Abstract. Currently, drylands occupy more than one-third of the global terrestrial
surface and are recognized as areas vulnerable to land degradation. The
concept of land degradation stems from the loss of an ecosystem's biological
productivity due to long-term loss of natural vegetation or depletion of
soil nutrients. Drylands' key role in the global carbon (C)
balance has been recently demonstrated, but the effects of land
degradation on C sequestration by these ecosystems still need to be
investigated. In the present study, we compared net C and water vapor
fluxes, together with satellite, meteorological and vadose zone (CO2,
water content and temperature) measurements, between two nearby
(∼ 23 km) experimental sites representing “natural”
(i.e., site of reference) and “degraded” grazed semiarid grasslands. We utilized
data acquired over 6 years from two eddy covariance stations located in southeastern Spain
with highly variable precipitation magnitude and distribution.
Results show a striking difference in the annual C balances with an average
net CO2 exchange of 196 ± 40 (C release) and −23 ± 2 g C m−2 yr−1
(C fixation) for the degraded and natural sites,
respectively. At the seasonal scale, differing patterns in net CO2
fluxes were detected over both growing and dry seasons. As expected, during
the growing seasons, greater net C uptake over longer periods was observed at
the natural site. However, a much greater net C release, probably derived
from subterranean ventilation, was measured at the degraded site during
drought periods. After subtracting the nonbiological CO2 flux from net
CO2 exchange, flux partitioning results point out that, during the 6
years of study, gross primary production, ecosystem respiration and water use
efficiency were, on average, 9, 2 and 10 times higher, respectively,
at the natural site versus the degraded site. We also tested
differences in all monitored meteorological and soil variables and CO2
at 1.50 m belowground was the variable showing the greatest intersite
difference, with ∼ 1000 ppm higher at the degraded site.
Thus, we believe that subterranean ventilation of this vadose zone CO2,
previously observed at both sites, partly drives the differences in C
dynamics between them, especially during the dry season. It may be due to enhanced
subsoil–atmosphere interconnectivity at the degraded site.
Concurrent Measurements of Soil and Ecosystem Respiration in a Mature Eucalypt Woodland: Advantages, Lessons, and Questions