Research Article

Effects of Biochar and Compost Aging on Soil Fertility and Radish Germination

Victor Gonzalez and Jihoon Kang*

School of Earth, Environmental and Marine Sciences, University of Texas Rio Grande Valley, Edinburg, Texas, USA

Received date: 07 Aug 2017; Accepted date: 23 Oct 2017; Published date: 30 Oct 2017

*Corresponding author: Jihoon Kang, School of Earth, Environmental and Marine Sciences, University of Texas Rio Grande Valley, Edinburg, Texas, USA, Tel: 956- 665-3526; E-mail: jihoon.kang@utrgv.edu
Abstract

There is increasing interest in using biochar as a soil amendment for urban agriculture. Soils in the Lower Rio Grande Valley in South Texas, USA, is naturally alkaline and growers have a concern on pH increase following the biochar amendment. Biochar aging with compost is considered to be a simple way for activating the biochar with nutrients to minimize potential harmful effect of fresh biochar amendment on plant growth. This study examined pH change in leachate eluted from the biochar aged with and without compost for a 2-week period and evaluated radish germination in an urban soil receiving the biochar and/or compost amendment in a pot experiment up to a 4-week growing period. The leachate pH irrigated with tap water (pH 7.0) during the aging period was in alkaline range with biochar being more alkaline (pH > 9.0) than compost (pH < 8.0). Radish germination experiment showed that the urban soil amended with compost (with and without aging) performed better in increasing stem length while the soil amended with 50:50 aged biochar and aged compost resulted in comparable growth of radish. Soil test analysis on the growing medium after the pot experiment showed that soils amended with biochar did not have appreciable level of plant available phosphorus (PO4-P=0) while all other medium that had received compost showed appreciable levels of plant available nitrogen (NO3-N=104 to 144 mg kg-1). Our results demonstrated that biochar is recommended to be blended and aged with compost for radish growth and the soil pH increase after the biochar addition may offset by soil buffering capacity in the long term.

Keywords: Biochar; Compost; Germination; Nitrogen; Phosphorus; Radish

Introduction

Adding carbon (C) into soil is beneficial for maintaining soil quality by improving soil physical, chemical and biological soil conditions [1]. In urban agriculture, typical amendment is compost derived from yard waste, which is produced when bacteria, yeast, and fungi break down organic materials by aerobic decomposition [2]. Biochar is another amendment that has been receiving scientific and public interests in recent years as a way to improve soil properties and sequester C to help mitigate climate change [3]. Like most charcoal, biochar is produced by heating biomass (e.g., wood and agricultural residue) with limited or no air to above 250 ºC, a process called pyrolysis [4]. Ideal feedstocks for biochar have 10- 20% moisture and high lignin content such as woody biomass while those for compost have 60-70% moisture, high nutrient levels, and low lignin content [5]. In general, biochar is rich in stable C that can last for hundreds to thousand years while compost is rich in labile C that breaks down relatively easily [6].

Biochar is a very porous material with negatively charged surface, absorbing water and dissolved nutrients up to five times its own weight [7]. It has been shown to improve crop productivity with its efficacy depending on different crops, biochar type, and its application rate according to a meta-analysis from previous published studies [8]. It can also improve biotic interactions including mycorrhizal fungi and biological N-fixers as well as increased water availability to plants [8].

However, if pure (fresh) biochar is incorporated into soil without activation or aging, it may lead to inhibition of plant growth at least temporarily because nutrients are fixed onto the biochar surface and become unavailable for plant uptake [7]. Field trials with fresh biochars has been in contrast with traditional practices such as Terra Preta in Amazonia where biochar was used in mixtures of manure, human faces, food waste and agricultural residue to increase agricultural yield in highly weathered soils [4,9]. Gardening communities also noted that adding fresh biochar is considered to be a disruptor for nutrient availability [10]. Prior to the land application of biochar, fresh biochar is recommended to be 1) loaded with nutrients and water, 2) colonized with microorganisms to ensure the fixed nutrients are more easily available to plants, and 3) aged by oxidation to increase its cation exchange capacity to maximum [7]. A range of methods to achieve these goals (e.g., liquid gold, compost tea, vermi compost, etc.) has been suggested and aging biochar with finished compost was considered to be the simplest method for the activation of biochar [7,10].

Considerable progress has been made in understanding biochar properties, sorption ability, and effects on plant growth when applied to soils [11,12]. One of the biochar properties important to agronomic applications is pH that can affect nutrient availability and soil microbial communities. In soils with low pH, some nutrients may reach toxic levels, and the activity of soil microbes may be reduced. Soils with a high pH generally have a lower availability of micro-nutrients [13]. Most biochars are alkaline (pH>7) and their use often raises soil pH further [14,15]. The pH of most soils in the Lower Rio Grande Valley (LRGV) in South Texas is alkaline (pH 7 to 8.5) and there is a concern from growers regarding the pH increase after biochar amendment [16].

