the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Oct 2023
Nitrifier denitrification potentially dominates N2O production in a sandy soil – results from different fertilization and irigation regimes in potato cropping in Germany
Laura Charlotte Storch
Katharina Schulz
Jana Marie Kraft
Annette Prochnow
Liliane Ruess
Benjamin Trost
Susanne Theuerl
Abstract. Spatial and temporal distribution of water and nitrogen supply affects soil-borne nitrous oxide (N2O) emissions. In this study, the effects of different irrigation technologies (no irrigation, sprinkler irrigation and drip irrigation) and nitrogen (N) application types (broadcasted and dissolved in irrigation water) on N2O emissions and the potentially underlying, genetically determined microbial processes were investigated over an entire season in potato cropping. N2O fluxes were highest during the first half of the season and mostly affected by the applied water volume rather than the N application types. The comparison of the different water application types revealed that nitrifier denitrification might potentially be the dominant source of N2O emissions, especially under sprinkler irrigation. The type of N fertilizer supply, broadcasted application or dissolved in irrigation water, showed only minor differences in the potential microbial community functionality. N2O production in both treatments was most likely also dominated by nitrifier denitrification, while the process of denitrification might be feasible too. Even though the current agronomic management measures generally meet the crop demand of water and N, it might be recommendable to adapt the time of application considering that potatoes mainly require N at later growth stages which could also reduce N2O emissions at the same time.
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Laura Charlotte Storch et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2277', Anonymous Referee #1, 13 Nov 2023
The authors investigate the influence of fertilization and irrigation on N2O emissions and on the abundance of functional genes. They present a comprehensive measurement program, although I would have chosen a different approach for studies on the question posed in the title. I therefore recommend changing the title and focusing on what is actually shown. The cited preliminary study Storch et al. 2023 could certainly serve as a model.
I think it is necessary for the authors to critically check all their statements again to see whether the suggestion of causality can really be maintained or whether only the correlation should be described. The assumptions made should also be discussed more in the context of existing experiments on the underlying processes. The previous isotope work in potato crops assumes denitrification, which, however, is particularly strong in the furrows and even stronger in the lanes, both of which have not been investigated if I understand correctly.
I would like to illustrate this with a few points below.
L16: In my opinion, the spatial distribution of soil moisture is not adequately described, especially the differences between ridges and furrows in potato crops is mentioned, but is not the subject of the study, at least not the results.
L18: I would rather speak here of genes potentially involved than of processes
L78ff: Please note the size of the plots.
L81: The amount of precipitation measured during the study period is at least as important as the annual precipitation.
L96: A continuous color scheme would be helpful, e.g. light and dark brown for ZI-ZN and ZI-N light and dark chalk blue for SI-ZN and SI-N light and dark green for DI-ZN and DI-N , a dark color (because irrigated) for F (e.g. purple or red) and black like black fallow for F-ZC. this can then also be used for the table.
L106 14.4+1.1 is unequal 15
L115ff: I haven't checked all the citations but here are two that are missing from the references Flessa et al 1998 and Linn&Doran 1984
L117: When was the N2O emission measured before or after the irrigation and if after how much time has passed in between?
Soil moisture results are not shown anywhere, not even in the supplement
L119: Are the samples only taken on the ridge?
L140: What is meant by development?
L152: I miss yield data. At least for the assessment of N2O emissions (different terms are used here: production emissions, fluxes, I would standardise this, I think it always means the same thing), it makes a big difference whether irrigation produces more yield.
167 Figure1: In any case, the uncertainties (e.g. SD) are missing here. From my experience of the spatial variability of N2O emissions, all these trends could also come from one variant. It would also be nice to mark the times of irrigation and the variants should be easier to recognise with colours.
L223 Figure 2: Perhaps the authors can make it easier for readers by marking the clusters they see (as is often done in principal component analyses), but for me it is quite mixed.
L237: Under the right conditions, a large proportion of N2O production can also be chemical. It should at least be discussed here why the authors think that it is not in their case.
L243: Higher soil moisture does not necessarily lead to anaerobic conditions. This is a complex interplay between O2 consumption and transport. Especially in well-structured soils, irrigation can have a only small and very short-term effect.
