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No abstract is provided for this article.

<p>Drought exerts a major control on the carbon (C) cycle of terrestrial ecosystems worldwide. However, the mechanisms and processes underpinning ecosystem responses remain uncertain, in particular in diverse, tall-growing ecosystems like tropical rainforests. In such ecosystems, trees are a predominant driver of ecosystem C cycling, as they link the major ecosystem fluxes, photosynthesis and ecosystem respiration through allocation and utilisation of recent assimilated C. Trees respond dynamically to drought, generally by reducing C assimilation and altering investments of recent C into metabolism, defence, growth and storage, which has consequences for the fate of C in the system. However, to date most of our understanding is derived from experiments on small trees and we lack an understanding of how whole-tree C allocation responds in diverse, stratified forest ecosystems.</p><p>To address this knowledge gap, we implemented a 9.5-week experimental drought in the world’s largest controlled growth facility, the Biosphere 2 Tropical Rainforest in Arizona, US. We continuously measured isotopic CO<sub>2</sub> fluxes of leaves, stem and soil as well as leaf and phloem non-structural carbohydrates across a range of canopy and understory forming trees during pre-drought and drought conditions. To study drought effects on the fate of recent photoassimilates, we labelled the entire ecosystem with a <sup>13</sup>CO<sub>2</sub> pulse during pre-drought and drought conditions and traced the carbon flow in leaf, stem and soil fluxes and non-structural carbohydrates of leaves and phloem.</p><p>Across all studied trees, drought generally reduced CO<sub>2</sub> uptake and metabolic activity in leaves, stems and soil. The phloem transport rates slowed down and the turnover of recent photoassimilates declined. As drought progressed respiration was increasingly fuelled by C reserves, as indicated by isotopic flux dynamics and a depletion of starch pools, particularly in leaves. Drought response patterns of fluxes, carbohydrate pools and C allocation dynamics were highly variable among trees. Interestingly, response diversity was not primarily explained by species identity, but likely related to a combination of functional and structural traits and the trees’ microenvironment within the forest. We conclude that the structural and functional composition of a forest is an important driver for tree C allocation and needs to be considered for understanding the mechanisms underpinning forest C dynamics in response to drought.</p>

For unfertile and degraded soils of sub-Saharan Africa, nitrogen (N) is often the most limiting growth factor restricting yields. The often-suggested exploitation of advantageous rhizosphere traits remains to be validated as a strategy to overcome N limitation, specifically when N deficiency co-occurs with further abiotic stresses such as water scarcity. To identify potential beneficial rhizosphere traits, three sorghum genotypes were cultivated in ‘mesocosms’ with a root-exclusion compartment under drought and optimal conditions. N mobilization and uptake were studied by 15N application coupled with 13CO2 labeling to track plant investment into rhizosphere traits. The uptake of 15Nmin by arbuscular mycorrhizal fungi (AMF) from the root-exclusion compartment was 4-12 times higher under drought compared to well-watered conditions. In addition, water stress enhanced below-ground allocation of recently assimilated carbon (C) into microbial biomass in both compartments. Under drought conditions, potential enzymatic reaction (Vmax) of chitinase and leucine amino peptidase declined and increased, respectively. This suggests that N acquisition from protein mineralization was relatively enhanced to that of chitin following moisture limitation. Substrate affinity (Km) of LAP was strongly reduced by drought compared to that of chitinase which displayed genotype-specific shifts in rhizosphere enzyme systems. We conclude that belowground C allocation to the rhizosphere microbiome activated the AMF symbiosis and its associated bacteria. This not only led to a shift in enzyme-driven exploitation of distinct organic N sources but also induced a strong increase in AMF-based Nmin acquisition from the hyphosphere. This trait plasticity in response to drought may be harnessed for stabilizing food production from low-fertile soil under the increasingly negative impacts of droughts under climate change.

No abstract is provided for this article.

