Interaction between N and C in Soil has Consequences for Global Carbon Cycling
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Interaction between N and C in Soil has Consequences for Global CarbonCyclingAuthor(s): Siegfried FleischerSource: Journal of Resources and Ecology, 3(1):16-19. 2012.Published By: Institute of Geographic Sciences and Natural Resources Research, Chinese Academy ofSciencesDOI: http://dx.doi.org/10.5814/j.issn.1674-764x.2012.01.003URL: http://www.bioone.org/doi/full/10.5814/j.issn.1674-764x.2012.01.003
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J. Resour. Ecol. 2012 3 (1) 016-019
March, 2012 Journal of Resources and Ecology Vol.3 No.1
Received: 2011-11-25 Accepted: 2011-11-30* The author: Siegfried FLEISCHER. Email: Siegfried.Fleischer@hh.se
1 IntroductionThe general view until now has been that CO2 emissions from soils (or from soil samples at the laboratory) can be used as a proxy for respiratory activity (Janssens et al. 2010). But gross heterotrophic respiration (GHR) within the soil, and net (NHR) which is CO2 emitted from the soil, were different as a result of within-soil CO2 reduction. Inhibition of N-cycling enabled measurement in the laboratory of a within-soil CO2-sink as the difference between GHR and NHR (Fleischer and Bouse 2008). The outcome of the initial rough estimate deserved further investigation, and resulted in the present broadened study. It is necessary to include within-soil CO2 sequestration to correctly understand the C-cycle.
The eddy covariance technique for flux measurements has largely contributed to increased knowledge of CO2-exchange between the atmosphere and terrestrial ecosystems. If this is the sole technique, however, land areas are black boxes not disclosing within-ecosystem
processes such as the within-soil process reported here. From the presentation of a global database on forest ecosystems (Lyussaert et al. 2007) it appeared that autotrophic, heterotrophic and ecosystem respirations have few direct observations and should be measured independently. They also suggested chamber measurements to increase knowledge on CO2-emissions from different types of land areas, but even with this method included, information on the underlying soil processes resulting in NHR is limited.
It has long been known that N-supply decreases the efflux of CO2 from soils, an effect often viewed upon as respiration impairment in some way (Janssens et al. 2010). Billing and Ziegler (2008) already followed this interpretation, but, importantly, they showed that elevated CO2 increased this process. An increase in NH4
+ does favour the chemoautotrophic process nitrification, but so does CO2 which is the sole carbon source for this process. This was our background when identifying the within-soil CO2-sink which could be prevented by specific inhibition
Interaction between N and C in Soil has Consequences for Global Carbon Cycling
Rydberg Laboratory, Halmstad University, P.O. Box 823, S-301 18 Halmstad, Sweden
Abstract: Energy-yielding processes in the N-cycle form important links with the global C-cycle. One example is demonstrated with the supply of nitrogen to soils, initially resulting in lowered CO2
emissions. This well known effect has mostly been interpreted as hampered or delayed soil respiration.
When added in surplus, however, nitrogen supply does not stabilize the minimum emissions initially
obtained, but gradually results in increased CO2 emissions. Specific inhibition of the CO2 consuming
process nitrification in soils, with surplus ammonium supply or with acetylene, mostly results in
additional CO2 emissions. The difference between this disclosed gross heterotrophic respiration
(GHR) and the net CO2 emission (NHR) is the result of a within-soil CO2-sink. Soil respiration solely
determined as CO2 emitted as NHR (the common situation) therefore may lead to misinterpretations
of the function of the soil system, especially in areas with high N-deposition. As a consequence, the
interpreted acclimation of the soil respiration response in a warmer world should be reconsidered.
The concept of respiration inhibition by nitrogen supply may also be questioned. Disregard of these
processes, including the indicated N-driven within-soil CO2-sink, may prevent adequate measures
counteracting climate change.
