3.1 Organic Amendment Properties
Composted municipal biosolids and biochars derived from pyrolysis of rice hulls and cotton field residues were applied as organic amendments to four soils at two rates each. The amendments varied in chemical composition (Table 3). Conversion of raw vegetative matter from rice hulls and cotton field residue during pyrolysis substantially altered their respective contents of carbon and nutrients, often resulting in greater concentrations afterwards.
Table 3. Chemical composition of organic amendments and the vegetative feedstocks used to create two biochars. Pre-pyrolysis is used to designate the vegetative feedstock source pyrolyzed to create each of the biochars.
Organic Amendment
|
pH
|
C
|
N
|
P
|
K
|
Ca
|
Mg
|
|
|
----- g kg-1 -----
|
------------ mg kg-1 -----------
|
Composted biosolids
|
6.86
|
508.2
|
15.0
|
11300
|
4630
|
89460
|
5340
|
Rice hulls (pre-pyrolysis)
|
--
|
347.2
|
4.8
|
970
|
2800
|
752
|
592
|
Cotton residue (pre-pyrolysis)
|
--
|
400.6
|
14.9
|
1342
|
8069
|
10606
|
3125
|
Rice hulls (pyrolyzed)
|
6.13
|
411.9
|
8.0
|
1509
|
5181
|
1486
|
1013
|
Cotton residue (pyrolyzed)
|
9.25
|
540.6
|
1.7
|
4262
|
20409
|
25979
|
7521
|
|
S
|
Na
|
Fe
|
Zn
|
Mn
|
Cu
|
B
|
|
----------------------- mg kg-1 ----------------------
|
Composted biosolids
|
8840
|
1050
|
10545
|
438
|
258
|
172
|
41
|
Rice hulls (pre-pyrolysis)
|
470
|
<140
|
25
|
24
|
96
|
2
|
2
|
Cotton residue (pre-pyrolysis)
|
1422
|
659
|
225
|
19
|
26
|
5
|
26
|
Rice hulls (pyrolyzed)
|
217
|
<140
|
116
|
40
|
222
|
6
|
3
|
Cotton residue (pyrolyzed)
|
1843
|
2117
|
1737
|
41
|
79
|
22
|
51
|
Amendment pH ranged from 6.13 (moderately acid) for the rice hull biochar to 9.25 (alkaline) for the cotton residue biochar. This is a substantial and relevant difference for biochars intended as a soil amendment. Carbon contents ranged from 411.9 g kg-1 in rice hulls to 540.6 g kg-1 in composted biosolids. Nitrogen (N) contents were relatively low, ranging from 1.7 g kg-1 in cotton residue to 15 g kg-1 in biosolids. Other nutrients, such as phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and boron (B) were generally well supplied by the amendments.
The conversion of vegetative matter by pyrolysis is well known to result in changes in nutrient content in the resultant biochars. Such changes are dependent upon the conditions (e.g. temperature and duration of pyrolysis) under which the biochar is created, and the properties of the feedstock used. The final biochar products are also known to vary in their benefits for plant growth. In biochars created from wheat and rice straw, Naeem et al. (2014) reported a positive relationship between temperatures (300, 400, and 500°C) and the chemical parameters pH, C, P, K, Zn, Mn, and Fe, as was observed in this study. There was a negative relationship between temperature and N. Kloss et al. (2012) found similar results in the same temperature range for K, Ca, Mg, and P, but noted a decrease in Ca and Mg as temperatures exceeded 500°C. These authors stressed that the conditions under which biochars are created should be considered in alignment with the intended application of the product. The chemical changes observed in biochars in this study, created at 350°C, are similar to those reported by others. Although not measured in this study, it is reasonable to presume that the presence of microorganisms in the biochars is substantially lower than that of the composted biosolids, due to the exposure to sterilizing high heat.
Table 4. Select chemical and physical properties related to the fertility of the soils used to grow cotton in the study.
Soil
|
pH
|
Cond
|
NO3-N
|
P
|
K
|
Ca
|
Mg
|
S
|
Na
|
|
|
µmhos/cm
|
--------------------- mg kg-1 ---------------------
|
Burleson
|
5.9
|
216
|
4
|
72
|
138
|
4026
|
538
|
30
|
20
|
Hearne
|
4.7
|
74
|
1
|
2
|
85
|
628
|
288
|
16
|
33
|
Pullman
|
7.7
|
282
|
19
|
30
|
514
|
5106
|
468
|
13
|
35
|
Wolfpen
|
6.8
|
95
|
1
|
7
|
28
|
636
|
27
|
7
|
1
|
|
Fe
|
Zn
|
Mn
|
Cu
|
Sand
|
Silt
|
Clay
|
Textural class
|
|
------------- mg kg-1 ------------
|
-------- % --------
|
|
Burleson
|
34.1
|
0.91
|
70.8
|
0.67
|
37
|
30
|
33
|
Clay Loam
|
Hearne
|
15.0
|
0.78
|
2.8
|
0.25
|
65
|
14
|
21
|
Sandy Clay Loam
|
Pullman
|
6.9
|
0.56
|
14.1
|
0.74
|
38
|
35
|
27
|
Loam
|
Wolfpen
|
12.9
|
0.11
|
6.3
|
0.17
|
73
|
20
|
7
|
Sandy Loam
|
3.2 Soils Properties
Four soils used to grow cotton in the greenhouse varied in chemical, physical, and mineralogical properties (Table 4). The Burleson soil is a clay loam rich in smectites. The Hearne soil is a sandy clay loam featuring kaolinitic minerals. The Pullman soil is a loam rich in illites. The Wolfpen soil is a sandy loam with very little (7 %) clay mineral content. Soil pH values range from 4.7 (strongly acidic) in the Hearne soil to 7.7 (moderately alkaline) in the Pullman soil. All soils are deficient in plant available N, measured as NO3-N. Only the Burleson soil sufficiently supplies P to plants without fertilization (critical level = 50 mg kg-1; Texas A&M AgriLife Soil Water and Forage Testing Laboratory, College Station, TX). The two sandiest soils, Hearne and Wolfpen, are deficient in plant available K (critical level = 125). Many of the soil nutrients present ranges that straddle the critical levels for each nutrient, however, fertilizer nutrients supplied were sufficient to address plant requirements. Differences measured in the physical and chemical properties presented contribute to differences in native fertility, as well as to degree of response to the chemical properties contributed by the organic amendments.
