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Increasing levels of atmospheric CO2 may change C and N dynamics in pasture ecosystems. The present study was conducted to examine the impact of four years of CO2 enrichment on soil and root composition and soil N transformation in natural pastureland. Plots of open-top growth chambers were continuously injected with ambient CO2 (350 µL L–1) and elevated CO2 (625 µL L–1). Soil cores exposed to ambient and elevated CO2 treatment were incubated and collected each year. Net N-mineralization rates in soil (NH4 +-N plus NO3ˉ–-N), in addition to total C and N content (%) of soil and root tissues were measured. Results revealed that elevated CO2 caused a significant reduction in soil NO3 (P < 0.05), however, no significant CO2 effect was found on total soil C and N content (%). Roots of plants grown under elevated CO2 treatment had higher C/N ratios. Changes in root C/N ratios were driven by changes in root N concentrations as total root N content (%) was significantly reduced by 30% (P < 0.05). Overall, findings suggest that the effects of CO2 enrichment was more noticeable on N content (%) than C content (%) of soil and roots; elevated CO2 significantly affected soil N-mineralization and total N content (%) in roots, however, no substantial change was found in C inputs in CO2-enriched soil.

Fire frequency might increase in many fire-dominated ecosystems of the world due to the combined effects of global warming, land-use change and increased human pressures. Understanding how changes in fire frequency can affect the main soil biogeochemical dynamics, as well as the microbial community, in the long term is utmost important. Here we determined the effect of changes in fire frequency and other fire history characteristics on soil C and N dynamics and the main microbial groups (using soil fatty acid profiles), in Pinus pinaster forests from central Spain. Stands were chosen to differ in the number of fires (1 to 3) occurred between 1976 and 2018, in the time elapsed since the last fire and the interval undergone between the last two consecutive fires. We found that, in general, most of the studied biogeochemical and microbial variables showed clear differences between unburned and burned stands. The time elapsed since the last fire was the most important fire history covariable and governed the main soil nutrient dynamics and microbial groups. Recovery to pre-fire values took 30–40 years. Increased wildfire frequency only modified total C and nitrification rate, but results were not consistent between stands burned twice and thrice. The time interval (years) between the last two fires was not a significant covariable. The fact that some stands burnt up to thrice in a period of 43 years supports the strong capacity of this ecosystem to recover, even under an increased fire frequency.

Key message Over-yielding of stand biomass did not occur in a tree polyculture comprised of Betula pendula, Alnus glutinosa and Fagus sylvatica selected for contrasting traits. This was due to antagonistic interactions between the component species. Fine root dynamics and soil C stocks were unaffected by species mixture. Context Increasing CO2 fixation in tree biomass through afforestation and forest management actions has potential for cost-effective climate mitigation. The influences of tree mixture on biomass production and subsequent soil C accumulation in polyculture still remain uncertain. Aims We studied the polyculture of Alnus glutinosa (L.) Gaertn, Betula pendula Roth and Fagus sylvatica L. in a plantation forest to examine the effectiveness of species mixtures as a tool for increased biomass production and soil C accumulation. Methods Tree biomass was estimated by developing species-specific allometric models and 3 years tree measurement. Fine root biomass and production were estimated by root coring and root-mesh methods. The ‘relative yield of mixture’ approach was used to examine the mixture effect. Results In mixture, an additive effect was observed in A. glutinosa (13% increase in basal diameter relative to the monoculture); however, there was no overall effect of mixture on total standing biomass due to the suppression of F. sylvatica (2.75 g m−2 reduction in woody biomass). Fine root biomass production showed no mixture effect. The quantity and quality of soil C (top 0.5 m) was not affected by tree mixture. Conclusion We conclude that the contrasting growth responses of the A. glutinosa, B. pendula and F. sylvatica in polyculture resulted in no over-yielding of standing biomass despite the complementary traits of the component species.

Primary succession is limited by both ecosystem development and plant dispersal, but the extent to which dispersal constrains succession over the long-term is unknown. We compared primary succession along two co-occurring arctic chronosequences with contrasting spatial scales: sorted circles that span a few meters and may have few dispersal constraints and glacial forelands that span several kilometers and may have greater dispersal constraints. Dispersal constraints slowed primary succession by centuries: plots were dominated by cryptogams after 20 years on circles but after 270 years on forelands; plots supported deciduous plants after 100 years on circles but after >400 years on forelands. Our study provides century-scale evidence suggesting that dispersal limitations constrain the rate of primary succession in glacial forelands.