L279 Figure 3: The graphic is difficult to read due to the very small numbers. Even if the representation in circles is nice, I would prefer small tables. The purpose of the picture in each subgraph at the top right is not clear to me.
L382: Are the differences significant enough to be mentioned in the conclusion? Especially when you consider that the measurements were only taken on the ridges.
L384ff: In this generality it does not help for future work, so it should definitely be sharpened up.
L391: I assume that JK is supposed to be JMK
Citation: https://doi.org/10.5194/egusphere-2023-2277-RC1 -
RC2: 'Comment on egusphere-2023-2277', Anonymous Referee #2, 25 Nov 2023
This study assesses the effects of different irrigation techniques (no irrigation, sprinkler irrigation and drip irrigation) and nitrogen (N) application types (broadcasted and dissolved in irrigation water) on N2O emissions and the potentially underlying, genetically determined microbial processes in a potato crop field. The title is indicating that nitrifier denitrification is identified as an important process for N2O emission. This is surely an interesting topic, and language quality of the manuscript is fine.
Two hypotheses are tested: First, the authors hypothesize that N2O flux rates differ between irrigation treatments due to differences in soil water content affecting denitrification, and second, that the addition of several small N doses due to fertigation will lead to lower N2O emission compared to broadcast N application due to better N use efficiency of the potato crops.
N2O emissions were measured on a weekly basis only. Due to the notorious temporal variability of N2O emissions with e.g., sharp short-lived peaks after irrigation or fertilization, this sampling design does not allow to accurately quantify cumulative emissions for the single treatments, and even less to differentiate N2O emissions between treatments such as e.g., fertigation and broadcast fertilization.
N2O measurements are combined with analysis of a set of nitrifier and denitrifier gene abundance. Generally it can be very valuable to link molecular analysis of functional N cycle gene abundance with N2O flux measurements. But to address the objectives of this study (in particular for N2O source process attributions as indicated already in the title) there are better methods, e.g., based on isotopic analysis of N2O and 15N labelling of process substrates. Since links between gene abundance, related processes and microbial activity are not trivial, and might be e.g. more pronounced for nitrifier genes than for denitrifier genes, the authors interpretations on origin of N2O appear overly speculative in my view. Furthermore, information on how the authors tried to bridge the huge spatial scales between mg-scale soil DNA analysis and chamber measurements is missing, information on soil moisture is hardly provided, and NUE seems to be not reported. Hence, it overall appears that this study is not able to satisfactorily address its goals, because of the chosen measurement design and due to the chosen methods.
Some specific comments:
L 35 The authors write “Current agricultural systems are characterized by a low nitrogen use efficiency (NUE), resulting inter alia in the loss of large amounts of actually available N through nitrate leaching and/or the generation of N2O (Wang et al., 2020; Wang et al., 2021; Menegat et al., 2022).” Statement is too general in my view, not all current agricultural systems show low N use efficiency. And cause and effect can be vice versa, low N use efficiency due to high losses also possible.
L 53 the denitrification pathway (NO3− to N2O/N2). This seems oversimplified, would write at least stepwise reduction of nitrate or nitrite to NO, N2O, N2.
L 60 given that nirk/nirS is explained here, nitrite should be mentioned earlier (see also comment to L 53).
Fig. 1 N2O emissions: The weekly measurements are not suitable to quantify N2O emissions across the treatments. Furthermore, only median values without uncertainty are given, and irrigation / fertilization events or climate information are not shown. And the figure setup does not allow to easily see differences between the treatments.
chapter 3.1 The entire discussion on causes of N2O differences across treatments reads odd given the weekly temporal resolution of measurements, which prevented accurate quantification of N2O emissions for the treatments.
Chapter 3.2: The statements on source processes of N2O seem pretty speculative and need to be reduced to their simple correlative nature.
Fig. 3: I like the figure design generally, but numbers and arrows are too small.
Citation: https://doi.org/10.5194/egusphere-2023-2277-RC2
Laura Charlotte Storch et al.
Laura Charlotte Storch et al.
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