the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Organic and inorganic nitrogen amendments suppress decomposition of biodegradable plastic mulch films
- 1Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Tennessee, United States
- apresent address: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan, United States
- 1Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Tennessee, United States
- apresent address: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan, United States
Abstract. Biodegradable mulch films (BDMs) are a sustainable and promising alternative to non-biodegradable polyethylene mulches used in crop production systems. Nitrogen amendments in the form of fertilizers are used by growers to enhance soil and plant-available nutrients, however, there is limited research on how these additions impact biodegradation of BDMs tilled into soils. A four-month soil microcosm study was used to investigate the effects of inorganic (ammonium nitrate) and organic (urea and amino acids) nitrogen application on biodegradable mulch decomposition. We investigated the response of soil bacterial, fungal and ammonia-oxidizing microbial abundance along with soil nitrogen pools and enzyme activities. Microcosms were comprised of soils from two diverse climates (Knoxville, TN, USA and Mount Vernon, WA, USA) and BioAgri, a biodegradable mulch film made of Mater-Bi®; a bioplastic raw material containing starch and poly(butylene adipate-co-terephthalate) (PBAT). Both organic and inorganic nitrogen amendments inhibited mulch decomposition, soil bacterial abundances and enzyme activities. The greatest inhibition of mulch biodegradation in TN soils was observed with urea amendment where biodegradation was reduced by about 6 % compared to the no-nitrogen control. In WA soils, all nitrogen amendments suppressed biodegradation by about 1 % compared to the no-nitrogen control. Ammonia monooxygenase amoA gene abundances were increased in TN soils in all treatments, but reduced for all treatments in WA soils. However, a significantly higher nitrate and lower ammonium concentration was seen for all nitrogen treatments compared to no-nitrogen controls in both TN and WA. This study suggests that addition of nitrogen, particularly inorganic amendments, could negatively affect mulch decomposition but that mulch decomposition does not negatively affect soil nitrification activity.
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Sreejata Bandopadhyay et al.
Status: open (until 15 Feb 2023)
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RC1: 'Comment on egusphere-2022-1333', Anonymous Referee #1, 21 Jan 2023
reply
Discussion of pre-print:
Organic and inorganic nitrogen amendments suppress decomposition of biodegradable plastic mulch films
by Bandopadhyay et al., submitted to EGU spheres
General comments
This is an interesting study assessing the effect of N fertilization of soils on biodegradation of biodegradable polymers and starch. The study is carefully conducted and designed and the statical analyses seems stringent and correct.
There are, however, a number of points the authors may want to consider addressing to improve the clarity and constrain the implications to what can be inferred from the experiments. These points are part of the specific comments made below. To highlight a few:
- The study set out to test the effect of fertilization (N) on biodegradation. It seems that this may warrant the use of soils that are N deprived (or span a gradient in N availability). Yet, the two soils chosen here have relatively high N contents to start. How generalizable are results then?
- The plastics contain starch and PBAT. Starch seems to degrade preferentially. So are we observing an effect of N on starch? Maybe the effect on PBAT is different?
- The duration of the experiment is “only” 16 weeks, while biodegradation in standard tests is followed over 2 years. Is it valid to infer that what one sees initially (during 4 of 24 months) is applicable to the remaining 20 months?
- The effect of N addition on the efficiency at which carbon is incorporated into microbial biomass is not addressed. Could it be that less CO2 formed because microbes build more biomass in a world of excess N?
- It seems that N addition affected also general soil activity, microbial abundance and enzyme activities. If this is true, then the reviewer suggests rethinking the title. The tile suggests that the effect is on plastic “only”. But one could argue: the soil simpl did not do well after N addition and, no wonder, did this also affect plastic biodegradation
More specific comments and more elaboration on these points are given below.
Specific comments
Line 13: microcosms: unclear what is meant (laboratory incubations?)
Line 19: the authors refer to “decomposition” because mineralization to CO2 was not followed?
Line 20: Now authors refer to “biodegradation”. Strictly speaking this is possibly only based on respirometric tests or if residual polymer is quantified
Line 23: suppression by 1%. That seems to be little. One % can be detected also given the variations between incubations?
Line 25: Are the nutrients themselves slowing down decomposition or does nutrient addition change other soil parameters that then affect degradation
Line 50: why is a “composting” standard cited when biodegradation is supposed to happen in the soil?
Line 54: not 90% biodegradation but rather 90% conversion to CO2. That is a difference because if some of the polymer carbon is used for biomass formation, mineralization to 90% may underestimate biodegradation
Line 75: shift in which direction (maybe say: C:N is lowered)
Line 83: “a few studies” … and which explanation was provided? Are results really mixed or do most studies show a positive effect of fertilization on organic matter decomposition?
Line 88: proposed … may .. – maybe better “… speculated that …”
Line 95: If the authors say “biodegradation”, then they need to conduct respirometric measurements. Otherwise, if weight loss (or similar) is measured, its strictly speaking not biodegradation but rather decomposition or dissipation
Table 1: C/N ratios are relatively small (around 10). Would one expect to see nitrogen limitation in such soils in the first place?
