in degraded or naturally poor soils by improving nutrient availability and mycorrhiza abundance (Chan et al., 2008, Alvum-Toll et al., 2011). Although it might not be needed as a soil conditioner in fertile temperate soils, an increment of the grassland carbon stocks by carbon amendment may act as carbon sink due to long C turnover times (Scurlock and Hall, 1998). Biochar use in grasslands may even be based on ancient soil types in temperate climates, i.e. chernozems, of which some are assumed to have developed under grassland (steppe) vegetation (Eckmeier et al., 2007). For anthropogenically used grasslands, which are typically used for livestock breeding with considerable amounts of manure and urine accumulation, positive biochar–slurry interactions may offer new ways for reducing GHG emissions (Winsley, 2007). Indeed, Bruun et al. (2011) showed in an incubation study that the addition of 3% fast pyrolysis biochar on a mass basis reduced CO2 and N2O emissions from a slurry amended soil significantly. Biochar and slurry can also reduce the wind erosion of biochar during application and alleviate the odor of the slurry (Blackwell et al., 2009). However, the promising idea of charging biochar with the nutrients contained in the slurry still has to be proven effective. Experiments with biochar and slurry showed that biochar can bind ammonia by surface interactions (Spokas et al., 2012). Furthermore, biochar reduced NO3 and total N leaching from manure-amended soil significantly (Laird et al., 2010, Ventura et al., 2013), with subsequent positive effects on plant-available nitrogen and thus plant growth. Concerning the N-efficiency of ecosystems, ammonia and denitrificatory N losses (including N2O emissions) are very important factors, as well as losses of NO3N by leaching, the main pathways for losses of N from an ecosystem. NH3 losses from grasslands can account for up to 28% (grazed pasture) or 27% (grassland fertilized with pig slurry) of the annual N input (Ball and Keeney, 1981, Pain et al., 1989), depending on farm management practices. N2O emissions can add up to 2–2.2% total N loss of added fertilizer of a grassland ecosystem (Velthof et al., 1996, Clayton et al., 1997).
However, results of biochar effects on ecosystems in temperate climates are still scarce, and the interactions of different biochar–slurry mixtures in the field still have to be elucidated. Consequently, the background of this study was to assess possible risks and chances of carbon amendment co-applied with slurry to a temperate grassland site with a focus on GHG- and ammonia emissions. We hypothesized that, first, the materials would degrade in the sequence of their carbonization grade: feedstock > hydrochar > biochar and that degradation would be measurable in the ecosystem respiration. To assess possible priming effects of biochar on hydrochar or vice versa, we introduced a mixed treatment in the incubation study. Second, we hypothesized that biomass growth will be negatively impacted by hydrochar application, as reported by others who found negative effects on plant germination and growth with hydrochar use in soils (Bargmann et al., 2012, Gajić and Koch, 2012). Third, that biochar will reduce N2O and CO2 emissions (Augustenborg et al., 2012, Case et al., 2012, Dempster et al., 2012), improve CH4 oxidation (Karhu et al., 2011, Liu et al., 2011), and that hydrochar will have rather adverse effects on the GHG balance (Kammann et al., 2012), as shown by incubation studies so far.
Section snippets
Laboratory incubation
A laboratory study was carried out with the same parameters as the field experiment but under controlled conditions. Soil for incubation was taken from the top 15 cm of the experimental field site prior to initiation of the field experiment. The grassland site in Linden, near Giessen, Germany (50̊32′N und 8̊41.3′E) has been managed extensively for decades as grassland with two cuts per year (Jäger et al., 2003). The soil, a haplic stagnosol (WRB, 2006), has a soil texture of 25% sand, 28% clay,
Statistics
Statistics were carried out using Microsoft Excel 2010, SigmaPlot 11.0 and 12.0 and IBM SPSS Statistics Versions 19 and 20. The differences in the cumulated gas fluxes before and after slurry addition, as well as differences in Nmin concentrations among the treatments, were analyzed by one way Anova. Potential differences of cumulated (linearly interpolated) vs measured carbon contents from the incubation were determined using a paired t-test and differences in pH and WHC were determined using
Water holding capacity
Generally, the addition of C-substrates led to an improvement of the soilś WHC. The addition of hydrochar alone (+14%) and mixture with biochar (+25%) led to significant increases of the WHC (t-test, p < 0.05).
CO2 emissions and carbon balance
Carbon dioxide (CO2) effluxes during the three months incubation period were in the order: feedstock > hydrochar > hydrochar–biochar mix > control ≥ biochar, p = 0.001) (Fig. 1A and B). In the period prior to slurry application, CO2 emissions were significantly reduced in the biochar treatment (81%
Soil and ecosystem respiration and C stability
We measured soil respiration in the incubation (experiment without plants) and ecosystem respiration (Reco) in the field (experiment with plant cover). In the field, plant growth in the GHG measurement plots was not significantly different among treatments. Thus, we argue that significant effects observed in the field were predominantly related to differences in soil respiration.
In the incubation study, soil CO2 efflux was reduced in the biochar treatment before slurry application, whereas
Conclusions
Our hypothesis of a degradation rate in the order biochar < hydrochar < feedstock was confirmed by the results of the incubation study and priming effects of biochar on hydrochar degradation or vice versa were not observed. In the field, increased CO2 emissions from the degradation of the carbon amendments have not been observed; biochar even lowered the emissions significantly compared to the control plots.
The hypothesis that hydrochar would reduce plant growth was confirmed by results from the
Acknowledgments
We thank the agricultural centre Eichhof, Bad Hersfeld, Germany, for providing us with Miscanthus feedstock from a long term field experiment. Hydrothermal carbonization was kindly carried out by Julian and Anke Schwark. The production of the biochar material was carried out by Helmut Gerber, Pyreg GmbH, Bingen. Slurry was kindly provided by the Upper Hardthof Farm (Giessen University, institute for animal breeding and domestic animal genetics). We acknowledge the support of R. Pfanschilling
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