Rooted Soil Layer Research Articles

Применение комбинированного орошения при возделывании сои на мелкоконтурных участках в условиях аридной зоны Поволжья

The purpose of the work was to improve the soybean irrigation system during cultivation in small-contour areas, based on the dynamics of moisture reserves in the root soil layer in the conditions of an arid growing season in the arid zone of the Volga region using combined (drip + sprinkler) irrigation, including differentiation of the timing and norms of irrigation by phases of culture development. The article describes the use of drip irrigation in a dry year, when cultivating soybeans in areas of incorrect configuration, having small contours, on which it is impossible to install wide-reach sprinklers. For cultivation in arid conditions of the Volga region, soybeans of the moisture-loving variety "Pokrovskaya" were selected. A description of the use of combined (drip + sprinkler) irrigation of soybeans is given. The dynamics of the moisture supply of crops during drip irrigation and irrigation with sprinklers was analyzed according to the phases of soybean vegetation in the conditions of the arid year of the arid zone, with GTC < 0.4. The use of drip irrigation technology in these conditions will make it possible to additionally introduce land plots into agricultural circulation, which will ensure an increase in the coefficient of land use of agricultural land and an additional yield of soybeans up to 3.4 tons/ha.

Alder-induced stimulation of soil gross nitrogen turnover in a permafrost-affected peatland of Northeast China

For the prediction of permafrost nitrogen (N) climate feedbacks, a better process-based understanding of the N cycle in permafrost ecosystems is urgently needed. Therefore, we characterized and quantified soil organic matter, gross soil microbial ammonification and nitrification and soil-atmosphere exchange of nitrous oxide (N2O) of boreal permafrost ecosystems on the southern edge of the Eurasian permafrost area in situ.Soil organic carbon (SOC) and total nitrogen (TN) stocks (top 0.5 m) of tree-free lowland peatland (LP) soils exceeded those of gravel-rich upland forest (UF) soils by an order of magnitude. Nuclear magnetic resonance spectroscopy revealed more recalcitrant organic matter at greater depth and more bioavailable organic matter substrates in upper peat horizons. In line with this result, gross ammonification and nitrification generally decreased with increasing sampling depth. Gross rates of mineral N turnover in active layers were comparable to those of temperate ecosystems. Despite substantial gross ammonification, the low nitrification:ammonification ratios and negligible soil N2O emissions depicted however a closed N cycle at UF and LP characterized by N limitation.In strong contrast, the lowland peat soils underneath alder trees (LA), being associated with diazotrophic bacteria in root nodules, showed an accelerated N turnover with very high gross rates of ammonification (3.1 g N m−2 d−1) and nitrification (0.5 g N m−2 d−1), exceeding those of UF and LP soils by an order of magnitude. This was accompanied by substantial N2O emissions comparable to temperate agricultural systems or tropical forests. The increase in gross soil microbial ammonification and nitrification was most pronounced in the rooted soil layer, where N inputs from biological N fixation almost doubled TN concentrations and halved SOC:TN ratios. The frozen ground of LA contained strongly increased ammonium concentrations that might be prone to release upon thaw via subsequent nitrification. This study shows that alder forests that further expand on permafrost-affected peatlands with global change create hot spots of soil mineral N turnover, thereby potentially enhancing permafrost N climate feedbacks via N2O emissions.

Effects of plastic film mulch biodegradability on nitrogen in the plant-soil system

The application of biodegradable film mulching (BFM) instead of non-biodegradable film mulching (NBFM) is a promising way to mitigate the negative impacts of residual film in agricultural mulching systems. But the effects of BFM on soil mineral nitrogen (N) are not known. To investigate the effects of BFM on N mineralization, nitrate (NO3−) accumulation and leaching, and plant N uptake, we conducted two-year field experiment with five treatments: no-mulching (No-M), white non-biodegradable film mulching (White-NotBioM), black non-biodegradable film mulching (Black-NotBioM), white biodegradable film mulching (White-BioM), and black biodegradable film mulching (Black-BioM). The net N mineralization in NBFM was greater than that in BFM due to the disintegration of biodegradable films in the middle and late stages of maize growth, resulting in a decrease in soil water content under BFM. Higher net N mineralization caused a higher NO3− accumulation in the topsoil (0–20 cm) under NBFM. The NO3− accumulation in the topsoil in Black-NotBioM was 23–88% higher than that in Black-BioM; while in White-NotBioM it was 16–63% higher than that in White-BioM. After two years of cropping, the NO3− accumulation in 100–180 cm (defined as N leaching in deep layers, NLD) in NBFM was 52–63% higher than that in BFM, implying that the higher NO3− accumulation in the topsoil in NBFM caused more N leaching. The yields and plant N uptake were similar between NBFM and BFM, but BFM had higher N harvest index values. Compared with NBFM, BFM showed less NO3− accumulation in the topsoil and less NLD, whereas yield, plant N uptake and net economic benefits were not reduced. Therefore, BFM, especially Black-BioM, could be an alternative to NBFM in maize production on the Loess Plateau. However, the higher N accumulation in root soil layer (0–100 cm) under Black-BioM should be accounted for in N fertilizer management.