Line 130: how could the authors be sure that the sole mass was mulch film and not including the mass of some adhering particles, etc.
Line 140: in some of the treatments, not only N but also labile C was added. How can the authors delineate the two? Sure, the organic N was added as an N source. But given that it is also a C source, is it not possible that labile C addition (in combination with N) supressed BDM decomposition because there was no need to access its carbon? What is the ration of BDMF added C versus fertilizer added C? Also, how far did fertilizer addition change the soil C:N?
Line 152: Do 16 weeks suffice if regulatory standards allow for 2 years?
Line 165; “rate equivalent”. Unclear what is meant. The authors always added the same amount of N?
Line 185: It seems that the mesocosms were set up to determine CO2 formation. The reviewer assumes that biodegradation was quantified from the excess CO2 formation in incubation vials with soil + BDMF relative to only soil (no BDMF) incubations. If correct, then the controls are subtracted from the incubations to arrive at polymer-derived CO2 formation.
However, what about the controls for N amendment? Were separate control incubations run for all N amendments? Because N amendment can change background mineralization (and organic N addition will for sure produce extra Co2)
Line 213: from “soil samples”. How close where these to the foil pieces or which soil mass was extracted? Quite clearly, if there is an effect of foils on soil microorganisms, it should be higher close to the foils. This means that a very reproducible soil sampling must have been done?
Line 235: was quenching by soil DOM accounted for? Were calibration curves of fluorescence standards prepared in soil extracts or pure buffer solutions?
Line 260: did the visual analysis confirm/agree with the mineralization data?
Line 274: The question above on the controls for soil + polymer + fertilizer refers to this equation. Essentially, were there no mulch + fertilizer controls?
Line 289: changes in the chemical bonding… which bonds are referred to? Polymer bonds?
Line 314: what are “main effects” and “interaction effects”?
Line 325: so could the nitrogen effect be, in fact, a soil pH effect?
Figure 1: soils are quite acidic (pH 5.25). is this normal for these soils? Are the soils not carbonated?
Line 365: Did the authors assess which components of the films degraded? And if nitrogen addition affected biodegradation of one component more than others? Also, biodegradation is inferred from Co2 formation. Is it possibly that microbes incorporated more C into their biomass in the presence of N, thus leading to an “apparent” decrease in biodegradation only?
Line 370 time and t being proportional: remove, since this indeed is the programmed heating rate
Line 378: and possibly more efficient biomass incorporation?
Line 382: “must occur”? unclear what is meant
Line 389: how was this calibrated? Simply relative peak areas? Do the authors know if responses scale linearly with mass?
Figure 2: what are these numbers relative to C added (mineralization in % of carbon added).
Figure 2a: very hard to read. Maybe pull apart in separate graphs (multipanel)
Figure 2: it seems that N additions not only affected polymer mineralization but also soil background mineralization. Did the authors check for that? This is important because then the conclusion shifts from “N addition slows down biodegradation of plastics” to “N addition slows down soil carbon respiration to CO2, including respiration of added plastic carbon”
Line 457: see comment above on effect of N on bacterial abundance and/or activity versus effect on film degradation
Line 527: It is unclear whether the 10% comes from starch or from PBAT?
Line 528: It should be mineralization not decomposition as biomass uptake is not determined (nor residual polymer quantified)
Line 531: since this study is on the “early stage” of biodegradation, how can the authors rule out that N addition has positive effects in later stages? Should this “early stage” fact not be stated more prominently as to be clear that the study was relatively short?
Line 353: if there is discrepancy, what does this tell us about using the photographic method to “quantify” biodegradation in field studies. Is this a reliable approach?
Line 539: but also of background soil organic matter decomposition?
Line 540: you mean by 1 percentage point (from 4 to 3%) or, as stated, by 1% (from 4% to 3.96%). Same for the 6%
Line 544: indeed, C:N was 10!
Line 547: see comment above. Is the conclusion then not rather: N addition lowers microbial abundance/activity and thereby also biodegradation of polymer? That conclusion is very different than how the title of the manuscript reads right now
Line 558: in this case its also not the N that does harm but rather the pH
Line 580: how could priming be avoided?
Line 583: what is meant by “labelled”? Any example?
Line 585: has this been done already?
Line 588: led to is better than “caused”. Because N addition unlikely was the cause for depolymerization
Line 630: see comment above on what the effect was: an effect on biodegradation of polymers or a general effect on soil activity. Also, if one wanted to test for N limitation (and a potential benefit of adding N), would one not chose soils with varying C:N ratios? Including soils that are N deprived?
Sreejata Bandopadhyay et al.
Data sets
Datasets associated with Organic and inorganic nitrogen amendments suppress decomposition of biodegradable plastic mulch films Sreejata Bandopadhyay https://figshare.com/s/d6eb6bfee547956dd3b7
Sreejata Bandopadhyay et al.
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