Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

Root nitrogen uptake capacity of Chinese fir enhanced by warming and nitrogen addition

There is a knowledge gap in the effects of climate warming and nitrogen (N) deposition on root N absorption capacity, which limits our ability to predict how climate change alters the N cycling and its consequences for forest productivity especially in subtropical areas where soil N availability is already high. In order to explore the effects and mechanism of warming and the N deposition on root N absorption capacity of Chinese fir (Cunninghamia lanceolata), a subtropical arbuscular mycorrhizal conifer, the fine root 15NH4+ and 15NO3- uptake kinetics at a reference temperature of 20 °C were measured across different seasons in a factorial soil warming (ambient, +5 °C) × N addition (ambient, +40 kg N ha-1 yr-1) experiment. The results showed that (i) compared with the control, warming increased the maximal uptake rate of NH4+ (Vmax,20 °C-NH4+) in summer, while N addition enhanced it in spring and summer; compared with non-warming treatments, warming treatments increased the uptake rate of NO3- at a reference concentration of 100 μmol (V100,20 °C-NO3-) in spring. (ii) The analysis of covariance showed that Vmax,20 °C-NH4+ was positively correlated with root mycorrhizal colonization rate (MCR) and V100,20 °C-NO3- was positively correlated with specific root respiration rate (SRR), whereas no N uptake kinetic parameter was correlated with specific root length, root N and non-structural carbon concentrations. Thus, our results demonstrate that warming-increased root NH4+ uptake might be related to warming-increased MCR, whereas warming-increased root NO3- uptake might be related to warming-increased SRR. We conclude that root NH4+ and NO3- uptake capacity of subtropical Chinese fir can be elevated under warming and N deposition, which could improve plantation productivity and mitigate N leaching loss and soil acidification.

Role of Tree Species, the Herb Layer and Watershed Characteristics in Nitrate Assimilation in a Central Appalachian Hardwood Forest

Forest plants that can assimilate nitrate may act as nitrate sink and, consequently, reduce nitrate losses from watershed ecosystems through leaching. This study, conducted at the Fernow Experimental Forest in West Virginia, quantified via nitrogen reductase activity (NRA) the nitrate assimilation of two tree species, red maple and sugar maple, and surrounding common herb-layer species at the tissue (foliage, roots) and plot level. NRA measurements were conducted in summer and spring. Furthermore, NRA was quantified under varying levels of soil nitrate availability due to fertilization, different stages in secondary forest succession, and watershed aspect. This study confirmed that NRA of mature maples does not respond to varying levels of soil nitrate availability. However, some herb-layer species’ NRA did increase with nitrogen fertilization, and it may be greater in spring than in summer. Combined with biomass, the herb layer’s NRA at the plot-level (NRAA) comprised 9 to 41% of the total (tree + herb-layer) foliar NRAA during the growing season. This demonstrates that the herb layer contributes to nitrate assimilation disproportionally to its small biomass in the forest and may provide a vernal dam to nitrate loss not only by its early presence but also by increased spring NRA relative to summer.

Quantifying nitrogen uptake and translocation for mature trees: an in situ whole-tree paired 15N labeling method

Nitrogen (N) is one of the major nutrients limiting plant growth in terrestrial ecosystems. To avoid plant-microbe competition, previous studies on plant N uptake preference often used hydroponic experiments on fine roots of seedlings and demonstrated ammonium preference for conifer species; however, we lack information about N uptake and translocation in the field. In this paper, we described a method of in situ paired 15N labeling and reported the rates and time course of N uptake and translocation by mature trees in situ. We added 15N-enriched ammonium or nitrate, together with the nitrification inhibitor dicyandiamide, to paired Larix kaempferi (Lamb.) Carr (larch) trees from 30-, 40- and 50-year-old plantations. Fine roots, coarse roots, leaves and small branches were collected 2, 4, 7, 14 and 30 days after labeling. Nitrate uptake and translocation averaged 1.59 ± 0.16 μg 15N g-1 day-1, which is slightly higher than ammonium (1.08 ± 0.10 μg 15N g-1 day-1), in all tree organs. Nitrate contributed 50-78% to N uptake and translocation, indicating efficient nitrate use by larch in situ. We observed no age effect. We suggest that sampling leaves after 4 days of 15N labeling is sufficient to detect mature tree N uptake preference in situ. Whole-tree 15N-ammonium recovery equaled that of 15N-nitrate 30 days after 15N addition, implying the importance of both ammonium and nitrate to mature larch N use in the long run. We conclude that our method is promising for studying mature tree N uptake preference in situ and can be applied to other conifer and broadleaf species. We suggest using highly enriched 15N tracer to overcome soil dilution and a nitrification inhibitor to minimize ammonium transformation to nitrate. Our study revealed mature tree N preference in situ and demonstrated the strong contribution of nitrate toward mature larch growth on soils rich in nitrate.

