Reduced Soil Temperature Research Articles

Moss regulates soil evaporation leading to decoupling of soil and near-surface air temperatures

PURPOSE: Moss covers a vast area from the polar to the tropics on a global scale and has important regulatory effects on the biogeochemical processes in the soil. Previous studies had shown that moss on soil could reduce soil temperature in the warm period and plays an important role in the stability of the soil carbon pool in the context of global warming. The cooling effect of soil temperature by moss may be due to its ability to accelerate soil water evaporation. Most vascular plants achieve limited homeothermy by transpiration, but moss has no organs which can regulate transpiration. Can moss also regulate soil water evaporation as the transpiration of vascular plants? The objective of this study was to determine the differences in temperature and evaporation between moss-covered and bare soil. MATERIALS AND METHODS: A year-round mesocosm experiment was carried out to investigate soil temperature and evaporation with and without moss. We used a high time-resolution instrument which could simultaneously monitor soil temperature beneath moss layers, photosynthetically active radiation, and near-surface air temperature (20 cm above the soil) in a 10-min interval. In the meantime, we used a chamber method to monitor soil evaporation and heat flux inside the mesocosms simultaneously. RESULTS AND DISCUSSION: We found that the evaporation of the moss increased drastically when near-surface air temperature exceeded 30 °C, which kept maximum soil temperature around 30 °C. This finding was different from many previous studies which reported that the evaporation of moss was always greater than bare soil or similar to open water surface. The phenomenon we found may be important for moss, which allowed moss to minimize the loss of water at high temperature and maintain a relatively constant temperature. It is known that moss is a C₃ plant, and the high-temperature threshold for the photosynthesis of C₃ vascular plants is also around 30 °C. Thus, the temperature-regulating behavior could improve the net carbon gain for moss and benefit for its survival. To our best knowledge, there is no research reporting this phenomenon. In the context of global warming, the temperature-regulating behavior of moss is very important for its controls on soil carbon dynamics and its other ecological functions, especially at lower latitudes with higher soil temperature. CONCLUSIONS: Our result showed that moss can also achieve limited homeothermy through soil water evaporation similarly as the vascular plant through transpiration, and the temperature threshold was around 30 °C. This homeothermy led to the decoupling of soil temperature and near-surface air temperature. How mosses elevate water evaporation in response to high temperature remains to be a challenge for future research.

Effects of afforestation on soil and ambient air temperature in a pair of catchments on the Chinese Loess Plateau

Climate models and satellite remote sensing have been used to determine the effects of afforestation on soil temperature at regional and global scales. However, the coarse spatial resolution of both methods have made them insensitive to local topography, which is a controlling factor of the surface energy budget, especially in areas with complex topography. While typically applied only locally at representative sites, long-term field measurements can shed light on the role of topography on soil temperature after afforestation and provide direct evidence to verify and calibrate the results from modeling and satellite observation. In this study, a pair of neighboring catchments (one afforested and the other with natural regrowth of grasses) on the Chinese Loess Plateau (CLP) was selected to assess the effects of afforestation on soil and ambient air temperature at different slope positions with an entire year of continuous measurements collected every 10 min. The results showed that the uphill slope in both catchments experienced higher soil and air temperatures than the downhill gully, where less solar radiation was received at the ground surface due to canopy interception and topographic shading. For example, the annual average soil (10–100 cm depth) and air temperature in the uphill slope in the forestland catchment was 1.09 and 1.22 °C higher than in the downhill gully, respectively. The effects of topography on soil and air temperature varied at different times of the year, and these effects interacted with the growth status of vegetation. In winter and spring, topography significantly affected soil and air temperature due to varied solar radiation received at different slope positions. By contrast, in summer, the effect of vegetation increased along with higher evapotranspiration and more solar radiation interception by the canopy, especially in the forestland catchment, which showed cooler soils than the other catchment during both the days and nights at all investigated slope positions. However, compared with the grassland catchment, the uphill slope of the forestland catchment had cooler air temperatures during the day, but warmer temperatures at night. We have concluded that both topography and afforestation influence the spatial variation and temporal dynamics of soil and air temperature, and they thereby the surface energy balance of the CLP. More local studies are warranted in order to continue to calibrate regional models and remote sensing data.

