Environ Monit Assess (2017) 189:176 DOI 10.1007/s10661-017-5881-y Soil organic matter as sole indicator of soil degradation S.E. Obalum & G.U. Chibuike & S. Peth & Y. Ouyang Received: 27 July 2016 /Accepted: 7 March 2017 # Springer International Publishing Switzerland 2017 Abstract Soil organic matter (SOM) is known to play properties as well. Thus, functions of SOM almost al- vital roles in the maintenance and improvement of many ways affect various soil properties and processes and soil properties and processes. These roles, which largely engage in multiple reactions. In view of its role in soil influence soil functions, are a pool of specific contribu- aggregation and erosion control, in availability of plant tions of different components of SOM. The soil func- nutrients and in ameliorating other forms of soil degra- tions, in turn, normally define the level of soil degrada- dation than erosion, SOM has proven to be an important tion, viewed as quantifiable temporal changes in a soil indicator of soil degradation. It has been suggested, that impairs its quality. This paper aims at providing a however, that rather than the absolute amount, temporal generalized assessment of the current state of knowl- change and potential amount of SOM be considered in edge on the usefulness of SOM in monitoring soil its use as indicator of soil degradation, and that SOM degradation, based on its influence on the physical, may not be an all-purpose indicator. Whilst SOM re- chemical and biological properties and processes of mains a candidate without substitute as long as a one- soils. Emphasis is placed particularly on the effect of parameter indicator of soil degradation is needed, SOM on soil structure and availability of plant nutrients. narrowing down to the use of its labile and microbial Although these properties are discussed separately, the components could be more appropriate, since early de- soil system is of dynamic and interactive nature, and tection is important in the control and management of changes in one property will likely affect other soil soil degradation. Keywords Aggregate stability. Land degradation . S. Obalum (*) : G. Chibuike Department of Soil Science, Faculty of Agriculture, University of Labile component . Organic carbon . Soil quality index Nigeria, Nsukka 410001, Nigeria e-mail: [email protected] S. Obalum Introduction e-mail: [email protected] One of the greatest problems facing agricultural soils of S. Peth Department of Soil Science, Faculty of Ecological Agriculture, the world is land/soil degradation. It can be triggered by University of Kassel, Nordbahnhofstr. 1a, 37213 Witzenhausen, many physical, chemical, biological and ecological pro- Germany cesses that lower the quality and potential productivity of the soil (Tefera et al. 2002;Anikwe2006;Lal2015). Y. Ou ya ng USDA Forest Service, Center for Bottomland Hardwoods Soil degradation thus poses a threat to ecological bal- Research, 775 Stone Blvd., Thompson Hall, Room 309, ance and environment safety. In sub-Saharan Africa, it is Mississippi State, MS 39762, USA a major cause of reduced agronomic productivity 176 Page 2 of 19 Environ Monit Assess (2017) 189:176 (Obalum et al. 2012a). About 85% of land degradation Mediterranean regions is caused by their low SOM worldwide is due to soil erosion and related problems levels, which result from intensive mechanized cultiva- (Mbagwu 2003a); hence, the two terms are often used tion in the regions. He argues that organic substances interchangeably. The importance of soil erosion is could be useful in assessing structural stability of the underlined by the fact that it is often not easily detected. soils, because these substances influence stability more Such difficulty in detecting soil erosion seems to wreck than any other stabilizing agent does. Generally, SOM more havoc than soil erosion itself, as the detection may level and stratification ratio are often cited as major not be timely enough to allow meaningful reclamation indicators of soil quality (Franzluebbers 2002; Krull programmes at a justifiable cost. Considering that soil et al. 2004;Blumetal.2014). erosion is the most widespread form of soil degradation Changes in SOM level could be manifested in a (Mbagwu 2003a;Biancalanietal.2012), the agro- number of ways, all of which relate to soil degradation. environmental cost of soil degradation speaks for itself. For instance, SOM loss reduces soil fertility and capac- Soil degradation, no doubt, is counterproductive. The ity to produce crops (Acton and Gregorich 1995;Plank selection of an appropriate indicator of soil quality under 2001), since SOM is a rich source of mineralizable different conditions could help in tackling the problem nitrogen (N), phosphorus (P), sulphur (S) and other of soil degradation. This is, however, a difficult task nutrients (Baldock and Broos 2011). Consequences of under field conditions where exceptionally large num- drastic changes in SOM status could also be well de- bers of soil properties interact in the naturally complex fined by measuring