Explore the vast range of titles available.

Semi‐arid grasslands on the Mongolian Plateau are expected to experience high inputs of anthropogenic reactive nitrogen in this century. It remains unclear, however, how soil organisms and nutrient cycling are directly affected by N enrichment (i.e., without mediation by plant input to soil) vs. indirectly affected via changes in plant‐related inputs to soils resulting from N enrichment. To test the direct and indirect effects of N enrichment on soil organisms (bacteria, fungi and nematodes) and their associated C and N mineralization, in 2010, we designated two subplots (with plants and without plants) in every plot of a six‐level N‐enrichment experiment established in 1999 in a semi‐arid grassland. In 2014, 4 years after subplots with and without plant were established, N enrichment had substantially altered the soil bacterial, fungal and nematode community structures due to declines in biomass or abundance whether plants had been removed or not. N enrichment also reduced the diversity of these groups (except for fungi) and the soil C mineralization rate and induced a hump‐shaped response of soil N mineralization. As expected, plant removal decreased the biomass or abundance of soil organisms and C and N mineralization rates due to declines in soil substrates or food resources. Analyses of plant‐removal‐induced changes (ratios of without‐ to with‐plant subplots) showed that micro‐organisms and C and N mineralization rates were not enhanced as N enrichment increased but that nematodes were enhanced as N enrichment increased, indicating that the effects of plant removal on soil organisms and mineralization depended on trophic level and nutrient status. Surprisingly, there was no statistical interaction between N enrichment and plant removal for most variables, indicating that plant‐related inputs did not qualitatively change the effects of N enrichment on soil organisms or mineralization. Structural equation modelling confirmed that changes in soil communities and mineralization rates were more affected by the direct effects of N enrichment (via soil acidification and increased N availability) than by plant‐related indirect effects. Our results provide insight into how future changes in N deposition and vegetation may modify below‐ground communities and processes in grassland ecosystems. A plain language summary is available for this article. Plain Language Summary

1. Mountain grasslands have recently been exposed to substantial changes in land use and climate and in the near future will likely face an increased frequency of extreme droughts. To date, how the drought responses of carbon (C) allocation, a key process in the cycle, are affected by land-use changes in mountain grassland is not known. 2. We performed an experimental summer drought on an abandoned grassland and a traditionally managed hay meadow and traced the fate of recent assimilates through the plant-soil continuum. We applied two ¹³CO₂ pulses, at peak drought and in the recovery phase shortly after rewetting. 3. Drought decreased total C uptake in both grassland types and led to a loss of above-ground carbohydrate storage pools. The below-ground C allocation to root sucrose was enhanced by brought, eapecially in the meadow, which also held larger root carbohydrate storage pools. 4. The microbial community of the abandoned grassland comprised more saprotrophic fungal and Gram(+) bacterial markers compared to the meadow. Drought increased the newly introduced AM and saprotrophic (A+S) fungi:bacteria ratio in both grassland types. At peak drought,¹³C transfer into AM and saprotrophic fungi, and Gram(-) bacteria was more strongly reduced in the meadow than in the abandoned grassland, which contrasted the patterns of the root carbohydrate pools. 5. In both grassland types, the C allocatioin largely recovered after rewetting. Slowest recovery was found for AM fungi and their ¹³C uptake. In contrast, all bacterial markers quickly recovered C uptake. In the meadow, where plant nitrate uptake was enhanced after drought, C uptake was even higher than in control plots. 6. Synthesis. Our results suggest that resistance and resilience (i.e. recovery) of plant C dynamics and plant-microbial interactions are negatively related, that is, high resistance is followed by slow recovery and vice versa. The abandoned grassland was more resistant to drought than the meadow and possibly had a stronger link to AM fungi that could have provided better access to water through the hyphal network. In contrast, meadow communities strongly reduced C allocation to storage and C transfer to microbial community in the brought phase, but in the recovery phase invested C resources in the bacterial communities to gain more nutrients for regrowth. We conclude that the management of mountain grasslands increases their resilience to drought.

Terrestrial ecosystems worldwide are receiving increasing amounts of biologically reactive nitrogen (N) as a consequence of anthropogenic activities. This intended or unintended fertilization can have a wide‐range of impacts on biotic communities and hence on soil respiration. Reduction in below‐ground carbon (C) allocation induced by high N availability has been assumed to be a major mechanism determining the effects of N enrichment on soil respiration. In addition to increasing available N, however, N enrichment causes soil acidification, which may also affect root and microbial activities. The relative importance of increased N availability vs. soil acidification on soil respiration in natural ecosystems experiencing N enrichment is unclear. We conducted a 12‐year N enrichment experiment and a 4‐year complementary acid addition experiment in a semi‐arid Inner Mongolian grassland. We found that N enrichment had contrasting effects on root and microbial respiration. N enrichment significantly increased root biomass, root N content and specific root respiration, thereby promoting root respiration. In contrast, N enrichment significantly suppressed microbial respiration likely by reducing total microbial biomass and changing the microbial community composition. The effect on root activities was due to both soil acidity and increased available N, while the effect on microbes primarily stemmed from soil acidity, which was further confirmed by results from the acid addition experiment. Our results indicate that soil acidification exerts a greater control than soil N availability on soil respiration in grasslands experiencing long‐term N enrichment. These findings suggest that N‐induced soil acidification should be included in predicting terrestrial ecosystem C balance under future N deposition scenarios.

