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Soil respiration (Rs), as a key process of carbon cycle in terrestrial ecosystems, has a direct impact on atmospheric CO2 concentration. How Rs responds to global change factors, such as rainfall changes and increased N deposition, has become a hot and difficult issue in the field of global change. Compared with the responses of Rs to the single factor of rainfall changes or increased N deposition, studying the response of Rs to the interaction of these two factors is more in line with natural environment, which can predict the future changes of soil carbon emission more accurately. At present, the related researches focused on different terrestrial ecosystems all over the world, and revealed the response mechanism from three aspects: soil, microorganism, and plant. Here, the research progress of soil respiration in response to the interaction of rainfall changes and increased N deposition in different terrestrial ecosystems was reviewed from the aspects of Rs and its components, factors related with soil properties, microorganisms and plant, and the deficiencies of current researches, and the research direction to be strengthened in the future were pointed out. Our review would provide a reference for further understanding the response law and the mechanism of soil respiration to the interaction between rainfall changes and increased N deposition.

[Effects of Short-term Exogenous Nitrogen and Carbon Input on Soil Respiration Under Changing Precipitation Pattern].

The responses of soil respiration to exogenous carbon (C) and nitrogen (N) inputs under changing precipitation patterns were explored via in-situ field experiments. In 2014, a typical temperate grassland on the Xilin River of Inner Mongolia was taken as the research site, and soil respiration was measured in the following treatments:addition of water alone (CK), addition of water + N fertilizer[CN, 2.5 g·(m2·a)-1], addition of water + labile C[CG, 24 g·(m2·a)-1], and addition of water + N fertilizer+ labile C[CNG, 2.5 g·(m2·a)-1+24 g·(m2·a) -1], and the correlations of soil respiration with soil temperature, soil moisture, soil dissolved organic C (DOC), and soil microbial biomass C (MBC) were analyzed. During the first water application event (FWE) with the frequency of natural precipitation, cumulative CO2 efflux over 168 hours significantly increased in the CG and CNG treatments, whereas there was no such change in the CN treatment. In addition, soil MBC contents in the CG and CNG treatments were significantly higher than that in the CK and CN treatments, and the correlation of average soil respiration rate with soil MBC content among these treatments was positively significant (P<0.05). In contrast with during the FWE, cumulative CO2 efflux over 168 hours and soil MBC content significantly decreased during the second water application event (SWE) with no natural precipitation (P<0.05), whereas soil DOC content significantly increased (P<0.05). The cumulative CO2 efflux over 168 hours significantly decreases in the CN and CG treatments (P<0.05).During both the water application events, soil respiration rate had a positive relationship with soil temperature and soil volume water content (P<0.05). Therefore, it is proposed that the distribution of natural precipitation influences soil water content, which controls the effects of exogenous C and N on soil respiration in semiarid grassland ecosystems.

[Effect of Irrigation Patterns on Soil CO2 and N2O Emissions from Winter Wheat Field in North China Plain].

The water-saving irrigation is the trend of modernized agriculture. This paper aimed to study the effect of water-saving irrigation on soil CO2 and N2O emissions. The field experiments were conducted under micro sprinkler irrigation of integrated water and fertilizer (MSI) and conventional flooding irrigation (FI) in winter wheat growth season in the west of North China Plain during 2013- 2014 using the static chamber method. This paper analyzed the seasonal variation of soil CO2and N2O emissions under MSI and FI, and then compared the soil CO2 and N2O emissions from treatments located in different vertical distance away from micro sprinkler pipe. Root exclusion was used to estimate the components of soil respiration and agricultural carbon sequestration intensity under MSI and FI in winter wheat field. The results indicated that: (1) The average soil CO2 emissions under MSI and FI were 418.19 mg (m² · h)⁻¹ and 372.14 mg · (m² · h)⁻¹ respectively with no significant difference, and cumulative CO2 emissions under MSI and FI were 2 150.6 g · m⁻² and 1 904.6 g · m⁻², respectively. (2) During returning green stage to harvest stage of winter wheat, the highest soil CO2 cumulative emissions were found at the closest site to the micro sprinkler irrigated pipes under MSI. However, there were no significant differences among spatial treatments. (3) Under MSI and FI, soil heterotrophic respiration (C) was 468.49 g · m⁻² and 427.31 g · m⁻², and the net primary productivity (3) was 1988.21 g · m⁻² and 1770.54 g · m⁻²; the carbon sink (C) during winter wheat growing season was 1 519.72 g · m⁻² and 1 343.24 g · m⁻², respectively. (4) The average N2O emissions under MSI and FI were 50.77 µg · (m² · h)⁻¹ and 28.81 µg · (m² · h)⁻¹ respectively with no significant difference. Cumulative N2O emission under MSI and FI was 272.67 mg · m⁻² and 154.08 mg · m⁻², respectively. (5) During returning green stage to harvest stage of winter wheat, the farther the distance away from the micro sprinkler irrigated pipes, the smaller the soil N2O emissions. Moreover, there were no significant differences among sptial treatment under MSI. Therefore, despite of the increase of soil CO2and N2O emissions, the intensity of carbon sink increased during the transformation from traditional flood irrigation to micro sprinkler irrigation in winter wheat fields.

