In terms of a sustainable energy and waste management, by products in form of aerobically composted or anaerobically digested organic materials can be used as cost-effective organic fertilizer in agriculture (Risberg et al., 2017). These organic residues can also be used as soil structure conditioner (Beck-Broichsitter et al., 2018) to improve the nutrient and water supply for plants (Duong et al., 2012), soil aeration (Reszkowska et al., 2011), and root growth (Möller and Müller, 2012). The impact of organic residues on soil chemical properties, for example, organic carbon (OC) and nutrient supply (Voelkner et al., 2015a) or wettability (Beck-Broichsitter et al., 2020b) is comprehensively analyzed, whereas a lack of information is still existing for physical properties including the plant available water capacity and air permeability.
The process of soil aeration is important for the root growth and the crop production of arable soils (Zhai and Horn, 2018; Beck-Broichsitter et al., 2020a). This includes a sufficient continuity of pores and a stable pore network (Horn et al., 2014), but tilled topsoils are known for discontinuities in the pore system through mechanically induced homogenization compared to non-tilled topsoils (Lipiec et al., 2003; Dörner et al., 2012; Assis et al., 2016). The effect of application of organic residues on the pore continuity can be evaluated using the c2 and c3 indices that consider the relation between air permeability, ka, and air-filled porosity, εa, of soils (Groenevelt et al., 1984; Ivelic-Sáez et al., 2015). The objective of the study is to determine the effect of application of compost, sewage sludge, and digestates containing maize, sugar beet, and winter wheat on nutrient availability, capacity parameters (AC, AWC), and intensity parameters (air permeability) of a glacial till-derived Ap horizon of a haplic Luvisol under agricultural use.
The authors hypothesize that the application of organic residues will increase the AC and plant AWC and decrease the pore continuity (c2 and c3 indices) of the untreated loam.
2 Materials and methods
2.1 Basic characteristics of soil material and organic residues
For the laboratory research, disturbed soil material was sampled from the Ap horizon (0–0.3 m in depth) of an agricultural-used and glacial till-derived haplic Luvisol (horizon sequence: Ap/E/Bt/Bw/C) (IUSS Working Group WRB, 2014), located at the research farm in Hohenschulen (54°31′28″N, 9°98′35″E) in Northern Germany. The silage from maize, sugar beet, and winter wheat were derived from the biogas plant in Schleswig-Holstein. The liquid digestates (i) 80% maize and 20% sugar beet (80m20b), (ii) 20% maize and 80% sugar beet (20m80b), and (iii) 20% winter wheat and 80% sugar beet (20w80b) were generated in a batch fermentation process. The compost (com) was produced by the local composting facility in Schleswig-Holstein, made out of shrub chippings (Beck-Broichsitter et al., 2018), whereas the sewage sludge (sl) and the associated chemical properties were derived from the municipal waste-water treatment plant in Schleswig-Holstein.
The texture of the soil material was classified as loam (FAO, 2006) consisting of 550 g kg−1 sand, 300 g kg−1 silt, and 150 g kg−1 clay with an OC content of 11.96 g kg−1, and pH of 6.79. The organic residues indicate pH values between 7.47 and 8.08 and, for example, total nitrogen contents between 1.2 and 4.0 kg m−3 organic mass−1 (Table 1).
Basic characteristics of loam and organic residues: (i) 80% maize and 20% sugar beet (80m20b), (ii) 20% maize and 80% sugar beet (20m80b), and (iii) 20% winter wheat and 80% sugar beet (20w80b); compost (com); and sewage sludge (sl) with two repeated measurements each and symbol ± corresponds to the standard deviation.