This study was designed to determine pH and soil nutrient status in nitrogen (N) and phosphorus (P) affected by biochar aged with and without a compost made from yard waste. Specific objectives were to 1) determine pH change during biochar and compost aging, and 2) evaluate the effect of fresh vs. aged biochar and/or compost on plant available N and P status, pH, and radish germination in a pot experiment.

Materials and Methods

Biochar, compost, and soil materials

A commercial biochar (Wakefield Agricultural Carbon LLC, Columbia, MO, USA) was used in this study. It was a commercial product sold for soil conditioner and it was mainly claimed to contribute to healthier soils and improve drought resistance according to the manufacturer. The biochar contained 84 % of carbon (C), 0.6 % of hydrogen (H), 4.8 % of oxygen (O) according to elemental analysis (Euro EA-CHNSO Elemental Analyzer, HEKAtech GmbH, Germany). Compost material used in this study was obtained from a local composting facility in McAllen, Texas. The compost sample was submitted to Texas Plant and Soil Analysis Laboratory (Edinburg, TX, USA) and nutrient analysis results including organic matter by loss-on-ignition are presented in Table 1.

Soil material used in this study was collected from the University of Texas Rio Grande Valley (UTRGV) campus, Edinburg, Texas. The soil was classified as being mostly deep, grayish-brown, and neutral to alkaline loams. The soil was air-dried and sieved through a 2-mm sieve. The texture was a clay loam with 43 % sand, 24 % silt and 33 % clay according to the hydrometer method [17]. The pH and electrical conductivity (EC) of biochar, compost, and soil materials were measured using a pH/EC meter (Extech Instruments, Waltham, MA, USA) in a 1:5 (w/v) suspension in deionized (DI) water.

Biochar and compost aging

The first part of this study was to produce aged biochar and/or compost samples. Five plastic cups (500 cm-3) with drain holes at the bottom (Figure 1) were filled with biochar and/or compost to employ five different ratios in volume basis: 1) 100 % biochar, 2) 25 % biochar + 75 % compost, 3) 50 % biochar + 50 % compost, 4) 75 % compost + 25 % biochar, and 5) 100 % compost. The mass of biochar and compost varied resulting in a range of biochar and/or compost materials (Table 2). One of the key component to enhance the biochar aging blended with compost is to provide enough moisture present so that nutrients can dissolve and the pores of the biochar can be charged [7,12]. All plastic containers were placed outside in the shade and were watered every other day up to 13 days with 200 ml of tap water that had a pH of 7.0. Leachate samples from each of the containers after watering were collected and measured for pH.

Variable

Value

Organic matter by loss-on-ignition (%)

12.08

Organic carbon by loss-on-ignition (%)

6.04

Carbon to nitrogen (C:N) ratio

30:01:00

Nitrogen (% N)

0.2

Phosphorus (% P)

0.19

Potassium (% K)

0.86

Table 1: Selected nutrient properties of compost material in this study

Treatmenta

Volume (cm-3)

Mass (g)

Density (g cm-3)

Biochar

Compost

Biochar

Compost

100 % Biochar

400

0

80

0

0.2

25 % Biochar + 75 % Compost

100

300

20

274

0.73

50 % Biochar + 50 % Compost

200

200

40

183

0.56

75 % Biochar + 25 % Compost

300

100

60

91

0.38

100 % Compost

400

0

0

365

0.91

Table 2: Experimental treatments of biochar and compost aging
Figure 1: Setup of aging biochar and compost samples: (1) 100 % Biochar, (2) 25 % Biochar + 75 % Compost, (3) 50 % Biochar + 50 % Compost, (4) 75 % Biochar + 25 % Compost, and (5) 100 % compost

Pot experiment

The second part of this study was to evaluate aged vs. fresh biochar and/ or compost amended into the study soil for radish germination (Figure 2). The radish was the Cherry Belle variety (Raphanus sativa) and this study focused on initial seed germination and stem development. Plastic cups in lieu of plant pots (150 cm-3 capacity) were filled with the soil amended with biochar and/or compost in varying ratios, having 7 different treatments in three replicated trials (21 pots in total) (Table 3). Overall amendment scheme was made on a volume basis, accounting for 50 % volume of soil and 50 % volume of biochar and/or compost receiving aged vs. fresh amendments. While total volume of the growing medium was 120 cm-3, the mass of pot materials ranged from 85 g to 140 g, resulting in a range of density. In each pot, the growing medium was mixed and two radish seeds were planted in 3.8 cm deep from the surface and watered up to 28 days.

