Combined effect of Biochar and Arbuscular Mycorrhizal Fungi on Potato Growth in Conditions of Drought and Low Phosphorous Availability

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

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

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.

Biochar and AMF table 1

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.

P and N uptake biochar

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.

Plant biomass biochar figure 1

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.

Discussion

From the data obtained the following general findings can be stated as:

  1. 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.
  2. The ability of AMF to work its magic in drought and low phosphorous conditions was impeded by the addition of biochar.
  3. 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?

 

 

 

How plant diversity benefits soil structure

Soil health and, particularly, the concern about increasing soil degradation and its effect on ecosystems, is a significant area of research. We’ve previously written about crop diversity and how it can have positive effects on soil organic matter and how crop residue return increases microbial biomass and soil health. However, a new article in Ecology Letters noted that many studies have focused on the biological effects of plant diversity on soil. Therefore, these researchers designed a study to look at the direct physical effects of species diversity on soil properties, leaving aside the biological effects.

Method

The experiment was conducted in two parts: a mesocosm trial and a field trial.

The mesocosm trial used 64 mesocosms with 24 plants in each. The type of plant added depended on the treatment. Six plants were used; two grasses (Lolium perenne and Anthoxanthum odoratum), two forbs (herbs) (Plantago lanceolata and Achillea millefolium) and two legumes (Trifolium repens and Lotus corniculatus). Therefore, some mesocosms were bare soil (control), 6 were monocultures of one of the six plants, and the remainder contained each available combination of two or more of the six plants. The 64 mesocosms were replicated again as some of the subsequent soil testing procedures were destructive. The 128 mesocosms were randomly arranged across 4 glasshouses and grown for 18 months after which the aboveground biomass was measured and soil cores were taken for root trait analysis. The soil cores were used to test for stability through three methods:

  1. slaking (rapid immersion in water);
  2. microcracking (slowly wetting); and
  3. mechanical breakdown (agitated in conical flask).

The replicated samples were tested for water conductivity (water flow through the soil) and soil strength, being its ability to withstand being displaced when a mechanical ram applied force to the soil column.

The field experiment used the grassland experiment set up at Jena in Germany in 2002. 82 plots with different treatments consisting of the same species used in the mesocosm experiment save for the L. perenne were combined with a number of other species to provide differing levels of diversity. Plots contained either 1, 2, 4, 8, 16 or 60 different species.

Topsoil was extracted and analysed for aggregate stability, root trait analysis, organic matter content and glomalin-related protein content in similar methods to those used in the mesocosm analysis.

Results and discussion

The data collected from the two experiments indicated that although species diversity increased aggregate stability, certain species of plants, particularly the grasses with their particular root traits, played the most significant role in this improved stability. With improved stability came improved productivity indicated by an increase in above-ground plant biomass significantly correlated with the improved aggregate stability.

Plant diversity Figure 1

Figure 1 from article. Figures (a) – (c) contain aggregate stability data from the mesocosm experiments while (d) – (f) contain the same data from the field experiment. Across the experiment, increasing plant diversity resulted in increased soil aggregate stability when assessed as a function of the remaining mean weight soil diameter.

Rooting strategies of the grasses resulted in a significant increase in resistance to slaking even when planted as a monoculture. The researchers hypothesise that this may be due to the grasses having finer root systems for greater resource uptake, their root length being the greatest and narrowest out of the 3 types of plants in the experiment. Across the experiment it was shown that root length density and root density were positively correlated with increased aggregate stability. Further, finer roots are decomposed easier than thicker roots which is likely to lead to higher organic matter which has independently been shown to increase aggregate stability.

L. perenne, one of the grasses, had the greatest root length and impacted on aggregate stability the most. When it was planted in a mesocosm with a diversity of other plants, the root length density of the mesocosm was not correlated with the soil aggregate stability but, in mesocosms with a diversity of plants but without L. perenne, the correlation between stability and root length density was again apparent (see Figure 2). The data points in the figure (at least for slaking aggregate stability) appear to generally show greater stability overall when L. perenne was included, indicating the significant effect the root system of this grass has on aggregates.

