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Long‐term carbon exchange in a sparse, seasonally dry tussock grassland
Author(s) -
Hunt John E.,
Kelliher Francis M.,
McSeveny Tony M.,
Ross Des J.,
Whitehead David
Publication year - 2004
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/j.1365-2486.2004.00842.x
Subject(s) - environmental science , eddy covariance , biomass (ecology) , grassland , soil respiration , ecosystem respiration , soil water , ecosystem , tussock , carbon dioxide , agronomy , carbon cycle , vegetation (pathology) , hydrology (agriculture) , soil science , ecology , biology , geology , medicine , geotechnical engineering , pathology
Rainfall and its seasonal distribution can alter carbon dioxide (CO 2 ) exchange and the sustainability of grassland ecosystems. Using eddy covariance, CO 2 exchange between the atmosphere and a sparse grassland was measured for 2 years at Twizel, New Zealand. The years had contrasting distributions of rain and falls (446 mm followed by 933 mm; long‐term mean=646 mm). The vegetation was sparse with total above‐ground biomass of only 1410 g m −2 . During the dry year, leaf area index peaked in spring (November) at 0.7, but it was <0.2 by early summer. The maximum daily net CO 2 uptake rate was only 1.5 g C m −2 day −1 , and it occurred before mid‐summer in both years. On an annual basis, for the dry year, 9 g C m −2 was lost to the atmosphere. During the wet year, 41 g C m −2 was sequestered from the atmosphere. The net exchange rates were determined mostly by the timing and intensity of spring rainfall. The components of ecosystem respiration were measured using chambers. Combining scaled‐up measurements with the eddy CO 2 effluxes, it was estimated that 85% of ecosystem respiration emanated from the soil surface. Under well‐watered conditions, 26% of the soil surface CO 2 efflux came from soil microbial activity. Rates of soil microbial CO 2 production and net mineral‐N production were low and indicative of substrate limitation. Soil respiration declined by a factor of four as the soil water content declined from field capacity (0.21 m 3 m −3 ) to the driest value obtained (0.04 m 3 m −3 ). Rainfall after periods of drought resulted in large, but short‐lived, respiration pulses that were curvilinearly related to the increase in root‐zone water content. Coupled with the low leaf area and high root : shoot ratio, this sparse grassland had a limited capacity to sequester and store carbon. Assuming a proportionality between carbon gain and rainfall during the summer, rainfall distribution statistics suggest that the ecosystem is sustainable in the long term.