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Developing global soil health metrics: Is SOM the
key to grassland soil health?
C. A. Horrocksa, J. Arangob, J. Núñezb, A....
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Developing global soil health metrics: Is SOM the key to grassland soil health? 

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A healthy agricultural soil can maintain productivity, whilst delivering essential ecosystem services. An unhealthy soil can result in erosion, compaction and reductions in above and below ground diversity, nutrient cycling efficiency, drought tolerance and yield1. Grasslands for livestock production cover 26 % of total global ice free land 2, improvement of grassland soil health is essential for global food security. We aimed to combine a range of ‘low-tech’ and ‘hi-tech’ methods to understand the effect of different forage varieties and management techniques on soil health in both temperate (UK) and tropical (Colombia) grasslands and to develop a scientifically ratified soil health assessment protocol for use by farmers globally.

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Developing global soil health metrics: Is SOM the key to grassland soil health? 

  1. 1. Developing global soil health metrics: Is SOM the key to grassland soil health? C. A. Horrocksa, J. Arangob, J. Núñezb, A. Arevalob, J.A. Cardosob, J.A.J. Dungaita Corresponding author: Claire.Horocks@Rothamsted.ac.uk aRothamsted Research, North Wyke, Okehampton, Devon b CIAT, Km 17 Recta Cali-Palmira CP 763537 Apartado Aéreo 6713, Cali, ColombiaC INTRODUCTION A healthy agricultural soil can maintain productivity, whilst delivering essential ecosystem services. An unhealthy soil can result in erosion, compaction and reductions in above and below ground diversity, nutrient cycling efficiency, drought tolerance and yield1. Grasslands for livestock production cover 26 % of total global ice free land 2, improvement of grassland soil health is essential for global food security. We aimed to combine a range of ‘low-tech’ and ‘hi-tech’ methods to understand the effect of different forage varieties and management techniques on soil health in both temperate (UK) and tropical (Colombia) grasslands and to develop a scientifically ratified soil health assessment protocol for use by farmers globally. RESULTS METHODS Soil was sampled from tropical forage field plots, 3 replicate plots of Brachiaria hybrid Mulato (Mu.), B.humidicola cv. CIAT 679 (Bh 679), B.humidicola cv CIAT 16888 (Bh 16888), and Panicum maximum (P. m), plus a bare soil control were sampled. Due to the tufted growth habit of Mu. and P. m, samples were collected from both within and between tussocks. Multiple proposed soil health metrics were assessed using ‘hi- tech’ and ‘low-tech’ methods. CONCLUSIONS a a a a ab ab b 0 1 2 3 4 5 6 7 Bh 679 Bh16888 PM within tussock PM between tussocks Mu. within tussock Mu. between tussocks Bare soil soilorganicmatter(%drymass) 0 20 40 60 80 100 120 Bare soil Bh 16888 Bh 679 Mu. P. m Lossinteamassafter1month(%drymass) Lipton Green Tea Lipton Redbush Tea Further Work • There was some evidence of an effect of tropical forage variety on SOM concentration qPCR of microbial markers - ‘Hi- tech’ metric • SOM showed a positive correlation with the ratio of fungal (18s) : bacterial (16S) marker genes • SOM showed a negative correlation with the functional gene AMOa, which plays a key role in nitrification • Increased SOM concentration increased aggregate stability in water, reducing the proportion lost through a fine mesh • Despite differences in microbial gene abundances the rate of decomposition of tea bags buried did not differ between forages SOM by loss on ignition Tea-bag test and ‘slake test’ – low tech FIELD SITE The data presented here are from a plot experiment at CIAT, Cali Colombia. The methods used have also been applied in livestock systems in the UK and Kenya. ‘Hi-Tech’ methods include: • QPCR to measure abundance of marker genes and functional gene • Next generation sequencing to measure fungal and bacterial diversity • Loss on ignition to measure total soil organic matter (SOM) • Gas chromatography to determine organic matter composition ‘Low-tech’ methods include: • An adapted slake test using a handmade sieve to measure aggregate stability in water • A soil crusher to measure aggregate friability • ‘Tea bag index’3- burial of tea bags to measure decomposition rates Collaboration with the NGO ‘Send a Cow’ to test farmer friendly soil health metrics in Kenya through peer farmer education. Figure 2. Correlation between soil organic matter concentration and gene copy number ratio of the 18S fungal marker and 16S bacterial marker gene. Figure 3. Correlation between soil organic matter concentration and total ammonia monooxygenase (AMOa) genes in bacteria and archaea. Figure 1. Mean (n=3) soil organic matter determined by loss on ignition from soils under 4 different tropical forages and bare soil. Error bars show ± 1 standard deviation. Figure 4 Correlation between soil organic matter concentration and % soil lost through a 50µm mesh on weight sieving. Figure 5. Mean (n=3) % mass loss of Green and Redbush tea buried for 1 month in soil under four tropical forages and a bare soil control. Error bars show ± 1 standard deviation. Handmade sieves for adapted ‘slake test’ Soil Crusher for measuring aggregate friability • Work in Colombia and the UK has shown differences in total SOM under different forage varieties • SOM shows a positive correlation with increased aggregate stability in water, and friability, indicators of improved soil health • Differences in total SOM also correlate with the abundance of key microbial marker genes but less so with potential decomposition (teabag test) • Ongoing work will identify differences in other soil health indicators in grassland systems and determine to what extent this can be predicted through measurement of SOM 1 Kibblewhite, M.G., Ritz, K. and Swift, M.J., 2008. Soil health in agricultural systems. Philosophical Transactions of the Royal Society B: Biological Sciences, 363(1492), pp.685-701. 2FAO, 2012. Livestock and landscapes. Available online http://www.fao.org/docrep/018/ar591e/ar591e.pdf accessed 12/07/2017. 3 The Teabag index. Available online http://www.teatime4science.org/colophon accessed 31/08/17. We gratefully acknowledge the support of the staff at Send a Cow and colleagues at CIAT Nairobi and CIAT Cali for all their ongoing support. This work was fund through a BBSRC Newton Fund Post Doctoral Fellowship.

A healthy agricultural soil can maintain productivity, whilst delivering essential ecosystem services. An unhealthy soil can result in erosion, compaction and reductions in above and below ground diversity, nutrient cycling efficiency, drought tolerance and yield1. Grasslands for livestock production cover 26 % of total global ice free land 2, improvement of grassland soil health is essential for global food security. We aimed to combine a range of ‘low-tech’ and ‘hi-tech’ methods to understand the effect of different forage varieties and management techniques on soil health in both temperate (UK) and tropical (Colombia) grasslands and to develop a scientifically ratified soil health assessment protocol for use by farmers globally.

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