Soil Organic Matter Quality in Soils With Different Levels of Manure Fertilisation

Most tasks that soil fulfils in agroecosystems depend on its soil organic matter (SOM) content and the quality of the said SOM. Organic matter, especially humus (part of SOM transformed by humification) through its complex influence on chemical, physical and biological properties, regulates soil productive output and resistance to degradation [Bakayoko et al. 2009, Kalembasa, Kalembasa 2016]. It is a strategic resource in maintaining high quality of soil. Soil users through influence on the quality and quantity of deposited SOM can significantly impact the quality and productive output of the soil environment. Research on the quality and quantity of organic matter should be the foundation of general evaluation of soil quality and its productive potential [Weber et al. 2018].

Over the past decades, a downward trend in organic matter content of arable soils has been observed in Poland. Among the potential causes of this profoundly adverse phenomenon most frequently mentioned are agricultural intensification and a disproportional share of crops whose cultivation leads to negative balance of SOM in overall crop structure especially cereals often grown in monocultures. Global causes most notably climate change and associated warming with prolonged periods of drought change air-to-water relations in soils contributing to higher levels of oxygenation and therefore accelerating processes of mineralisation of SOM [Gonet, Markiewicz 2007, Becher et al. 2013].

The scientific consensus states that the content of organic matter in soils is the result of a balance of processes leading to its accumulation and degradation, depending on the complex interaction of natural environmental conditions, modified by anthropogenic factors [Dorado et al. 2003, Mao et al. 2008, Dębska et al. 2016]. In arable soils, organic fertilisation is a significant modifying factor. The changing structure of agriculture in Poland, resulting mainly from the specialisation of production, also affects the balance of SOM. Farms specialised in crop production (with no livestock production) usually forgo organic fertilisation. Increase in livestock production often results in overproduction of natural fertilisers and a necessity to increase the frequency of natural fertiliser use on individual fields [Gonet, Markiewicz 2007].

The aim of the study was to assess the quality of organic matter in arable Luvisol with a varying degree of bovine manure fertilisation.

Soil research was conducted on the Ciechanów Upland in the Strzelnia village (53°02′N 20°37′E) located in the Grudusk commune (north-east of Mazowieckie Voivodeship). The research transect was marked covering Stagnic Luvisol [IUSS Working Group WRB 2015, Systematyka gleb Polski 2019] developed from glacial till. As the criterion for selecting sampling locations, three variants of SOM management were taken into account, where the most important determinant was the amount and frequency of bovine manure application. Representative soil samples were taken in spring (prior to pre-sowing treatments) from topsoil (up to 30 cm deep) (Tab. 1). Samples were collected from 20 points of an area of 1 ha by using the Egner stick. The selected soils used mineral fertilisation in accordance with agricultural engineering of arable crops.

After drying, the samples were crumbled up and sieved through a 2 mm sieve. The particle size distribution was determined by the combination of sedimentation and sieve method according to the Polish Society for Soil Science recommendations [Polish…2009]. For the analysis of chemical properties, a part of the sample (about 50 g) was grinded down in an agate mill (up to 0.25 mm in diameter of particle size). The following laboratory analyses were performed in prepared samples (in three repetitions): –

Soil pH was determined on the basis of measurement by the potentiometric method in 1 M KCl, with an air–dry soil solution ratio of 1:2.5 (m/v).

Soil organic matter content was calculated according to the following equation: SOM = TC · 1.724.

Sequential fractionation of organic matter was carried out (Fig. 1) based on the Schnitzer method [Dziadowiec, Gonet 1999, Becher 2013]. The name and symbol of the organic matter fraction and the method of preparation are presented in Table 2. The carbon ratio of fulvic to humic acids (C_HA / C_FA) was calculated.

Figure 1

Humic acid preparations were prepared according to the Schnitzer procedure [Dziadowiec, Gonet 1999]. Ash content (after heating up to 600°C) in humic acid preparations amounts to 1.38–2.09%. The following analyses were carried out for humic acids: –

Elemental composition [content of C, H and N – determined on the basis of elemental analysis (automatic CNH analyzer), content of O was determined from the difference]. Based on the share of atoms, the atomic ratio H/C and the degree of internal oxidation of the molecules were calculated: ω = [(2O + 3N) − H]/C.

