Biogeochemistry最新文献

Nitrogen (N) is a critical element in many ecological and biogeochemical processes in forest ecosystems. Cycling of N is sensitive to changes in climate, atmospheric carbon dioxide (CO₂) concentrations, and air pollution. Streamwater nitrate draining a forested ecosystem can indicate how an ecosystem is responding to these changes. We observed a pulse in streamwater nitrate concentration and export at a long-term forest research site in eastern North America that resulted in a 10-fold increase in nitrate export compared to observations over the prior decade. The pulse in streamwater nitrate occurred in a reference catchment in the 2013 water year, but was not associated with a distinct disturbance event. We analyzed a suite of environmental variables to explore possible causes. The correlation between each environmental variable and streamwater nitrate concentration was consistently higher when we accounted for the antecedent conditions of the variable prior to a given streamwater observation. In most cases, the optimal antecedent period exceeded two years. We assessed the most important variables for predicting streamwater nitrate concentration by training a machine learning model to predict streamwater nitrate concentration in the years preceding and during the streamwater nitrate pulse. The results of the correlation and machine learning analyses suggest that the pulsed increase in streamwater nitrate resulted from both (1) decreased plant uptake due to lower terrestrial gross primary production, possibly due to increased soil frost or reduced solar radiation or both; and (2) increased net N mineralization and nitrification due to warm temperatures from 2010 to 2013. Additionally, variables associated with hydrological transport of nitrate, such as maximum stream discharge, emerged as important, suggesting that hydrology played a role in the pulse. Overall, our analyses indicate that the streamwater nitrate pulse was caused by a combination of factors that occurred in the years prior to the pulse, not a single disturbance event.

Small artificial waterbodies are larger emitters of carbon dioxide (CO₂) and methane (CH₄) than natural waterbodies. The Intergovernmental Panel on Climate Change (IPCC) recommends these waterbodies are accounted for in national emission inventories, yet data is extremely limited for irrigated landscapes. To derive a baseline of their greenhouse gas footprint, we investigated 38 irrigation farm dams in horticulture and broadacre cropping in semi-arid NSW, Australia. Dissolved CO₂, CH₄, and nitrous oxide (N₂O) were measured in spring and summer, 2021–2022. While all dams were sources of CH₄ to the atmosphere, 52% of irrigation farm dams were sinks for CO₂ and 70% were sinks for N₂O. Relationships in the linear mixed effect models indicate that CO₂ concentrations were primarily driven by dissolved oxygen (DO), ammonium, and sediment carbon content, while N₂O concentration was best explained by an interaction between DO and ammonium. Methane concentrations did not display any relationship with typical biological variables and instead were related to soil salinity, trophic status, and size. Carbon dioxide-equivalent emissions were highest in small (< 0.001 km²) dams (305 g CO₂-eq m⁻² season⁻¹) and in those used for recycling irrigation water (249 g CO₂-eq m⁻² season⁻¹), with CH₄ contributing 70% of average CO₂-eq emissions. However, irrigation dams had considerably lower CH₄ emissions (mean 40 kg ha⁻¹ yr⁻¹) than the IPCC emission factor (EF) of 183 kg CH₄ ha⁻¹ yr⁻¹ for constructed ponds and lower N₂O EF of 0.06% than the indirect EF for agricultural surface waters (0.26%). This synoptic survey reveals existing models may be severely overestimating (4–5 times) farm dam CH₄ and N₂O emissions in semi-arid irrigation areas. Further research is needed to define these artificial waterbodies in emissions accounting.

