Seven years of carbon dioxide exchange over a degraded grassland and a cropland with maize ecosystems in a semiarid area of China

Abstract

Based on eddy covariance measurement over a degraded grassland and a cropland with maize (Zea mays) ecosystems from 2003 to 2009, carbon exchange processes and their responses to environmental factors in different temporal scales were analyzed in semiarid of China. Accounting for carbon export and import, NBP (net biome production) of cropland with maize ranged from 54.3 to 100.6g Cm−2yr−1. NBP remained positive indicating a carbon net loss from this ecosystem although NEE (net ecosystem exchange) was negative in most of years. Due to negligible carbon import and export, NBP of degraded grassland ecosystem was equal to NEE, with an average value of 138.4g Cm−2yr−1. The grassland ecosystem behaved as carbon source during the whole period. PPFD (incident photosynthetic active radiation) was the main driver for diurnal variation of NEE during growing season in most years. NDVI (normal difference vegetation index) was in accordance with seasonal patterns of NEE especially for cropland with maize ecosystem. Soil temperature at a depth of 5cm was also a main driver for seasonal variation of NEE at the degraded grassland ecosystem in normal precipitation years (2003 and 2005). Annual peak NDVI (NDVImax) was significantly correlated with annual NEE and GPP (gross primary productivity). The amount of growing season precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event (>1mmday−1) was associated with increasing annual carbon uptake. Drought in the early growing period is more critical to carbon dynamics of degraded grassland ecosystem while drought in the middle of growing season was more critical for cropland with maize ecosystem.