Linking nutrient resorption stoichiometry with plant growth under long-term nitrogen addition

Increased nitrogen(N)input can potentially lead to secondary phosphorus(P)limitation;however,it remains unclear whether differences in the plant's ability to cope with this P deficiency are related to their growth responses.Using a long-term experiment of N addition in a boreal forest,we explored the potential role of plant nutrient resorption efficiency and its stoichiometry in mediating plant growth responses to increased N input.We recorded the cover and measured the concentration and resorption efficiency of leaf N and P as well as the photosynthesis of a grass Deyeuxia angustifolia and a shrub Vaccinium vitis-idaea.The cover of the grass D.angustifolia increased with increasing N addition,while that of the shrub V.vitis-idaea decreased with N addition rate and almost disappeared from the high-level N addition over time.P resorption efficiency(PRE)increased in D.angustifolia but decreased in V.vitis-idaea with increasing leaf N:P which was increased by N addition for both species.In addition,photosynthesis increased linearly with N resorption efficiency(NRE)and PRE but was better explained by NRE:PRE,changing nonlinearly with the ratio in a hump-shaped trend.Furthermore,the variance(CV)of NRE:PRE for V.vitis-idaea(123%)was considerably higher than that for D.angustifolia(29%),indicating a more stable nutrient resorption stoichiometry of the grass.Taken together,these results highlight that efficient P acquisition and use strategy through nutrient resorption processes could be a pivotal underlying mechanism driving plant growth and community composition shifts under N enrichment.

Is Moss Stoichiometry Influenced by Microtopography in a Boreal Peatland of Northeast China？

To examine the effects of microtopography on the stoichiometry of carbon(C), nitrogen(N) and phosphorus(P) in mosses along the hummock-hollow gradient in boreal peatlands, we investigated species-level C?N, C?P and N?P ratios of five mosses(Sphagnum magellanicum, S. perichaetiale, S. palustre, S. girgensohnii and Aulacomnium palustre) in the hummocks, hollows and their intermediate zones, and then assessed community-level spatial patterns in a boreal ombrotrophic peatland of north of the Great Xing'an Mountain, Northeast China. The results show that at the species level, C?N, C?P and N?P ratios of the selected Sphagnum mosses remained stable in the hummock-hollow complexes due to unchanged C, N and P concentrations, whereas the non-Sphagnum moss(A. palustre) in the hummocks and intermediate zones had lower P concentrations and thus greater C?P ratios than that in the hollows. At the community level, moss N concentration and C?N ratio remained constant along the hummock-hollow gradient, whereas hummocks and intermediate zones had higher community-level moss C?P and N?P ratios than hollows because of greater C and lower P concentrations. These findings imply that the effects of microtopography on moss C?N?P stoichiometry are scale-dependent and reveal spatial heterogeneity in C and nutrient dynamics. These results provide a more comprehensive understanding of biogeochemical cycles in boreal peatlands.

Soil microbial respiration is regulated by stoichiometric imbalances: Evidence from a humidity gradient case

Humidity not only affects soil microbial respiration(SMR) directly, but, indirectly by regulating the availability of soil water and nutrients. However,the patterns of direct and indirect effects of humidity on SMR over large precipitation gradients remain unclear, limiting our understanding of the effects of precipitation changes on soil C cycle. Here, we investigated the relationships among humidity, soil nutrients, and SMR by identifying stoichiometric imbalances, microbial elemental homeostasis, and microbial C use efficiency along a precipitation gradient at a continental scale. The relationship between SMR and humidity index(HI) corresponded to a Richard’s curve with an inflection point threshold value of approximately 0.7. Soil microbial respiration increased with increasing humidity in drier areas(HI < 0.7), but tended to balance above this threshold. Increasing humidity exacerbated C:P and N:P imbalances across the selected gradient. Severe N and P limitations in soil microbial communities were observed in drier areas, while soil microbes suffered from aggravated P limitation as the humidity increased in wetter areas(HI > 0.7). Soil microbial communities regulated their enzyme production to maintain a strong stoichiometric homeostasis in drier areas;enzyme production, microbial biomass, and threshold elemental ratios were non-homeostatic under P limitation in wetter areas, which further contributed to the increase in SMR. Our results identified a moisture constraint on SMR in drier areas and highlighted the importance of nutrient(especially for P) limitations induced by humidity in regulating SMR in wetter areas. Understanding the modulation of SMR via soil enzyme activity may improve the prediction of soil C budget under future global climate change.

