We review 15 techniques for estimating missing values of net ecosystem CO 2 exchange (NEE) in edd... more We review 15 techniques for estimating missing values of net ecosystem CO 2 exchange (NEE) in eddy covariance time series and evaluate their performance for different artificial gap scenarios based on a set of 10 benchmark datasets from six forested sites in Europe.
Environmental science. Processes & impacts, Jan 12, 2016
Soil respiration is a major pathway in the global carbon cycle and its response to environmental ... more Soil respiration is a major pathway in the global carbon cycle and its response to environmental changes is an increasing concern. Here we explored how total soil respiration (RT) and its components respond to elevated acid rain in a mixed conifer and broadleaf forest, one of the major forest types in southern China. RT was measured twice a month in the first year under four treatment levels of simulated acid rain (SAR: CK, the local lake water, pH 4.7; T1, water pH 4.0; T2, water pH 3.25; and T3, water pH 2.5), and in the second year, RT, litter-free soil respiration (RS), and litter respiration (RL) were measured simultaneously. The results indicated that the mean rate of RT was 2.84 ± 0.20 μmol CO2 m(-2) s(-1) in the CK plots, and RS and RL contributed 60.7% and 39.3% to RT, respectively. SAR marginally reduced (P = 0.08) RT in the first year, but significantly reduced RT and its two components in the second year (P < 0.05). The negative effects were correlated with the decrea...
With the continuing increase in anthropogenic activities, acid rain remains a serious environment... more With the continuing increase in anthropogenic activities, acid rain remains a serious environmental threat, especially in the fast developing areas such as southern China. To detect how prolonged deposition of acid rain would influence soil organic carbon accumulation in mature subtropical forests, we conducted a field experiment with simulated acid rain (SAR) treatments in a monsoon evergreen broadleaf forest at Dinghushan National Nature Reserve in southern China. Four levels of SAR treatments were set by irrigating plants with water of different pH values: CK (the control, local lake water, pH≈4.5), T1 (water pH=4.0), T2 (water pH=3.5), and T3 (water pH=3.0). Results showed reduced pH measurements in the topsoil exposed to simulated acid rains due to soil acidification. Soil respiration, soil microbial biomass and litter decomposition rates were significantly decreased by the SAR treatments. As a result, T3 treatment significantly increased the total organic carbon by 24.5% in th...
A power function, Y=Y0 Mbeta, can be used to describe the relationship of physiological variables... more A power function, Y=Y0 Mbeta, can be used to describe the relationship of physiological variables with body size over a wide range of scales, typically many orders of magnitude. One of the key issues in the renewed power law debate is whether the allometric scaling exponent β equals 3/4 or 2/3. The analysis could be remarkably affected by sampling size, measurement error, and analysis methods, but these effects have not been explored systematically. We investigated the influences of these three factors based on a data set of 626 pairs of base metabolic rate and mass in mammals with the calculated β=0.711. Influence of sampling error was tested by re-sampling with different sample sizes using a Monte Carlo approach. Results showed that estimated parameter b varied considerably from sample to sample. For example, when the sample size was n=63, b varied from 0.582 to 0.776. Even though the original data set did not support either β=3/4 or β=2/3, we found that 39.0% of the samples supported β=2/3, 35.4% of the samples supported β=3/4. Influence of measurement error on parameter estimations was also tested using Bayesian theory. Virtual data sets were created using the mass in the above-mentioned data set, with given parameters α and β (β=2/3 or β=3/4) and certain measurement error in base metabolic rate and/or mass. Results showed that as measurement error increased, more estimated bs were found to be significantly different from the parameter β. When measurement error (i.e., standard deviation) was 20% and 40% of the measured mass and base metabolic rate, 15.4% and 14.6% of the virtual data sets were found to be significant different from the parameter β=3/4 and β=2/3, respectively. Influence of different analysis methods on parameter estimations was also demonstrated using the original data set and the pros and cons of these methods were further discussed. We urged cautions in interpreting the power law analysis, especially from a small data sample, and in selecting analysis methods.
