There have been some queries on the focus paddock organic carbon results, and we've just received the final pre-sowing 2009 data, so it seemed timely to update the website (below). An important thing to point out when comparing 'Till' and 'No Till' paddocks is that organic carbon levels will also vary with soil texture (naturally lower in sandier paddocks). Although the samples have been quite precisely located, there will also be measurement and sampling error in these results too.

Now that there is data from harvest 2008, and pre-sowing 2009, it's evident that measured organic carbon has tended to be higher at harvest. In most paddocks organic carbon measured pre-sowing has fallen between 2006 and 2009, in some paddocks markedly. 

 

How to explain this? Organic carbon (measured here as Oxidisable Organic Carbon, Walkley & Black 1934) is not a fixed part of the soil that rises or falls slowly depending on what establishment system is used. Soil organic carbon responds dynamically to water, nutrients and carbon inputs.

When the soil dries out, soil microbes (a part of organic carbon) die. When the soil wets up again, there is a flush of mineral nutrients (mineralisation) as the remains of the microbes become available to plants (and other microbes). If the soil stays wet long enough, microbe populations build and begin to use whatever nutrients they can find, with carbon as an energy source. The nutrients are built into living soil microbes and are unavailable to plants (immobilised).  

If there is plenty of carbon about (as there usually is after a crop), the size the soil microbe population can reach (and hence soil carbon) depends on nutrition.

 

Tillage has a reputation for reducing soil organic carbon because it exposes carbon to breakdown processes, but it is really the loss of mineral nutrients associated with that carbon - either by uptake in the plant or by leaching once nutrients are released into soil - that causes the long-term reduced soil organic carbon associated with tillage.

Where increases in soil organic carbon have been measured associated with no till in other countries, these are often conditional on adequate fertiliser inputs, particularly nitrogen. There is an extensive review on the topic in:

Alvarez R (2005) A review of nitrogen fertilizer and conservation tillage effects on soil organic carbon storage. Soil Use and Management 21, 38-52.

(links to the abstract)

In situations where fertiliser (nutrient) supply has been reduced because of poor seasons, soil organic carbon is as likely to fall in no-till as tilled systems, depending on how much nutrient is being taken out by crops and/or lost by leaching. The fact that soil organic carbon has been decreasing is actually an indication that it is doing its job (releasing nutrients to supplement plant demand). We have seen evidence of this in soil mineral nitrogen measurements.

Further comments would be welcome.