Nitrous oxide (N2O) is a potent greenhouse gas and causes stratospheric ozone depletion. While the emissions of N2O from soil are widely recognized, recent research has shown that terrestrial plants ...may also emit N2O from their leaves under controlled laboratory conditions. However, it is unclear whether foliar N2O emissions are universal across varying plant taxa, what the global significance of foliar N2O emissions is, and how the foliage produces N2O in situ. Here we investigated the abilities of 25 common plant taxa, including trees, shrubs and herbs, to emit N2O under in situ conditions. Using 15N isotopic labeling, we demonstrated that the foliage‐emitted N2O was predominantly derived from nitrate. Moreover, by selectively injecting biocide in conjunction with the isolating and back‐inoculating of endophytes, we demonstrated that the foliar N2O emissions were driven by endophytic bacteria. The seasonal N2O emission rates ranged from 3.2 to 9.2 ng N2O–N g−1 dried foliage h−1. Extrapolating these emission rates to global foliar biomass and plant N uptake, we estimated global foliar N2O emission to be 1.21 and 1.01 Tg N2O–N year−1, respectively. These estimates account for 6%–7% of the current global annual N2O emission of 17 Tg N2O–N year−1, indicating that in situ foliar N2O emission is a universal process for terrestrial plants and contributes significantly to the global N2O inventory. This finding highlights the importance of measuring foliar N2O emissions in future studies to enable the accurate assigning of mechanisms and the development of effective mitigation.
N2O, a potent greenhouse gas, plays a significant role in stratospheric ozone depletion. Identifying its sources is crucial for global emission management. Our study revealed widespread foliar N2O emissions in 25 plant taxa, originating mainly from nitrate reduction by endophytes. Globally, estimated foliar N2O emissions range from 1.01 to 1.21 Tg N2O–N year−1, constituting 6%–7% of total N2O emissions. This underscores the substantial impact of universal foliar N2O emissions on the global N2O inventory, emphasizing the importance of addressing this source in emission mitigation strategies.
Background
Cereal cultivars vary in root traits, and it can be proposed that wild forms and old cultivars are more adapted to using organic nitrogen (N) sources.
Aims
Investigating N uptake from ...cover crop (CC) rhizodeposits by wheat and barley of different domestication level and their wild relatives, we expected a more efficient N uptake by wild forms and old cultivars, indicated by higher N uptake from rhizodeposits per root biomass.
Methods
Eleven wheat and seven barley cultivars categorized into three domestication levels (wild, old, and modern) were grown under controlled conditions in pots with soil containing 15N‐labelled CC rhizodeposits. Shoot and root biomass, their 15N‐uptake, and a range of root traits were analyzed.
Results
Domestication level was a weak predictor of root architecture. N taken up from rhizodeposits was similar in all plants at 14% of plant N. Cultivars with largest values for most root traits (e.g., root length or volume) and root to shoot ratio also showed largest total N uptake and N uptake from rhizodeposits. However, N uptake efficiency (N uptake from rhizodeposits per root biomass) was smallest for these cultivars. Two modern wheat cultivars showed the largest N uptake efficiency from CC rhizodeposits, whereas in barley a wild and an old cultivar showed the largest efficiency.
Conclusions
In our study with a limited number of cultivars, domestication level is not related to the ability of cereals to take up N from CC rhizodeposits. The same proportion of N derived from rhizodeposits irrespective of cultivar or root traits indicates that the N provision from rhizodeposits was driven by other factors like mineralization. However, some cultivars had a larger N uptake efficiency.
Nitrifier denitrification is the reduction of nitrite (NO2−) by ammonia-oxidizing bacteria. This process may account for up to 100% of nitrous oxide (N2O) emissions from ammonium (NH4+) in soils and ...is more significant than classical denitrification under some conditions. Investigations of nitrifier denitrification have expanded in the last decade but many aspects are still not understood. In this review, we revisit our 2001 paper, present a comprehensive summary of current knowledge concerning nitrifier denitrification, and identify the many research needs. Nitrifier denitrification can be distinguished from other routes of N2O production using isotopic methods: either isotopomer techniques or a combination of 15N and 18O tracers. Our understanding of the regulation and conditions favouring nitrifier denitrification has improved over the last decade as a result of adopting molecular and modelling approaches. Environments low in oxygen, and especially those with fluctuating aerobic-anaerobic conditions, promote N2O production by nitrifier denitrification. Also, large NO2− concentrations, which often arise following inputs of ammonium or urea, may be linked to changes in aerobicity and high pH and favour nitrifier denitrification. The effects of temperature and carbon contents on nitrifier denitrification are incompletely understood and future research needs include: the study of pathways similar to nitrifier denitrification in archaea and nitrite oxidizers; the effects of interactions among microorganisms and between microorganism and plants; and the regulation and importance of the enzymes involved. A comparison and evaluation of the methods used for differentiating the sources of N2O is urgently needed. Furthermore, results from studies of freshwater and marine environments as well as wastewater treatment, where nitrifier denitrification is also known as nitrous aerobic denitritation (up to N2O) or aerobic denitritation (up to N2), will further advance our understanding.