Modelling of soil moisture change during the operation of the “Frigate” sprinkler

The irrigation process is one of the most important elements of technologies for growing crops on irrigated soils. The tasks of ensuring optimal soil water regime, eliminating gravita-tional water losses outside the zone of active moisture exchange and promoting the fullest use of plants of bioclimatic potential of the region require modeling of soil moisture dynamics in the process of wetting, as well as structures and technologies related to irrigation facilities. Soil moistening in the south of Ukraine is carried out mainly by sprinkling with the use of a Frigate sprinkler, which is largely close to the natural processes of moisture in soil structures, especially with the use of the latest Xi-Wob sprinklers, which provide uniform irrigation and uniform droplet size. The aim of the article is to develop a model of soil moisture change, which will determine the moisture content in both the root and near-surface layers of the soil and prognoses the movement parameters of the "Frigate" sprinkler elements. The designs of sprinklers currently available allow to provide uniform intensity of spraying of drops on all working area of "Frigate" sprinkler, therefore it is possible to choose a separate point for modeling. The data obtained for one point can be extended to the entire work area. The Darcy equation is used to mathematically describe the movement of water in the soil. The finite difference method was used to solve the Darcy equation based on the initial and boundary conditions. Assuming that within each soil element the humidity changes linearly, and replacing the partial derivatives of moisture in depth and time with the analogues, the discretized model of soil moisture dynamics in the final differences was obtained, which allows you to predict moisture at any time due to hydro-soil characteristics in previous time and boundary conditions. Discretization of the Darcy equation made it possible to obtain a model of the dynamics of the humidity of the root layer in the space of states. It turned out to be nonlinear due to the hydrophysical characteristics of the soil and was linearized by transformation into the Taylor series. The developed mathematical model of soil moisture change in the space of states allows to determine the moisture content in both the root and near-surface soil layer depending on its hydrological characteristics, type of cultivated plants, volume of water entering the fields from irrigation systems or in the form of random precipitation used to predict the movement parameters of the "Frigate" sprinkler elements.

Tomato fertigation in an open ground

Obtaining high and stable yields of tomato with good consumer qualities in the open field has now become problematic due to the fact that hybrids and varieties of tomato of an intensive type place high demands on the presence of moisture and nutrients in the root-soil layer. The required level of soil moisture in combination with the introduction of calculated doses of mineral fertilizers during the growing season provides for obtaining the planned yields of vegetables, including tomato. The most important resource for the further development of agriculture in the Russian Federation is irrigated land. Volgograd region is one of the largest irrigated agricultural regions in Russia. Growing of almost all vegetables in the region is conducted on irrigated lands. The use of only one irrigation without the use of fertilizers will not have a positive impact on the growth, development and productivity of plants. Fertigation is the most effective and economical way of delivering nutrients to the plant root system and allows to get a higher yields than other methods of fertilizer application. In fertigation systems, the control of optimal concentrations of fertilizers is easily achieved, their ratio and these parameters can be controlled automatically. The purpose of this work was to study the comparative efficiency of ammonium nitrate, calcium nitrate and a combination of calcium nitrate and ammonium chloride for tomato fertigation in combination with non-root dressings of complex water-soluble fertilizers containing macroand microelements in open feild on irrigated lighchestnut soil of dry steppe zone of chestnut soils in the Volgograd region. As a result, analysis of the use of mineral fertilizers for tomato fertigation in light chestnut soils of the Volgograd region was carried out. The positive effect of foliar dressing with microelements in the proposed dosages has been proven.

