The effect of biochars on maize (Zea mays) germination

Abstract

Biochar made from biosolids, corn stover, eucalyptus, fresh pine or willow pyrolysed at 550°C and incorporated into Manawatu fine sandy loam or Waitarere sand at rates from 0 to 10 t/ha did not significantly affect the germination or early growth (root and coleoptile length, and dry weight) of maize seeds. There were no interactions between type or rate of biochar with soil type. The results suggest that biochar incorporation prior to a maize crop should be investigated as a method of increasing stable soil carbon with the potential for mitigating carbon emissions.

Keywords:

• biochar
• black carbon
• carbon sequestration
• Zea mays
• maize

Introduction

Rising carbon dioxide concentrations in the atmosphere and consequent concerns about climate change make it imperative to reduce greenhouse gas (GHG) emissions (Lehmann 2007). New Zealand has an obligation under the Kyoto Protocol to reduce GHG emissions to the level they were in 1990 by 2012, offset those emissions through sequestration or, if this reduction target is not met, purchase emission units on the international market. It has been estimated that to correct the carbon imbalance with forestry, New Zealand would need a planting rate of 150 000 ha/yr. Another method gaining popularity in the agricultural sector, due to its apparent success in Australia and the USA, is the gain of carbon credits through increasing soil carbon. However, the drivers of carbon increase in New Zealand soils are still unknown (Kirschbaum et al. 2009) and exactly how soil carbon would be monitored for inclusion in the Kyoto Protocol is not yet clear. Difficulties with soil variability, knowledge of baseline stocks and prior rate of accumulation (as the intent of the Kyoto Protocol is to encourage a change in behaviour), and the extra problem of unknown drivers suggest that inclusion of soil carbon in the credit analysis could result in increased liability and is thus not yet advised (Parsons et al. 2009).

Part of the uncertainty associated with soil carbon is that in some forms it is not stable. Soil organic matter decomposes. Labile carbon, such as that in the microbial biomass, has a turnover of one to five years, whereas humic carbon may take decades to decompose and inert organic matter such as charcoal may take thousands of years (reviewed by Winsley 2007).

The longevity of charcoal in soil has led to the suggestion that the production of chars through the pyrolysis of biomass and incorporation of those chars into soils could be a feasible method of sequestering carbon (Lehmann et al. 2006; Swift 2001). Chars are already widely present in soils due to natural events, e.g. forest fires (Skjemstad et al. 1996) and anthropogenic processes, e.g. Amazonian terra preta soils (Winsley 2007).

As well as having the potential to sequester carbon, biochars have been mooted as having beneficial effects on soil quality parameters (Krull 2008; Lehmann et al. 2003). Biochar has been reported to increase water-holding capacity in sandy soils (Rasool et al. 2008), improve soil structure (Chan et al. 2008) and enhance chemical fertility (Lehmann 2007). In soils with low organic matter content, chars are an attractive option for increasing soil organic carbon by incorporating large amounts of biochar at crop establishment. There is therefore the potential to increase carbon sequestration in some soils while sustaining high primary production for the economy and minimising emissions trading scheme payments (Parsons et al. 2009).

There is potential to create chars from a wide range of feedstocks, for example grasses, prunings, crop waste and sewage sludge. However, some biosolid biochar feedstocks have been reported to contain high concentrations of heavy metals and toxic substances (Jones & Sewart 1997), which might reduce seed germination and seedling growth with consequent effects on crop establishment and yield. The research reported in this paper was established as a first step in measuring the impact of biochars on germination and seedling growth.

Materials and methods

Two soils were chosen for the germination trials: Manawatu fine sandy loam and Waitarere sand. Manawatu fine sandy loam is commonly used in maize cropping in the Manawatu region. Waitarere sand has potential for development for maize cropping if sufficient carbon content can be built up in the soil to improve water-holding capacity and so avoid moisture stress during dry summers.

