Climate change amplifies gross nitrogen turnover in montane grasslands of Central Europe in both summer and winter seasons

Abstract

The carbon- and nitrogen-rich soils of montane grasslands are exposed to above-average warming and to altered precipitation patterns as a result of global change. To investigate the consequences of climatic change for soil nitrogen turnover, we translocated intact plant-soil mesocosms along an elevational gradient, resulting in an increase of the mean annual temperature by approx. 2 °C while decreasing precipitation from approx. 1500 to 1000 mm. Following three years of equilibration, we monitored the dynamics of gross nitrogen turnover and ammonia-oxidizing bacteria (AOB) and archaea (AOA) in soils over an entire year. Gross nitrogen turnover and gene levels of AOB and AOA showed pronounced seasonal dynamics. Both summer and winter periods equally contributed to cumulative annual N turnover. However, highest gross N turnover and abundance of ammonia oxidizers were observed in frozen soil of the climate change site, likely due to physical liberation of organic substrates and their rapid turnover in the unfrozen soil water film. This effect was not observed at the control site, where soil freezing did not occur due to a significant insulating snowpack. Climate change conditions accelerated gross nitrogen mineralization by 250% on average. Increased N mineralization significantly stimulated gross nitrification by AOB rather than by AOA. However, climate change impacts were restricted to the 2-6 cm topsoil and rarely occurred at 12-16 cm depth, where generally much lower N turnover was observed. Our study shows that significant mineralization pulses occur under changing climate, which is likely to result in soil organic matter losses with their associated negative impacts on key soil functions. We also show that N cycling processes in frozen soil can be hot moments for N turnover and thus are of paramount importance for understanding seasonal patterns, annual sum of N turnover and possible climate change feedbacks.

Soil gross N ammonification and nitrification from tropical to temperate forests in eastern China

“…Less favourable climatic conditions (precipitation and temperature) have been shown to contribute to increased or decreased soil gross N ammonification (Baldos et al., ; Wang, Chen, et al., ). Although we identified significant relationships between actual soil temperature or moisture and GA and GN rates in primary and secondary forest soils along the transect, we were unable to detect a general climate effect.…”

“…Increases in labile organic C and N concentrations resulting from the lysis of dead microbial cells could increase gross N ammonification rates by increasing SM content (Borken & Matzner, ). Previous studies have reported that the relationship between SM content and N ammonification differs in different soils and that dry soil has an adverse on N ammonification (Paul et al., ; Rosenkranz et al., ) as substrate diffusion becomes limiting (Wang, Chen, et al., ). Drying below a certain threshold, which is soil texture‐dependent, generally decreases microbial activity, as can be observed by a decrease in heterotrophic respiration under lower SM content (Cheng, Wang, et al., ).…”

“…Detailed information on these procedures can be found in Wang, Chen, et al. (). There were two paired soil cores for quantifying GA and GN rates, which were labelled with 15 NH 4 + and 15 NO 3 − , respectively.…”

“…Two soil cores were labelled with 15 N–(NH 4 ) 2 SO 4 solution for the quantification of GA rate, and another two soil cores were labelled with 15 N–KNO 3 for determining GN rate. We used a multi‐needle injector system consisting of 10 simultaneously operated syringes with custom‐made side‐port cannulas to achieve reproducible homogenous distribution of 15 N label in the soil columns (described in detail by Wang, Chen, et al., ). In total, 2 mg N/kg dry soil of label was added, with a 15 N enrichment of 30 atom%.…”

Nitrogen mineralization, not N<sub>2</sub> fixation, alleviates progressive nitrogen limitation – Comment on “Processes regulating progressive nitrogen limitation under elevated carbon dioxide: a meta-analysis” by Liang et al. (2016)

“…Annual NO Liang et al (2016) combined lead to an increase in N availability of at most 4 kg N ha −1 yr −1 . Only few studies have reported annual gross N mineralization rates, which range between 120 and 450 kg N ha −1 yr −1 for temperate grasslands (Jamieson et al, 1999;Wang et al, 2016) and 800-1000 kg N ha −1 yr −1 in a Norway spruce forest (Rosenkranz et al, 2010). Assuming a 14 % increase in this rate under elevated CO 2 (as found for N-limited ecosystems by Rütting and Andresen, 2015), an additional amount of plant-available N of 17 to 140 kg N ha −1 yr −1 will be produced annually.…”