Soil Moisture Fluctuations Research Articles

Assessing Vegetation Response to Soil Moisture Fluctuation under Extreme Drought Using Sentinel-2

The aim of this study was to determine the extent to which Sentinel-2 Normalised Difference Vegetation Index (NDVI) reflects soil moisture conditions, and whether this product offers an improvement over Landsat-8. Based on drought exposure, cloud-free imagery availability, and measured soil moisture, five sites in the Southwestern United States were selected. These sites, normally dry to arid, were in various states of drought. A secondary focus was therefore the performance of the NDVI under extreme conditions. Following supervised classification, the NDVI values for one-kilometre radius areas were calculated. Sentinel-2 NDVI variants using Spectral Bands 8 (10 m spatial resolution), 5, 6, 7, and 8A (20 m spatial resolution) were calculated. Landsat-8 NDVI was calculated at 30 m spatial resolution. Pearson correlation analysis was undertaken for NDVI against moisture at various depths. To assess the difference in correlation strength, a principal component analysis was performed on the combination of all bands and the combination of the new red-edge bands. Performance of the red-edge NDVI against the standard near infrared (NIR) was then evaluated using a Steiger comparison. No significant correlations between Landsat-8 NDVI and soil moisture were found. Significant correlations at depths of less than 30 cm were present between Sentinel-2 NDVI and soil moisture at three sites. The remaining two sites were characterised by low vegetation cover, suggesting a cover threshold of approximately 30–40% is required for a correlation to be present. At all sites of significant positive moisture to NDVI correlation, the linear combination of the red-edge bands produced stronger correlations than the poorer spectral but higher spatial resolution band. NDVI calculated using the higher spectral resolution bands may therefore be of greater use in this context than the higher spatial resolution option. Results suggest potential for the application of Sentinel-2 NDVI in soil moisture monitoring, even in extreme environments. To the best of our knowledge, this paper represents the first study of this kind using Sentinel-2.

Functional roles of root plasticity and its contribution to water uptake and dry matter production of CSSLs with the genetic background of KDML105 under soil moisture fluctuation

ABSTRACTSoil moisture fluctuation (SMF) stress due to erratic rainfall in rainfed lowland (RFL) rice ecosystems negatively affect production. Under such condition, root plasticity is one of the key traits that play important roles for plant adaptation. This study aimed to evaluate root plasticity expression and its functional roles in water uptake, dry matter production and yield under SMF using three chromosome segment substitution lines (CSSLs) with major genetic background of KDML105 and a common substituted segment in chromosome 8. The CSSLs showed greater shoot dry matter production than KDML105 under SMF, which was attributed to the maintenance of stomatal conductance resulting in higher grain yield. The root system development based on total root length of the CSSLs were significantly higher than that of KDML105 due to the promoted production of nodal and lateral roots. These results implied that the common substituted segments in chromosome 8 of the 3 CSSLs may be responsible for the expression of their root plasticity under SMF and contributed to the increase in water uptake and consequently dry matter production and yield. These CSSLs could be used as a good source of genetic material for drought resistance breeding programs targeting rainfed lowland condition with fluctuating soil moisture environments and for further genetic studies to elucidate mechanisms underlying root plasticity.

Greenhouse Gas Emissions from Pecan Orchards in Semiarid Southern New Mexico

Greenhouse gas (GHG) emissions are fueling global climate change, with methane and nitrous oxide being the primary agricultural gases emitted. It has been shown that N2O emissions correlate to moisture content fluctuations; however, emissions from agricultural fields in the semiarid regions of the Southwest where rewetting events occur regularly are not well established. The scope of this study was to quantify GHG emissions in correlation to soil moisture fluctuations and fertilizer application. The study was conducted continuously in two pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards between Aug. 2010 and Aug. 2011 on a sandy loam soil (La Mancha) and a silty clay loam soil (Leyendecker), both under normal management practices. The small chamber technique was used to measure GHGs. Emissions varied greatly throughout the year. The largest flux of CO2 at La Mancha and Leyendecker both occurred during a drying event immediately following an irrigation event: 84,642.49 μg·m−2·h−1 and 30,338.24 μg·m−2·h−1, respectively. The net CH4 flux at Leyendecker and La Mancha was close to zero with the largest emissions occurring during wetting events. Results showed that N2O emissions were maintained near the baseline except for the few days following an irrigation event. The largest emission peak at La Mancha occurred after irrigation and nitrogen application: 322.06 μg·m−2·h−1. The largest emission peaks of 26.37 and 1.13 μg·m−2·h−1 at Leyendecker and La Mancha, respectively, occurred after irrigation, nitrogen application, and tillage. Nitrogen application was the driving factor affecting N2O emissions at La Mancha, whereas soil moisture content was the driving factor at Leyendecker. Emission factors (EFs) at La Mancha and Leyendecker were 0.49% and 0.05%, respectively. A thorough accounting of GHG emissions is necessary for budgeting and identifying mitigation policy.

