The Role of Soil Organic matter in Terra preta. How When Why

[Michaelangelica]

For me the interaction of charcoal with SOM is one of the main (Four?) pillars of Terra preta farming and gardening

Yet research on this is equivocal. Some say char speeds up the processes of CO2 release from the soil SOM and this precludes charcoal being used to ameliorate Global warming. Others claim the opposite. Others just say we need more research (fence sitters:)).

 

If TP is going to be promoted as a strategy for reducing Global Warming this important question needs to be addressed

 

Any thoughts?

Watch this space.

 

Of the roughly 1,500 Gt of C stored in organic matter in soils, an estimated 250-530 Gt resides in C pools with turnover times of decades or less. Models extrapolating the response of the fast-cycling soil C pool on the global scale show its importance in affecting atmospheric CO2. A great deal of work remains to be done to improve our assessments of the role of soils in the global C cycle.

Potential responses of soil organic carbon to global environmental change -- Trumbore 94 (16): 8284 -- Proceedings of the National Academy of Sciences

[Philip Small]

My sense is that sort term CO2 release speeds up, but is offset by increased CO2 sequestration increased above ground (and live root) biomass sequestration. Soil nitrous oxide release is also reduced. Char and soil change and microbial community adapts over time. Resulting soil exhibits slower CO2 release even though microbial biomass is more gigantic in TP soils.

 

Why does CO2 release speed up short term? For one, reduced nitrous oxide release means higher nitrogen use efficiency. When N is limiting, this is sure to increase microbial respiration above background, no? My sense is that with the liming effect of the ash component, bacterial activity is encouraged when soil pH is acidic, and alkaline ash influences pH higher. This is further encouraged by the presence of low-recalcitrance wood gas condensates aka wood vinegar. I also sense that fungi are stimulated.

 

Elaine Ingham (soilfoodweb) has pointed out that recalcitrant humus which can resist bacterial decomposition is more vulnerable to fungal breakdown. I sense this fungal effect in the short term CO2 release reported recently, but have difficulty visualizing what that mechanism is. I take comfort in Johannes Lehman's presentation that I saw in New Orleans about the surprising micro-heterogeneity in distribution of soil organic constituents and mineral nutrients. Whereas we soil scientists converse in standard (expected) C:N:P:S ratios for bacteria vs fungi vs humus vs wood vs straw, the reality demonstrated by Johannes Lehman is that those ratios vary wildly in a micro-scale survey through soil, with distinct sinks of C or N or P or S ocurring in separate locations. The expectation was that the nutrients would peak where the carbon peaked. Soil pH and pE micro-heterogeneity is likely orders of magnitude higher. Fungi can be stimulated by pH lowering effects of wood gas condensates at the same time that bacteria is stimulated by the pH increasing effects of the ash. Thats a start, but as with all things biochar the real story is likely to be far more complex and interesting.