changes in biological parameters soil to determine its quality. It becomes more difficult and structural stability (Gal et al. 2006). Weak soil when considering the many different functions of the structure and susceptibility to erosion, loss of plant soil each with its own specific requirements of soil nutrients and decline in soil biodiversity, to mention properties for optimum results. In essence, no single soil but a few, are all forms of soil degradation common in property can serve as indicator of soil quality (Acton and agricultural soils. As long as there remains no generally Gregorich 1995; USDA-NRCS 1996;Öztaş 2002). acceptable single soil property to monitor these phe- Hence, a set of soil properties is normally used as nomena, achieving sustainability especially in low- indicators of soil degradation (Doran and Parkin input agriculture would remain a mirage. With the rec- 1996). It is most appropriate to select soil properties that ognition of the important role of SOM in the mainte- vary with the particular function for which soil quality is nance of sustainable agricultural systems (Fernandes being evaluated (Anikwe 2006). et al. 1997), our success or failure in the use and man- Although a multifactorial concept, soil degradation agement of soils seems to revolve around our ability to still needs a single sensitive soil property to serve as its manage SOM. The failure-success paradigm here would indicator. Such an indicator can provide first-hand in- therefore depend on SOM concentration maintained in formation on the extent of major soil degradative pro- the soil at any point in time, which would give a clue as cesses that represent others. Soil organic matter (SOM) to the level of degradation of the soil. This paper is, seems promising in this regard due to its all-embracing therefore, aimed at exploring the potentialities of SOM influence on the physical, chemical and biological prop- for this onerous task of serving as an all-purpose indi- erties of soils (Krull et al. 2004), which makes it very cator of soil degradation, with emphasis on soil erosion sensitive to management, among other attributes. Ac- and plant nutrient depletion. cording to Plank (2001), there have been several at- tempts to use traditional test for SOM or some of its fractions as an index of soil productivity or sustainabil- An overview of soil degradation ity. Many authors (Mbagwu et al. 1991; Piccolo et al. 1996; Piccolo and Mbagwu 1997; Spaccini et al. 2002; Land degradation or soil degradation has been defined Obalum et al. 2011) have recognized that depletion of in many, at times discipline-oriented, ways; and this SOM, following decomposition and mineralization causes confusion, misunderstanding and misinterpreta- processes hastened by intensive agricultural activities, tion. The distinction between land and soil is clear in is the main cause of high rate of soil degradation. that land encompasses soil, suggesting that land degra- Mbagwu (2003b) indicates that the consistently weak dation is a broader concept than soil degradation. How- soil structure in many parts of the arid and semi-arid ever, because any effect on the soil usually spreads to Environ Monit Assess (2017) 189:176 Page 3 of 19 176 other land components (landscape, vegetation, water necessarily resist change; rather, they are in a stable and climate) in agricultural areas, soil degradation and steady-state condition with the new environment. This land degradation are often used interchangeably. Soil is never the case with the fragile ones which, under degradation is Bthe temporary or permanent lowering of stress, degrade to a new steady state. The altered state the productive capacity of soil caused by overgrazing, is unfavourable to plant growth just as the soil is less deforestation, inappropriate agricultural practices, over capable of performing environmental regulatory func- exploitation of fuel wood leading to desertification and tions. Resistant or stable soils can be regarded as not other man-induced activities^ (Mbagwu 2003a:15). Al- degraded, since they perform their normal functions; though soil degradation advances rapidly under anthro- they only need to be managed well to avoid falling pogenic activity but slowly under natural processes below this acceptable state. According to Mbagwu (Acton and Gregorich 1995), both situations reduce land (2003a), a soil is degraded if its productivity falls below quality which results in the loss of actual or potential the economic threshold even under favourable weather productivity of the land (Eswaran et al. 2001). conditions or with judicious input. He also opines that Several mechanisms initiate soil degradation. Such soil degradation is not always an absolute concept as mechanisms or processes of soil degradation can be long as degraded fragile soils respond to management grouped into three: physical, chemical and biological practises to allow for any land use or, even if they fail to (Mbagwu 2003a), although a fourth category, ecological respond to management, can be put to some other uses.