Plant Diversity Figure 2

Figure 2 from article. Without L. perenne (circles), increasing root length density, correlated with species diversity, resulted in increased resistance to being broken down. But the presence of L. perenne skewed this correlation (dots), seeming to have single-handed effect on increasing aggregate stability.

Adding to this finding that the effects were more complicated than just the effect of species diversity alone, the two legume plants resulted in reduced aggregate stability with associated reduced root length but thicker roots and greater root mass. Even in field plots with legumes included in the diversity treatments, a reduction in the aggregate stability of the treatment was observed compared to when the legumes were not included.

These abundant short, thick roots had other advantages though – water permeability and resistance to soil displacement from the force of the mechanical ram increased in the legume treatments.

Conclusion

What does this mean? As almost always, its complicated. However, we can be confident that increasing crop diversity is likely to result in increase soil aggregate stability. Aggregate stability increases the nutrient holding capacity of the soil, assisting plant growth. But some plants in the combination will have a greater affect than others on traits such as stability or resistance to displacement. If you have  sandy soil struggling to form or maintain aggregates and therefore losing nutrients, planting grasses are likely to help to a bigger extent than many other plant types. But if you have a clay soil that struggles to drain water and in which plants struggle to form a solid root system, a legume crop is more likely to increase water permeability and assist in soil aeration.

A point that is not obvious from the paper but could be of interest is whether, or how, legume crops adversely affect root length in a diverse planting. Grasses are considered to have their particular root strategy to scavenge as many nutrients as possible. But if planted next a legume, does the easy accessibility to nitrogen fixed by the root nodules of the legume plant result in the grass roots not growing to the depth they would have otherwise grown to? Perhaps this has been answered before but, if not, it could explain this particular piece of data.

A great experiment with some helpful data for farmers, agronomists and gardeners.

The Effects of Residue Return on Soil Microbial Biomass – A 30 Year Study

The role of microbial biomass (bacteria and fungi) in the soil is significant. Microbes help to decompose organic matter in the soil, recycle nutrients and to help the formation of soil aggregates. These activities increase the mineralisation and retention of elements in the soil required for plant growth. What effect returning crop residues to the soil in combination with different fertiliser use have on microbial biomass is therefore important for both crop health and to avoiding overuse of fertilisers and related financial and environmental consequences.

A study in the Journal of Agricultural Science looked at the effects of 30 years of returning maize to an experimental plot at the Jilin Agricultural University in combination with a variety of fertiliser treatments on the amount and composition of the microorganisms in the soil.

Using a split-plot design, the researchers had 3 residue return treatments of 0, 2.5 and 5 thousand kilograms of residue per hectare per year. Each plot treatment was repeated in triplicate and was subdivided into 4 square metre subplots. Each subplot was randomly treated with either no fertiliser, nitrogen only, potassium only, phosphorus only or with the various combinations of two or more of those fertilisers.

The soil was a clay loam soil and the subplots were divided by concrete barriers that were buried to a depth of 2 metres. On each plot a target of 60,000 maize plants were grown per hectare and after harvest the residues were dried, cut and incorporated back into the soil to a depth of 20cm at the various rates. On 8 May 2014, after 30 years of the soil be subjected to the various treatments, 4 soil sample cores were taken from each plot for analysis.

The hypothesis of the study were that:

  1. fertilisation would lead to significant changes in soil microbial communities to differing degrees dependent on the amount of maize residues also returned to the soil; and
  2. the soil microbial communities would be due to treatment-induced changes in the properties of the soil.

Results

Given the 24 different soil treatments there are a large number of statistically significant results reported on. Some of the more interesting or useful results were:

  • All fertiliser treatments resulted in a lowering of the soil pH. Plots with residue returned at 5000kg/ha had a significantly lower pH than the other residue treatments;
  • Fertilisation had no effect on organic carbon content of soils under control and 5,000kg/ha residue returns but a significant effect was seen when fertiliser was combined with 2,500kg/ha residue returns. Plots with residue return had higher organic carbon carbon than plots with none;
  • Fertilisers had differing effects on total nitrogen content depending on residue return but the more crop residues that were returned the higher the total N observed;
  • The carbon-to-nitrogen ratios were highest in plots without residue return and decreased as residue return increased;
  • Residue return resulted in a significant increase in available nitrogen and potassium, but not phosphorus.
  • A 5,000 kg/ha residue return resulted in a higher total and ratio of soil microbial groups than both the other residue treatments but the ratio of fungi to bacteria was reduced;
  • Fertiliser effect on soil microbes depended on the crop residue amount for a significant effect to be observed;
  • Residue return significantly effected fungi and bacteria and had an indirect effect on bacteria due to an increase on the soil fertility while fertiliser effected bacteria in the soil as a result of the decrease in pH;
  • Microbial community composition of plots with 5,000kg/ha returned were significantly different from the other residue treatments.

