Global Biogeochemical Cycles

Microbes influence soil organic matter decomposition and the long‐term stabilization of carbon (C) in soils. We contend that by revising the representation of microbial processes and their interactions with the physicochemical soil environment, Earth system models (ESMs) will make more realistic global C cycle projections. Explicit representation of microbial processes presents considerable challenges due to the scale at which these processes occur. Thus, applying microbial theory in ESMs requires a framework to link micro‐scale process‐level understanding and measurements to macro‐scale models used to make decadal‐ to century‐long projections. Here we review the diversity, advantages, and pitfalls of simulating soil biogeochemical cycles using microbial‐explicit modeling approaches. We present a roadmap for how to begin building, applying, and evaluating reliable microbial‐explicit model formulations that can be applied in ESMs. Drawing from experience with traditional decomposition models, we suggest the following: (1) guidelines for common model parameters and output that can facilitate future model intercomparisons; (2) development of benchmarking and model‐data integration frameworks that can be used to effectively guide, inform, and evaluate model parameterizations with data from well‐curated repositories; and (3) the application of scaling methods to integrate microbial‐explicit soil biogeochemistry modules within ESMs. With contributions across scientific disciplines, we feel this roadmap can advance our fundamental understanding of soil biogeochemical dynamics and more realistically project likely soil C response to environmental change at global scales.

Iron is hypothesized to be a limiting micronutrient for ocean primary production. This paper presents an analysis of the iron budget in the upper ocean. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite‐derived oceanic primary production and measured (Fe:C)_cellular ratios. The distributions of iron supply by upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO₃)_dissolved ratios to the nitrate supply and by assuming the soluble fraction of mineral aerosols. A lower bound on the rate of iron recycling in the photic zone was estimated as the difference between iron assimilation and supply. Global iron assimilation by phytoplankton for the open ocean was estimated to be 12 × 10⁹ mol Fe yr⁻¹. Atmospheric deposition of total Fe is estimated to be 96×10⁹ mol Fe yr⁻¹ in the open ocean, with the soluble Fe fraction ranging between 1 and 10% (or 1‐10 ×10⁹ mol Fe yr⁻¹). By comparison, the upwelling/entrainment supply of dissolved Fe to the upper ocean is small, ∼0.7×10⁹ mol Fe yr⁻¹. Uncertainties in the aeolian flux and assimilation may be as large as a factor of 5‐10 but remain difficult to quantify, as information is limited about the form and transformation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe :N) supply, confirms the production in the high‐nitrate low‐chlorophyll regions is indeed limited by iron availability.

The record of atmospheric dust deposition as recorded by a deep sea sediment trap in the Sargasso Sea is presented. The record is shown to be consistent with the limited available data on directly measured atmospheric dust loadings. The seasonality of the sediment trap dust flux is different from that of the atmospheric deposition as a result of seasonal biological cycles in the surface water. On the longer term the sediment trap dust flux undergoes quite large variations in the annual average flux from 3.6 to 9.4 mg m⁻² d⁻¹. These variations are shown to reflect changes in atmospheric transport efficiency from source regions in North Africa rather than changes in the strength of the dust source in that region. The changes in the dust inputs to this area of the Sargasso Sea appear not to have changed the flux of carbon reaching the deep water, and the implications of this are discussed.

The concentration of ¹⁸O in atmospheric CO₂ and H₂O is a potentially powerful tracer of ecosystem carbon and water fluxes. In this paper we describe the development of an isotope model (ISOLSM) that simulates the ¹⁸O content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic ¹⁸OC¹⁶O (hereinafter C¹⁸OO) fluxes; CO₂ oxygen isotope exchanges with soil and leaf water; soil CO₂ and C¹⁸OO diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of H₂¹⁸O and C¹⁸OO with the atmosphere. The isotope model is integrated into the land surface model LSM, but coupling with other models should be straightforward. We describe ISOLSM and apply it to evaluate (1) simplified methods of predicting the C¹⁸OO soil‐surface flux; (2) the impacts on the C¹⁸OO soil‐surface flux of the soil‐gas diffusion coefficient formulation, soil CO₂ source distribution, and rooting distribution; (3) the impacts on the C¹⁸OO fluxes of carbonic anhydrase (CA) activity in soil and leaves; and (4) the sensitivity of model predictions to the δ¹⁸O value of atmospheric water vapor and CO₂. Previously published simplified models are unable to capture the seasonal and diurnal variations in the C¹⁸OO soil‐surface fluxes simulated by ISOLSM. Differences in the assumed soil CO₂ production and rooting depth profiles, carbonic anhydrase activity in soil and leaves, and the δ¹⁸O value of atmospheric water vapor have substantial impacts on the ecosystem CO₂ flux isotopic composition. We conclude that accurate prediction of C¹⁸OO ecosystem fluxes requires careful representation of H₂¹⁸O and C¹⁸OO exchanges and transport in soils and plants.