Soil moisture, wind speed and depth hoar formation in the Arctic snowpack

ABSTRACTBasal depth hoar that forms in Arctic snowpacks often has a low thermal conductivity, strongly contributing to the snowpack thermal insulance and impacting the permafrost thermal regime. At Ward Hunt Island (Canadian high Arctic, 83°05′N, 74°07′W) almost no depth hoar was observed in spring 2016 despite favorable thermal conditions. We hypothesize that depth hoar formation was impeded by the combination of two factors (1) strong winds in fall that formed hard dense wind slabs where water vapor transport was slow and (2) low soil moisture that led to rapid ground cooling with no zero-curtain period, which reduced soil temperature and the temperature gradient in the snowpack. Comparisons with detailed data from the subsequent winter at Ward Hunt and from Bylot Island (73°09′N, 80°00′W) and with data from Barrow and Alert indicate that both high wind speeds after snow onset and low soil moisture are necessary to prevent Arctic depth hoar formation. The role of convection to form depth hoar is discussed. A simple preliminary strategy to parameterize depth hoar thermal conductivity in snow schemes is proposed based on wind speed and soil moisture. Finally, warming-induced vegetation growth and soil moisture increase should reduce depth hoar thermal conductivity, potentially affecting permafrost temperature.

Intercropping with two native woody shrubs improves water status and development of interplanted groundnut and pearl millet in the Sahel

To investigate the physiological responses of groundnut (Arachis hypogea) and pearl millet (Penisetum glaucum) that were intercropped with the native evergreen woody shrubs Piliostigma reticulatum (D.C.) Hochst and Guiera senegalensis J.F. Gmel compared to control crops throughout two growing seasons at two sites with contrasting climate and soil types in Senegal. Shrubs grown in groundnut and millet fields at higher than native density were coppiced annually with aboveground biomass returned to the soil and no additional fertilizer. Crop leaf area index (LAI), handheld normalized difference vegetation index (NDVI), leaf water potential, and soil moisture and temperature were monitored in 2012–2013. At the drier site, the presence of shrubs reduced soil temperature at 5 cm depth by up to 5 °C during early crop growth. Shrub presence increased LAI by up to 266%, NDVI by up to 217% and increased groundnut leaf water potential throughout the day at the wetter site. Shrub effects on crop physiology were stronger overall at the drier site. These results improve the understanding of how this unique agroforestry system alters the growing environment and the physiological response of associated crops throughout the season.

A proxy-year analysis shows reduced soil temperatures with climate warming in boreal forest

Scientists unequivocally agree that winter air temperature (TA) in northern high latitudes will increase sharply with anthropogenic climate change, and that such increases are already pervasive. However, contrasting hypotheses and results exist regarding the magnitude and even direction of changes in winter soil temperature (TS). Here we use field and satellite data to examine the ‘cold soil in a warm world’ hypothesis for the first time in the boreal forest using a proxy year approach. In a proxy warm year with a mean annual temperature similar to that predicted for ~2080, average winter TS was reduced relative to the baseline year by 0.43 to 1.22 °C in open to forested sites. Similarly, average minimum and maximum winter TS declined, and the number of freeze-thaw events increased in the proxy warm year, corresponding to a reduction in the number of snow-covered days relative to the baseline year. Our findings indicate that early soil freezing as a result of delayed snowfall and reduced snow insulation from cold winter air are the main drivers of reduced winter active-layer TS (at ~2-cm depth) under warming conditions in boreal forest, and we also show that these drivers interact strongly with forest stand structure.

