No abstract is provided for this article.
Global warming has already resulted in higher frequencies and severity of multiple abiotic and biotic stresses occurring concurrently or subsequently in farmers’ fields. This trend will likely amplify in the next decades. Yet, to date, the mechanisms determining interactions between abiotic and biotic stresses and their effects on crop performance under field conditions are unknown for most crops and stress combinations. Field data are particularly scarce and, hence, adequate modelling approaches do not exist so far. While crop‐growth models are the most appropriate tools for quantifying climate change effects, they remain largely radiation use efficiency (RUE)‐based, treating stress effects through empirical reductions in photosynthesis or yield (e.g., drought-related multipliers) rather than using explicit carbon reallocations. Critically, they ignore active defense sinks - the substantial fraction of assimilates moved into mucilage, phenolics and other biochemicals that protect plants under stress.This paper aims to describe a novel crop science and modelling approach, in which new empirical knowledge from the genetic to the field scale is integrated and formalized in the novel “MultiStress model” - implemented for maize.There are many examples of crop defence mechanisms towards multiple abiotic and biotic stressors and their interactions that come at carbon costs. Here, we focus on drought-response and illustrate the implementation of the MultiStress model for mucilage exudation under drought conditions. Many water-stressed plants including maize release root mucilage, a gelatinous polysaccharide that maintains rhizosphere moisture. This “hydraulic sponge” keeps soil around drying roots hydraulically conductive, facilitating higher water uptake in dry soil. Yet, the mucilage benefits come at a cost. It has been estimated that about 10–15% of total carbon assimilation may be diverted into mucilage under drought. This represents a large carbon sink that otherwise could have fueled grain production. Current crop models lack any pool for mucilage, so this carbon diversion is simply “lost” from the crop carbon budget. Empirical stress factors downscale growth but do not track where the saved carbon goes to. Most crop models impose a fractional yield loss under drought but cannot differentiate whether the plant invested extra carbon in mucilage or other survival mechanisms. This leads to misallocation of carbon, and overestimated yield and yield stability, since the metabolic cost of mucilage is never subtracted. The MultiStress model explicitly accounts for such carbon costs.Current process-based crop models are neither fit for generating the knowledge needed for assessing crop impacts of climate-induced multiple stress interactions; nor for the task of informing breeding of climate-resilient crop cultivars. Overcoming these challenges requires a renewal of crop science and modelling as shown and currently under development by the MultiStress Research Unit.
Biochemistry is an essential yet often undervalued aspect of soil ecology, especially in soil C cycling. We assume based on tradition, intuition or hope that the complexity of biochemistry is confined to the microscopic world, and can be ignored when dealing with whole soil systems. This opinion paper draws attention to patterns caused by basic biochemical processes that permeate the world of ecosystem processes. From these patterns, we can estimate activities of the biochemical reactions of the central C metabolic network and gain insights into the ecophysiology of microbial biosynthesis and growth and maintenance energy requirements; important components of Carbon Use Efficiency (CUE).The biochemical pathways used to metabolize glucose vary from soil to soil, with mostly glycolysis in some soils, and pentose phosphate or Entner-Doudoroff pathways in others. However, notwithstanding this metabolic diversity, glucose use efficiency is high and thus substrate use for maintenance energy and overflow respiration is low in these three soils. These results contradict current dogma based on four decades of research in soil ecology. We identify three main shortcomings in our current understanding of substrate use efficiency: 1) in numeric and conceptual models, we lack appreciation of the strategies that microbes employ to quickly reduce energy needs in response to starvation; 2) production of exudates and microbial turnover affect whole-soil CUE more than variation in maintenance energy demand; and 3) whether tracer experiments can be used to measure the long-term substrate use efficiency of soil microbial communities depends critically on the ability of non-growing cells to take up tracer substrates, how biosynthesis responds to these substrates, as well as on how cellular activities scale to the community level.To move the field of soil ecology forward, future research must consider the details of microbial ecophysiology and develop new tools that enable direct measurement of microbial functioning in intact soils. We submit that 13C metabolic flux analysis is one of those new tools.
