Potatoes are delicious. They are also the fourth most cultivated food crop in the world, led only by wheat, rice and maize. Therefore, not only are the delicious, they are important to global agriculture and hunger efforts.
However, they are also sensitive to drought and phosphorous deficiency, both of which could cause significant problems with future production.
A paper recently published in the Journal of Agronomy and Crop Science contains some useful information just in the introduction regarding the positive effects of arbuscular mycorrhizal fungi (AMF) on plant growth, particularly that it has been shown in a previous study by the same authors that the negative effects of drought and low phosphorous availability on potato plants can be reduced by inoculating the seed and soil with AMF. However, the authors of the paper sought to test whether using a biochar amendment, which has also previously been shown to have some positive effects on potatoes grown under salinity stress, will confer any additional resistance to drought and phosphorous stress when combined with AMF. The hole in the research regarding biochar effects in this setting rests on the differing results from previous experiments regarding the usefulness of biochar amendment and the need for a better understanding of how biochar affects AMF. This research could help to decide whether crop management can be optimised through a combination of the two treatments.
Another interesting piece of prior knowledge that was used in the experiment was the effect of using alternating partial root-drying irrigation, when one half of a plant’s root system is watered completely while the other half receives little water, and which half of the root system receives which treatment is alternated. The result is that the water deficient half of the root system activates the production of the stress-induced hormone abscisic acid causing partial closure of the stomata, increasing water use efficiency. Although this also results in reduced phosphorous uptake and reduced plant growth and crop yield, the addition of AMF to the soil compensates for and reverses these negative effects. This is due to AMF acting symbiotically with the plant to effectively increase the plant’s root system and therefore increase its ability to acquire water and nutrients.
The experiment tested 16 different treatments to understand the effect of biochar amendment on potato plants. The 16 treatments were made up of a combination of phosphorous fertilisation levels (P0 being reduced phosphorous addition, P1 being full fertilisation), irrigation amounts (FI being full irrigation, PRD being partial root-drying irrigation), AMF incorporation (M+ being AMF inoculation, M- being no inoculation) and biochar amendment (B+ and B-).
The biochar was created from pyrolyses of Birch wood at 500°C. The AMF species used was Rhizophagus irregularis. The experiment was carried out in a randomised pot experiment with the PRD treatments being performed by using plastic dividers in the pots that created a water-tight separation between the halves of the pot. One seed potato was added to each pot in a sandy loam soil with each treatment being performed in triplicate. Each treatment was applied for 30 days with the PRD being switched between pot halves of that treatment every 6 days. After 30 days the plants were harvested and plant total and root biomass, leaf area, phosphorous and nitrogen uptake, AMF colonisation of roots and water use efficiency were measured.
In a separate experiment, the researchers tested the ability of the biochar to adsorb mineral phosphorous and nitrogen in a water solution in order to understand the adsorption characteristics of biochar alone.
The results section, given the number of treatments, reads a bit like a shopping list. Table 1 gives a decent overview of the statistically significant effects of each treatment.
Table 1 from article – Statistical significance of treatment effects on ten measured criteria.
The most significant results for purpose of understanding the effects of biochar application were:
1. Effects on plant growth and AMF root colonisation
The researchers found that, save for in the P0 FI M- treatment, the application of biochar had a negative effect on potato plant growth. Where other experiments had found that the addition of AMF to plants under phosphorous and water stress reversed the negative effects of those forms of stress, the addition of biochar reversed the positive AMF effects. While the highest biomass was recorded in the P1 FI M+ & B- treatment, the lowest biomass recorded was in P0 PRD B+ treatments. Biochar amendment had no effect on root biomass.
Further, it was observed that biochar amendment restricted young potato plants from growing and resulted in the death of some.
2. Soil water dynamic and water use efficiency
The data from the water use efficiency in treatments when biochar wasn’t applied demonstrated that water use efficiency was increased under PRD irrigation conditions compared to full irrigation, in line with earlier research.
When biochar was applied, the effect of the treatment on water use efficiency was linked with the change in biomass. Therefore, the negative effects of biochar that lowered biomass in turn lowered water use. Therefore, in and of itself, biochar didn’t demonstrate any appreciable effect on water use efficiency.
3. Soil pH, water soluble phosphorous and acid phosphomonoesterases activity
Biochar lowered pH only in the P1 FI M- treatment, had no significant effect on pH in the other treatments and no significant difference between the presence and absence of biomass was found in relation to water soluble phosphorous.
4. Plant phosphorous and nitrogen uptake
Biochar treatments resulted in the decrease of phosphorous uptake in plants. The only exception was in the P0 FI M- treatment (therefore, it only seemed to benefit the crops if there was low phosphorous, no AMF but full irrigation). Across the range of treatments it appears that all the good work performed by AMF in assisting plant phosphorous uptake is undone by biochar addition.