Enhanced nitrogen (N) availability in temperate forests has altered ecosystem carbon (C) and N cycling. Recent research has shown that these alterations lead to reductions in belowground C allocation by trees and that the consequences of these reductions on soil C and nutrient cycling may vary by mycorrhizal type. We hypothesized that trees that associate with ectomycorrhizal fungi (ECM) would reduce C allocation towards roots and mycorrhizal fungi to a greater extent than trees that associate with arbuscular mycorrhizal fungi (AM) in response to > 25 years of N fertilization. We further hypothesized that N induced decoupling of roots and microbes in ECM trees would be evidenced by greater declines in extracellular enzyme activities. We measured belowground C allocation to fine root biomass and mycorrhizal colonization in 6 AM and 6 ECM dominated plots in the N fertilized and reference watersheds at the Fernow Experimental Forest in West Virginia, USA. We compared these to measurements of simple-C, complex-C, nitrogen, and phosphorus acquiring enzyme activities in organic horizon, bulk mineral, and rhizosphere soil fractions. N fertilization reduced fine root biomass and mycorrhizal colonization in both AM and ECM stands. We found more consistent reductions in enzyme activities in ECM soils than AM soils under N fertilization which may have been driven by greater declines in root-C transfer to soil microbes. This mechanism helps to explain variability in soil C cycling responses across N gradient and fertilization experiments and may prove useful in predicting the fate of soil C stocks in response to N deposition.

Iron (oxyhydr)oxides strongly adsorb phosphate and limit its bioavailability, but interactions between phosphate and various Fe (oxyhydr)oxides are poorly constrained in natural systems. An in-situ incubation experiment was conducted to explore Fe (oxyhydr)oxide transformation and effects on phosphate sorption in soils with contrasting saturation and redox conditions. Synthetic Fe (oxyhydr)oxides (ferrihydrite, goethite and hematite) were coated onto quartz sand and either pre-sorbed with phosphate or left phosphate-free. The oxide-coated sands were mixed with natural organic matter, enclosed in mesh bags, and buried in and around a vernal pond for up to 12 weeks. Redox conditions were stable and oxic in the upland soils surrounding the vernal pond but largely shifted from Fe reducing to Fe oxidizing in the lowland soils within the vernal pond as it dried during the summer. Iron (oxyhydr)oxides lost more Fe (− 41% ± 10%) and P (− 43 ± 11%) when incubated in the redox-dynamic lowlands compared to the uplands (− 18% ± 5% Fe and − 24 ± 8% P). Averaged across both uplands and lowlands, Fe losses from crystalline goethite and hematite (− 38% ± 6%) were unexpectedly higher than losses from short range ordered ferrihydrite (− 12% ± 10%). We attribute losses of Fe and associated P from goethite and hematite to colloid detachment and dispersion but losses from ferrihydrite to reductive dissolution. Iron losses were partially offset by retention of solubilized Fe as organic-bound Fe(III). Iron (oxyhydr)oxides that persisted during the incubation retained or even gained P, indicating low amounts of phosphate sorption from solution. These results demonstrate that hydrologic variability and Fe (oxyhydr)oxide mineralogy impact Fe mobilization pathways that may regulate phosphate bioavailability.

Global change drivers that modify the quality and quantity of litter inputs to soil affect greenhouse gas fluxes, and thereby constitute a feedback to climate change. Carbon cycling in the Yukon–Kuskokwim (Y–K) River Delta, a subarctic wetland system, is influenced by landscape variations in litter quality and quantity generated by herbivores (migratory birds) that create ‘grazing lawns’ of short stature, nitrogen-rich vegetation. To identify the mechanisms by which these changes in litter inputs affect soil carbon balance, we independently manipulated qualities and quantities of litter representative of levels found in the Y–K Delta in a fully factorial microcosm experiment. We measured CO₂ fluxes from these microcosms weekly. To help us identify how litter inputs influenced greenhouse gas fluxes, we sequenced soil fungal and bacterial communities, and measured soil microbial biomass carbon, dissolved carbon, inorganic nitrogen, and enzyme activity. We found that positive correlations between litter input quantity and CO₂ flux were dependent upon litter type, due to differences in litter stoichiometry and changes to the structure of decomposer communities, especially the soil fungi. These community shifts were particularly pronounced when litter was added in the form of herbivore feces, and in litter input treatments that induced nitrogen limitation (i.e., senesced litter). The sensitivity of carbon cycling to litter quality and quantity in this system demonstrates that herbivores can strongly impact greenhouse gas fluxes through their influence on plant growth and tissue chemistry.

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