ABSTRACT As the demand of legume crops such as pigeonpea [Cajanus cajan (L.) Millsp.] is increasi... more ABSTRACT As the demand of legume crops such as pigeonpea [Cajanus cajan (L.) Millsp.] is increasing in the United States, extending the growing areas and improving the yield of such crops become an important task. To understand how agricultural management could influence the ecophysiology, growth, and yield of pigeonpea, we conducted a three-year (2010-2012) field experiment in Nashville, TN. We used a split-plot design considering two planting dates (main factor, early and later planting), four pigeonpea varieties (sub-plot factor, G1, G2, W1 and W3) and three plant densities (sub-plot factor, 295,500, 145,250, and 96,833 pants/ha). Maximum net leaf photosynthesis, stomatal conductance, transpiration, water use efficiency (WUE), soil respiration, plant height, plant stem diameter, leaf area index (LAI), and seed yield were measured over the three years. Results showed there were significant differences in ecophysiology, growth and yield between planting dates, among varieties, and among years. Early planting produced higher leaf photosynthesis and yield compared to late planting. Among four varieties, G2 grow shorter and had lower LAI, but produced higher seed yield compared to other varieties. W1 grew taller than G2 and produced the second highest yield. Planting date did not show significant influences on leaf physiology, LAI and yield. All variables measured showed remarkable variations among three years. Both precipitation amount and pattern during the growing season had significant influences on plant growth and yield. Our results indicated that variety selection and early planting in the southeastern US may improve pigeonpea growth and yield. Irrigation during drought may also significantly enhance plant performance and yield.
Atmospheric Chemistry and Physics Discussions, 2014
Increases in observed atmospheric concentrations of the long-lived greenhouse gas nitrous oxide (... more Increases in observed atmospheric concentrations of the long-lived greenhouse gas nitrous oxide (N 2 O) have been well documented. However, information on eventrelated instantaneous emissions during fertilizer applications is lacking. With the development of fast-response N 2 O analyzers, the eddy covariance (EC) technique can be used to gather instantaneous measurements of N 2 O concentrations to quantify the exchange of nitrogen between the soil and atmosphere. The objectives of this study were to evaluate the performance of a new EC system, to measure the N 2 O flux with the system, and finally to examine relationships of the N 2 O flux with soil temperature, soil moisture, precipitation, and fertilization events. An EC system was assembled with a sonic anemometer and a fast-response N 2 O analyzer (quantum cascade laser spectrometer) and applied in a cornfield in Nolensville, Tennessee during the 2012 corn growing season (4 April-8 August). Fertilizer amounts totaling 217 kg N ha −1 were applied to the experimental site. Results showed that this N 2 O EC system provided reliable N 2 O flux measurements. The cumulative emitted N 2 O amount for the entire growing season was 6.87 kg N 2 O-N ha −1 . Seasonal fluxes were highly dependent on soil moisture rather than soil temperature. This study was one of the few experiments that continuously measured instantaneous, high-frequency N 2 O emissions in crop fields over a growing season of more than 100 days.
It is predicted that locust outbreaks will occur more frequently under future climate change scen... more It is predicted that locust outbreaks will occur more frequently under future climate change scenarios, with consequent effects on ecological goods and services. A field manipulative experiment was conducted to examine the responses of gross ecosystem productivity (GEP), net ecosystem carbon dioxide (CO2) exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) to locust outbreaks in a temperate steppe of northern China from 2010 to 2011. Two processes related to locust outbreaks, natural locust feeding and carcass deposition, were mimicked by clipping 80 % of aboveground biomass and adding locust carcasses, respectively. Ecosystem carbon (C) exchange (i.e., GEP, NEE, ER, and SR) was suppressed by locust feeding in 2010, but stimulated by locust carcass deposition in both years (except SR in 2011). Experimental locust outbreaks (i.e., clipping plus locust carcass addition) decreased GEP and NEE in 2010 whereas they increased GEP, NEE, and ER in 2011, leading to neutral ...
Models represent our primary method for integration of small-scale, processlevel phenomena into a... more Models represent our primary method for integration of small-scale, processlevel phenomena into a comprehensive description of forest-stand or ecosystem function. They also represent a key method for testing hypotheses about the response of forest ecosystems to multiple changing environmental conditions. This paper describes the evaluation of 13 stand-level models varying in their spatial, mechanistic, and temporal complexity for their ability to capture intra-and interannual components of the water and carbon cycle for an upland, oak-dominated forest of eastern Tennessee. Comparisons between model simulations and observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. Response surfaces of carbon and water flux as a function of environmental drivers, and a variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance.