Input of organic carbon (C) to the soil stimulates soil microbial activity leading to changes in turnover of soil organic matter, a phenomenon referred to

In the version of this article initially published, there was a mistake in Fig. 2f where 13 C-diacetyl was plotted instead of 13 C-enriched diacetyl ( 13 C/ 12 C + 13 C).This error led to the erroneous conclusion that there was 13 C-enriched diacetyl emitted from chambers that received 13 C1-pyruvate.The corrected version replaces Fig. 2i with 13 C-enriched diacetyl continuous flux data.Furthermore, since there was no 13 C-enriched diacetyl emitted from chambers that received 13 C1-pyruvate, Fig. 2, panel i (as well as panel h), was corrected to show only cumulative 13 C-fluxes from chambers that received 13 C2-pyruvate where there were 13 C-eniched fluxes.Fig. 3, panel a, was corrected to leave only a pink circle under "Diacetyl" indicating 13 C-enrichment from only chambers that received 13 C2-pyruvate.Original and corrected Fig. 2f-i and 3a panels are shown as, respectively, Fig. 1 and Fig. 2 below.Finally, in the Results section, the associated text that

Mucilage, a gelatinous substance comprising mostly polysaccharides, is exuded by maize nodal and underground root tips. Although mucilage provides several benefits for rhizosphere functions, studies on the variation in mucilage amounts and its polysaccharide composition between genotypes are still lacking. In this study, eight maize ( Zea mays L.) genotypes from different globally distributed agroecological zones were grown under identical abiotic conditions in a randomized field experiment. Mucilage exudation amount, neutral sugars and uronic acids were quantified. Galactose (∼39–42%), fucose (∼22–30%), mannose (∼11–14%), and arabinose (∼8–11%) were the major neutral sugars in nodal root mucilage. Xylose (∼1–4%), and glucose (∼1–4%) occurred only in minor proportions. Glucuronic acid (∼3–5%) was the only uronic acid detected. The polysaccharide composition differed significantly between maize genotypes. Mucilage exudation was 135 and 125% higher in the Indian (900 M Gold) and Kenyan (DH 02) genotypes than in the central European genotypes, respectively. Mucilage exudation was positively associated with the vapor pressure deficit of the genotypes’ agroecological zone. The results indicate that selection for environments with high vapor pressure deficit may favor higher mucilage exudation, possibly because mucilage can delay the onset of hydraulic failure during periods of high vapor pressure deficit. Genotypes from semi-arid climates might offer sources of genetic material for beneficial mucilage traits.

Ecosystem functions of tropical mountain ecosystems and their ability to provide ecosystem services are particularly threatened by the combined impact of climate and land-use change. Soils, as the linkage between abiotic and biotic components of an ecosystem, are strongly affected by these changes. To understand impacts of climate and land use changes on biodiversity and accompanying ecosystem stability and services at Mt. Kilimanjaro, detailed understanding and description of the current biotic and abiotic controls on ecosystem Carbon (C) and nutrient fluxes are needed. Therefore, we quantitatively described cycles of C and major nutrients (N, P, K, Ca, Mg, Mn, Na, S) on pedon and stand level scale along a 3500 m elevation gradient and in up to three stages of land-use intensification. Qualitative indicators (composition of soil organic matter and microbial communities) were used to relate pool changes to underlying processes. Annual pattern of litterfall and decomposition were closely related to rainfall seasonality and temperature. Several factors, such as decomposition rate, C & N contents, microbial biomass (MBC) and leaf litter quality, increased at mid elevation. This was reflected in shifts of soil organic matter composition and microbial communities controlling soil C stability. Land-use intensification led to 40-80% losses in topsoil C and MBC contents as well as an increased turnover through higher microbial demand for new C sources. In ecosystems with strong seasonal variations (savanna and alpine helichrysum cushion) the effectiveness of C storage and N turnover was strongly affected by spatial vegetation heterogeneity. Ecosystems at mid elevation (~2000 m) represent the interception zone of optimal moisture and temperature conditions. High inputs and fast turnover control the C sequestration in these ecosystems, while climatic restrains on input and decomposition limit the C turnover in soils at lower and higher elevation. Land-use intensification increases C and nutrient cycling, decreases stabilization from new C inputs through increased microbial C demand and thus decreases soil C storage.

No abstract is provided for this article.