Range Of Soil Water Contents Research Articles

A simple calibration of Hobo 4 channel analogue dataloggers for use with Watermark 253–L soil water potential sensors

The combination of granular matrix sensors with a Hobo 4 analogue channel datalogger provides a relatively inexpensive continuous soil water monitoring system. However, the datalogger excites all 4 channels concomitantly as it reads the sensor measurement of each channel in sequence. This results in localised electrolysis causing measurement bias error in Channels 2, 3 and 4. To evaluate this channel bias, Watermark granular matrix sensors were connected to Hobo 4 channel dataloggers to measure electrical conductance of the soil. This study formed part of a larger study aimed at understanding water use by Eucalyptus plantations at different soil depths. The measured soil conductivity was calibrated against the gravimetric method in soil derived from Natal Group Sandstone and Dwyka Tillite that occur in southern KwaZulu-Natal. The channels of a Hobo 4 channel datalogger were successfully calibrated against the gravimetric method for both soil types (R2 > 0.92). The voltage measurements of each channel increased in the order 1 < 2 < 3 < 4 for both soil types at a soil water content range of 12 to 44% and 6 to 46% for Dwyka Tillite and Natal Group Sandstone soils, respectively. Channel measurements were similar at soil water content ranges below 12 and 6% for tillite and sandstone soils, respectively. The study showed that Channels 2, 3 and 4 of the Hobo 4 channel datalogger are affected by electrolysis. If this analogue datalogger is used with these types of sensors, these channels need to be calibrated back to Channel 1.

Development of a low-cost automated calorimeter for determining soil specific heat

This paper describes the development of an automated calorimeter for measuring the specific heat (c) of soil samples. An isoperibolic calorimeter prototype was developed with low cost material. The data acquisition system was based on an Arduino microcontroller paired with modular sensors. For testing the developed system, c determination was performed soils of three textures and varying soil water and organic matter contents. Results of soil specific heat for the soil water content range varied from 945 to 1876 J kg−1 K−1 for sandy clay loam texture, from 736 to 1154 J kg−1 K−1 for sandy texture and from 1099 to 2296 J kg−1 K−1 for sandy clay texture. For several organic matter contents in the sandy clay soil, c varied from 1105 to 1793 J kg−1 K−1. The reliability of the automated system was assessed by comparison to measurements of temperature, voltage and electrical current by a calibrated equipment, whereas measured c was also compared against results estimated with a known worldwide equation, confirming the reliability of the developed prototype and system of data acquisition.

The role of soil hydraulic properties in crop water use efficiency: A process-based analysis for some Brazilian scenarios

The need for improvements in the water use efficiency by agricultural ecosystems requires a holistic assessment of the hydraulic functioning of cropped soils, taking into consideration the most relevant interactions and feedbacks that control the soil water budget. We implemented a mechanistic approach to isolate the effects of soil hydraulic properties (K-θ-h) of layered soils on water balance components and land and water productivity, adopting comprehensive scenarios of soil water availability and requirements. The agro-hydrological simulations were performed using the SWAP model integrated with the WOFOST crop growth module. The simulated scenarios included the rainfed crop growth of maize and soybean in three climate zones, evaluating the current climate scenarios as well as two future scenarios, a wetter and a drier one, totaling 108 scenarios simulated for 30 years each. Simulations were performed for six soils, grouped pairwise (3 × 2), where each pair represented the same soil group with two different long-term land uses: natural forest (proxy of a no-tillage system) and conventional agricultural use. The K-θ-h relationships were obtained simultaneously by inverse modeling for the full range of soil water contents commonly found in the domain of crop available water. The agro-hydrological simulations showed that the soil hydraulic properties affect dynamically water balance components and land productivity by relating soil hydraulic functioning to climate patterns and crop water requirements. In general, maize productivity was more sensitive to soil hydraulic properties under future climate scenarios than soybean. While land productivities of maize and soybean increased under the wetter climate scenario, water productivity of both crops was consistently reduced by both future climate scenarios. The K-θ-h of soils under conventional agricultural use over-performed their counterparts under long-term natural forest use, especially regarding land productivity during growing seasons with pronounced dry spells. Depending on the length and timing of drought stress during the growing season, the yield response is determined by soil-specific conditions strictly related to water availability. The long-term average revealed that the sampled loamy sand soils have more favorable hydraulic properties for crop growth; moreover, the reduced unproductive water losses, especially runoff, increased the dynamic water storage of those soils.