Most experimental studies measuring the effects of climate change on terrestrial C cycling have focused on processes that occur at relatively short time scales (up to a few years). However, climate-soil C interactions are influenced over much longer time scales by bioturbation and soil weathering affecting soil fertility, ecosystem productivity, and C storage. Elevated CO2can increase belowground C inputs and stimulate soil biota, potentially affecting bioturbation, and can decrease soil pH which could accelerate soil weathering rates. To determine whether we could resolve any changes in bioturbation or C storage, we investigated soil profiles collected from ambient and elevated-CO2plots at the Free-Air Carbon-Dioxide Enrichment (FACE) forest site at Oak Ridge National Laboratory after 11 years of 13C-depleted CO2 release. Profiles of organic carbon concentration, δ13C values, and activities of 137Cs, 210Pb, and 226Ra were measured to ∼30 cm depth in replicated soil cores to evaluate the effects of elevated CO2 on these parameters. Bioturbation models based on fitting advection-diffusion equations to 137Cs and 210Pb profiles showed that ambient and elevated-CO2 plots had indistinguishable ranges of apparent biodiffusion constants, advection rates, and soil mixing times, although apparent biodiffusion constants and advection rates were larger for 137Cs than for 210Pb as is generally observed in soils. Temporal changes in profiles of δ13C values of soil organic carbon (SOC) suggest that addition of new SOC at depth was occurring at a faster rate than that implied by the net advection term of the bioturbation model. Ratios of (210Pb/226Ra) may indicate apparent soil mixing cells that are consistent with biological mechanisms, possibly earthworms and root proliferation, driving C addition and the mixing of soil between ∼4 cm and ∼18 cm depth. Burial of SOC by soil mixing processes could substantially increase the net long-term storage of soil C and should be incorporated in soil-atmosphere interaction models.

Secondary forests comprise an increasing area of the tropics and play an important role in global carbon cycling. We compare above-ground biomass accumulation of both planted and naturally regenerating trees, as well as C in the top soil layer, in three restoration treatments replicated at 14, six to eight year old restoration sites in southern Costa Rica. Restoration strategies include: control (no planting), planting tree islands, and conventional, mixed-species tree plantations. We evaluate the importance of past land-use, soil nutrients, understory cover, and surrounding forest cover in explaining variation in above-ground biomass accumulation (ABA) rate across sites. Total ABA and planted tree ABA rate were highest in plantations, intermediate in islands, and lowest in control treatments, whereas ABA rate of naturally regenerating trees did not differ across treatments. Most ABA in plantations (89%) and islands (70%) was due to growth of planted trees. Soil carbon did not change significantly over the time period of the study in any treatment. The majority of across-site variation in both total and planted tree ABA rate was explained by duration of prior pasture use. Tree growth in the first two years after planting explained approximately two-thirds of the variation in ABA rate after 6-8. years. Soil nutrient concentrations explained relatively little of the variation in planted or naturally recruiting ABA rate. Our results show that planting trees substantially increases biomass accumulation during the first several years of forest recovery in former agricultural lands and that past-land use has a strong effect on the rate of biomass accumulation. Planting tree islands is a cost-effective strategy for increasing ABA and creating more heterogeneous habitat conditions than tree plantations. We recommend small scale planting trials to quickly assess potential biomass accumulation and prioritize sites for ecosystem service payments for carbon sequestration. © 2014 Elsevier B.V.

Ecosystems, 11 (6), ISSN:1432-9840, ISSN:1435-0629

Organic matter management is believed to solve many of the chemical and physical problems of coarse-textured, low fertility soils of Northeast Thailand. We tested the influence of different plant residues available in this area on soil C and N dynamics in upland (Oxic Paleustult) and lowland (Aeric Paleaquult) soils. Residues included groundnut (upland) or Sesbania rostrata stover (lowland), rice straw, Tamarindus indica and Dipterocarpus tuber-culatus leaves applied at 10 t ha−1 (dry matter). For the former three residues additional application rates of 20 t ha−1 were included as well as a mixture (50:50) of groundnut Sesbania — rice straw treatment. Groundnut stover and Sesbania had C/N ratios 17%) and polyphenol (>4.5%) contents. These latter residues, despite slow decomposition, apparently resulted in only moderate soil C (1 mm). Thus the mixture of groundnut/Sesbania with straw was among those residue treatments that led to the highest soil C (