Differential responses of short-term soil respiration dynamics to the experimental addition of nitrogen and water in the temperate semi-arid steppe of Inner Mongolia, China

Stimulation of soil respiration by elevated CO2 is enhanced under nitrogen limitation in a decade-long grassland study

Whether and how CO₂ and nitrogen (N) availability interact to influence carbon (C) cycling processes such as soil respiration remains a question of considerable uncertainty in projecting future C-climate feedbacks, which are strongly influenced by multiple global change drivers, including elevated atmospheric CO₂ concentrations (eCO₂) and increased N deposition. However, because decades of research on the responses of ecosystems to eCO₂ and N enrichment have been done largely independently, their interactive effects on soil respiratory CO₂ efflux remain unresolved. Here, we show that in a multifactor free-air CO₂ enrichment experiment, BioCON (Biodiversity, CO₂, and N deposition) in Minnesota, the positive response of soil respiration to eCO₂ gradually strengthened at ambient (low) N supply but not enriched (high) N supply for the 12-y experimental period from 1998 to 2009. In contrast to earlier years, eCO₂ stimulated soil respiration twice as much at low than at high N supply from 2006 to 2009. In parallel, microbial C degradation genes were significantly boosted by eCO₂ at low but not high N supply. Incorporating those functional genes into a coupled C-N ecosystem model reduced model parameter uncertainty and improved the projections of the effects of different CO₂ and N levels on soil respiration. If our observed results generalize to other ecosystems, they imply widely positive effects of eCO₂ on soil respiration even in infertile systems.

Managed grassland alters soil N dynamics and N2O emissions in temperate steppe

Reclamation of degraded grasslands as managed grasslands has been increasingly accelerated in recent years in China. Land use change affects soil nitrogen (N) dynamics and nitrous oxide (N₂O) emissions. However, it remains unclear how large-scale grassland reclamation will impact the grassland ecosystem as a whole. Here, we investigated the effects of the conversion from native to managed grasslands on soil N dynamics and N2O emissions by field experiments in Hulunber in northern China. Soil (0-10cm), nitrate (NO₃^-), ammonium (NH₄⁺), and microbial N were measured in plots in a temperate steppe (Leymus chinensis grassland) and two managed grasslands (Medicago sativa and Bromus inermis grasslands) in 2011 and 2012. The results showed conversion of L. chinensis grassland to M. sativa or B. inermis grasslands decreased concentrations of NO₃^--N, but did not change NH₄⁺-N. Soil microbial N was slightly decreased by the conversion of L. chinensis grassland to M. sativa, but increased by the conversion to B. inermis. The conversion of L. chinensis grassland to M. sativa (i.e., a legume grass) increased N₂O emissions by 26.2%, while the conversion to the B. inermis (i.e., a non-legume grass) reduced N₂O emissions by 33.1%. The conversion from native to managed grasslands caused large created variations in soil NO₃^--N and NH₄⁺-N concentrations. Net N mineralization rates did not change significantly in growing season or vegetation type, but to net nitrification rate. These results provide evidence on how reclamation may impact the grassland ecosystem in terms of N dynamics and N₂O emissions.

Effects and mechanism of freeze-thawing cycles on the soil N2O fluxes in the temperate semi-arid steppe

High nitrous oxide (N₂O) emissions during freeze-thawing period (FTP) have been observed in many different ecosystems. However, the knowledge about the dynamic of soil N₂O emissions and its main driving mechanism during the freeze-thawing processes in grassland ecosystem is still limited. An in-situ experiment was conducted during the FTP on the sites with 0 and 15% surplus of the average rainfall and two levels of N addition (0,10gN/(m²·year)) during growing season (marked as W0N0, W15N0, W0N10, W15N10, respectively) to explore the effects of water and N background on soil N₂O emissions during FTPs and the relationship between soil N₂O emissions and environmental factors. The results indicated that water and N treatments conducted during growing season did not show significant effect on the N₂O effluxes of FTP, but the soil mineral N contents of W0N10 treatment were significantly higher than those of W0N0, W15N0, W15N10 treatments (p<0.05). The soil PLFA concentrations of microbial groups monitored during 2015 spring freeze-thawing period (2015S-FTP) were lower than those during winter freeze-thawing period of 2014 (2014W-FTP), while cumulative soil N₂O emissions of 2015S-FTP were higher than those of 2014W-FTP. The correlations between soil N₂O effluxes and most of the measured environmental factors were insignificant, multiple stepwise regression analysis indicated that the soil temperature, soil NH₄⁺-N content and air temperature were the major environmental factors which significantly influenced the N₂O effluxes during 2014W-FTP, and air temperature and soil water content were the significant influencing factors during 2015S-FTP.

Divergent Effects of Nitrogen Addition on Soil Respiration in a Semiarid Grassland

Nitrogen (N) deposition has been steadily increasing for decades, with consequences for soil respiration. However, we have a limited understanding of how soil respiration responds to N availability. Here, we investigated the soil respiration responses to low and high levels of N addition (0.4 mol N m(-2) yr(-1) vs 1.6 mol N m(-2) yr(-1)) over a two-year period in a semiarid Leymus chinensis grassland in Inner Mongolia, China. Our results show that low-level N addition increased soil respiration, plant belowground biomass and soil microbial biomass carbon (MBC), while high-level N additions decreased them. Soil respiration was positively correlated with plant belowground biomass, MBC, soil temperature and soil moisture. Together plant belowground biomass and MBC explained 99.4% of variation in mean soil respiration, with plant belowground biomass explaining 63.4% of the variation and soil MBC explaining the remaining 36%. Finally, the temperature sensitivity of soil respiration was not influenced by N additions. Overall, our results suggest that low levels of N deposition may stimulate soil respiration, but large increases in N availability may decrease soil respiration, and that these responses are driven by the dissimilar responses of both plant belowground biomass and soil MBC.