In recent years, increased awareness of the potential interactions between rising atmospheric
CO2
concentrations (
CO2
) and temperature has illustrated the importance of multifactorial ecosystem ...manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and
CO2
manipulation, and compares it with those obtained in single factor
CO2
and temperature manipulation experiments. Across all combined elevated
CO2
and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the
CO2
‐only treatment than to those in the warming‐only treatment. In contrast to warming‐only experiments, both the combined and the
CO2
‐only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the
CO2
‐only treatment, possibly due to the warming‐induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor
CO2
treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated
CO2
and warming, i.e. the response to the combined treatment was usually less‐than‐additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long‐term) multifactor manipulation experiments. Because single factor
CO2
responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.
Sustained increased productivity of trees growing in elevated CO₂ depends in part on their stoichiometric flexibility, i.e., increasing their nutrient use efficiency, or on increased nutrient uptake ...from the soil. Phosphorus (P) may be a nutrient as limiting as nitrogen (N) in terrestrial ecosystems and may play a key-process in global terrestrial C storage. For this study archived litter and soil samples of two free air CO₂ enrichment (FACE) experiments were analyzed for C, N and P. Populus euramericana, nigra and alba and Betula pendula, Alnus glutinosa and Fagus sylvatica were grown in ambient and elevated CO₂ at respectively the Euro- and BangorFACE experiments. At EuroFACE, aboveground litter accumulated in L, F and H layers, while at BangorFACE almost all aboveground litter was incorporated into the mineral soil due to bioturbation. At EuroFACE, more P was lost from the F and H litter layers due to trees growing in elevated CO₂, while at BangorFACE more P was lost from the mineral soil. Results of this study imply that trees growing in elevated CO₂ were P limited at both experiments. Therefore, with increasing atmospheric CO₂, P may play a more pronounced role than previous thought in regulating secondary forest growth. Moreover, increased atmospheric CO₂ and ample N may allow a larger pool of P to become available for uptake due to, for instance, increased phosphatase activity resulting in increased organic matter turnover and biogenic weathering. Therefore, it may be postulated that under non-N-limited conditions, e.g., during regrowth, under high N deposition or in systems with high N₂-fixation, increased P availability and uptake may allow P-limited forests to sustain increased growth under increasing atmospheric CO₂.
Tropical forests store and sequester large amounts of carbon in above‐ and below‐ground plant biomass and soil organic matter (SOM), but how these are driven by abiotic and biotic factors remains ...poorly understood.
Here, we test the effects of abiotic factors (light variation, caused by logging disturbance, and soil fertility) and biotic factors (species richness and functional trait composition) on biomass stocks (above‐ground biomass, fine root biomass), SOM and productivity in a relatively monodominant Guyanese tropical rainforest. This forest grows on nutrient‐poor soils and has few species that contribute most to total abundance. We, therefore, expected strong effects of soil fertility and species’ traits that determine resource acquisition and conservation, but not of diversity. We evaluated 6 years of data for 30 0.4‐ha plots and tested hypotheses using structural equation models.
Disturbance increased productivity but decreased above‐ground biomass stocks. Soil phosphorus (P) enhanced above‐ground biomass and productivity, whereas soil nitrogen reduced fine root biomass. In contrast to expectations, trait values representing acquisitive strategies (e.g. high leaf nutrient concentration) increased biomass stocks, possibly because they indicate higher nutrient absorption and thus higher biomass build‐up. However, under harsh conditions where biomass increase is slow, acquisitive trait values may increase respiration and vulnerability to hazards and therefore increase biomass loss. As expected, species richness did not affect productivity.
We conclude that light availability (through disturbance) and soil fertility—especially P—strongly limit forest biomass productivity and stocks in this Guyanese forest. Low P availability may cause strong environmental filtering, which in turn results in a small set of dominant species. As a result, community trait composition but not species richness determines productivity and stocks of biomass and SOM in tropical forest on poor soils.
A plain language summary is available for this article.
Plain Language Summary
Tree occurrence in silvopastoral systems of Central America has been under pressure for various reasons including attempts to improve grassland productivity and the need for wood. However, scattered ...isolated trees are also recognized to provide ecosystem services like shade, fodder and fruits that are important to cattle in the dry season. In addition, trees may enhance the climate change mitigation potential of silvopastoral systems through increased carbon (C) uptake and subsequent soil carbon sequestration. Through differences in plant traits like nutrient uptake, canopy structure and litter quality, tree species may have an effect on C and nutrient cycling. Due to a prevailing north-easterly wind in the study area, three distinct areas associated with the impact of tree litter deposition were identified: (1) open pasture—no tree litter deposition; (2) tree canopy—above and belowground tree litter; and (3) leaf litter cone—aboveground tree litter deposition. Furthermore, the effect of tree species,
Guazuma ulmifolia
and
Crescentia alata
, were considered. The presence of trees, as compared to pasture, caused larger topsoil C, N and P contents. In the subsoil, C content was also larger due to tree presence. Soil fractionation showed that tree-induced larger litter input subsequently increased free and occluded OM fractions and ultimately increased stabilized SOM fractions. Therefore, trees were found to enhance soil C sequestration in these silvopastoral systems. This is also supported by the soil respiration data. Although the respiration rates in the pasture subplots were lower than in the leaf litter subplots, the difference was not significant, which suggests that part of the extra C input to the leaf litter subplots stayed in the soil. Nutrient cycling was also enhanced by tree presence, but with a clear differentiation between species.
