Carbon (C) sequestration in soils through the increase of the soil organic carbon (SOC) pool has generated broad interest to mitigate the effects of climate change. Biosolids soil application may represent a persistent increase in the SOC pool. While a vast literature is available on the value of biosolids as a soil conditioner or nutrient source in agricultural systems, there is still limited knowledge on soil sequestration mechanisms of biosolids-borne C or the main factors influencing this capacity. The emerging challenges posed by global environmental changes and the stringent needs to enhance C storage call for more research on the potential of soil biosolids incorporation as a sustainable C storage practice. This review addresses the potential of C sequestration of agricultural soils and opencast mines amended with biosolids and its biological regulation.
Increasing concern about global climate change has led to growing interest in developing feasible methods to reduce the atmospheric levels of greenhouse gases (GHGs) [
Soil C pool is the largest terrestrial C pool, constituting approximately two-thirds of the total C in ecosystems [
Soil organic carbon (SOC) includes plant, animal, and microbial residues in all stages of decomposition. SOC represents a balance between inputs, mostly via primary productivity or organic amendments, and outputs via decomposition [
Soil C sequestration is defined as any persistent increase in SOC originated from removing CO2 from the atmosphere. Increases in soil carbon storage may be accomplished by the production of more biomass. In this way, there is a net transfer of atmospheric CO2 into the soil C pool through the humification of crop residues, resulting in net carbon sequestration [
Soil C sequestration capacity reflects soil aptitude to retain and stabilize C. In the light of global change scenario, soil humification mechanisms have acquired renewed interest. Traditionally, the stability of organic compounds has been regarded as the main process controlling SOC retention, but recent analytical and experimental advances have demonstrated that molecular structure alone does not explain SOM stability [
Biosolids are typically made up of 40–70% organic matter, with organic C content ranging from 20–50%, total nitrogen (N) ranging from 2–5%, and a C/N ratio of about 10–20 [
Storage capacity of biosolids-borne C of typical soils of the Pampas region, Argentina, 360 days after biosolids application (application rate of 150 dry t/ha). Different letters for the same soil indicate significant differences at the 0.05 probability level
Many mathematical models have been proposed to describe the decomposition process of land applied organic materials, ranging from one to multicompartment models [
According to this model, organic C added through biosolids consisted of two fractions of different degree of biodegradability: a labile fraction (53–71%) that was quickly mineralized at a constant rate (
In general, C mineralization rates of freshly added organic sources have been found to be more rapid in soils with low than with high clay content [
Despite the protection provided by clay particles, many studies reported that mineralization rates of biosolids-borne C were not related to soil texture [
In turn, the effects of microbes on composition and recalcitrance of SOC have been shown to be as important as climatic and edaphic characteristics [
X-ray fluorescence analysis of dried sludge indicated that oxides of Si, Al, and Fe were the three main inorganic constituents of biosolids [
Scanning electron microscopy with X-ray microanalysis (SEM-EDS) image of biosolids (a) and X-ray diffraction (XRD) patterns of biosolids (b).
Some authors indicated that “the capacity of terrestrial ecosystems to store carbon is finite and the current sequestration potential primarily reflects depletion due to past land use” [
In recent years, there has been growing evidence that biosolids may be used to restore mine spoils or tailings. Rehabilitation of rocky materials exposed by mining typically involves physical amelioration and organic matter incorporation. The use of biosolids in such operations represents an opportunity to couple biosolids application with soil C sequestration. In several cases, this practice has led to high soil C accumulation, edaphic improvement, and an effective vegetation cover establishment [
Mined spoils in tropical regions present factors that can accelerate C flow, such as high temperatures, acidity, and low charges surfaces, and factors that may retard it, like clay texture, overburden materials, and anaerobic conditions due to waterlogging. Carbon accumulation in revegetated mining spoils suggests that retarding factors have prevailed under stripped soil situation. Common inorganic components present in mining spoils include aluminosilicates (allophone and imogolite) and Fe-(oxy) hydroxides (like ferrihydrite). Both of them have been reported to stabilize soil C [
Although mining spoils are not a suitable environment for plant development, the incorporation of biosolids has promoted plant colonization and unprecedented organic-C accumulation was reported in temperate and tropical regions [
Soil organic C dynamic depends on microbial activity, community composition, and soil enzyme activity [
Most of the existing information on soil microbial responses due to biosolids incorporation is related to the use of sewage sludge [
In most soils, microbial activity and proliferation is typically C-limited whereas N limitation hardly occurs [
Typical respiration trends of soils amended with sewage sludge enriched or not with trace elements (Cd 12 mg kg−1, Zn 300 mg kg−1). A lag phase can be observed before the onset of the C flush for metal spiked sludge.
Before the onset of CO2 release, an increase in the lag time may be observed in sludge amended soils [
There is still considerable controversy about the effects of biosolids applications on the native SOC pool [
Soil management practices may influence soil C storage after biosolids amendment. It has been widely reported that intensity of mineralization processes is much higher in soil surface layers than in deepest soil horizons [
Soil enzyme activity is responsible for most of the soil functionality, as it promotes the decomposition of organic matter and release nutrients in forms available to plants and microorganisms. Soil enzymes are actively released by proliferating soil microorganisms, plant roots, and soil fauna or passively released by dead microorganisms and root cell sloughing. A large fraction of the enzymes released in the extra- or pericellular space are stabilized by stable soil organic and inorganic phases [
Schematic contribution of biosolids-borne C to SOC fractions and to soil nutrient fluxes, in addition to biogenic extra C inputs in mine spoils revegetation projects. The dotted lines indicate the still poorly understood effect of oxydases on particulate C mineralization or the stabilization of mineralized soil C by chemisorption.
In any case, the presence of soil stabilized “enzymatic background” makes soil’s decomposition capacity only partially related to the actual levels of microbial activity in biosolids amended soils. As it is currently impossible to estimate the ratio between intra- and extracellular enzyme activity, it is not possible to precisely determine the actual contribution of active microorganisms to biosolids degradation.
In the light of global change scenario, increasing soil organic matter stocks has been identified as a feasible way for soil C sequestration. Many experiments indicated that application of biosolids to land or opencast mines resulted in an increase in carbon reserves of soils from different regions and under different management practices. Biosolids are typically made up of 40–70% organic matter, consisting of two fractions of different degree of biodegradability: a labile fraction that is quickly mineralized by soil microorganisms and a recalcitrant fraction, not available or resistant to soil microorganisms, responsible for soil organic carbon accumulation. The amount and proportion of recalcitrant C in biosolids are important attributes to predict the biological control of C storage in soils. It seems to be a direct relationship between C recalcitrance and mineralization: the higher the C recalcitrance in biosolids amended soils, the higher the metabolic energy needed for biosolids mineralization by soil microorganims. Monitoring microbial communities and soil enzyme activity may be used as ecological indicators of biosolids C stabilization in soil.
Many studies reported that mineralization rates of biosolids-borne C may not depend on soil texture and that slightly acid soils retained more biosolids-borne carbon than soils with a higher pH. Furthermore, amorphous iron and aluminum oxides usually found in biosolids would play an important role in soil organic C accumulation. Therefore, the capacity of soils to sequester biosolids borne C may not be finite. It is important to remark that the benefits associated with the use of biosolids for soil carbon sequestration are in addition to other benefits, like the improvement of soil quality in terms of physical, chemical and biological fertility, although the presence of contaminants may impact soil microbial communities on the long term.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research leading to this review has received funding from UBACYT 20620110200024, GEF UBA, and The Brazilian National Council for Scientific and Technological Development (CNPq).