Development of a pore gas pressure transducer used in unsaturated soils at high water content

Pore gas pressure in soil is an important parameter in many geoscience applications such as evaluating the effects of trapped pore gas pressure on water infiltration through soil mass, optimizing the design of gas extraction wells in landfills and assessing the performance of landfill covers in reducing landfill gas emission. In addition, it has been observed that pore gas pressure affects slope stability in unsaturated soils. However, the pore gas pressure build-up induced by water infiltration is generally ignored in most slope stability analysis by assuming gas pressure to be zero. Therefore, pore gas pressure measurement in soils is crucial to better understand the unsaturated soil behaviour. However, most of current measuring techniques of pore gas pressure are affected by water interruption during the measurement in unsaturated soils, especially at high water content. In this study, a novel gas pressure transducer was developed to measure the pore gas pressure in unsaturated soil within a wide range of water content. The newly developed pore gas pressure transducer mainly consists of an electrical pressure sensor package and an integrated membrane filter which can prevent water leaching through the membrane but allow gas to pass it freely. The performance of the gas pressure transducer was evaluated by a series of permeation tests. The results show that the developed gas pressure transducer has a good repeatability to monitor gas pressure and has a relatively fast response to the gas pressure change in compacted soils. This transducer is able to measure pore gas pressure range of 0~50 kPa of soils within a relatively high range of soil water content.

Diffusivity and sorptivity determination at different soil water contents from horizontal infiltration

Determination of soil hydraulic properties under non-saturated conditions is crucial in order to describe soil water dynamics in the field. The objectives of this work were: i- to determine soil water diffusivity (D) as a function of soil moisture with a simple lab set-up, based on Whisler et al. (1968) methodology, from soil moisture profiles obtained using capacitive sensors in three contrasting soils from Argentinean Pampas Region; and ii- to obtain a function that allows to determine sorptivity (S) values in the whole moisture range from diffusivity data. Additionally, horizontal infiltration data was used to obtain hydraulic conductivity function (K (h)) through inverse parameterization using HYDRUS 1-D. Horizontal infiltration was performed in order to determine D and S at different soil water content in 15 repacked columns from three different textured soils (loam, silty loam and sandy loam). The results indicate that the new approach is a suitable methodology for D (θ) and S (θ) determination. D values measured using the proposed laboratory setup ranged between 1 × 10−5 and 1.67 cm2 s−1, showing an exponential dependence with soil water content (r2 > 0.85). S values for different soil water contents ranged between 0.078 and 0.14 cm s−1/2. The highest value of S corresponded to the lowest soil water content, remaining practically constant in a large range of soil water content, and abruptly decreasing near saturation toward zero. The sandy loam soil showed higher values of D at saturation (D0) and total sorptivity (S0), as compared with the loam and silty loam soils. In conclusion, it was possible to determine D (θ) using a simple and robust setup, based on Whisler et al. (1968) method. Additionally, the proposed setup represents a simple way to measure soil S (θ) from horizontal infiltration, and to obtain K(h) functions through inverse parameterization.

Resilience of bacteria, archaea, fungi and N-cycling microbial guilds under plough and conservation tillage, to agricultural drought

Climate change causes droughts, which in turn cause significant physiological stress for soil microorganisms. In this study, we investigated how the abundance of total bacterial, crenarchaeal and fungal communities and the abundance of N-cycling microbial guilds responded to a severe agricultural drought event in a long-term experiment of minimum tillage (MT) and conventional ploughing (CT) at two soil depths. Drought, defined as a reduction of soil water content and increased soil temperature, significantly decreased the abundance of all the studied microbial communities. The data showed linear relationships between all dependent variables and soil water content and soil temperature for the examined range of soil water content (WHC 13–76%) and examined range of average daily soil temperature at 5 cm depth (17–30 °C). Thus, we found that the abundance of most studied microbial communities decreased by about 2% when water content decreased by 1 mass % and by about 10% when temperature increases by 1 °C. When comparing communities at average soil water content and average soil temperature, MT had higher average abundances of total bacterial and crenarchaeal 16S rRNA and fungal ITS genes in the 0–10 cm soil layer than did CT (1.9, 2.9 and 2.5 times, respectively), as well as AOA and AOB amoA (3.9 and 1.7 times, respectively), nirK, nirS, nosZI and nosZII genes (2.0, 1.8, 1.8 and 2.3 times, respectively); while significant differences between MT and CT in the 10–20 cm soil layer were found only in the average abundance of crenarchaeal 16S rRNA and crenarchaeal amoA genes (3.5 and 2.7 times greater under MT than CT). Regardless of the weather conditions during our study, the abundances of all communities were greater under MT 0–10 than under CT 0–10. After three weeks of severe drought, the greatest decrease in the abundance of all communities, bacterial and archaeal N-cycling guilds as well as total prokaryotes and fungi, was observed under MT 0–10. However, after only a few rainfall events, all communities under both tillage systems reached their initial abundance, demonstrating a high resilience.

Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes

SummaryThis review examines the interactions between soil physical factors and the biological processes responsible for the production and consumption in soils of greenhouse gases. The release of CO2 by aerobic respiration is a non‐linear function of temperature over a wide range of soil water contents, but becomes a function of water content as a soil dries out. Some of the reported variation in the temperature response may be attributable simply to measurement procedures. Lowering the water table in organic soils by drainage increases the release of soil carbon as CO2 in some but not all environments, and reduces the quantity of CH4 emitted to the atmosphere. Ebullition and diffusion through the aerenchyma of rice and plants in natural wetlands both contribute substantially to the emission of CH4; the proportion of the emissions taking place by each pathway varies seasonally. Aerated soils are a sink for atmospheric CH4, through microbial oxidation. The main control on oxidation rate is gas diffusivity, and the temperature response is small. Nitrous oxide is the third greenhouse gas produced in soils, together with NO, a precursor of tropospheric ozone (a short‐lived greenhouse gas). Emission of N2O increases markedly with increasing temperature, and this is attributed to increases in the anaerobic volume fraction, brought about by an increased respiratory sink for O2. Increases in water‐filled pore space also result in increased anaerobic volume; again, the outcome is an exponential increase in N2O emission. The review draws substantially on sources from beyond the normal range of soil science literature, and is intended to promote integration of ideas, not only between soil biology and soil physics, but also over a wider range of interacting disciplines.

Analisis Konsumsi Air Sayuran Organik dalam Rumah Tanaman

Sayuran organik yang dibudidayakan di rumah tanaman memerlukan pengaturan pemberian air irigasi untuk mempertahankan kadar air tanah berdasarkan fase tumbuh tanaman. Salah satu kendala dalam pemberian irigasi adalah tidak adanya parameter yang digunakan untuk menentukan jadwal dan jumlah air irigasi yang diberikan. Penelitian ini dilaksanakan untuk mengetahui apakah pelaksanaan irigasi yang digunakan di lahan penelitian dapat memenuhi kebutuhan air tanaman serta jumlah konsumsi airnya berdasarkan pendekatan perubahan kadar air. Pada penelitian ini dilakukan pengamatan kelembaban tanah setiap hari dan menganalisis aliran air tanah berdasarkan parameter sifat fisik dan hidrolika tanah. Hasil penelitian menunjukkan bahwa kadar air tanah berada diantara kapasitas lapang dan titik layu permanen namun 87% dari masa tanam berada dibawah nilai readily avaiable water (RAW). Kondisi ini menunjukkan pemberian air irigasi pada lahan penelitian masih belum sepenuhnya optimal. Berdasarkan analisis perubahan kadar air tanah, konsumsi air selama masa tanam sayuran kailan di lahan penelitian adalah 55 mm selama masa tanam dengan laju konsumsi air 1,1 mm/hari. Kebutuhan air di lahan penelitian untuk mencapai kondisi RAW adalah 130 mm.

Morphophysiological response of young olive trees to verticillium wilt under different surface drip irrigation regimes

