Abstract
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−1 year−1) or nitrification (19 to 31 kg N ha−1 year−1) do not at all reflect the pronounced dynamics of gross rates of ammonification (215–240 kg N ha−1 year−1) or nitrification (362–417 kg N ha−1 year−1).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.
Highlights
Biogeochemical nitrogen (N) cycling in terrestrial ecosystems is complex, since it comprises many players ranging from microorganisms to higher plants performing and involving a wide range of processes with very different magnitudes of N turnover (Schimel and Bennett 2004, Booth et al 2005, Kreutzer et al 2009, Rennenberg et al 2009, Butterbach-Bahl et al 2011)
(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
Temporal dynamics of N turnover With a hitherto not realized temporal resolution of mostly sub-monthly measurements (8–38 day intervals) over a period of 14 months we followed the temporal dynamics of soil gross and net microbial N turnover in typical steppe soils of Inner Mongolia
Summary
Biogeochemical nitrogen (N) cycling in terrestrial ecosystems is complex, since it comprises many players ranging from microorganisms to higher plants performing and involving a wide range of processes with very different magnitudes of N turnover (Schimel and Bennett 2004, Booth et al 2005, Kreutzer et al 2009, Rennenberg et al 2009, Butterbach-Bahl et al 2011). The terrestrial N cycle is dominated by the soil microbial N turnover processes of ammonification (the conversion of organic N compounds to ammonium), nitrification (conversion of organic N or ammonium to nitrate), and the subsequent allocation of bioavailable N such as ammonium and nitrate to plants and microorganisms as well as to N loss pathways. Soil nitrogen cycling regulates plant productivity, and ecosystem N retention and loss. The current state of knowledge on N cycling may correspond to the state of knowledge on the carbon (C) cycle several decades ago (Schlesinger 2009)
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