Towards comparable assessment of the soil nutrient status across scales-Review and development of nutrient metrics

Leaf trait responses to global change factors in terrestrial ecosystems

Global change influences plant growth by affecting plant morphology and physiology. However, the effects of global change factors vary based on the climate gradient. Here, we established a global database of leaf traits from 192 experiments on elevated CO2 concentrations (eCO2), drought, N deposition, and warming. The results showed that the leaf mass per area (LMA) significantly increased under eCO2 and drought conditions but decreased with N deposition, whereas eCO2 levels and drought conditions reduced stomatal conductance and increased and decreased photosynthetic rates, respectively. Leaf dark respiration (Rd) increased in response to global change, excluding N deposition. Leaf N concentrations declined with eCO2 but increased with N deposition. Leaf area increased with eCO2, N deposition, and warming but decreased with drought. Leaf thickness increased with eCO2 but decreased with warming. eCO2 and N deposition enhanced plant water-use efficiency (WUE), eCO2 and warming increased photosynthetic N-use efficiency (PNUE), while N fertilization reduced PNUE significantly. eCO2 produced a positive relationship between WUE and PNUE, which were limited under drought but increased in areas with high humidity and high temperature. Trade-offs were observed between WUE and PNUE under drought, N deposition, and warming. These findings suggest that the effects of global change factors on plants can be altered by complex environmental changes; moreover, diverse plant water and nutrient strategy responses can be interpreted against the background of their functional traits.

Spatial-temporal variability in under-canopy soil fertility and nutritional contents of cashew trees in Makonde Plateau of southeastern Tanzania

This study was conducted out to evaluate the soil fertility in under the cashew tree canopy and nutritional contents of cashew trees on the Makonde Plateau in southeast Tanzania. Seven villages were included, all of which were geographically close to one another yet had the same agro-ecology. The cashew plant samples were taken above the same longitudinal segment as the soil samples, which were taken in under canopies of cashew trees. Nitrogen, Phosphorus, Potassium, Magnesium, Calcium, and micro nutrients (Iron, Zinc, Sodium, and Copper) were all examined in the samples. However, soils were analysed for Organic Carbon, Organic Matter, and pH with soil samples taken at two different depths of 0-30 cm and 30-50 cm, these tests were conducted during the wet and dry seasons. The results confirmed that Calcium, Magnesium, Sodium, and Iron varied significantly with soil depth, as well as with soil depth, seasons, and their interactions, for Organic Carbon and Organic Matter. During the wet season, Nitrogen, Phosphorus, and Zinc concentrations in cashew trees were all statistically higher. Nitrogen and Phosphorus were significantly strong and positively (r = 0.95) correlated in cashew plants with respect to all other nutrients, suggesting synergistic effects. These results imply that macro nutrients including Nitrogen, Phosphorus, Potassium, Magnesium, and Calcium, and micronutrients such as Iron, Zinc, Sodium and Copper limit cashew production in the research area. It is necessary to determine site-specific recommendations and dosages for these nutrients.

When things get MESI: The Manipulation Experiments Synthesis Initiative-A coordinated effort to synthesize terrestrial global change experiments

Responses of the terrestrial biosphere to rapidly changing environmental conditions are a major source of uncertainty in climate projections. In an effort to reduce this uncertainty, a wide range of global change experiments have been conducted that mimic future conditions in terrestrial ecosystems, manipulating CO₂ , temperature, and nutrient and water availability. Syntheses of results across experiments provide a more general sense of ecosystem responses to global change, and help to discern the influence of background conditions such as climate and vegetation type in determining global change responses. Several independent syntheses of published data have yielded distinct databases for specific objectives. Such parallel, uncoordinated initiatives carry the risk of producing redundant data collection efforts and have led to contrasting outcomes without clarifying the underlying reason for divergence. These problems could be avoided by creating a publicly available, updatable, curated database. Here, we report on a global effort to collect and curate 57,089 treatment responses across 3644 manipulation experiments at 1145 sites, simulating elevated CO₂ , warming, nutrient addition, and precipitation changes. In the resulting Manipulation Experiments Synthesis Initiative (MESI) database, effects of experimental global change drivers on carbon and nutrient cycles are included, as well as ancillary data such as background climate, vegetation type, treatment magnitude, duration, and, unique to our database, measured soil properties. Our analysis of the database indicates that most experiments are short term (one or few growing seasons), conducted in the USA, Europe, or China, and that the most abundantly reported variable is aboveground biomass. We provide the most comprehensive multifactor global change database to date, enabling the research community to tackle open research questions, vital to global policymaking. The MESI database, freely accessible at doi.org/10.5281/zenodo.7153253, opens new avenues for model evaluation and synthesis-based understanding of how global change affects terrestrial biomes. We welcome contributions to the database on GitHub.

