Examination of the negative correlation between leaf δ15N and the N:P ratio across a northeast-southwest transect in China.

Nitrogen (N) and phosphorus (P) are two crucial limiting mineral elements for terrestrial plants. Although the leaf N:P ratio is extensively used to indicate plant nutrient limitations, the critical N:P ratios cannot be universally applied. Some investigations have suggested that leaf nitrogen isotopes (δ15N) can provide another proxy for nutrient limitations along with the N:P ratio, but the negative relationships between N:P and δ15N were mainly limited to fertilization experiments. It will obviously benefit the study of the nature of nutrient limitations if the relationship could be explained more generally. We analyzed leaf δ15N, N, and P contents across a northeast-southwest transect in China. Leaf δ15N was weakly negatively correlated with leaf N:P ratios for all plants, while there was no correlation between them for various plant groups, including different growth forms, genera, and species across the entire N:P range. This suggests that the use of leaf δ15N in indicating the shift of nutrient limitations across the whole N:P range still requires more validated field investigations. Notably, negative relationships between δ15N and N:P hold for plants with N:P ratios between 10 and 20 but not for plants with N:P ratios lower than 10 or higher than 20. That is, changes in leaf δ15N along with the N:P ratio of plants that are co-limited by N and P can exhibit variations in plant nutrient limitations, whereas plants that are strictly limited by N and P cannot. Moreover, these relationships are not altered by vegetation type, soil type, MAP, or MAT, indicating that the use of leaf δ15N in reflecting shifts in nutrient limitations, depending on the plant nutrient limitation range, is general. We examined the relationships between leaf δ15N and the N:P ratio across an extensive transect, providing references for the widespread use of leaf δ15N in reflecting shifts in nutrient limitation.

Levels and variations of soil bioavailable nitrogen among forests under high atmospheric nitrogen deposition.

To examine the perturbation of atmospheric nitrogen (N) deposition on soil N status and the biogeochemical cycle is meaningful for understanding forest function evolution with environmental changes. However, levels of soil bioavailable N and their environmental controls in forests receiving high atmospheric N deposition remain less investigated, which hinders evaluating the effects of enhanced anthropogenic N loading on forest N availability and N losses. This study analyzed concentrations of soil extractable N, microbial biomass N, net rates of N mineralization and nitrification, and their relationships with environmental factors among 26 temperate forests under the N deposition rates between 28.7 and 69.0 kg N ha-1 yr-1 in the Beijing-Tianjin-Hebei (BTH) region of northern China. Compared with other forests globally, forests in the BTH region showed higher levels of soil bioavailable N (NH4+, 27.1 ± 0.8 mg N kg-1; NO3-, 7.0 ± 0.8 mg N kg-1) but lower net rates of N mineralization and nitrification (0.5 ± 0.1 mg N kg-1 d-1 and 0.4 ± 0.1 mg N kg-1 d-1, respectively). Increasing N deposition levels increased soil nitrification and NO3- concentrations but did not increase microbial biomass N and N mineralization among the study forests. Soil moisture and C availability were found as dominant factors influencing microbial N mineralization and bioavailable N. In addition, by budgeting the differences in soil total N densities between the 2000s and 2010s, atmospheric N inputs to the forests were more retained in soils than lost proportionally (84% vs. 16%). We concluded that the high N deposition enriched soil N without stimulating microbial N mineralization among the study forests. These results clarified soil N status and the major controlling factors under high anthropogenic N loading, which is helpful for evaluating the fates and ecological effects of atmospheric N pollution.

Relationships between leaf δ15 N and leaf metallic nutrients.

RATIONALE: Nitrogen (N) isotopic ratios (δ15 N values) of plants are primarily determined by the δ15 N values of their N sources. Metallic nutrients affect plant N uptake. However, there is little knowledge of the relationships between leaf δ15 N values and leaf metallic nutrients. The δ15 N values are often lower in soil nitrate (NO3 - ) than in ammonium (NH4 + ) due to large isotopic fractionation during nitrification. Plants acquire more NO3 - than NH4 + when accumulating high potassium (K), calcium (Ca) and magnesium (Mg) to maintain charge balance. In addition, plants that absorb more NO3 - than NH4 + increase the soil pH and decrease the availability of iron (Fe), manganese (Mn) and zinc (Zn). We therefore hypothesized that leaf δ15 N values correlate negatively with K, Ca and Mg contents, while positively with Fe, Mn and Zn contents. METHODS: Leaves of non-N-fixing plants were sampled across an approx. 6000 km transect in China and their δ15 N values and metallic nutrient content were determined using elemental analyzer/isotope ratio mass spectrometry. RESULTS: Inconsistent with the hypothesis, leaf δ15 N values correlated positively with leaf K, Ca and Mg, indicating higher δ15 N values of soil NO3 - than NH4 + . Higher δ15 N values of soil NO3 - revealed stronger denitrification than nitrification in the study regions because isotopic fractionation occurs during both processes. Leaf δ15 N values correlated negatively with Fe, relating to decreases in soil Fe availability, which might be attributed to oxidation of Fe2+ to Fe3+ supplying electrons for denitrification, while greater uptake of NO3 - than NH4 + of plants increases soil pH. Leaf δ15 N values correlated positively with Zn and did not correlate with Mn. These observed relationships between leaf δ15 N values and metallic nutrients, except Mn, were independent of vegetation or soil types. CONCLUSIONS: This study has enriched our knowledge of associations between metallic nutrients and N cycling in plant-soil systems, especially for the roles of Fe in soil N transformations and K, Ca and Mg in plant N uptake.

