Environmental Engineering Reference
In-Depth Information
The NPP is allocated to the production of biomass (wood, leaves, roots, fruits,
seeds etc.) and respired back to the atmosphere mainly due to decomposition by
microbial activities. The quantity of carbon lost by respiration for heterotrophic
organisms is defined heterotrophic respiration (Rh) and the difference between
NPP and Rh is the Net Ecosystem Production
NEP
=
NPP
−
RH
=
GPP
−
TER
The NEP represents the net ecosystem carbon sink or source due to physi-
ological processes and it is also named Net Ecosystem Exchange (NEE) when it is
quantified using measurements of CO
2
exchanges between ecosystem and atmos-
phere (while it is called NEP when measured using inventory approaches). The
two terms are somehow interchangeable but in general with opposite signs: a flux
of carbon from the atmosphere to the ecosystem is positive in NEP and negative in
NEE. The sum of the two respiration components (Rh and Ra) represents the total
ecosystem respiration (TER).
The NEP is however different from the long term carbon balance of the ecosys-
tem because there can be changes in the carbon stocks due to episodic losses by
natural or anthropogenic disturbances and management practices. For this reason
the Net Biome Production (NBP) is defined as (Schulze et al.
2000
):
NEP
=
NPP
−
CO
2
LOSSES DUE TO DISTURBANCES
There are different possible approaches and methods to measure the fluxes of
energy and greenhouse gases (GHGs) in terrestrial ecosystems, ranging from
inventory approaches to chambers measurements and ecosystem scale techniques
such as the Eddy Covariance method (Aubinet et al.
2012
).
The Eddy Covariance methodology has been developed in the early '90s and
has been widely applied at global level. It is based on high frequency (10 Hz)
measurements of wind speed, temperature and gas concentration using a three-axis
sonic anemometer (which measures the wind speed along the three axis) and a fast
response gas analyzer, typically an Infra Red Gas Analyzer (IRGA) for CO
2
and
H
2
O, even if new systems have been recently developed and commercialized to
measure high frequency concentrations of other gases such CH
4
, N
2
O and O
3
.
With the Eddy Covariance technique it is possible to measure the Net
Ecosystem Exchange (NEE) of a GHG of a given surface extended around the
monitoring tower (the footprint). The extension and shape of the footprint is func-
tion of the wind speed, wind direction and the difference between the measure-
ment and canopy heights and it has generally a radius between few hundred meters
and one kilometer around the measurement point. The Eddy Covariance technique
is the only method available today to continuously measure the net ecosystem
exchanges at ecosystem level and in a not-destructive way.
In addition, for CO
2
NEE measurements, there is the possibility to statistically par-
tition the net carbon fluxes measured into its major components as the gross primary
production (GPP) and the ecosystem respiration (Reichstein et al.
2005
; Lasslop et al.
2010
) allowing a better interpretation of the fluxes in terms of ecosystem processes.
Nowadays, more than 500 sites exist globally, organized in regional networks
contributing to the global network FLUXNET (
http://luxnet.ornl.gov/
) with the