1. Drought periods are projected to become more severe and more frequent in many European regions. While effects of single strong droughts on plant and microbial carbon (C) dynamics have been studied in some detail, impacts of recurrent drought events are still little understood. 2. We tested whether the legacy of extreme experimental drought affects responses of plant and microbial C and nitrogen (N) turnover to further drought and rewetting. In a mountain grassland, we conducted a l3C pulse-chase experiment during a naturally occurring drought and rewetting event in plots previously exposed to experimental droughts and in ambient controls (AC). After labelling, we traced 13C below-ground allocation and incorporation into soil microbes using phospholipid fatty acid biomarkers. 3. Drought history (DH) had no effects on the standing shoot and fine root plant biomass. However, plants with experimental DH displayed decreased shoot N concentrations and increased fine root N concentrations relative to those in AC. During the natural drought, plants with DH assimilated and allocated less 13C below-ground; moreover, fine root respiration was reduced and not fuelled by fresh C compared to plants in AC. 4. Regardless of DH, microbial biomass remained stable during natural drought and rewetting. Although microbial communities initially differed in their composition between soils with and without DH, they responded to the natural drought and rewetting in a similar way: gram-positive bacteria increased, while fungal and gram-negative bacteria remained stable. In soils with DH, a strongly reduced uptake of recent plant-derived 13C in microbial biomarkers was observed during the natural drought, pointing to a smaller fraction of active microbes or to a microbial community that is less dependent on plant C. 5. Synthesis. Drought history can induce changes in abovevs. below-ground plant N concentrations and affect the response of plant C turnover to further droughts and rewetting by decreasing plant C uptake and below-ground allocation. DH does not affect the responses of the microbial community to further droughts and rewetting, but alters microbial functioning, particularly the turnover of recent plant-derived carbon, during and after further drought periods.

Studies of plant resource‐use strategies along environmental gradients often assume that dry matter partitioning represents an individual's energy investment in foraging for above‐ versus below‐ground resources. However, ecosystem‐level studies of total below‐ground carbon allocation (TBCA) in forests do not support the equivalency of energy (carbon) and dry matter partitioning, in part because allocation of carbon to below‐ground pools and fluxes that are not accounted for by root biomass (e.g., mycorrhizal hyphae, rhizodeposition; root and soil respiration) can be substantial. Here, we apply this reasoning to individual plants in controlled environments and ask whether dry matter partitioning below‐ground (root mass fraction, RMF) accurately reflects TBCA in studies of optimal partitioning theory. We quantified the relationship between RMF and TBCA in individual plants, using 311 observations from 51 studies that simultaneously measured both allocation variables. Our analysis included tests of whether the RMF‐TBCA relationship depended on mutualist soil microbes, plant growth form, age and study methodology including isotopic pulse–chase duration. We found that RMF was a poor proxy for below‐ground energy investment. This disconnect of RMF and TBCA was driven in part by plants of low RMF (<0.4) exhibiting significantly higher rates of root and soil respiration per unit root mass than plants of high RMF. Root colonization by mutualist microbes, including arbuscular mycorrhizal fungi and nitrogen‐fixing bacteria, increased TBCA by 5%–7%, and TBCA was lower in grasses than other species by 9%–16%. These patterns were evident for relationships assessed both within and between species. We conclude that optimal partitioning studies of plants along environmental gradients are likely to underestimate plant energy allocation below‐ground if the C costs of root and soil respiration are ignored, especially under conditions favouring low RMF. Because energy rather than biomass better reflects how assimilated C supports fitness, this omission of respired C suggests existing studies misrepresent the significance of below‐ground processes to plant function. A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

$\\bullet$ Although arbuscular mycorrhizal (AM) fungi are a major pathway in the global carbon cycle, their basic biology and, in particular, their respiratory response to temperature remain obscure. $\\bullet$ A pulse label of the stable isotope 13C was applied to Plantago lanceolata, either uninoculated or inoculated with the AM fungus Glomus mosseae. The extra-radical mycelium (ERM) of the fungus was allowed to grow into a separate hyphal compartment excluding roots. We determined the carbon costs of the ERM and tested for a direct temperature effect on its respiration by measuring total carbon and the $^{13}C:^{12}C$ ratio of respired CO2. With a second pulse we tested for acclimation of ERM respiration after 2 wk of soil warming. $\\bullet$ Root colonization remained unchanged between the two pulses but warming the hyphal compartment increased ERM length. δ13C signals peaked within the first 10 h and were higher in mycorrhizal treatments. The concentration of CO2 in the gas samples fluctuated diurnally and was highest in the mycorrhizal treatments but was unaffected by temperature. Heating increased ERM respiration only after the first pulse and reduced specific ERM respiration rates after the second pulse; however, both pulses strongly depended on radiation flux. $\\bullet$ The results indicate a fast ERM acclimation to temperature, and that light is the key factor controlling carbon allocation to the fungus.