Albuquerque et al. (2014) found biochars from different feedstocks differentially altered soil pH, EC, N, and P, and that rate of application was a significant factor. These authors found that the resultant biochar type was a significant effect on the growth of sunflower biomass. Ameloot et al. (2015) reported on biochars created from poultry litter and pine chips feedstocks at two different temperatures used to amend two acidic silt loams contrasting in organic carbon content. The poultry litter biochar contained approximately half the carbon of the pine chips biochar and approximately 16 times as much N. Supply of N was enhanced from poultry litter biochar in the soil with greater native C content and immobilization of N was exacerbated in the soil with lower native C. Immobilization, the incorporation of N into microbial biomass, prevents plant uptake of N from soil. This occurs when the supply of N is too low, relative to the supply of C, to provide both the C and N needs of microorganism cell growth and division (Hart et al. 1994). Therefore, amendment with biochars which are uniformly in excess of the C:N ratio required for N release (<20:1), will result in N immobilization and reduced plant uptake (Havlin et al. 2013). These results illustrate the complexity of interactions between biochar amendment and soil properties that will need to be accounted for as use becomes more widespread.
Table 5. ANOVA p-value results for treatments’ and their interactions’ (parameters) effect on shoot (top plant matter) height and mass, root mass, and shoot mass to root mass (S:R) ratio. Model tested in ANOVA was in the order presented in the table from top to bottom: y = carbon rate soil carbon*rate carbon*soil rate*soil carbon*soil*rate.
Parameter
|
df
|
Shoot height
|
Shoot mass
|
Root mass
|
S:R ratio
|
full model
|
27
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment
|
3
|
<0.0001
|
<0.0001
|
0.0005
|
0.0288
|
rate
|
1
|
0.3901
|
0.7989
|
0.3936
|
0.9088
|
soil
|
3
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment*rate
|
2
|
0.0008
|
0.3007
|
0.4854
|
0.2117
|
amendment*soil
|
9
|
0.0848
|
0.4898
|
0.0066
|
<0.0001
|
rate*soil
|
3
|
0.9226
|
0.0181
|
0.0046
|
0.4848
|
amendment*soil*rate
|
6
|
0.0189
|
0.1973
|
0.1078
|
0.0010
|
3.3 Plant Shoot and Root Biomass
Cotton plants grown for six weeks in the greenhouse were evaluated for differences by treatment and treatment combination effects on shoot and root growth (Table 5). Organic amendment (carbon source) was significant (α = 0.05) for differences in shoot height, shoot mass, root mass, and the ratio of shoot to root mass (S:R). Rate of application (554 or 1662 mg kg-1 soil) was not a significant affect for any plant growth parameter. Soil series was a highly significant effect for all measured properties (p < 0.0001). Organic amendment and soil interaction was significant for root mass and S:R only.
Comparison of means for difference from the control within each soil indicates that different growth parameters are differentially affected by the organic amendment treatments Table 6), though organic amendments were generally responsible for improvements to plant performance. Of the four growth parameters measured, shoot height for six-week-old cotton plants most frequently differed from the control. In no case were shoot heights for any organic amendment numerically lower than the control. In the Pullman soil, all treatments were significantly associated with taller plants. The Pullman soil also saw the highest frequency of significant shoot mass increases with amendment. Similar to shoot height, shoot mass was never numerically decreased with amendment. Root mass was improved in all cases, with the small exception of two treatments in the Wolfpen soil. Root mass was most frequently significantly improved in the Burleson soil.
Differences in the distribution of carbon resources between the shoot and root system are difficult to determine in terms of favorability for production systems. Generally, fewer roots are required to access water and nutrients when supplies of each are plentiful and located near the base of the plant (Johnson and Thornley 1987). However, when nutrients are deficient and/or diffuse in the soil volume, more roots are required to contact and intercept immobile species (Blum, 1996). There is a general trade-off in the plant between acquisition of soil resources and internal allocation of soil and photosynthetic resources. Brouwer (1963) described an equilibrium between shoots and roots that favored greater yield when root systems were more prolific.