Fine root dynamics responses to nitrogen addition depend on root order, soil layer, and experimental duration in a subtropical forest

Elevated atmospheric N deposition has been well documented to enhance fine root production in N-limited temperate forests, but how fine roots respond to N deposition in N-rich tropical and subtropical forests remains poorly understood. The sequential coring and minirhizotron methods were applied to quantify fine root biomass, production, and turnover of a N-rich but P-limited subtropical forest in southern China and to assess the responses of these root variables to a gradient of N additions (control (0), low-N (35), medium-N (70), and high-N (105 kg N ha−1 year−1)) during the first 3 years of experimentation. The high- and medium-N additions significantly reduced fine root diameter by about 30% but increased the specific root length by 20–105%, i.e., fine roots became thinner and longer under the experimental N addition. Both low- and medium-N additions generally stimulated fine root production (10–88%) and turnover (3–40%), whereas high-N suppressed them by 32–70% and 8–54%, respectively, varying with sampling season and estimation method. The stimulatory effects were presumably ascribed to the increased fine root growth for P acquisition, the suppressive effect, to the deleterious damage to the root health and micronutrient availability. Overall, the N effects were more pronounced in the surface (0–10 cm) than in the deeper (10–40 cm) soil layers and for the first-order than the higher-order fine roots. Our results indicate that lower-order absorptive fine roots are responsive to elevated N deposition, and complex responses could emerge due to the interactive influences of the N deposition rate, seasonality, and soil depth.

Mechanized application of ameliorants for preservation of soil moisture on cultivated lands

During the drought season agronomic crops get into a critical situation when a lack of moisture and nutritional chemicals can result in plant wilting and crop loss. To prevent the following situation, it is proposed to apply liquid ameliorants in the process of subsurface tillage which can accumulate moisture in a soil. The introduction of ameliorants offered to carry out work on liquid ameliorants directly into the root soil layer, to create favorable conditions for the development of the plant. Also, when making soil ameliorants preventing the leaching of their streams generated from rainfall and meltwater. Experimental studies have shown that zero tillage with simultaneous application of liquid ameliorants (in the process of experiment was used urea-ammonium nitrate) can increase soil moisture at cultivation depth from 22 to 47%. Moisture of the soil was determined in two months after cultivation and applying of liquid ameliorants according to the National Standard 28268-89 “Soils. Methods of moisture determination, maximum hydroscopic moisture and permanent wilting coefficient of plants”. In this case, received values of filtration filters and soil porosity show qualitative soil cultivation for crop growing.

Mechanized application of ameliorants for preservation of soil moisture on cultivated lands

During the drought season agronomic crops get into a critical situation when a lack of moisture and nutritional chemicals may lead to a wilt of plants and crop loss. For preventing of the following situation it is proposed to apply liquid ameliorants in the process of subsurface tillage which can accumulate moisture in a soil. The introduction of ameliorants offered to carry out work on liquid ameliorants directly into the root soil layer, to create favorable conditions for the development of the plant. Also, when making soil ameliorants preventing the leaching of their streams generated from rainfall and meltwater. Experimental studies have shown that zero tillage with simultaneous application of liquid ameliorants (in the process of experiment was used urea-ammonium nitrate) can increase soil moisture at cultivation depth from 22 to 47%. Moisture of soil determined after two months after cultivation and applying of liquid ameliorants according to the National Standard 28268-89 “Soils. Methods for determination of moisture, maximum hydroscopic moisture and moisture of steady plant fading”. In this case, received values of filtration filters and soil porosity show qualitative soil cultivation for crop growing.

Impact of Irrigation Methods on Soil Salt Content and Their Differences in Whole Cotton Growing Season in Arid Area of Northwest China

Abstract: Research for changes of soil water and salt is an important content of land sciences and agriculture sciences in arid and semi arid regions. In this paper, sampling in actual agricultural fields, laboratory analysis of soil samples and statistical analysis methods are used to quantitatively analyze soil salinity changes under different irrigation methods throughout the cotton growing season in Shihezi reclamation area. The results show that irrigation methods play an important role in soil salt content in the surface soil (0–20 cm) and sub-deep soil (40–60 cm), followed by deep soil layer (60–100 cm) and root soil layer (20–40 cm). Furrow irrigation yields the maximum soil salt content in deep layer (60–100 cm) or sub-deep layer (40–60 cm) and the maximum salinity occurs in the first half of the cotton growing season (June or earlier). In contrast, drip irrigation yields the maximum soil salinity in the root layer (20–40 cm) or sub-deep (40–60 cm), and this usually appears in the second half gro...