The effects of irrigation on verticillium wilt in olive, in terms of morphological, biomass and physiological parameters were evaluated on pot‐grown trees maintained in the field for 3 years. Plants inoculated and noninoculated with Verticillium dahliae were irrigated to high and low range of soil water content (HR and LR) at daily (DF; about 2 days/event), weekly (WF) and daily‐weekly (DWF) drip‐irrigation frequency. Morphological parameters, relative biomass and biomass water‐use efficiency were higher at LR than at HR (with few exceptions) and at DF than at other frequencies in noninoculated and inoculated plants, but the fungus reduced those parameters by 17.0–38.5%. Lower root weight ratio, relative biomass and shoot length as area originated at HR in noninoculated plants, could be favourable to the accumulation of root infections and the amount of fungus per tissue length in inoculated plants because higher infection was known at HR. Moreover, higher aerial biomass and length promoted by irrigation at DF could prevent the more severe expression of symptoms, which occurred at WF and DWF in the presence of Verticillium. Negative correlations were found between indicated parameters and disease. Lower water stress (SΨ), and higher stomatal conductance (gs) and net photosynthesis at DF in noninoculated plants could limit the disease by improving water status, as SΨ was increased by the fungus only at WF and DWF, and gs and disease were negatively correlated. LR‐DF treatment minimized the disease and kept the growth, water‐use efficiency and physiological parameters in inoculated plants to levels close to noninoculated plants.

Pedotransfer Functions of Soil Thermal Conductivity for the Textural Classes Sand, Silt, and Loam

Core Ideas The work describes the estimation of soil thermal conductivity (λ) from easily available soil data. Measured high‐resolution λ(θ) data were used to improve the accuracy of the model of Lu et al. (2014). Reliable pedotransfer functions estimated soil thermal conductivity of classes Sand, Silt and Loam. Good quality λ data need precise, quantitative measurements and accurate, model‐based estimations. Data of soil thermal conductivity (λ) covering the whole range of soil water content (θ) are needed in many environmental disciplines. However, laboratory measurements of λ(θ) are time consuming and costly. Therefore the use of pedotransfer functions is promising. In this study, we describe the procedure of deriving such λ(θ) pedotransfer functions for the FAO texture groups (TG) of Sand, Silt and Loam. First, high‐resolution λ(θ) measurements from saturation to dryness of 43 soils were conducted by two laboratory methods: the evaporation approach and single‐point measurements of λ at distinct water contents. Measurements were obtained for a set of sandy, silty, loamy, and clayey soils, and for two urban soils and two peat soils. The slope and magnitude of the λ(θ) curves show typical dependencies on bulk density (ρb) and soil texture. In a second step, the λ(θ) data were used to evaluate the λ(θ) model as suggested by Lu et al. (2014). Fitting this model to the complete data set yielded a poor agreement with RMSE = 0.53 W m‐1 K‐1. However, satisfying results were obtained by dividing the data into TGs Sand, Silt and Loam resulting in RMSEs of 0.18, 0.11 and 0.24 W m‐1 K‐1, respectively. Estimated λ values at low soil moisture (wilting point) were hardly different among texture classes, but λ differences become highly relevant when soil moisture increased to field capacity. The derived pedotransfer functions are an easy‐to‐use approach providing reliable λ(θ) curves for sandy, silty and loamy soils, when only basic soil information (texture and bulk density) are available.

Effects of soil drought stress on photosynthetic gas exchange traits and chlorophyll fluorescence in Forsythia suspensa

To clarify the changes in plant photosynthesis and mechanisms underlying those responses to gradually increasing soil drought stress and reveal quantitative relationships between photosynthesis and soil moisture, soil water conditions were controlled in greenhouse pot experiments using 2-year-old seedlings of Forsythia suspensa (Thunb.) Vahl. Photosynthetic gas exchange and chlorophyll fluorescence variables were measured and analyzed under 13 gradients of soil water content. Net photosynthetic rate (P N), stomatal conductance (g s), and water-use efficiency (W UE) in the seedlings exhibited a clear threshold response to the relative soil water content (R SWC). The highest P N and W UE occurred at R SWC of 51.84 and 64.10%, respectively. Both P N and W UE were higher than the average levels at 39.79% ≤ R SWC ≤ 73.04%. When R SWC decreased from 51.84 to 37.52%, P N, g s, and the intercellular CO2 concentration (C i) markedly decreased with increasing drought stress; the corresponding stomatal limitation (L s) substantially increased, and nonphotochemical quenching (N PQ) also tended to increase, indicating that within this range of soil water content, excessive excitation energy was dispersed from photosystem II (PSII) in the form of heat, and the reduction in P N was primarily due to stomatal limitation. While R SWC decreased below 37.52%, there were significant decreases in the maximal quantum yield of PSII photochemistry (F v/F m) and the effective quantum yield of PSII photochemistry (ΦPSII), photochemical quenching (q P), and N PQ; in contrast, minimal fluorescence yield of the dark-adapted state (F 0) increased markedly. Thus, the major limiting factor for the P N reduction changed to a nonstomatal limitation due to PSII damage. Therefore, an R SWC of 37.52% is the maximum allowable water deficit for the normal growth of seedlings of F. suspensa, and a water content lower than this level should be avoided in field soil water management. Water contents should be maintained in the range of 39.79% ≤ R SWC ≤ 73.04% to ensure normal function of the photosynthetic apparatus and high levels of photosynthesis and efficiency in F. suspensa.