residue return figure 1

Figure from article. Effect of residue return amount and mineral fertiliser application on microbial biomass.

Discussion

The main point raised by the researchers in the discussion was that there seems to be a threshold over which the rate of residue return will have a significant effect on microbial biomass, as can be seen in figure 1 (a) above. The biomass amounts between the control and 2,500kg/ha residue return treatment show no significant effect but a visible effect can be seen when the 5,000kg/ha of residue was returned. Whilst it has been previously explained that an increase in residue causes an increase in organic carbon, in turn increasing biomass, in this experiment the increase in residue amount and increase in biomass did not correlate with the changes in organic carbon amounts in the soil.

If such a threshold is confirmed, knowing this threshold under different soil types, different crop types and environmental conditions could be an important piece of knowledge to farmers in adjusting their fertiliser rates for a given rate of residue return.

Conclusion

Soil nutrient composition is effected by many factors. Tillage, crop rotation, soil type and texture are just a few of these. Understanding how two agronomic treatments such as residue retention and fertiliser will interact and adjusting application rates of the fertiliser will certainly assist food production. Whilst there is further research required to confirm the threshold observed in this study, whether different crops have different effects and whether crop rotations alter the impact are just a few. But this study can be used to guide farmers and agronomists in broadly understanding the effect of residue retention.

 

Nitrogen Use Efficiency – Current Knowledge and Future Development

The significance of nitrogen to agricultural science is a recurring theme for the Legume Laboratory, having recently been the focus of articles about the possibility of artificial nitrogen fixation and estimating nitrogen mineralisation rates from legume crops.

A review piece was recently published in the journal Agronomy about nitrogen use in cereal crops, focusing on our current knowledge of how to increase efficiency in its uptake and identifying areas where developing technology may result in further efficiency improvements.

Wheat crops (and plants generally) require nitrogen to:

  1. establish and maintain photosynthetic capacity and activity;
  2. maximise the number and size of seeds; and
  3. increase the quality of crop products.

Nitrogen is highly mobile in soil and the efficiency of plant uptake depends on many variables including the type of crop and environmental conditions such as soil type and rainfall. For example, rice is known to have the lowest rate of uptake amongst cereal crops while barley has the highest.

The literature review doesn’t delve into the research behind many of its statements but it is a great starting point for some basic knowledge and further research into different sources and uses of nitrogen fertilisers, different application methods and current and developing technologies to diagnose nitrogen status in crops.

agronomy-06-00025-g001-1024

Mind-map figure. Left side – inorganic nitrogen fertiliser sources split based on nitrogen content. Right side – current and potential technologies for nitrogen sources and in-crop monitoring. Figure reproduced from article.

Fertiliser sources

The article describes the pros and cons of different inorganic nitrogen sources besides their nitrogen content. For example:

  • Anhydrous ammonia, which contains the highest nitrogen content of inorganic fertilisers, is a gas requiring pressurised storage, specialised equipment for storage, handling and application and can lower soil pH undesirably;
  • Aqua ammonia has high ammonia content but is volatile at temperatures above 10ºC and needs to be injected into soil;
  • Ammonium nitrate combines two different forms of nitrate, reportedly improves baking quality of wheat, but has low nitrogen content compared to other sources.
  • Ammonium sulfate contains both nitrogen (but in a low amount compared to other sources) and sulfate and is useful for acid-requiring crops and high pH soils;
  • Ammonium chloride contains a low concentration of nitrogen but one that is suitable for chloride responsive crops such as cereals and coconut but not for crops that cannot tolerate the chloride. It also lowers soil pH.
  • Urea, the most widely used nitrogen fertiliser source due to ease of manufacture, transport, and low cost, is also volatile, phytotoxic to susceptible crops and can be toxic to germinating seedlings when emerging.

