Xiujun Wang

Regulation of phytoplankton carbon to chlorophyll ratio by light, nutrients and temperature in the Equatorial Pacific Ocean: a basin-scale model

Abstract. The complex effects of light, nutrients and temperature lead to a variable carbon to chlorophyll (C:Chl) ratio in phytoplankton cells. Using field data collected in the Equatorial Pacific, we derived a new dynamic model with a non-steady C:Chl ratio as a function of irradiance, nitrate, iron, and temperature. The dynamic model is implemented into a basin-scale ocean circulation-biogeochemistry model and tested in the Equatorial Pacific Ocean. The model reproduces well the general features of phytoplankton dynamics in this region. For instance, the simulated deep chlorophyll maximum (DCM) is much deeper in the western warm pool (~100 m) than in the Eastern Equatorial Pacific (~50 m). The model also shows the ability to reproduce chlorophyll, including not only the zonal, meridional and vertical variations, but also the interannual variability. This modeling study demonstrates that combination of nitrate and iron regulates the spatial and temporal variations in the phytoplankton C:Chl ratio in the Equatorial Pacific. Sensitivity simulations suggest that nitrate is mainly responsible for the high C:Chl ratio in the western warm pool while iron is responsible for the frontal features in the C:Chl ratio between the warm pool and the upwelling region. In addition, iron plays a dominant role in regulating the spatial and temporal variations of the C:Chl ratio in the Central and Eastern Equatorial Pacific. While temperature has a relatively small effect on the C:Chl ratio, light is primarily responsible for the vertical decrease of phytoplankton C:Chl ratio in the euphotic zone.

Role of ocean biology-induced climate feedback in the modulation of El Nino-Southern Oscillation

Received 4 November 2008; revised 17 December 2008; accepted 29 December 2008; published 10 February 2009. (1 )E l Nino-Southern Oscillation (ENSO) properties can be modulated by many factors; most previous studies have focused on physical aspects of the climate system in the tropical Pacific. Ocean biology-induced feedback (OBF) onto physics and bio-climate coupling have been the subject of much recent interest, revealing striking model dependence and even conflicting results. Current satellite data are able to resolve the space-time structure of oceanic signals both in biology and physics, providing an opportunity for quantifying their relationships. Here we use the biological signature from satellite ocean color data to estimate interannual variability of the attenuation depth of solar radiation (Hp), a field linking ocean biology and physics. We then apply a singular value decomposition (SVD) analysis to interannual Hp and sea surface temperature (SST) anomaly fields to derive an empirical Hp model which is incorporated in a hybrid coupled ocean-atmosphere model of the tropical Pacific to represent the OBF. It is shown that the OBF can have significant effects on ENSO behaviors, including its amplitude, oscillation periods and seasonal phase locking. Citation: Zhang, R.-H., A. J. Busalacchi, X. Wang, J. Ballabrera-Poy, R. G. Murtugudde, E. C. Hackert, and D. Chen (2009), Role of ocean biology-induced climate feedback in the modulation of El Nino-Southern Oscillation, Geophys. Res. Lett., 36, L03608,

Modeling seasonal phosphate export and resupply in the Subantarctic and Polar Frontal zones in the Australian sector of the Southern Ocean

We developed and applied a one-dimensional (z) biophysical model to the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) to simulate seasonal phosphate export production and resupply. The physical component of our model was capable of reproducing the observed seasonal amplitude of sea surface temperature and mixed layer depth. In the biological component of the model we used incident light, mixed layer depth, phosphate availability, and estimates of phytoplankton biomass from the Sea-viewing Wide Field-of-view Sensor to determine production and tuned the model to reproduce the observed seasonal cycle of phosphate. We carried out a series of sensitivity studies, taking into account uncertainties in both physical fields and biological formulations (including potential influence of iron limitation), which led to several robust conclusions (as represented by the ranges below). The major growing season contributed 66–76% of the annual export production in both regions. The simulated annual export production was significantly higher in the PFZ (68–83 mmol P m−2) than in the SAZ (52–61 mmol P m−2) despite the PFZs having lower seasonal nutrient depletion. The higher export production in the PFZ was due to its greater resupply of phosphate to the upper ocean during the September to March period (27–37 mmol P m−2) relative to that in the SAZ (8–15 mmol P m−2). Hence seasonal nutrient depletion was a better estimate of seasonal export production in the SAZ, as demonstrated by its higher ratio of seasonal depletion/export (64–78%) relative to that in the PFZ (34–47%). In the SAZ, vertical mixing was the dominant mechanism for supplying phosphate to the euphotic zone, whereas in the PFZ, vertical mixing supplied only 37% of the phosphate to the euphotic zone and horizontal transport supplied the remaining 63%.

