Soil Organic Matter. What is it How to build it. Clive Kirkby

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1 Soil Organic Matter What is it How to build it Clive Kirkby CSIRO Plant Industry Charles Sturt University EH Graham Centre for Agricultural Innovation John Kirkegaard Alan Richardson Graeme Batten Chris Blanchard Len Wade Lots of carbon in soils however But lots also lost if lots has been lost it would seem reasonable to Terrestrial Atmosphere Some researchers living think have estimated we should 7 Bt that agricultural soils have (with lost biomass the up proper to 5% management) of their carbon since 5Bt mechanised be able to agriculture put it back began Soil Organic Material 15 Bt 3-4 t/ha carbon lost to improve soil quality & help mitigate climate change of carbon leading to soil degradation and a loss of soil fertility

2 Where is this soil C? Humus = passive fraction Soil Science Society of America glossary defines soil organic matter as The organic Plant fraction material of the soil (dead but active) exclusiveof un-decayed plant and (up to %) animal residues and is considered synonymous with humus Living (up to 1%) heavy fraction or the fine fraction (FF-SOM) Soil Organic Matter or humus (very dead) (up to 95%) How do we increase humus-c C levels Kirkegaard has compared min-till and Luo So, conventional et in al. the In the (1) absence absence cultivation compared of of composts large combined no-till amounts and & conventional manures, with of stubble it would Rumpel incorporation, compost seem et cultivation al. that (8) manures stubble increasing compared at 69 retained most paired SOM common wheat on levels sites the stubble is surface way not as burning and suggested stubble and retention burning to increase simple for as France soil-c years levels for at 31 Harden, is years. to Although no-till altered NSW. vertical distribution of soil-c Even retaining (more after near 31 crop years surface) residues there there was was no In all difference treatments no difference in soil-c and/or stock levels + in soil-c between have levels gone no-till downand best yields conventional reducing come reduce from tillage cultivation stubble burn with conventional cultivation

3 Soil Organic Material And Carbon NOT THE SAME THING The stuff in soil is ORGANIC MATERIAL NOT CARBON We measure soil C because we want to estimate the amount of organic material in the soil We estimate FF-SOM by measuring FF-C Organic material ALWAYScontains other elements as well as carbon What other elements and how much CHNOPS - The OM Building Blocks Element Human Alfalfa Insect Bacteria Fungi Is it the same for soil organic matter Carbon SOM is just 19.37dead 11.34bits 6.1& pieces 1.14 of 6.38 organisms so a good place to start looking when literature trying to figure out Sulphur which are.64 the important.1.14 elements.3.6 is For to each look Australian organism at the mainland composition (disregarding states H and of live O) organisms We Nitrogen tested this 5.14 by analysing.83 soils 1.5data 3.4 found in.66 the Phosphorus Also analysed soils collected from four of the five Hydrogen Oxygen not only do C, N, P, S contribute most of the mass Total C:N:P:S ratios are approximately constant

4 Total soil C:N 598 soils from various countries 59 Australian soils r =.99 r =.91 Total soil C (%) Total soil N (%) FF-SOM (passive SOM) has constant C:N:P:S ratio C N P S humus soils from various countries 15 Australian soils collected from four of the five mainland states 531 soils from various countries 15 Australian soils collected from four of the five mainland states Total soil C (%) r =.93 r = Total soil N (%) Total soil S (%) Details in Kirkby et al. 11

