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Summary: The link between above‐ and belowground communities is a key uncertainty in drought and rewetting effects on forest carbon (C) cycle.In young beech model ecosystems and mature naturally dry pine forest exposed to 15‐yr‐long irrigation, we performed 13C pulse labeling experiments, one during drought and one 2 wk after rewetting, tracing tree assimilates into rhizosphere communities.The 13C pulses applied in tree crowns reached soil microbial communities of the young and mature forests one and 4 d later, respectively. Drought decreased the transfer of labeled assimilates relative to the irrigation treatment. The 13C label in phospholipid fatty acids (PLFAs) indicated greater drought reduction of assimilate incorporation by fungi (−85%) than by gram‐positive (−43%) and gram‐negative bacteria (−58%).13C label incorporation was more strongly reduced for PLFAs (cell membrane) than for microbial cytoplasm extracted by chloroform. This suggests that fresh rhizodeposits are predominantly used for osmoregulation or storage under drought, at the expense of new cell formation. Two weeks after rewetting, 13C enrichment in PLFAs was greater in previously dry than in continuously moist soils. Drought and rewetting effects were greater in beech systems than in pine forest.Belowground C allocation and rhizosphere communities are highly resilient to drought. [ABSTRACT FROM AUTHOR]

Nitrogen (N) resorption is an important pathway of N conservation, contributing to an important proportion of plant N requirement. However, whether the ratio of N resorption to N requirement may be affected by environmental factors, mycorrhizal types or atmospheric CO2 concentration remains unclear. Here, we conducted a meta‐analysis on the impacts of environmental factors and mycorrhizal types on this ratio. We found this ratio in ectomycorrhizal (EM) trees decreased with mean annual precipitation, mean annual temperature, soil total N content and atmospheric CO2 concentration and was significantly lower than that in arbuscular mycorrhizal (AM) trees. An in situ15N tracing experiment further confirmed that AM trees have a stronger reliance on N resorption than EM trees. Our study suggests that AM and EM trees potentially have different strategies for alleviation of progressive N limitation, highlighting the necessity of incorporating plant mycorrhizal types into Earth System Models. [ABSTRACT FROM AUTHOR]

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer‐term (decadal to century) feedbacks between climate, CO2 and plant N uptake could emerge to reduce ecosystem‐level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N2 fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ15N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem‐level N cycling responses to rising atmospheric CO2 concentrations. We find that pre‐industrial increases in global atmospheric CO2 concentrations corresponded with a decrease in the δ15N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ15N of Sphagnum decreases with increasing N2‐fixation rates. These findings suggest that plant‐microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO2 concentrations, highlighting an ecosystem‐level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome. [ABSTRACT FROM AUTHOR]

Nitrogen (N) resorption is a strategy for plant N conservation through which plants withdraw N from senescing leaves prior to litterfall and its underlying mechanisms are important for better understanding of N cycling. However, most current studies focused on the impacts of soil and leaf nutrients on leaf N resorption efficiency (NRE), and plant physiological regulation that is species dependent is still unclear.Here, we conducted a field experiment to investigate the variations of leaf NRE along an altitudinal gradient in a temperate forest of Northeastern China.Results showed that leaf NRE of Quercus mongolica and Fraxinus mandshurica increased with altitude, while leaf NRE of Tilia amurensis, Acer mono and Acer pseudosieboldianum exhibited an opposite trend, although the relationships were not significant for F. mandshurica and A. mono. The inconsistent responses of leaf NRE of different species to increasing altitude were primarily due to the effect of leaf Glutamate dehydrogenase (GDH), an enzyme responsible for N translocation. Leaf GDH activity in senescing leaves explained the variation of NRE more than soil and climate factors did, suggesting that different plant species had different physiological regulation strategies for their N conservation under similar environment.Synthesis. Our study highlights the role of leaf enzyme as a pivotal regulator of leaf NRE and helps us better understand and predict N cycling under climate change in forest ecosystems. [ABSTRACT FROM AUTHOR]

Above and belowground compartments in ecosystems are closely coupled on daily to annual timescales. In mature forests, this interlinkage and how it is impacted by drought is still poorly understood. Here, we pulse‐labelled 100‐year‐old trees with 13CO2 within a 15‐year‐long irrigation experiment in a naturally dry pine forest to quantify how drought regime affects the transfer and use of assimilates from trees to the rhizosphere and associated microbial communities. It took 4 days until new 13C‐labelled assimilates were allocated to the rhizosphere. One year later, the 13C signal of the 3‐h long pulse labelling was still detectable in stem and soil respiration, which provides evidence that parts of the assimilates are stored in trees before they are used for metabolic processes in the rhizosphere. Irrigation removing the natural water stress reduced the mean C residence time from canopy uptake until soil respiration from 89 to 40 days. Moreover, irrigation increased the amount of assimilates transferred to and respired in the soil within the first 10 days by 370%. A small precipitation event rewetting surface soils altered this pattern rapidly and reduced the effect size to +35%. Microbial biomass incorporated 46 ± 5% and 31 ± 7% of the C used in the rhizosphere in the dry control and irrigation treatment respectively. Mapping the spatial distribution of soil‐respired 13CO2 around the 10 pulse‐labelled trees showed that tree rhizospheres extended laterally 2.8 times beyond tree canopies, implying that there is a strong overlap of the rhizosphere among adjacent trees. Irrigation increased the rhizosphere area by 60%, which gives evidence of a long‐term acclimation of trees and their rhizosphere to the drought regime. The moisture‐sensitive transfer and use of C in the rhizosphere has consequences for C allocation within trees, soil microbial communities and soil carbon storage. [ABSTRACT FROM AUTHOR]