The growth response of the hyphae of mycorrhizal fungi has been determined, both when plant and fungus together and when only the fungus was exposed to a temperature change. Two host plant species, Plantago lanceolata and Holcus lanatus, were grown separately in pots inoculated with the mycorrhizal fungus Glomus mosseae at 20/18 °C (day/night); half of the pots were then transferred to 12/10 °C. Plant and fungal growth were determined at six sequential destructive harvests. A second experiment investigated the direct effect of temperature on the length of the extra‐radical mycelium (ERM) of three mycorrhizal fungal species. Growth boxes were divided in two equal compartments by a 20 µm mesh, allowing only the ERM and not roots to grow into a fungal compartment, which was either heated (+8 °C) or kept at ambient temperature. ERM length (LERM) was determined on five sampling dates. Growth of H. lanatus was little affected by temperature, whereas growth of P. lanceolata increased with temperature, and both specific leaf area (SLA) and specific root length (SRL) increased independently of plant size. Percentage of colonized root (LRC) and LERM were positively correlated with temperature when in symbiosis with P. lanceolata, but differences in LRC were a function of plant biomass. Colonization was very low in H. lanatus roots and there was no significant temperature effect. In the fungal compartment LERM increased over time and was greatest for Glomus mosseae. Heating the fungal compartment significantly increased LERM in two of the three species but did not affect LRC. However, it significantly increased SRL of roots in the plant compartment, suggesting that the fungus plays a regulatory role in the growth dynamics of the symbiosis. These temperature responses have implications for modelling carbon dynamics under global climate change.

Here, soil CO₂ efflux, minirhizotron fine root production (FRP), and estimated total below-ground carbon allocation (TBCA) were examined along an elevation and hybridization gradient between two cottonwood species. FRP was 72% greater under high-elevation Populus angustifolia, but soil CO₂ efflux and TBCA were 62% and 94% greater, respectively, under low-elevation stands dominated by Populus fremontii, with a hybrid stand showing intermediate values. Differences between the responses of FRP, soil CO₂ efflux and TBCA may potentially be explained in terms of genetic controls; while plant species and hybridization explained variance in carbon flux, we found only weak correlations of FRP and TBCA with soil moisture, and no correlations with soil temperature or nitrogen availability. Soil CO₂ efflux and TBCA were uncorrelated with FRP, suggesting that, although below-ground carbon fluxes may change along environmental and genetic gradients, major components of below-ground carbon flux may be decoupled.

1. Intensively managed pine plantations in the south-eastern United States can play an important role in global carbon sequestration both through accumulation of carbon in wood used in long-lasting products as well as through increased soil carbon storage. Fertilization and understorey-elimination are two commonly used intensive management practices in the south-eastern United States that have the potential to increase carbon storage in vegetation and affect soil carbon. 2. In this study, we assessed the effects of these practices on carbon accumulation in vegetation biomass and in the soil of 17-year-old slash pine Pinus elliottii plantations in the flatwoods of northern Florida, USA. 3. Three treatments, fertilization, understorey-elimination, and fertilization plus understorey-elimination, were evaluated and compared with an untreated control. 4. All three treatments increased above-ground biomass accumulation compared with the untreated control; understorey-elimination also increased biomass of the forest floor litter, with or without fertilization. 5. Although understorey-elimination increased above-ground production, as a result of reduced below-ground production total net primary production was decreased in plots from which the understorey was eliminated. 6. Soil carbon storage was lower in plots where the understorey was eliminated, with or without fertilization. This appeared to be the result of reduced fine root growth and mortality but also may have reflected reduced litterfall inputs early in the rotation. 7. Our results indicate that intensive management of pine plantations on sandy flat-woods soils can increase carbon sequestration, but these increases will be the result of increased carbon accumulation in biomass and its long-term uses rather than through increased soil carbon.

The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO₂ concentration ([CO₂]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO₂ enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO₂] and to predict the likely effect of elevated [CO₂]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (L(D)) to account for differences in growing-season length. Moreover, elevated [CO₂]-induced enhancement of TBCA in the combined data decreased from approximately equal to 50% (700 g C m⁻² y⁻¹) at the lowest L(D) to approximately equal to 30% (200 g C m⁻² y⁻¹) at the highest L(D). The consistency of the trend in TBCA with L and its response to [CO₂] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.