Lower Net Ecosystem Production Research Articles

Blue carbon stores in tropical seagrass meadows maintained under green turtle grazing

Seagrass meadows are important sites for carbon storage. Green turtles (Chelonia mydas) are marine megaherbivores that consume seagrass throughout much of their global range. With successful conservation efforts, turtle abundance will increase, leading to more meadows being returned to their natural grazed state. There is concern this may lead to a loss of carbon stored in these systems, but the effects of green turtle grazing on seagrass ecosystem carbon dynamics have not been investigated. Here we experimentally show that despite 79% lower net ecosystem production (NEP) following grazing (24.7 vs. 119.5 mmol C m−2 d−1) in a Caribbean Thalassia testudinum seagrass meadow, grazed areas maintained net positive metabolic carbon uptake. Additionally, grazing did not change the meadow production to respiration ratio, indicating it did not stimulate remineralization of sediment carbon stores. Compared to other published estimates of seagrass NEP (median: 20.6 mmol C m−2 d−1), NEP in grazed Caribbean T. testudinum meadows is similar to that in many other ungrazed systems. Our results demonstrate that while grazing does decrease potential future carbon sequestration as a result of lower NEP, it does not promote a metabolic release of current carbon stocks.

Seasonal variability of evapotranspiration and carbon exchanges over a biomass sorghum field in the Southern U.S. Great Plains

The eddy covariance method was used to investigate carbon fluxes and evapotranspiration (ET) from a high biomass forage sorghum (Sorghum bicolor L.) field in the Southern U.S. Great Plains for three growing seasons (2013–2015). Above normal precipitation and narrow row spacing (50 cm) led to higher biomass production (25 Mg ha−1) and leaf area index (LAI = 7.2) development in 2014. This also resulted in higher carbon uptake or net ecosystem production (NEP) and ET during that year. Early and late season precipitation enhanced ecosystem respiration (Reco) resulting in lower NEP in 2015. Shorter growing season (119 days) also contributed to lower cumulative NEP in 2015. Estimated gross primary production (GPP) in 2014 (1780 g m−2) was 10% higher than the GPP in 2013 (1591 g m−2) and 24% higher than the GPP in 2015 (1353 g m−2). During all growing seasons, the site was a source of carbon (negative NEP) at the beginning and transitioned to a sink (positive NEP) later in the season. Biomass-GPP relationship indicated that approximately 65% of total GPP was allocated to above ground biomass (AGB). Average monthly ecosystem WUE (expressed as gross carbon gain per unit of ET) ranged from 1.7 g mm−1 to 4.2 g mm−1. Results from our study indicate that weather conditions, growing season length and crop management are important factors in determining the magnitude of carbon uptake and release, and ET of this cellulosic biofuel feedstock crop in the Southern U.S. Great Plains.

Influence of climate variability on seasonal and interannual variations of ecosystem CO2 exchange in flooded and non-flooded rice fields in the Philippines