In the current study, nutrients and water were supplied in sufficiency. Therefore, an interpretation could be put forth that plants with a larger S:R ratio more efficiently accessed those nutrients and water, supplying the photosynthetic activity of the plant with less energy invested below the surface. On the other hand, a larger root system with a smaller S:R ratio could indicate an investment (i.e. insurance) by the plant in buffering against future abrupt change in nutrient and/or water availability, and will provide continued carbon inputs from active exudation and post-harvest decomposition (Johnson et al. 2006). For the purposes of improving rain fed cotton production in Mali and neighboring countries, it may be presumed that both early shoot development and S:R ratio should be considered important attributes (Brown and Scott, 1984). Short-term productivity is never unimportant, yet insurance against environmental stress and natural redistribution of soil carbon will provide lasting benefits.
Table 6. Cotton plant shoot (top matter) height and mass, root mass, and shoot mass to root mass (S:R) ratio for plants grown in four soils with three organic amendments at two rates each. Bolded numbers with asterisks (*) indicate significant difference from the control treatment within each soil by Dunnett’s test for comparison of means (α = 0.05). µ = mean and σ = 1 standard deviation for the mean of 5 replicates.
Soil
|
Amendment
|
Rate
|
Shoot Height
|
|
Shoot Mass
|
|
Root Mass
|
|
S:R ratio
|
|
|
|
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
|
|
mg kg-1
|
cm
|
|
g
|
|
g
|
|
|
|
Burleson
|
Control
|
0
|
26.2
|
1.6
|
2.26
|
0.37
|
0.53
|
0.14
|
4.38
|
0.47
|
Burleson
|
Compost
|
554
|
*32.7
|
2.4
|
3.10
|
0.37
|
*0.81
|
0.07
|
3.82
|
0.18
|
Burleson
|
Compost
|
1662
|
26.5
|
1.9
|
2.27
|
0.36
|
0.67
|
0.16
|
*3.45
|
0.45
|
Burleson
|
Rice Hull Biochar
|
554
|
29.7
|
1.6
|
2.74
|
0.45
|
*0.91
|
0.11
|
*3.05
|
0.64
|
Burleson
|
Rice Hull Biochar
|
1662
|
31.1
|
4.3
|
2.80
|
0.36
|
*0.78
|
0.09
|
*3.60
|
0.34
|
Burleson
|
Cotton Biochar
|
554
|
29.8
|
2.3
|
3.03
|
0.67
|
*0.80
|
0.16
|
3.78
|
0.23
|
Burleson
|
Cotton Biochar
|
1662
|
*32.6
|
4.9
|
3.01
|
0.74
|
*0.85
|
0.15
|
*3.52
|
0.36
|
Hearne
|
Control
|
0
|
22.1
|
1.0
|
1.28
|
0.39
|
0.65
|
0.13
|
1.95
|
0.41
|
Hearne
|
Compost
|
554
|
*26.5
|
2.0
|
1.66
|
0.22
|
1.02
|
0.39
|
1.83
|
0.71
|
Hearne
|
Compost
|
1662
|
22.7
|
0.3
|
*1.94
|
0.29
|
1.21
|
0.30
|
1.69
|
0.53
|
Hearne
|
Rice Hull Biochar
|
554
|
23.5
|
0.7
|
1.53
|
0.26
|
1.18
|
0.13
|
1.29
|
0.14
|
Hearne
|
Rice Hull Biochar
|
1662
|
*25.2
|
1.2
|
1.53
|
0.10
|
1.19
|
0.28
|
1.32
|
0.20
|
Hearne
|
Cotton Biochar
|
554
|
*27.6
|
0.7
|
1.71
|
0.55
|
0.88
|
0.23
|
2.02
|
0.65
|
Hearne
|
Cotton Biochar
|
1662
|
*29.9
|
0.8
|
*2.28
|
0.54
|
*1.43
|
0.66
|
1.88
|
0.90
|
Pullman
|
Control
|
0
|
26.2
|
2.4
|
1.43
|
0.21
|
0.51
|
0.06
|
2.77
|
0.15
|
Pullman
|
Compost
|
554
|
*34.0
|
3.0
|
*2.12
|
0.42
|
0.68
|
0.07
|
3.13
|
0.38
|
Pullman
|
Compost
|
1662
|
*39.9
|
2.0
|
1.84
|
0.34
|
0.56
|
0.09
|
3.26
|
0.17
|
Pullman
|
Rice Hull Biochar
|
554
|
*35.7
|
2.6
|
*2.09
|
0.15
|
0.49
|
0.09
|
*4.35
|
0.70
|
Pullman
|
Rice Hull Biochar
|
1662
|
*32.5
|
2.3
|
1.79
|
0.40
|
0.50
|
0.18
|
*3.74
|
0.98
|
Pullman
|
Cotton Biochar
|
554
|
*34.9
|
2.7
|
*2.00
|
0.22
|
0.67
|
0.14
|
3.04
|
0.38
|
Pullman
|
Cotton Biochar
|
1662
|
*35.8
|
3.4
|
1.93
|
0.37
|
0.47
|
0.14
|
*4.23
|
0.60
|
Wolfpen
|
Control
|
0
|
27.0
|
1.6
|
1.88
|
0.28
|
0.82
|
0.07
|
2.29
|
0.28
|
Wolfpen
|
Compost
|
554
|
30.6
|
2.6
|
2.08
|
0.50
|
0.85
|
0.21
|
2.46
|
0.18
|
Wolfpen
|
Compost
|
1662
|
30.1
|
4.1
|
2.33
|
0.30
|
0.90
|
0.13
|
2.60
|
0.24
|
Wolfpen
|
Rice Hull Biochar
|
554
|
*32.8
|
4.2
|
2.46
|
0.49
|
0.64
|
0.13
|
*3.88
|
0.39
|
Wolfpen
|
Rice Hull Biochar
|
1662
|
30.4
|
4.5
|
*2.56
|
0.44
|
0.76
|
0.11
|
*3.38
|
0.34
|
Wolfpen
|
Cotton Biochar
|
554
|
31.0
|
1.9
|
2.44
|
0.27
|
0.90
|
0.06
|
2.72
|
0.32
|
Wolfpen
|
Cotton Biochar
|
1662
|
*33.0
|
3.1
|
2.48
|
0.24
|
0.89
|
0.12
|
*2.81
|
0.26
|
3.4 Whole Plant Nutrient Content
The ANOVA results for affect of treatments on macro and micronutrients are presented in Table 7. Amendment was significant for the observed differences in all nutrients at α = 0.05 with the exception of K (p = 0.0621) and Mn (p = 0.1136). Rate of amendment application was not responsible for differences in the assimilation of any nutrient. Soil was a significant factor for all nutrients (p < 0.0001). Interactive effects for amendment and soil were significant determinants for differences in all nutrients.
