Water-saving rice production. - A heretic s point of view -

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1 production - A heretic s point of view - Mathias Becker University of Bonn, Germany The historic dimension Benefits Water economy Trade-offs Weeds Nutrients Soil organic C N use efficiency Trace gases Conclusions and outlook years ago: Early agriculture.. key development in the rise of sedentary human civilization years ago: Neolithic revolution.. domestication of rice in Pearl River Valley, China years ago:.. first cultivation of rice in the Indus Valley. O. rufigogon O. sativa ago field bunding, soil puddling and transplanting early irrigation systems in Sri Lanka years ago spread of irrigation and paddy production in E and SE Asia More recent innovations Liebig Mineral fertilizers Darwin & Mendel Basis for modern breeding Haber and Bosch Synthetic N fertilizers J.v.Liebig C. Darwin G. Mendel F. Haber C. Bosch 3 4 1

2 Production (mio t) / Irrigated area (%) The green revolution High-yielding, short cycled, and input-responsive genotypes Irrigation paved the way for the success of the green revolution Doubling of yields ascribed to irrigation (CGIAR / CIAT, 1998) The green revolution Yield increases correlated with share of irrigated land area Water supply Weed control Soil nutrient supply N use efficiency Yield increase Rice grain yield (t/ha) Irrigation was main contributor to avert global hunger crisis 5 6 Emerging water shortages declining groundwater levels more drought events delayed onset of rains Water-savingrice production Plastic mulching (He et al., 2013) Aerobic rice (Bouman et al., 2005) Deficit irrigation (Fereres et al., 2006) Raised-bed systems (Choudhury et al. 2007) AWD (Carrijo et al, 2017) Alternate Wetting and Drying Aims: Standardized rainfall anomalies Less water Higher WUE Do we throw our hard-earnedyield gains over boardto save water? Whereand when does water-saving work? 7 8 2

3 The traditional paddy system Deficit irrigation or AWD Traditional irrigatedrice permanent flooding AWD rice intermittent irrigation +5cm (Ψ = 0kPa) +5cm cm Ψ-20kPa ±0cm cm Ψ-20kPa Large waterconsumption (2,500 L / kg grain) High and ± staple yields Reduced water consumption (-25%) (Norton et al., 2017) Weeds, nutrient deficiency, NUE, yield? 9 10 Carrijo et al. (2017) Field Crops Res. 203, ) Less lodging Higher C-remobilization More roots More productive tillers No major yield penalty Carrijo et al. (2017) Field Crops Res. 203, ) Less lodging Higher C-remobilization More roots More productive tillers No major yield penalty -5 kpa reduces water consumption by 25% increases water productivityby 23% -10 kpa -20 kpa Yield penalty of 3-23% (ns) f(drying intensity)

4 Carrijo et al. (2017) Field Crops Res. 203, ) Less lodging Higher C-remobilization More roots More productive tillers No significant yield penalty Carrijo et al. (2017) Field Crops Res. 203, ) Less lodging Higher C-remobilization More roots More productive tillers No major yield penalty Mainly wet season data (53 of 56 studies in WS) Only published findings (no publication on complete failure) Yield penalty canreach 50% in alcaline clay soils with low OM CH 4, N 2 O How about yield losses due to Weeds P & micronutrient availability N use efficiency (losses) How about effects on global climate? Weeds Comparison of rainfed (temporary soil drying) and permanentlyflooded rice ph, P, Mn, Fe, Zn Temporary aerobic soils affect ph and nutrients Cumulative weed biomss Humid Humid forest Moist Sub-humid savanna Semi-arid Dry savanna Rainfed Irrigated Rainfed Irrigated Rainfed Irrigated Wet-dry Flooding Wet-dry Flooding Wet-dry Flooding (t/ha) Flooded soils tend tobe ph neutral Becker et al., ,92 0,80 1,58 0,86 0,73 0,66 Becker and Johnson, ,23 1,00 1,78 1,29 2,25 1,02 Kent et al., ,81 1,11 Becker et al., ,96 0,89 1,38 0,88 Mean difference Yield gap due to weeds 0,23 0,49 1,23 0,15 0,45 1,02 Permanent flooding reducesweed biomass and increases grain yields (Becker et al., 2003; Howell et al. 2015) 15 In a flooded soil Redox (Eh) -110 mv ph of acid soil 6.4 ph of alcaline soil 6.4 The carbonate buffering system in flooded soil results in a neutral ph value Do water-savingpractices offset carbonate buffering? In moist soil Redox (Eh) +250 mv ph of acid soil 5.6 ph of alcaline soil is

5 ph: Nurient availability Acid: Ca, Mg, S; alcaline: P, Mn, Zn Nutrient availability N (NH 4+ ) N, K, S, Ca, Mg P, B (Norton et al., 2017) Fe, Mn, Zn, Cu Mo Al Water-saving (temporary drying) effects in acid soil ReducedS uptake ReducedCa, Mg uptake Water-savingcan change soil ph and induce nutrient deficiency? Nutrient availability Soil organic C C stores and sequestration in wetlands -P (Kirk, G. 2004) Water-saving (temporary soil drying) effects in alcaline soil (Catling, 1999; Kirk, 2004) Reduced P uptake Reduced Mn, Zn uptake -P -Mn ~ 300 peta-grams (10 15 ) of C stored in wetlands and rice fields ~ 400 tera-grams (10 12 ) sequestered annually 14% of global C stored in wetlands and rice paddy soils (Wojick 1999 ) Can this ecosystem service be maintained underwater-saving?