Soluble soil aluminum alters the relative uptake of mineral nitrogen forms by six mature temperate broadleaf tree species: possible implications for watershed nitrate retention

Increased availability of monomeric aluminum (Al³⁺) in forest soils is an important adverse effect of acidic deposition that reduces root growth and inhibits nutrient uptake. There is evidence that Al³⁺ exposure interferes with NO₃^- uptake. If true for overstory trees, the reduction in stand demand for NO₃^- could increase NO₃^- discharge in stream water. These effects may also differ between species that tolerate different levels of soil acidity. To examine these ideas, we measured changes in relative uptake of NO₃^- and NH₄⁺ by six tree species in situ under increased soil Al³⁺ using a ¹⁵N-labeling technique, and measured soluble soil Al levels in a separate whole-watershed acidification experiment in the Fernow Experimental Forest (WV). When exposed to added Al³⁺, the proportion of inorganic N acquired as NO₃^- dropped 14% across species, but we did not detect a reduction in overall N uptake, nor did tree species differ in this response. In the long-term acidification experiment, we found that soluble soil Al was mostly in the free Al³⁺ form, and the concentration of Al³⁺ was ~65 μM higher (~250%) in the mineral soil of the acidified watershed vs. an untreated watershed. Thus, increased levels of soil Al³⁺ under acidic deposition cause a reduction in uptake of NO₃^- by mature trees. When our ¹⁵N uptake results were applied to the watershed acidification experiment, they suggest that increased Al³⁺ exposure could reduce tree uptake of NO₃^- by 7.73 kg N ha^-1 year^-1, and thus increase watershed NO₃^- discharge.

Soil solution and sugar maple response to NH(4)NO (3) additions in a base-poor northern hardwood forest of Québec, Canada

Nitrogen additions (NH4NO3) at rates of three- and ten-fold ambient atmospheric deposition (8.5 kg ha(-1) year(-1)) were realised in an acid- and base-poor northern hardwood forest of Québec, Canada. Soil solution chemistry, foliar chemistry, crown dieback and basal area growth of sugar maple (Acer saccharum Marsh.) were measured. Except for a transitory increase of NO3 and NH4 concentrations, there was no persistent increase in their level in soil solution 3 years after N treatments, with the exception of one plot out of three, that received the highest N addition, beginning to show persistent and high NO3 concentrations after 2 years of N additions. Three years of N additions have significantly increased the N DRIS index of sugar maple but not N foliar concentration. Potassium, Ca and Mn foliar concentrations, as well as P and Ca DRIS indices, decreased in treated plots after 3 years. No treatment effect was observed for basal area growth and dieback rate. One unexpected result was the significant decrease in foliar Ca even in the treated plots that received low N rates, despite the absence of significant NO3-induced leaching of Ca. The mechanism responsible for the decrease in foliar Ca is not known. Our results, however, clearly demonstrate that increased N deposition at sites with low base saturation may affect Ca nutrition even when clear signs of N saturation are not observed.

Plant and soil natural abundance delta (15)N: indicators of relative rates of nitrogen cycling in temperate forest ecosystems

Watersheds within the Catskill Mountains, New York, receive among the highest rates of nitrogen (N) deposition in the northeastern United States and are beginning to show signs of N saturation. Despite similar amounts of N deposition across watersheds within the Catskill Mountains, rates of soil N cycling and N retention vary significantly among stands of different tree species. We examined the potential use of delta (15)N of plants and soils as an indicator of relative forest soil N cycling rates. We analyzed the delta (15)N of foliage, litterfall, bole wood, surface litter layer, fine roots and organic soil from single-species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine root and organic soil delta (15)N values were highest within sugar maple stands, which correlated significantly with higher rates of net mineralization and nitrification. Results from this study suggest that fine root and organic soil delta (15)N can be used as an indicator of relative rates of soil N cycling. Although not statistically significant, delta (15)N was highest within foliage, wood and litterfall of beech stands, a tree species associated with intermediate levels of soil N cycling rates and forest N retention. Our results show that belowground delta (15)N values are a better indicator of relative rates of soil N cycling than are aboveground delta (15)N values.

Plant and soil natural abundance δ 15N: indicators of relative rates of nitrogen cycling in temperate forest ecosystems

Watersheds within the Catskill Mountains, New York, receive among the highest rates of nitrogen (N) deposition in the northeastern United States and are beginning to show signs of N saturation. Despite similar amounts of N deposition across watersheds within the Catskill Mountains, rates of soil N cycling and N retention vary significantly among stands of different tree species. We examined the potential use of delta (15)N of plants and soils as an indicator of relative forest soil N cycling rates. We analyzed the delta (15)N of foliage, litterfall, bole wood, surface litter layer, fine roots and organic soil from single-species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine root and organic soil delta (15)N values were highest within sugar maple stands, which correlated significantly with higher rates of net mineralization and nitrification. Results from this study suggest that fine root and organic soil delta (15)N can be used as an indicator of relative rates of soil N cycling. Although not statistically significant, delta (15)N was highest within foliage, wood and litterfall of beech stands, a tree species associated with intermediate levels of soil N cycling rates and forest N retention. Our results show that belowground delta (15)N values are a better indicator of relative rates of soil N cycling than are aboveground delta (15)N values.