The article discusses the advances in matching as closely as possible in time the release of nitrogen from fertiliser to when the plant requires it. A method such as urease inhibition controls nitrogen release and has been found in studies to increase potato tuber yield and nitrogen use efficiency while also decreasing nitrification. However, as with many fertilisers, the positive effects are affected by the status of the soil prior to treatment, it is difficult to predict the time-frame and release rate of nitrogen, and yield increase doesn’t consistently match the use of nitrogen inhibitors.

The use of microogranisms is a potential nitrogen source which could increase crop yields. Plant growth-promoting bacteria have been shown to promote growth in row or horticultural crops, effecting the production of growth regulators that stimulate the plant uptake of nutrients.

New sources of nitrogen such as nanofertilisers have the potential to deliver more nitrogen from lower dose amounts but require more research on possible adverse effects. Coating seeds in nutrients appears to have some potential to improve nitrogen uptake, and recapturing nitrogen lost from agricultural fields or after food consumption could be the basis of new fertiliser production.

Nitrogen fertiliser application and management

Different fertiliser sources, different crops and different application times require different methods of application. The tables below set out the different nitrogen sources and applications methods for different application times to match nitrogen availability with crop demand (top) and the effects of management practices on nitrogen nutrition.

agronomy figures

 Tables sourced from article.

Diagnosing crop nitrogen needs

Matching nitrogen supply to crop requirements is an area where significant improvements in efficiency can be found. Diagnosing when crops require nitrogen application during growth is limited to a few methods such as sap nitrate tests (high accuracy but labour intensive and destructive to the plant) and optical sensors (non-destructive and reliable but cannot detect over-fertilisation). The development of satellite and drone technology has opened up the possibility of remote sensing nitrogen status. Developing accurate methods of optically determining whether, and how much, nitrogen is needed by a particular crop is both a matter of technology development and increased knowledge of nitrogen uptake pathways and efficiencies.

agronomy

Crop nitrogen assessment techniques. Figure from article.

Conclusions

The article provides some useful knowledge that can be used now to help match nitrogen source, application method and application time. But more importantly it shows where the limits of our knowledge are and what technological improvements could assist nitrogen supply and crop production and its a pretty safe bet that the authors have written the review as a springboard for further research.

Incorporating suitable fertilisers for particular crops, soil types or environmental conditions at suitable times will improve crop production, particularly throughout the developing world. Research into the matrix of factors that effect nitrogen uptake in crops to elucidate the conditions that enhance efficiency would build on our current knowledge, and the ability to deliver nitrogen with maximum efficiency at times predicated on real-time in-crop knowledge of nitrogen content will significantly increase food production, reduce the costs and side-effects of fertiliser production and mitigate the environmental effects caused by the nitrification of over-supplied fertiliser.

Estimating Legume Nitrogen Mineralisation Rates

The Australian Society of Agronomy held its 17th Australian Agronomy Conference last year, and the result is a smorgasbord of papers freely available here. From pest management to climate change there is a paper to suit every interest.

One paper which is of use to grain growers in South East Australia is “Legume effects on available soil nitrogen and comparisons of estimates of the apparent mineralisation of legume nitrogen“: in short, if a farmer rotates a legume crop through their field, how can they estimate the nitrogen available to subsequent crops grown in the same field.

Background

The introduction states three important factors in the decomposition of organic nitrogen and mineralisation, being:

  1. The amount of rainfall, which stimulates microbial activity;
  2. The amount of legume residues present; and
  3. The nitrogen content and quality of the residues.

Using these factors, the paper reports on a field experiment conducted in southern NSW that compared the soil nitrogen in fields treated as follows:

  1. Lupin grown for grain;
  2. Lupin grown for brown manure (the crop was killed before seed maturation to retain maximum nitrogen in the plant;
  3. Canola grown for grain;
  4. Wheat grown for grain.

By calculating the amount of residues of each treatment left after harvest and measuring the percentage of nitrogen content in those residues, the study used the data obtained along with the rainfall throughout the period to develop a method for grain farmers to estimate soil nitrogen content, in turn assisting growers to estimate fertiliser needs for crops grown following a legume rotation.

