Leaf N resorption efficiency and litter N mineralization rate have a genotypic tradeoff in a silver birch population

Effects of climate, soil, and leaf traits on nutrient resorption efficiency and proficiency of different plant functional types across arid and semiarid regions of northwest China

BACKGROUND

Plant nutrient resorption is crucial for the efficient conservation of nutrients. However, the mechanisms through which abiotic and biotic factors control nutrient resorption remain controversial. We investigated leaf nitrogen (N) and phosphorus (P) resorption efficiency (NRE and PRE) and resorption proficiency, as well as the underlying mechanisms for each plant functional type (PFT: non-legume herbs, non-legume shrubs, and legumes) by collecting green and senescent leaves of 59 species covering 106 sites from arid and semiarid regions of northwest China.

RESULTS

Legumes had much lower leaf NRE and much higher senesced leaf N than the other two PFTs; they had similar leaf PRE to non-legume shrubs. Non-legume herbs exhibited the highest leaf P resorption. Climate, particularly temperature, increased leaf N resorption in non-legume herbs; however, climate, particularly decreasing precipitation, decreased leaf P resorption in legumes. Leaf nutrient resorption in non-legume shrubs decreased with increasing soil fertility, but leaf NRE in legumes increased. Leaf traits contributed more to leaf N and P resorption than climate and soil. Senesced leaf N and P concentrations increased along the resource-conservative to resource-acquisitive strategy axis. There were strong negative relationships between leaf NRE and senesced leaf N concentration and between leaf PRE and senesced leaf P concentration, in which legumes had a lower slope than non-legumes.

CONCLUSIONS

These findings suggest that ecological strategies and N-fixing plant types modulate nutrient resorption. Plants with the resource-conservative strategy are highly proficient in nutrient resorption. We highlight the importance of leaf economics traits and spectrum in regulating leaf nutrient resorption in drylands in the context of global climate change, potentially modulating plant traits and community composition. The higher proficient and efficient N and P resorption of plant species suggests the crucial importance of nutrient resorption in the nutrient cycling of harsh drylands.

Assessing community assembly controls over community-scale nutrient resorption responses to nitrogen deposition

Nutrient resorption is a fundamental physiological process in plants, with important ecological controls over numerous ecosystem functions. However, the role of community assembly in driving responses of nutrient resorption to perturbation remains largely unknown. Following the Price equation framework and the Community Assembly and Ecosystem Function framework, we quantified the contribution of species loss, species gain, and shared species to the reduction of community-level nutrient resorption efficiency in response to multi-level nitrogen (N) addition in a temperate steppe, after continuous N addition for seven years. Reductions of both N and phosphorus (P) resorption efficiency (NRE and PRE, respectively) were positively correlated with N addition levels. The dissimilarities in species composition between N-enriched and control communities increased with N addition levels, and N-enriched plots showed substantial species losses and gains. Interestingly, the reduction of community-scale NRE and PRE mostly resulted from N-induced decreases in resorption efficiency for the shared species in the control and N-enriched communities. There were negative correlations between the contributions of species richness effect and species identity effect and between the number and identity of species gained for the changes in both NRE and PRE following N enrichment. By simultaneously considering N-induced changes in species composition and in species-level resorption, our work presents a more complete picture of how different community assembly processes contribute to N-induced changes in community-level resorption.

Soil organic matter, rather than temperature, determines the structure and functioning of subarctic decomposer communities

The impacts of climate change on ecosystem structure and functioning are likely to be strongest at high latitudes due to the adaptation of biota to relatively low temperatures and nutrient levels. Soil warming is widely predicted to alter microbial, invertebrate, and plant communities, with cascading effects on ecosystem functioning, but this has largely been demonstrated over short-term (<10 year) warming studies. Using a natural soil temperature gradient spanning 10-35°C, we examine responses of soil organisms, decomposition, nitrogen cycling, and plant biomass production to long-term warming. We find that decomposer organisms are surprisingly resistant to chronic warming, with no responses of bacteria, fungi, or their grazers to temperature (fungivorous nematodes being an exception). Soil organic matter content instead drives spatial variation in microorganism abundances and mineral N availability. The few temperature effects that appear are more focused: root biomass and abundance of root-feeding nematodes decrease, and nitrification increases with increasing soil temperature. Our results suggest that transient responses of decomposers and soil functioning to warming may stabilize over time following acclimation and/or adaptation, highlighting the need for long-term, ecosystem-scale studies that incorporate evolutionary responses to soil warming.

Elevated atmospheric humidity prolongs active growth period and increases leaf nitrogen resorption efficiency of silver birch

Climate models predict increasing amounts of precipitation and relative atmospheric humidity for high latitudes in the Northern Hemisphere. Therefore, tree species must adjust to the new climatic conditions. We studied young silver birches (Betula pendula Roth) in a long-term (2012-2018) free air humidity manipulation experiment, with the aim of clarifying the acclimation mechanisms to elevated relative atmospheric humidity. In 2016-2018, stem radial increment (measured by dendrometers) and leaf abscission were monitored, and the leaf N and P resorption efficiencies were determined. Biomass allocation was estimated, and the seasonal dynamics of foliar NPK storage was assessed. Humidification increased N resorption efficiency by 11%. The annual means of N resorption efficiency varied from 41 to 52% in control and from 50 to 59% in humidified stands. The P resorption efficiency was strongly affected by weather conditions and varied between years from 25 to 66%. Higher foliar NPK storages at the end of growing season and delayed leaf fall allowed to extend the growth period in humidified plots, which resulted in a week longer stem radial growth. Although stem diameter growth of humidified birches recovered after 5 years, tree height retardation persisted over the seven study years, resulting in increased stem taper (diameter to height ratio) under humidification. Additionally, humidification increased the share of the bark in stem biomass and the number of branches per crown length. The acclimation of silver birches to increased air humidity entails changes in forest N cycle and in birch timber quality.