Biochars containing abundant and diverse functional groups can influence soil nitrous oxide (N
2
O) emissions. It remains unclear how these functional groups alter soil N
2
O emissions and which ...specific functional groups are responsible. In this study, biochars containing differing functional group compositions were initially screened for their ability to mitigate N
2
O emissions. Then, the mechanisms of the targeted functional group were assessed by loading the targeted functional group onto graphene, to exclude the potential effects of other functional groups, in order to test specific functional group effects on N
2
O production and reduction in an agricultural soil and on a typical denitrifier strain (
Thiobacillus denitrificans
). The results showed that the abundance of the C=O (carboxyl and carbonyl functional groups) was significantly correlated (
P
< 0.01) with soil N
2
O emissions and the N
2
O/(N
2
O+N
2
) emission ratio, indicating that the carboxyl functional group plays a significant role in reducing the mitigation potential of biochar on soil N
2
O emissions. Compared to the carboxyl-free graphene, the carboxyl-loaded graphene significantly decreased the genetic potential for N
2
O reduction and increased the N
2
O/(N
2
O+N
2
) emission ratio of the soil and the
Thiobacillus denitrificans
strain. The addition of an electron transfer inhibitor (N, N′-dicyclohexylcarbodiimide) eliminated the negative effect of the carboxyl functional group in terms of reducing N
2
O emissions, suggesting that the inhibitory effect of the carboxyl functional group on N
2
O reduction was likely caused by disruption of the electron transfer from biochar to N
2
O-reducing microbes. Our results imply that the carboxyl functional group on biochar’s surface can negatively affect the potential of biochar to mitigate soil N
2
O emissions.
Nitrous oxide (N
O) is a potent greenhouse gas and causes stratospheric ozone depletion. While the emissions of N
O from soil are widely recognized, recent research has shown that terrestrial plants ...may also emit N
O from their leaves under controlled laboratory conditions. However, it is unclear whether foliar N
O emissions are universal across varying plant taxa, what the global significance of foliar N
O emissions is, and how the foliage produces N
O in situ. Here we investigated the abilities of 25 common plant taxa, including trees, shrubs and herbs, to emit N
O under in situ conditions. Using
N isotopic labeling, we demonstrated that the foliage-emitted N
O was predominantly derived from nitrate. Moreover, by selectively injecting biocide in conjunction with the isolating and back-inoculating of endophytes, we demonstrated that the foliar N
O emissions were driven by endophytic bacteria. The seasonal N
O emission rates ranged from 3.2 to 9.2 ng N
O-N g
dried foliage h
. Extrapolating these emission rates to global foliar biomass and plant N uptake, we estimated global foliar N
O emission to be 1.21 and 1.01 Tg N
O-N year
, respectively. These estimates account for 6%-7% of the current global annual N
O emission of 17 Tg N
O-N year
, indicating that in situ foliar N
O emission is a universal process for terrestrial plants and contributes significantly to the global N
O inventory. This finding highlights the importance of measuring foliar N
O emissions in future studies to enable the accurate assigning of mechanisms and the development of effective mitigation.
Dairy livestock production systems rely on high‐quality forage legumes, which are widely present in grassland swards all over Europe. A future climatic scenario with higher average annual ...temperatures and lower precipitation is expected to affect grassland productivity in general and the productivity of the most important forage legume species Trifolium repens in particular. One way to cope with such constraints is the adoption of currently underutilized minor legume species with a higher tolerance towards drought stress. Therefore, the present study investigated legume species with lower moisture requirements than T. repens, these are Lotus corniculatus, Medicago falcata, Medicago lupulina and Onobrychis viciifolia in comparison with T. repens. Legumes were grown in containers as monocultures in a roofed open‐sided greenhouse under conditions of optimal water supply or periodic drought stress. Generally, drought stress decreased the biomass production, but species differed in their reaction. Particularly, M. lupulina and L. corniculatus had lower relative biomass losses (−26%) compared to T. repens (−43%, as averaged over drought stress periods). However, in overall biomass production T. repens still was at one level with M. lupulina and L. corniculatus under drought stress. This was related to high stomata control as indicated by the intrinsic water‐use efficiency. We conclude that there are promising future options of forage legumes as alternatives to T. repens.
Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO3 –) reduction. However, few studies have confirmed this in a soil ...matrix hitherto. This study investigated if, and how, an electric potential affected NO3 – reduction in a soil matrix. The results showed that, compared to a control treatment, applying an electric potential of −0.5 V versus the standard hydrogen electrode (SHE) significantly increased the relative abundance of NO3 –-reducing microbes (e.g., Alcaligenaceae and Pseudomonadaceae) and the abundances of the nrfA, nirK, nirS, and nosZ genes in soil matrices. Meanwhile, the electric potential treatment doubled the NO3 – reduction rate and significantly increased the rates of production of ammonium (NH4 +), dinitrogen (N2), and nitrous oxide (N2O). The amount of NO3 –-N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased N2O, N2, NH4 +, and nitrite (NO2 –) pools. An open-air experiment showed that the electric potential treatment promoted soil NO3 – reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO3 – reduction via both denitrification and dissimilatory NO3 – reduction to ammonium (DNRA) in the soil matrix. The mechanisms revealed in this study have implications for the future development of potential techniques for enhancing NO3 – reduction in the vadose zone and consequently reducing the risk of NO3 – leaching.
Agriculture and land use change has significantly increased atmospheric emissions of the non-CO
2
green-house gases (GHG) nitrous oxide (N
2
O) and methane (CH
4
). Since human nutritional and ...bioenergy needs continue to increase, at a shrinking global land area for production, novel land management strategies are required that reduce the GHG footprint per unit of yield. Here we review the potential of biochar to reduce N
2
O and CH
4
emissions from agricultural practices including potential mechanisms behind observed effects. Furthermore, we investigate alternative uses of biochar in agricultural land management that may significantly reduce the GHG-emissions-per-unit-of-product footprint, such as (i) pyrolysis of manures as hygienic alternative to direct soil application, (ii) using biochar as fertilizer carrier matrix for underfoot fertilization, biochar use (iii) as composting additive or (iv) as feed additive in animal husbandry or for manure treatment. We conclude that the largest future research needs lay in conducting life-cycle GHG assessments when using biochar as an on-farm management tool for nutrient-rich biomass waste streams.
PDF