Effects of water table management on least limiting water range and potato root growth

Soil physical quality indicator, Least Limiting Water Range (LLWR) is the range of soil water content, where water, oxygen and mechanical resistance are not limiting factors to root growth. The objective of this study was to evaluate LLWR, soil water availability, potato root growth and tuber yield under different water table management levels. Water table level was managed targeting 0.36 and 0.76m below the soil surface, denoted as high (HI) and low (LO) level, respectively. Undisturbed soil core samples were obtained in the 0–0.15, 0.15–0.30 and 0.30–0.45m soil layers to assess the LLWR. Root parameters were assessed using mini-rhizotrons installed into the soil along the potato row. Overall, LLWR decreased in depth due to a decrease in soil organic matter and an increase soil bulk density. The LO resulted in a narrower range for LLWR than HI. In the 0–0.15m soil layer, the soil water content in the HI treatment fell inside the LLWR limits with high frequency during the growing season, but both water table levels resulted in similar root growth. In contrast, in the 0.15–0.30 and 0.30–0.45m soil layers, soil water content fell inside LLWR more frequently in the LO than HI treatment. The LO management increased potato root length and surface area in the 0.15–0.30m soil layer compared to HI; while in the 0.30–0.45m soil layer, roots were not present in the HI likely due to the soil water content that was above LLWR. Optimal potato root growth was observed when the soil water content fell within the LLWR range at the highest frequency during the season. Despite impacts on the root system, similar tuber yields were achieved between LO and HI treatments. Nevertheless, the treatment HI used three times more water to supply the crop water requirement than LO.

Chlorophyll fluorescence upper-to-lower-leaf ratio for determination of irrigation time for Pentas lanceolata

The objective of this study was to use nondestructive measurements as the precise irrigation indices for potted star cluster (Pentas lanceolata). Drought stress was imposed on plants for 0, 3, 5, 7, 12, and 16 d by withholding water. Measurements were conducted on the third leaf counted from the apex (upper leaves) and on the third leaf from the bottom (lower leaves). Within the range of soil water content (SWC) from 10 to 45%, leaf water potential (WP), SWC, and soil matric potential (SMP), chlorophyll fluorescence, photochemical reflectance index (PRI), adjusted normalized difference vegetation index (aNDVI), and the reflectance (R) at 1950 nm (R1950) were measured. The plants reached the temporary wilting point at −3.87 MPa of leaf WP; the maximal fluorescence yield of the light-adapted state (Fm′) ratio of upper-to-lower leaves was 1.7. When the Fm′ ratio was 1.3, it corresponded to lower-leaf WP 8.904; it was the time to irrigate. In conclusion, the Fm′ ratio of upper-to-lower leaves was shown to be a nondestructive predictor of leaf WP and can be used to estimate irrigation timing.

SoilFlex‐LLWR: linking a soil compaction model with the least limiting water range concept

AbstractSoil compaction impacts growing conditions for plants: it increases the mechanical resistance to root growth and modifies the soil pore system and consequently the supply of water and oxygen to the roots. The least limiting water range (LLWR) defines a range of soil water contents within which root growth is minimally limited with regard to water supply, aeration and penetration resistance. The LLWR is a function of soil bulk density (BD), and hence directly affected by soil compaction. In this paper, we present a new model, ‘SoilFlex‐LLWR’, which combines a soil compaction model with the LLWR concept. We simulated the changes in LLWR due to wheeling with a self‐propelled forage harvester on a Swiss clay loam soil (Gleyic Cambisol) using the new SoilFlex‐LLWR model, and compared measurements of the LLWR components as a function of BD with model estimations. SoilFlex‐LLWR allows for predictions of changes in LLWR due to compaction caused by agricultural field traffic and therefore provides a quantitative link between impact of soil loading and soil physical conditions for root growth.