6 Rainfed Irrigated Rainfed Irrigated Rainfed Irrigated Soil organic C Comparison of rainfed and irrigated rice N use efficiency Comparison of rainfed and irrigated rice Alternate Humid forest Permanent Moist s Rainfed wetting Irrigated flooding Rainfed Soil organic Carbon Becker and Johnson, (%) (% 1,53 1,79 1, a Becker et al., ,23 1,93 1,78 Becker and Johnson, 2001b 1,81 2,54 1,38 Touré et al., ,46 Niang et al., ,70 2,10 1,5 Mean difference 0,52 0,3 Humid forest Moist savanna Dry savanna Humid Humid forest Moist - -Sub-humid savanna Dry - Semi-arid savanna Temporary Permanent Temporary Permanent Temporary Permanent drying flooding drying flooding drying flooding Mineral N use efficiency (kg grain / kg N) % -60% -52% (n=216) (n=96) Becker et al., 2003 With temporary aerobic conditions: higherrespiration, 0.5% less SOC With temporary aerobic soil 50% lower mineral N fertilizer use efficiency Soil N and NUE Effect of soil aeration on soil N mineralization Soil N and NUE Effect of soil aeration on soil N mineralization Wetland soil type Soil aeration Net mineralization (mg day -1 ) NH 4+ -N NO 3- -N Total N min Wetland soil type Soil aeration Net mineralization (mg day -1 ) NH 4+ -N NO 3- -N Total N min Sandy clay ph 5.6 Flooded Aerobic Sandy clay ph 5.6 Flooded Aerobic Clay loam ph 6,5 Flooded Aerobic Clay loam ph 6,5 Flooded Aerobic Clay ph 7.3 Flooded Aerobic Clay ph 7.3 Flooded Aerobic With temp. aerobic soil, high N mineralization but dominated by NO 3 With temporary aerobic soil, N fraction dominated by NO 3 Akter and Becker, 2017 Akter and Becker,

7 CH 4 flux (mg m -2 ) N 2 O flux (mg m -2 ) Nitrate reductase NRase activity (mmol NO 2- h -1 plant -1 ) Genotype differencesin onset and activity NRase Low nitrate assimiltaion two days after nitrate supply Nitrate reductase NRase activity (mmol NO 2- h -1 plant -1 ) Genotype differencesin onset and activity NRase Low nitrate assimiltaion two days after nitrate supply Is this sufficient? Is this sufficient? No! Maize and many weeds can assimilate nitrate earlierand more than rice Ouko et al., 2009 Ouko et al., 2009 Avoid the oxidation of NH 4 Keep Eh <200 mv Add nitrificationinhibitors Trace gases Flooded Aerobic Akter et al., 2017 Trace gases Flooded Aerobic Akter et al., RF_T3 Sandy RF_T5 clay, ph RF_T6 5.6 RM_T3 Clay loam, RM_T5pH RM_T6 6.5 RC_T3 Clay, RC_T5 ph 7.3 RC_T6 0 RF_T3 Sandy RF_T5 clay, ph RF_T6 5.6 RM_T3 Clay loam, RM_T5pH RM_T6 6.5 RC_T3 Clay, RC_T5 ph 7.3 RC_T6 Cumulative CH 4 fluxes under field capacity and flooded soil condition Cumulative N 2O fluxes under field capacity and flooded soil conditions Highest CH 4 fluxes from anaerobic (flooded) clay soils with high SOM High N 2 O emissions from aerobic clay soils with high SOM

8 Trace gasses CH 4 = 25 x CO 2; N 2O = 298 x CO 2 Akter et al., 2017 Water saving and efficient use of water are possible (Yushi et al. 2012) Improved S and K availability Less alcohol, organic acids, H 2S (Sun et al. 2012) Accelerated SOM mineralization Reduced P-Mn-Zn availability (Kirk, 2004) Improved root growth Higher microbial activity (Yang et al. 2004) Reduced N assimilation (Moyad et al., 2013) Micronutruient deficiencies (Kreye & Bouman., 2011) Less CH 4 emissions (Wassmann et al., 2010 Chu et al., 2015) More weeds Higher global warming potential (Akter et al., 2017) Cumulative (CO 2 + CH 4 + N 2 O) GWP (t CO 2-C eq ha -1 ) Soil drying increases GWP of clay soils with high SOM 29 Less water use and improved WUE justify the water-saving hype? 30 Under unfavorable conditions.. marginal soils (extreme ph, high clay content, low SOC) dry climatic conditions (high ET, high VPD) unreliable water supply (no short-term access to irrigation) labor-scarce environments (no timely intervention).. trade-offs outweigh benefits of water-saving Under favourableconditions.. Good soils (high SOM), reliable water irrigation supply Labor availability for timely interventions.. there is no need for water-saving technologies - A heretic s point of view - Mathias Becker University of Bonn, Germany Water economy is not all! For high rice yields, irrigation is required! In case of water scarcity, forget about rice (grow less water-demanding crops)