We review the current understanding of the Dole effect (the observed difference between the δ¹⁸O of atmospheric O₂ and that of seawater) and its causes, extend the record of variations in the Dole effect back to 130 kyr before present using data on the δ¹⁸O of O₂ obtained from studying the Vostok ice core (Sowers et al., 1993), and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present‐day Dole effect, +20.8‰, is considerably lower than the observed value, +23.5‰, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05‰ lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only ±0.2‰, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical term reflects variations in low‐latitude land hydrology and productivity or possibly variability in low‐latitude oceanic productivity.

Investigations focusing on the temperature sensitivity of microbial activity and nutrient turnover in soils improve our understanding of potential effects of global warming. This study investigates the temperature sensitivity of C mineralization, N mineralization, and potential enzyme activities involved in the C and N cycle (tyrosine amino‐peptidase, leucine amino‐peptidase, ß‐glucosidase, ß‐xylosidase, N‐acetyl‐ß‐glucosaminidase). Four different study sites in the Austrian alpine zone were selected, and soils were sampled in three seasons (summer, autumn, and winter). A simple first‐order exponential equation was used to calculate constant Q₁₀ values for the C and N mineralization over the investigated temperature range (0–30°C). The Q₁₀ values of the C mineralization (average 2.0) for all study sites were significantly higher than for the N mineralization (average 1.7). The Q₁₀ values of both activities were significantly negatively related to a soil organic matter quality index calculated by the ratios of respiration to the organic soil carbon and mineralized N to the total soil nitrogen. The chemical soil properties or microbial biomass did not affect the Q₁₀ values of C and N mineralization. Moreover, the Q₁₀ values showed no distinct pattern according to sampling date, indicating that the substrate quality and other factors are more important. Using a flexible model function, the analysis of relative temperature sensitivity (RTS) showed that the temperature sensitivity of activities increased with decreasing temperature. The C and N mineralization and potential amino‐peptidase activities (tyrosine and leucine) showed an almost constant temperature dependence over 0–30°C. In contrast, ß‐glucosidase, ß‐xylosidase, and N‐acetyl‐ß‐glucosaminidase showed a distinctive increase in temperature sensitivity with decreasing temperature. Low temperature at the winter sampling date caused a greater increase in the RTS of all microbial activities than for the autumn and summer sampling dates. Our results indicate (1) a disproportion of the RTS for potential enzyme activities of the C and N cycle and (2) a disproportion of the RTS for easily degradable C compounds (ß‐glucose, ß‐xylose) compared with the C mineralization of soil organic matter. Thus temperature may play an important role in regulating the decay of different soil organic matter fractions due to differences in the relative temperature sensitivities of enzyme activities.

Surface sediments were obtained from a matrix of 76 sample sites in the inner shelf mud belts of the East China Sea (ECS) for a comprehensive study of the distribution, composition, deposition flux, and fate of polycyclic aromatic hydrocarbons (PAHs). The sampling sites covered an area of ~80,000 km² extending ~1000 km from the mouth of the Yangtze River to the Min River in the inner shelf. The total deposition flux of the 16 USEPA priority PAHs (16 PAHs) of the Yangtze estuarine‐inner shelf was estimated to be 152 t/yr, accounting for ~38% of the total annual input of the 16 PAHs into the ECS. This indicates that the Yangtze estuarine‐inner shelf is one of the largest sinks of land‐based PAHs in the world. Principal component analysis indicated that the 16 PAHs in the northern Yangtze estuarine mud area were mostly phenanthrene while shifting to high‐molecular‐weight PAHs in the southern Min‐Zhe coastal mud area. The positive matrix factorization model revealed that the deposition flux of low‐molecular‐weight (LMW) PAHs decreased from north to south, most likely due to the mass transfer between the resuspended sediments triggered by the East Asian monsoon and the water columns, as the resuspended sediments are transported southward. This release of LMW PAHs from the sediments to the water columns could become an important secondary PAH source in ECS.

