Matthew H. Turnbull

Nitrogen (N) and phosphorus (P) are soil macronutrients that influence ecosystem productivity through strong impacts on plant metabolism. The influence of nutrient supply on the relationships between leaf respiration rate (R) and leaf N concentration ([N]) has been widely investigated. By contrast, how root R responds to variations in nutrient availability and whether there remains a general response across a wide range of species is less well known. We conducted an experiment assessing the effects of N and P supply on root R in 10 woody plant species, with root R being determined by the in vivo rate of oxygen (O₂) consumption. Maximum R (R_max) was also quantified by O₂ uptake in the presence of an exogenous substrate and a respiratory uncoupler Our results showed that high‐N and high‐P supply significantly stimulated mass‐based root R in woody plants, with the effects of N supply significant only when P supply was high. The promoting effect of high‐P treatment remained consistent despite N supply. Root R‐[N] bivariate relationships were altered by nutrient availability across all species, with higher root R at a given root [N] under low‐ than high‐N supply. Similarly, root R at a given P concentration ([P]) was higher under low‐ than high‐P supply. Root R_max was significantly higher than in vivo R for all nutrient treatments, showing that in vivo root R was limited by substrate supply and/or adenylates, with no significant difference in R/R_max ratios among nutrient treatments. These results indicate that ecosystem models should consider different scaling relationships linking root R to root N or P concentrations for woody species when predicting the effects of nutrient availability on carbon cycle dynamics and climate–biosphere feedback. Read the free Plain Language Summary for this article on the Journal blog.

Summary: Nonstructural carbohydrate (NSC) concentrations might reflect the strategies described in the leaf economic spectrum (LES) due to their dependence on photosynthesis and respiration. We examined if NSC concentrations correlate with leaf structure, chemistry, and physiology traits for 114 species from 19 sites and 5 biomes around the globe. Total leaf NSC concentrations varied greatly from 16 to 199 mg g⁻¹ dry mass and were mostly independent of leaf gas exchange and the LES traits. By contrast, leaf NSC residence time was shorter in species with higher rates of photosynthesis, following the fast‐slow strategies in the LES. An average leaf held an amount of NSCs that could sustain one night of leaf respiration and could be replenished in just a few hours of photosynthesis under saturating light, indicating that most daily carbon gain is exported. Our results suggest that NSC export is clearly linked to the economics of return on resource investment.

An approach to improving radiation use efficiency (RUE) in wheat is to screen for variability in rates of leaf respiration in darkness (R_dark). We used a high-throughput system to quantify variation in R_dark among a diverse range of spring wheat genotypes (301 lines) grown in two countries (Mexico and Australia) and two seasons (2017 and 2018), and in doing so quantify the relative importance of genotype (G) and environment (E) in influencing variations in leaf R_dark. Through careful design, residual (unexplained) variation represented less than 10% of the total observed. Up to a third of the variation in R_dark (and related traits) was under genetic control. This suggests opportunities for breeders to use R_dark as a novel selection tool. In addition, E accounted for more than half of the total variation in area-based rates of R_dark. Here, the day of measurement was crucial, suggesting that day-to-day variations in the environment influence rates of R_dark measured at a common temperature. Overall, this study provides new insights into the role G and E play in determining variation in rates of leaf R_dark of one of the most important cereal crops, with implications for future improvements in carbon use efficiency and yield.

A significant warming effect on arctic tundra is greening. Although this increase in predominantly woody vegetation has been linked to increases in gross primary productivity, increasing temperatures also stimulate ecosystem respiration. We present a novel analysis from small-scale plot measurements showing that the shape of the temperature- and light-dependent sink-to-source threshold (where net ecosystem exchange (NEE) equals zero) differs between two tussock tundra ecosystems differing in leaf area index (LAI). At the higher LAI site, the threshold is exceeded (i.e the ecosystem becomes a source) at relatively higher temperatures under low light but at lower temperatures under high light. At the lower LAI site, the threshold is exceeded at relatively lower temperatures under low light but at higher temperatures under high light. We confirmed this response at a single site where LAI was experimentally increased. This suggests the carbon balance of the tundra may be sensitive to small increases in temperature under low light, but that this effect may be significantly offset by increases in LAI. Importantly, we found that this LAI effect is reversed under high light, and so in a warming tundra, greater vegetation cover could have a progressively negative effect on net carbon uptake.

