Author : Rujirat Wongchandaeng

Summary : This study was conducted at the International Rice Research Institute in Los Bafios, Laguna,
Philippines. First experiment aimed to determine the soil nitrogen supply capacity in Binangonan
clay and Sariaya Sandy loam soils. An anaerobic incubation experiment was conducted in the
green house to determine the soil nitrogen (N) mineralization and the N supplying ability of
paddy soil without N fertilization. The second experiment aimed to evaluate the effects of biochar
made from rice straw on the N dynamics and balance on lowland rice soils. The pot experiment
was conducted in a screenhouse under an anaerobic condition during two rice growing seasons.
The experiment was arranged in a completely randomized design (CRD) in three replications
with two biochar levels (10 and 40 ton ha-1). Two dosages ofbiochar were applied once in the wet
season. The basic characteristics of selected rice soils showed that the Binangonan clay soil had
higher cation exchange capacity (CEC) and clay content. The Sariaya sandy loam soil, on the
other hand, had higher total N and total C. The amount of N mineralized during cropping season
of rice was estimated to be between 51.9 mg N kg-1 to 141.7 mg N kg-1 in Binangonan clay soil
and between 62.7 mg N kg-1 to 77.83 mg N kg-1 in Sariaya sandy loam soil. The N mineralization
was related to total N and NH4+-N contents. Under anaerobic condition, the Sariaya sandy loam
soil had higher N supplying capacity than the Binangonan clay soil. N-supply measured as NH4+-
N in sandy loam soil was higher at 0-30 days and observed at adequate level for N uptake during
vegetative stage while the Binangonan clay soil was higher at 0-60 days and very poor in N
supplying capacity. The second experiment showedthat rice straw biochar increased the soil pH,
CEC, total organic carbon, and total N contents; and decreased the soil bulk density, particularly
with at higher application rate. Application of biochar was greatly pronounced on NH4+-N during
anaerobic condition in biochar-amended soil at 40 ton ha' in combination with chemical fertilizer
and cow manure in both soils. In clay soil, plant N uptake and grain yield significantly increased
by biochar rate at 40 ton ha-1 with chemical fertilizer and the use of chemical fertilizer alone in
both cropping seasons. In sandy loam soil, the added biochar at 10 and 40 ton ha-1 in combination with chemical fertilizer and cow manure had markedly increase N uptake in both seasons. However, the added biochar at 10 ton ha-1 in combination with chemical fertilizer showed higher yield during the WS cropping while a lower yield was observed in the DS cropping as affected by added chemical fertilizer. The nitrous oxide emission between treatments was statistically significant with urea having the highest peak, however, the amount was negligible when compared to nitrogen output. Results from the estimation of N balances had guided the
appropriate biochar application rate based on higher yield and less N losses. For clay soil, the use
of straw biochar rate of 40 ton ha-1 with chemical fertilizer together with N management
significantly improve the N output from residual N is recommended. For sandy loam soil, the
application of rice-straw biochar at 10 ton ha-1 in combination with chemical fertilizer and cow
manure need additional 64.3 kg ha-1 N to meet the target yield.