Seasonal rainfall in the Philippines is known to be modulated by ENSO phenomenon, with El Niño frequently contributing to reduced rainfall and drought while La Niña resulting in excessive rainfalls, floods and more intense typhoons. The alterations in rainfall patterns can have considerable feedback on solar radiation, air temperature, and soil moisture which can affect the ecosystem CO2 exchange. In this paper, we assessed the effects of the ENSO events (2008–mid 2010) on the seasonal climate conditions and determined how it affected the gross primary production (GPP), ecosystem respiration (Re), and net ecosystem production (NEP) of two contrasting rice environments: flooded and non-flooded.The 2008 dry season (DS) was under a La Niña event while the 2008 wet season (WS) was a neutral one with strong tropical cyclones associated during the wet season. The 2009DS was also La Niña while the 2009WS was El Niño; however, the northern part of the Philippines experienced strong tropical cyclones. The 2010DS was under an El Niño event.The La Niña in 2008DS resulted in about 15% lower solar radiation (SR), 0.3°C lower air temperature (Ta) and 60% higher precipitation compared to the 28-year climate normal patterns. Both flooded and non-flooded rice fields had lower NEP in 2008 DS (164 and 14gCm−2, respectively) than in 2008 WS (295 and 82gCm−2, respectively) because the climate anomaly resulted in SR – driven decrease in GPP. The La Niña in 2009DS even resulted in 0.2°C lower air temperature and 40% more precipitation than the 2008DS La Niña. This cooler temperature resulted in lower Re in flooded rice fields while the higher precipitation resulted in higher GPP in non-flooded fields since the climate was favorable for the growth of the aerobic rice, the ratoon crops and the weeds. This climate anomaly benefitted both flooded and non-flooded rice fields by increasing NEP (351 and 218gCm−2, respectively). However, NEP decreased in 2009WS in both flooded and non-flooded rice fields (225 and 39gCm−2, respectively) due to the devastating effects of the strong tropical cyclones that hit the northern part of the Philippines. On the other hand, the El Niño event during 2010DS resulted in about 6% higher solar radiation, 0.4°C higher air temperature and 67% lower precipitation than the 28-year climate normal pattern. The NEP of flooded and non-flooded rice fields were closely similar at 187 and 174gCm−2, respectively. This climate anomaly resulted in Ta – driven increase in Re, as well as vapor pressure deficit (VPD) – driven decrease in GPP in flooded rice fields. The GPP and Re in non-flooded rice fields were less sensitive to higher VPD and higher Ta, respectively. It appears that the net ecosystem CO2 exchange in non-flooded rice field was less sensitive to an El Niño event.

西南高山地区净生态系统生产力时空动态

西南高山地区生态系统类型丰富、地形复杂, 是响应全球气候变化的重点区域,对全球气候变化具有重要的指示作用。应用生态系统模型(Carbon Exchange between Vegetation, Soil, and the Atmosphere,CEVSA)模型估算了1954-2010年西南高山地区净生态系统生产力(NEP)的时空变化,分析了其对气候变化的响应。结果表明:(1) 1954-2010年西南高山地区NEP平均为29.7 g C·m^-2·a^-1,其中低海拔地区常绿针叶林和常绿阔叶林NEP较高,而高海拔地区的草地覆盖类型NEP较低。(2)西南高山地区NEP总量的变动范围为-8.36-29.4Tg C/a,平均每年吸收碳15.4Tg C;NEP年际下降趋势显著(<em>P</em><0.05),平均每年减少0.187Tg C,下降显著的区域占研究地区总面积的35.2%(<em>P</em><0.05),其中草地(-0.526 g C·m^-2·a^-2, <em>P</em><0.01)和常绿针叶林(-0.691 g C·m^-2·a^-2, <em>P</em><0.01)下降趋势极为显著。(3) 年NEP总量的年际变化与年平均温度呈负相关(<em>r</em>=-0.454,<em>P</em><0.01),与年降水量呈正相关(<em>r</em>=0.708,<em>P</em><0.01),与温度显著负相关的区域占60.3%(<em>P</em><0.05),与降水显著正相关的区域占52.1%(<em>P</em><0.05),其中草地和常绿针叶林均与温度极显著负相关(<em>r</em>=-0.603,<em> P</em><0.01; <em>r</em>=-0.485, <em>P</em><0.01),而与降水量极显著正相关(<em>r</em>=0.554, <em>P</em><0.01; <em>r</em>=0.749, <em>P</em><0.01)。(4)西南高山地区是明显的碳汇区,但是由于土壤异养呼吸(HR, heterotrophic respiration)的增长速度大于净初级生产力(NPP, net primary production)的增长速度,最近20a有部分地区开始由碳汇转为碳源。;There are various types of ecosystems and complex landform in alpine area of southwestern China, which make it very sensitive to global climate change. Quantifying the spatial and temporal variation of terrestrial ecosystem carbon sink in this place requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. A process-based biogeochemical model CEVSA(Carbon Exchange between Vegetation, Soil, and the Atmosphere) was used to estimate temporal and spatial variations of net ecosystem production (NEP) in alpine area of southwestern China from 1954 to 2010. In this study, the set of independent environmental variables to run the model consisted of climate (ten days mean value of temperature, precipitation, relative humidity, cloudiness), atmospheric CO₂ concentration, soil and vegetation types at a spatial resolution of 0.1°. We first ran CEVSA model with average climate data during the period 1954-2010 until an ecological equilibrium was reached, then made dynamic simulations from 1954 to 2010 with transient changes in climate at a time-step of ten days.To achieve the results, various kinds of computer software were applied, such as ANUSPLIN4.1, Arcgis9.3, SPSS18.0, Fortran 90. The results showed: (1)The mean value of NEP was 29.7 g C·m^-2·a^-1 during the period 1954-2010 in alpine area of southwestern China, with higher NEP in the evergreen needle-leaved and evergreen broadleaved tree cover at low altitude and lower NEP in herbaceous cover at high altitude of study area. (2) Annual total NEP ranged from -8.36 to 29.4 Tg C/a with a mean value of 15.4 Tg C/a. NEP showed a statistically significant decreasing trend with a reduction rate of 0.187 Tg C/a (<em>P</em><0.05) during the period 1954-2010 and the significant decreasing area of NEP accounted for 35.2% (<em>P</em><0.05). The decreasing trend was highly significant in herbaceous cover(-0.526 g C·m^-2·a^-2, <em>P</em><0.01) and evergreen needle-leaved tree cover (-0.691 g C·m^-2·a^-2,<em> P</em><0.01). (3)Annual total NEP was significantly negatively correlated with annual mean temperature <em>(AMT) </em>(<em>r</em>=-0.454, <em>P</em><0.01) and positively correlated with annual total precipitation <em>(ATP)</em>(<em>r</em>=0.708, <em>P</em><0.01). The negative correlation (<em>P</em><0.05) of annual NEP and <em>AMT </em>existed in 60.3% of the study region while the positive correlation (<em>P</em><0.05) of annual NEP with <em>ATP</em> existed in 52.1% of the study region, respectively. Herbaceous cover and evergreen needle-leaved tree cover as the two main vegetation types in study area were both correlated negatively with <em>AMT</em>(<em>r</em>=-0.603,<em> P</em><0.01; <em>r</em>=-0.485, <em>P</em><0.01) and positively with <em>ATP</em>(<em>r</em>=0.554, <em>P</em><0.01; <em>r</em>=0.749,<em> P</em><0.01), reaching an extremely remarkable level. (4)Alpine area of southwestern China acted as a carbon sink, however, a small fraction of terrestrial ecosystems here shifted from carbon sinks into carbon sources in recent 20 years, for that the NPP (net primary production) growth rate was less than that of the soil HR (heterotrophic respiration).These estimates indicate that the carbon sequestration capacity in alpine area of southwestern China was weakened by the ongoing climate change.