[Responses of ecosystem carbon budget to increasing nitrogen deposition in differently degraded Leymus chinensis steppes in Inner Mongolia, China].

Based on a field manipulative nitrogen (N) addition experiment, the effects of atmospheric N deposition level change on the plant biomass and net primary productivity (NPP), soil respiration (Rs) and net ecosystem exchange (NEE) were investigated respectively in 2009 and 2010 in two differently degraded Leymus chinensis steppes in Inner Mongolia of China, and the difference in the response of NEE to equal amount of N addition [10 g x (M2 x a)(-1), MN] between the two steppes was also discussed. The results indicated that for the light degraded Leymus chinensis steppe (site A) , the average plant aboveground biomass (AGB) in MN treatment were 21.5% and 46.8% higher than those of CK in these two years. But for the moderate degraded Leymus chinensis steppe (site B), the N addition decreased the plant AGB and ANPP in 2009, while showed positive effects in 2010. N addition increased the belowground biomass (BGB) of the both sites and belowground NPP (BNPP) of site B in both years, but decreased the BNPP of site A in 2010. The increase of N input in the two steppes did not change the seasonal variation of Rs. The cumulative annual soil C emissions in MN treatment in site A showed an increase of about 14.6% and 25.7% of those in the CK respectively for these two years, while were decreased by about 10.4% and 11.3%, respectively in site B. The NEE of MN treatments, expressed by C, for the two steppes were 59.22 g x (m2 x a)(1) and 166.68 g x (m2 x a)(-1), as well as 83.27 g x (m2 x a)(-1) and 117.47 g x (m2 x a)(-1), respectively in these two years. The increments in NEE originated from N addition for these two years were 15.79 g x (M2 x a)(-1) and 82.94 g x (M2 x a)(-1) in site A and 74.54 g x (M2 x a)(-1) and 101.23 g x (M2 x a)(-1) in site B. The N input per unit could obtain greater C sink effect in the steppe with lower initial N level.

[Effects and mechanism of freeze-thawing cycles on key processes of nitrogen cycle in terrestrial ecosystem].

As a widespread natural phenomenon in the soil of middle and high latitude as well as high altitude, freeze-thawing cycles have a great influence on the nitrogen cycle of terrestrial ecosystem in non-growing season. Freeze-thawing cycles can alter the physicochemical and biological properties of the soil, which thereby affect the migration and transformation of soil nitrogen. The impacts of freeze-thawing cycles on key processes of nitrogen cycle in terrestrial ecosystem found in available studies remain inconsistent, the mechanism is still not clear, and the research methods also need to be further explored and innovated. So it is necessary to sum up and analyze the existing achievements in order to better understand the processes of soil nitrogen cycle subjected to freeze-thawing cycles. This paper reviewed the research progress in China and abroad about the effects and mechanisms of freeze-thawing cycles on key processes of nitrogen cycle in terrestrial ecosystem, including mineralization, immobilization, nitrification and denitrification, N leakage and gaseous loss, and analyzed the deficiencies of extant research. The possible key research topics that should be urgently paid more attention to in the future were also discussed.

[Reponses of soil total organic carbon and dissolved organic carbon to simulated nitrogen deposition in temperate typical steppe in Inner Mongolia, China].