The experiment methodology and results

The soil was tested in April 2011 for nitrogen levels prior to growing each of the treatment crops. In April 2012, after each crop was harvested (and the brown manure crop was terminated), the soil was tested again and then all fields were planted with the same cultivar of wheat. In April 2013, the soil was once again tested.

The testing in April 2012 showed that the nitrogen content was 3 to 5 times higher in the lupin crop treatments than in the wheat treatments, and that the brown manure treatment had the highest quality of nitrogen content.

Further testing in April 2013 confirmed the increased nitrogen findings in the lupin treatments, but also showed that the mineralisation of nitrogen by microbial activity continued through the subsequent wheat treatment sown in 2012.  This resulted in a finding that an equivalent of 4 to 5 kg of nitrogen per tonne of the residue dry matter from Lupin treatments in 2011 remained in the soil and had become available for subsequent plant growth.

The conclusion

Using the data obtained and the rainfall measurements, the researchers delineate some useful rules to estimate soil nitrogen content.

The simplest estimate was to assume approximately 10kg of additional nitrogen have been added to the soil per hectare for each tonne of shoot residue. Using a rough estimate of the percentage of dry mass which is harvested as grain, and which farmers will know at the end of a growing season, the equation suggested for farmers to calculate the expected mineral nitrogen available for growing is given as:

Nitrogen (kg/ha) = 20 x tonne grain yield per hectare

The researchers do point out that rainfall between legume harvest and the following growing season will affect microbial activity and therefore mineralisation rate. Given the variation in rainfall, the equation is likely to be applicable only to areas with similar rainfall patterns to those in South East Australia.

Why is it useful?

Being able to adjust fertiliser rates has the obvious economic benefits for farmers. But it also has environmental effects. Over-application of nitrogen fertiliser increases nitrification of unused nitrogen, resulting the production of the greenhouse gas nitrogen dioxide, and when excess nitrogen runs off into adjoining streams, increased bacterial growth results in eutrophication and resultant negative effects on aquatic wildlife.

For the scientifically minded home gardener or market gardener, weighing your legume crops or green manure and calculating the total weight per hectare can give a rough idea of the nitrogen added to your soil and give greater confidence in the amount of fertiliser required in following growing seasons.

 

 

 

Simulating Growing Food on Mars

We’ve previously written about the challenges presented by trying to grow crops in Mars like conditions. What we discover from these challenges have the potential to increase our growing potential on earth as well as prepare for the possibility of living on another planet.

An article in Science Daily discussed an ongoing experiment by the Wageningen University and Research Centre. The article is based on, we presume, a press release or similar as there is no mention of a published paper.

The article cites the results of the second round of an experiment using soil simulating that which may be found on Mars or the Moon and comparing the growth of different food crops in that soil with the same crops grown in normal compost soil. The researchers found no statistical difference between the experiment group and compost grown crops in their latest experiment.

However, the researchers mention that a previous similar experiment failed, with all crops grown in the extraterrestrial soil having died. This time they added chopped grass to the extraterrestrial soil and grew the plants in flat trays instead of small pots.

If both of these changes were made to all experimental species, the advantage provided by one or the other variable cant be distinguished. The article talks about a problem with watering being fixed as a result of the changes, but this could have been due to either variable (flat trays losing less water than pots or organic matter improving soil structure and water holding capacity) or a combination of both. Further, the addition of organic matter in itself will improve the potential for the crops to grow  independent of water retention.

This issue may be clarified in a subsequent paper, but could be quite important. If the improvement is due to water retention alone and the improvement was found in experimental crops grown in a flat tray without organic matter being added, this may be of more significance than the improvement being due to the addition of organic matter. If the improvement is independent of the addition of organic matter, the ability of the extraterrestrial soil to produce food without addition is of great advantage to any future expedition. However, if organic matter is required independent of the type of vessel the crop is grown in, then we have a problem.

