Biochar in rice-based systems: effects and opportunities SM Haefele C Knoblauch AA Marifaat Y Konboon
Introduction
Rice residues in Asia Annual production: 550 Mt rice straw 110 Mt rice husk Traditionally and in extensive systems used as fertilizer, fuel, fodder, etc. About 1/3 of the biomass. In intensive systems residue removal often a problem; mostly incorporated or burned. About 2/3 of the biomass;
Advantages of bioenergy/biochar in ricebased systems Residues are a by-product of rice production, their use does not compete with food production; Even complete removal of residues does not result in decreasing SOM in irrigated rice; Removal of rice residues from flooded fields directly reduces CH 4 emission; The high cropping intensity in intensive rice systems maximizes biomass production per area;
Bioenergy/char developments Increasing use of rice husk accumulating at rice mills, char produced is used as organic fertilizer or dumped; Cambodia: 30 kw, 0.5 t h -1 husk, 70 kus$; Thailand: 100 kw at big rice mills, mostly combustion; Myanmar: 5-10 kw, 0.1-0.2 t h -1 husk, 5-7 kus$;
Objectives Test the stability of charred rice residues in rice production systems; Evaluate the agronomic effects of biochar applications; Evaluate the effect on greenhouse gas emissions from flooded rice;
Materials and Methods
Field experiments Field trial at 3 sites with RCB design and 3 replications: T1 T2 T3 T4 T5 T6 Rice husk None None CRH CRH RH RH Fertilizer None med. NPK None Med. NPK None Med. NPK Initial application of rice husk: CRH: 41 t ha -1 (16.4 t ha -1 C) RH: 49 t ha -1 (17.9 t ha -1 C) Thereafter, all above-ground residues removed.
Site characteristics IRRI Siniloan Ubon Irrigated lowland rice 2 crops per year Fertile clay soil, ph 7 Poorly drained Umbrisols Rainfed upland rice 1 crop per year Poor clay soil, ph 4.5 Well drained Ferralsols Rainfed lowland rice 1 crop per year Very poor sand soil, ph 4 Well drained Acrisols
Biochar used Weight loss during carbonization: 68% Loss of C: 65% Loss of N: 72% C Si N RH g/kg 362 95 6.9 CRH g/kg 398 204 5.9 CRH has: BD of 0.150 kg l -1, CEC of 1 cmol 100g -1 a high water holding capacity; P K Ca Mg 1.4 4.5 0.6 0.8 4.4 10.3 1.5 2.5
CH 4 and CO 2 measurements CH 4 and CO 2 emissions were measured during the 2007 wet season at IRRI in: the old experiment (established WS 2005); the new experiment (established WS 2007); Emission rates were determined once a week;
Results
Treatment effect on mean rice yield (t ha -1 ) at all three sites 6 seasons 3 seasons 3 seasons IRRI Siniloan Ubon 0F_control 2.85 b 3.04 a 2.26 c 0F_CRH 2.78 b 3.31 a 2.42 bc 0F_RH 2.69 b 3.31 a 2.27 c F_control 4.34 a 3.03 a 2.64 b F_CRH 4.15 a 3.22 a 3.23 a F_RH 4.19 a 3.26 a 2.87 b
Soil C concentration and treatment dependent C losses at IRRI 6 5 4 T1 T2 T3_CRH T4_CRH T5_RH T6_RH C after 2.5 years: T3/4: 17.9 t C ha -1 T5/6: -0.2 t C ha -1 TOC (%) 3 Sampling depth: 0-0.2 m 2 1 Topsoil (0-0.1 m) 0 May Sep Jan May Sep Jan May Sep Jan 2005 2006 2007
Soil C concentration and treatment dependent C losses at Siniloan TOC (%) 8 6 4 T1 T2 T3 T4 T5 T6 C after 2.5 years: T3/4: 11.9 t C ha -1 T5/6: 4.8 t C ha -1 Sampling depth: 0-0.3 m 2 Topsoil (0-0.1 m) 0 May Sep Jan May Sep Jan May Sep Jan 2005 2006 2007
Soil C concentration and treatment dependent C losses at Ubon 2.0 1.5 Topsoil (0-0.1 m) T1 T2 T3_CRH T4_CRH T5_RH T6_RH C after 2.5 years: T3/4: 9.1 t C ha -1 T5/6: 4.1 t C ha -1 TOC (%) 1.0 Sampling depth: 0-0.3 m 0.5 0.0 May Sep Jan May Sep Jan May Sep Jan 2003 2004 2005
Seasonal CH 4 emission right after RH/CRH application at IRRI CH 4 emission (µmol m -2 h -1 ) 800 600 400 200 New experiment Control CRH 25000 20000 15000 10000 5000 New experiment RH GWP 23x higher 0 Jun Jul Aug Sep Oct 0 Jun Jul Aug Sep Oct 2007 wet season 2007 wet season CH 4 emissions of the Control and CRH treatment are not significantly different, but much below the emission of the RH treatment (28x)
Seasonal CH 4 emission 2 years after RH/CRH application at IRRI CH 4 emission (µmol m -2 h -1 ) 800 600 400 200 Old experiment Control CRH RH GWP is 2x higher 0 Jun Jul Aug Sep Oct 2007 wet season 2 years after RH/CRH application, the CH 4 emissions of the Control and CRH treatment are still considerably below the RH treatment.
Seasonal C losses (kg ha -1 ) from CH 4 and CO 2 emissions at IRRI and their GWP C emission as CH 4 C emission as CO 2 Directly after RH/CRH application Control 49 647 CRH 70 544 RH 1970 3796 2 years after RH/CRH application Control 21 963 CRH 26 1200 RH 79 1338 C emission Total 696 614 5767 984 1226 1416 CDE GWP RH/CRH 6859 8407 194413 x 23 5466 6773 12095 x 2 CDE: Carbon dioxide equivalent for 100 y; GWP: Global warming potential;
Conclusions Rice husk biochar did not affect rice yield on fertile, irrigated soils and in an upland system; it did increase yields on poor rainfed lowland soils. Soil C from carbonized rice husk seems to be stable in irrigated systems. In rainfed systems, the C losses seem to be caused by illuviation into deeper soil horizons.
Conclusions Bioenergy/biochar in rice systems offer climate change mitigation by C neutral energy production, by reducing CH 4 emission, and by carbon sequestration; Bioenergy/biochar systems provide energy and new income sources for rural areas; The challenge ahead is to develop options for straw use and get access to a C credit system;