Nitrogen and water availability control plant carbon storage with warming

Plants may slow global warming through enhanced growth, because increased levels of photosynthesis stimulate the land carbon (C) sink. However, how climate warming affects plant C storage globally and key drivers determining the response of plant C storage to climate warming remains unclear, causing uncertainty in climate projections. We performed a comprehensive meta-analysis, compiling 393 observations from 99 warming studies to examine the global patterns of plant C storage responses to climate warming and explore the key drivers. Warming significantly increased total biomass (+8.4 %), aboveground biomass (+12.6 %) and belowground biomass (+10.1 %). The effect of experimental warming on plant biomass was best explained by the availability of soil nitrogen (N) and water. Across the entire dataset, warming-induced changes in total, aboveground and belowground biomass all positively correlated with soil C:N ratio, an indicator of soil N availability. In addition, warming stimulated plant biomass more strongly in humid than in dry ecosystems, and warming tended to decrease root:shoot ratios at high soil C:N ratios. Together, these results suggest dual controls of warming effects on plant C storage; warming increases plant growth in ecosystems where N is limiting plant growth, but it reduces plant growth where water availability is limiting plant growth.

Seasonal variations of some soil nutrients in a natural and an agricultural olive grove in Adana, Turkey

The bioavailability and cycling of nutrients in soil are two of the most important factors for healthy plant growth and development in natural and agricultural ecosystems. Seasonal variations of some soil macronutrient (phosphorus and potassium) and micronutrient (copper, manganese, and zinc) contents were investigated in a natural olive (Olea europaea L.) grove (NO) and an agricultural olive gene garden (OGG) located in Adana, Turkey. Soils were sampled at 0-10 cm and at 10-20 cm depth in the months of November, February, May, and August between 2013 and 2015. Soil phosphorus, potassium, copper, manganese, and zinc contents were in the range between 0.37 and 8.65 mg kg^-1, 181.81 and 1030.67 mg kg^-1, 1.41 and 2.74 mg kg^-1, 13.88 and 51.06 mg kg^-1, and 0.39 and 2.27 mg kg^-1, respectively. All soil nutrients significantly decreased as soil depth increased in all sampling times (P < 0.05). In general, significant seasonal effects were observed in all soil nutrients at 0-10 cm depth that was more variable than at 10-20 cm depth. Soil phosphorus negatively and positively correlated with soil potassium in NO and in OGG at 0-10 cm depth, respectively (P < 0.05). Soil zinc was negatively and positively correlated with soil phosphorus in NO and in OGG at 10-20 cm depth, respectively (P < 0.05). In conclusion, soil depth might be a more important factor than seasonality on the vertical distribution of soil nutrients in olive groves. In addition, correlations between soil nutrients in this study should be taken into consideration for the optimum management of agricultural practices in biological olive groves.

Long-term ecosystem nitrogen limitation from foliar δ15 N data and a land surface model

The effect of nutrient availability on plant growth and the terrestrial carbon sink under climate change and elevated CO₂ remains one of the main uncertainties of the terrestrial carbon cycle. This is partially due to the difficulty of assessing nutrient limitation at large scales over long periods of time. Consistent declines in leaf nitrogen (N) content and leaf δ¹⁵ N have been used to suggest that nitrogen limitation has increased in recent decades, most likely due to the concurrent increase in atmospheric CO₂ . However, such data sets are often not straightforward to interpret due to the complex factors that contribute to the spatial and temporal variation in leaf N and isotope concentration. We use the land surface model (LSM) QUINCY, which has the unique capacity to represent N isotopic processes, in conjunction with two large data sets of foliar N and N isotope content. We run the model with different scenarios to test whether foliar δ¹⁵ N isotopic data can be used to infer large-scale N limitation and if the observed trends are caused by increasing atmospheric CO₂ , changes in climate or changes in sources and magnitude of anthropogenic N deposition. We show that while the model can capture the observed change in leaf N content and predict widespread increases in N limitation, it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ¹⁵ N observed in the data across the globe. The addition of an observation-based temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ¹⁵ N. Our results show that leaf δ¹⁵ N observations cannot, on their own, be used to assess global-scale N limitation and that using such a data set in conjunction with an LSM can reveal the drivers behind the observed patterns.