Levels and variations of soil organic carbon and total nitrogen among forests in a hotspot region of high nitrogen deposition.

Human activities have distinctly enhanced the deposition levels of atmospheric nitrogen (N) pollutants into terrestrial ecosystems, but whether and to what extents soil carbon (C) and N status have been influenced by elevated N inputs remain poorly understood in the 'real' world given related knowledge has largely based on N-addition experiments. Here we reported soil organic C (OC) and total N (TN) for twenty-seven forests along a gradient of N deposition (22.4-112.9 kg N/ha/yr) in the Beijing-Tianjin-Hebei (BTH) region of northern China, a global hotspot of high N pollution. Levels of soil TN in forests of the BTH region have been elevated compared with investigations in past decades, suggesting that long-term N deposition might cause soil TN increases. Combining with major geographical and environmental factors among the study forests, we found unexpectedly that soil moisture and pH values rather than N deposition levels were major regulators of the observed spatial variations of soil OC and TN contents. As soil moisture and pH values increased with mean annual precipitation and temperature, respectively, soil C and N status in forests of the BTH region might be more responsive to climate change than to N pollution. These evidence suggests that both N deposition and climate differences should be considered into managing ecosystem functions of forest resources in regions with high N pollution.

The biomass accumulation and nutrient storage of five plant species in an in-situ phytoremediation experiment in the Ningxia irrigation area.

Phytoremediation has been widely used and is considered an environmentally friendly and efficient method for mitigating nitrogen (N) and phosphorus (P) loads. However, the technique is rarely employed in the Ningxia irrigation area, which suffers from serious N and P pollution. To investigate ways of protecting the aquatic environment in this region, we conducted in-situ experiments along an agricultural ditch in 2014 and 2015. During the pre-experiment in 2014, five single species floating-bed systems (Zizania latifolia, Oryza sativa, Ipomoea aquatica, Lactuca sativa and Typha latifolia) and one multi-species floating-bed system with three replicates were evaluated over about two months. I. aquatica performed best with respect to biomass accumulation and nutrient storage among all plant systems. Multi-species system was not superior to single species systems: 42% and 37% of the N and P storage in the multi-species system were achieved by I. aquatica. In the formal experiment during 2015, I. aquatica was tested again and performed excellently with respect to biomass production (1.06 kg/m2), N (27.58 g/m2) and P (2.34 g/m2) uptake. Thus, this study demonstrated that I. aquatica could be used to reduce N and P loads under saline and alkaline conditions in the Ningxia irrigation area.

Performance of five plant species in removal of nitrogen and phosphorus from an experimental phytoremediation system in the Ningxia irrigation area.

Agricultural non-point source (ANPS) pollution is an important contributor to elevated nitrogen (N) and phosphorus (P) in surface waters, which can cause serious environmental problems. Considerable effort has therefore gone into the development of methods that control the ANPS input of N and P to surface waters. Phytoremediation has been extensively used because it is cost-effective, environmentally friendly, and efficient. The N and P loads from agricultural drainage are a potential threat to the water quality of the Yellow River in Ningxia, China. Yet, phytoremediation has only rarely been applied within the Ningxia irrigation area. In an experimental set-up, five species (Ipomoea aquatica, IA; Lactuca sativa, LS; Oryza sativa, OS; Typha latifolia, TL; Zizania latifolia, ZL) were evaluated for their ability to reduce N and P loads over 62 days and five observation periods. Total N and P concentrations, plant biomass, and nutrient content were measured. The results showed that OS, LS, and IA performed better than ZL and TL in terms of nutrients removal, biomass accumulation, and nutrients storage. The highest overall removal rates of N and P (57.7 and 57.3%, respectively) were achieved by LS treatment. In addition, plant uptake contributed significantly to nutrient removal, causing a 25.9-72.0% reduction in N removal and a 54.3-86.5% reduction in P removal. Thus, this study suggests that OS, LS, and IA would be more suitable than ZL and TL for controlling nutrient loads in the Ningxia irrigation area using phytoremediation.

Accounting for the effect of temperature in clarifying the response of foliar nitrogen isotope ratios to atmospheric nitrogen deposition.

Atmospheric nitrogen deposition affects nitrogen isotope composition (δ15N) in plants. However, both negative effect and positive effect have been reported. The effects of climate on plant δ15N have not been corrected for in previous studies, this has impeded discovery of a true effect of atmospheric N deposition on plant δ15N. To obtain a more reliable result, it is necessary to correct for the effects of climatic factors. Here, we measured δ15N and N contents of plants and soils in Baiwangshan and Mount Dongling, north China. Atmospheric N deposition in Baiwangshan was much higher than Mount Dongling. Generally, however, foliar N contents showed no difference between the two regions and foliar δ15N was significantly lower in Baiwangshan than Mount Dongling. The corrected foliar δ15N after accounting for a predicted value assumed to vary with temperature was obviously more negative in Baiwangshan than Mount Dongling. Thus, this suggested the necessity of temperature correction in revealing the effect of N deposition on foliar δ15N. Temperature, soil N sources and mycorrhizal fungi could not explain the difference in foliar δ15N between the two regions, this indicated that atmospheric N deposition had a negative effect on plant δ15N. Additionally, this study also showed that the corrected foliar δ15N of bulk data set increased with altitude above 1300m in Mount Dongling, this provided an another evidence for the conclusion that atmospheric N deposition could cause 15N-depletion in plants.