Plasticity in nodal root elongation through the hardpan triggered by rewatering during soil moisture fluctuation stress in rice

Rainfed lowland (RFL) rice fields have hardpans and experience soil moisture fluctuations (SMF) stress, which influence root system development. Here, we clarify the expression and timing of the plasticity in nodal root elongation through the hardpan under SMF and its contribution to shoot growth using a shallow-rooting IR64 and its deep-rooting introgression line, YTH304. Under SMF, soil moisture content had negative relationship with soil penetration resistance, regardless of hardpan bulk densities. YTH304 had greater root system below the hardpan than IR64 in hardpan with 1.50 but not in 1.70 g cm−3 bulk density (BD). YTH304 had greater plasticity in nodal root elongation through the hardpan than IR64 under SMF, which was clearly expressed during rewatering. YTH304 also had greater soil water uptake below the hardpan during drought and greater shoot growth than IR64. The results imply that deep root system development during SMF was due to the plasticity in nodal root elongation through the hardpan expressed during rewatering rather than during drought periods. This is against the long standing belief that active root elongation through the hardpan happens during drought. This also implies a need to revisit current root screening methods to identify rice lines with good hardpan penetration ability.

Root plasticity under fluctuating soil moisture stress exhibited by backcross inbred line of a rice variety, Nipponbare carrying introgressed segments from KDML105 and detection of the associated QTLs

In rainfed lowland rice ecosystem, rice plants are often exposed to alternating recurrences of waterlogging and drought due to erratic rainfall. Such soil moisture fluctuation (SMF) which is completely different from simple or progressive drought could be stressful for plant growth, thereby causing reduction in yield. Root plasticity is one of the key traits that play important roles for plant adaptation under such conditions. This study aimed to evaluate root plasticity expression and its functional roles in dry matter production and yield under SMF using Nipponbare, KDML 105 and three backcross inbred lines (BILs) and to identify QTL(s) associated with root traits in response to SMF at two growth stages using Nipponbare/KDML105 F2 plants. A BIL, G3-3 showed higher shoot dry matter production and yield than Nipponbare due to its greater ability to maintain stomatal conductance concomitant with greater root system development caused by promoted production of nodal and lateral roots under SMF. QTLs were identified for total nodal root length, total lateral root length, total root length, number of nodal roots, and branching index under SMF at vegetative and reproductive stages. The QTLs detected at vegetative and reproductive stages were different. We discuss here that relationship between root system of G3-3 and the detected QTLs. Therefore, G3-3 and the identified QTLs could be useful genetic materials in breeding program for improving the adaptation of rice plants in target rainfed lowland areas.

Genotypic variations in the plasticity of nodal root penetration through the hardpan during soil moisture fluctuations among four rice varieties

Rainfed lowland rice fields are characterized by soil moisture fluctuations (SMF) and the presence of hardpan that impedes deep rooting and thus limits water extraction from deep soil layer during the periods of drought. In this study, we used rootboxes with three layers; shallow layer, artificial hardpan, and deep and wet layer below the hardpan, to evaluate differences in the plasticity of nodal roots elongation through the hardpan and promote root branching below the hardpan in response to SMF among four rice varieties; Sasanishiki, Habataki, Nipponbare, and Kasalath. Experiments were conducted during the summer and autumn seasons. Plasticity was computed as the difference in root traits within each variety between the SMF and continuously well-watered treatments. In both experiments, Habataki consistently tended to exhibit higher root plasticity than the other three varieties by increasing number of nodal roots that penetrated the hardpan during rewatering period in SMF, when the soil moisture increased and penetration resistance decreased. This root plasticity then contributed to greater water use at the deeper soil during the subsequent drought period and overall shoot dry matter production. Habataki had significantly higher δ13C value in roots at deep layer than roots at the shallow and hardpan layers under SMF, which may indicate that these were relatively newly grown roots as a consequence of root plasticity. This study also indicates that CSSLs derived from Sasanishiki and Habataki varieties may be suitable for the analysis of QTLs associated with root plasticity expression in rainfed lowland with hardpan and experiencing SMF.