Một trong những nguyên nhân chính dẫn đến hiệu suất sử dụng nitơ (N) thấp ở cây trồng là sự bay hơi của amoniac (NH₃) từ phân bón. Thông tin được lấy từ 1667 phép đo sự bay hơi NH₃ được ghi trong 148 tài liệu nghiên cứu đã được tóm tắt để đánh giá ảnh hưởng đến sự bay hơi NH₃ của loại cây trồng, loại phân bón, cùng lượng và cách thức áp dụng, nhiệt độ, cũng như carbon hữu cơ trong đất, kết cấu, pH, CEC, phương pháp đo lường và vị trí đo lường. Bộ dữ liệu đã được tóm tắt theo ba cách: (1) bằng cách tính trung bình cho mỗi yếu tố được đề cập, trong đó các kết quả từ mỗi tài liệu nghiên cứu có trọng số như nhau; (2) bằng cách tính giá trị trung bình có trọng số được điều chỉnh cho các đặc điểm không cân bằng của dữ liệu thu thập; và (3) bằng cách phát triển một mô hình tóm tắt sử dụng hồi quy tuyến tính dựa trên giá trị trung bình có trọng số về sự bay hơi NH₃ và bằng cách tính tổn thất bay hơi NH₃ toàn cầu từ việc áp dụng phân bón với dữ liệu có độ phân giải 0.5° về sử dụng đất và đất đai. Tổn thất trung bình tính được của NH₃ từ việc áp dụng phân N tổng hợp toàn cầu (78 triệu tấn N mỗi năm) và phân động vật (33 triệu tấn N mỗi năm) tương ứng là 14% (10–19%) và 23% (19–29%). Ở các nước đang phát triển, do nhiệt độ cao và việc sử dụng phổ biến urê, amoni sulfat, và amoni bicarbonat, tổn thất bay hơi NH₃ ước tính từ phân bón tổng hợp là 18%, và ở các nước công nghiệp hóa là 7%. Tổn thất ước tính của NH₃ từ phân động vật là 21% ở các nước công nghiệp hóa và 26% ở các nước đang phát triển.

Trong thập kỷ qua, một kho thông tin lớn về phát thải từ các loại đốt sinh khối khác nhau đã được tích lũy, phần lớn là kết quả từ các hoạt động nghiên cứu của Chương trình Địa cầu Sinh học Quốc tế/ Hóa học Khí quyển Toàn cầu Quốc tế. Tuy nhiên, thông tin này chưa sẵn có một cách dễ dàng đối với cộng đồng hóa học khí quyển vì nó bị phân tán trên một số lượng lớn các tài liệu và được báo cáo bằng nhiều đơn vị và hệ thống tham chiếu khác nhau. Chúng tôi đã đánh giá một cách có phê phán những dữ liệu hiện có và tích hợp chúng vào một định dạng nhất quán. Dựa trên phân tích này, chúng tôi trình bày một tập hợp các hệ số phát thải cho một loạt các loại chất phát thải từ các vụ cháy sinh khối. Trong những trường hợp dữ liệu không có sẵn, chúng tôi đã đề xuất các ước lượng dựa trên các kỹ thuật ngoại suy thích hợp. Chúng tôi đã đưa ra các ước lượng toàn cầu về phát thải từ cháy rừng đối với các loại chất quan trọng phát thải từ những kiểu đốt sinh khối khác nhau và so sánh các ước lượng của chúng tôi với kết quả từ các nghiên cứu mô hình hóa ngược.

A fast method has been proposed [Antoine and Morel, this issue] to compute the oceanic primary production from the upper ocean chlorophyll‐like pigment concentration, as it can be routinely detected by a spaceborne ocean color sensor. This method is applied here to the monthly global maps of the photosynthetic pigments that were derived from the coastal zone color scanner (CZCS) data archive [Feldman et al., 1989]. The photosynthetically active radiation (PAR) field is computed from the astronomical constant and by using an atmospheric model, thereafter combined with averaged cloud information, derived from the International Satellite Cloud Climatology Project (ISCCP). The aim is to assess the seasonal evolution, as well as the spatial distribution of the photosynthetic carbon fixation within the world ocean and for a “climatological year”, to the extent that both the chlorophyll information and the cloud coverage statistics actually are averages obtained over several years. The computed global annual production actually ranges between 36.5 and 45.6 Gt C yr⁻¹ according to the assumption which is made (0.8 or 1) about the ratio of active‐to‐total pigments (recall that chlorophyll and pheopigments are not radiometrically resolved by CZCS). The relative contributions to the global productivity of the various oceans and zonal belts are examined. By considering the hypotheses needed in such computations, the nature of the data used as inputs, and the results of the sensitivity studies, the global numbers have to be cautiously considered. Improving the reliability of the primary production estimates implies (1) new global data sets allowing a higher temporal resolution and a better coverage, (2) progress in the knowledge of physiological responses of phytoplankton and therefore refinements of the time and space dependent parameterizations of these responses.