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

Planktonic gross primary production (GPP), community respiration (CR), and nitrification (NIT) were measured monthly in the Scheldt estuary by the oxygen incubation method in 2003. No significant evolution of planktonic GPP was observed since the 1990s with high rates in the freshwater area (salinity 0; 97 6 65 mmol C m 22 d 21 ) decreasing seaward (22-37 mmol C m 22 d 21 ). A significant decrease of NIT was observed with regard to previous investigations although this process still represents up to 20% of total organic matter production in the inner estuary. Planktonic CR was highest in the inner estuary and seemed to be mainly controlled by external organic matter inputs. Planktonic net community production was negative most of the time in the estuary with values ranging from 2300 to 165 mmol C m 22 d 21 . Whole estuary net ecosystem production (NEP) was investigated on an annual scale using the results mentioned above and published benthic metabolic rates. A NEP of 239 6 8 mmol C m 22 d 21 was estimated, which confirms the strong heterotrophic status of this highly nutrified estuary. NEP rates were computed from June to December 2003 to compare with results derived from the Land-Ocean Interaction in the Coastal Zone budgeting procedure applied to dissolved inorganic phosphorus and carbon (DIP and DIC). DIP budgets failed to provide realistic estimates in the inner estuary where abiotic processes account for more than 50% of the nonconservative DIP flux. DIC budgets predicted a much lower NEP than in situ incubations (2109 6 31 versus 242 6 9 mmol C m22 d21) although, as each approach is associated with several critical assumptions, the source of