High adaptability of Pinus sylvestris var.mongolica to drought-induced soil nutrient deficiency

Background:Drought can exert a profound influence on soil nutrient availability,and understanding whether and how tree species adapt to this change is a critical priority for predicting the consequence of climate change on forest structure and function.The objective of this study was to examine the adaptability of Mongolian pine(Pinus sylvestris var.mongolica)to drought‑induced changes in soil nutrient availability from the perspective of root functions.Methods:We conducted a 7‑year precipitation manipulation experiment with three levels of throughfall reduction(0%,30%,and 50%)to simulate different drought intensities.We measured soil physicochemical properties and fineroot nutrient concentrations and biomass,and calculated the stoichiometric homeostatic regulation coefficient(1/H)of fine roots.Results:Drought reduced soil organic carbon(C),nitrogen(N),phosphorous(P)and inorganic N concentrations,as well as ratios of total N to total P,and available N to available P in the 0–20 cm soil layer.In contrast,drought had no significant effect on fine‑root N and P concentrations,and fine‑root biomass in the 0–40 cm soil layer.Fine roots displayed high homeostatic regulation coefficients of N(with 1/H values of 0.19 and 0)and P(with 1/H values of 0.33 and 0)concentrations in 0–20 and 20–40 cm soil layers,respectively.Conclusions:Our results indicate that drought leads to soil nutrient deficiency and the decoupling between N and P cycling,and provide evidence that Mongolian pine has high adaptability to drought‑induced decrease in soil nutrient availability by maintaining great fine‑root biomass to ensure sufficient nutrient uptake.

Effects of nitrogen addition on plant-soil-microbe stoichio-metry characteristics of different functional group species in Bothriochloa ischemum community

Nitrogen(N)deposition,the source of N input into terrestrial ecosystems,is exhibiting an increasingly serious impact on the biogeochemical cycle and functional stability of ecosystems.Grasslands are an important component of terrestrial ecosystems and play a key role in maintaining terrestrial ecosystem balance.Therefore,it is critical to understand the effects of nitrogen addition on grassland ecosystems.We conducted gradientN addition experiments(0,3,6,and 9 g N m^(-2)2 y^(-1))for threeyears ingrassland communities with similar site conditions.We utilized four typical herbaceous plants,including the dominant species Bothriochloa ischemum(B.ischemum)and companion species Stipa bungeana(S.bungeana),Artemisia gmelinii(A.gmelinii),and Cleistogenes squarrosa(C.squarrosa),to explore how different plant-soil-microbe systems respond to N addition.Stoichiometric homeostasis analysis demonstrated that both plants and microbes were strictly homeostatic.However,the companion species were found to be more susceptible to P dominant species.Furthermore,aggravated overlap in stoichiometric niches between plant species were observed at the N6 and N9 levels.Vector analysis indicated that the vector angle was>45°regardlessof plant species and N levels,suggesting that there was a strong Plimitation in the rhizosphere microbial community.Variation partitioning analysis revealed that the Composite roots exhibited a greater effect(explaining 34.7% of the variation)on the rhizosphere microbes than on the Gramineae,indicating that there may be more intense nutrient competition in its rhizosphere.Ingeneral,the effects of N addition on species were different a cross functional groups,with a significant positive effect on the Gramineae(B.ischemum,S.bungeana,and C.squarrosa)and a significant negative effecton the Compositae(A.gmelinii),which should be fully considered in the future ecological management and restoration.