• Bert Quin

Soil Carbon and its Role in Soil Fertility

Grazed pastures comprise over 80% of NZ’s agricultural land.

When these pastures were converted from native forests, bush and tussock, sown with ryegrass and rhizobia-treated clover seed, and fed with P, S and trace elements, soil organic matter levels (and of course the organic carbon, nitrogen and sulphur they contained) were observed to increase steadily, for decades in many cases, before reaching a new, apparently sustainable, plateau. Soil biological activity, most easily measured by counting earthworm numbers, also increased substantially, particularly if earthworms had been artificially introduced to an area. The same trends happen, but over a shorter timeframe, after heavily cropped land is returned to pasture.

One of the major foundations to these higher soil organic matter levels was the fixation of atmospheric N by the rhizobia bacteria in clover root nodules. Decaying plant matter then provided C, N and other nutrients for soil micro-organisms and other plants.

But in recent years there is increasing concern and evidence that soil organic matter contents are declining under pasture. Large quantities of soluble organic C and N are being leached from the soil, and no longer being replenished.

Why? There is no simple answer, but global warming is certainly not anything like the whole answer. My belief is that we have, for the past 30 years or so, been changing the dynamics of what was a sustainable system.

In the drive for more production, we have increased the proportion of pasture being grown that gets consumed, through subdivision and rotational grazing management…

In the drive for more production, we have increased the proportion of pasture being grown that gets consumed, through subdivision and rotational grazing management. We have also pushed grass production per hectare – at the expense of clover – by rapidly increasing the use of nitrogen fertilisers, especially urea and DAP. This has increased net shoot to root growth ratios, as root senescence occurs after grazing. More and more of the nutrient content of the total pasture production grown, especially C, N and K, is being returned, very unevenly, in dung and urine, from which much of the C and N is leached in soluble form.

A recent Landcare study of 6 dairy farms in the Waikato found that 3 farms had experienced substantial net losses in total soil C and N over the last 20 years, ranging from 1 to 9 tonnes C per hectare annually, and 0.2 to 0.3 tonnes N/ha/yr. Two were more or less in balance, and one showed substantial gains.

A follow-up AgResearch study on the sames farms, reported in February 2011, looked at how much soluble organic C and N was been lost by leaching through the soils. They found that this loss mechanism alone could explain 20-30% of the total soil C and N losses. They also found that, surprisingly at first, the poorer the soil drainage, the greater the C and N leaching loss. It was postulated that the lower infiltration rates of the poorly draining soils was, because it allowed ponding of water after rainfall events, had the potential for anaerobic conditions, which is known to increase decomposition of soil organic matter.

The increase in stocking rates ( greatly accelerated in the case of dairy farms by the use of imported palm kernel), is a prime cause of soil compaction, anaerobic soil conditions, shallower rooting and lower soil soil C and N-accumulating biological activity, further reducing the ability of excreted C and N to be captured by microbes before it is leached.

We need to find ways of stimulating soil microbial activity and making sure that we our maintaining, and increasing where possible, the organic matter content and biological activity of our soils. Better grazing management, such as the use of stand-ff pads from which excreta is collected for spraying back, is part of the answer. Another, equally important, part is to determine what soil amendments and what forms of nutrients most cost-effectively assist soil microbes and fauna to do their job. This is the challenge that Fertilizer NZ has taken up. It is in fact a challenge for all New Zealanders, because our soil is our greatest resource. We cannot directly influence the weather, or prevent natural disasters like the terrible Christchurch earthquake, but we can learn out to best nourish and protect our soil.