A multi-scale analysis of a terrestrial carbon budget - Is New Zealand a source or sink of carbon?

Interest in national carbon (C) budgets has increased following the signing of the Kyoto Protocol as countries begin to develop source/sink C inventories. In this study, specific-site measurements, regional databases, satellite observations, and models were used to test the hypothesis that New Zealand's terrestrial ecosystems are C neutral because C uptake by planted forests and scrub is roughly balanced by C losses from indigenous forests and soils. Net ecosystem C balance was estimated from the difference between net primary production (NPP) and heterotrophic soil respiration The productivity portion of the CASA model and NOAA-AVHRR imagery were used to estimate national NPP (128 +/- 14 Mt C per year). Main sources of uncertainty were the coarse spatial scale (1 x 1 km(2) grid cells), and the general lack of information on photosynthetically active radiation, light-use efficiency, and below-ground C allocation for the major vegetation types: indigenous and exotic forests, scrub, and grasslands (improved, unimproved and tussock). Total soil CO2-C production predicted from an Arrhenius-type function coupled to climate and land-cover data was 380 +/- 30 Mt C per year, suggesting that New Zealand's terrestrial ecosystems may be either (a) a net source of atmospheric CO2 or (b) roughly in C balance if ca. 252 Mt CO2-C per year (66%) can be attributed to roots. Soil moisture limitations on respiration were small, reducing the national value to 365 +/- 28 Mt C per year. Differences between NPP and heterotrophic soil respiration were -29 Mt C per year for improved pastures, -8 Mt C per year for indigenous forests, and +4 Mt C per year for planted forests; the large negative value for improved grasslands may be due to under-estimation of NPP and root respiration. Soil C losses to coastal waters, as estimated from a consideration of all the major erosion processes, were ca. 3-11 Mt C per year. These national-scale estimates of ecosystem C balance were in general agreement with those based on plot-scale data for some major ecosystems including planted forests (4 hit C per year vs 3.7 Mt C per year, respectively) and indigenous forest (-8 Mt C per year vs ca. -2.8 Mt C per year, respectively). Poor agreement for forest regenerating after land abandonment (-17 hit C per year vs +3 hit C per year) was probably due to an underestimate of NPP at the national scale, Overall, the results suggest that New Zealand is a net C source, despite the fact that some ecosystems are accumulating C. For some land-use types, using the balance between NPP and soil respiration at the national scale to estimate the net ecosystem C balance may be too coarse, and studies of land-use changes at finer spatial scales are needed to reduce uncertainties in national-scale C balance estimates. (C) 2000 Elsevier Science B.V. All rights reserved.