5 Which dead organisms make FF-SOM Reference Methodology Comments from papers Bol et al. (9) Pyrolysis-GC/MS-IRMS Plant-derived organic matter was not stored in soils, but was transformed to microbial remains, mainly in the form of carbohydrates and proteins and held in soil by organo-mineral Derrien et al. (6) Dijkstra et al. (6) 13 C labelling particle size fractionation (GC/C-IRMS) interactions. Answer Considering the typical lability of carbohydrates, the relatively great age of carbohydrate carbon may be explained by physical or chemical protection from degradation, as well as by recycling of soil organic matter carbon by soil microbes. Chloroform fumigation extraction Soil microbial biomass although plant material 13 C and 13 C, (crop 15 N enriched. residues) 15 N isotope analysis is the initial possibility input of OM stabilisation material in the form of microbial carbon. Young SOM more 13 C and 15 N enriched than fresh litter inputs. The older, more stable SOM, more 13 C and 15 N enriched than young SOM. Suggests microbial biomass could be source of stabilised SOM. Gleixner et al. (1) Pyrolysis-GC/MS-IRMS Some of the more resistant pyrolysis products were derived from proteins. This supports the Gleixner et al. () Pyrolysis-GC/MS-IRMS Results suggest that mainly recycling of C in carbohydrates (in addition to physical and chemical protection) is responsible for SOM stabilisation. Pyrolysis products probably related to soil organisms and their extracellular polymeric Guggenberger et al. (1994/95) Kramer et al. (3) substances FF-SOM is dead microbes Size fractionation 13 C CPMAS-NMR analysis 13 C, 15 N, isotope analysis 13 C CPMAS-NMR analysis The amount of alkyl-c correlated well with the resynthesis of carbohydrates. Microbially derived sugars accounted for a large portion of the clay associated carbon. These results imply that humification (and the concomitant stabilization of soil-c) at our site resulted from microbial alteration of organics rather than from accumulation of recalcitrant compounds. Aggregate coatings mainly microbial metabolites or debris, not plant debris. Lehmann et al. (7) FTIR near-edge X-ray absorption fine structure (NEXAFS) Liang and Balser (8) Amino sugar assay Our study supports the hypothesis that microbial residues are refractory and that they contribute to terrestrial carbon sequestration. Miltner et al. (9) 13 C CPMAS-NMR analysis The nuclear magnetic resonance (NMR) spectra of the soils were very similar and indicate that the residues of the degraded microbial biomass were very similar to those of the SOM and are a significant source for the formation of the SOM. Simpson et al. (7) Sollins et al. (6) Yao and Shi (1) D-NMR Density fractionation 13 C, 15 N, 14 C isotope analysis Continuous flow isotope ratio mass spectrometry (CF-IRMS) Microbial biomass was found to contribute >5% of the extractable soil organic matter fractions ~ 45% of the humin fraction and accounted for >8% of the soil nitrogen. Age of SOM increased with aggregate density. 13 C and 15 N enrichment increased with density. 13 C and 15 N accumulate preferentially as a result of microbial processing. Suggests material in dense, older, fractions largely of microbial origin. Microbial processing played an important role in organic matter stabilization in turfgrass ecosystems. Bol et al. (9) Kramer et al. (3) Lehmann et al. (7) Pyrolysis- GC/MS-IRMS 13 C, 15 N, isotope analysis 13 C CPMAS- NMR analysis FTIR near-edge X-ray absorption fine structure (NEXAFS) Plant-derived organic matter was not stored in soils, but was transformed to microbial remains These results imply that humification at our site resulted from microbial alteration of organics rather than from accumulation of recalcitrant plant compounds. Aggregate coatings mainly microbial metabolites or debris, not plant debris.

6 Hypothesis Thus it would seem to get more The FF-SOM conversion we of need crops to residues get more to dead microbial biomass may be limited microbes by nutrient availability in the soil and not just carbon inputs But to microbes get more are dead more microbes nutrient rich we must than first crop get more residues live ones Tested with laboratory incubation studies and a field trial Lab incubations: incubate soil + stubble with/without nutrients C N P S Field wheat trial 1, incorporate microbes stubble 1, with/without 1,5 nutrients st incubation study Incubate soils from four different agricultural areas (different clay and starting FF-C values) with equivalent of 1 t/ha wheat straw with and without extra nutrients 5 kg N/ha; kg P; 13 kg/s Seven incubation cycles, each 3 months long, new straw and nutrients added at beginning of each cycle

7 Change in microbial biomass-c Microbial biomass-c (µg g -1 soil) 1 control (zero nutrients) 1X nutrients X nutrients 1 8 Improving microbe food quality (not amount) 6 4 has led to an increase in microbe numbers Hamilton Harden Buntine Leeton Soil Microbial biomass vs Increase in humus-c Microbial biomass-c (µg g -1 soil) Microbial biomass 1 control (zero nutrients) 1X nutrients 1 X nutrients Improving 8 microbe food quality (not amount) 6 not only led to an increase in microbe 4 numbers but to an increase in FF-C as well 5 Increase in FF-C Increase in humus-c (as % of straw-c added) Hamilton Harden Buntine Leeton Soil

