Abstract
Anaerobic digestion (AD) can generate biogas while simultaneously producing digestate which can be used as fertilizer. Feedstocks used for AD influence digestate composition, which in turn may affect carbon (C) and nitrogen (N) turn-over in soils and subsequently influence nitrous oxide (N2O) emissions after soil application. Assessment of greenhouse gas emissions from digestates can help to evaluate the overall sustainability of an agricultural production system. The objective of this study was therefore to evaluate and understand the effect of differences in digestate composition on in situ N2O emissions within the 1st weeks after application of seven digestates. The digestates were derived from different feedstocks and 15N-labeled, either in total N or only in ammonium-N. Therefore, the experimental design enabled us to differentiate between potential N2O-N sources (i.e., digestate N or soil N). Furthermore, it allowed to distinguish to some extent between organic-N and ammonium-N as potential N sources for denitrification. Digestates were homogeneously incorporated into the upper 5 cm of microplots in an arable Haplic Luvisol in South Germany at a rate of 170 kg N ha−1. After application, N2O fluxes were measured for ~60 days (May-July) using the closed chamber method in 2 experimental years. Mainly due to higher precipitations in the 1st year, cumulative N2O emissions were higher (312–1,580 g N2O-N ha−1) compared to the emissions (133–690 g N2O-N ha−1) in the 2nd year. Between 16–33% (1st year) and 17–38% (2nd year) of N2O emissions originated from digestate N, indicating that digestate application triggered N2O production and release mainly from soil N. This effect was strongest immediately after digestate application. It was concluded that the first (short term) peak in N2O emissions after digestate application is largely related to denitrification of soil-N. However, the experimental setup does not allow to differentiate between the different denitrification pathways. Weather conditions showed a substantial effect on N2O emissions, where the correlation between N2O and CO2 flux rates hinted on denitrification as main N2O source. The effect of digestate composition, particularly organic N from the digestate, on soil N2O emissions seems to be of minor relevance.
Highlights
In the EU, about 180 million tons of anaerobic digestate are estimated to be produced per year, most of which is used as organic fertilizer (Corden et al, 2019)
−3.45 1.91 1.16 5.48 7.34 3.42 4.62 8.87 −3.88 −0.55 −1.44 0.56 −1.59 −0.14 0.76 −1.49 kg ha−1 139 c 583 b 634 ab 599 ab 808 a 618 ab 540 b 447 b 62.3 b 322 a 314 a 303 a 278 a 325 a 258 a 231 a Different letters indicate significant differences at p < 0.05 (Tukey Test). §indirect emissions only based on NH3 loss, nitrate leaching was not accounted for
The major finding of this study was the large share of N2ON from the soil pool, showing that digestate application triggers “enhanced soil-derived N2O.”
Summary
In the EU, about 180 million tons of anaerobic digestate are estimated to be produced per year, most of which is used as organic fertilizer (Corden et al, 2019). Digestates have been shown to have the potential to substitute mineral fertilizers and contribute to a sustainable soil management (Gutser et al, 2005; Cavalli et al, 2016; Verdi et al, 2019). Application of organic as well as mineral nitrogen (N) fertilizers is known to increase greenhouse gas (GHG) emissions from soils. About 60% of anthropogenic N2O emissions are emitted by agricultural soils (Ciais et al, 2013), it is of high relevance to assess N2O in relation to fertilizer application. Studies have shown that digestates might lead to a higher risk of N2O formation than manures, which is related to the higher share of ammonium Stinner, 2009).
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