Root-zone Cooling Evaluation Using Heat Pump for Greenhouse Strawberry Production

Ground-source heat pumps (GSHPs) have been used to chill water to facilitate cooling of ‘Natsuakari’ strawberry (Fragaria ×ananassa) grown within containers during the summer. Two types of soil containers and cooling systems have been considered. In one system, cold-water tubes were placed under as well as over the top of the soil, whereas the other cooling system used cold water passing through tubes placed under the soil and within the irrigation channel to facilitate bottom irrigation. The cooling efficiency of each system was evaluated by observing temperature relationships between greenhouse air and soil. The relationship was represented by means of an elliptic curve, the geometric center and tilt angle of which indicated representative daily soil temperatures and degree of temperature stability, respectively. Both values were observed to be lower for the bottom irrigation system during the two plant growth periods considered in this study, thereby indicating that colder and relatively constant soil temperatures can be maintained via greater heat convection. This greater cooling method was facilitated by rapid transfer of cold water through the bottom irrigation channel into the root zone, resulting from reduction in soil moisture content induced by plant transpiration in addition to heat conduction from the soil to the cooling tube. Measured soil temperatures for the buried-tube system were observed to be coldest when the tube was chilled considerably (9.4 °C). Although the setup of the considered bottom watering system was rather sensitive in that the system required maintenance of a constant water level throughout the container, both systems effectively produced cooler soil temperatures compared with the case in which no GSHP was used.

Effect of long-term straw mulching and no-tillage on physical properties of meadow soil in cold region.

The effects of different straw returning technology and farming system on soil physical properties is not clear in Heilongjiang Province, which is located in middle temperature zone with large amount of straw. Here, the effects of straw mulching on soil physical properties in meadow soil under no tillage conditions were studied in field experiments for eight consecutive years (2010-2017). The no straw covering (0%), 30% coverage (30%), 60% coverage (60%) and 100% coverage (100%) were disposed in the experiment. The results showed that straw mulching under no tillage significantly increased soil bulk density by 0.10-0.20 g·cm-3, which increased with the increases of coverage amount. Straw mulching increased soil solid fraction by 2.5%-7.8%. Soil temperature decreased with the increases of coverage amount, and this trend was more apparent on the surface of soil. The temperature reduction in 0-5 cm soil layer was 1.87-2.90 ℃. Soil water content significantly increased with the increases of straw mulching, with an enhancement of 6.4%-10.2%. Straw mulch decreased the total porosity and diameter of >0.05 mm aeration pores, increased the effective pore size of 0.05-0.002 mm, with the magnitude of such effects being positively dependent on coverage amount. There was no significant effect of straw mulching on inactive porosity of soil. Long-term straw mulching increased soil compaction and soil moisture, reduced soil temperature and total soil porosity, and increased soil available porosity in 0-5 cm soil layer.

Use of Cardboard Disks for Crowning Seedlings in Reforestation

ABSTRACT The present study aimed to evaluate the effect of chemopreventive or water-repellent substances on the durability of cardboard disks used to crown forest tree species. The experiment presented a randomized block design with four repetitions and four sampling times (63, 133, 260 and 365 days) and was installed in Seropedica, RJ, in an area with Urochloa humidicola . Bamboo poles were used to simulate seedlings in the field and define the position of the experimental units. The treatments consisted of in natura cardboard, or treated with CuSO4 solution, pinus shellac, CuSO4 + pinus shellac, CuSO 4 + Sika®, pinus shellac + Sika®, and a second control with manual crowning. The cardboard crowning was effective at suppressing grass growth over all periods evaluated, independent of the treatment. After a year in the field, the cardboard disks treated with CuSO4 solution presented higher physical integrity with only 25% mass loss, against 60% mass loss in the control. The cardboard crowning also reduced soil temperature at the crown area by up to 15 °C in the 0 to 10 cm layer, in comparison with manual crowning. The results of this study indicate that the cardboard crowning method is efficient and has the potential to reduce the cost of weed control in reforestation.

Sand mulching and its relationship with soil temperature and light environment in the cultivation of Lilium longiflorum cut flower

Abstract Lilium longiflorum is one of the most important cut flowers both in the world and in Argentina. To plan a commercial crop, it is necessary to understand tools that relate environmental variables to crop growth and development. One of these tools is leaf expansion, which can be determined from the evolution of the leaf area index (LAI). The technique known as sand mulching substantially modifies the soil temperature but can also alter other environmental variables such as light quality. The objective of this work was to determine whether the application of sand mulching affects the growth rate and leaf expansion of a lilium cut flower crop. The experiment was carried out in a greenhouse at the Faculty of Agronomy, University of Buenos Aires, Argentina, where a lilium crop was planted in the soil, adding a 5-cm layer of fine sand over the entire soil surface to reduce soil temperature. Plant height and number of leaves were measured twice a week and the LAI was estimated by spectral reflectance once a week. The cycle length was also recorded, and the accumulation of dry matter was measured at the end of the experiment. The soil temperature was lower in the first month of cultivation, and there were differences in the light environment and in the water status of the crop. Although there were no significant differences in the cycle length or leaf appearance, sand mulching led to differences in the stem growth rate, LAI and final plant height, resulting in a better quality of the product.