Key words: soil respiration; carbon sink; CO2 emissions; temperature response; climate change; N-supply
Siegfried FLEISCHER: Interaction between N and C in Soil has Consequences for Global Carbon Cycling 17
of nitrification (Fleischer and Bouse, 2008). A strong CO2-sink did not generally seem to coincide with increased NO3
- in the soil water. A possible interpretation was that repeated N-cycling generated the energy needed for the extensive CO2-reduction in different soils, as such an expression of gross nitrification. Terrestrial sinks as a result solely of the extensive photosynthetic CO2 uptake into green plants above the soil surface was questioned.
These initial findings are the basis for the hypothesis that N-supply under current atmospheric N-deposition conditions does not decrease heterotrophic soil respiration it enforces the within-soil CO2-sink, in this way leading to decreased CO2-emissions. At very high N-supply rates nitrification is gradually inhibited, and CO2-emissions are again increasing (Fleischer and Bouse 2008), thus contradicting the prevailing interpretation that heterotrophic respiration is inhibited by N-supply (Olsson et al. 2005; Janssens et al. 2010). Results presented here support the role of the within-soil CO2-sink.
2 MethodsTwenty-five to forty composite soil samples (25 mm) from the 05 cm horizon (in a vertical profile 05, 510 and 1015 cm) were transported at sampling temperatures and sieved (2 mm) immediately after arrival to the laboratory. Possible remains of fine roots were included in the saprophytic system. Preparation and incubation for 37 hours in darkness of each soil sample included series of 12 ml exetainers without (references), and with additions of NH4
+ to increase nitrification, and with acetylene (0.8%2.0%) to inhibit nitrification, and followed the procedures by Fleischer and Bouse (2008). The study is based on 225 triplets (each with 3 references and 3 with acetylene or N-additions), 8 with five replicates and 8 duplicates. It covered frost free conditions in soils from 2004 to 2009, from central Europe (5009 N) to northern Scandinavia (6603 N), and two sampling occations in a temperate rainforest in Washington State, USA (4752 N). Compared to the previous study (Fleischer and Bouse 2008) the weight of high atmospheric N-deposition areas has been lowered from 25% to 15% and the share of cropland, represented by the Mellby research fields in SW Sweden (5620N, 1259 E) is 37% which is close to the European situation.
CO2-production in the saprophytic soil system was measured with GS-chromatography on Hay Sep Q in the atmosphere from the sealed exetainers. Calculation of the soil CO2-sink followed (GHR-NHR)/GHR*100 (percent of GHR).
3 ResultsSpecific inhibition of nitrification in forest, agricultural or grassland soils (sieved) did on most occasions result in increased emissions of CO2. The difference between this GHR and NHR in the soil saprophytic system (the within-soil CO2-sink) was the result of N-cycling generating CO2 uptake in darkness. Intermittently, in one fourth of the samples, inhibition of this process resulted
in decreased CO2-emissions, instead of an increase (Fig. 1). Experimentally this situation can be induced by high N-loading, exclusively by nitrate. The average within-soil CO2-sink of all forest sites was 5.11% (n=120), farmland 4.80% (n=76), grassland 4.53% (n=11) and of all sites 4.90% (sieved soils, n=207). The sink is also found in deeper soil layers (Fig. 2).
Inhibition of nitrification in soil kept at different temperatures disclosed GHR. Additional supply of NH4
+ resulted in a more pronounced CO2-sink at high temperatures (Fig. 3).
4 DiscussionA strong within-soil CO2-sink driven by nitrogen cycling is localized within the saprophytic system and seems to be of the same magnitude in forest, grassland and agricultural soils. Provided that NHR averages 50% of the 80.4 PgC emitted from the soil surface globally (Raich et al, 2002), and the average sink obtained in this subsystem (4.9%) is representative for all land areas (tropics given the same value in this generalization), GHR amounts to 42.27 Pg
Fig. 1 Magnitude of the CO2 sink / additional source in the soil saprophytic system, determined after inhibition of nitrification in sieved forest, grassland