Five feedstocks (biosolids, corn stover, eucalyptus, fresh pine and willow) were pyrolysed at 550°C in a gas-fired rotating drum kiln at Massey University (Aitkenhead et al. 2009). Portions of Manawatu fine sandy loam and Waitarere sand were incubated at field capacity at ambient temperature with four rates (0, 2.5, 5.0 or 10.0 t/ha) of individual biochars. The amounts of biochar incorporated were calculated on the 200 mm ploughing depth commonly used in the district (Aslam et al. 1999; Horne et al. 1992; Sparling et al. 1992). After 21 days, to allow equilibration and mitigation of the liming effect of biochars (Laird 2008), 250 g samples of each soil–biochar mix and a control of soil only were spread thinly and evenly on moist Anchor regular weight seed germination paper (ISTA 2009). For each of the four replicates of each treatment, fifty maize seeds (cultivar N48K2, high-quality seed, thousand seed weight = 347.7 g, germination rate >90%) were distributed evenly over the germination paper, which was then rolled, placed in a basket, sealed in a plastic bag and incubated at 25°C for five days (ISTA 2009). Due to time constraints, two replicates were held at 5°C for 48 hours before processing. For all four replicates, germination percentage and coleoptile length were assessed. After washing, seedlings were separated into coleoptile, root and seed, and were then dried at 65°C for 48 hours before dry weights were recorded. The effect of biochar feedstock or rate of application on the germination of maize or on seedling growth response were analysed by an analysis of variance (ANOVA) using SAS (Release 8.02, SAS Institute Inc., Gary, North Carolina). Where significant effects were detected in the ANOVA (P=0.05), means were compared using Duncan's multiple range test. For data where no significant effects were observed, means and standard errors are presented.

Results and discussion

There were no effects of biochar feedstock or rate of biochar application on germination of maize; all germination rated were greater than 96% (average 98% (±0.2%)). Biochar feedstock at any rate did not affect the dry weight of coleoptiles (46.5 (±0.46) mg), roots (23.1 (±0.26) mg), remaining seed (208.3 (±1.62) mg) or coleoptile length (135.1 (±1.34) mm).

Soil type had a significant effect on seed weight and coleoptile weight and length, but not on root weight. Coleoptile weight and length were greater (seed weight conversely lower) in the Manawatu soil than in the Waitarere soil (Table 1). The slower growth of maize in Waitarere soil may affect emergence, particularly under poorer sowing conditions. There were no significant interaction effects of type or rate of biochar with soil type.

Table 1  Response of Zea mays seedling components to different soil types; means with the same letter within a column are not significantly different.

	Weight (mg)
Soil type	Coleoptile	Root	Seed	Coleoptile length (mm)
Waitarere	44.7a	22.7a	211.7a	129.8a
Manawatu	48.4b	23.6a	204.1b	140.8b

These results suggest that, in ideal conditions, biochars from the feedstocks tested could be incorporated during ploughing up to a rate of 10 t/ha in Manawatu fine sandy loam or Waitarere sand three weeks before maize is to be sown.

Sand country in the Manawatu covers approximately 90 000 ha (Muckersie & Shepherd 1995) and has potential for use as dairy support (i.e. providing dry matter for dairy herds located elsewhere). A maize crop, followed by pasture establishment, with biochar incorporation at each stage, could be a practical way of incorporating stable soil carbon, achieving soil improvement and carbon sequestration at the same time. The next steps in this work will be to examine the effect of biochars on maize establishment under field conditions and also to investigate the effect of biochars on pasture species such as ryegrass and white clover which are likely to follow a maize crop during dairy conversion.

Acknowledgements

The authors thank Ruth Morrison for her technical advice, Bob Toes for soil collection, Palmerston North City Council for supplying the biosolids, and William Aitkenhead and Peter Bishop for biochar production. Thanks also go to William Aitkenhead, Mandy Liesch, Sapari Mat and Michael Walker for assistance with trial setup and seedling measurements.