Increasing atmospheric reactive nitrogen (N) deposition due to human activities could change N cycling in terrestrial ecosystems. However, the differences between the fates of deposited [ABSTRACT FROM AUTHOR]

Excessive application of nitrogen fertilizer in agricultural soils is considered as a main cause of ecological problems such as nitrogen leaching, which has become an important limitation to improving nitrogen use efficiency in agricultural production. Because of its physical, chemical, and biological stability, biochar when applied to soils can play a key role in nutrient cycling, potentially affecting nitrogen retention and improving soil fertility. However, experimental results were variable and dependent on the experimental set-up, soil properties, and fertilizer application rates and biochar were set up at the high and low levels, which possess a big blind area. A laboratory experiment was conducted to investigate the effects of biochar additions on dynamic changes of nitrogen under artificial rainfall conditions by adding 0, 0.5%, 2%, 4%, 6%, and 8% biochar (the biochar was prepared by pyrolyzing corn straw at 500-550℃ under oxygen limited condition) into soil columns collected from red dryland of southern China under urea application condition (300 kg/hm2). The results showed that the speed of leaching of NO3 - and total nitrogen was delayed by biochar applications. Compared with urea application treatment, the total nitrogen and NH4 + contents in leachate were significantly decreased by 4.02% and 8.93% when ≥2% biochar was applied into soil, and the NO3 - contents of leachate was obviously decreased by 3.23%-11.87% when the biochar ratio reached 4%. But there was no significant effect of adding small amount of biochar (such as 0.5%) on N leaching compared with urea application treatment. Leaching losses of NO3 - had been found to be highest because the NO3 - was mobile, and may not be taken up by plants. NO3 - leaching amount accounted for 84%-90% of the total nitrogen leaching amount, while NH4 + leaching only accounted for 0.4%-2%; The total nitrogen content of different soil layers in each treatment was similar regardless of the fertilization and biochar applications. While the total nitrogen contents in soils were markedly different among the treatments. Compared with urea application treatment, the content of soil total nitrogen was significantly increased by 4.62% to 10.62% when biochar addition was ≥2%, and the relationship between addition amounts of biochar (x) and the content of soil total nitrogen (y) followed an index equation: y=1.6e0.0845x (R2=0.99, n=24). The results above suggested that biochar could be used as a potential additive for nutrient retention in order to increase the utilization efficiency of chemical fertilizers. In conclusion, biochar additions could decrease the leaching amount of NH4 +, NO3 - and total nitrogen and partly increased the concentration of soil total nitrogen. However, the low biochar addition had no significant effects on reducing nitrogen leaching and increasing soil total nitrogen content. Therefore, the purpose to obviously decrease nitrogen leaching and increase the content of soil total nitrogen could be realized only when biochar addition was above 2% in dryland soil. Nonetheless, the effect of biochar addition on reducing soil nitrogen leaching needs to be further verified before biochar can be applied to agricultural fields on a large scale. [ABSTRACT FROM AUTHOR]

Drought limits the metabolic activity of plants and soil organisms, but drought effects on forest's C cycle will ultimately depend on the resilience of forests to drought – the ability of forests to recover from and to adapt to drought. Here, we synthesize various studies on the effects of drought on the forest C cycle at different time scales: (1) In 13C-pulse labelling experiments at the tree level in young and mature forests, we have tracked the fate of recent assimilates in ecosystems during drought and the recovery phase following rewetting. (2) In a naturally dry pine forest, we have conducted a 15 year long irrigation experiment to study how long-term water limitation affects C-pools and fluxes.At the annual time scale, photosynthesis and soil respiration were strongly suppressed by drought. Following rewetting, however, both C fluxes recovered rapidly and even exceeded C fluxes of the permanently moist control, almost compensating for the reduced C fluxes during drought. The 13C pulse labelling experiment in young model forests tracking recent assimilates from leafs to the soil revealed that during drought C allocation to the belowground was strongly reduced. In contrast, after rewetting in the recovery phase, substantially greater amounts of 13C were allocated belowground indicating that trees primarily invested their assimilates into their rhizosphere, probably to regain root functions after drought. In the mature pine forest, tracing of a 13C pulse for one year indicated a similarly sensitive response of ecosystem C cycling to soil moisture. Allocation of recently assimilated C to the belowground showed a tipping point at soil moisture contents around 15%, which is also the critical soil moisture for microbial activity in the soils. This finding strongly suggests that the moisture dependent C use by rhizosphere communities drives tree's utilization of recent assimilates. In the 15 year long experiment in the dry pine forest, irrigation strongly increased forest productivity and more than doubled C accumulation in forest biomass. Soil carbon fluxes were also strongly increased: C inputs by litterfall but also C outputs by soil respiration increased by a factor of two to three. The accelerated C cycling in the soil was also mirrored by compositional shifts in the soil microbiome. Irrigation promoted bacteria and fungi with more copiotrophic life style strategies. Determination of soil organic matter pools revealed a C loss in the organic layer under irrigation but a C gain in the mineral soil, resulting in small net effects.Overall our results show that C fluxes in the forest ecosystem respond very sensitive to drought with distinct tipping points. Rewetting following drought leads to a recovery of forests. Nevertheless, in the long-term, C accumulation in biomass is suppressed by repeated summer drought, while net effects on C balance of soils are much smaller. [ABSTRACT FROM AUTHOR]