Table 7. ANOVA p-value results for treatments’ and their interactions’ (parameters) effect on whole plant nutrient content. Model tested in ANOVA was in the order presented in the table from top to bottom: y = carbon rate soil amendment*rate amendment*soil rate*soil amendment*soil*rate.
Factor
|
df
|
N
|
P
|
K
|
Ca
|
Mg
|
S
|
full model
|
27
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment
|
3
|
<0.0001
|
0.0080
|
0.0621
|
<0.0001
|
0.0001
|
0.0181
|
rate
|
1
|
0.2361
|
0.5668
|
0.4644
|
0.1037
|
0.1130
|
0.8586
|
soil
|
3
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment*rate
|
2
|
0.0363
|
0.4331
|
0.4619
|
0.1276
|
0.2698
|
0.0494
|
amendment*soil
|
9
|
<0.0001
|
0.0217
|
0.0005
|
<0.0001
|
<0.0001
|
0.0216
|
rate*soil
|
3
|
0.1010
|
0.0618
|
0.2064
|
0.1878
|
0.8714
|
0.7205
|
amendment*soil*rate
|
6
|
0.2287
|
0.1853
|
0.4771
|
0.0086
|
0.0742
|
0.3512
|
Factor
|
df
|
S
|
Zn
|
Fe
|
Cu
|
Mn
|
B
|
full model
|
27
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment
|
3
|
0.0181
|
0.0084
|
0.0055
|
<0.0001
|
0.1136
|
<0.0001
|
rate
|
1
|
0.8586
|
0.5938
|
0.1961
|
0.2020
|
0.4778
|
0.1534
|
soil
|
3
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
<0.0001
|
amendment*rate
|
2
|
0.0494
|
0.5975
|
0.0980
|
0.5356
|
<0.0001
|
0.7553
|
amendment*soil
|
9
|
0.0216
|
0.0249
|
0.0012
|
<0.0001
|
0.0083
|
0.0006
|
rate*soil
|
3
|
0.7205
|
0.1522
|
0.2306
|
0.2045
|
0.7905
|
0.0338
|
amendment*soil*rate
|
6
|
0.3512
|
0.5328
|
0.0306
|
0.0076
|
<0.0001
|
0.3441
|
Table 8 presents the results of Dunnett’s test for comparison of means for differences in macronutrient assimilation from the control treatment. Soil series is evident as a determinant for N assimilation. The Wolfpen soil shows the strongest effect of amendment in depressing N uptake, as compared to the control. Regardless of the soil series, however, N uptake is almost uniformly depressed in the presence of organic amendments. The high rate of compost application in the clayey Burleson soil is the sole exception (35.2 mg N kg-1 compared to 31.6 mg N kg-1). The Wolfpen soil exhibited the strongest effect of organic amendment on P uptake as well. Generally, organic amendments resulted in numeric, if not significant increases in P assimilation by the plants. Fewer differences or clear trends are visible across soil for K, Ca, Mg, and S. There are sparse examples of nutrient uptake depression for each. However, K uptake, which is important to bole development in cotton, was significantly improved in the Pullman soil in the low rates for all three amendments.
Table 8. Whole plant tissue (top matter) macronutrient analysis results for cotton plants grown in four soils with three organic amendments at two rates each. Bolded numbers with asterisks (*) indicate significant difference from the control treatment within each soil by Dunnett’s test for comparison of means (α = 0.05). µ = mean and σ = one standard deviation for the mean of five replicates.
Soil
|
Amendment
|
Rate
|
N
|
|
P
|
|
K
|
|
Ca
|
|
Mg
|
|
S
|
|
|
(soil appl.)
