Assessment of effects of the rising atmospheric nitrogen deposition on nitrogen uptake and long-term water-use efficiency of plants using nitrogen and carbon stable isotopes

Comprehensive Analysis of Nitrogen Deposition in Urban Ecosystem: A Case Study of Xiamen City, China

Atmospheric nitrogen deposition can supply nitrogen for ecosystems while posing a serious threat to ecological security. An assessment of the ecological risks caused by atmospheric nitrogen deposition is critical for urban sustainable development. Based on “Ecological Risk Analysis” and the “Driver-Pressure-State-Response (DPSR) framework,” this paper established a comprehensive ecological risk assessment model and assessed the ecological risk of nitrogen deposition in Xiamen City, China. The results showed that the risk from nitrogen deposition to the forest ecosystem is high due to the impact of nitrogen deposition on the residual rate of litter and survival rate of seedlings. The risks to freshwater and marine ecosystems were determined to be high and moderate, respectively, due to the promotion of eutrophication by nitrogen. The risk to farm ecosystems was low due to the impact on weeds. The proportion of high-risk areas in Xiamen City was 37.1%. Among the districts of Xiamen City, Tong’an and Xiang’an had the highest proportion of high-risk areas (48%) and low-risk areas (31.8%), respectively.

Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest

Nitrogen (N) deposition (NDEP ) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A 15 N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy NDEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of NDEP on the relative change in C (∆C/∆N) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used 15 N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this 15 N to an equivalent amount in simulated NDEP treatments. By the end of the experiment, three times as much 15 N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more 15 N tracer from litter decomposition than from simulated mineral NDEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more 15 N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of NDEP on forest ∆C/∆N may not be apparent from mineral 15 N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of NDEP is its influence on the rate of N release from litter.

The key factor limiting plant growth in cold and humid alpine areas also plays a dominant role in plant carbon isotope discrimination

Many environmental factors affect carbon isotope discrimination in plants, yet the predominant factor influencing this process is generally assumed to be the key growth-limiting factor. However, to our knowledge this hypothesis has not been confirmed. We therefore determined the carbon isotope composition (δ(13)C) of plants growing in two cold and humid mountain regions where temperature is considered to be the key growth-limiting factor. Mean annual temperature (MAT) showed a significant impact on variation in carbon isotope discrimination value (Δ) irrespective of study area or plant functional type with either partial correlation or regression analysis, but the correlation between Δ and soil water content (SWC) was usually not significant. In multiple stepwise regression analysis, MAT was either the first or the only variable selected into the prediction model of Δ against MAT and SWC, indicating that the effect of temperature on carbon isotope discrimination was predominant. The results therefore provide evidence that the key growth-limiting factor is also crucial for plant carbon isotope discrimination. Changes in leaf morphology, water viscosity and carboxylation efficiency with temperature may be responsible for the observed positive correlation between Δ and temperature.

Response of plants' water use efficiency to increasing atmospheric CO2 concentration

This study assesses plants' adaptation to the elevated atmospheric CO(2) concentrations (c(a)) using 83 tree-ring δ(13)C series from the mid- to high-latitudes of the northern hemisphere. We found that the variation of Δ with the atmospheric CO(2) concentration is nonlinear and that the range of Δ change is relatively small. After 1950, the mean increase in Δ is 0.43‰, corresponding to the average coefficient of Δ-c(a) relationship to be about 0.006‰/ ppmv CO(2). In contrast to the changes in Δ, intercellular CO(2) concentration (c(i)) and intrinsic water-use efficiency (W(i)) both increase linearly with c(a). For the past two and a half centuries, changes in the intercellular CO(2) concentration (c(i)) and intrinsic water-use efficiency (W(i)) are, on average, both about 30%, while the mean change of the c(i)/c(a) ratio is 3%. Most changes have occurred after 1950. W(i) responds to c(a) linearly with sensitivities ranging from 0.06 to 0.6 μmol CO(2)/mmol H(2)O ppmv(-1), and an average 0.33 μmol CO(2)/mmol H(2)O ppmv(-1) during the past 50 years. Statistical analysis shows that the increase in c(a) accounts for 98% of the W(i) variation. The remaining small variance is explained by altitude and temperature. Trees at higher elevations show slightly higher increase in W(i), and they are also more sensitive to the CO(2) increase than trees at lower altitudes. Trees growing at low temperature environments are slightly more sensitive to CO(2) increase than those at higher temperature sites. No significant relationship between precipitation and plants' W(i) response to the atmospheric CO(2) increase is found with these data. Although the temperature and altitude both impact the W(i) response to elevated CO(2), the size of the impact is physically small and can be omitted from ecological models.