C
.
alata
(Jícaro) enhanced available and stabilized forms of organic N, while
G
.
ulmifolia
(Guácimo) enhanced available soil P and stabilized organic P.
Forest ecosystems are important sinks for rising concentrations of atmospheric CO 2 . In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental ...forests were grown under atmospheric concentrations of CO 2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO 2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO 2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO 2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO 2 . Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO 2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO 2 . global change net primary production
Elevated CO
2
(eCO
2
) can stimulate plant productivity and increase carbon (C) input to soils, but nutrient limitation restricts productivity. Despite phosphorus (P)-limited ecosystems increasing ...globally, it is unknown how nutrient cycling, particularly soil microbial extra cellular enzyme activity (EEA), will respond to eCO
2
in such ecosystems. Long-term nutrient manipulation plots from adjacent P-limited acidic and limestone grasslands were exposed to eCO
2
(600 ppm) provided by a mini-Free Air CO
2
Enrichment system. P-limitation was alleviated (35 kg-P ha
−1
y
−1
(P35)), exacerbated (35 kg-N ha
−1
y
−1
(N35), 140 kg-N ha
−1
y
−1
(N140)), or maintained (control (P0N0)) for > 20 years. We measured EEAs of C-, N- and P-cycling enzymes (1,4-β-glucosidase, cellobiohydrolase, N-acetyl β-D-glucosaminidase, leucine aminopeptidase, and acid phosphatase) and compared C:N:P cycling enzyme ratios using a vector analysis. Potential acid phosphatase activity doubled under N additions relative to P0N0 and P35 treatments. Vector analysis revealed reduced C-cycling investment and increased P-cycling investment under eCO
2
. Vector angle significantly increased with P-limitation (P35 < P0N0 < N35 < N140) indicating relatively greater investment in P-cycling enzymes. The limestone grassland was more C limited than the acidic grassland, characterised by increased vector length, C:N and C:P enzyme ratios. The absence of interactions between grassland type and eCO
2
or nutrient treatment for all enzyme indicators signaled consistent responses to changing P-limitation and eCO
2
in both grasslands. Our findings suggest that eCO
2
reduces C limitation, allowing increased investment in P- and N-cycle enzymes with implications for rates of nutrient cycling, potentially alleviating nutrient limitation of ecosystem productivity under eCO
2
.
Graphic abstract
Purpose
The tropical phosphorus cycle and its relation to soil phosphorus (P) availability are a major uncertainty in projections of forest productivity. In highly weathered soils with low P ...concentrations, plant and microbial communities depend on abiotic and biotic processes to acquire P. We explored the seasonality and relative importance of drivers controlling the fluctuation of common P pools via processes such as litter production and decomposition, and soil phosphatase activity.
Methods
We analyzed intra-annual variation of tropical soil phosphorus pools using a modified Hedley sequential fractionation scheme. In addition, we measured litterfall, the mobilization of P from litter and soil extracellular phosphatase enzyme activity and tested their relation to fluctuations in P- fractions.
Results
Our results showed clear patterns of seasonal variability of soil P fractions during the year. We found that modeled P released during litter decomposition was positively related to change in organic P fractions, while net change in organic P fractions was negatively related to phosphatase activities in the top 5 cm.
Conclusion
We conclude that input of P by litter decomposition and potential soil extracellular phosphatase activity are the two main factors related to seasonal soil P fluctuations, and therefore the P economy in P impoverished soils. Organic soil P followed a clear seasonal pattern, indicating tight cycling of the nutrient, while reinforcing the importance of studying soil P as an integrated dynamic system in a tropical forest context.
Extracellular enzymes (EE) play a vital role in soil nutrient cycling and thus affect terrestrial ecosystem functioning. Yet the drivers that regulate microbial activity, and therefore EE activity, ...remain under debate. In this study we investigate the temporal variation of soil EE in a tropical terra-firme forest. We found that EE activity peaked during the drier season in association with increased leaf litterfall, which was also reflected in negative relationships between EE activities and precipitation. Soil nutrients were weakly related to EE activities, although extractable N was related to EE activities in the top 5 cm of the soil. These results suggest that soil EE activity is synchronized with precipitation-driven substrate inputs and depends on the availability of N. Our results further indicate high investments in P acquisition, with a higher microbial N demand in the month before the onset of the drier season, shifting to higher P demand towards the end of the drier season. These seasonal fluctuations in the potential acquisition of essential resources imply dynamic shifts in microbial activity in coordination with climate seasonality and resource limitation of central-eastern Amazon forests.
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