Effects of biochar on spatial and temporal changes in soil temperature in cold waterlogged rice paddies

Soil temperature is an important factor constraining plant growth, large fluctuations of which generally result in low crop yield. However, the investigation of the effect of biochar on soil temperature, especially in cold waterlogged paddies, has been limited. This study investigated the effect of bamboo biochar (BB), rice straw biochar (RB), and rice straw (RS) on temporal variations in soil temperature at different depths, compared with no exogenous organic matter amendment soil as control treatment (CK), in a cold waterlogged paddy in Zhejiang Province, China. A logger with probes was used to record soil temperatures at the depths of 5 cm, 10 cm, and 20 cm at 08:00 h, 14:00 h, and 22:00 h daily from July 1 to Oct 31, 2012. Soil physicochemical properties and rice yield were analyzed after the harvest. The results showed that the application of BB and RB significantly reduced the soil temperature at 5- and 10-cm depths by 0.11 °C–0.21 °C on average, whereas RS significantly reduced soil temperature at a 5-cm depth by 0.19 °C and increased the average temperature at a 20-cm depth by 0.06 °C. Moreover, the average soil temperature related to BB, RB, and RS treatment significantly increased at 08:00 h during the rice-growing season by 0.15 °C, 0.43 °C, and 0.52 °C and significantly decreased at 14:00 h by 0.51 °C, 0.73 °C, and 0.87 °C, respectively, as compared with that in CK. Overall, both biochar and RS reduced the differences between day and night soil temperatures by 0.66 °C–1.39 °C, thereby regulating diurnal soil-temperature fluctuations, especially at a 5-cm depth. The reduced differences were likely attributed to the relatively lower bulk density (8.87–17.1%) and higher water content (9.75–14.3%) in the biochar-amended soil as compared with that in control soil. Furthermore, RB had a significant stronger positive effect on grain yield than BB, because it regulated soil temperature more effectively. These results suggested the feasibility of applying RB to cold waterlogged paddies to improve soil properties and crop yield.

Effect of tillage and crop residue on soil temperature following planting for a Black soil in Northeast China

Crop residue return is imperative to maintain soil health and productivity but some farmers resist adopting conservation tillage systems with residue return fearing reduced soil temperature following planting and crop yield. Soil temperatures were measured at 10 cm depth for one month following planting from 2004 to 2007 in a field experiment in Northeast China. Tillage treatments included mouldboard plough (MP), no till (NT), and ridge till (RT) with maize (Zea mays L.) and soybean (Glycine max Merr.) crops. Tillage had significant effects on soil temperature in 10 of 15 weekly periods. Weekly average NT soil temperature was 0–1.5 °C lower than MP, but the difference was significant (P < 0.05) only in 2007 when residue was not returned in MP the previous autumn. RT showed no clear advantage over NT in increasing soil temperature. Higher residue coverage caused lower soil temperature; the effect was greater for maize than soybean residue. Residue type had significant effect on soil temperature in 9 of 15 weekly periods with 0–1.9 °C lower soil temperature under maize than soybean residue. Both tillage and residue had small but inconsistent effect on soil temperature following planting in Northeast China representative of a cool to temperate zone.

Niche partitioning of intertidal seagrasses: evidence of the influence of substrate temperature.

