As far as I can judge, the authors adequately addressed nearly all of the reviewers‘ comments.
The manuscript also profited from moving detailed methods description to an appendix. However, I do not think that the approach to the appendix is optimal: Large parts are exact duplicates of the methods section contained in the manuscript, with more or less extensive additions (in some sub-chapters, no addition at all, in some just a few sentences, in some, extensive methods description). This makes it hard for the reader to find the information that is not already contained in the manuscript. I would recommend re-writing the appendix to only contain additional information.
Please find some detailed comments below. Line numbers refer to the submitted manuscript (L) or to the authors’ response (A):
Comments relating to methods/interpretation:
L208f. “They were fed extra protein (standard cake concentrate) to reduce weight loss during lactation”: Please be more specific about the type and amount of additional feed – “standard cake concentrate” is rather ambiguous. Moreover, should not this additional N input into the system be considered in the budgets?
L210f.: So were these measured life weights actually used for the calculations? In this case, it would not be necessary to use assumptions and estimates of life weight gain, or standard values for LSU (as LSU could be calculated exactly).
L293-295: Vegetation, including legume content, was only assessed in the initial two years? How was vegetation change accounted for (e.g. potential increase of legumes)? That the vegetation must have changed seems to be implied in A32f.: “The field has been ploughed and reseeded to improve the quality of the grassland, but only in March 2011.” If there was still a cover of 99% Lolium perenne, no resowing to improve the grassland would have been necessary.
L538ff. “Approximately 85% of total harvested N is used to feed livestock (Sutton 2011). A measure to reduce N pollution could therefore be the reduction of meat consumption or a larger fraction of meat produced from grassland only (Smith et al. 2013).”
According to the source (and to A244), this refers to 85% of the harvested *and imported* crop N *in the EU*. Both informations (EU and including import) should be added. Further, the original source (Sutton, M. A. et al. (eds) The European Nitrogen Assessment (Cambridge Univ. Press, 2011)) should be cited. From the reference given, it is not clear if “crop N” includes grassland biomass or only arable crops. If grassland biomass is included, it does not make sense to offer “a larger fraction of meat produced from grassland only” as a solution (which, moreover, does not seem to be included in Smith et al. (2013)). If grassland biomass is not included, the argument also does not work: In this study, animal only/predominantly were fed grassland biomass, and still there was considerable N pollution.
L547ff. “However, leaching from our study was high compared to the Swiss NitroEurope site, where a maximum loss of 0.35 g N m-2 y-1 was estimated from an ungrazed grass/clover sward, despite comparable annual rainfall and N inputs (Ammann et al., 2009). This difference can be explained by the different plant cover and management. It has been shown that clover introduction can reduce leaching (Owens et al., 1994), whereas grazing tends to increase leaching (Cuttle and Scholefield, 1995). Grazed grasslands tend to have higher N leaching rates than cut grasslands since highly concentrated N deposited in urine is inefficiently recovered by herbage and prone to leaching.”
This argument does not convince me at all. In my opinion, the two sites are simply not comparable at all, since the actual nitrogen input received by the plant-soil system is much larger in this study. According to [Amman et al., Agriculture, Ecosystems and Environment 133 (2009) 150–162], about 15 g m-2 a-1 were removed from the site in harvested biomass at the Swiss site, compared to an average of 3.3 g m-2 a-1 in this study – the N in the biomass eaten by the grazing animals was largely returned as immediately available urinary N, in addition to the 22 g m-2 a-1 of fertilizer applied. Any differences in leaching are far more likely to be caused by the ratio between fertilizer input and removal of N in plant/animal products, than by the effect of grazing per se, or the presence of legumes. In fact, even though the fertilizer application rates in this study follow Scottish recommendations, I would not consider them sustainable management.
Further, the study of Owens et al. (1994) cited above (and several others) consider legumes that are introduced to replacing fertilizer inputs. The sentence above could easily be understood as if introducing legumes would reduce leaching even when N fertilizer level is the same (as it is in the study by Amman et al., Agriculture, Ecosystems and Environment 133 (2009) 150–162].
Additionally, I think that [Amman et al., Agriculture, Ecosystems and Environment 133 (2009) 150–162] should be cited here, since the paper of Ammann et al. (2009) included in the references does not give contain any values of nitrate leaching.
L662 “Due to the C export from harvest, C sequestration tends to be lower in cut systems.”: Compared to grazed systems? Please provide references for this.
