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Nitrogen (N) is the primary growth-limiting nutrient in many terrestrial ecosystems, and therefore plant production per unit N taken up (i.e., N use efficiency, NUE) is a fundamentally important component of ecosystem function. Nitrogen use efficiency comprises two components: N productivity (A(N), plant production per peak biomass N content) and the mean residence time of N in plant biomass (MRT(N)). We utilized a five-year fertilization experiment to examine the manner in which increases in N and phosphorus (P) availability affected plant NUE at multiple biological scales (i.e., from leaf to community level). We fertilized a natural gradient of nutrient-limited peatland ecosystems in the Upper Peninsula of Michigan, USA, with 6 g N x m(-2) x yr(-1), 2 g P x m(-2) x yr(-1), or a combination of N and P. Our objectives were to determine how changes in carbon and N allocation within a plant to leaf and woody tissue and changes in species composition within a community, both above- and belowground, would affect (1) NUE; (2) the adaptive trade-off between the components of NUE; (3) the efficiency with which plants acquired N from the soil (N uptake efficiency); and (4) plant community production per unit soil N availability (N response efficiency, NRE). As expected, N and P addition generally increased aboveground production and N uptake. In particular, P availability strongly affected the way in which plants took up and used N. Nitrogen use efficiency response to nutrient addition was not straightforward. Nitrogen use efficiency differed between leaf and woody tissue, among species, and across the ombrotrophic-minerotrophic gradient because plants and communities were adapted to maximize either A(N) or MRT(N), but not both concurrently. Increased N availability strongly decreased plant and community N uptake efficiency, while increased P availability increased N uptake efficiency, particularly in a nitrogen-fixing shrub. Nitrogen uptake efficiency was more important in controlling overall plant community response to soil N availability than was NUE, and above- and belowground community N uptake efficiencies responded to nutrient addition in a similar manner. Our results demonstrate that plants respond to nutrient availability at multiple biological scales, and we suggest that N uptake efficiency may be a more representative measurement of plant responses to nutrient availability gradients than plant NUE.

Plants and mycorrhizal fungi form mutualistic relationships that affect how resources flow between organisms and within ecosystems. Common mycorrhizal networks (CMNs) could facilitate preferential transfer of carbon and limiting nutrients, but this remains difficult to predict. Do CMNs favour fungal resource acquisition at the expense of plant resource demands (a fungi-centric view), or are they passive channels through which plants regulate resource fluxes (a plant-centric view)? We used stable isotope tracers (13CO2 and 15NH3), plant traits, and mycorrhizal DNA to quantify above- and below-ground carbon and nitrogen transfer between 18 plant species along a 520-km latitudinal gradient in the Pacific Northwest, USA. Plant functional type and tissue stoichiometry were the most important predictors of interspecific resource transfer. Of ‘donor’ plants, 98% were 13C-enriched, but we detected transfer in only 2% of ‘receiver’ plants. However, all donors were 15N-enriched and we detected transfer in 81% of receivers. Nitrogen was preferentially transferred to annuals (0.26 ± 0.50 mg N per g leaf mass) compared with perennials (0.13 ± 0.30 mg N per g leaf mass). This corresponded with tissue stoichiometry differences. Synthesis Our findings suggest that plants and fungi that are located closer together in space and with stronger demand for resources over time are more likely to receive larger amounts of those limiting resources. Read the free Plain Language Summary for this article on the Journal blog.

Climate change will directly affect carbon and nitrogen mineralization through changes in temperature and soil moisture, but it may also indirectly affect mineralization rates through changes in soil quality. We used an experimental mesocosm system to examine the effects of 6-year manipulations of infrared loading (warming) and water-table level on the potential anaerobic nitrogen and carbon (as carbon dioxide (CO2) and methane (CH4) production) mineralization potentials of bog and fen peat over 11 weeks under uniform anaerobic conditions. To investigate the response of the dominant methanogenic pathways, we also analyzed the stable isotope composition of CH4 produced in the samples. Bog peat from the highest water-table treatment produced more CO2 than bog peat from drier mesocosms. Fen peat from the highest water-table treatment produced the most CH4. Cumulative nitrogen mineralization was lowest in bog peat from the warmest treatment and lowest in the fen peat from the highest water-table treatment. As all samples were incubated under constant conditions, observed differences in mineralization patterns reflect changes in soil quality in response to climate treatments. The largest treatment effects on carbon mineralization as CO2 occurred early in the incubations and were ameliorated over time, suggesting that the climate treatments changed the size and/or quality of a small labile carbon pool. CH4 from the fen peat appeared to be predominately from the acetoclastic pathway, while in the bog peat a strong CH4 oxidation signal was present despite the anaerobic conditions of our incubations. There was no evidence that changes in soil quality have lead to differences in the dominant methanogenic pathways in these systems. Overall, our results suggest that even relatively short-term changes in climate can alter the quality of peat in bogs and fens, which could alter the response of peatland carbon and nitrogen mineralization to future climate change. [ABSTRACT FROM AUTHOR]

Leaching of dissolved organic matter (DOM) and the associated nutrient elements can be a significant form of loss from developing ecosystems. We studied how the adsorption of dissolved organic C (DOC) and N (DON) changes during soil development and determined which soil characteristics control adsorption. We sampled 77, 255, 616, and about 1200+ yr-old andesitic soils at five depths and did adsorption isotherm experiments fit to a modified Langmuir equation. We measured DOC and DON in soil solution at the 10- to 20-, 40-, and 150-cm soil depths during the snowmelt period to compare with adsorption experiments. Ability of the soils to adsorb DOM increased with soil age. Regression analyses were performed between adsorption capacity or the null point concentration of either DOC or DON and the independent variables soil organic C (SOC), N, allophane, oxalate extractable Fe, crystalline Fe, and specific surface area. The best relationships were found between adsorption capacity and the allophane/SOC ratio (r² = 0.88), and between the null point concentration of DOC or DON and the SOC/a!lophane ratio (DOC: r² = 0.85; DON: r² = 0.77). Stepwise multiple regression indicated that oxalate-extractable Fe and specific surface area contributed only small increases in the multiple R². High correlations between the nullpoint adsorption of DOC or DON and the DOC (r² = 0.92) or DON (r² = 0.86) field soil solution concentrations indicated that results obtained in laboratory experiments were applicable to field conditions. The cause of the increased ability of the soils to adsorb and retain DOM during soil development appears to be an increase in allophane concentrations. [ABSTRACT FROM AUTHOR]