| 14 May 2025 | Subsequently updated
Nitrous oxide emission from pigeon pea – maize rotation in response to conservation agriculture and biochar amendments in a Ferralsol, northern Uganda
Abstract. Smallholder agriculture in sub-Saharan Africa (SSA) commonly involves limited use of mineral or organic fertilizer, often resulting in severe nutrient limitation. Conservation Agriculture (CA), including crop rotation with legumes and biochar amendments, has been advocated to enhance soil fertility and plant available nitrogen (N). However, CA may affect nitrous oxide (N2O) emissions even in unfertilized agroecosystems. N2O is an important greenhouse gas, and understanding the trade-offs between N2O emissions and crop yields in N-poor agroecosystems in SSA is essential. Here we studied crop yield, soil N and N2O emissions in a double cropping system (pigeon pea – maize rotation) throughout two consecutive cropping seasons (April–October 2023 and October 2023–January 2024) in a Ferralsol in Northern Uganda. The study, conducted at a site which had been left fallow for 3 years, involved pairwise comparison of conventionally tilled systems under crop rotation (Conventional) and continuous maize monocropping (ConventMM). In addition, the effect of tillage systems (Conventional, CA and CA+biochar) under pigeon pea – maize rotation was investigated. We defined CA as reduced tillage with planting basins and crop residue retention, whereas conventional tillage involved overall ploughing. Grain yield was not significantly affected by rotation or tillage system. N2O fluxes were small, ranging from 1.02–51.19 µg N m2 h-1 over the entire period. Short-lived emission peaks were observed following pigeon pea harvest in the crop rotation, which were absent in maize monocropping. Overall, across all seasons, cumulative growing-season (279 days) N2O emissions ranged from 0.44–1.11 kg N ha-1. Biochar amendments in CA systems did not affect daily N2O emissions in planting basins. In the first season, yield-scaled N2O emissions and N yield scaled N2O emissions were significantly smaller in CA systems with biochar compared to conventional tillage, suggesting that CA and biochar was effective in minimising emissions without penalising pigeon pea productivity.
Review on the manuscript ‚Nitrous oxide emission from pigeon pea – maize rotation in response to conservation agriculture and biochar amendments in a Ferralsol, northern Uganda by Namatsheve et al.
This study investigates the effects of conservation agriculture (CA), both with and without biochar amendments, compared to conventional tillage in an unfertilised maize-pigeon pea rotation in Uganda. It also compares these treatments with a maize monoculture treatment.
The manuscript significantly contributes to the scientific knowledge base by presenting a novel dataset.
The introduction shows the relevance of the study and the basis of existing literature. The study design and hypotheses are well described. The data is highly valuable. It contains 17 observations of N2O emissions and its driver variables per treatment over the course of one year (two seasons) in Uganda. The methods are sound, and well described. The results are well described and the discussion a good start, but both need some revisions as suggested below.
I recommend to publish the manuscript after major revisions.
L30: Overall, across all seasons, cumulative growing-season (279 days) N2O emissions ranged from 0.44 – 1.11 kg N ha-1
RC: Since this is a major finding, you should give the values specifically per treatment in the abstract.
L127: Table S19
RC: You start with Table S19. You should instead name your Tables differently and make sure to have them ordered in the same way as they occur in your text. You start with table S1 here and so on.
Next table is S8 in your manuscript. Make sure this is S2 in the updated MS.
L130: RC: Please add average precipitation sums (for example 5 or 10 years) in order to show how usual the 2023 conditions were.
L135: Is it known how long the maize and cassava production have been practiced?
L160: For CA, weeds were controlled by spraying glyphosate at a rate of 1.03 L ha-1, immediately after sowing and hand pulling throughout the season.
RC: Set in context how realistic glyphosate spraying would be under farmers practice conditions.
L 198: RC Write: Plot wise sampling in 0 – 20 cm was carried out at the onset (April 2023) and end (October 2023) of the first growing season from planting basins in CA and CA+Biochar treatments and in the planting rows in conventional treatments to assess the effect of different treatments on soil properties.
Leave out the next sentence.
L234: by (Žurovec et al., 2017).