I am also not quite convinced by the explanation given: In grazed systems, biomass is consumed by the animal on the field, C is either respired (on the field) or immediately returned to the field in excrements. In cut systems, biomass is consumed by the animal off-site, C is respired off-site or is excreted. If the excrements are returned to the field, this should result in approximately the same balance as in cut systems. If excrements are not returned to the field, this is not specific to cutting versus grazing, but means that the field under consideration is “sponsoring” the carbon budget of another site with the organic fertilizer it produces. I am not saying that there are no system-inherent differences regarding C sequestration between cut and grazed systems, only that the reasons for such differences given here do not seem convincing to me.
Other comments:
Throughout manuscript: SI unit for year is “a”, not “yr” (unless Biogeosciences prefers "yr"?)
Throughout manuscript: Please check that there always is a space between a value and a unit for all units apart from “%” (but not before “%”).
Abstract: Several instances of “m2” instead of “m-2”.
L37: It is not clear to which emission the value “6.4 g m-2 yr-1” refers: NH3 emissions? Sum of N2, NOx and NH3 emissions?
L37: “efficiency of N use by animal products”: Is this averaged over the “only-grazed” years, or over all years?
L42: Without proper context, “harvest” can be misleading, since it could include “harvested by the grazing animal”.
L44: Comma after “fermentation”.
L52: Better write “Estimated enteric fermentation”, as the current formulation suggests actual measurements.
L63 “developed countries”: Should this not be “developing countries”?
L69-71 “After the conversion to intensive N management, the tight coupling of the N and C cycles becomes disrupted, leading to emissions of N2O and CH4 at rates which may outweigh the benefits of C sequestration.”: In their answers to the reviewers, the authors explain why this can be the case, but they should still provide references for this statement (i.e. studies where this was the case) directly in the text.
L107: “south of” with lower case “s”
L109: In my opinion, “5.4 ha” would be sufficiently precise.
L109 “has been under permanent grassland management for more than 20 years” (and equivalent phrase in Appendix): Better give an absolute date (“since at least 19xy”) or clarify “before the start of the experiment”, because this could also mean “20 years including the experimental duration”, or even “20 years up to now”.
L111 “with clover”: should be “white clover”
L118: Superfluous space and full stop.
L125f. “( Table 4 and 5 125 and in Fig. 1 a) and b).”: Please check formatting.
L134 “(covering an area of 5.4 ha)”: now duplicate
L146: Space after “bones)”
L210 “animals were weight”: should be “weighed”
L246: Having further headings under the level three header (2.8.1) should maybe be avoided.
L294 “quadrates”: should be “quadrats”
L391, L394. Please either give “average annual stocking densities” with the unit “LSU ha-1”, or “stocking rates” with the unit “LSU ha-1 a-1” (stocking density as an instantaneous measure without time unit, stocking rate with a time unit).
L398, 399, 401: For non-SI units like “application” or “cut”, an exponent should not be used.
L404 “3 2.”: should be “3.2”
L435 “export through grazing animals were” should be “export through grazing animals was”
L516f “lies within the range of N applied in other European studies with similar management”. I recommend leaving this part out, as it does not really convey much information: If the management was similar, this includes similar fertilization (as fertilization is part of management). Moreover, At least two of the cited studies consider grasslands along a gradient of management intensity, so that it does not really make sense to say that the management of this study was similar.
L520 “Pheonix et al.” should be “Phoenix et al.”
L521f. “As our experimental field was sown as a grass mixture (without clover) the legume fraction was less than 1%”: At 20 to 30 years after sowing, the seed mixture originally used normally does not explain much of the species composition in permanent grasslands, as these are typically colonized by ubiquitous grassland species that are adapted to the site and the management. I would assume that low pH and the very high N fertilization level are the main reason for the absence of clover, not the seed mixture. As the reason for the low legume cover is not pertinent to the statement made here, I recommend leaving out this part of the sentence.
L523 “(NEU)” should be “(NUE)”
L660 “the inclusion [...] lead to”: should read “the inclusion […] leads to”
L745 Full stop and space before “Methane”.
L754: “changing land use from cropland to pasture in the attempt to reduce C losses from soils might lead to increased CH4 losses from grazing animals if the total number of animals increases rather than animals are fed in a different way.” : The meaning of this sentence is not immediately clear –“fed in a different way”? Do is mean that CH4 emissions may increase if cropland is converted to grassland in order to feed more animals, and not in order to substitute arable crops by grass as animal feedstuff?
L765f. “care must be taken to preserve C loss by management options” : What is meant by “preserving C loss”, and would not the management options for reducing C loss and increasing C stocks be the same or at least overlapping?
L773 “NOxNH4“ : elsewhere „NOx/NH3“
Table 1: Explanation of “LSU” should include to how many kg live weight 1 LSU corresponds. |
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