Characteristics of soil nematode communities under conditions of woody plant introduction

Nematode communities in the rooted soil layer under the crowns of introduced tree species on Valaam Island (Republic of Karelia) have been studied by conventional nematological methods to evaluate their taxonomic diversity, population density, and eco-trophic structure. On the whole, 49 nematode genera have been recorded, with their total abundance varying from 1209 to 2548 ind./100 g soil. The abundance and proportion of plant parasites in the soil nematode assemblage have proved to be higher under introduced trees, compared to natural forest biocenoses.

Estimation of total available water in the soil layer by integrating actual evapotranspiration data in a remote sensing-driven soil water balance

Summary The total available water ( τ ) by plants that could be stored in its root soil layer is a key parameter when applying soil water balance models. Since the transpiration rate of a vegetation stand could be the best proxy about the soil water content into the root soil layer, we propose a methodology for estimating τ by using as basic inputs the evapotranspiration rate of the stand and time series of multispectral imagery. This methodology is based on the inverted formulation of the soil water balance model. The inversion of the model was addressed by using an iterative approach, which optimizes the τ parameter to minimize the difference between measured and modeled ET. This methodology was tested for a Mediterranean holm oak savanna (dehesa) for which eddy covariance measurements of actual ET were available. The optimization procedure was performed by using a continuous dataset (in 2004) of daily ET measurements and 16 sets of 8 daily ET measurements, resulting in τ values of 325 and 305 mm, respectively. The use of these τ values in the RSWB model for the validation period (2005–2008) allowed us to estimate dehesa ET with a RMSE = 0.48 mm/day. The model satisfactorily reproduces the water stress process. The sensitivity of τ estimates was evaluated regarding two of the more uncertain parameters in the RSWB model. These parameters are the average fraction of τ that can be depleted from the root zone without producing moisture stress ( p τ ) and the soil evaporation component. The results of this analysis indicated relatively little influence from the evaporation component and the need for adequate knowledge about p τ for estimating τ .

Remote Sensing for Crop Water Management

Advancements on Earth Observation science and technology in the last decades have made possible the operative use of dense time series of multispectral imagery at high spatial resolution [5-30 m] to monitor crop development across its growing season at a suitable scale. These time series of images, jointly with meteorological data are able to provide accurate maps of daily evapotranspiration and so crop water requirements by using the remote sensing-based approach crop coefficient, Kc, and reference evapotranspiration, ETo, where Kc is derived from spectral reflectances and ETo from meteorological data. A water balance in the root soil layer enables us to calculate irrigation water requirements at appropriate scale for monitoring water management near- real time. This approach could be coupled to the remote sensing-based surface energy balance which uses surface temperature as primary input. But what we could call «remote sensing-driven crop water management» requires at least two steps more to be placed into the day-to-day routine on farming irrigation: On the one hand, for planning irrigation the users require the forecasting of crop water requirements for the week ahead; it can be achieved by extrapolating crop coefficient trend and by using weather forecasting for ETo estimation. On the other hand, decision makers in charge of irrigation require access to this information in an easy-to-use way on real time. It can be achieved through leading edge webGIS tools, which facilitates co-creation and collaboration with stakeholders.

Remote Sensing for Crop Water Management

Advancements on Earth Observation science and technology in the last decades have made possible the operative use of dense time series of multispectral imagery at high spatial resolution [5-30 m] to monitor crop development across its growing season at a suitable scale. These time series of images, jointly with meteorological data are able to provide accurate maps of daily evapotranspiration and so crop water requirements by using the remote sensing-based approach crop coefficient, Kc, and reference evapotranspiration, ETo, where Kc is derived from spectral reflectances and ETo from meteorological data. A water balance in the root soil layer enables us to calculate irrigation water requirements at appropriate scale for monitoring water management near- real time. This approach could be coupled to the remote sensing-based surface energy balance which uses surface temperature as primary input. But what we could call «remote sensing-driven crop water management» requires at least two steps more to be placed into the day-to-day routine on farming irrigation: On the one hand, for planning irrigation the users require the forecasting of crop water requirements for the week ahead; it can be achieved by extrapolating crop coefficient trend and by using weather forecasting for ETo estimation. On the other hand, decision makers in charge of irrigation require access to this information in an easy-to-use way on real time. It can be achieved through leading edge webGIS tools, which facilitates co-creation and collaboration with stakeholders.