Tabelle 1. Grundlegende Eigenschaften des Lehms und der organischen Rückstände: (i) 80 % Mais und 20 % Zuckerrübe (80m20b), (ii) 20 % Mais und 80 % Zuckerrübe (20m80b) und (iii) 20 % Weizen und 80 % Zuckerrübe (20w80b), Kompost (com) und Klärschlamm (sl) mit jeweils zwei Messwiederholungen und das Symbol ± entspricht der Standardabweichung.
|pHCaCl2 (−)||6.79 ± 0.3||8.08 ± 0.6||7.47 ± 0.3||7.74 ± 0.3||7.72 ± 0.3||7.78 ± 0.4|
|OC (g kg−1)||11.96 ± 2.3||-||-||-||-||-|
|dm (%)||-||52.0 ± 4||-||5.51 ± 0.2||5.38 ± 0.3||5.38 ± 0.1|
|NH4-N (kg m3 om−1)||-||-||-||1.95 ± 0.3||2.09 ± 0.3||2.08 ± 0.4|
|Ntotal (kg m3 om−1)||-||1.24 ± 0.2||4.02 ± 0.3*||2.82 ± 0.1||2.87 ± 0.2||2.32 ± 0.1|
|P (g kg dm−1)||-||1.97 ± 0.2||2.46 ± 0.2*||23.5 ± 1.1||23.6 ± 1.8||27.3 ± 1.9|
|K (g kg dm−1)||-||5.95 ± 0.5||3.49 ± 0.3*||71.9 ± 3.4||75.1 ± 2.9||74.4 ± 3.1|
|Mg (g kg dm−1)||-||4.03 ± 0.4||1.96 ± 0.1*||10.1 ± 0.9||10.4 ± 1.1||13.0 ± 0.8|
|Ca (g kg dm−1)||-||21.9 ± 1.6||-||28.3 ± 1.4||28.7 ± 2.1||31.5 ± 1.8|
|Na (g kg dm−1)||-||-||-||3.24 ± 0.2||3.03 ± 0.1||3.59 ± 0.1|
|C/N (–)||10 ± 0.9||-||-||8.02 ± 0.3||7.97 ± 0.2||9.98 ± 0.2|
2.2 Sample preparation and laboratory analysis
The organic residues were air dried, sieved (≤2 mm), and mechanically mixed with the loam (θ of approx. 0.1 cm3 cm−3) to simulate the annual application rates for fertilizer in 0.3-m Ap horizon with 30 Mg dry mass ha−1 for compost and 30 m3 moist mass ha−1 for digestates and sewage sludge. After the application of the organic residues, particle density (ρs; g cm−3), using pycnometer method; OC (g kg−1), using coulometric carbon dioxide (CO2) measurement; potential cation exchange capacity, CECpot (cmolc kg−1), including natrium (Na), potassium (K), magnesium (Mg), and calcium (Ca), using barium chloride method; texture, using combined sieve and pipette method; soil pH values (in 0.01M CaCl2 solution); and saturated hydraulic conductivity, Ks (cm d−1), using by steady-state flow method were analysed following Hartge and Horn (2016).
Furthermore, the loam and the residue mixtures were compacted to a dry bulk density, ρb (g cm−3) of approximately 1.45 g cm−3 by a load frame (Instron 8871, Norwood, USA) with a pressing force of 5 kN, resulting in 8–10 soil cores (diameter: 5.5 cm; height: 4 cm) each.
2.3 Soil water retention characteristics
The volumetric water content (θv) for the different drying stages was from the soil cores with 8–10 replicates per depth by a combined pressure plate (saturated, −60 hPa, and −300 hPa) and ceramic vacuum outflow method (−15,000 hPa) as well as oven dried for 24 hours at 105°C. The air-filled porosity, εa, was calculated as follows:
The soil porosity, ε, was calculated as:
The AC (cm3 cm−3) and the plant AWC (cm3 cm−3) were calculated using the following equations:
2.4 Air permeability and pore continuity indices
The air conductivity, kl, was simultaneously determined from the same samples (100 cm3) as used for measuring soil water retention characteristics at −60, −300, and −15,000 hPa and in dry stage (105°C, 24 h) using an air flow meter with different scales between 0.1 and 10 L m−1 (Key Instruments, Trevor, USA) (for more details, see Zhai and Horn, 2018):
The pore continuity was determined using the c2 and c3 indices as proposed by Groenevelt et al. (1984) and Zhai and Horn (2018). The indices are calculated as relationship between ka and εa and also εa2 in the following form (Ball et al., 1988; Dörner and Horn, 2006):
Furthermore, soils with similar pore size distribution and porosity continuity have analog c2 values, whereas soils with similar c3 values only have analog pore size distribution (Groenevelt et al., 1984). In additionally, differences between c2 and c3 are related to differences in the pore continuity, independent from the pore size distribution (Dörner and Horn, 2006).