On-farm assessment of a new early-maturing drought-tolerant rice cultivar for dry direct seeding in rainfed lowlands

Dry direct seeding of rice (DDSR) is becoming a common practice in drought-prone lowland areas where there is insufficient labor for transplanting, but early-season drought often causes poor crop establishment, which allows subsequent weed infestation. Although early-maturing drought-tolerant cultivars have been released in tropical Asia in the last decade, almost all farmers in these areas still use cultivars selected for irrigated lowlands. The objective of our study was to compare a new drought-tolerant cultivar (Rc348) and a popular cultivar of farmers (Rc10) in DDSR under rainfed lowland conditions. On-farm experiments in three villages in northern Luzon, the Philippines, showed that the yield of Rc10, but not Rc348, was negatively associated with soil drying. Although their average yields were comparable (Rc348, 3.03 t ha−1; Rc10, 3.00 t ha−1), Rc348 yielded 34% more with moderate to severe weed infestation or soil dryness (3.12 vs. 2.33 t ha−1). Weed infestation increased with increasing soil dryness, but the weed pressure was more severe for Rc10 than for Rc348. Rc348 had a higher seedling emergence percentage, number of seedlings m−2, and ground cover at 30 days after sowing than Rc10 in drought-prone fields at upper (drier) positions in the toposequence. These results were validated by an on-station experiment with controlled drought stress at the International Rice Research Institute. We suggest that the adoption of newly released cultivars from the breeding programs for rainfed rice with reliable seedling emergence and early vigor in the presence of fluctuating soil moisture would stabilize the yield of DDSR in the target drought-prone areas.

Influence of Soil Moisture Status on Soil Cadmium Phytoavailability and Accumulation in Plantain (Plantago lanceolata)

The effect of fluctuating soil moisture cycles on soil cadmium (Cd) phytoavailability was investigated in a pot trial with two contrasting soils (Kereone (Allophanic), total Cd 0.79 mg kg−1; and Topehaehae (Gley), total Cd 0.61 mg kg−1) that were either sown with plantain (Plantago lanceolata) or left unseeded. Varying soil moisture contents were established using contrasting irrigation regimes: “flooded” (3 days flooded and then 11 days drained); or “non-flooded” (irrigation to 70% of potted field capacity every 7 days). Overall, there was no significant difference in mean 0.05 M CaCl2 soil extractable Cd concentrations or plant tissue Cd concentrations between flooded and non-flooded irrigation. However, there was a consistent trend for an increase in soil extractable Cd concentrations following irrigation, regardless of the irrigation regime. Mean soil extractable Cd and plant tissue Cd concentrations were significantly greater (approximately 325% and 183%, respectively) for the Topehaehae soil than the Kereone soil, despite the lower soil total Cd concentration of the Topehaehae soil. These results indicate that Cd solubility is sensitive to increases in soil moisture following periods of soil drainage, but insensitive to short-term periods of soil saturation. Plant tissue Cd concentrations in Cd-sensitive forage crops such as plantain are likely to be greater following large rainfall events over summer and autumn. This has the potential to increase animal dietary Cd exposure and rate of liver/kidney Cd accumulation.

Particulate organic carbon is a sensitive indicator of soil degradation related to overgrazing in Patagonian wet and mesic meadows

Patagonian meadows are key for the development and sustainability of rural environments. However, they have been degraded due to a combination of weather conditions and overgrazing. Total soil organic carbon (TOC) has been found as a moderate indicator of meadow soil degradation by long-term heavy grazing. The aim of this study was to evaluate the particulate organic carbon (POC) (a more labile fraction of the soil organic matter) as an ecological indicator for assessing soil changes in natural Patagonian meadows, related to different long-term grazing managements. We worked in three locations along a precipitation gradient in Northern Patagonia. Each location has wet and mesic meadows with good and poor grassland condition, attributed to two long-term grazing pressures (light and heavy). We sampled soil throughout the plant-growing season following the natural soil moisture fluctuations, and quantified the POC content, the POC:TOC ratio, and the F>53 proportion. Along the different sampling dates, significant differences between good versus poor grassland condition were found in many cases in POC, a few in POC:TOC ratio, and none in F>53 proportion. The POC varies over time, in association with changes in the soil moisture (adjusted R2 vary between 0.76 and 0.99); therefore, the sampling date must be standardized (we recommend dry periods). The POC appears to be a sensitive indicator when assessing the effect of different soil management practices on soil quality, an important step for meadow conservation, restoration and sustainable use, but further research is needed to validate this exploratory study.

