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RIVM rapport:Abstract niet beschikbaar, The integrated nitrous oxide (N2O) and methane (CH4) grassland project aims to estimate and explain emissions of these greenhouse gases from two ecosystems, namely drained agricultural peat soil under grass at the experimental farm Zegveld and undrained peat in the nature preserve Nieuwkoopse Plassen. Peat soils were chosen because of their expected considerable contribution to the greenhouse gas budget considering the prevailing wet and partial anaerobic conditions. The emission dynamics of these ecosystems are considered representatives of large peat areas because the underlying processes are rather general and driven by variables like organic matter characteristics, water and nutrient conditions and type of vegetation. The research approach comprises measerements and modelling at different integration levels relating to the microbiology of the production and consumption of N2O and CH4 (laboratory studies), their movement through peat soil (rhizolab and field studies), and the resulting fluxes (field studies). Typical emissions from drained soil were 15-40 kg ha-1 y-1 N2O and virtually zero for CH4. The undrained soil in the nature preserve emitted 100-280 kg ha-1 y-1 CH4, and probably little N2O. The process knowledge collected and partly integrated in the models helps to explain these data. For example, the low methane emissions from drained peat can more coherently be understood and extrapolated because (i) upper soil layers are aerobic, thus limiting methane production and stimulating methane oxidation, (ii) absence of aerenchymatous roots of wetland plants that connect deeper anaerobic soil layers where methane is produced to the atmosphere and supply labile carbon, (iii) a low methane production potential in deep layers due to the low decomposability of organic matter, and (iv) long anaerobic periods needed in the topsoil to develop a methane production potential.

This thesis describes model development and experimentation on the comprehension and prediction of methane (CH 4 ) emissions from rice paddies. The large spatial and temporal variability in CH 4 emissions and the dynamic non-linear relationships between processes underlying CH 4 emissions impairs the applicability of empirical relations. Mechanistic concepts are therefore starting point of analysis throughout the thesis.The process of CH 4 production was investigated by soil slurry incubation experiments at different temperatures and with additions of different electron donors and acceptors. Temperature influenced conversion rates and the competitiveness of microorganisms. The experiments were used to calibrate and validate a mechanistic model on CH 4 production that describes competition for acetate and H 2 /CO 2 , inhibition effects and chemolithotrophic reactions. The redox sequence leading eventually to CH 4 production was well predicted by the model, calibrating only the maximum conversion rates.Gas transport through paddy soil and rice plants was quantified by experiments in which the transport of SF 6 was monitored continuously by photoacoustics. A mechanistic model on gas transport in a flooded rice system based on diffusion equations was validated by these experiments and could explain why most gases are released via plant mediated transport. Variability in root distribution led to highly variable gas transport.Experiments showed that CH 4 oxidation in the rice rhizosphere was oxygen (O 2 ) limited. Rice rhizospheric O 2 consumption was dominated by chemical iron oxidation, and heterotrophic and methanotrophic respiration. The most abundant methanotrophs and heterotrophs were isolated and kinetically characterised. Based upon these experiments it was hypothesised that CH 4 oxidation mainly occurred at microaerophilic, low acetate conditions not very close to the root surface. A mechanistic rhizosphere model that combined production and consumption of O 2

Eddy correlation measurements of CO2 exchange were made in intensively managed peat pastures at 2 different groundwater tables during most of a growing season. F was separated into a respiratory and an assimilatory CO2 flux. The fit of the Arrhenius temperature response to Fr showed that Fr was generally higher at low groundwater tables. The fit of a hyperbolic irradiance response to Fa showed that Fa was also generally higher at low groundwater tables, more than compensating for higher Fr. The daily CO2 balance suggested that the low groundwater pasture was a larger CO2 sink or a smaller CO2 source than the high groundwater pasture during the measurement period. The difference in Fr between the groundwater tables was substantially less than the factor 2 difference in level of soil subsidence. This suggests that oxidization of soil organic matter was a relatively minor factor in soil subsidence.

Introduction and objectivesGood comprehension of the energy and mass cycles and their effect on climate dynamics is crucial to understanding, predicting and anticipating ecological changes due to possible future climate perturbations. Here direct and long-term flux density measurements of greenhouse gases from various ecosystems provide means to supply such fundamental knowledge. For the global water vapour and carbon cycles, however, the interactions between different spatial scales become important, where extrapolating from canopy flux density measurements to global budgets lead to practical and theoretical problems. This thesis focuses on the direct and long-term measurement of surface flux densities and interaction processes at the canopy (< 1 km scale within the framework of the Surface Layer Integration Measurements and Modelling (SLIMM) project. Furthermore, some characteristics and limitations of the scintillation technique are studied in two field experiments in New Zealand.As indicated in Chapter 1, the first objective of this project was the direct and continuous long-term measurement of the surface flux densities of radiation, momentum, heat, water vapour and carbon dioxide (CO 2 ) to study the effect of biological and climatic processes that regulate carbon dioxide exchange of this ecosystem at the canopy scale. At the same time these data were used to study the effect of plant related and environmental conditions on the interaction of carbon dioxide and water vapour exchange, to satisfy the second objective of the thesis. The third objective focussed on the prospect of obtaining both the spatial averaged sensible heat flux density and momentum flux density from scintillation measurements.Generally, a compromising point measurement of the mean horizontal wind speed or friction velocity is used to calculate the sensible heat flux density from the temperature structure parameter. By using two scintillometers at two heights, point measurements to obtain th