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
|
mg kg-1
|
------------------------------------------- g kg-1 in tissue --------------------------------------------
|
Burleson
|
Control
|
0
|
31.6
|
5.0
|
2.84
|
0.41
|
22.72
|
2.14
|
18.99
|
2.34
|
6.21
|
0.40
|
6.81
|
1.39
|
Burleson
|
Compost
|
554
|
29.0
|
4.5
|
2.94
|
0.36
|
20.33
|
1.42
|
16.60
|
1.29
|
5.41
|
0.77
|
5.71
|
0.66
|
Burleson
|
Compost
|
1662
|
35.2
|
3.9
|
2.94
|
0.66
|
23.25
|
1.25
|
19.80
|
1.52
|
6.01
|
0.49
|
6.75
|
0.58
|
Burleson
|
Rice Hull Bioch.
|
554
|
29.6
|
2.8
|
3.03
|
0.14
|
20.71
|
1.41
|
15.61
|
3.18
|
5.24
|
0.87
|
6.39
|
0.90
|
Burleson
|
Rice Hull Bioch.
|
1662
|
29.4
|
3.0
|
2.82
|
0.53
|
*19.48
|
0.53
|
15.52
|
2.14
|
5.29
|
0.86
|
5.70
|
0.58
|
Burleson
|
Cotton Bioch.
|
554
|
28.9
|
6.4
|
2.90
|
0.21
|
21.31
|
2.09
|
17.65
|
2.83
|
5.88
|
1.03
|
6.56
|
0.97
|
Burleson
|
Cotton Bioch.
|
1662
|
26.8
|
3.7
|
2.83
|
0.20
|
20.56
|
2.37
|
*14.70
|
2.11
|
5.04
|
0.68
|
5.90
|
0.83
|
Hearne
|
Control
|
0
|
45.4
|
7.1
|
1.50
|
0.39
|
24.69
|
1.86
|
23.15
|
4.46
|
7.69
|
1.91
|
13.24
|
3.31
|
Hearne
|
Compost
|
554
|
*20.3
|
10.7
|
1.61
|
0.19
|
28.53
|
5.67
|
*10.88
|
1.91
|
*4.57
|
0.50
|
*6.68
|
3.27
|
Hearne
|
Compost
|
1662
|
32.4
|
11.7
|
2.00
|
0.19
|
29.75
|
3.16
|
*14.77
|
2.63
|
*5.27
|
0.96
|
10.11
|
3.55
|
Hearne
|
Rice Hull Bioch.
|
554
|
39.1
|
3.9
|
1.64
|
0.44
|
27.68
|
1.52
|
*16.03
|
3.95
|
6.22
|
0.99
|
10.29
|
1.69
|
Hearne
|
Rice Hull Bioch.
|
1662
|
39.4
|
5.8
|
1.83
|
0.61
|
27.91
|
4.11
|
*13.77
|
1.48
|
*4.96
|
0.33
|
9.85
|
3.48
|
Hearne
|
Cotton Bioch.
|
554
|
32.4
|
9.8
|
1.42
|
0.58
|
26.07
|
5.01
|
19.28
|
5.38
|
6.46
|
1.29
|
9.84
|
3.43
|
Hearne
|
Cotton Bioch.
|
1662
|
34.9
|
5.0
|
1.23
|
0.29
|
25.62
|
2.94
|
17.76
|
2.12
|
6.40
|
0.70
|
8.95
|
1.07
|
Pullman
|
Control
|
0
|
23.6
|
4.3
|
1.82
|
0.27
|
26.56
|
3.15
|
20.70
|
1.75
|
5.96
|
0.53
|
4.17
|
0.42
|
Pullman
|
Compost
|
554
|
*18.8
|
1.2
|
2.26
|
0.45
|
*30.57
|
1.58
|
20.01
|
1.43
|
6.36
|
0.59
|
3.98
|
0.18
|
Pullman
|
Compost
|
1662
|
20.8
|
2.5
|
2.07
|
0.24
|
29.40
|
1.57
|
20.51
|
1.87
|
5.86
|
0.23
|
4.59
|
0.54
|
Pullman
|
Rice Hull Bioch.
|
554
|
23.5
|
2.2
|
*2.50
|
0.36
|
*32.60
|
2.07
|
23.36
|
1.55
|
6.48
|
0.38
|
4.77
|
0.53
|
Pullman
|
Rice Hull Bioch.
|
1662
|
*18.9
|
1.0
|
1.79
|
0.21
|
29.57
|
2.99
|
20.29
|
2.85
|
5.86
|
0.70
|
4.04
|
0.36
|
Pullman
|
Cotton Bioch.
|
554
|
21.1
|
2.7
|
2.36
|
0.30
|
*31.50
|
2.05
|
22.23
|
2.55
|
6.36
|
0.56
|
4.63
|
0.83
|
Pullman
|
Cotton Bioch.
|
1662
|
21.8
|
1.8
|
2.30
|
0.44
|
29.95
|
1.84
|
22.25
|
3.30
|
6.34
|
0.87
|
4.91
|
0.65
|
Wolfpen
|
Control
|
0
|
30.1
|
3.2
|
1.16
|
0.22
|
11.53
|
0.97
|
17.33
|
1.81
|
2.63
|
0.29
|
5.62
|
1.49
|
Wolfpen
|
Compost
|
554
|
*24.2
|
2.2
|
*1.70
|
0.15
|
11.64
|
1.42
|
18.98
|
3.11
|
2.52
|
0.23
|
5.28
|
1.73
|
Wolfpen
|
Compost
|
1662
|
*21.9
|
3.0
|
1.59
|
0.10
|
10.53
|
0.44
|
13.69
|
2.50
|
*2.04
|
0.20
|
4.96
|
1.95
|
Wolfpen
|
Rice Hull Bioch.