Spatial and temporal variability of soil nitric oxide emissions in N-saturated subtropical forest

Acidic subtropical forest soils, receiving high atmospheric nitrogen (N) deposition, are characterized by fast N turnover, making them potential “hot spots” for nitrous oxide (N2O) and nitric oxide (NO) emissions. Production of NO in soils is mediated by microbial nitrification and denitrification and/or chemical decomposition of microbially produced nitrite (NO2 −). In monsoonal forests, soil microbial processes are exposed to strong soil moisture and temperature fluctuations, known to cause pronounced variability in NO emission. We quantified in situ fluxes of NO throughout two summers in the N-saturated TieShanPing forest, Chongqing, SW China. Additional N2O measurements were carried out in the second summer to infer likely source processes. To address spatial variability, fluxes were measured along two transects, one covering well-drained Acrisols on a forested hill slope, and the other hydrologically connected terraces with poorly drained Cambisols situated in a groundwater discharge zone. Hill slope sites emitted on average 48.4 ± 60.6 µg NO–N m−2 h−1 in the relatively wet summer of 2015 and 88.4 ± 94.6 µg NO–N m−2 h−1 in the dry summer of 2016, illustrating the strong effect of soil moisture regime. The NO emission in the groundwater discharge zone, measured in the dry summer of 2016 only, while it was flooded in the summer of 2015, was significantly smaller (26.4 ± 52.7 µg N m−2 h−1). The mean N2O-N emission on the hillslope (18.1 ± 13.9 µg N m−2 h−1) was smaller than NO-N flux, while it was larger (38.3 ± 39.3 µg N m−2 h−1) in the groundwater discharge zone. The temporal variability of the NO flux was largely explained by soil moisture fluctuations, whereas soil organic matter pool sizes contributed most to spatial variability. Hourly measurements showed that the NO flux doubled with 8 °C increase of soil temperature, indicating that soil temperature was another important driver for NO emissions. Our study indicates that acid N-saturated subtropical forest soils are a large source for NO, particularly during dry summers.

Using BDS SNR Observations to Measure Near-Surface Soil Moisture Fluctuations: Results From Low Vegetated Surface

The feasibility of using China’s Beidou Navigation Satellite System (BDS) signal-to-noise ratio (SNR) data for estimating soil moisture is demonstrated in this letter. Previous studies for the Global Navigation Satellite System (GNSS) multipath for estimating soil moisture have concentrated on the Global Positioning System (GPS) SNR data. The SNR data recorded by BDS receivers can also be strongly impacted by the relative permittivity, which however, has not been comprehensively investigated. This letter used a commercially available geodetic-quality BDS/GPS receiver to collect BDS SNR data on both B1 and B2 frequencies. Two methods were investigated to demonstrate the relationship between BDS SNR metrics and soil moisture. One was the phase method proposed by Larson et al. , and the other was an interference model proposed by Peng et al. The results show that both B1 and B2 frequencies perform well to reflect the moisture fluctuations, particularly for reflecting big precipitation events. Comparisons between BDS B1 and B2 with GPS L2C and L5 demonstrate the results from these two constellations are comparable. BDS could be a new data source for producing global high-temporal soil moisture products using GNSS-based approaches.

Effects of Clear Polymer Film on Emergence and Survival of Direct Sown Native Vegetation

AbstractClear, degradable polymer films are used with agricultural crops to alter the germination and growing environment and extend the length of the growing season. The film acts like a greenhouse, elevating temperatures and improving soil water. We examined the potential of polymer film to improve emergence and persistence of direct‐seeded native vegetation. A seed mix comprising three tree species and two shrubs was sown in early August 2015, with and without polymer film. The effect of duration of polymer film (from 0–10 weeks) on emergence and persistence was tested, and the effects of polymer film on temperature and soil moisture were monitored. Our hypothesis that polymer film would improve emergence was partially supported, with between 6 and 10 weeks of polymer film coverage increasing emergence of small‐seeded species above the level of the Control. The polymer film increased temperatures by an average of 5 °C and volumetric water content by up to 10%, compared with ambient conditions, and reduced fluctuations in soil moisture, which may have contributed to the increased emergence observed. Emergence was low across all treatments (range 0·2–47%). Our hypothesis that polymer film would improve persistence was not supported, with seedling numbers declining rapidly following removal of the polymer film and no differences between treatments in percentage persistence by the end of the experiment. We concluded that polymer film is a potentially useful tool for improving emergence of smaller‐seeded species in direct seeding for land restoration, although provided no benefit for emergence of the larger‐seeded species. Copyright © 2017 John Wiley & Sons, Ltd.