There is increasing evidence that the accessibility of soil organic matter (SOM) to microbial decomposers is more important than chemical recalcitrance for regulating SOM stability. We show that the rapid reduction in SOM decomposition following the addition of sorptive mineral phases to soils in laboratory conditions leads to decreased accessibility of SOM to microbial decomposers due to the formation of organo‐mineral complexes. We manipulated SOM accessibility in a short‐term microcosm experiment by adding different proportions of a sorptive mineral material derived from an aluminium‐rich allophanic soil to a constant mass of soil to determine the effects on SOM decomposition after 1, 4 and 8 days. The decrease in SOM decomposition with increasing proportion of added sorptive mineral phase occurred within 1 day and did not change further at 4 and 8 days. In a second experiment, we added three proportions of the sorptive mineral phases (0%, 15% and 50%) to three soils with different carbon (C) concentrations and measured rates of SOM decomposition, changes in water extractable C, the formation of organo‐mineral complexes inferred from pyrophosphate‐extractable aluminium, and the natural abundance ¹³ C isotopic composition of CO ₂ derived from SOM decomposition. We confirmed that the proportional decreases in SOM decomposition with increasing organo‐mineral complexes and decreasing microbial access to SOM was the same for the three soils, suggesting that the effects are independent of soil C concentration and pH. We also showed that the short‐term reductions in SOM accessibility led to microbial decomposition of more ¹³ C enriched substrates, suggesting preferential stabilisation of plant‐derived ( ¹³ C depleted) substrates. Our study demonstrated that SOM accessibility and decomposition could be reduced rapidly and proportionally to the amount of added sorptive mineral phases resulting from increased organo‐mineral interactions irrespective of the initial soil organic carbon concentration. Highlights: Addition of sorptive mineral phases reduced short‐term soil organic matter (SOM) decomposition by the same proportion for three soils. Relatively ¹³ C depleted SOM was preferentially adsorbed onto the mineral phases. The reduction in SOM decomposition was attributed to reduced microbial access due to increased organo‐mineral interactions. The effects occurred rapidly and proportionally to the amount of added sorptive mineral phases.

Summary: Short‐term temperature response curves of leaf dark respiration (R–T) provide insights into a critical process that influences plant net carbon exchange. This includes how respiratory traits acclimate to sustained changes in the environment. Our study analysed 860 high‐resolution R–T (10–70°C range) curves for: (a) 62 evergreen species measured in two contrasting seasons across several field sites/biomes; and (b) 21 species (subset of those sampled in the field) grown in glasshouses at 20°C : 15°C, 25°C : 20°C and 30°C : 25°C, day : night. In the field, across all sites/seasons, variations in R₂₅ (measured at 25°C) and the leaf T where R reached its maximum (T_max) were explained by growth T (mean air‐T of 30‐d before measurement), solar irradiance and vapour pressure deficit, with growth T having the strongest influence. R₂₅ decreased and T_max increased with rising growth T across all sites and seasons with the single exception of winter at the cool‐temperate rainforest site where irradiance was low. The glasshouse study confirmed that R₂₅ and T_max thermally acclimated. Collectively, the results suggest: (1) thermal acclimation of leaf R is common in most biomes; and (2) the high T threshold of respiration dynamically adjusts upward when plants are challenged with warmer and hotter climates.

In the present study we investigated variations in leaf respiration in darkness (RD) and light (RL), and associated traits in response to season, and along a gradient of soil moisture, in Mediterranean woodland dominated by holm oak (Quercus ilex L.) in central and north-eastern Spain respectively. On seven occasions during the year in the central Spain site, and along the soil moisture gradient in north-eastern Spain, we measured rates of leaf RD, RL (using the Kok method), light-saturated photosynthesis (A) and related light response characteristics, leaf mass per unit area (MA) and leaf nitrogen (N) content. At the central Spain site, significant seasonal changes in soil water content and ambient temperature (T) were associated with changes in MA, foliar N, A and stomatal conductance. RD measured at the prevailing daily T and in instantaneous R–T responses, displayed signs of partial acclimation and was not significantly affected by time of year. RL was always less than, and strongly related to, RD, and RL/RD did not vary significantly or systematically with seasonal changes in T or soil water content. Averaged over the year, RL/RD was 0.66 ± 0.05 s.e. (n = 14) at the central Spain site. At the north-eastern Spain site, the soil moisture gradient was characterised by increasing MA and RD, and reduced foliar N, A, and stomatal conductance as soil water availability decreased. Light inhibition of R occurred across all sites (mean RL/RD = 0.69 ± 0.01 s.e. (n = 18)), resulting in ratios of RL/A being lower than for RD/A. Importantly, the degree of light inhibition was largely insensitive to changes in soil water content. Our findings provide evidence for a relatively constrained degree of light inhibition of R (RL/RD ~ 0.7, or inhibition of ~30%) across gradients of water availability, although the combined impacts of seasonal changes in both T and soil water content increase the range of values expressed. The findings thus have implications in terms of the assumptions made by predictive models that seek to account for light inhibition of R, and for our understanding of how environmental gradients impact on leaf trait relationships in Mediterranean plant communities.