|
554
|
*25.4
|
3.1
|
*1.69
|
0.24
|
13.59
|
1.72
|
*12.48
|
1.98
|
2.57
|
0.36
|
4.60
|
0.77
|
Wolfpen
|
Rice Hull Bioch.
|
1662
|
26.9
|
2.9
|
*1.79
|
0.21
|
*14.17
|
1.22
|
*11.80
|
1.48
|
2.44
|
0.41
|
4.94
|
1.01
|
Wolfpen
|
Cotton Bioch.
|
554
|
*23.7
|
2.8
|
*1.72
|
0.33
|
12.58
|
1.51
|
14.88
|
2.08
|
2.64
|
0.43
|
5.44
|
0.88
|
Wolfpen
|
Cotton Bioch.
|
1662
|
*22.5
|
1.8
|
*2.06
|
0.43
|
13.94
|
2.15
|
*13.36
|
1.83
|
2.65
|
0.31
|
4.85
|
0.78
|
Table 9 presents the results of Dunnett’s test for comparison of means for differences in micronutrient assimilation. There was no effect on Zn assimilation. Plant uptake of Fe and Cu however, was differentially affected by soil. Amendments improved Fe uptake in the Pullman soil and depressed uptake in the Wolfpen soil. Amendments depressed Cu uptake in the Burleson soil and the Hearne soil, while cotton grown in the Pullman and Wolfpen soils were almost uniformly unaffected. No effect was present for Mn. A small number of examples indicated depression of B uptake in the sandier soils, Hearne and Wolfpen.
Table 9. Whole plant tissue (top matter) micronutrient analysis results for cotton plants grown in four soils with three organic amendments at two rates each. Bolded numbers with asterisks (*) indicate significant difference from the control treatment within each soil by Dunnett’s test for comparison of means (α = 0.05). µ = mean and σ = one standard deviation for the mean of five replicates.
Soil
|
Amendment
|
Rate
|
Zn
|
|
Fe
|
|
Cu
|
|
Mn
|
|
B
|
|
|
|
(soil appl.)
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
µ
|
σ
|
|
|
mg kg-1
|
---------------------------- mg kg-1 in tissue --------------------------
|
Burleson
|
Control
|
0
|
25.7
|
5.5
|
37
|
17
|
6.4
|
1.2
|
76
|
15
|
62
|
5
|
Burleson
|
Compost
|
554
|
22.1
|
2.4
|
26
|
1
|
*5.2
|
0.8
|
66
|
15
|
63
|
6
|
Burleson
|
Compost
|
1662
|
26.9
|
2.8
|
34
|
4
|
*4.9
|
0.8
|
88
|
20
|
67
|
8
|
Burleson
|
Rice Hull Bioch.
|
554
|
23.1
|
2.2
|
*25
|
3
|
5.8
|
0.4
|
71
|
11
|
64
|
4
|
Burleson
|
Rice Hull Bioch.
|
1662
|
22.5
|
2.7
|
*21
|
1
|
6.5
|
0.8
|
67
|
12
|
58
|
7
|
Burleson
|
Cotton Bioch.
|
554
|
23.5
|
4.3
|
30
|
5
|
*5.2
|
0.7
|
86
|
44
|
64
|
11
|
Burleson
|
Cotton Bioch.
|
1662
|
20.8
|
3.2
|
32
|
2
|
*5.3
|
0.2
|
62
|
11
|
55
|
2
|
Hearne
|
Control
|
0
|
40.4
|
11.0
|
176
|
53
|
11.4
|
2.1
|
66
|
16
|
112
|
28
|
Hearne
|
Compost
|
554
|
27.2
|
6.3
|
88
|
28
|
*6.7
|
1.2
|
87
|
25
|
*60
|
13
|
Hearne
|
Compost
|
1662
|
33.7
|
6.7
|
78
|
10
|
*8.4
|
1.0
|
81
|
7
|
*73
|
17
|
Hearne
|
Rice Hull Bioch.
|
554
|
37.8
|
6.4
|
87
|
25
|
9.4
|
1.5
|
65
|
12
|
88
|
23
|
Hearne
|
Rice Hull Bioch.
|
1662
|
39.5
|
9.2
|
125
|
35
|
10.2
|
1.5
|
96
|
46
|
78
|
20
|
Hearne
|
Cotton Bioch.
|
554
|
29.7
|
9.1
|
189
|
93
|
9.5
|
2.3
|
51
|
19
|
80
|
17
|
Hearne
|
Cotton Bioch.
|
1662
|
33.8
|
7.2
|
140
|
46
|
*8.1
|
0.9
|
43
|
10
|
92
|
18
|
Pullman
|
Control
|
0
|
11.4
|
2.1
|
26
|
7
|
5.9
|
0.7
|
44
|
8
|
67
|
5
|
Pullman
|
Compost
|
554
|
11.9
|
2.8
|
31
|
3
|
5.8
|
0.7
|
35
|
4
|
62
|
8
|
Pullman
|
Compost
|
1662
|
10.9
|
1.5
|
*32
|
3
|
5.5
|
0.6
|
41
|
6
|
59
|
11
|
Pullman
|
Rice Hull Bioch.
