In the literature many studies have demonstrated that organic substrates added to degraded soil improve quality, resulting first at all in increase of organic matter content and stabilize soil structure. Garcia et al.
[35] identify some main functions of organic amendments in soils: (1) promotion of soil aggregation; (2) provision of plant nutrients; and (3) a reduction in water content loss, in addition to other beneficial functions. The many other authors find several samples described in literature which confirm the positive effect on soil physical, chemical and microbiological properties. The study of Soria et al.
[36] evaluates the effects of technosols made with different organic amendments (waste of gardening, greenhouse horticultural, stabilized sewage sludge and two mixtures of sludge with both vegetable composts) to restore degraded soils in a semiarid limestone quarry. Amended technosols after 6 and 18 months increased water retention capacity, electrical conductivity, total organic carbon and nitrogen, as compared to not amended and natural soils. In turn Arif et al.
[37] carried out 5-year consecutive application of fresh industrial sludge (FIS) and composted industrial sludge (CIS) to restore soil functions at surface (0–15 cm) and subsurface (15–30 cm) of the degraded agricultural land. The authors found that sludge amendments increased such soil parameters like total organic carbon (TOC), soil available nitrogen (SAN), soil available phosphorus (SAP) and soil available potassium (SAK) at 0–15 cm depth. Taking into consideration of microbial activities they noted significant increase of value of dehydrogenase (DHA), β-glucosidase (BGA) and alkaline phosphatase (ALp) after FIS and CIS applications. However, other enzymes, such as urease activity (UA) and acid phosphatase (ACp), were significantly reduced compared to control soil. Moreover, sludge amendments significantly increased microbial biomass nitrogen (MBN) and microbial biomass phosphorus (MBP). Significant changes were noted in the increase population of soil culturable microflora (bacteria, fungi and actinomycetes) after sludge application into soil
[37][38]. Composts produced from biodegradable waste not only help to improve soil fertility and plant yield, but also are able to control of soil erosion, biocontrol of diseases and bioremediation
[39]. The optimal rates (not greater than 50 t ha
−1) of different organic amendments can improve physical (soil structure, permeability, water holding capacity, etc.) and chemical (pH, cation exchangeable capacity, etc.) soil properties, favoring plant growth and microbial activity, without any risks for the environment (subsoil and groundwater contamination)
[40]. Other studies have indicated the effectiveness of organic matter addition on increase surface roughness resulting in a large decline in soil erosion rates
[41]. The use organic matter on salt-degraded soils caused following benefits: (i) aggregate formation, (ii) pores: soil aeration and plant root prolongation, (iii) water leaching
[42]. Several advantages of using of biochar on degraded land identified IPCC special report
[43]: (i) improved nutrient use efficiency due to reduced leaching of nitrate and ammonium and increased availability of phosphorus in soils with high phosphorus fixation capacity, (ii) management of heavy metals and organic pollutants: through reduced bioavailability of toxic elements, (iii) stimulation of beneficial soil organisms, (iv) improved porosity and water-holding capacity, (v) amelioration of soil acidification.
2.1. Soil Organic Matter (SOM)
Healthy soils are able to store significant quantities of carbon (C) in the form of soil organic carbon (SOC) or soil organic matter (SOM). Navarro-Pedreño et al.
[44] noted that around 45% of the mineral soils in Europe have low or very low organic carbon content (0–2%) and 45% have a medium content (2–6%). SOC is included as a metrics for the regular assessment of land degradation in reporting for SDG target 15.3
[45]. The source of carbon in the soil is above and below ground plant biomass, animal residues, organic products of edaphone and biomass introduced in the form of fertilizers (manure, slurry, compost or green fertilizers). As a result of the mineralisation of organic compounds in the soil, under aerobic conditions, the available nutrients for plants are created, but at the same time the production of CO
2 increases. However, in the process of humification, i.e., chemical, biological and biochemical transformations of various degrees of advancement, humus and other non-humus substances (fats, carbohydrates and lignins) are formed. One should strive for humification processes (accumulation of organic matter) to prevail over mineralization processes. Soil organic matter (SOM), and in it organic carbon, determines the physical, chemical and biological properties. It is one of the main components of forming soil fertility and influences the formation and durability of soil aggregates. Its content determines soil sorption capacity, water retention, biodiversity and soil density. Organic fertilisation is necessary to maintain and improve soil fertility, although affects yields more slowly and to a lesser extent
[46]. Ngo et al.
[47] compared several additives (mineral fertilizers, buffalo manure, compost, vermicompost and biochar) to check effect on degraded soils. They found the synergistic effects between plants and different organic amendments on carbon storage and soil organic matter composition. The biowaste compost (BWC) amended soils were assessed during 180 days under arid ambient conditions and in comparison with control soil
[48]. It was shown a significant increase in SOM and SOC in dependence on used BWC quantities to 120 days, and then decrease in SOM and SOC levels.