Partial drying accelerates bacterial growth recovery to rewetting

Fluctuations in soil moisture create drying-rewetting events affecting the activity of microorganisms. Microbial responses to drying-rewetting are mostly studied in soils that are air-dried before rewetting. Upon rewetting, two patterns of bacterial growth have been observed. In the Type 1 pattern, bacterial growth rates increase immediately in a linear fashion. In the Type 2 pattern, bacterial growth rates increase exponentially after a lag period. However, soils are often only partially dried. Partial drying (higher remaining moisture content before rewetting) may be considered a less harsh treatment compared with air-drying. We hypothesized that a soil with a Type 2 response upon rewetting air-dried soil would transform into a Type 1 response if dried partially before rewetting. Two soils were dried to a gradient of different moisture content. Respiration and bacterial growth rates were then measured before and during 48 h after rewetting to 50% of water holding capacity (WHC). Initial moisture content determined growth and respiration in a sigmoidal fashion, with lowest activity in air-dried soil and maximum above ca. 30% WHC. Partial drying resulted in shorter lag periods, shorter recovery times and lower maximum bacterial growth rates after rewetting. The respiration after rewetting was lower when soil was partially dried and higher when soils were air-dried. The threshold moisture content where transition from a Type 2 to a Type 1 response occurred was about 14% WHC, while >30% WHC resulted in no rewetting effect. We combine our result with other recent reports to propose a framework of response patterns after drying-rewetting, where the harshness of drying determines the response pattern of bacteria upon rewetting dried soils.

Soil Moisture Stochastic Model in Pinus tabuliformis Forestland on the Loess Plateau, China

As an important restrictive factor of ecological construction on the Loess Plateau, the study of soil moisture dynamics is essential, especially under the impact of climate change on hydrological processes. In this study, the applicability of the Laio soil moisture stochastic model on a typical plantation Pinus tabuliformis forestland on the Loess Plateau was studied. On the basis of data concerning soil properties, climate, and plants of the typical forestland during the period 2005–2015 in the Chinese National Ecosystem Research Network (Ji County Station) in Ji County, Shanxi, model results were acquired and compared with observed soil moisture from 2005 to 2015 in the study area. The genetic algorithm method was used to optimize model parameters in the calibration process. In the calibration and validation periods, the relative error between numerical characteristics of simulated and observed soil moisture values was mostly within 10%, and model evaluation index J was close to 1, indicating that the Laio model had good applicability in the study area. When calibrating the model, it was recommended to use soil moisture data with a sampling interval of no more than 10 days so as to reduce the loss of soil moisture fluctuation information. In the study area, the Laio model was strongly sensitive to variations of input parameters, including maximum evapotranspiration rate Emax, average rainfall depth α, and average rainfall frequency λ, which should be paid more attention for stable and reliable simulation results. This study offers a method to obtain soil moisture data at ungauged sites. Results from this study provide guidance for Laio model application on the Loess Plateau.

Soil heterotrophic CO2 emissions from tropical high-elevation ecosystems (Páramos) and their sensitivity to temperature and moisture fluctuations

Increasing temperatures and changes in the intensity and frequency of precipitations may impact the ability of tropical high-elevation Andean ecosystems (Páramos) to store and retain carbon (C). We, therefore, examined how warming and fluctuations in soil moisture could influence soil CO2 emissions from heterotrophic respiration (RH, the result of microbial respiration), of two Páramos of contrasting climatic regimes within their area of distribution. We here show high sensitivity of both RH and Q10 under warmer and fluctuating moisture conditions. Together with the high rates of C-normalized RH compared to other soil C-rich ecosystems from higher latitudes (2 μmol gC−1 h−1 versus 0,4 μmol gC−1 h−1, respectively) our results evidenced how soil heterotrophic-derived CO2 emissions could potentially increase under expected climate scenarios, eventually altering the capacity of soil Páramos to sequester C.