|
554
|
13.0
|
2.4
|
*36
|
3
|
*6.2
|
0.4
|
43
|
5
|
59
|
6
|
Pullman
|
Rice Hull Bioch.
|
1662
|
9.4
|
1.2
|
*38
|
2
|
4.8
|
0.5
|
36
|
3
|
63
|
6
|
Pullman
|
Cotton Bioch.
|
554
|
12.1
|
1.4
|
*37
|
3
|
5.7
|
0.5
|
42
|
8
|
67
|
7
|
Pullman
|
Cotton Bioch.
|
1662
|
12.0
|
1.9
|
*39
|
3
|
5.7
|
0.6
|
38
|
6
|
65
|
14
|
Wolfpen
|
Control
|
0
|
20.1
|
3.3
|
49
|
11
|
5.6
|
0.8
|
293
|
83
|
84
|
13
|
Wolfpen
|
Compost
|
554
|
19.0
|
2.1
|
*36
|
5
|
5.7
|
0.8
|
186
|
81
|
74
|
15
|
Wolfpen
|
Compost
|
1662
|
16.4
|
3.5
|
*28
|
5
|
5.0
|
0.7
|
216
|
71
|
*55
|
9
|
Wolfpen
|
Rice Hull Bioch.
|
554
|
21.9
|
4.7
|
*29
|
4
|
6.0
|
1.0
|
203
|
82
|
63
|
12
|
Wolfpen
|
Rice Hull Bioch.
|
1662
|
24.0
|
4.1
|
*28
|
2
|
5.3
|
1.0
|
347
|
54
|
*54
|
15
|
Wolfpen
|
Cotton Bioch.
|
554
|
18.6
|
5.6
|
*33
|
5
|
5.2
|
1.1
|
308
|
42
|
71
|
12
|
Wolfpen
|
Cotton Bioch.
|
1662
|
17.6
|
3.9
|
*31
|
6
|
5.2
|
1.0
|
184
|
91
|
*58
|
10
|
Whole plant N in this study (20.8 - 45.4 mg N kg-1) was in the range of 1.6 to 4.5 mg N kg-1 reported by others for young cotton plants (Joham 1950; Harris 1960). The general depression of uptake of N in amendment treatments raises the concern that additional N fertilizer may be required in the field to ensure early nutritional needs for cotton crops are met. Whole plant P was generally increased by amendment. However, the values obtained in this study (1.2 - 3.0 mg P kg-1) were lower than those from the same studies (3.15 - 5.1 mg P kg-1). Other macronutrients reported by Joham (1950) and Harris (1960) for young whole plants including K (20.8 - 46.1 mg kg-1), Ca (15.1 - 34.6 mg kg-1), and Mg (4.0 - 5.36 mg kg-1). However, these nutrients were well under the range for many treatments in the Wolfpen soil.
When relating the nutrient uptake results to shoot matter, the depression of N in every case may be at least partially explained as a dilution effect arising from the increase in shoot height and mass (Table 6 and Table 8). This statement does not imply a prediction on sufficiency or deficiency beyond the six-week period of this study. However, with P uptake there was a general positive coincidence between P increase and shoot increases. Because growth (as shoot height, shoot mass, and root mass) was generally stimulated by amendment, depressions and increases in the uptake of all other nutrients may be considered through the same lens.
There is a lack of data from recent studies on whole plant nutrients at early stages of growth. There were no identifiable sources of information on whole plant S, or the micronutrients Zn, Fe, Cu, Mn, and B. Early growth of cotton biomass and nutrient uptake can substantially influence later development stages, this study provides important data in that regard that is currently in short supply.
3.5 AMF/Root Association
Mutualism between cotton roots and AMF occurs only when there is an unsatisfied need that AMF can provide to that plant (Hale et al. 2011). Therefore, harnessing the benefits of this relationship towards cotton production following biochar amendment in sub-Saharan Africa will depend upon better understandings the complex nature of the system. For example, if biochar amendment were to suppress the association between AMF and roots, then our understanding of the benefits of biochar would perhaps need to be tempered. On the other hand, if biochar were to uniformly stimulate AMF association with cotton roots, the attendant benefits would be expected, including increased P acquisition, drought resistance, and pest tolerance (Watt et al. 2006).
Multicollinearity evaluation resulted in different choices for the model depending on whether PCR, VIF, or ridge regression were employed. For example, P supplied via carbon amendment, and not soil test P, was identified as highly related by PCR. On the other hand, ridge regression identified only soil-associated properties as significant at α = 0.05. Ultimately, carbon amendment P content was included (p = 0.0547) in the final regression model as the data appear to support the inclusion of factors other than those exclusively associated with the four soils (Figures 1 and 2). Note that an intercept term was not significant for the model. Regression parameter estimates from the SAS software REG procedure are presented in Table 10.
Table 10. Ridge regression parameter estimates, standard errors, and probabilities (p-values) for relating measured AMF % density in cotton roots to soil and amendment properties.