The influence of soil temperature on rhizome depths of four intertidal seagrass species was investigated in central Queensland, Australia. We postulated that certain intertidal seagrass species are soil temperature-sensitive and vertically stratify rhizome depths. Below-ground vertical stratification of intertidal seagrass rhizome depths was analysed based upon microclimate (soil temperature) and microhabitat (soil type). Soil temperature profiles exhibited heat transfer from surface layers to depth that varied by microhabitat, with vertical stratification of rhizome depths between species. Halodule uninervis rhizomes maintain a narrow median soil temperature envelope; compensating for high surface temperatures by occupying deeper, cooler soil substrates. Halophila decipiens, Halophila ovalis and Zostera muelleri rhizomes are shallow-rooted and exposed to fluctuating temperatures, with broader median temperature envelopes. Halodule uninervis appears to be a niche specialist, with the two Halophila species considered as generalist niche usage species. The implications of niche use based upon soil temperature profiles and rhizome rooting depths are discussed in the context of species' thermal tolerances and below-ground biomass O2 demand associated with respiration and maintenance of oxic microshields. This preliminary evidence suggests that soil temperature interaction with rhizome rooting depths may be a factor that influences the distribution of intertidal seagrasses.

Prescribed Fire in Grassland Butterfly Habitat: Targeting Weather and Fuel Conditions to Reduce Soil Temperatures and Burn Severity

Prescribed burning is a primary tool for habitat restoration and management in fire-adapted grasslands. Concerns about detrimental effects of burning on butterfly populations, however, can inhibit implementation of treatments. Burning in cool and humid conditions is likely to result in lowered soil temperatures and to produce patches of low burn severity, both of which would enhance survival of butterfly larvae at or near the soil surface. In this study, we burned 20 experimental plots in South Puget Sound, Washington, USA, prairies across a range of weather and fuel conditions to address the potential for producing these outcomes. Risk to diapausing butterfly larvae, assessed by measuring subsurface soil temperatures and heat dosages, was lower when air temperature was less than 26°C and dead fuel moisture was greater than 9 %. The likelihood that unburned or low-severity patches would be left was affected by dead fuel moisture, but also required pre-existing fuel discontinuities. Burns conducted in the morning hours during the summer drought season (the main prescribed-fire season in this system) were cooler and had lower severities. This research increases our understanding of how fine fuel moisture and fuel continuity during grassland burning can affect fire intensity, severity, and spread. It also provides support for burning earlier in the day as a way to increase burn heterogeneity and has allowed us to create recommendations for burning in sensitive butterfly habitat.

Effects of tillage and mulching measures on soil moisture and temperature, photosynthetic characteristics and yield of winter wheat

In order to find out the appropriate measures for soil water preservation and wheat yield increase, a three–year field experiment was conducted in Yuzhou Experimental Station of Water Saving Agriculture in the west of Henan Province, China, during the growing seasons of 2014–2015, 2015–2016 and 2016–2017 to study the effects of different tillage modes and soil mulching measures on dynamics of soil moisture and temperature, photosynthetic characteristics and yield of winter wheat (Triticum aestivum L.). The results showed that straw coverage and film mulching measures under the conventional tillage and no tillage modes increased soil moisture by 4.4%–27.3%,13.4%–41.8% and 6.2%–46.7%, 2.4%–80.2%, respectively. The treatments of soil surface coverage measures under both conventional tillage and no tillage modes at tillering stage increased the temperature of soil layer at 10cm and 15cm depth by 9.1%–18.2% and 10.0%–40.0% in 2014–2015, while mulching measures under the conventional tillage obviously increased soil temperature at wintering stage but decreased soil temperature at booting and mature stages in 2016–2017. However, mulching measures under no tillage mode reduced soil temperature in shallower layer during seedling establishment stage in the three–year period of 2014–2017. Moreover, the two mulching measures raised net photosynthetic rate (Pn) and water use efficiency (WUE) of functional leaves under the conventional tillage mode and reduced transpiration rate (Tr) at the filling stage. Except straw coverage measures under no tillage in 2015–2016, the combination of mulching measures under no tillage reduced Pn, Tr and WUE of wheat leaves at filling stage as compared to those without mulching measures. Under the conventional tillage mode, straw coverage increased wheat yield by 6.4%–9.3%, mainly due to the increase in the number of grains per panicle. Under no tillage mode, both straw coverage and mulching measures reduced wheat yield to different extents and film mulching measures was more effective than straw coverage. The combination of conventional tillage and straw mulching resulted in a yield of 8425.9–8888.9kghm−2, so this combination was recommended as the optimal treatment under the conditions set in this study.