Herbivores suppress soil microbes to influence carbon sequestration in the grazing ecosystem of the Trans-Himalaya

Understanding factors that regulate carbon (C) pools is of high importance for offsetting greenhouse-gas emissions. Soils represent a vast C pool, whose size and stability are strongly influenced by land-use. Grazing, by native herbivores and livestock, is the predominant land-use across over 40% of the terrestrial surface and influences over 109 Mg of soil-C annually in the world’s dry regions. The interactions between plants, grazers, and soil microbes, is of critical importance for this soil-C pool. However, soil microbial responses to grazing, and associated feedbacks, remain poorly understood. Grazing management policies are unable to adequately accommodate key interactions that are important for effective ecosystem stewardship. After 10-yr of experimental herbivore-exclusion in the semiarid Trans-Himalayan ecosystem, we measured grazer effects on soil microbial abundance in n=30 herbivore exclosures, each paired with an adjacent control plot using substrate-induced respiration, microbial-carbon, and microbial-nitrogen (SIR, MBC, MBN). We found that grazing reduced soil microbial biomass by 13–16%, over the course of the vegetation growing season. But, the strength and direction of grazer effects varied through time at different points in the growing season. Grazing also shifted fungal:bacterial ratio towards dominance by fungi which were more tolerant of periodic dry-down and seasonal fluctuations in soil moisture than bacteria. So, grazer influence on microbial abundance and community composition may collectively play crucial roles in net soil-C dynamics. But, this effect is constrained by environmental factors, such as moisture availability. The projected climatic trend in the Trans-Himalaya is towards progressively wetter conditions, and this may counter grazer effect on microbes, alter microbial communities, and ultimately impact potential soil-C storage. So, accounting for projected changes in precipitation, in addition to managing stocking density of herbivores, may also be crucial for these large soil-C pools.

ASSESSMENT OF GROUNDWATER RESPONSE AND SOIL MOISTURE FLUCTUATIONS IN THE MUGELLO BASIN (CENTRAL ITALY)

Extreme meteorological events such as heavy rainstorms are considered to increase due to global warming. The consequences of such events can be manifold, and might cause massive interferences of the hydrological system of a landscape. Particularly the intramontane basins of the Apennine in Italy are frequently threatened by extreme rainfall events that cause severe damage on buildings and infrastructure. Moreover, the lithological and geomorphological settings of these basins, which depict the products of a complex landscape history, amplify these threats. In order to develop possible mitigation strategies, it is crucial to assess landscape functioning by analysing hydrological processes of the landscape system. In this study, we conducted spatially distributed and dynamic hydrological modelling on a catchment in the intramontane basin of the Mugello valley in Tuscany, Italy. Foremost, measurements of saturated hydraulic conductivity and texture analyses were performed to estimate both infiltration and hydraulic conductivity of the surface and topsoil, respectively. We regionalised the collected data with a stochastic gradient treeboost method for the whole catchment. Soil depth was estimated with a simple sine-cosine-slope relation, whereas, hydropedologic parameters for the hydrological model were estimated with pedotransferfunctions applied on the collected infiltration data. We modelled a period of 100 days, representing each day per time step. A synthetic rainfall period was compiled based on measured data from meteorological stations within the Mugello basin. To produce a reliable synthetic rainfall data set, the estimated precipitation values were set in comparison to calculated return periods for extreme events of all available meteorological station. To assess the diversity of the hydrological response of several locations in the catchment, six semirandom test locations were located on hillslopes and spots were sedimentation is apparent. The results show that groundwater and soil moisture fluctuations appear to be significantly different for both hillslopes and areas were sediments are deposited. The differences cannot be explained by the topographical settings but rather by the approximated thickness of the weathered zone and the spatial diversity of the hydropedological properties of the soil.

Sensing soil moisture and vegetation using GNSS-R polarimetric measurement

GNSS-Reflectometry is an efficient tool for remote sensing and plays a key role in several applications. The estimation of soil moisture and vegetation in the land field is attracting widespread interest in hydrology, climatology and carbon cycles. In order to investigate the scattering polarization properties from different types of surface environments, an airborne measurement was performed, equipped with a new 4-channel prototype for collecting the direct, reflected left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) signals. Both the reflected LHCP and RHCP signals were acquired at the same time by a dual polarization antenna. A data averaging procedure was used to reduce the incoherent part of the received power and two reflected signals were normalized by direct signals obtained from each front-end (FE). Then three polarimetric observables were used to analyze vegetation biomass and soil moisture fluctuations. It was concluded that the polarimetric ratio (PR) is sensitive to soil moisture content (SMC) and considerably independent of roughness and vegetation biomass. The trunk component is confirmed to be the most important factor affecting the amplitude of scattering polarizations. Furthermore, the measurement results showed that the PR variation between different elevation angles was affected by roughness and biomass. The PR variation in forests with big biomass shows the least amount of changes when compared to other geographical environments. The results show another possibility of further geophysical parameter evaluations employing polarimetric applications in GNSS-R.

