Author : Yin Yin Thant

Summary : Two consecutive greenhouse experiments were conducted to investigate the
effects of corn cob and rice straw biochars on soil properties, microbial activity, rice yield
and greenhouse gas (methane and nitrous oxide) emissions in rainfed rice cultivation. The
experiments were conducted at International Rice Research Institute, Los Banos,
Philippines. In wet season (WS, 2016), biochar addition increased soil pH and decreased
soil bulk density with or without chemical fertilizer and cow manure. The lowest soil
bulk density was observed in rice straw biochar (40 t ha-1) together with cow manure and
chemical fertilizer. All treatments with added biochar, chemical fertilizer and cow
manure significantly enhanced total nitrogen, soil organic carbon, available phosphorus,
exchangeable potassium and nitrogen uptake by rice plants when compared to the control
treatment.The highest total nitrogen was exhibited in rice straw biochar (40 t ha-1) with
chemical fertilizer treatment. Soil organic carbon and exchangeable potassium were
highest in rice straw biochar (40 t ha-1) together with cow manure and chemical fertilizer
while corncob biochar (40 t ha-1) with cow manure and chemical fertilizer showed
highest available phosphorus and nitrogen uptake in WS, 2016. Fluorescein Diacetate
microbial activity was measured at flowering and harvesting stages and the results
indicated that 40 t ha-1 of corn cob and rice straw biochar combinations with cow manure
and chemical fertilizer stimulated higher microbial activity while the treatments with only
chemical fertilizer and cow manure incorporation showed lower activity in WS 2016.
Emissions of methane and nitrous oxide were monitored in a closed chamber method
throughout the whole rice growing season at two weeks intervals. Chemical fertilizer
alone, single cow manure addition and combination of them favored CH4 emission and
showed higher rate of emission compared to all biochar containing treatments. The
emission rate was lowest in rice straw biochar (40 t ha-1) alone application. In contrast,
higher N20 emission was occurred in rice straw biochar (10 t ha-1) with cow manure and
chemical fertilizer applications compared to chemical and cow manure containing
treatments while the lowest emission was found in corncob biochar (40 t ha-1) alone
applied treatment in wet season. Rice straw and corncob biochar combination at the rate
of 40 t ha-1 with chemical fertilizer and cow manure gave the highest grain yield and no
significant differences with the grain yield of other biochar containing combinations. In
dry season (DS, 2016), soil pH increased with rice straw and corncob biochar application.
Lower soil bulk density was observed in all biochar containing treatments in comparison
with other treatments and corncob biochar (40 t ha-1) together with cow manure and chemical fertilizer showed lowest bulk density. Soil total nitrogen and available
phosphorus were also highest in corncob biochar (40 t ha-1) together with cow manure
and chemical fertilizer. Mixed application of rice straw biochar (40 t ha-1) with cow
manure and chemical fertilizer showed highest soil organic carbon and exchangeable
potassium. The highest nitrogen uptake was observed in the treatment of corncob biochar
(40 t ha-I) together with cow manure and chemical fertilizer. Fluorescein Diacetate
microbial activity was highest in treatments with corncob biochar (40 t ha-1) with
chemical fertilizer at flowering stage. Similarly, application of rice straw biochar (40 t ha-1) added with cow manure and chemical fertilizer demonstrated similar effect at
harvesting stage. Single chemical fertilizer, cow manure alone and their combination
showed higher methane emission rate than biochar containing treatments. The
combination of rice straw biochar (40 t ha-1) with cow manure and chemical fertilizer
showed the lowest emission rate of methane. For N20 emission, the highest emission rate
was observed in corncob biochar (10 t ha-1) together with cow manure and chemical
fertilizer followed by rice straw biochar (10 t ha-1) together with cow manure and
chemical fertilizer. On the other hand, the lowest N20 emission rate was found in rice
straw biochar alone with the rate of 40 t ha-1 in DS 2016. The highest grain production
was resulted from rice straw biochar (40 t ha-1) combination with cow manure and
chemical fertilizer and corncob biochar (40 t ha-1) together with cow manure and
chemical fertilizer. Thus, this study suggests that biochar was more efficient when added
together with organic and inorganic amendment to improve rice production.