Plant community change mediates the response of foliar δ(15)N to CO 2 enrichment in mesic grasslands

Fiber fractions, multielemental and isotopic composition of a tropical C4 grass grown under elevated atmospheric carbon dioxide

BACKGROUND

Brazil has the largest commercial herd of ruminants with approximately 211 million head, representing 15% of world's beef production, in an area of 170 million hectares of grasslands, mostly cultivated with Brachiaria spp. Although nutrient reduction due to increased atmospheric carbon dioxide (CO2) concentration has already been verified in important crops, studies evaluating its effects on fiber fractions and elemental composition of this grass genus are still scarce. Therefore, a better understanding of the effects of elevated CO2 on forage quality can elucidate the interaction between forage and livestock production and possible adaptations for a climate change scenario. The objective of this study was to evaluate the effects of contrasting atmospheric CO2 concentrations on biomass production, morphological characteristics, fiber fractions, and elemental composition of Brachiaria decumbens (cv. Basilisk).

METHODS

A total of 12 octagonal rings with 10 m diameter were distributed in a seven-ha coffee plantation and inside each of them, two plots of 0.25 m2 were seeded with B. decumbens (cv. Basilisk) in a free air carbon dioxide enrichment facility. Six rings were kept under natural conditions (≈390 μmol mol-1 CO2; Control) and other six under pure CO2 flux to achieve a higher concentration (≈550 μmol mol-1 CO2; Elevated CO2). After 30 months under contrasting atmospheric CO2 concentration, grass samples were collected, and then splitted into two portions: in the first, whole forage was kept intact and in the second portion, the leaf, true stem, inflorescence and senescence fractions were manually separated to determine their proportions (%). All samples were then analyzed to determine the fiber fractions (NDF, hemicellulose, ADF, cellulose, and Lignin), carbon (C), nitrogen (N), potassium (K), calcium (Ca), sulfur (S), phosphorus (P), iron (Fe), and manganese (Mn) contents and N isotopic composition.

RESULTS

Elevated atmospheric CO2 concentration did not influence biomass productivity, average height, leaf, stem, senescence and inflorescence proportions, and fiber fractions (p > 0.05). Calcium content of the leaf and senescence portion of B. decumbens were reduced under elevated atmospheric CO2 (p < 0.05). Despite no effect on total C and N (p > 0.05), lower C:N ratio was observed in the whole forage grown under elevated CO2 (p < 0.05). The isotopic composition was also affected by elevated CO2, with higher values of δ15N in the leaf and stem portions of B. decumbens (p < 0.05).

DISCUSSION

Productivity and fiber fractions of B. decumbens were not influenced by CO2 enrichment. However, elevated CO2 resulted in decreased forage Ca content which could affect livestock production under a climate change scenario.

Bacterial community response to a preindustrial-to-future CO2 gradient is limited and soil specific in Texas Prairie grassland

Rising atmospheric CO₂ concentration directly stimulates plant productivity and affects nutrient dynamics in the soil. However, the influence of CO₂ enrichment on soil bacterial communities remains elusive, likely due to their complex interactions with a wide range of plant and soil properties. Here, we investigated the bacterial community response to a decade long preindustrial-to-future CO₂ gradient (250-500 ppm) among three contrasting soil types using 16S rRNA gene amplicon sequencing. In addition, we examined the effect of seasonal variation and plant species composition on bacterial communities. We found that Shannon index (H') and Faith's phylogenetic diversity (PD) did not change in response to the CO₂ gradient (R²  = 0.01, p > 0.05). CO₂ gradient and season also had a negligible effect on overall community structure, although silty clay soil communities were better structured on a CO₂ gradient (p < 0.001) among three soils. Similarly, CO₂ gradient had no significant effect on the relative abundance of different phyla. However, we observed soil-specific variation of CO₂ effects in a few individual families. For example, the abundance of Pirellulaceae family decreased linearly with CO₂ gradient, but only in sandy loam soils. Conversely, the abundance of Micromonosporaceae and Gaillaceae families increased with CO₂ gradient in clay soils. Soil water content (SWC) and nutrient properties were the key environmental constraints shaping bacterial community structure, one manifestation of which was a decline in bacterial diversity with increasing SWC. Furthermore, the impact of plant species composition on community structure was secondary to the strong influence of soil properties. Taken together, our findings indicate that bacterial communities may be largely unresponsive to indirect effects of CO₂ enrichment through plants. Instead, bacterial communities are strongly regulated by edaphic conditions, presumably because soil differences create distinct environmental niches for bacteria.

Biochar addition induced the same plant responses as elevated CO2 in mine spoil

Nitrogen (N) limitation is one of the major constrain factors for biochar in improving plant growth, the same for elevated atmospheric carbon dioxide (CO₂). Hence, we hypothesized that (1) biochar would induce the same plant responses as elevated CO₂ under N-poor conditions; (2) elevated CO₂ would decrease the potential of biochar application in improving plant growth. To test these hypotheses, we assessed the effects of pinewood biochar, produced at three pyrolytic temperatures (650, 750 and 850 °C), on C and N allocation at the whole-plant level of three plant species (Austrostipa ramossissima, Dichelachne micrantha and Isolepis nodosa) grown in the N poor mine spoil under both ambient (400 μL L^-1) and elevated (700 μL L^-1) CO₂ concentrations. Our data showed that biochar addition (1) significantly decreased leaf total N and δ¹⁵N (P < 0.05); (2) decreased leaf total N and δ¹⁵N more pronouncedly than those of root; and (3) showed more pronounced effects on improving plant biomass under ambient CO₂ than under elevated CO₂ concentration. Hence, it remained a strong possibility that biochar addition induced the same plant physiological responses as elevated CO₂ in the N-deficient mine spoil. As expected, elevated CO₂ decreased the ability of biochar addition in improving plant growth.