Seasonality of soil microbial nitrogen turnover in continental steppe soils of Inner Mongolia

Wu, H
Dannenmann, M
Wolf, B
Han, X G
Zheng, X
Butterbach-Bahl, K
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Annual estimates and seasonal patterns of gross nitrogen turnover in terrestrial soils are poorly understood due to the lack of experimental evidence. Based on year round sampling in wintergrazed and ungrazed steppe soils of Inner Mongolia, we show that measurements of net rates of ammonification (-9 to - 6 kg N/ ha/year) or nitrification (19 to 31 kg N/ ha/year) do not at all reflect the pronounced dynamics of gross rates of ammonification (215 - 240 kg N / ha / year) or nitrification (362 - 417 kg N / ha / year). Four different seasons with characteristic functional patterns of N turnover were identified: (1) Growing season dynamics as characterized by drying/rewetting cycles and negatively correlated temporal courses of net microbial growth and periods with intensive gross ammonification, contributing 40 - 52% and 29 - 32% to cumulative annual gross ammonification and nitrification, respectively. Net N mineralization was almost exclusively observed during the growing season. (2) Microbial N dynamics during the autumn freeze-thaw period was characterized by a sharp decline in microbial biomass in conjunction with a peak of gross nitrification contributing 19 - 36% to cumulative annual fluxes. (3) During winter at constantly frozen soil, a net build-up of microbial biomass was observed, whereas gross N turnover rates were low, contributing 7 - 10% and 6 - 11% to cumulative annual gross ammonification or gross nitrification, respectively. (4) The spring freeze-thaw period showed extremely dynamic changes in gross N turnover and soil nitrate concentrations. This period contributed 34 - 44% and 21 - 46% to cumulative annual gross ammonification and nitrification, respectively. This study highlights that freeze-thaw cycles are key periods for understanding patterns and magnitudes of gross N turnover in semi-arid continental steppe ecosystems. The results further imply that the observed patterns of microbial biomass and gross N turnover dynamics are likely the consequence of a seasonal succession of microbial communities and turnover of microbial biomass. Our findings emphasize the necessity for high resolution studies on gross N turnover as a prerequisite to infer functioning and annual budgets of ecosystem N cycling.
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Grassland Society of Southern Africa

The Grassland Society of Southern Africa (GSSA) is involved and concerned with the science and practice of range and pasture management. This broad field involves primarily the use and conservation of natural resources. It encompasses applied fields such as livestock production, wildlife management, nature conservation, water catchment management and range and mine-dump rehabilitation. The disciplines include, amongst others, ecology, botany zoology, range and pasture science, animal science, soil science and genetics. This collection includes journal articles from the African Journal of Range and Forage Science as well as related articles and reports from throughout the Southern African region.