Search for: All records

Unprecedented nitrogen (N) inputs into terrestrial ecosystems have profoundly altered soil N cycling. Ammonia oxidizers and denitrifiers are the main producers of nitrous oxide (N₂O), but it remains unclear how ammonia oxidizer and denitrifier abundances will respond to N loading and whether their responses can predict N‐induced changes in soil N₂O emission. By synthesizing 101 field studies worldwide, we showed that N loading significantly increased ammonia oxidizer abundance by 107% and denitrifier abundance by 45%. The increases in both ammonia oxidizer and denitrifier abundances were primarily explained by N loading form, and more specifically, organic N loading had stronger effects on their abundances than mineral N loading. Nitrogen loading increased soil N₂O emission by 261%, whereas there was no clear relationship between changes in soil N₂O emission and shifts in ammonia oxidizer and denitrifier abundances. Our field‐based results challenge the laboratory‐based hypothesis that increased ammonia oxidizer and denitrifier abundances by N loading would directly cause higher soil N₂O emission. Instead, key abiotic factors (mean annual precipitation, soil pH, soil C:N ratio, and ecosystem type) explained N‐induced changes in soil N₂O emission. Altogether, these findings highlight the need for considering the roles of key abiotic factors in regulating soil N transformations under N loading to better understand the microbially mediated soil N₂O emission.

 

Ecosystem functional responses such as soil CO₂emissions are constrained by microclimate, available carbon (C) substrates and their effects upon microbial activity. In tropical forests, phosphorus (P) is often considered as a limiting factor for plant growth, but it is still not clear whether P constrains microbial CO₂emissions from soils. In this study, we incubated seven tropical forest soils from Brazil and Puerto Rico with different nutrient addition treatments (no addition, Control; C, nitrogen (N) or P addition only; and combined C, N and P addition (CNP)). Cumulative soil CO₂emissions were fit with a Gompertz model to estimate potential maximum cumulative soil CO₂emission (C_m) and the rate of change of soil C decomposition (k). Quantitative polymerase chain reaction (qPCR) was conducted to quantify microbial biomass as bacteria and fungi. Results showed that P addition alone or in combination with C and N enhancedC_m, whereas N addition usually reducedC_m, and neither N nor P affected microbial biomass. Additions of CNP enhancedk, increased microbial abundances and altered fungal to bacterial ratios towards higher fungal abundance. Additions of CNP, however, tended to reduceC_mfor most soils when compared to C additions alone, suggesting that microbial growth associated with nutrient additions may have occurred at the expense of C decomposition. Overall, this study demonstrates that soil CO₂emission is more limited by P than N in tropical forest soils and those effects were stronger in soils low in P.

A laboratory incubation study was conducted with nitrogen, phosphorus or carbon addition to tropical forest soils. Soil CO₂emission was fitted with a Gompertz model and soil microbial abundance was quantified using qPCR. Phosphorus addition increased model parametersC_mand soil CO₂emission, particularly in the Puerto Rico soils. Soil CO₂emission was more limited by phosphorus than nitrogen in tropical forest soils.

 

Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO 2 that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the feedbacks from other limiting resources and plant community change, which remain poorly understood for soil CO 2 efflux, J CO2 , a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO 2 enrichment gradient (250 to 500 µL L −1 ) for eight years to grassland plant communities on soils from different landscape positions. We identified the trajectory of J CO2 responses and feedbacks from other resources, plant diversity [effective species richness, exp(H)], and community change (plant species turnover). We found linear increases in J CO2 on an alluvial sandy loam and a lowland clay soil, and an asymptotic increase on an upland silty clay soil. Structural equation modeling identified CO 2 as the dominant limitation on J CO2 on the clay soil. In contrast with theory predicting limitation from a single limiting factor, the linear J CO2 response on the sandy loam was reinforced by positive feedbacks from aboveground net primary productivity and exp(H), while the asymptotic J CO2 response on the silty clay arose from a net negative feedback among exp(H), species turnover, and soil water potential. These findings support a multiple resource limitation view of the effects of global change drivers on grassland ecosystem carbon cycling and highlight a crucial role for positive or negative feedbacks between limiting resources and plant community structure. Incorporating these feedbacks will improve models of terrestrial carbon sequestration and ecosystem services. 

Grazing can affect plant community composition and structure directly by foraging and indirectly by increasing wind erosion and dust storms and subsequently influence ecosystem functioning and ecological services. However, the combined effects of grazing, wind erosion, and dust deposition have not been explored. As part of a 7-year (2010–2016) field manipulative experiment, this study was conducted to examine the impacts of grazing and simulated aeolian processes (wind erosion and dust deposition) on plant community cover and species richness in a temperate steppe on the Mongolian Plateau, China. Grazing decreased total cover by 4.2%, particularly the cover of tall-stature plants (> 20 cm in height), but resulted in 9.1% greater species richness. Wind erosion also reduced total cover by 17.0% primarily via suppressing short-stature plants associated with soil nitrogen loss, but had no effect on species richness. Dust deposition enhanced total cover by 5.7%, but resulted in a 7.3% decrease in species richness by driving some of the short-stature plant species to extinction. Both wind erosion and dust deposition showed additive effects with grazing on vegetation cover and species richness, though no detectable interaction between aeolian processes and grazing could be detected due to our methodological constraints. The changes in gross ecosystem productivity, ecosystem respiration, and net ecosystem productivity under the wind erosion and dust deposition treatments were positively related to aeolian process-induced changes in vegetation cover and species richness, highlighting the important roles of plant community shifts in regulating ecosystem carbon cycling. Our findings suggest that plant traits (for example, canopy height) and soil nutrients may be the key for understanding plant community responses to grassland management and natural hazards.