Organic matter, at least in the quantities required to grow crops, will be self limiting on another celestial body. Even if the first pioneers started with significant organic matter to grow crops, the recycling of that organic matter (using crop residues, food waste, faecal waste etc) will not replace what they started with when they grow subsequent crops. As the organic matter dwindles, making the assumption that the chopped grass added in this experiment was the variable that had the most effect on the survival of the crops, so will the crop growth and food producing ability reduce. Eventually, experiment one will repeat itself, and crops will either die quickly or fail for initiate.

This seems like a small point overlooked that may have significant consequences on what we learn from this experiment. The Wageningen University and Research Centre looks like does some really interesting, well thought out work, and perhaps a proper recital of the methods and materials in a published paper would show these concerns unfounded.

What is the effect of leaving some of the vegetable crops up over the winter—how does that improve soil conditions?

Great information for improving your soil…by not doing much at all!

Soils Matter, Get the Scoop!

Intentionally or unintentionally, many gardeners have left plants in their gardens over the winter. This is actually a good thing…and something everyone should consider on a yearly basis!

Scientists – specifically agronomists and soil scientists – refer to the plant “litter” that remains after a harvest as “residue”. Leaving the residues in place over the winter, instead of pulling them up or tilling them into the soil surface, provides numerous benefits for the soil and your garden.

Sustaining life through soil protection leads to a bright future Farmers keep crop residues on their fields for the same reason home gardeners should consider them!  Credit: Fabian Fernandez

  • Plant residues reduce erosion and the loss of valuable topsoil. Residues cover soil and protect it during the non-growing season. Crop residues catch rainfall. This reduces the impact that individual rain droplets have with the soil surface. Residues also slow down any flow of melting snow over the soil. Both help protect the soil…

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The Role of Fungi in Assisting Crop Access to Nitrogen

Researchers from the South Dakota State University have written a detailed review of the role of Arbuscular Mycorrhizal Fungi (“AM”), a fungus known to form a mutual association with a large percentage of land plants, in giving crops greater access to nitrogen in the soil.

AM are obligate biotrophs, requiring carbon from their host plants in order to complete their life cycles. Plants transfer carbon to AM, which form large networks between each other and numerous plant hosts and act as an extension of plant roots, in exchange for the fungi delivering a number of required nutrients to the plant host.

The role of AM in providing phosphate to host plants is well studied; being a relatively immobile nutrient, areas of phosphate depletion around the plant roots arise, and the AM provides an alternative method of access that results in the downgrade in activity of the plant uptake pathways.

But the role of AM in passing nitrogen to plants is unknown.

Roots of plants that form associations with AM have both plant uptake pathways and mycorrhizal uptake pathways for the intake of nutrients. Mycorrhizal uptake pathways are regions that form intimate connections between the fungi and plant roots, allowing the passing of nutrients between them.

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Mycorrhizal uptake pathway and the Mycorrhizal interface. Sourced from article.

The review ultimately finds that the research into nitrogen uptake via AM are inconsistent in their findings and that further research is needed for a strong conclusion to be reached.

The researchers note that recent studies suggest that AM has something in the order of a five times greater affinity for ammonium than the plant uptake pathways, meaning that AM is more adept at sequestering nitrogen from the soil in low nitrogen conditions. Further, there is increasing evidence of the existence of pathways available for the transport of nitrogen through AM hyphae to their host plants and that some plants down-regulate their own N uptake pathways when AM-associated uptake is occurring.

Despite the findings of a positive relationship, the results of the studies reviewed show that no general rule is applicable. Whilst some plants are able to efficiently take up nitrogen via their associated AM network, other studies showed lower efficiency or an efficiency that varied as a result factors that aren’t completely understood.

Understanding with some clarity the plants and the environmental circumstances that result in significant nitrogen uptake via AM communities, and particularly the signals emitted by plants to direct this to commence, could provide a significant advance in the efficient use of nitrogen fertiliser. If certain crops are identified as having positive nitrogen uptake pathways via AM when a certain signal is emitted, and we are able to turn that signal on as required, nitrogen fertiliser utilisation could be increased significantly as can the problems of nitrification and eutrophication of nearby waterways be reduced.

Definitely a great area of research that could significantly improve food production if a breakthrough in our knowledge can be made.