The following exponential relation between ka and εa was suggested by Ball et al. (1988):
2.5 Statistical analysis
The statistical software R (R Development Core Team, 2014) was used to evaluate the data. The data were tested for normal distribution and heteroscedasticity on the Shapiro–Wilk test and graphical residue analysis. An analysis of variance (ANOVA) was conducted with p<0.05 followed by Tukey's HSD (honestly significant difference) test (*p≤0.05, **p≤0.01, ***p≤0.001, and ****p≤0.0001) following Hasler and Horton (2008) to evaluate the differences between loam and the loam residue mixture in basic soil characteristics (see Table 2), AC, plant AWC (see Table 3), and their interaction terms (twofold and threefold), respectively. The coefficient of determination (r2) is an index for the goodness of fit.
Soil characteristics after the application of the organic residues to loam: (i) 80% maize and 20% sugar beet (80m20b), (ii) 20% maize and 80% sugar beet (20m80b), and (ii) 20% winter wheat and 80% sugar beet (20w80b); compost (com); and sewage sludge (sl) for two repeated measurements each, and symbol ± corresponds to the standard deviation. Differences in mean values compared to loam were significant at p<0.05 (*), p<0.01 (**), p<0.001 (***), and p<0.0001 (****) for ANOVA.
Tabelle 2. Bodeneigenschaften nach der Zuführung der organischen Rückstände zum Lehm: (i) 80 % Mais und 20 % Zuckerrübe (80m20b), (ii) 20 % Mais und 80 % Zuckerrübe (20m80b) und (iii) 20 % Weizen und 80 % Zuckerrübe (20w80b), Kompost (com) und Klärschlamm (sl) mit jeweils zwei Messwiederholungen und das Symbol ± entspricht der Standardabweichung. Abweichungen zum Mittelwert des Lehms sind bei der ANOVA signifikant bei p<0,05 (*), <0,01 (**), <0,001 (***), <0,0001 (****).
|pHCaCl2 (−)||6.79 ± 0.3||6.87 ± 0.3||6.75 ± 0.3||6.62 ± 0.4||6.59 ± 0.6||6.65 ± 0.3|
|CECpot (cmolc kg−1)||9.39 ± 1.4||11.1 ± 1.5*||9.78 ± 0.7||10.2 ± 0.8||9.58 ± 0.6||9.68 ± 0.6|
|Na (cmolc kg−1)||0.11 ± 0.1||0.12 ± 0.1||0.11 ± 0.1||0.25 ± 0.1****||0.12 ± 0.1||0.12 ± 0.1|
|K (cmolc kg−1)||0.64 ± 0.1||0.83 ± 0.1**||0.68 ± 0.1||1.23 ± 0.1****||1.24 ± 0.2****||1.19 ± 0.2****|
|Mg (cmolc kg−1)||2.35 ± 0.1||2.38 ± 0.3||2.39 ± 0.2||2.58 ± 0.3||2.61 ± 0.2||2.51 ± 0.3|
|Ca (cmolc kg−1)||7.71 ± 0.9||8.56 ± 0.7||8.21 ± 1.1||8.24 ± 0.9||8.31 ± 0.8||7.97 ± 0.9|
|OC (g kg−1)||12.4 ± 1.4||13.5 ± 1.9||12.2 ± 1.1||14.1 ± 1.7||14.3 ± 1.5*||13.2 ± 1.2|
|Ntotal (g kg−1)||1.19 ± 0.1||1.28 ± 0.1||1.31 ± 0.1||1.52 ± 0.2||1.61 ± 0.1||1.48 ± 0.1|
|C/N ratio||10 ± 0.4||10 ± 0.6||10 ± 0.6||9 ± 0.3||9 ± 0.4||9 ± 0.3|
Porosity, ε, air capacity, AC, plant available water capacity, AWC, and saturated hydraulic conductivity, Ks, after the application of the organic residues to the loam for 8–10 repeated measurements each. Differences in mean values compared to loam were significant at p<0.05 (*), p<0.01 (**), p<0.001 (***), and p<0.0001 (****) for ANOVA. The symbol ± corresponds to the standard deviation; abbreviations of residues are listed in Table 1.