Hotspots are characterized by an increased availability of nutritional elements compared to the surrounding bulk soil, which enhances microbial activity. However, the shifts in nutrient stoichiometry, when hotspot formation is initiated by a non-microbial organic matter source (rhizodeposits, litter, feces & mucus, percolating dissolved organic matter), are highly hotspot-specific. This results in contrasting microbial dynamics in the rhizo-hyphosphere, the detritusphere, the drilosphere and further biopores of soil animals, and in preferential flow pathways.Experiments and models of microbial growth-death dynamics have recently improved our understanding of how element stoichiometry shapes element allocation in microbial metabolism. This holds specifically true for to the two contrasting pathways of microbial growth – intracellular element storage versus the investment of C, nutrients and energy in replicative microbial growth. Both growth modes involve synthesis of organic polymers – either storage or structural cellular polymers. However, the nature of these two types of polymers is highly contrasting with regards to their elemental but also their molecular diversity, such that the two growth modes generate distinct differences in the molecular compositon of the cellular biomass. We can therefore expect that the stoichiometric differences of nutrient hotspots will drive differences in the molecular diversity of the microbial biomass, and so ultimately the successively accumulating necromass.Whereas controlled incubation experiments demonstrate how element allocation to storage and replicative growth depends on nutrient stoichiometry, we lack an understanding of how growth modes are distributed among hotspots in situ. Besides developing this conceptual understanding, we aim to shed light on the implications of differences in cell physiology among hotspots, which includes i) the turnover rate of microbial biomass due to contrasting resistance to stress (e.g. starvation), ii) the molecular composition of the microbial cells and iii) the resulting chemical properties and molecular diversity of necromass. These factors strongly influence the formation rates, qualities and persistence of necromass-derived soil organic matter arising in these hotspots. Furthermore, the accrual of organic matter shapes microbial resource availability, including element stoichiometry, in the hotspot, as well as the physico-chemical microbial habitat properties. In consequence, a feedback loop between microbial growth- and turnover-based organic matter formation and the initial processes, that trigger the hotspot formation elaborates. Thus, although hotspot formation is always initiated by non-microbial organic matter input, the characteristics of the established soil hotspots are ultimately linked to the microbial necromass’ molecular and elemental diversity, which is the direct product of the hotspots’ microbial metabolism and growth mode. This study aims to relate the nutrient enriching processes (rhizodeposits, litter, feces & mucus, percolating DOM) and soil-intrinsic feedbacks to the dominant microbial growth modes and resulting properties of the organic matter in soil hotspots. 
In the original version of this article, equations 4 and 9 unfortunately contained errors
Land use change and agricultural intensification in developing countries affect terrestrial carbon (C) stocks, CO2 efflux, microbial communities and overall soil health. This study assesses the effects of four land use types typical for northern Ethiopia (forests, exclosures, grazing lands and intensively cultivated croplands) on various soil health indicators. We quantified and compared microbial biomass carbon (MBC), water extractable organic carbon (WOC), metabolic quotient (qCO2), substrate use efficiency (SUE) and dynamics of 14C-labelled glucose added to soil. Irrespective of the land use, MBC but not SUE decreased 2- to 8-fold with increasing depth, demonstrating the C limitation of subsoil microbial communities under all land use forms. Sandy soils, however, which permit seepage and leaching of WOC into lower layers and promote subsoil microbial communities, adapted to frequent input of easily accessible C substrates. Significantly higher qCO2 were recorded in subsoils compared to topsoils, especially in croplands with low MBC. In croplands, high glucose-14C incorporation (≈20%) into their low microbial biomass indicates a high SUE and reflects a better nutrient supply of these microbial communities. Mineralization of up to 95% of 14C-labeled glucose in topsoils of forest and grazing lands was higher than in croplands, and exclosures never reached the level of natural ecosystems. This demonstrates that 6–10 years of exclosure establishment does not result in soil microbial communities and soil C dynamics resembling those of natural forests. Our study demonstrates that land use can negatively affect the ecological performance of microbial communities and that these impacts are more severe in sandy than in clayey soils. Mitigation strategies such as minimum tillage or residue retention in intensively cultivated croplands can increase microbial abundance and activity and help ensure environmental sustainability and mitigation of climate change. Nonetheless, such measures need to be carefully accompanied by monitoring indicators of soil health to confirm the sustainability of the chosen mitigation strategies.
