Microbial responses to P addition in six South African forest soils

Forests growing on highly weathered soils are often phosphorus (P) limited and competition between geochemical and biological sinks affects their soil P dynamics. In an attempt to elucidate the factors controlling the relative importance of these two sinks, we investigated the relationship of between soil microbial growth kinetics and soil chemical properties following amendments with C, N and P in six South African forest soils. Microbial growth kinetics were determined from respiration curves derived from measurements of CO₂ effluxes from soil samples in laboratory incubations. We found that microbial growth rates after C + N additions were positively related to NaOH-extractable P and decreased with soil depth, whereas the lag time (the time between substrate addition and exponential growth) was negatively related to extractable P. However, the growth rate and lag time were unrelated to the soil’s sorption properties or Al and Fe contents. Our results indicate that at least some of the NaOH-extractable inorganic P may be biologically available within a relatively short time (days to weeks) and might be more labile than previously thought. Our results also show that microbial utilization of C + N only seemed to be constrained by P in the deeper part of the soil profiles.     

A pre‐adaptive approach for tropical forest restoration during climate change using naturally occurring genetic variation in response to water limitation

Effective reforestation of degraded tropical forests depends on selecting planting material suited to the stressful environments typical at restoration sites that can be exacerbated by increased duration and intensity of dry spells expected with climate change. While reforestation efforts in nontropical systems are incorporating drought‐adapted genotypes into restoration programs to cope with drier conditions, such approaches have not been tested or implemented in tropical forests. As the first effort to examine genetic variation in plant response to drought in a tropical wet forest, we established a watering experiment using five replicated maternal lines (i.e. seedlings from different maternal trees) of five dipterocarp species native to Borneo. Apart from the expected species level variation in growth and herbivory (3‐fold variation in both cases), we also found intraspecific variation so that growth in some cases varied 2‐fold, and herbivory 3‐fold, among genetically different maternal lines. In two species we found that among‐maternal line variation in growth rate was negatively correlated with tolerance to water limitation, that is, the maternal lines that performed the best in the high water treatment lost proportionally more of their growth during water limitation. We argue that selection for tolerance to future drier conditions is not only likely to impact population genetics of entire forests, but likely extends from forest trees to the communities of canopy arthropods associated with these trees. In tropical reforestation efforts where increased drought is predicted from climate change, including plant material resilient to drier conditions may improve restoration effectiveness.     

A trait‐based plant economic framework can help increase the value of reforestation for conservation

While reforestation is gaining momentum to moderate climate change via carbon sequestration, there is also an opportunity to use tree planting to confront declining global biodiversity. Where tree species vary in support of diversity, selecting appropriate species for planting could increase conservation effectiveness. We used a common garden experiment in Borneo using 24 native tree species to examine how variation among tree species in their support of beetle diversity is predicted by plant traits associated with “acquisitive” and “conservative” resource acquisition strategies. We evaluate three hypotheses: (1) beetle communities show fidelity to host identity as indicated by variation in abundance and diversity among tree species, (2) the leaf economic spectrum partially explains this variation as shown by beetle preferences for plant species that are predicted by plant traits, and (3) a small number of selected tree species can capture higher beetle species richness than a random tree species community. We found high variation among tree species in supporting three highly intercorrelated metrics of beetle communities: abundance, richness, and Shannon diversity. Variation in support of beetle communities was predicted by plant traits and varied by plant functional groups; within the dipterocarp family, high beetle diversity was predicted by conservative traits such as high wood density and slow growth, and in non‐dipterocarps by the acquisitive traits of high foliar K and rapid growth. Using species accumulation curves and extrapolation to twice the original sample size, we show that 48 tree species were not enough to reach asymptote levels of beetle richness. Nevertheless, species accumulation curves of the six tree species with the highest richness had steeper slopes and supported 33% higher richness than a random community of tree species. Reforestation projects concerned about conservation can benefit by identifying tree species with a disproportional capacity to support biodiversity based on plant traits.     

The influence of soil frost on the quality of dissolved organic carbon in a boreal forest soil: combining field and laboratory experiments

Riparian soils exert a major control on stream water dissolved organic carbon (DOC) in northern latitudes. As the winter climate in northern regions is predicted to be particularly affected by climate change, we tested the sensitivity of DOC formation to winter conditions in riparian soils using an 8?year field-scale soil frost manipulation experiment in northern Sweden. In conjunction with the field experiment, we also carried out a laboratory experiment based on three levels of four winter climatic factors: frost intensity, soil water content, frost duration and frequency of freeze–thaw cycles. We evaluated changes in lability of DOC in soil solution from lysimeter samples taken at different depths (10–80?cm) as well as from DOC extracted from soils in the laboratory, using carbon-specific ultraviolet absorbance at 254?nm (sUVA₂₅₄). In the field, significantly more labile DOC was observed during the spring and summer from upper horizons of frost-exposed soils, when compared to controls. In addition, the amount of labile DOC was positively correlated with frost duration at a soil depth of 10?cm. In the laboratory, frost intensity was the factor that had the greatest positive influence on DOC lability; it also reduced the C:N ratio which may indicate a microbial origin of the DOC. The laboratory experiment also demonstrated significant interactions between some of the applied climatic factors, such as frost intensity interacting with water content. In combination, field and laboratory experiments demonstrate that winter soil conditions have profound effects on DOC-concentration and quality during subsequent seasons.     

Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.     

Microbially Available Phosphorus in Boreal Forests: Effects of Aluminum and Iron Accumulation in the Humus Layer

Soil microorganisms play an important role in the mobilization of phosphorus (P), and these activities may be beneficial for plant P utilization. We investigated the effects on microbial P availability of different combinations of aluminum and iron (Al + Fe) concentrations and different P pools in humus soils from boreal forest ecosystems. We measured respiration rates in laboratory incubations before and after additions of glucose plus (NH₄)₂SO₄ (Glu+N), with or without a small dose of KH₂PO₄. Glu+N was added in excess so that the availability of the inherent soil P would be growth-limiting for the microorganisms. The exponential increases observed in microbial growth after substrate additions (Glu+N) was slower for humus soils with high Al+Fe concentrations than for humus soils with low Al+Fe concentrations. Adding a small dose of KH₂PO₄ to humus soils with high Al+Fe concentrations did, however, increase the exponential growth, measured as the slope of the log-transformed respiration rates, by more than 200%. By contrast, the average increase in exponential growth was only 6% in humus soils with low Al+Fe concentrations. Almost eight times more carbon dioxide (CO₂) was evolved between the substrate additions and the point at which the respiration rate reached 1 mg CO₂ h^–1 for soils with high Al+Fe concentrations compared to humus soils with low Al+Fe concentrations. The amount of CO₂ evolved was positively related to the Al+Fe concentration of the humus soils (r ² = 0.86, P < 0.001), whereas the slope was negatively related to Al+Fe concentration (r ² = 0.70, P < 0.001). Easily available P forms were negatively related to the Al+Fe concentration, whereas organic P showed a strong positive relationship to Al+Fe (r ² = 0.85, P < 0.001), suggesting that other forms of P, as well as inorganic P, are affected by the increased sorption capacity. The results indicate that P mobilization by microorganisms is affected by the presence of sorption sites in the humus layer, and that this capacity for sorption may relate not only to phosphate but also to organic P compounds.