Soil properties as key predictors of global grassland production: Have we overlooked micronutrients?

Fertilisation experiments have demonstrated that nutrient availability is a key determinant of biomass production and carbon sequestration in grasslands. However, the influence of nutrients in explaining spatial variation in grassland biomass production has rarely been assessed. Using a global dataset comprising 72 sites on six continents, we investigated which of 16 soil factors that shape nutrient availability associate most strongly with variation in grassland aboveground biomass. Climate and N deposition were also considered. Based on theory-driven structural equation modelling, we found that soil micronutrients (particularly Zn and Fe) were important predictors of biomass and, together with soil physicochemical properties and C:N, they explained more unique variation (32%) than climate and N deposition (24%). However, the association between micronutrients and biomass was absent in grasslands limited by NP. These results highlight soil properties as key predictors of global grassland biomass production and point to serial co-limitation by NP and micronutrients.

Fertilized graminoids intensify negative drought effects on grassland productivity

Droughts can strongly affect grassland productivity and biodiversity, but responses differ widely. Nutrient availability may be a critical factor explaining this variation, but is often ignored in analyses of drought responses. Here, we used a standardized nutrient addition experiment covering 10 European grasslands to test if full-factorial nitrogen, phosphorus, and potassium addition affected plant community responses to inter-annual variation in drought stress and to the extreme summer drought of 2018 in Europe. We found that nutrient addition amplified detrimental drought effects on community aboveground biomass production. Drought effects also differed between functional groups, with a negative effect on graminoid but not forb biomass production. Our results imply that eutrophication in grasslands, which promotes dominance of drought-sensitive graminoids over forbs, amplifies detrimental drought effects. In terms of climate change adaptation, agricultural management would benefit from taking into account differential drought impacts on fertilized versus unfertilized grasslands, which differ in ecosystem services they provide to society.

The role of climate, foliar stoichiometry and plant diversity on ecosystem carbon balance

Global change is affecting terrestrial carbon (C) balances. The effect of climate on ecosystem C balance has been largely explored, but the roles of other concurrently changing factors, such as diversity and nutrient availability, remain elusive. We used eddy-covariance C-flux measurements from 62 ecosystems from which we compiled information on climate, ecosystem type, stand age, species abundance and foliar concentrations of N and P of the main species, to assess their importance in the ecosystem C balance. Climate and productivity were the main determinants of ecosystem C balance and its stability. In P-rich sites, increasing N was related to increased gross primary production and respiration and vice versa, but reduced net C uptake. Our analyses did not provide evidence for a strong relation between ecosystem diversity and their productivity and stability. Nonetheless, these results suggest that nutrient imbalances and, potentially, diversity loss may alter future global C balance.

Conventional analysis methods underestimate the plant-available pools of calcium, magnesium and potassium in forest soils

The plant-available pools of calcium, magnesium and potassium are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex. In numerous forest ecosystems, despite very low plant-available pools, elevated forest productivities are sustained. We hypothesize that trees access nutrient sources in the soil that are currently unaccounted by conventional soil analysis methods. We carried out an isotopic dilution assay to quantify the plant-available pools of calcium, magnesium and potassium and trace the soil phases that support these pools in 143 individual soil samples covering 3 climatic zones and 5 different soil types. For 81%, 87% and 90% of the soil samples (respectively for Ca, Mg and K), the plant-available pools measured by isotopic dilution were greater than the conventional exchangeable pool. This additional pool is most likely supported by secondary non-crystalline mineral phases in interaction with soil organic matter and represents in many cases (respectively 43%, 27% and 47% of the soil samples) a substantial amount of plant-available nutrient cations (50% greater than the conventional exchangeable pools) that is likely to play an essential role in the biogeochemical functioning of forest ecosystems, in particular when the resources of Ca, Mg and K are low.