The growing medium was analyzed for selected nutrients (N and P) using a commercial soil test kit (SMART 3 Electronic Soil Lab, LaMotte, Chestertown, MD, USA) after the pot experiment. The LaMotte Soil Lab uses the principles of Mehlich-1 extraction for plant available ammonium nitrogen (NH4-N), nitrate nitrogen (NO3-N), and phosphate phosphorus (PO4-P). At the end of experiment, the sprouted biomass was weighed and their stem length was measured by a ruler. One-way analysis of variance (ANOVA) was performed using the PROC GLM procedure in SAS University Edition (SAS Institute Inc., Cary, NC, USA). The treatment factor was soil amendment (Table 3) with dependent variables of pH, NH4-N, NO3-N, PO4-P, stem length, and biomass after the pot experiment. Statistical differences between treatment means were tested with the Waller-Duncan k-ratio t-test at a probability level of 0.05.

Results and Discussion

pH change during aging period

The pH of leachate samples from various ratio of biochar to compost (Figure 3) generally followed the original pH of the tested materials (Table 4). There was no abrupt change in the leachate pH except day 15 when the samples received natural rainfall with pH of 6.2. Note that tap water (pH = 7.0) was irrigated for day 1 through day 14. Biochar showed an increase in pH during the 2-week aging period (9.08-9.24) while compost showed relatively steady level of pH (7.88-7.96). There was a general trend that the more biochar, the greater pH in the leachate samples.

Plant available nutrients and plant biomass affected by biochar and compost amendment

The types of biochar and compost amendment had significant effects on pH, NH4-N, NO3-N, PO4-P, and stem length (p < 0.001) while its effect on biomass was not statistically significant (p = 0.08) (Table 5).Nitrogen

Amendmenta

Soil (g)

Biochar (g)

Compost (g)

Density (g cm-3)

 

 

Fresh

Aged

Fresh

Aged

 

100 % Fresh Biochar

70

15

0

0

0

0.71

100 % Aged Biochar

70

0

15

0

0

0.71

100 % Fresh Compost

70

60

0

0

0

1.08

100 % Aged compost

70

0

60

0

0

1.08

50:50 Fresha

70

15

0

30

0

0.96

50:50 Ageda

70

0

15

0

30

0.96

Soil only

140

0

0

0

0

1.17

Table 3: Experimental treatments of biochar and compost aging

 

pH

EC (µS)

Biochar

9.08

1058

Compost

7.96

1184

Soil

7.6

577

Table 4:pH and electrical conductivity (EC) of biochar, compost and soil materials

Dependent variable

DFa

SSb

MSc

F-value

p-value

pH

6

3.18

0.53

568.13

<0.001

NH4-N

6

0.09

0.01

11.3

0.0001

NO3-N

6

13,918

2,320

366.4

<0.001

PO4-P

6

1.04

0.17

388.4

<0.001

Stem length

6

43.51

7.25

3.24

0.0439

Biomass

6

0.87

0.14

2.69

0.08

Dependent variable

DFa

SSb

MSc

F-value

p-value

pH

Model

6

3.18

0.53

568.13

<0.001

Error

14

Corrected Total

20

NH4-N

Model

6

0.09

0.01

11.3

0.0001

Error

Corrected Total

NO3-N

Model

6

13,918.31

2,319.72

366.38

<0.001

Error

88.64

6.33

Corrected Total

14,006.95

PO4-P

Model

6

1.044

0.174

388.39

<0.001

Error

0.007

0.0005

Corrected Total

1.051

Stem length

Model

6

43.51

7.25

3.24

0.0439

Error

Corrected Total

Biomass

Model

6

0.87

0.14

2.69

0.08

Error

Corrected Total

Table 5: One-way ANOVA of soil amendment effects on pH, plant availablesoil nutrients, stem length, and biomass after pot experiment
aDF = degree of freedom; bSS = sum of squares; cMS = Mean of square
Figure 2: Pot experiment setup. Photo was taken 3 days after being planted
Figure 3: Leachate pH change during biochar and/or compost aging. All data points received tap water (pH = 7.0)

is critical for plant growth and it is a key component of the chlorophyll (green color) in plants. When leaves contain sufficient N, photosynthesis rate increases [13]. According to the glossary of Soil Science Society of America, mineralization is defined as “the conversion of an element from an organic form to inorganic state as a result of microbial activity” [18]. Once mineralized, NH4 can be taken up by plants, nitrified, immobilized by soil microorganism, lost as a gas by NH4 volatilization, held as an exchangeable ion by clays or fixed in the inter layers of certain clay minerals [19]. Often, the NH4 is rapidly converted to nitrate by the microbial process of nitrification by obligate, aerobic, chemoautotrophic bacteria [19]. In this study, plant available NH4-N level in the soil after the pot experiment was relatively low (0.09 to 0.28 mg kg-1) (Table 6). The NO3-N level, however, was two-fold higher in the pot soils amended with biochar and compost (105 to 133 mg kg-1) than those with soil only (60 mg kg-1). The lowest levels of NH4-N and NO3-N were found with the control medium (soil only). The pot soils amended with fresh compost resulted in significantly higher NO3-N than those amended with aged compost while opposite was true for the soils amended with biochar (i.e., aged biochar > fresh biochar). In both amendments, the difference in NO3-N between fresh vs. aged amendment was relatively small (< 13 mg kg-1).

