Litter decomposition and C and N dynamics as affected by N additions in a semi-arid temperate steppe, Inner Mongolia of China

Litter decomposition is the fundamental process in nutrient cycling and soil carbon（C） sequestration in terrestrial ecosystems. The global-wide increase in nitrogen（N） inputs is expected to alter litter decomposition and,ultimately, affect ecosystem C storage and nutrient status. Temperate grassland ecosystems in China are usually N-deficient and particularly sensitive to the changes in exogenous N additions. In this paper, we conducted a 1,200-day in situ experiment in a typical semi-arid temperate steppe in Inner Mongolia to investigate the litter decomposition as well as the dynamics of litter C and N concentrations under three N addition levels（low N with 50 kg N/（hm2？a）（LN）, medium N with 100 kg N/（hm2？a）（MN）, and high N with 200 kg N/（hm2？a）（HN）） and three N addition forms（ammonium-N-based with 100 kg N/（hm2？a） as ammonium sulfate（AS）, nitrate-N-based with 100 kg N/（hm2？a） as sodium nitrate（SN）, and mixed-N-based with 100 kg N/（hm2？a） as calcium ammonium nitrate（CAN）） compared to control with no N addition（CK）. The results indicated that the litter mass remaining in all N treatments exhibited a similar decomposition pattern： fast decomposition within the initial 120 days, followed by a relatively slow decomposition in the remaining observation period（120–1,200 days）. The decomposition pattern in each treatment was fitted well in two split-phase models, namely, a single exponential decay model in phase I（〈398 days） and a linear decay function in phase II（≥398 days）. The three N addition levels exerted insignificant effects on litter decomposition in the early stages（〈398 days, phase I; P〉0.05）. However, MN and HN treatments inhibited litter mass loss after 398 and 746 days, respectively（P〈0.05）. AS and SN treatments exerted similar effects on litter mass remaining during the entire decomposition period（P〉0.05）. The effects of these two N addition forms differed greatly from those of CAN after 746 and 1,053 days, respectively（P〈0.05）. During the decomposition period, N concentrations in the decomposing litter increased whereas C concentrations decreased, which also led to an exponential decrease in litter C：N ratios in all treatments. No significant effects were induced by N addition levels and forms on litter C and N concentrations（P〉0.05）. Our results indicated that exogenous N additions could exhibit neutral or inhibitory effects on litter decomposition, and the inhibitory effects of N additions on litter decomposition in the final decay stages are not caused by the changes in the chemical qualities of the litter, such as endogenous N and C concentrations. These results will provide an important data basis for the simulation and prediction of C cycle processes in future N-deposition scenarios.

Effects of water and nitrogen addition on vegetation carbon pools in a semi-arid temperate steppe

Global change will lead to increases in regional precipitation and nitrogen（N） deposition in the semi-arid grasslands of northern China. We investigated the responses of vegetation carbon（C） pools to simulated precipitation and N deposition increases through field experiments in a typical steppe in Inner Mongolia. The treatments included NH4NO3 addition at concentrations of 0（CK）, 5（LN, low nitrogen）, 10（middle nitrogen, MN）, and 20（HN,high nitrogen）（g m^（-2）a^（-1）） with and without water. After three consecutive years of treatment, from 2010 to 2012,water addition did not significantly change the size of the total vegetation C pools, but it significantly decreased the ratio of root：shoot（R：S）（P = 0.05） relative to controls. By contrast, N addition significantly increased the total vegetation C pools. The C pools in the LN, MN and HN treatments increased by 22, 39 and 44 %, respectively. MN produced the largest effect among the N concentrations,although differences between N-added treatments were not significant（P ？ 0.05）. N addition significantly reduced the ratio of root：shoot（R：S）（P = 0.03）. However, there were no significant interactive effects of water and N addition on the vegetation C pools.

Environmental benefits and farmers' adoption of winter cover crops in the North China Plain

The introduction of cover crops into monoculture systems to improve soil health has been widely adopted worldwide. However, little is known about the environmental risks and application prospects of different cover crops in spring maize(Zea mays L.) monocultures proposed in the North China Plain. A pot experiment was conducted to evaluate the effects of different winter cover crops on subsequent maize yield, soil fertility, and environmental risks of nitrogen(N) loss, and a questionnaire survey was conducted to examine factors influencing farmers' willingness to adopt cover crops in the North China Plain. Based on the same fertilization regime during the maize growing period, four winter cover crop treatments were set up, including bare fallow, hairy vetch(Vicia villosa Roth.), February orchid(Orychophragmus violaceus), and winter oilseed rape(Brassica campestris L.). The results indicated that winter cover crops significantly increased subsequent maize yield and soil organic carbon, total N, and microbial biomass carbon and N compared with the bare fallow treatment.The incorporation of cover crops led to a negligible increase in nitrous oxide(N_(2)O) emissions and had a very limited effect on ammonia(NH_(3)) emissions.The incorporation of February orchid and winter oilseed rape decreased nitrate leaching compared with the hairy vetch treatment in the maize growing season.The N losses via N_(2)O and NH_(3) emissions and N leaching accounted for 71%–84% of the N surplus. However, yield increase and environmental benefits were not the main positive factors for farmers to accept cover crops. Financial incentive was rated by 83.9% of farmers as an “extremely important” factor, followed by other costs, when considering winter cover cropping. These results indicate that the environmental benefits depend on the type of cover crop. Maintaining high levels of soil fertility and maize yield, providing sufficient subsidies, and encouraging large-area cultivation of cover crops are critical measures to promote winter cover cropping in the North China Plain.

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

High nitrous oxide （N20） emissions during freeze-thawing period （FFP） have been observed in many different ecosystems. However, the knowledge about the dynamic of soil N20 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 （0,10 g N/（m2-year）） during growing season （marked as WON0, WISN0, WONI0, WISNI0, respectively） to explore the effects of water and N background on soil N20 emissions during FTPs and the relationship between soil N20 emissions and environmental factors. The results indicated that water and N treatments conducted during growing season did not show significant effect on the N20 effluxes of FTP, but the soil mineral N contents of WONI0 treatment were significantly higher than those of WON0, WI5N0, WI5NI0 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 N20 emissions of 2015S-FTP were higher than those of 2014W-FFP. The correlations between soil N20 effluxes and most of the measured environmental factors were insignificant, multiple stepwise regression analysis indicated that the soil temperature, soil NH$-N content and air temperature were the major environmental factors which significantly influenced the N20 effluxes during 2014W-FTP, and air temperature and son water content were the significant influencing factors during 2015S-FTP.