Based on a field manipulative nitrogen (N) addition experiment, the effects of atmospheric N deposition level change on the contents, inter-annual variation and profile distribution of soil total organic carbon (TOC) and dissolved organic carbon (DOC) were investigated from May, 2008 to October, 2011 in a temperate typical steppe in Inner Mongolia of China, and the relationship between TOC and DOC was also discussed. The treatments in the manipulative experiment included N additions at rates of 0, 5, 10, and 20 g x (m2 x a)(-1), representing the control (CK), low N (LN), medium N (MN), and high N (HN) treatment, respectively. The results indicated that the concentrations of soil TOC and DOC decreased progressively with soil depth in all cases except for the DOC at 10-20 cm depth in individual years. The increase of N input in typical steppe did not change the vertical distribution of soil TOC and DOC, but reduced the vertical variation of TOC and increased the vertical variation of DOC in the surface soil horizon. In addition, the contents of soil TOC and DOC at 0- 10 cm and 10- 20 cm soil layers changed insignificantly after the continuous increase in anthropogenic N input for four years. The soil organic C density of 0-20 cm soil layer for different N treatment levels varied between 3.9 kg x m(-2) and 5.6 kg x m(-2), and the soil organic C densities of fertilized treatments in the first two years were similar to or slightly lower than those of CK, while in the following two years, the increase in N deposition gradually played a positive role in increasing soil organic C density, but the differences in soil TOC and DOC contents between CK and fertilized plots were not significant (P > 0.05). The ratio of soil DOC to TOC (DOC/TOC) varied from 0.32% to 1.09%. The increase in N deposition generally lowered the proportion of DOC in soil TOC, which was conducive to the accumulation of soil organic C. The change of soil DOC was positively correlated with that of TOC (P < 0.01). The temporal variations of soil DOC in different N treatments were all far greater than those of TOC, and the soil DOC was the important sensitive indicator for predicting and evaluating the response of soil C pool to the change in atmospheric N deposition in the temperate grassland ecosystem.

[Microbial response mechanism for drying and rewetting effect on soil respiration in grassland ecosystem: a review].

As one of the most important and wide distribution community type among terrestrial ecosystems, grassland ecosystem plays a critical role in the global carbon cycles and climate regulation. China has extremely rich grassland resources, which have a huge carbon sequestration potential and are an important part of the global carbon cycle. Drying and rewetting is a common natural phenomenon in soil, which might accelerate soil carbon mineralization process, increase soil respiration and exert profound influence on microbial activity and community structure. Under the background of the global change, the changes in rainfall capacity, strength and frequency would inevitably affect soil drying and wetting cycles, and thus change the microbial activity and community structure as well as soil respiration, and then exert important influence on global carbon budget. In this paper, related references in recent ten years were reviewed. The source of soil released, the trend of soil respiration over time and the relationship between soil respiration and microbial biomass, microbial activity and microbial community structure during the processes of dry-rewetting cycle were analyzed and summarized, in order to better understand the microbial response mechanism for drying and rewetting effecting on soil respiration in grassland ecosystem, and provide a certain theoretical basis for more accurate evaluation and prediction of future global carbon balance of terrestrial ecosystems and climate change.

[Soil net nitrogen mineralization in the typical temperate grassland].

Soil net nitrogen mineralization rate of three types temperate grassland in Inner Mongolia, China was studied using the resin-core technique. The major results include: the net nitrogen mineralization rate of the Stipa baicalensis meadow grassland, the Aneulolepidum Chinense grassland and the Stipa krylovii grassland were 0.333 kg x (hm2 x d)(-1), 0.316 kg x (hm2 x d)(-1) and 0.211 kg x (hm2 x d)(-1) respectively during the field incubation period of July to October, 2005; Soil net nitrogen mineralization accumulation and rates was remarkably different between phase incubation and continuous incubation during the same field incubation period; Rainfall was one of the main factors affecting nitrogen mineralization. The net nitrogen mineralization rate was correlated with the change of soil moisture in all of the measured sites, the correlation coefficient was 0.80, 0.61 and 0.56.

[Study of distinguish root respiration from total soil respiration by root exclusion method in the temperate semi-arid grassland in Inner Mongolia, China].