Carbon Dioxide Fluxes and Concentrations in a Cotton Field in Northwestern China:Effects of Plastic Mulching and Drip Irrigation

Abstract In northwestern China, there has been a change from traditional cultivation system (TC) with no mulching and flood irrigation to a more modern cultivation system (MC) using plastic film mulching with drip irrigation. A field study was conducted to compare soil CO 2 concentrations and soil surface CO 2 fluxes between TC and MC systems during a cotton growing season. CO 2 concentrations in the soil profile were higher in the MC system (3107-9212 μL L −1 ) than in the TC system (1275-8994 μL L −1 ) but the rate of CO 2 flux was lower in the MC system. Possible reasons for this included decreased gas diffusion and higher soil moisture due to the mulching cover in the MC system, and the consumption of soil CO 2 by weathering reactions. Over the whole cotton growing season, accumulated rates of CO 2 flux were 300 and 394 g C m −2 for the MC and TC systems, respectively. When agricultural practices were converted from traditional cultivation to a plastic film mulching system, soil CO 2 emissions could be reduced by approximately 100 g C m −2 year −1 in agricultural lands in arid and/or semi-arid areas of northern and northwestern China.

Modeling the upper ocean dynamics in the Subantarctic and Polar Frontal zones in the Australian sector of the Southern Ocean

A one-dimensional (1-D) mixed layer model (the Chen scheme) was applied in the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) to simulate the upper ocean dynamics. The model was forced with 4 years data of the heat fluxes, freshwater fluxes, and wind stresses from the National Centers for Environmental Prediction. In both the SAZ and PFZ, the 1-D model was capable of reproducing the amplitude of the seasonal sea surface temperature (SST) and the seasonally of the mixed layer depth (MLD). The shallowest MLD was found in January-February (20 m in the SAZ, 35 m in the PFZ), and the deepest MLD was found between August and October (600 m in the SAZ, 160 m in the PFZ). The summer MLD was shallower in the SAZ than in the PFZ due to the lower wind stress. However, the shallower winter MLD in the PFZ than in the SAZ was due to the strong stratification in the water below the mixed layer. In the SAZ, variability in the wind stress was the dominant term driving the fluctuation in MLD in the summer, but variability in the heat flux was the major factor controlling the timing of the deepening and shoaling of the mixed layer in the winter. In the PFZ both the variability in the wind stress and the heat flux dominated the variability of the MLD in both the summer and the winter.

Contributions of wheat and maize residues to soil organic carbon under long-term rotation in north China

Soil organic carbon (SOC) dynamics in agro-ecosystem is largely influenced by cropping. However, quantifying the contributions of various crops has been lacking. Here we employed a stable isotopic approach to evaluate the contributions of wheat and maize residues to SOC at three long-term experimental sites in north China. Soil samples were collected from 0–20, 20–40, 40–60, 60–80 and 80–100 cm after 13 and 20 years of wheat-maize rotation, and SOC and its stable 13C composition were determined. Our data showed that the δ13C value of SOC varied, on average, from −22.1‰ in the 0–20 cm to −21.5‰ in the 80–100 cm. Carbon input through maize residues ranged from 35% to 68% whereas the contribution of maize residues to SOC (0–40 cm) ranged from 28% to 40%. Our analyses suggested that the retention coefficient was in the range of 8.0–13.6% for maize residues and 16.5–28.5% for wheat residues. The two-fold higher retention coefficient of wheat versus maize residues was due to the differences in the quality of residues and probably also in the temperature during the growing season. Our study highlighted the importance of crop management on carbon sequestration in agricultural lands.

