J . Sci. Food -4gric. 1982, 33, 249-252

Effects of Season, Cultivation and Nitrogen Fertiliser on the Size of the Soil Microbial Biomass

James M. Lynch and Lynda M. Panting

Agriculturd Research Council Letconzbe Laboratory, Wantage, 0.xfordshir.e OX12 9JT

(Manuscript received 23 September 1981)

In arable soil, the biomass in the surface 5 cm of soil, as measured by the fumigation/ respiration technique, increased when roots grew, reached a maximum around the time of maximum root production and thereafter declined. Rotovation of the soil in autumn resulted in a smaller biomass than in undisturbed soil, but only in the spring. Straw on the soil surface seemed not to affect the biomass, but it may have caused a transient increase in early autumn. Application of nitrogen fertiliser in- creased the biomass in soil after direct-drilling cereals but not after ploughing.

1. Introduction

The fumigation/respiration technique,l whereby a soil is fumigated with chloroform and the subsequent flush in COZ from the killed organisms is measured, provides a convenient means to assess the effect of agricultural practices on the size of the soil microbial biomass. Using intact cores of soil, it has been shown that the microbial biomass is greater in direct-drilled than ploughed soil, in grassland than arable soil, in the late spring/early summer than winter, and in the presence of straw from a preceding crop than after burning it.3,3 Some further observations of these effects are now re:ported.

Direct-drilled winter cereals sometimes require a larger than usual nitrogen fertiliser input for maximum yield, possibly because soil nitrogen is mineralised more slowly than in soil that has been ploughed.3 However, another possibility, that a greater microbial biomass in the direct-drilled soil might compete more effectively with plant roots for available N, has been examined.

2. Materials and methods

The method was based on that described by Jenkinson and Powlson,' but to lessen soil disturbance intact cores were taken from between the crop rows; COZ production was measured by gas chroma- tography.2, The microbial biomass ( B ) of the surface 5 cm of soil was calculated from the relation- ship: Bk = X - x = F, where F is the flush of COz produced from the dead biomass, X the amount of COZ produced in the 10 days following fumigation, x the amount of COZ produced by non-fumiga- ted soil in the period and k (0.41) the proportion of fumigated organisms mineralised to COz during incubation at 22"C.'j

The soils sampled were both clays (classification : UK, Stagnogley ; USDA, Typic Haplaquept ; FAO/UNE.SCO, Eutric Gley~ol ) .~ At least eight replicate cores (half of which were fumigated) were taken from single plots. The air temperature was recorded, this being close to the temperature at the soil surface.

2.1. Effects of rotovation, season and straw The soil sampled was Lawford series of pH 6.5 from Northfield, Challow, Oxfordshire. Straw from the preceding crop (winter oilseed rape, Brassica nupus) was generally burnt, but to test the effect

0022-5142/82/03oCro249 $02.00 0 1982 Society of Chemical Industry

249

250 J. M. Lynch and L. M. Panting

of straw it was chopped and left on the surface of some plots. On 24 September 1979, some plots were rotovated (cultivation to a depth of ca 10 cm). Winter wheat (Triticurn aestivurn) was sown on 30 October 1979.

2.2. Effects of fertiliser and cultivation The soil sampled was Denchworth series of pH 6.3 from Cornpton Besuchamp, Ashbury, Oxford- shire. Straw from the preceding crop (winter oilseed rape) was burnt. On 28 September 1979, some plots were ploughed (cultivation to a depth of ca 20 cm). The winter wheat crop was sown on 13 October 1979 and 24 kg N ha-l (as ammonium nitrate) applied to the seed-bed. In some plots further applications of 47 and 70 kg N ha-' were made on 5 March 1980 and 18 April 1980, respec- tively (fertiliser added treatment).

3. Results 3.1. Effects of season The size of the soil biomass remained relatively constant between autumn and early spring, increased to reach a maximum around late spring and thereafter declined to its original level (Figure 1). This seasonal change did not seem to depend solely on temperature.

500 t '0° i

I loo i QI J -I

r -7 No" Dec ' Jan Feb Mar Apr May Juo Jul

1979 1980

Figure 1. Biomass in the clay soil (Lawford series) between November 1979 and July 1980. The soil was direct-drilled with a winter wheat crop on 30 October 1979. The bars indicate standard error; 0 , bio- mass and 0, temperature.

Table 1. The effect of ammonium nitrate fertiliser on the biomass of direct-drilled and ploughed soil. The winter wheat crop was sown in the soil (Denchworth series) on 13 October 1979 and Nitram (24 kg N ha-' as NH4N03) applied to the seed-bed. Further applications of 47 and 70 kg N ha-l were made on 5 March 1980 and 18 April

1980, respectively

Date Cultivation Fertiliser

Microbial biomass and standard error'

(mg C 100 g-l dry soil)

Microbial biomass &5 cm

(kg C ha-')

Soil organic C in microbial biomass (% g/g)

20 March 1980 Direct drilled

Ploughed

17 July 1980 Direct drilled

Ploughed

Added None Added None Added None Added None

8 4 2 6 .8" 58 +9.0b 57 * 3.90 5 5 2 1 . 7 b 99 k 7.6" 7 8 + 4 . 9 b 6 3 + 4 . 4 c 60 3 .9c

365 254 237 230 435 345 270 254

2.1 1 . 5 1.4 1 .4 2 . I 2.1 1 . 5 1 . 5

* On each sampling date, results not followed by the same letter are significantly different I P = 0 . 0 9 .