How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P, and sulfur-S is fundamental for ... more How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P, and sulfur-S is fundamental for understanding nutrient cycling in terrestrial ecosystems. We compiled a global database of C, N, P, and S concentrations in soils and microbes and developed relationships between them by using a power function model. The C:N:P:S was estimated to be 287:17:1:0.8 for soils, and 42:6:1:0.4 for microbes. We found a convergence of the relationships between elements in soils and in soil microbial biomass across C, N, P, and S. The element concentrations in soil microbial biomass follow a homeostatic regulation curve with soil element concentrations across C, N, P and S, implying a unifying mechanism of microbial assimilating soil elements. This correlation explains the well-constrained C:N:P:S stoichiometry with a slightly larger variation in soils than in microbial biomass. Meanwhile, it is estimated that the minimum requirements of soil elements for soil microbes are 0.8 mmol C Kg(-1) dry so...
For the purposes of forest restoration, carbon (C) fixation, and economic improvement, eucalyptus... more For the purposes of forest restoration, carbon (C) fixation, and economic improvement, eucalyptus (Eucalyptus urophylla) has been widely planted in South China. The understory of eucalyptus plantations is often occupied by a dense community of the fern Dicranopteris dichotoma, which intercepts tree canopy leaf litter before it reaches the ground. To understand the effects of this interception of litterfall on C cycling in eucalyptus plantations, we quantified the mass of intercepted litter and the influences of litterfall interception on litter decomposition and soil respiration. The total mass of E. urophylla litterfall collected on the understory was similar to that collected by the traditional litter trap method. All of the eucalyptus litterfall is intercepted by the D. dichotoma canopy. Of the litterfall that was intercepted by D. dichotoma, 20-40% and 60-80% was intercepted by the top (50-100 cm) and bottom (0-50 cm) of the understory canopy, respectively. Intercepted litterfal...
Eos, Transactions American Geophysical Union, 2008
A US National Science Foundation workshop brought together more than 40 researchers from differen... more A US National Science Foundation workshop brought together more than 40 researchers from different disciplines to discuss applications of data assimilation techniques in ecology, define major scientific questions to be addressed, and identify future research challenges. The field of ecology has become a data-rich enterprise, due largely to rapid development of measurement sensors and long-term accumulation of data from research networks. With the implementation of the National Ecological Observatory Network (NEON), a network with ...
We review 15 techniques for estimating missing values of net ecosystem CO 2 exchange (NEE) in edd... more We review 15 techniques for estimating missing values of net ecosystem CO 2 exchange (NEE) in eddy covariance time series and evaluate their performance for different artificial gap scenarios based on a set of 10 benchmark datasets from six forested sites in Europe.
Environmental science. Processes & impacts, Jan 12, 2016
Soil respiration is a major pathway in the global carbon cycle and its response to environmental ... more Soil respiration is a major pathway in the global carbon cycle and its response to environmental changes is an increasing concern. Here we explored how total soil respiration (RT) and its components respond to elevated acid rain in a mixed conifer and broadleaf forest, one of the major forest types in southern China. RT was measured twice a month in the first year under four treatment levels of simulated acid rain (SAR: CK, the local lake water, pH 4.7; T1, water pH 4.0; T2, water pH 3.25; and T3, water pH 2.5), and in the second year, RT, litter-free soil respiration (RS), and litter respiration (RL) were measured simultaneously. The results indicated that the mean rate of RT was 2.84 ± 0.20 μmol CO2 m(-2) s(-1) in the CK plots, and RS and RL contributed 60.7% and 39.3% to RT, respectively. SAR marginally reduced (P = 0.08) RT in the first year, but significantly reduced RT and its two components in the second year (P < 0.05). The negative effects were correlated with the decrea...
With the continuing increase in anthropogenic activities, acid rain remains a serious environment... more With the continuing increase in anthropogenic activities, acid rain remains a serious environmental threat, especially in the fast developing areas such as southern China. To detect how prolonged deposition of acid rain would influence soil organic carbon accumulation in mature subtropical forests, we conducted a field experiment with simulated acid rain (SAR) treatments in a monsoon evergreen broadleaf forest at Dinghushan National Nature Reserve in southern China. Four levels of SAR treatments were set by irrigating plants with water of different pH values: CK (the control, local lake water, pH≈4.5), T1 (water pH=4.0), T2 (water pH=3.5), and T3 (water pH=3.0). Results showed reduced pH measurements in the topsoil exposed to simulated acid rains due to soil acidification. Soil respiration, soil microbial biomass and litter decomposition rates were significantly decreased by the SAR treatments. As a result, T3 treatment significantly increased the total organic carbon by 24.5% in th...