In the examined soil horizons of the accumulation of organic matter, a similar content of granulometric fractions was found, enabling them to be classified as loam (Tab. 3). Similar pH values were determined (5.17–5.45). The obtained values suggest similar conditions for the transformation of organic matter in the selected research objects.

Clear differences between the studied soils were found in the SOM content. In the soil intensively fertilised with manure, a significantly higher content of SOM was found. The lowest content of organic matter was found in the soil where for dozen or so years before sampling, no organic fertilisation was performed. The obtained results confirm the universally recognised fact that the organic fertilisation has a positive impact on the positive balance of organic matter in soil [Turski 1988, Howard et al. 1998, Dębska et al. 2016].

The fractional composition of SOM is commonly presented in the form of the quantity and share of carbon in the distinguished fractions, in accordance with the adopted fractionation method [Becher 2013]. In the studied soils, a diversified amount and share (in TC) of SOM carbon fraction was found (Tab. 4). The obtained results allow fractions to be ordered in the following order of decreasing quantitative significance: CHUMIN>CHA>CFA>CBIT>CDEC.{{\rm{C}}_{{\rm{HUMIN}}}} > {{\rm{C}}_{{\rm{HA}}}} > {{\rm{C}}_{{\rm{FA}}}} > {{\rm{C}}_{{\rm{BIT}}}} > {{\rm{C}}_{{\rm{DEC}}}}.

A fraction of low quantitative significance in the studied soils is the fraction separated in the first fractionation step (using 0.05 M H₂SO₄) – operationally named post decalcification (C_DEC). This fraction is mainly represented by simple organic compounds, low molecular weight, potentially labile in the soil environment and susceptible to mineralisation [Dębska 2004, Becher 2013]. The largest share of this fraction was found in the soil intensively fertilised with manure (sample I). In other soils, the share of this fraction was comparable.

The soil bitumen fraction (lipid fraction extracted using organic solvents) is formed, among others, by waxes, tars, resins, fatty acids and their esters, and many other aliphatic and high energy compounds [Wiesenberg at al. 2004, Becher, Kalembasa 2006]. In the studied soils, a varied amount and share of C_BIT were observed. Unlike C_DEC, the amount of this fraction was clearly the smallest in soil heavily fertilised with manure. Literature reports suggest that the biological activity of soils, combined with the intensification of organic matter mineralisation, may lead to a decrease in the share of carbon that can be separated with organic solvents. Some authors also see the reason for the variation in the amount of bitumen carbon in the saponification of fatty acids with calcium and magnesium cations found in soils [Neves et al. 2009, Kalembasa, Becher 2006].

The carbon extracted from the tested soil samples with an alkaline solution (0.1 M NaOH) is represented by humic substances (C_HS), which are then separated into fulvic acids (C_FA) and humic acids (C_HA). As can be seen in the data contained in the table, a significant part (about half) of the total amount of SOM in the studied soils is accumulated in extractable (0.1 M NaOH) humic substances. The highest share of humic substances was found in soil deprived of manure fertilisation. Among humic substances separated with 0.1 M NaOH, humic acids predominated as evidenced by the values of the carbon ratio of humic acids to fulvic acids (C_HA/C_FA) above 1. The lowest value of this ratio was found in the soil intensively fertilised with manure (1.38) and the highest in soil not fertilised with this natural fertiliser (1.68). Generally, the prevalence of humic acids is an advantageous quality feature of soil humus because humic acids are more stable in the soil environment and determine the most important functions of humus in the soil environment [Becher et al. 2013, Weber et al. 2018].