SWAMP: A soil layer water supply model for simulating macroscopic crop water uptake under osmotic stress

Models like SWAP, HYDRUS and SALTMED compute crop water uptake under osmotic stress with a dimensionless piecewise linear or S-shaped reduction function. Parameters for these functions, to reduce water uptake, corresponds normally to the Maas and Hoffman salinity threshold and slope values. Unfortunately, extensive crop- and site-specific calibration of the parameters is required. This is because these values, amongst other reasons, serve only as guidelines and express salt tolerance at a time and root-zone average soil salinity and not local total potential heads. In this paper an alternative model (Soil WAter Management Program, SWAMP), that does not rely on these parameters and functions were presented and evaluated. The algorithm used by SWAMP to simulate the water supply of a rooted soil layer and hence water uptake, under decreasing matric potentials was adapted to include the effect of decreasing osmotic potentials. Data from a lysimeter trial was used to evaluate SWAMP. The model was calibrated to represent the soil conditions of the trial, i.e. peas and maize were irrigated with EC's between 20 and 600mSm−1 and grown in sand to sandy loam soils with water tables of the same quality. Under these osmotic stress conditions, SWAMP was able to simulate weekly water uptake of both crops grown on both soils well, i.e. the aggregated accuracy, correlation and pattern performance (ISWAMP) were above 75%. No macro-pattern was observed. Thus, the water uptake residuals contain no structure that is not accounted for in the algorithm and parameters. No extensive calibration was necessary because the parameters for the algorithm were calculated from easily measured inputs. From the three most sensitive parameters, only the critical leaf water potential of a crop might be difficult to obtain. SWAMP contains default values for a number of crops. A model is, therefore, presented that simulate the change in osmotic stress with changing soil water content and that does not rely on the salinity threshold and slope parameters.

Conceptual approach to lateral salinity gradients around roots of salt-sensitive and salt-tolerant crops under irrigation conditions

Under controlled crop irrigation with brackish water, a vertical soil salinity gradient develops in the rooted soil layer. Furthermore, in periods of soil water depletion, due to ion exclusion by roots, a transpiration-driven, lateral build-up of a salinity gradient between the soil fraction adjacent to roots and distant, occurs. Root morphological traits determine the volume of the soil fraction, in which salt accumulation occurs (rhizospheric soil volume). The increase of soil salinity near roots is greater, when plant roots form a small rhizospheric soil volume (e.g. leek). When roots form a larger rhizospheric soil volume, salts are distributed throughout a larger soil volume, which improves conditions for root water uptake and crop salt tolerance (e.g. rape).

Conceptual approach to lateral salinity gradients around roots of salt-sensitive and salt-tolerant crops under irrigation conditions

Under controlled crop irrigation with brackish water, a vertical soil salinity gradient develops in the rooted soil layer. Furthermore, in periods of soil water depletion, due to ion exclusion by roots, a transpiration-driven, lateral build-up of a salinity gradient between the soil fraction adjacent to roots and distant, occurs. Root morphological traits determine the volume of the soil fraction, in which salt accumulation occurs (rhizospheric soil volume). The increase of soil salinity near roots is greater, when plant roots form a small rhizospheric soil volume (e.g. leek). When roots form a larger rhizospheric soil volume, salts are distributed throughout a larger soil volume, which improves conditions for root water uptake and crop salt tolerance (e.g. rape).

On‐Farm Evaluation of the Improved Soil Nmin–based Nitrogen Management for Summer Maize in North China Plain

The improved soil Nmin–based N management is a promising approach to precision N management, which determines the optimum side‐dress N rates based on N target values and measured soil nitrate N content in the root soil layer at different growth stages. A total of 148 on‐farm N‐response experiments, in seven key summer maize (Zea mays L.) production regions of North China Plain (NCP) from 2003 to 2005, were conducted to evaluate the Nmin–based N management compared to traditional farmer's N practices. The recommended N rates based on the improved soil Nmin method were not significantly different ( ≤31 kg N ha−1) from those determined by yield response curves (n = 13). The average N rate determined with the soil Nmin method (157 kg N ha−1) was significantly lower than farmer's practice (263 kg N ha−1), while maize grain yield was 0.4 Mg ha−1 higher than farmer's N practice (8.5 Mg ha−1) across all sites (n = 148). As a result, the improved soil Nmin–based N management significantly increased net economic gains by $202 ha−1, reduced residual nitrate N content and N losses by 44 kg N ha−1 and 65 kg N ha−1, respectively, and improved recovery N efficiency, agronomic N efficiency and N partial factor productivity by 16%, 6 kg kg−1 and 36 kg kg−1, respectively, compared with farmer's N practice. We conclude that the improved soil Nmin–based N management can be applied for summer maize production in NCP for improved N use efficiency and reduced environmental contamination.