A power function, Y=Y0 Mbeta, can be used to describe the relationship of physiological variables... more A power function, Y=Y0 Mbeta, can be used to describe the relationship of physiological variables with body size over a wide range of scales, typically many orders of magnitude. One of the key issues in the renewed power law debate is whether the allometric scaling exponent β equals 3/4 or 2/3. The analysis could be remarkably affected by sampling size, measurement error, and analysis methods, but these effects have not been explored systematically. We investigated the influences of these three factors based on a data set of 626 pairs of base metabolic rate and mass in mammals with the calculated β=0.711. Influence of sampling error was tested by re-sampling with different sample sizes using a Monte Carlo approach. Results showed that estimated parameter b varied considerably from sample to sample. For example, when the sample size was n=63, b varied from 0.582 to 0.776. Even though the original data set did not support either β=3/4 or β=2/3, we found that 39.0% of the samples supported β=2/3, 35.4% of the samples supported β=3/4. Influence of measurement error on parameter estimations was also tested using Bayesian theory. Virtual data sets were created using the mass in the above-mentioned data set, with given parameters α and β (β=2/3 or β=3/4) and certain measurement error in base metabolic rate and/or mass. Results showed that as measurement error increased, more estimated bs were found to be significantly different from the parameter β. When measurement error (i.e., standard deviation) was 20% and 40% of the measured mass and base metabolic rate, 15.4% and 14.6% of the virtual data sets were found to be significant different from the parameter β=3/4 and β=2/3, respectively. Influence of different analysis methods on parameter estimations was also demonstrated using the original data set and the pros and cons of these methods were further discussed. We urged cautions in interpreting the power law analysis, especially from a small data sample, and in selecting analysis methods.
ABSTRACT As the demand of legume crops such as pigeonpea [Cajanus cajan (L.) Millsp.] is increasi... more ABSTRACT As the demand of legume crops such as pigeonpea [Cajanus cajan (L.) Millsp.] is increasing in the United States, extending the growing areas and improving the yield of such crops become an important task. To understand how agricultural management could influence the ecophysiology, growth, and yield of pigeonpea, we conducted a three-year (2010-2012) field experiment in Nashville, TN. We used a split-plot design considering two planting dates (main factor, early and later planting), four pigeonpea varieties (sub-plot factor, G1, G2, W1 and W3) and three plant densities (sub-plot factor, 295,500, 145,250, and 96,833 pants/ha). Maximum net leaf photosynthesis, stomatal conductance, transpiration, water use efficiency (WUE), soil respiration, plant height, plant stem diameter, leaf area index (LAI), and seed yield were measured over the three years. Results showed there were significant differences in ecophysiology, growth and yield between planting dates, among varieties, and among years. Early planting produced higher leaf photosynthesis and yield compared to late planting. Among four varieties, G2 grow shorter and had lower LAI, but produced higher seed yield compared to other varieties. W1 grew taller than G2 and produced the second highest yield. Planting date did not show significant influences on leaf physiology, LAI and yield. All variables measured showed remarkable variations among three years. Both precipitation amount and pattern during the growing season had significant influences on plant growth and yield. Our results indicated that variety selection and early planting in the southeastern US may improve pigeonpea growth and yield. Irrigation during drought may also significantly enhance plant performance and yield.
Atmospheric Chemistry and Physics Discussions, 2014
Increases in observed atmospheric concentrations of the long-lived greenhouse gas nitrous oxide (... more Increases in observed atmospheric concentrations of the long-lived greenhouse gas nitrous oxide (N 2 O) have been well documented. However, information on eventrelated instantaneous emissions during fertilizer applications is lacking. With the development of fast-response N 2 O analyzers, the eddy covariance (EC) technique can be used to gather instantaneous measurements of N 2 O concentrations to quantify the exchange of nitrogen between the soil and atmosphere. The objectives of this study were to evaluate the performance of a new EC system, to measure the N 2 O flux with the system, and finally to examine relationships of the N 2 O flux with soil temperature, soil moisture, precipitation, and fertilization events. An EC system was assembled with a sonic anemometer and a fast-response N 2 O analyzer (quantum cascade laser spectrometer) and applied in a cornfield in Nolensville, Tennessee during the 2012 corn growing season (4 April-8 August). Fertilizer amounts totaling 217 kg N ha −1 were applied to the experimental site. Results showed that this N 2 O EC system provided reliable N 2 O flux measurements. The cumulative emitted N 2 O amount for the entire growing season was 6.87 kg N 2 O-N ha −1 . Seasonal fluxes were highly dependent on soil moisture rather than soil temperature. This study was one of the few experiments that continuously measured instantaneous, high-frequency N 2 O emissions in crop fields over a growing season of more than 100 days.