8 Not only did FF-C C increase but FF-N, P, S as well.5. Carbon Nitrogen.15 FF-C (%) Thus to build FF-C levels you must have the N,.. P and S as well.4 Phosphorus Sulphur.5 FF-N (%) FF-P (%) FF-S (%).1.5. zero 1X X zero 1X X. Nutrient treatment When crop residues are retained in the field You are adding a large amount of material to this pool However it doesn t stop soil microbes from eating this Crop residues Living (up to 1%) Soil Organic Matter or humus (very dead) (up to 95%)

9 nd incubation study Incubate four soils with equivalent of 1 t/ha wheat straw with and without extra nutrients Straw labelled with 13C isotope to enable old humus-c and new straw-c to be tracked independently Only one incubation cycle Net change in FF-C C (1 st expt) Change in FF-C as % of straw-c added nutrients mean 6% mean 15% Hamilton Harden Buntine Leeton Soil

10 Total soil C lost or retained Chane in carbon (mg kg soil -1 ) new C gained old C lost net change -7% % % Harden error bars are SE soil alone (control) + nutrients Total soil C lost or retained Chane in carbon (mg kg soil -1 ) new C gained old C lost net change -11% % % Hamilton error bars are SE +7-3 soil alone (control) + nutrients

11 Chane in carbon (mg kg soil -1 ) Total soil C lost or retained new C gained old C lost net change -9% % % -3 Buntine error bars are SE +14 soil alone (control) + nutrients Chane in carbon (mg kg soil -1 ) Chane in carbon (mg kg soil -1 ) Total soil C lost or retained new C gained Nutrient addition Hamilton always old C lost net humification increased the new C retained Nutrient addition always increased the net C level Buntine Nutrient addition had a varied effect on the old C lost Harden Leeton It - is just as important to understand why we still lose old humus as it is how to make new soil alone soil alone (control) + nutrients (control) humus + nutrients

12 Field trial Incorporate straw with or without extra nutrients Harden field plan Block 4 Block 3 Block Block Burn Cultivate Burn Direct Drill 3 Incorporate 4 Bash Cultivate 5 Standing Direct Drill 6 Mulch Direct Drill 7 Bash Direct Drill On average 5. kg N; kg P; 1.3 kg S per tonne straw

13 Depth (1 cm increments) Change in C after 3 years Carbon (t/ha) total C t/ha % of C is below 3 cm stubble + nutrients stubble Change in C after 3 years 8 6 1x stubble 1x stubble 1x nutrients Carbon t ha -1 4 Results variable across the site Block

14 % of soil & gravel in the profile (soil is that part of material < mm diam) 1 8 Block 1 % soil + gravel %C C 45 t/ha.3 %C C 51 t/ha 34% gravel 66% soil Block 4 19% gravel 81% soil soil depth (1 cm increments) Change after 5 years

15 Depth (1 cm increments) Change in C after 3 years Carbon (t/ha) total C t/ha % of C is below 3 cm stubble + nutrients stubble Change in C after 5 years Carbon (t/ha) Depth (1 cm increments) stubble + nutrients stubble

16 Carbon: 1 and no supplementary 33 t/ha nutrients 1 34 t/ha C (t/ha) 3 1 block 1 block block 3 block 4 Carbon: 1 and t/ha 1 41 t/ha + supplementary nutrients C (t/ha) 3 1 block 1 block block 3 block 4

Carbon: 1 and 1 Depth (1 cm intervals) Carbon (t/ha) 4 6 8 1 1 14 16 1 3 4 5 6 7 8 9 1 11 1 13 14 15 16 mean 1 (+) nutrients (37. t/ha) (-) nutrients (33.1 t/ha) 4.

17 Carbon: 1 and 1 Depth (1 cm intervals) Carbon (t/ha) mean 1 (+) nutrients (37. t/ha) (-) nutrients (33.1 t/ha) 4.1 t/ha (+) nutrients increased FF-C by 4.1 t/ha over 3 years (compared to (-) nutrients) Depth (1 cm intervals) mean 1 (+) nutrients (4.5 t/ha) (-) nutrients (34.3 t/ha) 6. t/ha (+) nutrients increased FF-C by.1 t/ha over years (compared to (-) nutrients) Summary FF-SOM always contains C, N, P and S and in constant proportions Supplementary nutrients led to an increase in microbe numbers and faster stubble breakdown Supplementary nutrients led to an increase in FF-SOM measured by measuring FF-C C levels Thanks for listening Appears to work in lab and field

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