A simple approach to predicting unsaturated hydraulic conductivity based on empirically scaled microscopic characteristic length

A simple and empirical scaling approach is proposed for predicting the unsaturated hydraulic conductivity (Kus) based on the microscopic characteristic length (λ). The approach was tested on 19 different soils. The results showed that the proposed approach was useful for a wide range of soil water content, except at relatively high soil moisture. The predicted λ(θ) values were overestimated by 2%, which led to 5% overestimation in prediction of Kus(θ) at soil matric potentials of less than –0.1 MPa. However, the predictions of Kus(θ) were not satisfactory at soil matric potentials of greater than –0.1 MPa. The results also revealed that the proposed approach was accurate for predicting Kus(θ) in the dry range of soil water content. Future models should be developed based on the empirical and physical scaling approaches to improve prediction of soil hydraulic properties at high water contents. This means that new scaling factors need to incorporate more information about the pore geometry.Editor D. Koutsoyiannis; Associate editor A. Carsteanu

Impact of expected climate change on soil water regime under different vegetation conditions

A mathematical model was applied for the Bukk Mountains (Hungary) to evaluate the effects of climate change on soil water balance elements and soil water regime. Model runs using SWAP model were performed for combinations of four distinctive soil types and three land use systems of arable land, grassland, and forest. The temporal variation of soil water regime under changing climatic conditions was examined considering no land cover change occurring in the future. The climate data consisted of the predictions of two regional climate models, the Swiss CLM and the Swedish RCA. The RCA results showed 45% to 50% and the CLM showed 5% to 14% higher future precipitation outlook compared to present conditions. Considering different land use types, the projected number of days with soil moisture deficit was the highest in forest ecosystems for both the upper 50 cm soil layer and the whole soil profile, which could be as high as 61% of days below optimal soil water content range. Our results showed increased water fluxes, especially in deep percolation in far future period and a strong influence of soil properties on the changes in the climate model results, indicating significant long-term effects of climate change on soil water regime.

PTFs for predicting LLWR from various soil attributes including cementing agents

Least limiting water range (LLWR), the range of soil water content at which plant growth is least limited by water potential, soil aeration or soil mechanical resistance is routinely calculated from water release curve (WRC) and soil resistance curve (SRC). There is no enough information about the effect of various soil attributes including cementing agents (metal oxides, carbonates and organic carbon) on LLWR. The present study evaluates the effect of several soil characters, including cementing agents, texture, sodium adsorption ratio (SAR), bulk density (Db) and cation exchange capacity (CEC) on LLWR and develops proper pedotransfer functions (PTF) for its prediction. Disturbed and undisturbed samples of 32 soils with wide range of properties were collected from Ahar, Horand and Tabriz regions, northwest of Iran. Undisturbed soil samples were equilibrated to matric pressures of 0.001, 0.004MPa in hanging columns and of 0.01, 0.03, 0.1, 0.5, 1.5MPa in pressure plate and the equivalent water contents were measured gravimetrically. Penetration resistance of each sample at the mentioned matric pressures was measured by a hand cone penetrometer. Soil water contents at field moisture capacity (θfc) and permanent wilting point (θwp) (matric pressures of 0.01 and 1.5MPa, respectively) were predicted using WRC; water content at 2MPa penetration resistance (θsr) was estimated from SRC. Water content at 10% air filled porosity (θafp) was taken as θs-0.1 and LLWR computed from the above moisture coefficients. The relative influence of soil characters, as independent variables on the moisture coefficients (θafp, θfc, θsr, θwp) and on LLWR was evaluated separately using multiple linear stepwise regression and then appropriate pedotransfer functions were developed to predict LLWR. The relative influences of soil attributes on the moisture coefficients and on LLWR were not similar. Clay content, bulk density (Db) and ammonium oxalate extractable iron produced considerable effects on LLWR. Grouping the examined soil samples according to Db or clay content led to more accurate prediction of LLWR (R2=0.86 and 0.76, respectively) than forcing all samples in a single group (R2=0.31). In samples with Db≥1.4Mg/m3, citrate-bicarbonate-dithionate extractable aluminum, as cementing agent, turned to be the second most influential soil attribute (after clay) on LLWR while the latter was not affected by calcium carbonate equivalent. Even though SAR significantly (P<0.01) affected both θwp and θfc, its net effect on LLWR was insignificant.