In order to determine the possibility of measuring the root respiration by using root exclusion method, we conducted field experiments to measured total soil respiration and net soil respiration in three temperate grassland communities including Leymus chinensis free-grazed steppe, stipa grandis free-grazed steppe and Leymus chinensis degenerated free-grazed steppe in the growing season, 2005. At the same time, the proportion of the root respiration to the total soil respiration was estimated in different steppes by this method separately. The results indicated that the root exclusion method was very operable in estimating the proportion of the root respiration to the total soil respiration in temperate grassland communities, and the proportion varied from 25% - 45% with the mean value of 35.66%. The precision of this proportion was markedly improved compared with the correlation studies of both here and abroad, and it was provided with superior applied cost in the field of carbon cycle of grassland ecosystem.

[Study on the free-grazed steppes root respiration of growing season in the Xilin River Basin, China].

In order to determine the proportion of the root respiration to the total soil respiration, field experiments were condusted to measured soil respiration in three temperate grassland communities including Leymus chinensis free-grazed steppe, stipa grandis free-grazed steppe and Leymus chinensis degenerated free-grazed steppe using root biomass ratiocination method in the growing season, 2005. At the same time, seasonal variation including root respiration, total soil respiration and proportion of the root respiration to the total soil respiration in different grassland communities were compared, and the influence of root biomass, water and temperature to these processes were expounded. The results indicated that the rule of the root respiration and total soil respiration in the three steps was a one-peak pattern with the season dynamic, the maximum rate was in the late of July, but in Leymus chinensis degenerated free-grazed steppe, the rate was obviously larger than the other two types between July and August; The proportion of the root respiration to the total soil respiration obviously changed in the growing season, and the proportion was varied from 40% - 50 % with the mean value of 40.3 % , but it was very different among three steps because of the regional differentiations.

Emission characteristics of carbon dioxide in the semiarid Stipa grandis steppe in Inner Mongolia, China.

Using the static opaque chamber method, the soil respiration rates (SR) were measured through the continuous experiments in situ in semiarid Stipa grandis steppe in Xilin River Basin of Inner Mongolia, China from June 2001 to June 2003, in parallel, the difference between the SR and the ecosystem respiration rates (TER) were compared. The results indicated that the seasonal variations of the SR and TER were obvious with higher emissions in growing season and a relatively low efflux level in non-growing season, furthermore, the negative effluxes were found in the observation site in winter; the annual CO2 efflux of total ecosystem ranged from 160.5 gC/(m2 x a) to 162.8 gC/(m2 x a) and that of soil ranged from 118.7 gC/(m2 x a) to 152.3 gC/(m2 x a). The annual SR accounted for about 74.0% to 93.5% of the annual TER, but the results of Analysis of Variance (ANOVA) indicated that the difference between the annual average TER and SR did not reach the significance level of 0.05. The TER was under similar environmental controls as SR, in growing seasons of drought years, the variations of soil moisture at 0-10 cm and 10-20 cm depth could account for 79.1%-95.6% of the changes of the SR and TER, but in non-growing season, more than 75% of the variations of the SR and TER could be explained by the changes of the ground temperature of soil surface layers.

[Analyses of the correlation between the fluxes of CO2 and the distribution of C & N in grassland soils].

Using static dark enclosed chamber technique, the emissions of CO2 from typical temperate grasslands such as Stipa baicalensis, Leymus chinensis, Stipa grandis and Stipa krylovii in Xilin River Basin of Inner Mongolia were measured. And the contents of organic carbon and total nitrogen in different soil layers were also studied to analyze the correlation between the emissions of CO2 and the contents of C & N. The results showed that the emissions of CO2 from the 4 grassland communities were significantly positively correlated with soil organic carbon and total nitrogen in different layers of grassland soils, and the correlation coefficients were almost all above 0.8. That meant the flux of CO2 is severely influenced by the contents of soil organic carbon and total nitrogen when environmental factors were similar. The decreases of soil organic carbon and total nitrogen with the depth into soils (more than 70% of the contents centralizes above 0-30 cm layer) showed that most of them came from the decomposition of organic matter on the earth's surface. In the 0-100 cm layer of grassland soils, the decreases of amount of soil organic carbon and total nitrogen from Stipa baicalensis, Stipa grandis and Leymus chinensis grasslands to Stipa krylovii grassland orderly showed that the impacts of precipitation, evaporation and dryness index on the distribution of soil organic carbon and total nitrogen were important.