Spatial Heterogeneity of Soil Respiration Response to Precipitation Pulse in a Temperate Mixed Forest in Central China

Water availability is one of the fundamental drivers for biological activities and terrestrial carbon cycling. Although the response of soil respiration to precipitation has been well documented in arid and semiarid ecosystems, our understanding of its pattern in forests is rather limited. This study was conducted to examine the difference of precipitation effect on soil respiration under different canopy conditions in a temperate coniferous (Pinus armandii Franch) and broadleaved (Quercus aliena var. acuteserrata) mixed forest in Central China. The results showed that precipitation significantly reduced soil temperature, but increased soil volumetric water content and soil respiration (6.0%-35.3%). Precipitation caused a greater increment in soil respiration beneath the canopy of broadleaved trees (24.2%) than that beneath coniferous ones (13.5%). Precipitation-induced increase in soil respiration was consistently lower beneath the canopy of small size classes (7.1%-32.6%) than large size classes (9.5%-33.3%). Mean soil respiration of forest gaps increased 22.4% following precipitations. Our study highlights the positive response of soil respiration to precipitation pulses in water-unlimited ecosystems. The findings suggest that the spatial heterogeneity of soil respiration to precipitation pulse under different canopy conditions should be emphasized while assessing terrestrial carbon cycling and its feedback to climate change.

Open‐top chambers for temperature manipulation in taller‐stature plant communities

Abstract Open‐top chambers simulate global warming by passively increasing air temperatures in field experiments. They are commonly used in low‐stature alpine and arctic ecosystems, but rarely in taller‐stature plant communities because of their limited height. We present a modified International Tundra Experiment (ITEX) chamber design for year‐round outdoor use in warming taller‐stature plant communities up to 1.5 m tall. We report a full year of results for the chambers' effects on air and soil temperature, relative humidity, and soil moisture in a northern hardwood forest clearing and a southern early successional grassland site located in Michigan, USA. Detailed construction plans are also provided. The chambers elevated daytime air temperatures at 1 m height by 1.8°C above ambient levels, on average over an entire year, at both the northern and southern site. The chambers did not affect relative humidity at either site. The chambers did not alter average soil temperature or moisture at the northern site and reduced soil temperatures and soil moisture at the southern site. The chambers increased variability in soil freeze/thaw cycles at both sites. The chambers achieved predicted levels of warming for mid‐century (2046–2065) scenarios consistent with the majority of representative concentration pathways in the International Panel on Climate Change Fifth Assessment Report, with minimal experimental artefact. This design is a valuable tool for examining the effects of in situ warming on understudied taller‐stature plant communities and creates the opportunity to expand future comparisons across a diversity of systems.

Mulching type-induced soil moisture and temperature regimes and water use efficiency of soybean under rain-fed condition in central Japan

Soybean (Glycine max) is a high water-demand crop and grown under moderate temperature in Japan. To protect the crop from hot summer and to utilize rainfall for its cultivation, selection of appropriate mulching material(s) is crucial. For optimum production of the crop, soil moisture and temperature regimes as well as water use efficiency (WUE) of the crop were investigated under straw, grass, paper, plastic and bare soil (control) mulching under rain-fed condition at Gifu university farm in Japan. The mulching treatments, compared to the control, lowered soil temperature by 2°C at 5cm depth and 0.5°C at 15 and 25cm depths. The plastic and straw mulching stored the highest quantity of soil moisture at 5 and 15cm depths; the bare soil stored the lowest quantity. At 25cm depth, soil-moisture content was the highest under paper mulch but invariable under the other mulches. Plastic mulching reduced evaporation rate from the soil surface and, consequently, the reduced soil-water consumption (SWC) from the root zone augmented WUE of soybean. The paper mulching, by conserving soil-moisture and reducing soil temperature, provided better crop growth attributes, while the plastic mulching improved WUE of green soybean. Therefore, the plastic mulch performed best in reducing soil-water consumption and increasing WUE, while the paper mulch was good for soil-moisture conservation and temperature modification that increased soybean yield.