Biochar mitigates salinty stress in potato

Increasing salinity poses a problem for many growers. Areas of land clearing can lead to increased water table heights, pushing salt and ions higher into the soil. Dams and lakes with reduced water intake resulting in lower water levels can also lead to increased concentrations of salt within the water (in turn affecting irrigation water) and in the surrounding soil.

Salinity affects plants in two ways; the increase in salt concentration in the soil surrounding a plant inhibits the plant’s ability to take up water, and the build up of sodium ions within plants can be toxic.

Biochar is a charcoal substance created by heating organic materials such as wood and green waste to high temperature in the absence of oxygen. For more information on Biochar and its uses see here. Biochar is known to improve degraded soils and, in the context of salinity, is able to adsorb salt and reduce its uptake in plants.

A recent article in the Journal of Agronomy and Crop Science tested the effects of biochar amendment on potato plants irrigated with increasingly saline water. Over three months the researchers grew potatoes in pots, one set without biochar amendments and one set with amendment, and subjected plants within each group to irrigation water with either no additional salt, 25 mili Molar (mM) of salt or 50mM of salt. They then measured a number of parameters relating to tuber yield, photosynthetic rate and gas exchange amongst other parameters to assess the various affects of the increasing salt concentration between the two groups.

The increase in salinity on potatoes without biochar amendment had the expected effects, reducing the number of tubers per plant and the weight of the tubers, reduced root length and volume, decreased the rate of photosynthesis, reduced the density and aperture of leaf stomata and increased the concentration of salt and potassium ions in the leaves.

For plants grown in amended soil, these negative effects were reduced, many of them in a statistically significant way.

  1. Tuber yield increased across all treatment groups although only the 25mM treatment group had a statistically significant change.
  2. The researchers found a significant positive linear relationship between stomatal conductance and photosynthesis rates, suggesting that where stomata are negatively effected by a treatment, a reduction in photosynthesis (and therefore growth and health of the plant) will follow. Biochar amendment was found to improve both these measurements compared to plants grown with the same level of salinity in soil lacking the amendment.
  3. Salt adsorption was significantly reduced in plants grown with the biochar amendment.

Interestingly, nitrogen and carbon content in plant leaves where reduced under saline conditions with little difference in plants with biochar amendment.

In their discussion the researchers estimated that approximately 97% of the salt was adsorbed from soil treated with the 50mM solution. While this has immediate positive effects on plant growth, over time it is expected that the capacity of the biochar to continue adsorbing the salt will reduce. Similar studies performed by the same researchers using drought stress found similar effects in plants under stress and similar improvements with biochar activation. The researchers also note the limits of their study and urge testing their results under field conditions.

To conclude, biochar addition to saline soils appears to have the ability to improve the viability of potato plants but repeat applications are likely to be required.

 

Improving Plant Health through Microbiome Selection

When looking for methods of improving crop tolerance to drought, saline soils, certain pathogens and other agricultural concerns, attention is mostly focused on such possible solutions as closely related but better performing species of plant, directly attacking the cause of the stress or searching out transgenic solutions.

But the symbiotic relationship between the plant and its microbiome may hold some secrets that could help us adapt some of the most significant issues in agriculture. If certain microbiomes assist a host plant to survive or even prosper with such stressors as reduced water availability or increased salt concentration when compared to other microbiomes, we can potentially inoculate soils with the right species of microorganisms to maintain or increase our food production productivity.

A review paper in the journal Trends in Microbiology looked at a number of experimental methods in selecting for beneficial microbiomes by assessing the effect of different soils on a particular host trait. The paper describes a number of methods that have been used, the advantages and disadvantages of each, as well as experiment design considerations that have been taken from previous research in the area.

One method that appeared to garner the most favour by the writers was growing a subject plant species in a variety of soils, then selecting lines that met a particular criteria eg number of flowers. The microbiome of selected lines are then inoculated into sterilised soil and re-sown with a genetically identical host plant species. This method results in a representative sample of all microorganisms in the selected line being retained and allow the host plant to continue selecting for the most beneficial (or excluding the non-beneficial) microorganism.

Microbiome engineering

As gene sequencing technology continues to improve, the possibility of identifying the different species and ratios of species in the most beneficial microbiomes is continuing to grow, in turn improving precision in food production practices.