| At present, there has been a common view that increased N deposition has been affecting the stability of some forest ecosystems. Numerous studies have shown that increased N deposition could significantly influence soil carbon and nitrogen cycles as well as soil microbial characteristics in forest ecosystems, resulting in negative effects on the the stability of forest ecosystems. Soil microbial communities are the most active part of the soil, which is closely correlated with soil fertility and health, understanding the variations in soil microbial biomass and community structure can help us to explain the functional changes of forest ecosystems. Soil could produce or consume greenhouse gases responding to the change of N deposition, which are the key contents in the researches of ecosystem feedback processes. In subtropical China, in order to decrease the negative effects induced by inefficient plantations, promote multi-purpose managements and improve the ecological function and economic value of plantations, many indigenous valuable broad-leaf tree species(including N-fixing tree species) are being developed for the construction of plantations in production practice. However, the response mechanisms of young plantations of different tree species(e.g., N-fixing tree species versus non-N-fixing tree species) ecosystems in subtropical China to increased N depositions are still inappropriate. Therefore, a field N addition experiment was established in two adjacent young plantations of non-N-fixing Castanopsis hystrix(CH) and N-fixing Erythrophleum fordii(EF) in subtropical China, following four-levels of N treatments(Control, no N addition; Low-N, 50 kg N ha-1yr-1; Medium-N, 100 kg N ha-1yr-1; and High-N, 150 kg N ha-1yr-1 experimental inputs) with 17 months duration, in order to study the effects of increased N additions on soil main carbon and nitrogen turnover processes using the methods of elemental analysis, gas chromatographic technique and phospholipid fatty acid(PLFA). The main contents of field N addition experiments were summarized as follows:(1) effects of increased N additions on soil chemical properties in two young plantations;(2) effects of increased N additions on soil greenhouse gas emissions in two young plantations;(3) effects of increased N additions on soil microbial community structure in two young plantations. The field N addition experiment was to evaluate the stability of two young plantation ecosystems under increased N addition using soil microbial community structure as an indicator and provide data references and theoretical foundation for the sustainable development of forestry as well as environmental protection. Because N additions may have indirect effects on microbial communities by altering plant community composition and/or the amounts and types of plant C inputs to soils, reducing microbial activity levels, another 16-day incubation experiment was performed with soils collected from the organic layer of control plots in above-mentioned field experiments and fertilized with a solution of ammonia nitrate. The fertilizer was supplied at an equivalent of 0g N m-2(N0), 5g N m-2(N5), 10 g N m-2(N10), 15 g N m-2(N15) and 25 g N m-2(N25). The incubation was carried out in PVC cylinders with gas sampling device under optimal conditions and microbial community parameters were characterized by using phospholipid fatty acid(PLFA) analysis on days 2(t2), 6(t6), and 16 days(t16) after N addition. Evolved CO2 were determined by gas chromatograph technique. The main purpose of incubation experiment was to determine the direct effects of increased N additions on soil microbial communities alone without C inputs of plants and examine the early responses of microbial respiration to N additions as well as the relationship between microbial respiration and community structure. The main results are as follows:(1) Medium-N and High-N treatments significantly decreased soil(0-10cm) pH, but increased the contents of NH4+-N and NO3--N. All the N addition treatments did not significantly influence the content of soil organic carbon in 0-10 cm, 10-20 cm and 20-30 cm layers, but Medium-N and High-N treatments significantly increased the contents of soil extractable DOC in two plantations. Moreover, all of three N addition treatments had no significant effects on the contents of soil total N, total P, available P and total K in two plantations in each soil layer.(2) Medium-N and High-N treatments significantly decreased soil CO2 emission rate by 11% and 15% in C. hystrix plantation at the end of observation time, while only High-N treatment significantly reduced soil CO2 emission rate by 21% in E. fordii plantation compared to the controls. All levels of N additions significantly increased soil N2 O emission rate by 285%,562% and 835%, respectively in relative to the control plots in C. hystrix plantation. while in E. fordii plantation, N2 O emission rate was significantly enhanced by Medium-N and High-N treatments(252% and 770%, respectively). In addition, Medium-N and High-N treatments significantly reduced CH4 uptake rate by 30% and 69%, but such inhibitions of CH4 uptake were stronger in E. fordii plantation(67% and 93% under Medium-N and High-N treatments, respectively). Soil CO2 emission rate had relatively stronger correlations with soil temperature than soil moisture in two plantations. To a certain degree, N addition weakened the relationships between soil N2 O and CH4 emission rate and soil hydrothermal factors in C. hystrix plantation, especially for the relationships between N2 O emission rate and soil temperature as well as soil moisture, however, such an inclination was not that significant in E. fordii plantation.(3) Medium-N and High-N treatments significantly decreased soil microbial total PLFAs by 23.5% and 32.1% in C. hystrix plantation, which was reduced only by High-N treatment in E. fordii plantation. Bacterial PLFAs and fungal PLFAs were also negatively affected by N additions, but Medium-N and High-N treatments significantly increased biomass ratio of fungal to bacterial(F:B) in both the plantations. Nonmetric multidimensional scaling and multi-response permutation procedure analysis showed that soil microbial community structure in C. hystrix plantation was significantly different from that in E. fordii plantation under natural(control) conditions(A=0.65, p<0.05). All the N addition treatments significantly influenced soil microbial community structure in two plantations(p<0.001). Moreover, the redundancy analysis demonstrated that the shift of soil microbial community structure under N additions was significantly correlated to the variations in NH4+-N, microbial biomass C and soil pH(p<0.05). These results suggest that N additions could significantly influence the stability of soil microbial community and thus negatively affect soil health and fertility in the young plantations of C. hystrix and E. fordii.(4) The results of short-term incubation experiment showed that N10, N15 and N25 treatments significantly increased microbial respiration rate in soil CH during the incubation by an average of 35%, 69%, and 94%, respectively, compared to the controls. While in soil EF, only N25 treatment significantly enhanced respiration rate by an average of 23% relative to the controls. Soil microbial biomass were significantly reduced by N addition in two soils, but they happened at different sampling date, and microbial communities in soil EF had resilience to recover their biomass to the control level from the negative effect of the perturbations caused by N inputs. Moreover,we found that N10, N15 and N25 treatments significantly increased the physiological stress indexes for microbial communities, which were mainly correlated with soil pH. Principal component analysis showed that all the N addition treatments significantly altered microbial community structure at each sampling time in two soils. Pearson’s correlation analysis revealed that microbial respiration rate in soil CH was significantly related to soil extractable DOC, F:B, microbial community structure as well as physiological stress indexes of microbial communities, but the role of these factors to regulate microbial respiration could change with incubation time. However, these factors were not significantly correlated with microbial respiration rate in soil EF, which was mainly due to the unique microbial community structure, and the mechanism calls for a better understanding. During observation time, soil microbial respiration rate were not or negatively related to soil total microbial biomass in two studied soils, which indicated that soil microbial biomass should not be utilized as the sole predictor of microbial respiration, when comparing soils with different community structures and levels of physiological stress under N additions during the early stage in the studied subtropical soils, because soil microbial community structure and physiological status of microbial communities could significantly influence microbial respiration rate as well. |
PDF Full Text Request