Phosphorus is considered second to N as the most essential nutrient to ensure the hardy growth of the plant and activity of the cells [20]. In this study, plant available PO4-P in the growing medium was not detected (zero) with soil only or soil amended with 100 % biochar (Table 6). Poor stem development and lower biomass of sprouted radish was found in these treatments. It was notable that radishes grew taller in soils amended with aged biochar and/or compost (e.g., stem length from 3.8 to 4.7 cm). It is important to note that there was a heat wave (~ 37 ºC) that came through during the fourth week of the pot experiment. The average temperature during the weeks prior to the heat wave were approximately 27-29 ºC. The heat wave caused nearly all radishes to wilt and some to die (e.g., soil only) as the pot experiment was conducted in outdoor. For the better control of experimental treatments, such a heat wave should have been avoided. Radish seed in the control pot (soil only) from trial 1 did not grow at all while the seed from trial 2 and 3 died by the heat wave. This suggests that plants were less able to recover from heat and drought when they were grown in soils without amendment. The alkaline pH (8 to 9) from biochar and compost themselves did not greatly increase soil pH (6.7 to 7.6) probably due to the buffering capacity of the soil and slightly acidic rain water (pH 6.2).

Conclusions

The objective of this study was to examine pH change during biochar aging blended with and without compost, and the efficacy of radish seed

 

pH

NH4-N

NO3-N

PO4-P

Stem length

Biomass

(mg kg-1)

(mg kg-1)

(mg kg-1)

(cm)

(g)

100 % Fresh biochar

6.69 ± 0.04e

0.27 ± 0.02a

130.9 ± 1.2b

0e

3.2 ± 0.3ab

0.25 ± 0.23ab

100 % aged biochar

6.44 ± 0.01f

0.14 ± 0.01bc

143.6 ± 1.5a

0e

3.8 ± 0.5a

0.44 ± 0.30ab

100 % fresh compost

7.07 ± 0.02d

0.28 ± 0.02a

116.3 ± 2.1d

0.42 ± 0.02b

2.9 ± 1.7ab

0.85 ± 0.10a

100 % aged compost

7.64 ± 0.02a

0.14 ± 0.03bc

104.8 ± 3.1e

0.35 ± 0.02c

4.8 ± 1.6a

0.42 ± 0.16ab

50 % fresh biochar + 50 % fresh compost

7.16 ± 0.01c

0.17 ± 0.02c

125.5 ± 2.1c

0.60 ± 0.02a

2.8 ± 0.1ab

0.42 ± 0.20ab

50 % aged biochar + 50 % aged compost

7.37 ± 0.03b

0.20 ± 0.06b

132.6 ± 2.2b

0.19 ± 0.03d

4.7 ± 1.9a

0.42 ± 0.09ab

Soil only

7.40 ± 0.02b

0.09 ± 0.04c

59.8 ± 1.7f

0e

0b

0b

Table 6: Plant available soil nutrients, pH, stem length, and biomass after pot experiment. All values (mean ± standard deviation) are averages of three replicated trials. Note that in each column, same means with the same letter are not significantly different at a probability level of 0.05

germination in an alkaline urban soil amended with the biochar and/or compost. Results showed that leachate pH during the aging period was dependent on the original pH of biochar and compost materials with biochar being more alkaline (pH > 9.0) than compost (pH < 8.0) in the leachate samples. With increasing ratio of biochar in the biochar + compost mixture, there was increase in the leachate pH. Pot experiment demonstrated that radish germination was more effective with compost amendment (both fresh and aged). The study soil amended with 50 % aged biochar + 50 % aged compost was also favorable for the radish germination. The soil amended with biochar alone had no detectable PO4-P after the pot experiment regardless of aging. At the end of the pot experiment, there was no appreciable pH increase in the soil amended with biochar, indicating soil itself may have buffer capacity to neutralize salts derived from biochar. Current study simulated only seed germination period in a pot experiment scale. Future study should investigate the synergistic effect of biochar and compost aging in a large scale and longerterm to better understand the nutrient dynamics and plant growth under natural condition.

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