Root water uptake and profile soil water as affected by vertical root distribution

Water uptake by plant roots is a main process controlling water balance in field profiles and vital for agro-ecosystem management. Based on the sap flow measurements for maize plants (Zea mays L.) in a field under natural wet- and dry-soil conditions, we studied the effect of vertical root distribution on root water uptake and the resulted changes of profile soil water. The observations indicate that depth of the most densely rooted soil layer was more important than the maximum rooting depth for increasing the ability of plants to cope with the shortage of water. Occurrence of the most densely rooted layer at or below 30-cm soil depth was very conducive to maintaining plant water supply under the dry-soil conditions. In the soil layers colonized most densely by roots, daytime effective soil water saturation (S (e)) always dropped dramatically due to the high-efficient local water depletion. Restriction of the rooting depth markedly increased the difference of S (e) between the individual soil layers particularly under the dry-soil conditions due likely to the physical non-equilibrium of water flow between the layers. This study highlights the importance of root distribution and pattern in regulating soil water use and thereby improving endurance of plants to seasonal droughts for sustainable agricultural productivity.

Modeling methane fluxes in wetlands with gas‐transporting plants: 3. Plot scale

A process model based on kinetic principles was developed for methane fluxes from wetlands with gas‐transporting plants and a fluctuating water table. Water dynamics are modeled with the 1‐D Richards equation. For temperature a standard diffusion equation is used. The depth‐dependent dynamics of methane, oxygen, molecular nitrogen, carbon dioxide, soil carbon, electron acceptors in oxidized and in reduced form are affected by transport processes and kinetic processes. Modeled transport processes are convection and diffusion in the soil matrix, ebullition, and plant‐mediated gas transport. Modeled kinetic processes are carbon mineralization, aerobic respiration, methane production, methane oxidation, electron acceptor reduction, and electron acceptor reoxidation. Concentration gradients around gas‐transporting roots in water‐saturated soil are accounted for by the models from the two previous papers, ensuring an explicit connection between process knowledge at the kinetic level (millimeter scale) and methane fluxes at the plot scale. We applied the model to a fen, and without any fitting, simulated methane fluxes are within 1 order of magnitude of measured methane fluxes. The seasonal variations however, are much weaker in the simulations compared to the measurements. Simulated methane fluxes are sensitive to several uncertain parameters such as the distribution over depth of carbon mineralization, the total pool size of reduced and oxidized electron acceptors, and the root‐shoot ratio. Because of the process‐based character of the model it is probable that these sensitivities are present in reality as well, which explains why the measured variability is usually very high. Interestingly, heterogeneities within a rooted soil layer seem to be less important than heterogeneities between different soil layers. This is due to the strong influence of the interaction between water table and profile scale processes on the oxygen input to the system and hence on net methane production. Other existing process models are discussed and compared with the presented model.

Rice roots and methanogenesis in a paddy soil: ferric iron as an alternative electron acceptor in the rooted soil

Irrigated rice fields are among the major sources of the greenhouse gas CH 4. To get a better understanding of the controls of CH 4 production we used microcosms planted with rice and studied emission, porewater chemistry and the potential role of ferric iron as a competitive electron acceptor. CH 4 emission from rice microcosms peaked after 60 d when porewater CH 4 concentrations in the unrooted soil also reached their highest values. A rooted soil layer with low CH 4 concentrations and high E h developed on top of the lower unrooted soil. When measured in vitro, methanogenesis in the rooted upper soil layer started at a very low rate, but increased dramatically after 25–40 d when ferric iron reduction had stopped. If mixtures of the upper soil layer and of lower unrooted soil layer were incubated, the length of the lag phase depended linearly on the proportion of soil from the upper layer. In the lower soil layer, methanogenesis started immediately at high rates. The gross mineralization rate in both soil layers was identical. There was no difference or change in the numbers of culturable methanogens between the two layers, either at the beginning or at the end of the experiment. Hence, the lag phase in methane production by the upper soil layer may have been caused either by competition for substrates or by direct inhibition, but not by population growth. O 2 release from rice roots controls methanogenesis in the rooted upper soil layer either directly or by the oxidation of ferrous iron. Our data suggest that the presence of ferric iron, resulting from the input of oxygen via the roots, results in a shift of electron flow from methanogenesis to ferric iron reduction. This interaction can be assumed to be of major importance for the biogeochemistry of CH 4 in wetland ricefields.