Inhibition of microbial biofuel production in drought-stressed switchgrass hydrolysate.

BackgroundInterannual variability in precipitation, particularly drought, can affect lignocellulosic crop biomass yields and composition, and is expected to increase biofuel yield variability. However, the effect of precipitation on downstream fermentation processes has never been directly characterized. In order to investigate the impact of interannual climate variability on biofuel production, corn stover and switchgrass were collected during 3 years with significantly different precipitation profiles, representing a major drought year (2012) and 2 years with average precipitation for the entire season (2010 and 2013). All feedstocks were AFEX (ammonia fiber expansion)-pretreated, enzymatically hydrolyzed, and the hydrolysates separately fermented using xylose-utilizing strains of Saccharomyces cerevisiae and Zymomonas mobilis. A chemical genomics approach was also used to evaluate the growth of yeast mutants in the hydrolysates.ResultsWhile most corn stover and switchgrass hydrolysates were readily fermented, growth of S. cerevisiae was completely inhibited in hydrolysate generated from drought-stressed switchgrass. Based on chemical genomics analysis, yeast strains deficient in genes related to protein trafficking within the cell were significantly more resistant to the drought-year switchgrass hydrolysate. Detailed biomass and hydrolysate characterization revealed that switchgrass accumulated greater concentrations of soluble sugars in response to the drought and these sugars were subsequently degraded to pyrazines and imidazoles during ammonia-based pretreatment. When added ex situ to normal switchgrass hydrolysate, imidazoles and pyrazines caused anaerobic growth inhibition of S. cerevisiae.ConclusionsIn response to the osmotic pressures experienced during drought stress, plants accumulate soluble sugars that are susceptible to degradation during chemical pretreatments. For ammonia-based pretreatment, these sugars degrade to imidazoles and pyrazines. These compounds contribute to S. cerevisiae growth inhibition in drought-year switchgrass hydrolysate. This work discovered that variation in environmental conditions during the growth of bioenergy crops could have significant detrimental effects on fermentation organisms during biofuel production. These findings are relevant to regions where climate change is predicted to cause an increased incidence of drought and to marginal lands with poor water-holding capacity, where fluctuations in soil moisture may trigger frequent drought stress response in lignocellulosic feedstocks.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0657-0) contains supplementary material, which is available to authorized users.

Camelina growth and yield response to sowing depth and rate in the northern Corn Belt USA

Camelina (Camelina sativa L.) is gaining interest as a productive alternative oilseed crop for biofuels, bioproducts, and healthy food-use applications. Developing sound agronomic practices for its production is key to optimizing its seed oil yield potential. Plant stand establishment of camelina has been problematic in some environments, which may be related to its small seed size and current recommendation for shallow sowing. Shallow sowing can diminish seed to soil contact and expose seeds to large soil temperature and moisture fluctuations, which greatly affect germination and seedling emergence. A study was conducted in 2011 and 2012 on a Barnes loam soil in western Minnesota to examine the effects of sowing spring camelina at soil depths of 1, 2, and 4cm at sowing rates of 2, 3, and 6kgha−1. Seedling emergence increased with sowing rate, but averaged across depths it had no effect on seed yield. Seedling emergence between the 1 and 2cm sowing depths did not differ in 2012, while stands were reduced by an average of 29% at the 2cm depth in 2011, but seed yields did not differ between the two sowing depths in either year. However, averaged across sowing rates, sowing at 4cm led to a 64% stand reduction and a significant 22% loss of yield compared with the 1 and 2cm depths. Neither sowing depth nor rate affected seed oil content or harvest index. Results show that camelina has exceptional yield compensation capacity at low plant densities and can be sown deeper than commonly recommended (i.e., ≤1.0cm) without compromising yield in the northern Corn Belt. Sowing to a depth of 2cm may be beneficial for certain soils prone to large temperature and moisture fluctuations.