The study described in this thesis focuses on the understanding of growth of leaf area and biomass in wheat and sunflower under low phosphorus conditions.Chapters 2 and 3 address the question whether P-deficiency limits leaf area expansion directly by inhibiting the individual leaf expansion, or through its effects on the availability of assimilates for growth. Experimental and simulation techniques were used to find an answer to this question. It was concluded that in sunflower the effects of P on sink size and assimilate supply were important for determining plant leaf area expansion (Chapter 2). Instead, wheat responded to a low P supply mainly by reducing sink size (Chapter 3). A sensitivity analysis of the effects of different model parameters at different levels of P supply, on the simulated total leaf area, showed that the value of the phyllochron, in wheat and sunflower, and tillering in wheat were crucial in determining the total leaf area.Chapter 4 focuses on the effects of P on the generation of leaves and tillers in wheat. Here, it was shown that the emergence of main stem leaves in wheat plants grown under P-deficient conditions was delayed (higher phyllochron) as both, the rate of leaf primordia initiation in the apex, and the rate of individual leaf expansion were reduced.In Chapter 5, the effects of P and assimilate supply on tillering were studied in a field experiment. P deficiencies directly altered the normal pattern of tiller emergence by slowing down the emergence of leaves on the main stem, and by reducing the maximum rate of tiller emergence. In general terms, assimilate supply seemed to play a minor role determining tillering. Including the effects of P on the phyllochron and on the maximum tillering rate in a morphogenetic model of the generation of leaves and tillers, it was possible to explain the dynamics of tiller emergence of a field experiment.In Chapter 6, a comparative study of the response to P in wheat and sunflower is presented.

For a long time, the emphasis in silviculture in Western Europe was solely on even-aged, monospecific stands; many empirical stand-level growth models were developed and successfully used for managing such stands. In contrast, no generally accepted growth and yield approach has emerged so far for mixed forests. Moreover, the inexhaustible number of species combinations, management regimes, and site-dependent interactions make an empirical approach less suitable.In the present study, a mechanistic model was developed that simulates growth and yield in mixed forest stands. Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) and beech ( Fagus sylvatica L.) were used in this research. In the model, tree growth is dependent on radiation availability. Stand development is largely driven by competition for radiation. A spatial module was developed to investigate the effects of tree and stand characteristics on radiation interception. The study showed that in heterogeneous stands a spatial approach is needed to account for competition between trees.Growth of the trees was estimated using the radiation-use efficiency concept (RUE). Results revealed that detailed process models can be used to estimate RUE and that it is a suitable tool for (mixed) forest modelling.To describe the distribution of the dry matter growth, a separate module was developed using functional relationships between tree components: the dry matter distribution is driven by the aim to maintain structural balances within the tree. The study showed that this approach is able to reproduce the development of an individual forest tree. The approach was thus considered very suitable for modelling the effects of between-tree competition for resources on growth and development of mixed forest stands.The overall growth model, COMMIX, was applied to investigate the effects of stand composition on mixed stand productivity, using a replacement series. Analysis showed that the productivity of mixed forest stands is ge

This thesis represents detailed research on the "rice-soils-weather" system of the Ebro Delta (Spain) providing knowledge on how temperature, radiation, soil properties and farm management determine rice growth. After an introductory chapter, the findings are developed step-by-step. (i) Chapter 2 is an overview of the conditions in the study area. (ii) Chapter 3 focuses on the study of the soils creating a soil geographic database of the northern part of the Ebro Delta. Four properties were used for defining soil units; soil development, drainage status, texture and salinity. (iii)Chapters 4 and 5 deal with rice growth under weather limited conditions. Potential productions of 13000 kg ha -1were estimated, while maximum yields of 11000 kg ha -1were recorded at field level. In Chapter 4, the phenological development, the daily dry matter production and the leaf area development of the rice crop were modelled. In Chapter 5, the use of remote sensing techniques at field level for monitoring the rice crop status were tested. (iv)Chapters 6 and 7 analyze rice growth in relation to soil properties and farm management. Two soil factors were found to dominate the effects on yield: one was topsoil CEC (in strong association with clay content) with a positive effect, and the other one was soil salinity with a negative effect. High groundwater tables did not have significant effects on rice yield, except when the water had a high salt content. Four main groups of causes within the cropping status limited rice growth: potassium and zinc shortage where a strong antagonism of either factor with sodium was observed, low plant establishment where a minimum number of 160-180 plants m -2was necessary to maximize yield, and length of the growing season, especially the length of the pre-heading period in which the potential size of the crop was primarily determined.Potassium and zinc shortages in the plant were mainly induced by soil salinity. High K in saline soils did not increase K

