| Human activities have significantly altered global and regional cycles of nitrogen (N) in the last several decades due to combustion of fossil fuels, intensive agriculture and stock breeding. The amount of human-made reactive N which is more than the natural production, released into the atmosphere. Nitrogen (N) deposition has been widely considered as an environmental problem that leads to destruction of ecological balance such as biodiversity loss, base cation loss, soil acidification, but increases net primary productivity of N-limited forest ecosystems. Greater N deposition has a significantly effect on soil respiration.The dynamic of soil respiration after simulated nitrogen deposition in different litter treatment subplots was studied in a natural Pinus tabulaeformis forest in Taiyueshan, Shanxi Province. Three litter treamtent subplots including control (C), litter exclusion (B), litter and root exclusion (A) were established. Nitrogen levels were implemented as: CK (0kg·hm-2·a-1), LN (50kg·hm-2·a-1), MN (100kg·hm-2·a-1) and HN (150kg·hm-2·a-1). Relationships of soil respiration to soil temperature, soil humidity, microbial biomass, soil enzyme activity and fine root biomass after each nitrogen application were studied. The main results were as follows:(1)The results showed that soil respiration rates in different litter treatment subplots were the highest on the first day and then gradually decreased after each nitrogen application. Soil respiraton rates were steady after the third day for the litter exclusion (B) and litter and root exclusion (A) subplots, however, the respiration rate in control (C) subplot consistently decreased with nitrgen application. External nitrogen inhibited the soil respiration rate in litter and root exclusion (A) subplot, but promoted respiraiton in litter exclusion (B) and control (C) subplots. The variation of soil microbial biomass C and N was similar with soil respiration rate. The relationships between soil respiration and soil enzyme acitivity, and soil moisture were not significant, while soil respiration correlated significantly with soil temperature (P<0.05). The hybrid model Rs=aebTWc with soil temperature, soil moisture appeared to have better precision than the single factor model in predicting the soil respiration. Nitrogen deposition decreased the determination coefficient R2of Rs=aebT of different litter treatments. Furthermore, nitrogen deposition reduced Q10in the litter and root exclusion (A) and litter exclusion (B) subplots, but did not significantly reduce the Q10in the control (C) subplot.(2)Simulated nitrogen deposition promoted soil respiration in litter removal (B), control (C) subplots and litter respiration at an early stage, but the promotion weakened over time. Litter removal treatment decreased the average rates of soil respiration by28.99%, nitrogen deposition decreased the fluctuation percent of soil respiration after removing the litter. Soil respiration was associated significantly with soil temperature at5cm depth (P<0.05), which explained37.3%-62.2%of the seasonal variation of soil respiration, and litter removal treatment decreased the R2of all nitrogen levels. Four two-variable linear/non-linear models with soil temperature and soil moisture appeard to have precision when used to predict the soil respiration, with a R2ranging from0.676to0.856, and nitrogen deposition decreased the R2of litter removal treatments, but it did not significantly affect the R2of control treatments. Simulated nitrogen deposition increased soil microbial biomass C, N, a linear relationship was found between soil respiration and microbial biomass C, N. It was suggested that nitrogen deposition and litter were two important factors affecting soil CO2efflux in Pinus tabulaeformis forests.(3) Microbial biomass at10-20cm depth was lower than that at0-10cm depth under the same N application rate. Low nitrogen levels increased soil microbial biomass carbon, but high nitrogen levels decreased it. Microbial biomass nitrogen showed a trend of decrease with the increment in the nitrogen level. Soil microbial biomass C, N showed obvious seasonal variation.(4)N applications significantly increased fine root biomass. Root morphological characteristics including length/biomass, surface area/biomass, volume/biomass, tips/biomass of Pinus tabulaeformis seedling within different treatment presented significant seasonal changes (P<0.01), with no significant differences between treatments (P>0.05), but these root morphological characteristics of small fine root responded obviously to nitrogen (P<0.05). |
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