Tomoyoshi Hirota

Soil Frost Control: Its Application to Volunteer Potato Management in a Cold Region

The earlier onset of persistent snowcover since the late 1980s has narrowed the time window for soil-surface cooling without insulating snowcover, drastically reducing the soil frost depth in eastern Hokkaido, Japan. In crop fields managed by rotation, small potato tubers left unharvested in the fall survive the winter and emerge as weeds during spring–summer (volunteer potatoes). To eliminate them, soil frost depths are manipulated by artificially controlling snowcover thickness, guided by numerical soil-temperature model prediction. Field trials demonstrated that soil frost depths were predicted within accuracy of several centimeters. The optimal soil frost depth of 0.3–0.4 m is proposed as a compromise between the elimination of volunteer potatoes and permissible soil frost depth to prevent negative effects on agriculture in the following spring. The numerical model also facilitates decision-making related to the work schedule of snow plowing practices (yukiwari in Japanese). This method is adopted by local potato producers, who manage farmland on a large scale. This method represents a new agricultural technology that is useful for adaptation to climate change.

Effects of saturated hydraulic conductivity on volunteer potato (Solanum tuberosum L.) tuber survival

Abstract In situ observations were conducted at eight fields in the Tokachi region of the northernmost island of Japan to assess the effects of soil physical conditions on the survival of volunteer potato (Solanum tuberosum L.) tubers. Statistical analyses revealed a positive correlation between the survival rate and the lowest value of saturated hydraulic conductivity to a depth of 0.5 m, except for data from one field where potato tubers were killed by low temperatures below –3°C during winter and from another field with poor drainage despite high soil permeability to 0.5 m in depth. Incubation experiments indicated that potato tubers survived in saturated soil at 0 and 6°C, which respectively matched soil temperatures during the snowmelt period and 1 week thereafter. This result suggests that potato tubers can survive prolonged exposure to moist surface soil caused by large amounts of snowmelt water during the spring snowmelt period. The incubation experiment results also indicated that potato tubers were killed in saturated soil at 15°C. This temperature was almost equal to the average air temperature during the period when volunteer potato tubers were sprouting in the field. Therefore, this result implies that the potato tubers will be killed after a substantial amount of rainfall during the subsequent warm period at poorly drained fields.

Water and nitrate movements in an agricultural field with different soil frost depths: field experiments and numerical simulation

Abstract To evaluate water and anion movement in an agricultural field in different frost conditions, a paired-plot field experiment was conducted at an agricultural site in northern Japan where a significant decrease in the frost depth has occurred during the past 20 years. Snow cover was removed to enhance soil freezing in one plot (treated plot), whereas natural conditions were maintained in a control plot. The maximum frost depth was 0.43 m in the treated plot and 0.11 m in the control plot, which induced substantial differences in water movement throughout the winter. A substantial amount of water moved upward before the onset of snowmelt. However, nitrate did not move markedly before the snowmelt period in either plot. The amount of snowmelt infiltration in the control plot was larger than in the treated plot. Correspondingly, the peak of nitrate content in the control plot was deeper than that in the treated plot after the snowmelt period. Soil freezing, snow accumulation and snowmelt processes were simulated reasonably well using a one-dimensional numerical model: Simultaneous Heat and Water (SHAW). Nevertheless, the model performed poorly for simulating soil thawing and soil water movement, suggesting a need for improvement.

Optimum soil frost depth to alleviate climate change effects in cold region agriculture

On-farm soil frost control has been used for the management of volunteer potatoes (Solanum tuberosum L.), a serious weed problem caused by climate change, in northern Japan. Deep soil frost penetration is necessary for the effective eradication of unharvested small potato tubers; however, this process can delay soil thaw and increase soil wetting in spring, thereby delaying agricultural activity initiation and increasing nitrous oxide emissions from soil. Conversely, shallow soil frost development helps over-wintering of unharvested potato tubers and nitrate leaching from surface soil owing to the periodic infiltration of snowmelt water. In this study, we synthesised on-farm snow cover manipulation experiments to determine the optimum soil frost depth that can eradicate unharvested potato tubers without affecting agricultural activity initiation while minimising N pollution from agricultural soil. The optimum soil frost depth was estimated to be 0.28–0.33 m on the basis of the annual maximum soil frost depth. Soil frost control is a promising practice to alleviate climate change effects on agriculture in cold regions, which was initiated by local farmers and further promoted by national and local research institutes.

