10a - soil organic carbon

distribution of SOC

not evenly distributed globally - disproportionately higher in northern hemisphere

soil microbial carbon pump

emphasises the importance of soil microbial anabolism (metabolic pathways that construct molecules from smaller units) and its contributions to soil carbon formation and stabilisation, which can be applied for elucidating the source, formation and sequestration of soil organic carbon

2 types of SOC

POM and MAOM

POM

particulate organic matter - lasts 1-50 years, lighter, more complex, carbon and nitrogen (much more C)

MAOM

mineral-associated organic matter - lasts 10-1000 years, heavier, less complex, carbon and nitrogen (~ 60:40)

why does the northern hemisphere have more organic matter?

Cold/wet weather = more conserved soil organic matter, more favourable conditions for soil microbes

soil organic carbon stocks

arise from dynamic balance between primary productivity inputs from plants and carbon losses arising from biological respiration, losses increased through practices such as ploughing which increase respiration

impact of more clay

ability to store and sequester organic carbon (also influenced by management e.g. reduced by intensive ploughing)

organic matter inputs

Doesn't necessarily improve yields, but improves soil texture

- Some improvement in potato yield (soil is turned over several times before and during growing)

measuring organic carbon in soil - loss-on-ignition (LOI)

heat soil and vaporise soil organic matter (mass lost = assumed organic matter)

- Some issues with this including inconsistent temperatures between labs, some mass lost may be water etc

measuring organic carbon in soil - dry combustion and elemental analysis

most accurate and direct measure, oven temperatures over 1000 °C, so more expensive. Also prone to error due to carbonate minerals. Provides estimates of carbon and nitrogen directly so no conversion factor is required

measuring organic carbon in soil - Reflectance fourier transform infrared spectroscopy (IR)

little sample preparation, high throughput, still undergoing development

measuring organic matter in soil - hot water extraction

extracts most accessible organic carbon with potassium permanganate