Continuing the theme of increasing biomass production discussed briefly in a recent post which claimed:

Producing more biomass is the primary need in a world with a growing population. You can work your way around to understanding this in a crab-wise fashion by thinking of it as energy efficiency: making better use of the sunlight that falls on the planet. It's the ultimate solar power hack.

Oliver notes that increased CO2 concentrations have had a measurable effect on production.

everyone studying carbon dioxide levels agrees that there are "biological sinks" -- places where more carbon-dioxide means more biomass, either because of the direct carbon-dioxide-fertilisation effect (it is, after all, plant food) or because the climatic effects are to the benefit of plants. Growth in sinks = growth in biomass. And a billion tonnes of carbon or so flowing into sinks every year will add up, over time. No denying that.

Oliver cites posts that seem to have am anti-IPCC agenda, and the research papers those posts were based on, though the spin of those posts sometimes exceeds what can can be inferred from the papers, and selects data points that support the agenda. It's not all benefit, and there is reason to suspect that things will not be so beneficial in future if present trends continue, as it seems they must.

Philip works a different angle.

Most TP enthusiasts, myself included, are convinced that the most mysterious effects from adding charcoal relate to soil biology, more than they relate to direct physical and chemical effects, although those realms play important roles also. And, in keeping with my previous post, it seems clear to me that increased energy efficiency is a critical bit here. Plants and microbes are growing more biomass with less effort for reasons that can't be entirely explained by traditional nutrient-based perspectives. Yes, the charcoal adds potassium, yes it raises soil pH, yes it increases soil water and nutrient holding capacity. But the results speak to more, much more.

The behavior of charcoal amended soil seems to defy the limits of the soil-biology system understood by traditional science. However, it would be entirely foolish to think that simple soil nutritional requirements are not still in play. Nutrient deficiencies limit living systems. Charcoal may promote efficiencies that help stretch the budget in regards to those limits, but in the end, the most limiting nutrient before adding charcoal is probably still going to be the most limiting nutrient after adding charcoal.

Increased CO2 concentrations increase productivity, but mostly where CO2 is the limiting nutrient. It also helps plants use less water, since they don't have to "breathe" so hard, and so evapotranspire so much water, in a richer atmosphere. But at some point other nutrients will become the limits. Perhaps lack of nitrogen or any of the other primary and secondary nutrients will be the weak link. Similarly, biochar boosts production, and does so in some fascinating ways the challenge current understandings of soil chemistry and biology, but again there are limits.

What got me thinking about this was consulting soil scientist Doug Edmeades’ posts on soil organic matter. The first, Carbon farming: take-off or rip-off, explored how carbon sequestration efforts can cut both ways. The second, Soil Organic Matter Matters, hits on the most-limiting-nutrient.

Pasture plants need 16 nutrients. Without all 16 the clover will disappear, the pasture will be N deficient, the quality grasses will fail, pasture production would collapse followed by a need to cut back the stocking rate and, given sufficient years, a farm would be back to native pastures and bush. In the process soil carbon levels would decline.

I've made similar arguments, but not as well and not with equal authority. These fellows know their stuff. This isn't speculative, it's historical.

Collapsed pasture production is no idle threat. We know that the collapse of legumes in pasture systems in Europe and in the eastern US helped motivate the expansion of the western US. Against that historical backdrop, Benjamin Franklin famously demonstrated sulfur deficiency when he added gypsum to alfalfa to form the words "This has been plastered". Doug Edmeades mentions this because soil carbon sequestration enthusiasts seem to have temporarily lost track of these limits. The same caution applies to charcoal.

The goal of increased biomass production is valid and important, but it isn't as easy as it is sometimes claimed to be. Production can only increase to the level of the limiting nutrient. Adding more of some nutrient already in adequate supply won't help. And, once you do add more of the limiting nutrient the limit shifts elsewhere, it doesn't go away, at least not on large scale, in the field, applications. At some point the ultimate limit, photoperiod, is reached.

I'm grappling with the implications of combined effects. The Doug Edmeades’ posts explore some the research and observations about the limits of soil carbon accumulation. In well managed systems a point is apparently reached where no more carbon will accumulate from on-site production. It could still rise if biochar was added from off site, but normal accumulation of humus reaches a saturation point.

Some cited research claims that there is even a small loss after that point. Pastures that have long been under good management, and so have ample carbon, have had a tiny loss in the recent decades. That is the same time period when we have had some increase in warmth and some increase in CO2 concentrations. I suspect that these things are related, and that fuller understanding of how all of these factors interact is required to formulate optimal management systems.