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Non-PR, IFPRI4; GRP38, EPTD

Under rainfed conditions, root development of trees can be very unpredictable and variable, depending on the amount and distribution of rainfall received. This becomes more critical when the rainfall is seasonal and the soil has a high clay content. Our investigation dealt with the root development of Acacia mangium established as plantation forest on a soil with heavy clay texture in Kemasul Forest Reserve, Malaysia. The distribution of active roots was measured at 9- and 21-month-old plantations using the radioactive 32P injection method. Growth at different distances from the tree base and at different soil depths was studied. After nine months of field planting, we found that roots were mostly concentrated at the surface within 1000 mm distance from the tree base. At one year after the first measurement, roots were traced as far as 6400 mm away. A large part of these roots, however, were detected within 3700 mm distance in the upper 300mm soil. At this stage, roots still did not go deeper than 450 mm depth, probably due to the high clay content at lower depth and low pH. This rapid root growth indicates that below-ground competition can be very intense if this species is established as a mixed-species plantation.

Under rainfed conditions, root development of trees can be very unpredictable and variable, depending on the amount and distribution of rainfall received. This becomes more critical when the rainfall is seasonal and the soil has a high clay content. Our investigation dealt with the root development of Acacia mangium established as plantation forest on a soil with heavy clay texture in Kemasul Forest Reserve, Malaysia. The distribution of active roots was measured at 9- and 21-month-old plantations using the radioactive 32P injection method. Growth at different distances from the tree base and at different soil depths was studied. After nine months of field planting, we found that roots were mostly concentrated at the surface within 1000 mm distance from the tree base. At one year after the first measurement, roots were traced as far as 6400 mm away. A large part of these roots, however, were detected within 3700 mm distance in the upper 300mm soil. At this stage, roots still did not go deeper than 450 mm depth, probably due to the high clay content at lower depth and low pH. This rapid root growth indicates that below-ground competition can be very intense if this species is established as a mixed-species plantation.

A 2 yr field study on the influence of N fertilization and rainfall on groundwater pollution was carried out in the sandy area of Belgium. The NO inf3 sup− -N and Cl− content of the groundwater at 0.5, 1.0, 1.5, and 2.0 m depths was monitored every two weeks on a field, grown with barley in 1980 and with maize in 1981. Turnips for cattle feed were grown in between the two crops. The total annual rainfall during the period under study was about 800 mm. The NO inf3 sup− -N content at all depths was at all times above 11.3 mg NO inf3 sup− -N dm−3, the WHO safe limit. Fluctuation of the NO inf3 sup− -N content occurred mainly at 0.5 and 1.0 m. The concentration at 1.5 and 2.0 m depths was higher most of the time than at 0.5 and 1.0 m. Leaching of NO inf3 sup− -N into deeper layers occurred when there was heavy rainfall. There was no important loss of NO inf3 sup− -N through denitrification at 1.5 and 2.0 m depths.

A simple model to predict nitrate leaching developed by Bums (1974) was tested. The measured and calculated nitrate concentrations in 100'cm wet sandy soil profiles were compared: the results showed reasonably good agreement between the two values. Adjusting the initial conditions and reducing the segment size from 10 to 5 cm did not improve the accuracy of the prediction. Better agreement was, however, obtained when only the samples above the groundwater table were considered.

The study of the relation between N fertilisation and groundwater pollution was carried out on two fields in the sandy area of Belgium during a two-year period. The fields under study were grown with maize and barley and received fertiliser N and manure at normally applied rates. The N03- -N and Cl “content of the groundwater at 0.5, 1.0, 1.5 and 2.0 m depth was monitored every two weeks. The total rainfall during the two consecutive years was about 800 mm. On one of the fields, the N03-N content at all depths was most of the time above 11·3 mg N03-N dm3, the WHO safe limit. However, on the other field the nitrate level was much lower, except at 0·5 m depth. The difference between the two fields was attributed to differences in the rate of N input. The fluctuations during the two-year study were influenced by N fertilisation and rainfall distribution. The N03- -N/CI- ratio indicated that denitrification in the deeper zones of the soil was not prominent.

A 2 yr field study on the influence of N fertilization and rainfall on groundwater pollution was carried out in the sandy area of Belgium. The NO inf3 sup− -N and Cl− content of the groundwater at 0.5, 1.0, 1.5, and 2.0 m depths was monitored every two weeks on a field, grown with barley in 1980 and with maize in 1981. Turnips for cattle feed were grown in between the two crops. The total annual rainfall during the period under study was about 800 mm. The NO inf3 sup− -N content at all depths was at all times above 11.3 mg NO inf3 sup− -N dm−3, the WHO safe limit. Fluctuation of the NO inf3 sup− -N content occurred mainly at 0.5 and 1.0 m. The concentration at 1.5 and 2.0 m depths was higher most of the time than at 0.5 and 1.0 m. Leaching of NO inf3 sup− -N into deeper layers occurred when there was heavy rainfall. There was no important loss of NO inf3 sup− -N through denitrification at 1.5 and 2.0 m depths.

A simple model to predict nitrate leaching developed by Bums (1974) was tested. The measured and calculated nitrate concentrations in 100'cm wet sandy soil profiles were compared: the results showed reasonably good agreement between the two values. Adjusting the initial conditions and reducing the segment size from 10 to 5 cm did not improve the accuracy of the prediction. Better agreement was, however, obtained when only the samples above the groundwater table were considered.

The study of the relation between N fertilisation and groundwater pollution was carried out on two fields in the sandy area of Belgium during a two-year period. The fields under study were grown with maize and barley and received fertiliser N and manure at normally applied rates. The N03- -N and Cl “content of the groundwater at 0.5, 1.0, 1.5 and 2.0 m depth was monitored every two weeks. The total rainfall during the two consecutive years was about 800 mm. On one of the fields, the N03-N content at all depths was most of the time above 11·3 mg N03-N dm3, the WHO safe limit. However, on the other field the nitrate level was much lower, except at 0·5 m depth. The difference between the two fields was attributed to differences in the rate of N input. The fluctuations during the two-year study were influenced by N fertilisation and rainfall distribution. The N03- -N/CI- ratio indicated that denitrification in the deeper zones of the soil was not prominent.