2.2. Carbon Sequestration and Climate Mitigate
Increasing the organic matter content in soils can be fundamental in reducing CO
2 concentration in the atmosphere. Two processes can be defined: carbon biosequestration in plants and carbon sequestration in soil. Photosynthesis reduces the amount of carbon dioxide in the atmosphere: thanks to chlorophyll, plants absorb CO
2 from the atmosphere and as a result of biochemical transformations convert it into organic compounds necessary for their life processes. Increasing the yields of selected plants with appropriate agrotechnology helps to reduce CO
2 emissions into the atmosphere (biosequestration). An effective reduction of the carbon dioxide content in the atmosphere can be achieved by sequestering CO
2 in SOM
[49]. An increase in soil organic matter content in Europe is estimated to have the potential to absorb about 0.8% of the current CO
2 emissions from the burning of fossil fuels in the world, improving compliance with the international Kyoto Protocol
[50].
Figure 4 shows the good agricultural practices described in the literature that influence the SOM content of soils.
Figure 4. Methods of increasing carbon deposit in the soil
[51][52].
The initiative to increase the global soil organic matter resource by 0.4% (four per mille) per year as compensation for global greenhouse gases (GHGs) emissions from anthropogenic sources was initiated during COP 21. Most carbon sequestration research only considers 30 cm of topsoil because it is considered that farming techniques have the greatest impact on this layer. It is estimated that agricultural land accumulates around 600 Gt C in a 1 m thick soil layer. Increasing SOC inventories by 4 per mille (around 2.5 Gt C per year) could offset about 30% of global greenhouse gas emissions
[53]. It has been found that by using best land management practices, a C absorption rate of up to 10 per mille can be achieved in the first 20 years for soils with low initial SOC resources
[49]. Earlier investigations on impact of biosolids on soil organic carbon buildup in calcareous strip-mined land in soil was done in Illinois
[54]. Biosolids were applicated at a cumulative loading rate from 455 to 1654 Mg ha
−1 (dry wt.) for 8 to 23 yr in rotation from 1972 to 2004. Over a 34 years reclamation period, mean net soil carbon accumulation rate was 1.73 (0.54 to 3.05) Mg carbon ha
−1 yr
−1 in biosolids amended fields compared with 0.07 to 0.17 Mg carbon ha
−1 yr
−1 in fertilizer controls. Soil carbon accumulation rate was significantly correlated with biosolids application rate, expressed as (Equation (4)):
where:
-
y is the annual net soil carbon sequestration (Mg C ha −1 yr −1),
-
x is annual biosolids application (Mg ha −1 yr −1, dry wt.).
Placek et al.
[55] proposed several factors for calculating of carbon sequestration in degraded soil of zinc smelter and post-mining areas: (i) carbon management index (CMI), (ii) carbon lability (L), (iii) soil organic carbon (SOC) pool; (iv) carbon stock (C stock); (v) carbon sequestration rate (C sequestration); (vi) soil organic carbon build up rate (SOC build up rate). The authors used municipal compost, lacustrine chalk and coal slurry, the improvement of soil fertility and soil quality (increase value of total Kjeldahl nitrogen TN, total carbon TC, total organic carbon TOC). They found that CMI and SOC sequestration rate were the best methods to determine carbon sequestration in the soil during conducted pot experiment.
2.4. Plant Ecosystem Restoration
Brownfield sites are usually devoid of vegetation. There are many publications in the literature confirming the fact that organic fertilization usually creates favorable conditions for plant growth and vitality
[59]. Due to the fact that organic amendments provide both nutrients and water, the plants have better conditions to adopt to live in difficult degraded soil ecosystems. Usually many benefits are noted, such as an increase of total biomass weight, root and stem length and diameter, the number of leaves and foliar area. Some authors describe specific reactions of plants for organic amendment to degraded/contaminated soils. Khan et al.
[60] used hard wood biochar (HWB), bagasse (BG), rice husk (RH) and maize comb waste (MCW) to chromite mine degraded soil containing Cd. The results indicated that the biochar added to soil, significantly increased chlorophyll contents (20–40%) and biomass (40–63%) of tomato and cucumber. Moreover, HWB was the most effective at reducing Cd bioavailability and significantly decrease Cd levels in vegetables. Good sample of such effect was induced phytoremediation carried out on degraded terrain around zinc mill (Miasteczko Slaskie, Silesia Region, Poland). The soil is characterized by extremally high concentration of heavy metals (mainly Zn, Pb, Cd) and totally degraded. Addition of sewage sludge in the dose 30 t ha
−1 boosted the survival of trees such as Scots pine, birch, beech and oak (
Figure 5). Similar results were obtained with use of municipal green waste (MGW) on degraded former opencast coal land on the margins of UNESCO’s Blaenavon Industrial Landscape World Heritage site in southeast Wales
[61]. The application of MGW into soil significantly supported the growth of Silver Birch (Betula pendula, Roth) and European Larch (
Larix decidua), but no Common Alder (
Alnus glutinosa (L.) Gaertn.)
[61]. There are many advantages of use the so called “aided phytostabilizaion technologies”. The technologies are widely proposed as a suitable strategy ecosystem plant revegetation. As a results of organic additives, metals bioavailability can be reduced. At the same time tolerant plant species find the suitable conditions to growth and further improves the soil characteristics boosted by the increase soil organic matter and biological activity
[62].
. Mycorrhized Scots pine growing in a plot in the field growing on control soil (a) and on soil enriched with sewage sludge (b), (photo M. Kacprzak).
Application of organic fertilizers to degraded soil caused that the plants receive better conditions to develop and produce better defense responses, then are in general less susceptible to infection by phytopathogens such
Pythium,
Phytophthora and
Fusarium spp.
[63].