Field Application of Organic Fertilizers Triggers N2O Emissions From the Soil N Pool as Indicated by 15N-Labeled Digestates

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.

Organic Matter Composition of Digestates Has a Stronger Influence on N2O Emissions than the Supply of Ammoniacal Nitrogen

“…Consequently, it can be concluded that, in the case of dried solid, the addition of easily degradable C in combination with soil-derived NO 3 − triggered the N 2 O production and release. This notion is consistent with previous findings, based on experiments with 15 N-labeled manures [40,47,57,58]. It highlights the complexity of the presented issue and the importance of additional determination of the responsible N source.…”

“…Increasing fluxes after fertilizer application and in connection with soil wetting (i.e., rainfall) (Figure 1) have already been reported in literature [22,[36][37][38][39]. In accordance with Hayakawa et al [22] and Häfner et al [40], the highest N 2 O peaks were observed within the first rainfall event (week 1). Hayakawa et al [22] assumed denitrifying activity as the main driver, since they measured a simultaneous increase in CO 2 release.…”

“…The subsequent peaks in N 2 O fluxes determined during the following weeks can be partly associated with further rainfall events, which was also described in previous studies [40,[45][46][47]. During rather dry periods organic decomposition was reduced [48].…”

“…Hayakawa et al [22] assumed denitrifying activity as the main driver, since they measured a simultaneous increase in CO 2 release. The simultaneous CO 2 release was also noted by Häfner et al [40], who stated that at least part of the emitted CO 2 originates from digestate carbonate C, indicating that such a CO 2 release cannot only be assigned to the decomposition of organic matter. In the present study, the N 2 O release directly after fertilization might have been a result of nitrification followed by denitrification, which was triggered by the high initial NH 4 + content of digestate.…”

The Effectiveness of Digestate Use for Fertilization in an Agricultural Cropping System

“…However, a good proportion of nutrients such as N, phosphorous (P), and potassium (K) are retained [ 8 ]. The mineral contents and characteristics of digestate depend, most of the time, on the characteristics of the substrate and the modality of digestion [ 9 ]. These defined attributes, such as organic matter content, NH 4 , the C/N ratio, and N content present in the different substrates or feedstocks that form the digestates, will show differences in efficiencies and productivity in plants and soils, therefore making it the main source of this research.…”