| Worldwide, great quantities of nitrogen(N) are directly applied to soils in the form of fertilizer, annually. Thus, understanding the effects of such N addition on the ecology system carbon(C) cycles are important. Farmland ecosystems are the most active C sinks, and the C fixation capacity in these systems is largely dependent on fertilization and field management. Therefore, understanding the effects of N fertilization on the C and N cycles benefits both soil productivity and environmental quality. The present study investigated the C cycle with winter wheat(Triticum aestivum L.) in areas that had received different N fertilization rates since 2004. Our results reveal the effects of long-term N addition on wheat field C balance, soil C efflux, soil organic C physical chemistry stability mechanism, and clear the mircobal mechanism under long-term N addition. Our study clarify the C and N cycles under continuous N addition, and the results of this study may provide guidelines for farmers in the studied region regarding the choice of an appropriate N application regime to achieve higher soil fertility while becoming more environmentally friendly. The main conculusions of our study were showed bellow:(1) Long-term suitable N application can increase crop C pool and soil C stockWheat community C pool ranged between 3.47 to 6.0 Mg C ha-1, nitrogen could improve the wheat production and C sequestration ability, 180 kg ha-1 application rate can reach the highest yield and the net C storage in wet year, but in the drought year, drought-tolerant variety in the 270 kg ha-1 treatment had the highest yield and the net C storage in dryland farming system. The drought-tolerant variety(Changhan No.58(CH)) showed a yield advantage in drought year with high N fertilizer rates; the N rate of 270 kg ha-1 for CH was better to avoid the risk of reduced production due to lack of precipitation; however, under conditions of better soil moisture, the N rate of 180 kg ha-1 was more economic. The effects of long-term N fertilization with no tillage and without residues on the profile distribution of soil organic carbon(SOC) and N concentrations and stocks and their interrelationships were evaluated for two wheat cultivars. N fertilization could affect C and N distribution at the 0-120 cm soil depth, and C and N levels were positively correlated in the 0-30 cm layer, but this relationship weakened as the soil depth increased. N fertilization had a large influence on C and N stocks and were affected by the wheat cultivars. Cropping wheat with N fertilization rates under 270 kg ha-1 could lead to C fixation in soil, and when N was present in excess of the level required by the crop to achieve the maximum yield, the soil became a C source and the N stock decreased after long-term N fertilization. The results of this study may provide guidelines for farmers in the studied region regarding the choice of an appropriate N application regime to achieve higher soil fertility while becoming more environmentally friendly.(2) Soil C and N fractions responded differently to the addition of NAll labile fractions responded differently to the addition of N in the 0-200 cm soil profiles after 10 years of N fertilization. Our results indicate that N fertilization may impact C and N cycling by changing certain fractions of soil organic matter. Light fraction of carbon(LFOC), light fraction of nitrogen(LFON), heavy fraction of carbon(HFOC) and heavy fraction of nitrogen(HFON) were constant among the different N rates. However, dissolved organic carbon(DOC) decreased in the 180, 270 and 360 kg ha-1 treatments in the 20-200 cm soil layers, and easily oxidizable carbon(EOC) was the highest in 90 kg ha-1 treatment and decreased in 0-20 cm layer of the high-N treatments. Both dissolved organic nirogen(DON) and inorganic N(NO3- and NH4+) increased as the N rates increased. Nitrate was the most sensitive fraction under N fertilization. The LFOC, LFON and EOC were the most sensitive fractions in the surface soil layer, DOC and DON appeared to be good indicators for the subsurface layers. Changes in these fractions were primarily affected by microbial activity, which can be affected by N availability and soil pH. The dynamics of soil C and its fractions are complex under N application, and clarifying these dynamics requires long-term studies and the analysis of many factors and deeper soil layers. Our study provides a clear picture of the distribution and sensitivity of fractions in soil profiles with different N fertilization rates, and clarify the C and N cycles under N addition.(3) Long-term N addition changed the soil microbial community structure and function, which cause the change of soil C and N cycleAfter 10 years of N addition in the wheat field, soil physical-chemical properties(TN, DON, NO3-, BD and SOC) were changed, and these changes correlated with the soil microbial communities, which could explained 61.7% of the change in the bacterial community and 66.7% of the change in fungal community in our study. N addition increased bacteria: fungi ratios, and high N addition had a significant effect on certain microbial species abundance. This community shift also correlated with changes in microbial activity, as determined by the respiration rates, which were higher in the N90 and N180 treatments than in the others treatments, and led to a lower CUE:NUE ratios. Moreover, a fertilization rate of 180 kg ha-1 N per year was the threshold in our study that reduced microbial activity after long-term N fertilization, however, whether this threshold is similar to those in other ecosystems remains to be seen.(4) N addition altered the autotrophic respiration to heterotrophic respiration ratio and resulted in different soil C emissionsCharacterization of soil respiration(Rs) and its components’ dynamics under long-term N fertilization is necessary to understand the mechanisms that regulate soil C emissions. In the farmland planted with winter wheat which had been fertilized with N for 10 years, the seasonal and diurnal responses of Rs and its components(heterotrophic respiration(Rh) and autotrophic respiration(Ra) were continuously monitored 3 years under different N fertilization rates. The results showed that the seasonal dynamics of Rs and its components were regulated by biotic factors(leaf area index and biomass) and abiotic factors(soil temperature and moisture), of which the leaf area index played an important role. N addition altered the Ra-to-Rs ratio and resulted in different Rs responses. Moderate N addition(180 kg ha-1) increased the C efflux, but long-term, excess N fertilization(360 kg ha-1) caused no significant change in Rs compared to the control, which indicated that there was a threshold N rate for the change in Rs. Finally, diel Rs and soil temperature were decoupled in farmland. Higher Rs was observed when the temperature increased, which demonstrates the hysteresis effect. Moreover, N fertilization increased the peak Rs values in the diurnal pattern.(5) N addition improved the net ecosystem productivity in wheat ecosystem, and soil C balance were controlled by moisture and N applicationFrom the perspective of the C fixation and emission, wheat farmland ecosystem had a positive net ecosystem productivity(NEP) in dry wheat field, which means that wheat farmland ecosystem in this area is the "sink" of atmospheric CO2. The NEP in no N fertilization treatment were ranged from 1917 to 4341 kg C ha-1 in three years,and in N ferilizaiton treatments were ranged from 4721 to 10134 kg C ha-1; N addition could increased wheat net ecosystem productivity significantly in the growing seaseons in wet year. And the soil C balance results indicated that C sequestration in wheat farmland with appropriate nitrogen application amount. Soil C balance were both controlled by soil moisture and N application, over amount of nitrogen(360 kg ha-1) rate could led C deficit in the growing year with sufficient rainfall, and finally led the C stock decrease after ten years N application. |
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