According to organic residue decomposition research, initial stages of humification are characterised by intensive formation of fulvic acids [Dębska 2004]. Probably the lowest C_HS/C_FA value in the soil intensively fertilised with manure is a consequence of the systematic inflow of ‘fresh’ organic matter and the initial stages of humification [Aoyama, Kumakura 2001]. The carbon remaining in the sample after extraction (residual fraction) was included in the humin fraction (C_HUMIN). This fraction consists of humic substances most durable and resistant to microbiological decomposition and humic substances related to the solid phase of the soil. In the studied soils, this fraction constituted the most quantitatively important component of organic matter. One of the factors contributing to the large quantity of this fraction may be the presence of the clay mineral fraction, which, as is known from scientific studies, can be a factor ‘preserving’ humic substances in the soil. It is possible due to the formation of persistent organic-mineral compounds [Stevenson 1994, Weber et al. 2018].

Fractionation of organic matter has proven that in the studied soils, humification significantly promotes the formation of humic acids, i.e. humus fraction composed of high molecular weight and relatively stable organic compounds [Turski 1988, Stevenson 1994]. In this paper, qualitative studies of humic acid preparations isolated from the topsoil of the studied soils were undertaken (Tab. 5). Elemental composition is presented in the form of the share of atoms of individual elements (atomic %).

Research has shown differences between the properties of humic acids, especially between soils heavily fertilised with manure and other soils. First of all, humic acids of soil fertilised with manure were characterised by a lower share of carbon as well as a higher share of hydrogen and nitrogen. As a reflection of the elemental composition in humic acids of this soil, a higher value of the H/C ratio and a lower degree of internal oxidation (parameter ω, taking into account the entire elemental composition of organic compounds) were obtained. Positive values of the degree of internal oxidation of molecules suggest that humification of organic matter in the studied soils occurs under conditions of good oxygenation.

The value of the H/C ratio is inversely proportional to the aromaticity of organic compounds that constitute humic acids [Gonet 1989, Dębska et al. 2009, Amir et al. 2010]. This may suggest that humic acids extracted from soil fertilised with manure contain a higher proportion of aliphatic groups in relation to the other tested soils. The greater share of nitrogen in the humic acids of this soil is probably the result of the ‘availability’ of simple forms of nitrogen originating from manure mineralisation at the condensation stage of the humification process. The possibility of incorporating nitrogen from the mineralisation of organic matter into the structure of humic substances is also suggested by other authors [Aoyama, Kumakura 2001, Becher 2013].

Suggestions regarding the chemical nature of humic acids resulting from the analysis of elemental composition were confirmed by spectrophotometric studies in the visible light spectrum. Higher values of spectrophotometric coefficients were found in humic acids of manure fertilised soil.

Numerous studies on the chemical nature of humic acids suggest that these substances in advanced stages of humification (‘mature’), characterised by a higher molecular weight and a greater degree of condensation of aromatic elements of the structure, show lower spectrometric coefficient values compared to acids in the initial stages of humification (‘young’) [Chen et al. 1977, Kumada 1987, Howard et al. 1998, Senesi et al. 2003, Dębska 2004]. The obtained results suggest that humic acids of soil fertilised with manure annually show a lower degree of humification, i.e. they are less ‘mature’, with a lower molecular weight and less condensation of the aromatic part of their structure. This may be the result of the humification process of organic compounds systematically inflowing after manure fertilisation. Scientific research on the dynamics of changes in humic acid parameters at various stages of organic matter decomposition confirms the truth of this suggestion [Gonet 1989, Aoyama, Kumakura 2001, Szombathova et al. 2004].

The research revealed a clear effect of manure fertilisation on the quantity and quality of organic matter accumulated in the topsoil level of Luvisols. In the soil fertilised with manure every year for 15 years, in relation to the soil where this fertilisation was applied every 4 years and soil without organic fertilisation, a much larger amount of organic matter was found. The organic matter of this soil was characterised by a higher proportion of labile forms of organic matter (post decalcification fraction and fulvic acid fraction) and a lower proportion of bitumen fraction as well as a lower carbon ratio of the humic acid fraction to the fulvic acid fraction. It was found that in soil intensively fertilised with manure, more organic matter occurs in the initial stages of humification.

Humic acids extracted from the soil intensively fertilised with manure, in relation to the remaining ones, showed features indicating a lower degree of their humification. Humic acids of this soil probably have a lower molecular mass and less condensation of the aromatic part of their structure.