Training of Growers and Extension Agents in the Dominican Republic on Managing Heat Stress of Bell Pepper (Capsicum annum L.) Grown in High Tunnels

The Dominican Republic is located in the Hispaniola Island in the Caribbean region. Its climate differs substantially over short distances and is the most diverse of the Caribbean. Annual average temperature is 25 °C, ranging from 18 °C (at higher elevations) to 28 °C (at sea level). Average annual rainfall is 1500 mm. Although sugar, coffee, cocoa, and tobacco are the principal cash crops of the country, peppers are increasing in popularity on the country’s exportation market. Bell peppers grown in high tunnels and greenhouses may be impacted by high temperatures that result in reduced fruit yield and quality. In this article, we make a short review of cooling techniques for high tunnels and share the experience of training bell pepper growers and extension personnel from the Dominican Republic regions of San José de Ocoa and Constanza on how to manage heat stress of bell peppers grown inside high tunnels. Bell pepper plants inside high tunnels showed symptoms of heat stress, including reduced plant and fruit size, small fruit number, and a mild leaf chlorosis. In both regions, but particularly in the warmer region, San José Ocoa, because of occurrences of high temperatures inside the high tunnels, growers were advised to use well-ventilated high tunnels, such as structures with plastic film on the top and screen net on the sides. Other strategies such as shade nets (30% to 40% shade) placed on top of the high tunnel or whitewash paint applied on the high tunnel cover may also help reduce air and soil temperatures inside the tunnel. Use of white or silver reflective mulch, instead of the commonly used black mulch, may provide additional reduction of soil temperature. In conclusion, high tunnel structures developed for temperate regions, with limited to poor ventilation, may result in excessively high temperatures inside the high tunnels and thus were not recommended for the Dominican Republic regions of this study. More research is necessary on adequate high tunnel design and cooling techniques for high tunnel production, particularly in tropical regions.

LEGUMINOUS COVER CROPS FOR BANANA PLANTATIONS IN SEMI-ARID REGIONS

ABSTRACT High temperatures and low rainfall characterize the Brazilian semiarid regions. This regional climate demands the adoption of practices that increase the efficiency and sustainability of local farming. This study aimed to assess the ability of two perennial herbaceous leguminous species, calopo and tropical kudzu, to provide permanent soil cover in banana plantations in Jequitinhonha Valley, northeast Minas Gerais state, Brazil. To this end, we evaluated the differences of calopo and tropical kudzu in soil cover capacity and the amount of senescent phytomass deposited on the soil surface, nutrient content in senescent phytomass, as well as their effects on temperature and soil moisture, compared with bare soil in two experimental sites. The results showed that, compared with tropical kudzu, calopo had a higher soil cover capacity and was more effective at increasing organic material and nutrients in the soil owing to the relatively higher amount of senescent phytomass deposited on the soil surface. However, both calopo and tropical kudzu reduced soil temperature and increase soil moisture compared with bare soil. Overall, we concluded that these species can deposit high levels of senescence in the soil, providing several benefits to the cultivation system of banana plants in the semiarid regions.

Effects of snow absence on winter soil nitrogen dynamics in a subalpine spruce forest of southwestern China

The lack of snow cover due to winter climate change has great potential to impact winter soil nitrogen cycling in boreal forests. A snow manipulation was conducted in a Tibetan spruce forest to explore the effects of snow absence on winter soil nitrogen dynamics by shelter method. Snow absence on average reduced soil temperatures at the depths of 0cm and 5cm by 1.44°C and 0.33°C, respectively, throughout the winter. Moreover, snow absence increased soil frost and freeze-thaw cycles. Soil net nitrogen mineralization and labile nitrogen pools (ammonium, nitrate and dissolved organic nitrogen) were higher in the snow absence plots compared to control plots. Snow absence increased soil microbial biomass carbon but did not affect microbial biomass nitrogen. Nevertheless, soil enzyme activities involved in nitrogen cycles were often lowered by snow absence over the winter. The results noted in this study suggest that warming-induced absence in seasonal snowpack may stimulate winter soil nitrogen availabilities by changing soil microhabitats, which has important implications for soil biogeochemical cycles in the subalpine forest ecosystems on the eastern Tibetan Plateau.