Variable
|
Parameter Estimate
|
Standard Error
|
p-value
|
Carbon amendment P
|
+1.411
|
0.698
|
0.0547
|
Soil test P
|
-0.486
|
0.137
|
0.0017
|
Soil pH
|
+2.153
|
0.976
|
0.0372
|
% Clay
|
+1.129
|
0.354
|
0.0040
|
ANOVA
|
Sum of Squares
|
r2
|
p-value
|
Model (n = 28)
|
26634
|
0.8614
|
< 0.0001
|
Regression results indicate that total P in the carbon amendments were positively related with AMF root colonization, while soil test P was negatively related. Soil pH and % clay content of the soils were both positively related to AMF root colonization in the range of values included in this study. Other factors evaluated (amendment C and N) were not significant. Because AMF are frequently associated with biotic and abiotic stress resistance in major crop plants, a better understanding of early association patterns as a function of soil property and amendments will be important to managing them for crop production benefits.
In our study, AMF inoculants were prepared from fresh soils taken from the same sites as the processed soils used to grow cotton. It was our intention to work only with native populations, as exotic species have occasionally been shown to be less beneficial to plant growth and less inter-functional with other native macro and microflora (Klironomos 2003; Koziol and Bever 2017; Kouadio et al. 2017; Rezacova et al. 2020). Therefore, soil specific effects are to be expected, by design. However, the current study supports positive correlation between AMF/root association and clay content, plant available soil P, and pH reported in previous studies.
Previous research has indicated that AMF colonization of plant roots is negatively related to plant available P in soils, whether native to the soil or supplied as fertilizer (Douds and Schenck 1990; Konvalinkova et al. 2017). However, P supplied to soil in organic amendments was demonstrated to stimulate AMF association with roots. The chemical forms in plant residues, manures, and composts are primarily organic P compounds (Sadras 2006; Azuara et al. 2013). The chemical forms in biochar are primarily insoluble inorganic P complexes with aluminium (Al), Ca, Mg, and Fe that are more stable than organic P (Uchimiya et al. 2015). Neither are immediately plant available, and therefore require some energy to mineralize or solubilize. Hence, the assistance to plants from AMF towards increasing available P favors increased mutualistic interaction. This mechanism explains the contrasting negative effect of native soil P with the positive effect of amendment P on AMF / root association.
There is very little evidence directly comparing soil pH as a factor in AMF association with plant roots. Graw (1979) reported that AMF efficiency in increasing P uptake from Ca2+ and Al3+ complexed phosphate (PO43-) applied to soil in Mexican marigold (T. minuta L.) and nigerseed (G. abyssinica) over the range of 4.4 to 6.6 pH. The range of soil pH values in this study was wider (4.7 to 7.7), extending the evidence for this effect. Carrenho et al. (2007), however, reported that liming to increase soil pH (4.5-7.0) did not result in significant effect for AMF colonization of peanut (A. hypogaea L. variety Tatú), sorghum (S. bicolor (L.) Moench variety AG 1017), or corn (Z. mays L. variety IAC Taiúba) in the field. This may have been an artifact of ecosystem perturbations caused by introduction of the calcareous dolomite liming agent (CaMg(CO3)2), rather than a response to soil pH. Porter et al. (1987) found that different species of AMF associated with cereal crops predominated in acidic soils (A. laevis) and alkaline soils (Glomus sp. WUM 3) in western Australian, supporting the concept that external adjustments to soil pH may not favour the activity of native adapted AMF species.
The body of knowledge on AMF association affected by soil clay content is not well developed. Our study found that soil clay was positively related to AMF association with cotton roots. Porter et al. (1987) found a weak positive relationship with clay content. Carrenho et al. (2007) reported a negative effect for soil clay on AMF colonization, stating that sandy soils stimulated development of AMF association while clayey soils inhibited it. There is a need to continue filling the knowledge gap in this specific area of inquiry.
Post-hoc simple regression analysis for direct relationships between AMF as an independent variable and the plant performance parameters nutrient assimilation, shoot height, shoot mass, root mass, and S:R ration indicated that there were relationships only for N (17.19 + 0.19*AMF; model p = 0.0189) and shoot mass (2.61 - 0.02; model p = 0.0065). Although a relationship between AMF association and P uptake was expected, it was not supported by the data in this study. Increased N uptake associated with AMF has been reported for celery (Ames et al. 1983) and lettuce (Tobar et al. 1994; Azcon et al. 2008). Hodge et al. (2001) attributed improved N uptake to improved organic matter degradation. Contrary to the negative effects regarding shoot mass, Konvalinková et al. (2017) reported increased shoot mass following inoculation with AMF in leeks (A. porrum) and ryegrass (L. perenne), but not in medic (M. truncatula). Their study was performed in the glass house in pots filled with sand and zeolite mix. Plants were harvested at 63 days, as opposed to 48 in our study. The effects in very young plants, where AMF associations are only recently established, may reflect larger initial percentage investments of plant photosynthetic product than those in more mature stages of growth development.
Brundrett et al. (1985) reported that AMF hyphae penetrated leek root cells within 2 days, that arbuscules (structures for transfer of products between plant and host) were formed within 4 days, and that vesicles (structures for storage of product) were formed within 5 days of inoculation. It is clear that mutualistic associations between AMF and host plant can establish very rapidly. The observations in this study were that hyphae, arbuscules, and vesicles were present all present in cotton roots harvested at 6 weeks. However, it was beyond the scope of this study to further evaluate allocation of resources between host and AMF symbiont as a function of plant developmental stage.