Carbon accumulation in arid croplands of northwest China: pedogenic carbonate exceeding organic carbon

Soil carbonate (SIC) exceeds organic carbon (SOC) greatly in arid lands, thus may be important for carbon sequestration. However, field data for quantifying carbonate accumulation have been lacking. This study aims to improve our understanding of SIC dynamics and its role in carbon sequestration. We analyzed two datasets of SOC and SIC and their 13C compositions , one with over 100 soil samples collected recently from various land uses in the Yanqi Basin, Xinjiang, and the other with 18 archived soil samples from a long-term experiment (LTE) in Pingliang, Gansu. The data from the Yanqi Basin showed that SOC had a significant relationship with SIC and pedogenic carbonate (PIC); converting shrub land to cropland increased PIC stock by 5.2 kg C m−2, which was 3.6 times of that in SOC stock. The data from the LTE showed greater accumulation of PIC (21–49 g C m−2 year−1) than SOC (10–39 g C m−2 year−1) over 0–20 cm. Our study points out that intensive cropping in the arid and semi-arid regions leads to an increase in both SOC and PIC. Increasing SOC through straw organic amendments enhances PIC accumulation in the arid cropland of northwestern China.

Relative contribution of maize and external manure amendment to soil carbon sequestration in a long-term intensive maize cropping system

We aimed to quantify the relative contributions of plant residue and organic manure to soil carbon sequestration. Using a 27-year-long inorganic fertilizer and manure amendment experiment in a maize (Zea mays L.) double-cropping system, we quantified changes in harvestable maize biomass and soil organic carbon stocks (0–20 cm depth) between 1986-2012. By employing natural 13C tracing techniques, we derived the proportional contributions of below-ground crop biomass return (maize-derived carbon) and external manure amendment (manure-derived carbon) to the total soil organic carbon stock. The average retention of maize-derived carbon plus manure-derived carbon during the early period of the trial (up to 11 years) was relatively high (10%) compared to the later period (22 to 27 years, 5.1–6.3%). About 11% of maize-derived carbon was converted to soil organic carbon, which was double the retention of manure-derived carbon (4.4–5.1%). This result emphasized that organic amendments were necessary to a win-win strategy for both SOC sequestration and maize production.

Spatial and temporal variability of the phytoplankton carbon to chlorophyll ratio in the equatorial Pacific: A basin‐scale modeling study

[1] The relationship between phytoplankton carbon biomass and chlorophyll is nonlinear because of the complex impacts of light, nutrient conditions, and temperature. A basin-scale ocean circulation-biogeochemistry model implemented with a dynamic phytoplankton model is employed to explore the spatial and temporal variability of the phytoplankton carbon to chlorophyll (C:Chl) ratio in the equatorial Pacific Ocean. The dynamic model computes the phytoplankton C:Chl ratio as a function of light, nitrate, iron, and temperature. The model reproduces well the general features of phytoplankton dynamics in this region, e.g., the deeper chlorophyll maximum (DCM) in the western warm pool and shallower DCM in the upwelling region. The model predicts large spatial and temporal variations of the C:Chl ratio. The mixed layer C:Ch1 ratio increases from 150 in the warm pool, whereas subsurface ratio is ~50 below 100 m. The model produces a weak seasonality in the mixed layer C:Chl ratio but strong interannual variability that is associated with the El Nino-Southern Oscillation (ENSO) events. The warm pool has strong anomalies during the cold phase of the ENSO with a reduced C:Chl ratio and a shoaled DCM. However, the upwelling region reveals strong anomalies during the warm phase of the ENSO, showing an increased C:Chl ratio in the euphotic zone and a deepened DCM. The predicted large spatial and temporal variations of the C:Chl ratio have potential implications for the carbon uptake in the equatorial Pacific.

Evaluation of the CENTURY Model Using Long-Term Fertilization Trials under Corn-Wheat Cropping Systems in the Typical Croplands of China

Soil organic matter models are widely used to study soil organic carbon (SOC) dynamics. Here, we used the CENTURY model to simulate SOC in wheat-corn cropping systems at three long-term fertilization trials. Our study indicates that CENTURY can simulate fertilization effects on SOC dynamics under different climate and soil conditions. The normalized root mean square error is less than 15% for all the treatments. Soil carbon presents various changes under different fertilization management. Treatment with straw return would enhance SOC to a relatively stable level whereas chemical fertilization affects SOC differently across the three sites. After running CENTURY over the period of 1990–2050, the SOC levels are predicted to increase from 31.8 to 52.1 Mg ha−1 across the three sites. We estimate that the carbon sequestration potential between 1990 and 2050 would be 9.4–35.7 Mg ha−1 under the current high manure application at the three sites. Analysis of SOC in each carbon pool indicates that long-term fertilization enhances the slow pool proportion but decreases the passive pool proportion. Model results suggest that change in the slow carbon pool is the major driver of the overall trends in SOC stocks under long-term fertilization.