Tabelle 3. Porosität, ε, Luftkapazität, AC, nutzbare Feldkapazität, nFK, und gesättigte Wasserleitfähigkeit, Ks, nach der Zuführung der organischen Rückstände zum Lehm mit jeweils 8–10 Messwiederholungen. Abweichungen zum Mittelwert des Lehms sind bei der ANOVA signifikant bei p<0,05 (*), <0,01 (**), <0,001 (***), <0,0001 (****). Das Symbol ± entspricht der Standardabweichung; Abkürzungen sind in Tabelle 1 aufgeführt.
|(cm3 cm−3)||(cm3 cm−3)||(cm3 cm−3)||(cm d−1)|
|Loam||0.462||0.142||0.143||54 ± 11|
|com||0.438||0.154||0.114**||58 ± 12|
|sl||0.439||0.151||0.122*||40 ± 14|
|80m20b||0.447||0.143||0.130||55 ± 21|
|20m80b||0.473||0.191***||0.111***||59 ± 23|
|20w80b||0.445||0.122||0.145||79 ± 31|
3.1 Basic soil characteristics after application of organic residues
The results show that the OC content of the loam was improved through application of organic residue (12.4 up to 14.3 g kg−1). There are very small differences in weak acidic pH values between 6.59 and 6.87, whereas application of compost (com) increases and application of digestate decreases the pH value (Table 2). The calcium (Ca) and especially the potassium (K) content significantly increased through application of organic residue from 0.64 up to 1.24 cmolc kg−1. The C/N ratios between 9 and 10 indicate easily decomposable organic substances and, therefore, a rapid available nutrient source for plants.
3.2 Soil water retention characteristics and air permeability after application of organic residues
The results indicate medium to high total porosities and the application of organic residues slightly decreased the porosity values, except for 20m80b that in turn increased the AC value of up to 0.191 cm3 cm−3 and decreased the AWC value of down to 0.111 cm3 cm−3 compared to loam, while 20w80b shows an opposite trend (Table 3). The AC values can be classified as medium to high, while the AWC values are at medium to low level. The Ks values between 40 cm d−1 and 79 cm d−1 can be classified as high without any significant changes compared to the loam.
In the range of εa values between 0.14 and 0.22 cm3 cm−3, the ka values are nearly identical, whereas the ka values increase up to 433 μm2 for compost (com) at the dry stage between εa values of 0.45 and 0.48 cm3 cm−3 (Figure 1).
The c2 and c3 values in the range between −60 and −15,000 hPa show significant differences between the loam and the loam residue mixtures resulting in differences in pore size distribution and porosity continuity (Table 4). On the other side, the c2 and c3 values in the dry stage (105°C) are not very pronounced, resulting in a similar pore continuity and pore size distribution.
Pore continuity indices c2 and c3 after the application of organic residues for 8–10 repeated measurements each at pressure heads, h, of −60, −300, and −15,000 hPa and in dry stage (105°C). Differences in mean values compared to loam were significant at p<0.05 (*), p<0.01 (**), p<0.001 (***), and p<0.0001 (****) for ANOVA. The symbol ± corresponds to the standard deviation, and r2 indicates the coefficient of determination; abbreviations of residues are listed in Table 1.
