the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Oct 2024
Managing Soil Nitrogen Surplus: The Role of Winter Cover Crops in N2O Emissions and Carbon Sequestration
Abstract. Cover crops are acclaimed for enhancing the environmental sustainability of agricultural practices by aiding in carbon (C) sequestration and reducing losses of soil mineral nitrogen (SMN) after harvest. Yet, their influence on nitrous oxide (N2O) emissions—a potent greenhouse gas—presents a complex challenge, with findings varying across different studies. This research aimed to elucidate the effects of various winter cover crops—winter rye (frost-tolerant), saia oat (frost-sensitive grass), and spring vetch (frost-sensitive legume)—against a control of bare fallow on SMN dynamics, N2O emissions and C sequestration.
While cover crops efficiently lowered SMN levels during their growth, they also increased N2O emissions in comparison to bare fallow conditions. Notably, winter frost events triggered significant emissions from the frost-sensitive varieties. Moreover, the practices of residue incorporation and soil cultivation were associated with increased N2O emissions across all cover crop treatments. Winter rye, distinguished by its high biomass production and nitrogen (N) uptake, was linked to the highest cumulative N2O emissions, highlighting the impact of biomass management and cultivation techniques on N cycling and N2O emissions.
Cover crop treatment lead to a slight increase in direct N2O emissions (4.5±3.0, 2.7±1.4, and 3.1±3.8 kg N2O-N ha-1 for rye, oat, and vetch, respectively) compared to the fallow (2.6±1.7 kg N2O-N ha-1) over the entire trial period (16 months). However, the potential of non-legume cover crops to reduce indirect N2O emissions compared to fallow (0.3±0.4 and 0.2±0.1 kg N2O-N ha-1 a-1 for rye and oat respectively) and their contribution to carbon sequestration (120–150 kg C ha-1 a-1 over a period of 50 years when growing cover crops every fourth year) might partially counterbalance these emissions. Thus, while cover crops offer environmental benefits, their net impact on N2O emissions necessitates further exploration into optimized cover crop selection and management strategies tailored to specific site conditions to fully leverage their ecological advantages.
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Status: open (until 04 Jan 2025)
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RC1: 'Comment on egusphere-2024-2849', Chantal Hendriks, 05 Dec 2024
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With pleasure I read the manuscript entitled ‘Managing Soil Nitrogen Surplus: The Role of Winter Cover Crops in N2O Emissions and Carbon Sequestration’. There are still quite some knowledge gaps to fill regarding this topic and the authors contributed to bridging some of these gaps. In general, I think the manuscript is publishable after some minor revisions. Especially the carbon modelling needs more explanation. More specific comments are listed in attached document.
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RC2: 'Comment on egusphere-2024-2849', Anonymous Referee #2, 20 Dec 2024
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The present manuscript undertakes the estimation of the impact of multiple cover crop species on direct and indirect N2O emissions, and C sequestration. The paper addresses these aspects in the context of a crop rotation that goes beyond the cover cropping phase (including sugar beet and winter wheat), a relevant aspect when studying the impact of this practice. It also considers multiple variables for a more comprehensive assessment of environmental and productivity outcomes of cover cropping. I consider this paper is suitable for publication with minor revisions, which I share below. Congratulations on this great work!
Title: would it be possible to more clearly reflect the geographical and methodological scope of the work conducted in the title of the paper? The current title may lead readers to think this paper is a comprehensive review on the effect of cover crops on N2O emissions and SOC sequestration across multiple locations, growing conditions, practices, etc.
Line 19: consider adding a few sentences in the abstract that describe how the experiment was set up.
Lines 75-77: could this sentence be included in another paragraph in order to avoid having a paragraph with a single sentence?
Line 81: what do you mean by “soil organic model”? Should that be “soil organic C model”?
Lines 103-104: would it be possible to add the geographic coordinates of the sites?
Line 131: Is the total of 180 kg N/ha the addition of soil N plus fertilizer N? Would it be possible to clarify that? Were all site*year combinations sampled the same way in the spring for SMN? Also, could the German fertilizer ordinance be cited?
Line 146: Why were the first 3 cm of soil not considered in the measurements of bulk density? Also, why were the soil layers considered for bulk density measurements not continuous? There seems to be a gap between the 1st and 2nd layer, as well as between the 2nd and 4th layer.
Line 149: should g/cm3 be g cm-3? Line 152 seems to have similar opportunities to refine the notation used.
Section 2.2: were soil samples for SMN and WFPS taken every time that N2O was measured?
Section 2.3: how many chambers per plot were there in the study? One? Consider clarifying. Also, were gas samples taken at specific time intervals? Consider clarifying. Also, consider indicating how many times N2O was sampled across the site and years. Consider including similar information on the number of samples taken for other variables, like SMN, WFPS, etc.
Line 177/178: should “...normal distribution of data…” be “normal distribution of model residuals…” instead?
Line 191: extra comma after 2015.
Line 197-198: is the application of digestate a common practice in the region, or why was it included?
Line 197: what does “cover crop/bare fallow” in CR1 and CR2 mean exactly? Could that be clarified?