No abstract is provided for this article.
To explore carbon (C), nitrogen (N), and phosphorus (P) dynamics during leaf litter decomposition, we investigated the temporal variability of soil microbial biomass and associated soil enzyme activities. Our 342-day leaf litter (Quercus wutaishanica) decomposition experiment at the Loess Plateau (China) sheds light on how soil microorganisms – mainly the soil microbial biomass C, N, P, and soil enzyme activities – maintain element homeostasis of C, N, and P by producing soil enzymes during various phases of litter decomposition. Overall, the highest soil enzyme activities were measured in summer, whereas soil microbial biomass carbon (MBC) and soil microbial biomass nitrogen (MBN) were highest in winter. Soil water content and soil temperature had significant effects on soil enzyme activities and stoichiometry. The results indicate that the stoichiometry of extracellular enzyme activities (lnBG:lnNAG:lnAP) changes significantly between individual stages of litter decomposition. The resources available for microorganisms were restricted by C and P, while P limitation progressively decelerated during decomposition and C limitation was enhanced at the latest stage. Strong positive correlations between ecological indicators of microbial element limitation and soil enzyme activities indicated that microbes allocate C and nutrients towards soil enzyme production to mine for scarce nutrients. This adaptation strategy enabled the soil microorganisms to maintain element homeostasis. Soil MBC, MBN, soil microbial biomass phosphorus (MBP), MBC:MBN, MBC:MBP, and MBN:MBP rarely showed significant correlations with soil or leaf litter C, N, P, C:N, C:P, and N:P. This suggests no resource dependency of microbial element composition but supports the concept of element homeostasis of soil microorganisms. Besides compensating for element imbalances by adjusting soil enzyme production, the microorganisms also adjusted element use efficiencies such as C use efficiency. We conclude that the maintenance of element homeostasis by soil microorganisms induces a tight co-regulation of enzymes involved in covering their C and nutrient supply during successive litter degradation.
Short rotation coppices (SRCs) are a promising alternative for environmental-friendly biomass production. However, profound understanding of nitrogen (N) uptake and allocation dynamics and their interaction with biomass production of individual tree species is required for their sustainable management. In-situ 15N soil pulse labeling of the widely applied willow cv. Tordis and poplar cv. Max 1 allowed tracing their uptake of Nmin and to evaluate the effect of N nutrition on their growth. A pulse of either 15NH4NO3 or NH4 15NO3 was applied to the soil of four replicate trees of each species in a pot experiment. Leaf, twigs, stem, root were analyzed to quantify the uptake and allocation of 15N after labeling. Summarizing all compartments of poplar, almost all of 15NO3 − (97%) from the N soil pool could be recovered, but only a third of the 15NH4 + (34%). In contrast, willow incorporated exactly the same amount of 15N (49%) from both tracers i.e. showing no preference for a certain Nmin species. Poplar did not only have the higher Nmin uptake but also showed a higher total biomass (12.2 g·tree−1) production than willow (10.2 g·tree−1) in first 56 days, which goes along which its higher allocation of N into leaves. We conclude that the poplar cv. Max 1 might be a better choice for biomass production, especially at arable sites with high N contents as well as for protecting from all negative impacts of non-closed N cycles as typical for classical agricultural managed sites (e.g. nitrate leaching or N2O emissions).