Tabelle 4. Porenkontinuitätsindizes c2 und c3 nach der Zuführung der organischen Rückstände für jeweils 8–10 Messwiederholungen bei Druckstufen, h, von −60 hPa, −300 hPa, −15000 hPa und im getrockneten Zustand (105°C). Abweichungen zum Mittelwert des Lehms sind bei der ANOVA signifikant bei p<0,05 (*), <0,01 (**), <0,001 (***), <0,0001 (****). Das Symbol ± entspricht der Standardabweichung und r2 dem Korrelationskoeffizienten; Abkürzungen sind in Tabelle 1 aufgeführt.
|Residue||c2 (μm2)||c3 (μm2)|
For better understanding of the model parameters, M, N, and εb, the ka and εa values were fitted with Eq. 10 and εb was estimated with Eq. 11. The fitted parameters for loam and organic residue mixtures are listed in Table 5. A positive linear ka/εa relationship (r2: 0.96–0.99) was found, and the blocked air-filled pore space, εb, of the loam with 0.0079 cm3 cm−3 was not significantly affected through the application of organic residue.
Model parameter, LogM, N, and blocked air-filled pore space, εb, after the application of organic residues for 8–10 repeated measurements each at pressure heads, h, of −60, −300, and −15,000 hPa and in dry stage (105°C). Differences in mean values compared to loam were significant at p<0.05 (*), p<0.01 (**), p<0.001 (***), and p<0.0001 (****) for ANOVA. The symbol ± corresponds to the standard deviation, and r2 indicates the coefficient of determination; abbreviations of residues are listed in Table 1.
Tabelle 5. Modellparameter, Log M, N and blockiertes Porenvolumen, εb, nach der Zuführung der organischen Rückstände für jeweils 8–10 Messwiederholungen bei Druckstufen, h, von −60 hPa, −300 hPa, −15000 hPa und im getrockneten Zustand (105°C). Abweichungen zum Mittelwert des Lehms sind bei der ANOVA signifikant bei p<0,05 (*), <0,01 (**), <0,001 (***), <0,0001 (****). Das Symbol ± entspricht der Standardabweichung und r2 dem Korrelationskoeffizienten; Abkürzungen sind in Tabelle 1 aufgeführt.
4.1 Basic soil characteristics after application of organic residues
The results of the study presented in Table 2 indicate lower nitrogen contents, Ntotal, of 1.3 g kg−1 after the application of compost and sewage sludge compared to the digestates with Ntotal values between 1.5 and 1.6 g kg−1 through treatment-derived NH3 and NH4+ losses (Möller and Müller, 2012; Stoknes et al., 2016). The nutrient contents (Na, K, Mg, and Ca) and, therefore, the cation exchange capacity, CECpot, were higher after the application of organic residues with values between 9.6 and 11.1 cmolc kg−1 than for the loam with 9.4 cmolc kg−1 as also proposed by Ojeda et al. (2015). The increase in pH values through the application of compost considering the initial pH of 8.1; thus, this liming effect was confirmed in several studies (Ojeda et al., 2015), also for other organic residues (e.g., biochar). The composition of organic matter was not directly investigated, but Tambone et al. (2010) proposed that the chemical composition did not greatly contribute to compare digestates with compost or sewage sludge. It should also be taken into account that the dose of the applied organic residues affects its use as soil conditioner and fertilizer each (Govasmark et al., 2011) and the additional supply of OC that contains different numbers of polar and non-polar functional groups can negatively affect the wettability of the loam. Thus, also the local connectivity of flow pathways is reduced (Beck-Broichsitter et al., 2020b). Furthermore, the potential risk of soil acidification after organic residue amendment (Voelkner et al., 2015a) and the effect on wettability and dispersion (Voelkner et al., 2015b) of aggregated structured soils compared to the initially homogenized loam used in this study should be validated under field conditions.
