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:

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 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.

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.