Line 201: consider clarifying that by “treatment” you mean the cover crop treatments tested in the field experiments. In the same line, consider replacing “have been quantified” for “were quantified”.
Line 203: is this first order kinetic modeling approach the one used by RothC and C-Tool? Please consider clarifying, or better integrating the paragraphs to more clearly explain how the modeling was conducted.
Line 212: should “variant-specific” be “variety-specific”?
Lines 225-227: does this mean that four years of weather data (2018-2021) were repeated for 50 years to conduct the simulation? If that was the case, how representative the weather during those 4 years is from general climate patterns in the areas of the study? I think a larger time period should probably be considered for a 50-year simulation. I think this point should definitely be reevaluated. Furthermore, should future climate scenarios consider the impacts of climate change (change in temperature and rainfall patterns, for example)?
Lines 234 and 235: seems like soil temperature was measured, but I do not think that is listed in the materials and methods section. Could that be added?
Line 250: how were the crop phases defined? Do they go from planting until harvest? Consider clarifying somewhere in the methods.
Lines 261, 262: I imagine N uptake was estimated based on DM yield and N content, correct? If that is the case, should 100% of the N content estimated for vetch be considered N uptake? Or is part of that N coming from N fixation? Please revise and clarify.
General methods: was sugar beet and winter wheat yield measured? In case yes, was there any impact of the treatments? If yes/no, consider including a few sentences in the results and discussion sections related to these points.
Lines 262, 263: you indicate “A strong correlation was observed between cover crop DM formation and both C content and N uptake (r>0.9).” Is this because of the way in which N uptake is calculated?
Lines 291, 292: you indicate: “At the end of the cover crop phase in April, SMN levels were lowest in rye, with some variation in other treatments across site-years (Figure 1, Table S2).”; are the values in figure 2 averages for the entire phase, or values associated with sampling at the end of each phase? The figure caption suggests it is the average across the entire phase. Please clarify.
Section 3.5: I really liked this section! In my opinion, it is super useful for the reader to have this recap!
Figure 2 and 3: would it help improve the clarity and comparability across panels to use the same Y axis scale for all panels, or at least for panels a, b and c?
Table 3: why was each cover crop specie included as a separate fixed effect in the linear mixed-effect model? Should cover crop be included as a categorical fixed effect, with three levels (one for each of the three species)? What is the impact in your model of including them separately? Also, seems like different models were fitted for each crop phase, correct? Consider clarifying this in the methodology.
Table 3: conditional R2 describes model fit considering both fixed and random effects. Please consider calculating marginal R2 that consider only model fixed effects.
Table 3: how was the number of observations for each phase calculated? Also, were soil samples for SMN taken every time that N2O was measured? It is not clear from the methodology, and this modeling approach assumes there is SMN, WFPS and soil temperature values associated with all N2O measurements. If not, how was this approached?
Lines 389-390: no need to re-state how the calculation was conducted.
Lines 435 to 444: authors discuss the impacts of cover crops on soil properties that contribute to larger soil water holding capacity. However, this is likely not the explanation for the results observed in the present study (no differences between treatments), as cover crops were implemented only for a single cropping season in each of the sites. Consider narrowing your discussion and emphasizing aspects that may further explain what was observed in your study.
Lines 481-482: this other paper reports similar findings: https://www.sciencedirect.com/science/article/abs/pii/S0167880921004540
Lines 509-511: consider adding a citation to this sentence.
Lines 530-535: what about the impact of freeze-thaw periods on N2O emissions? You mention that soil temperature dynamics affect N2O emissions (line 535), but this is not discussed in the specific context of your study. For example, could plots of frost-sensitive cover crops exhibit more freeze-thaw cycles due to less biomass insulation of the soil, and therefore more N2O emissions? This point is discussed to some degree later in the section. Consider expanding the discussion, and potentially merging paragraphs addressing this topic so overall flow is improved.
Line 536-541: these sentences do not seem to fit very well together, as authors discussed fertilizer application, organic amendment application and tillage practices in a few sentences; consider rephrasing.
Line 592: Authors mention “frost-induced N2O emissions”; how were those emissions defined or calculated? Did you consider variations in soil temperature above and below zero during the time of the year in which freeze-thaw emissions are likely to occur?
Lines 627-633: great to see these lines discussing the limitations of the potential impact of cover crops on mitigating indirect N2O emissions. I also wonder how this mitigation potential would look like when considering an LCA approach for the entire crop rotation you studied. If the N in the cover crop biomass mineralizes again during the sugar beet and/or winter wheat phase, could we still affirm these indirect emissions were mitigated? Consider discussing this point.
Related to the previous point, I wonder if it would be possible to incorporate what the net GHG emissions in CO2-eq for each treatment would look like, when considering N2O emissions, C sequestration, and potentially avoided indirect N2O emissions (although not sure if I would bring in this last piece, per item mentioned above). Consider incorporating this into your results and discussion.
Citation: https://doi.org/10.5194/egusphere-2024-2849-RC2
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Managing Soil Nitrogen Surplus: The Role of Winter Cover Crops in N2O Emissions and Carbon Sequestration Victoria Nasser https://doi.org/10.25625/HFEDA7
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