Effects on soil microbial biomass 251

3.2. Effects of rotovation and straw No significant difference was observed from soil which had been direct-drilled in January and July but ;zit the March sampling, the biomass was 40% less in the rotavated soil [47 (s.e. k 6.6) mg C 100 g-' dry soil] than in direct-drilled soil [28 (s.e. k 5.9) mg C 100 g-1 dry soil]. On none of the sampling occasions was there any significant effect of leaving straw on the surface compared with burning it.

3.3. Effects of fertiliser and cultivation After N fertiliser was applied in the spring to the Denchworth series soil, the biomass as measured in both March and July had increased where the soil had been direct-drilled, but not where it had been ploughed (Table 1). Where no spring fertiliser was applied at the March sampling no significant effect of cultivation on biomass was detected, but at the July sampling the biomass was greater where the soil had been direct-drilled than where it had been ploughed.

4. Discussion

The seasonal change in biomass and the greater quantity in direct-drilled compared with ploughed soil closely resembled earlier observations.2 Although temperature probably has some influence on the biomass, there is no reason to doubt the earlier conclusion that the biomass size is linked primarily to the availability of substrates derived from plant roots.

That rotovation affected biomass at the spring sampling only is puzzling because this shallow cultivation practice could be expected to produce effects intermediate between ploughing and direct drilling. Straw on the soil surface or incorporated by rotovation did not affect the size of the biomass, contrary to earlier observations.3 The discrepancy may be attributable to the difference in autumn conditions which in the present experiment were ideal for straw decomposition. This may have led to a transient increase in biomass that could not persist for want of substrate. Even so, the biomass remained relatively constant for most of the year, possibly because it largely represented the autochthonous population* which needs a small energy supply. i n contrast, the increase in bio- mass in response to fresh substrates may be zymogenous or fermentative and this will die out rapidly unless its energy demands are met.

The increase in biomass associated with N fertiliser application suggests that a fraction of the biomass is N-limited. This increase, although evident in direct-drilled soil at both sampling times, will need to be investigated in other soils to test the generality of the effect. The specificity of the effect for direcc-drilled soil and not ploughed may reflect different microbial populations and/or enzyme activities The N may provide a priming action to induce the breakdown of the native soil organic matter (humus). Whatever the interpretation, the effect could be important agriculturally as a possible explanation for the greater N requirement when crops are direct-drilled instead of ploughed. However, other explanations are possible: there could be greater losses due to leaching in the direct-drilled areas and there can be greater denitrification of available N in the spring after direct drilling.9 The extra biomass following fertiliser additions was on average 100 kg C ha-1 in the sur- face 5 cni. if the C :N ratio of microbial cells is assumed to be about 5: 1, this means that about 20 kg N ha-' is immobilised in the microbial biomass. This is similar to the extra 25 kg N ha-1 that is often recommended should be added to the seed-bed when direct drilling.10 i f that N were in the autochthonous biomass, its release would take much longer than if it were in the zymoge- nous population. Little is known about the re-cycling of N from biomass, but clearly such informa- tion is desirable from both basic and practical standpoints.

References 1.

2. 3.

Jenkinson, D. S . ; Powlson, D. S . The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol. Biorhem. 1976, 8, 209-213 Lynch, J. M.; Panting, L. M. Cultivation and the soil biomass. Soil Biol. Biochem. 1980, 12, 29-33. Lynch, J. M.; Panting, L. M. Variations in the size of the soil biomass. Soil Biol. Biochem. 1980, 12, 547-550.

252 J. M. Lynch and L. M. Panting

4. Dowdell. R. J. ; Cannell, R. Q. Effect of ploughing and direct drilling on soil nitrate content. J . Soil Sci. 1975,

5. Lynch, J. M.; Panting, L. M. Measurement of the microbial biompss in intact cores of soil. Microb. Ecol. 1981, I, 229-234.

6. Anderson, J. P. E.; Domscb, K. H. Mineralization of bacteria and fungi in chloroform-fumigated soils. Soil Biol. Biochem. 1978,10,207-213.

7. Cannell, R. Q.; Ellis, F. B.; Christian, D. G.; Graham, J. P.; Douglas, J. T. The growth and yield of winter cereals after direct drilling, shallow cultivation and ploughing in non-calcareous clay soils 1974-8. J. agric. Sci., Camb. 1980,94,345-359.

8. Winogradsky, S. Sur la microflore autochtone de la terre arable. C.r . hebd. skanc. Acad. Sci., Paris 1924, 178, 1236-1239.

9. Dowdell, R. J. Fate of nitrogen applied to agricultural crops with particular reference to denitrification. Phil Trans. R . SOC. Lond. 1981, In press.

10. Ministry of Agriculture, Fisheries and Food. Fertilizer Recommendations for Agriciiltiiral and Horticiiltiiral Crops Her Majesty’s Stationery Office, London, 1979.

26,53-61.