Evidence for biochar sorption of allelochemicals produced by strawberry guava

Strawberry guava has been repeatedly suggested to be allelopathic, but experimental evidence is limited.  Here we present some lab trial results that confirm allelopathy, and also indicate that the allelochemical(s) produced are sorbed (or otherwise immobilized) by biochars.

Methods:

Leaves of strawberry guava were obtained from dense thickets of the species in Plaine Champagne, Mauritius. Leaves were air-dried for 24 h, shredded and soaked in deionized water to prepare 0.1 g/mL and 0.3 g/mL concentrations of leaf extract solutions. After 48 h, the leaf extract solutions were filtered and the filtrate was collected to be treated with maize (Zea mays) and coconut (Cocos nucifera) biochar. Different dosages of biochar were added to each concentration of leaf extract filtrate to obtain 0 (control), 0.002 and 0.02 g biochar/mL filtrates. Each treatment solution containing biochar was thoroughly mixed for 24 h. Each treatment solution containing a mixture of biochar and leaf extract was filtered and the filtrate was collected for germination experiments. 

Coconut biochar (BC1) was obtained from coconut husks pyrolyzed at ~350°C for 3 h in a small-scale batch pyrolysis system. Maize biochar (BC2)  was obtained by pyrolizing maize biomass at 350°C for 3 h in a lab pyrolysis system. Maize and coconut biochars were pre-washed with deionized water for 24 h to remove leachates that can potentially have negative effects on germination and plant growth.

The germination experiment consisted of a 3-way factorial combination of two concentrations of leaf extract (0.1 g/ml and 0.3 g/ml), two types of biochar (coconut and maize feedstocks), and three biochar concentrations (0, 0.002, and 0.02 g/ml).  In addition there were two controls: deionized water and leachates from each biochar (at 0.02 g/ml only) washed with deionized water, thus giving a total of 2 x 2 x 3 + 3 = 15 treatments. 

Results:

High dosages of strawberry guava extract strongly inhibit germination and seedling development (see figure).  The most dramatic effects are seen for cotyledon development.  At high extract dosages only ~20-25% of seedlings developed cotyledons (Fig. f,h), but treatment with high dosages of biochar increased this to >60%.  In the case of  maize-feedstock biochar (BC2) applied at a high dosage, radish seeds showed germination and cotyledon development rates similar to controls (>95%).  Wow!

(Note that we've left out all the "boring" statistical analysis for this note -- those Experiment.com enthusiasts who are into linear mixed models and such will have to wait for the publication...).

 

Biochar treatments "rescue" maize and radish seeds from strawberry guava leaf extracts.  Means shown ± 1 SE.

Below is a photo from the germination experiment showing inhibition in radish seeds by strawberry guava leaf extract and a major "rescue" effect by biochar!

 

(Columns) Left to right: (1) Deionized water, (2) biochar leachate, (3) leaf extract of strawberry guava, (4) strawberry guava leaf extract with 0.002 g/ml biochar, and (5) strawberry guava leaf extract with 0.02 g/ml biochar

Discussion:

These results give us confidence that biochars act to sorb (or otherwise immobilize) allelochemicals produced by strawberry guava.  These results are exciting, as there have been few viable management alternatives to dealing with the devastating impacts of this invasive species.  We focused specifically on feedstocks (coconut husks and maize stems) that would be readily available in Mauritius and other small island nations facing similar impacts.  The obvious follow-up is to establish field trials to see how well this will work.  We need your help to do this!