It is predicted that locust outbreaks will occur more frequently under future climate change scen... more It is predicted that locust outbreaks will occur more frequently under future climate change scenarios, with consequent effects on ecological goods and services. A field manipulative experiment was conducted to examine the responses of gross ecosystem productivity (GEP), net ecosystem carbon dioxide (CO2) exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) to locust outbreaks in a temperate steppe of northern China from 2010 to 2011. Two processes related to locust outbreaks, natural locust feeding and carcass deposition, were mimicked by clipping 80 % of aboveground biomass and adding locust carcasses, respectively. Ecosystem carbon (C) exchange (i.e., GEP, NEE, ER, and SR) was suppressed by locust feeding in 2010, but stimulated by locust carcass deposition in both years (except SR in 2011). Experimental locust outbreaks (i.e., clipping plus locust carcass addition) decreased GEP and NEE in 2010 whereas they increased GEP, NEE, and ER in 2011, leading to neutral ...
Models represent our primary method for integration of small-scale, processlevel phenomena into a... more Models represent our primary method for integration of small-scale, processlevel phenomena into a comprehensive description of forest-stand or ecosystem function. They also represent a key method for testing hypotheses about the response of forest ecosystems to multiple changing environmental conditions. This paper describes the evaluation of 13 stand-level models varying in their spatial, mechanistic, and temporal complexity for their ability to capture intra-and interannual components of the water and carbon cycle for an upland, oak-dominated forest of eastern Tennessee. Comparisons between model simulations and observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. Response surfaces of carbon and water flux as a function of environmental drivers, and a variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance.
How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P, and sulfur-S is fundamental for ... more How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P, and sulfur-S is fundamental for understanding nutrient cycling in terrestrial ecosystems. We compiled a global database of C, N, P, and S concentrations in soils and microbes and developed relationships between them by using a power function model. The C:N:P:S was estimated to be 287:17:1:0.8 for soils, and 42:6:1:0.4 for microbes. We found a convergence of the relationships between elements in soils and in soil microbial biomass across C, N, P, and S. The element concentrations in soil microbial biomass follow a homeostatic regulation curve with soil element concentrations across C, N, P and S, implying a unifying mechanism of microbial assimilating soil elements. This correlation explains the well-constrained C:N:P:S stoichiometry with a slightly larger variation in soils than in microbial biomass. Meanwhile, it is estimated that the minimum requirements of soil elements for soil microbes are 0.8 mmol C Kg(-1) dry so...
For the purposes of forest restoration, carbon (C) fixation, and economic improvement, eucalyptus... more For the purposes of forest restoration, carbon (C) fixation, and economic improvement, eucalyptus (Eucalyptus urophylla) has been widely planted in South China. The understory of eucalyptus plantations is often occupied by a dense community of the fern Dicranopteris dichotoma, which intercepts tree canopy leaf litter before it reaches the ground. To understand the effects of this interception of litterfall on C cycling in eucalyptus plantations, we quantified the mass of intercepted litter and the influences of litterfall interception on litter decomposition and soil respiration. The total mass of E. urophylla litterfall collected on the understory was similar to that collected by the traditional litter trap method. All of the eucalyptus litterfall is intercepted by the D. dichotoma canopy. Of the litterfall that was intercepted by D. dichotoma, 20-40% and 60-80% was intercepted by the top (50-100 cm) and bottom (0-50 cm) of the understory canopy, respectively. Intercepted litterfal...
Eos, Transactions American Geophysical Union, 2008
A US National Science Foundation workshop brought together more than 40 researchers from differen... more A US National Science Foundation workshop brought together more than 40 researchers from different disciplines to discuss applications of data assimilation techniques in ecology, define major scientific questions to be addressed, and identify future research challenges. The field of ecology has become a data-rich enterprise, due largely to rapid development of measurement sensors and long-term accumulation of data from research networks. With the implementation of the National Ecological Observatory Network (NEON), a network with ...
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