4.2 Effect of organic residue application on water retention characteristics and air permeability
The results presented in Table 3 indicate that the application of organic residues increased the ACs and decreased the plant AWC especially in case of digestate containing 20% maize and 80% sugar beet (20m80b). The results regarding the AWC values are contrary to the initial hypotheses and findings of Ojeda et al. (2015) and Beck-Broichsitter et al. (2018), except for digestate containing 20% wheat and 80% sugar beet, that slightly increased the AWC of the loam. Thus, the application of organic residues can potentially compensate low air capacities (Jasinska et al., 2006), whereas the decrease in AWC values can be attributed to the decrease in volume fraction of medium pores (−300 < h < −15,000 hPa) through homogenization as stated by Hu et al. (2009). However, wheat-containing digestate (20w80b) seems to slightly improve or not deteriorating the AWC as hypothesized before. It should be noted that the increase in the air capacities indicates an increase in the volume fraction of pores available for air flow or soil aeration, but it is not identical with the accessibility of oxygen for plant roots (Reszkowska et al., 2011; Beck-Broichsitter et al., 2020a).
The application of organic residues, however, significantly lowered the pore continuity at pressure heads of −60 and −300 hPa, except digestate 20w80b. The air-filled coarse pores may not contribute to the flow when they are discontinuous (Lipiec et al., 2003; Dörner and Horn, 2006). In the dry stage, the pore continuity indices presented in Table 4 show no significant differences; thus, the formation of shrinkage cracks seems to become more important for the connectivity of the pore system (Beck-Broichsitter et al., 2020b). However, compared to aggregated, structured soils in the field, the initially homogenized samples in this study tend to a higher shrinkage crack formation potential (Beck-Broichsitter et al., 2018) resulting in a comparatively higher air-filled porosity, more pronounced preferential flow paths and, therefore, overestimated air permeabilities. By the way, the slope factor N derived by Eq. 10 was significantly lower for digestate 20w80b than for loam and other residues; thus, the pore connectivity increases slowly with increase in air-filled pore porosity (Ball et al., 1988).
The blocked air-filled porosities presented in Table 5, between 0.0072 and 0.0084 cm3 cm−3, are nearly equal and on a very low level compared to the porosities in Table 2; thus, almost all pores of loam and loam residues mixtures are available for gas flow as also proposed for tilled topsoils (Dörner et al., 2012). This may be related to the increase in OC content that contributes a better quality of porous media (Assis et al., 2016). On the other side, tillage-induced homogenization can interrupt the functional pore system (Petersen et al., 2008), and compared to non-tillage conditions, the blocked air-filled porosity could be higher (Dörner and Horn, 2006; Dörner et al., 2012). In terms of the study results, the initial re-compaction of the soil cores to 1.45 g cm−3 may positively affected the rearrangement of the soil particles and pore connectivity (Horn et al., 2014) that could be another reason for the very low εb values.
The objective of the study was to determine the effect of application of compost, sewage sludge, and digestates containing maize, sugar beet, and winter wheat on nutrient content, air capacity, plant available water capacity, and air permeability.
The results suggest that the application of organic residues can increase the nutrient content (Na, K, Mg, and Ca) and the air capacity of the loamy soil, whereas the plant available water capacity and the pore continuity may decrease. It can also not be concluded that a higher air capacity automatically increases the pore continuity and, therefore, the air permeability. However, the wheat-containing digestate 20w80b shows an opposite trend, slightly improving the plant available water capacity, whereas the air capacity is significantly lower compared with compost, sewage sludge, maize-containing digestates, and the untreated loam.
Further research is needed to investigate the application of organic residues under field conditions considering tilled and non-tilled conditions.
This research was supported by the Innovation Foundation Schleswig-Holstein and the ZMD Rastorf GmbH, Germany.
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