We developed a numerical model to simulate N flow in an agricultural paddy field and upland field area and analysed scenarios for recycling the agricultural runoff, including field drainage, from an agricultural area with an irrigation/drainage system. In it, we considered N removal in paddy fields, a regulating reservoir, and canals. The results of analysis indicated that a large amount of the effluent load occurred during the transplanting period and just after fertilisation, and that recycling could reduce the effluent N load. In the case where paddy fields occupied the area, the effluent N load would be equal to the inflowing N load (net zero effluent) at a 48 % recycling rate. However, in the case where upland fields (33 %) and paddy fields (67 %) occupied, 95 % recycling rate was needed to achieve the net zero effluent. To implement the knowledge for reduction of effluent load from an agricultural area, we should solve the matter that the water environment deteriorates as pollutants accumulate with intensive recycling of runoff.
Increase of reactive nitrogen associated with production of food and feed has increased N load into aquasphere and atmosphere. This has induced the problems such as eutrophication, acidification and global warming. What will be going on in future? In order to predict it and to mitigate food production managements, monitoring of N loads in various scales is required. How shall we conduct them? This paper will focus its discussion on the monitoring of NO3--N load from soils based on the previous studies which were conducted in the different scales, such as soil structure level, field level and basin level.
An overlay of cattle manure compost, several centimeters thick, on a lowland soil-column with a depth of 24 cm, and watering at a rate of 39 mm a day resulted in significant reduction of water-percolation in one or two weeks. With watering, large amounts of dissolved or dispersed organic matter and K+ were released from the compost. The K+ was exchanged with Ca2+ in the soil column and the degree of Ca2+-saturation of the organic matter increased with downward movement and the organic matter was retained in the soil. The precipitated or flocculated organic matter with Ca2+ might have clogged the capillary pores of the soil. Formation of bubbles due to biological activities and swelling of soil colloids with increased K+ saturation might also have been involved in the reduction of water-percolation. No significant reduction of water-percolation occurred when an Ap horizon soil of nonallophanic Andisol was placed in the column. Ion exchange of Ca2+ with K+ proceeded less in the nonallophanic Andisol than in the lowland soil. Aggregates are more stable and bulk density was lower in the nonallophanic Andisol than in the lowland soil. These properties were favorable for the nonallphanic Andisol to keep its high water-percolation. Other changes in soil chemical properties were an increase in labile P and a pH rise. The increase in labile P took place in the upper part of the nonallophanic Andisol column due to its high P retention capacity whereas an increase in the labile P was also observed in the lower part of the lowland soil column.
Tidal flats in Imazu and Kafuri Bay in Fukuoka Prefecture, Japan, were well known as the nest-sites of horseshoe crabs, but their nesting has declined because of water and sediment pollution of the bay. To rehabilitate the polluted beach in the tidal flat, placement of fresh sand on the beach was performed by Fukuoka City, resulting in an increase of nesting of the horseshoe crab. We compared the factors that potentially affect nesting at Imazu and Kafuri beach : morphology, exposed time, grain-size distribution, water content, oxygen concentration, and redox potential. The elevation of the beach was higher at Imazu than at Kafuri, leading to a longer exposed time at Imazu than Kafuri. This induced low water content, and high oxygen concentration and redox potential at Imazu compared with Kafuri though the grain-size distribution and organic matter were almost the same at both beaches. We suggest that sand placement is an effective procedure to rehabilitate nesting sites of the horseshoe crab in terms of exposed time of the sites, water content, oxygen concentration, and temperature.
We continue the long term experiment on nitrate removal in the paddy field plots. Natural water with high nitrate concentration of approximately 20 mg L-1 was supplied to each plot continuously more than ten years. Nitrate removal rates gradually decreased in the first few years, but now they keep the ability of nitrate removal. In this paper, the decrease of nitrate concentration and water temperature in the flow process was measured at 3 hours intervals. Water temperature was lowest in the first block and it gradually increased through flow process (Fig. 2). The values of nitrate concentration decreased linearly and did not change so much by time as shown in Fig. 4. Average nitrate removal rate (R) of non-plant plot was 0.61 g m-2 d-1. The coefficients of nitrate removal rate (a) were calculated by the equation (9) and the average coefficient of nitrate removal rate (a) was 0.038 m d_1. The coefficient of nitrate removal rate of the first block was smaller than the values of other blocks due to low water temperature (Fig. 9).
Influence of flow rate on nitrate removal in the flow process was examined in a paddy field plot. Flow rate was changed for three stages. In the first stage flow rate was controlled at 0.040 m d-1, in the second stage at 0.073 m d-1 and in the third stage at 0.136 m d-1. Corresponding to the increase of flow rate, water flows through a plot rapidly. The retention time is 26.3 hours in the first stage, 16.5 hours in the second stage and 9 hours in the third stage. With the increase of flow rate, the nitrate concentrations of outflow water gradually increased. The decrease of nitrate concentration through the flow process also changed. Coefficients of nitrate removal rate (a) changed in a day, but the daily average values (a) in each stage were almost constant.
Neutralization effects of acidity of rain were examined by cover plants on slope land. Slope gradient was 10 degrees. Cynodon dactylon Pers, Dichondr a repens Forst and Artemisia princes Pampan were selected as
cover plants. The pH value of rain less than pH 5.6 was observed 10 times among 13 times of sampling, and
it was ranged from 4.39 to 5.92. The main results of this experiment were as follows :
①The pH value of surface runoff increased than that of each rain. The average value of [H+] of surface runoff was a value from which 47% of [H+」of rain was subtracted. This means that cover plants have the neutralization effects of acid rain.
②The neutralization capability of acid rain was explained that cation leached from the leaf parts of cover plants has played the role of counteractive.
③There was the difference in the pH value of surface runoff between Atremisa princes and Cynodon dactylon. Therefore it was shown that the neutralization effects of acid rain of Atremisa princes was larger than that of Cynodon dactylon.