Further, biochar addition decreased total nitrogen uptake in all treatments (save for FI P0 M- and P1 FI M+) and the decreased nitrogen uptake was more pronounced under PRD irrigation.
Figure 5 from article – (a) Plant P uptake (mg plant−1) and (b) Plant N uptake (mg plant−1) as affected by P fertilization levels (P0: without P fertilizer, P1: with P fertilizer), inoculation treatments (M−: mycorrhiza free substrate, M+: Rhizophagus irregularis), irrigation treatments (FI: full irrigation and PRD: partial root-zone drying irrigation) and biochar treatments (B−: without biochar, B+: with biochar). Error bars indicate S.E. (n = 2–3). Different letters on top of columns are indicating significant differences (P < 0.05) between B− and B+ treatments within same irrigation, P fertilization and inoculation treatments.
5. Biochar adsorption of nitrogen and phosphorous in aqueous solution
Biochar didn’t show any adsorption of nitrogen and a 0.96% adsorption of phosphorous after 24 hours in an aqueous solution.
Pot experiment findings generally
In all, the researchers found that the change in total biomass of the potato plants was linearly related to the change in phosphorous uptake in the plants across each of the different treatment types but the change of nitrogen uptake, although less significantly effected by the treatments, had a stronger linear relationship with change in biomass.
Figure 1 (from article) (a) Total biomass of plant (g plant−1), (b) Leaf area (cm2 plant−1), (c) Root biomass (g plant−1) and (d) AMF root colonization (%) as affected by P fertilization levels (P0: without P fertilizer, P1: with P fertilizer), inoculation treatments (M−: mycorrhiza free substrate, M+: Rhizophagus irregularis), irrigation treatments (FI: full irrigation and PRD: partial root-zone drying irrigation) and biochar treatments (B−: without biochar, B+: with biochar). Error bars indicate S.E. (n = 2–3). Different letters on top of columns are indicating significant differences (P < 0.05) between B− and B+ treatments within same irrigation, P fertilization and inoculation treatments
Like we said, a lot of results, and many of the generalisations that could be made had one or more exceptions.
From the data obtained the following general findings can be stated as:
- AMF is brilliant as it has the ability reverse the effects that drought and phosphorous deficiency has on phosphorous and nitrogen uptake and, by extension, plant biomass.
- The ability of AMF to work its magic in drought and low phosphorous conditions was impeded by the addition of biochar.
- Biochar did work to increase biomass in one scenario, when AMF was not present, phosphorous was low and there was full irrigation. Why this it so? We don’t know.
The effect of biochar cant be put down to its adsorption of phosphorous and has only minimal adsorption of nitrogen. So, why are we seeing these negative effects?
In relation to reduce biomass, the researchers hypothesised that the effect of biochar on the soil structure, particularly additional porosity, would alter water and nutrient retention rates in the soil, lowering accessibility to it for plants and AMF. However, unknown factors external to the experiment were also considered possible causes of the reduced biomass effect. How the altered water retention capacity of the biochar amended soil could cause this effect is difficult to pin down. The soil water content didn’t appear to be affected by the biochar amendment, nor was the pH particularly changed, both factors which can see reduced time for nitrogen and phosphorous to be available to the plants. However, biochar is hydrophobic which may cause water to permeate lower in the soil much more quickly, taking with it the nitrogen and making it more difficult for the plant to access.
But what about the effect of biochar on the AMF? Colonisation of plant roots didn’t seem to be affected by the biochar amendment, so there may be some effect of the biochar on the ability of the AMF’s nutrient gathering or its symbiosis with the plant. However, this has been left unanswered and further study is required on how the two interact.
What to do with this information?
It is rare (perhaps a first) that we write about a negative study (although no study except a poorly designed one is truly negative – we always learn something). But, whether you’re a spud farmer or just growing tubers in the backyard, this article teaches us that:
1. AMF are important and can reverse the effects of reduced water and phosphorous content in the soil but adding biochar as well wont help.
2. If you have full irrigation but low phosphorous and no additional AMF or method to incorporate AMF into your crop, adding some biochar may help increase phosphorous uptake;
3. Biochar addition too early in the potato plant growth stages may delay growth and potentially kill your plant;
4. PRD combined with AMF is useful way (more practicable if growing potatoes in a pot) to reduce water use but not lose biomass;
5. If you are a research, there is a big gap in our knowledge – what effect does biochar have on AMF – does it affect AMF uptake of nutrients in the soil, does affect the symbiosis between the AMF and the plant roots, or something else altogether?