Effect of litter layer on soil–atmosphere N2O flux of a subtropical pine plantation in China

Forest soils are important sources for nitrous oxide (N2O), but how the surface litter layer affects these sources is still unclear. Seasonal rainfall in the subtropical monsoon climate provides a unique opportunity to examine soil–atmosphere N2O flux under a wide range of soil water content. We studied this question over 3 years using a litter removal method in a 20-year-old pine plantation (Pinus elliottii) in subtropical China. Annual mean chamber-based soil–atmosphere N2O fluxes of the control (FCK) and litter-free (FLF) treatments were 6.07 and 5.17 μg N2O m−2 h−1, respectively. Removal of the litter layer reduced 15% of soil N2O emissions, suggesting the mineral soil as the dominant factor that determines soil N2O emissions. Seasonal FCK and FLF were both significantly influenced by water-filled pore space (WFPS) but not by soil temperature (TS). However, FCK and FLF were both correlated with TS during the wet season (January–June) but not during the dry season (July–December). During the wet season, FCK and FLF were 84% and 132% higher than during the dry season, respectively. In contrast, seasonal litter-based N2O fluxes (FCK−LF = FCK − FLF) were not correlated with WFPS and TS. During the dry season, however, a positive relationship was observed for FCK−LF and WFPS. In the context of climate change and human activities, future changes in soil environment and surface litter management will alter the strength of soil N2O emissions of the subtropical pine forests in China.

The litter layer acts as a moisture-induced bidirectional buffer for atmospheric methane uptake by soil of a subtropical pine plantation

Forest soils are well known sinks for atmospheric methane (CH4), but how the surface litter layer controls gas diffusion into the mineral soil is still unclear. Seasonal rainfall in the humid climate provides a unique opportunity to examine uptake of atmospheric CH4 under a wide range of soil water content (SWC). We studied this question using a litter removal method in a 20-year-old slash pine (Pinus elliottii) plantation in subtropical China during 2005–2007. Soil-atmosphere CH4 fluxes of the control (FCK) and litter-free (FLF) treatments and their differences (litter-affected CH4 flux, FCK–LF = FCK − FLF) were all significantly influenced by SWC and not by soil temperature. Litter layer reduced atmospheric CH4 uptake by soil when SWC was below 15.8 vol%, and increased atmospheric CH4 consumption by soil when SWC was above this value. We concluded that the litter layer acts as a moisture-induced bidirectional buffer for atmospheric CH4 uptake by soils in a subtropical humid pine plantation. However, the removal of the litter layer had a minimal effect (+0.7%) on annual atmospheric CH4 uptake by soil, through compensating effects during the wet and dry seasons. Therefore, in the context of climate change, future changes in SWC will alter the strength of atmospheric CH4 uptake by soils of subtropical pine plantations.

Fungi mediate nitrous oxide production but not ammonia oxidation in aridland soils of the southwestern US

Nitrification and denitrification are key processes that control nitrogen (N) availability and loss in terrestrial systems. Nitrification in soils is thought to be performed primarily by chemoautotrophic prokaryotes such as bacteria and archaea, and denitrification by bacterial heterotrophs. However, recent work in drylands suggests that fungi may play a larger role in N transformations than previously recognized. Arid and semi-arid ecosystems experience extreme temperatures and low moisture conditions, both of which favor growth and survival of some fungi, but the extent of fungal nitrification and denitrification in these soils has not been explored. We investigated the role of fungi in nitrate (NO3−) and nitrous oxide (N2O) production in soils from regions across the southwestern US. Soils were collected from urban and Sonoran Desert sites within the Phoenix metropolitan area in Arizona, and from grasslands in Arizona and New Mexico. Rates of potential nitrification were measured in soils subjected to biocide treatments (fungal and bacterial inhibitors) and acetylene, an inhibitor of autotrophic ammonia oxidation. Nitrous oxide production was measured from soils incubated at two moisture levels in the presence of the biocide treatments. Rates of potential nitrification decreased significantly in all soils with additions of acetylene, indicating that autotrophic microorganisms may dominate ammonia oxidation and NO3− production rather than heterotrophs (e.g., fungi). Similarly, fungi played a minor role in N2O production in urban turfgrass soils that were highly managed with irrigation and fertilizers. In contrast, fungi were major sources of N2O production in desert and semi-arid grassland soils across a range of soil water content. We conclude that fungi are likely responsible for denitrification in aridland soils, and that land-use changes associated with urbanization alter the biotic pathways responsible for N cycling.