The study described in this thesis focuses on a quantification of leaf wetness formation in paddy rice crops based on insight in the physical processes of the formation of leaf wetness. For this purpose, experimental research was conducted in a tropical paddy rice field.A shielding (nocturnal net radiative loss) experiment was designed to investigate the dependence of dew formation on nocturnal net radiative loss. A simple method was developed to estimate dew amount and duration using the nightly total net radiative loss. The shielding experiment unexpectedly provided a feasible method for experimentally distinguishing dew from guttation. For a better understanding of the characteristics of the formation of leaf wetness in paddy rice crops, warm water flooding both in the paddy rice field and in a phytotron was conducted to investigate the effects of water temperature on dew formation and guttation. The experimental data indicated an optimum paddy water temperature of 30°C for rice leaf guttation. The effect of water temperature on dew formation depended on the weather condition above the canopy, especially the thermal stratification condition, net radiation and vapour pressure deficit. A simple device, the dewball, was designed to observe dew formation and to determine its threshold value of nocturnal net radiative loss from top leaves. This was shown to be a simple and accurate method for estimating daily dew duration on top leaves. The possibility of using the mean diurnal patterns of weather variables to estimate dew formation was analysed by comparing the dew estimated using a simulation model to the measured results. This helped to understand to what extent the diurnal patterns will give as good an estimate of dew formation as when the observed hourly weather data are used. Based on the observation results on the dewball and the results of the shielding experiment, a simple approach to estimate daily dew duration on top leaves of the paddy rice crops was formu

Research topicsThe relationships between climate and both phenology and growth of some important European tree species were studied to evaluate the potential impacts of climate change on trees and forests in Europe. In order to make such assessments, insight is required on the mechanisms how climatic variables interact with plant processes. The topics addressed in this study were: (1) the modelling of phenology, (2) the consequences of climate change on spring frost damage, (3) the importance of phenotypic plasticity, (4) the importance of phenology on the effects of climate change on growth of monospecies deciduous forests, and (5) the importance of phenology on the effects of climate change on growth of mixed-species deciduous forests.Modelling phenologyTo evaluate the impacts of climate change on growth of temperate deciduous tree species, the onset and cessation of the growth must be accurately described. A review is presented on eight models predicting the date of leaf unfolding depending on temperature. These models were fitted using 57 years of observations on the date of leaf unfolding of Fagus sylvatica in The Netherlands, and used to predict 40 years of similar observations collected in Germany. As conflicting experimental evidence exist on the role of photoperiod on leaf unfolding of Fagus sylvatica, photoperiod was incorporated into each of these models.The timing of leaf unfolding could best be described by a model in which the effects of chilling temperatures (-5 to +10°C) and forcing temperatures (>0°C) operate sequentially in time, according to a triangular and logistic function, respectively. Including photoperiod reduced the predicting power of this model.Spring frost damageTwo studies presented in literature evaluate the effect of increasing winter temperature on the probability of spring frost damage to trees. However, one study predicted an increase, while the other predicted a decrease in the probability of spring frost damage. It is unclear wh

Accurate prediction of crop phenology is important not only for modelling purposes but also for crop improvement and management actions. The objective of this study was to develop a model for predicting phenological development to flowering in rice ( Oryza sativa L.).Data from the literature were analysed to evaluate a basic equation for crop development. The Beta function, commonly used as a skewed probability density function in statistics, was found to accurately describe responses of rice development rate to both temperature and photoperiod. Controlled-environment experiments were then conducted for detailed understanding of the physiological basis of photothermal responses of preflowering development in rice. Effects of day and night temperature on development to flowering were found to be different. The plants did not respond to photoperiod throughout the entire preflowering period, instead, the photoperiod- sensitive phase was sandwiched by two photoperiod-insensitive phases. It appeared that responses to both day and night temperature were stronger during the photoperiod-sensitive phase than during the two photoperiod-insensitive phases. The results also indicated that leaf appearance occurring simultaneously during the preflowering development had a different thermal response from that of the development per se .Based on the experimental results, a detailed model for photothermal responses of flowering in rice was developed, using the Beta function. The model, referred to as the three-stage Beta (3s-Beta) model, describes different photothermal responses during the photoperiod-sensitive phase and the photoperiod-insensitive phases of preflowering ontogeny. Using parameter values derived from controlled-environment experiments, the 3s-Beta model adequately predicted rice flowering dates observed in field conditions. It preformed better than several existing models over a wide range of environments.In order to assist new plant type design, the 3s-Beta model w