Winter Nocturnal Air Temperature Distribution for a Mesoscale Plain of a Snow-Covered Region: Field Meteorological Observations and Numerical Simulations

AbstractWinter air temperatures strongly affect crop overwintering and cold resource usage. To clarify how winter air temperature distributions are formed in a mesoscale plain, field observations and simulations were conducted for the Tokachi region in Japan. Results elucidating the winter climate within the plain revealed that the winter mean air temperature at each site was correlated closely with the mean daily minimum air temperature. The daily minimum air temperature was not correlated with altitude, suggesting that local variation of the daily minimum temperature influences the temperature distribution. Observations at different distances from the upwind mountains revealed that nocturnal air temperatures were higher for stronger winds closer to the mountain foot. Low temperatures associated with wind speed suggest that radiative cooling strongly affects the temperature distribution. Wind and temperature conditions in the boundary layer influence the degree of drop in nocturnal air temperature and it...

Estimating NEE in a wheat-planted plot with an automatically controlled chamber

Abstract We observed carbon dioxide (CO2) flux at two experimental plots (wheat (Triticum aestivum L.) -planted and bare) for a year using an automatically controlled chamber. At each plot, two chambers were installed at six observation points by rotation. Consequently, the total installment duration at each point was one-third of the entire experimental period. Although we manually moved the chambers periodically, they hampered wheat growth and reduced the dried weight of harvested wheat by 65%. However, they did not influence the carbon (C) content ratio of harvested wheat. The rate of decrease of soil water contents after rainfall in the wheat plot was higher than that in the bare plot, especially after the canopy height reached around 30 cm. The maximum gap of soil water content at 5 cm depth between the two plots was about 5%. Wheat mitigated the increase of soil temperature in the daytime. The gap of soil temperature at 2 cm depth between the two plots sometimes exceeded 10°C. Considering the difference between dried weights of harvested wheat per unit ground area inside and outside the chamber collar, the annual net ecosystem exchange (NEE), whole ecosystem respiration and gross primary production were estimated respectively as –103 g C m−2 y−1, 831 g C m−2 y−1 and–934 g C m−2 y−1. The absolute values of each were smaller than those reported from past studies. Adding the exported carbon of harvested wheat (364 g C m−2) and subtracting the imported carbon of the seeds (3.1 g C m−2) to the NEE, net biome production across the ground surface was 259 g C m−2. It was greater than that in the bare plot (187 g C m−2). Although further improvements of measurements and more accurate estimated equations are necessary to evaluate the carbon budget correctly with chamber measurements, our chamber measurement captured the NEE variation, responding to seasonal, meteorological and biological changes.

Possible Change of Water and Nitrate Cycles Associated with the Frost-Depth Decrease Under Climate Change

Amounts of snowfall in early winter have increased in recent years because of climate change, thereby insulating soil from cold air and decreasing the soil frost depth during the last 20 years in the Tokachi region of northernmost Japan. To reveal soil water movement in recent winters, field observations were conducted for 5 years at an experimental field in this region. Soil froze to 0.2 m during two winters and a frozen soil layer was absent in three other periods. Substantial amounts of water infiltrated to the deep soil layer in both frozen and unfrozen winters, suggesting that the thin frozen layer did not impede the snowmelt infiltration in recent years. To simulate past soil frost conditions, we removed snow in early winter and replaced it after the soil frost depth reached the desired value. An approximately 0.4-m-thick frozen layer greatly impeded snowmelt infiltration during the spring snowmelt period. During the spring snowmelt period, the nitrate at the control plot, where natural snow conditions were maintained throughout winter and maximum soil frost depth was approximately 0.1 m, was carried deeper than that at the snow-removal treatment plot because of the greater amount of snowmelt infiltration at the control plot. These results imply a dramatic change of snowmelt infiltration and nitrate movement during the spring snowmelt period in this region, presumably resulting from climate change.