RC: write by Žurovec et al. (2017).
L246: ∑(𝑓𝑖 + (𝑓𝑖+1 )/2 × (𝑡𝑖+1 − 𝑡𝑖) × 24 × 10−5
RC: There is a bug in this formula, there are opening up three brackets but only two of them close again. Make sure to provide a correct and well formatted equation.
L255: (NO3-N and NH₄⁺-N)
RC: Use NO3- here and elsewhere (Text & Fig. to ensure consistency.
L281-283: Adjust the text, since it is the same as before:
RC: My suggestion: For calculating yield scaled N2O emissions, the same scaling factor as for cumulative N2O fluxes of 0.12 for basin and 0.88 for interrows in CA treatments, and 0.50 for inrows and interrows in conventional treatments were applied.
L297: RC: The statistical approach makes sense. Nevertheless I wonder how residuals data can be normal without any transformation, this is very unusual for N2O, could you show the residual plot?
L310
3.0 Result -> 3 Results
L316 : SOC ranged from 1.25 – 2.23% and biochar significantly increased C
RC: use SOC consistently since this is the abbreviation you introduced.
Figure 1: The insert in (a) shows mean ± se N2O fluxes during peak emission on 18 October 2023.
RC: This is the same as you see when looking at this peak day in the time series plot, so I do not see the additional value to much. Well if you want to pronounce it a lot you can keep it. I would rather show the cumulative values - but this might show up later in a separate Figure?
Table 3: Please round the yields meaningfully (Consider the accuracy of measurements!). Give no decimal number here if you have kg /ha. Same for N yields.
Figure 5+ 6 : Add the weighted cumulative values to the plots as a third colour besides basin/inrow and interrow, since this is the main result that should be also show here.
Indicate Tukey-HSD letters also in Fig 5
L388: Write two sentences, else it is confusing: Chamber position did not significantly explain variations in cumulative N2O emissions. There were no significant differences (p > 0.05) between conventionally tilled pigeon pea – maize rotation and conventionally tilled continuous maize monocropping (Table S17, S18, Fig. 5, Fig S3)
L477: corelated -> correlated: Please check spelling (and grammar) throughout the manuscript- I didn’t.
L481: probably immobilizing available N rather than releasing it for microbial N transformations; RC: I know what you mean but… The phrase “releasing it for microbial N transformations” is a bit unclear — microbes don't exactly "release" N for other microbes; it may be better to phrase it differently:
Better: This suggests that microbes in these soils compete effectively for mineral nitrogen, likely immobilizing it and thereby reducing its availability for microbial processes such as nitrification and denitrification
L485: Our results imply that the process of symbiotic N fixation per se, and residue
retention do not affect soil mineral N and N2O emissions in unfertilized soils with inherently low N. Rochette et al. (2004) also reported that there is considerable uncertainty related to the emissions of N2O from soils under legumes, and the soil mineral N alone was a poor indicator of N2O emissions for two seasons in acidic soils in Canada.
RC: 1) It is well-known that N2 fixation itself does not affect N2O emissions, please rephrase so this gets clear.
2) In many studies soil mineral N is a bad indicator for N2O emissions. This is nothing particular to legumes. Please adjust.
L536: Lentz et al. (2014) also found that biochar reduced N2O emissions by 50%, indicating that biochar inhibited nitrification and N immobilisation.
RC: Did they measure nitrification and immobilisation? It is a bit contrary to the study by Munera-Echeverri et al. (2022), so the effect of biochar on nitrification and the difference between these studies should be explained.
L554 – 561:
End the paragraph with your study, not with the numbers of another study. Put the following sentence to line 554: Our results are in line with other studies in SSA, although they applied mineral N fertilizer; for example, Shumba et al. (2023) reported yield scaled emissions of 0.09 – 0.19 in maize after applying 58 kg N ha-1 in a Ferralsol and Lixisol in Zimbabwe.
Rephrase to make it consistent again
Then end the discussion with this: These practices were effective in minimising emissions without penalising pigeon pea productivity, supporting CA as a sustainable agricultural practice.
L569: Typo: Yield-scaled