The Origin of Soil Organic Matter

The origin of soil organic matter (SOM) varies considerably depending of the particular ecosystem. For instance, in the natural forest or grassland systems, SOM is mostly in situ produced from biomass sources (Fig. 22.1). The largest contribution to maintenance and/or rising (accumulation) of SOM comes from above-ground plant biomass production such as leaf and woody litter in addition to belowground root necromass and exuded organic substances (Kalbitz et al. 2000) . For instance, leaf contribution in overall litterfall biomass production in forest/woodland

Fig. 22.1 Possible sources of SOM for commonly cultivated mineral soil (adapted from Ondrasek 2008 and references therein)

ecosystems is 70-75%, in wooded grasslands 50-60% and in sclerophyllous Mediterranean forests 54% (Matthews 1997).

Contrary, in today's modern and intensive agro-ecosystems, with possible several crop rotations over the short period, such as associated rain-fed cereal and irrigated horticultural production, the main part of above-ground biomass and litterfall (e.g. yield, straw) is exported from the system Ondrasek et al. (in press). To prevent depletion and retain its multifunctional role, SOM must be replenished, i.e. introduced into the arable top-soil layer through (1) specific land use and management practices and/or (2) other natural and/or anthropogenic exogenous resources (Fig. 22.1).

Anthropogenic exogenous sources of SOM are different sorts of organic wastes and/or co-products from urban (e.g. biosolids from sewage sludge) and peri-urban or rural activities such as agriculture (e.g. animal manures/slurries) and industry (e.g. biosolids from industrial wastewaters) (Fig. 22.1). In the last several decades there have been increasing environmental regulations that have resulted in more animal wastes treatment options, and thus affecting characteristics of residues that are subsequently applied to land. According to recent estimations (Van-Camp et al.

2004) around 1.65 billion tonnes of exogenous SOM are produced in the European Union (EU) each year, from which 61% represent animal wastes, 25% crop residues, 7% industrial wastes and 7% urban and municipal wastes (e.g. sewage sludge, biowastes and green wastes). The predominant application of exogenous SOM on arable land areas in EU is (on a weight basis) in the form of animal manure and slurries (97%), whereas almost a negligible portion comes from industrial wastes (2%) and sewage sludge (1%) application.

Some of mentioned organic wastes (precursors of organic soil amendments) before application must be stabilised, i.e. degraded and decomposed to a certain degree through composting or similar processes, to final product such as biosolids. Data about biosolids production and management practices across Europe and the USA (e.g. Epstein 2003; Ondrasek 2008 and references therein) show that in EU countries, from the total biosolids production (cca 7.4 million dry tones per year) 42% is disposed of in landfills, 36% is used in agriculture and 11% is incinerated, whereas in the USA, from total production (cca 4.1 million dry tones per year) 60% of bio-solids is beneficially used (e.g. land application), 17% is disposed and 22% is incinerated.

NBluMHy-occurrina 'loodfid cwlarWa Ç Initial JtnU»* son Q Atter -'CJ ji;

Fig. 22.2 Illustration of creation of Jendek soil and its decomposition over time in Neretva R. swamp, Croatia according to data from references cited in the text

NBluMHy-occurrina 'loodfid cwlarWa Ç Initial JtnU»* son Q Atter -'CJ ji;

Fig. 22.2 Illustration of creation of Jendek soil and its decomposition over time in Neretva R. swamp, Croatia according to data from references cited in the text

A great potential of natural exogenous resources of SOM in food (agriculture) and wood (forestry) production over the millennia have been exploited from highly rich organic Histosols of peatlands and heathlands. Both systems were used as indigenous (after drainage/drying) or exogenous (as soil amendments) SOM sources for different purposes in crop production (nutrients source) and animal (bedding material, feed supplements) food, as well for wood (forestation) production. Heathlands are unique ecosystems found over the world in temperate upland regions, characterised by low-growing vegetation and organically enriched soils, which develop because environmental factors such as waterlogging and acidity constrain biomass decomposition (Holden et al. 2007). Peatlands are distributed just on around 3% of the global land surface (Strak 2008), but comprise 20-30% of the global, and ~50% of the UK's soil carbon stock (Yallop and Clutterbuck 2009) . At the European scale, the current area of peatlands is estimated at 340,000 km2 (Byrne et al. 2004), of which almost 50% has already been artificially drained for forestry (90,000 km2), agriculture (65,000 km2) and peat extraction (2,300 km2).

One of the oldest examples of exogenous SOM application (peat, charcoal, animal residues, etc.) to naturally occurring nutrient poorly Oxisol was found in more than 10,000-year-old man-made Terra Preta Anthrosols of Amazonia (Woods et al. 2006). Similar examples of improvement of naturally low-fertile Arenosols with different organic amendments (peat, manures, forest litter) can be recognised in European Plaggic and Terric

Anthrosols that were up to several thousand years old (e.g. Blume and Leinweber 2004). Unusual, if not unique, process of creating organic man-made soils (Jendek) was started in swamps of Neretva River valley (Croatia) more than 100 years ago (Fig. 22.2) . Over decades that initially organic soil (>30% OM) was totally changed (up to 1-m depth) in most physical, chemical and biological properties, and became mineral soil (<2% OM) due to intensive land management practices and favourable natural conditions (i.e. Mediterranean climate) for decomposition and mineralization of indigenous OM (Fig. 22.2).

Land use and management practices may also be a powerful tool for SOM conservation and/or even improvement, when an application of exogenous organic materials is restricted or non-achievable (e.g. small livestock production) Ondrasek et al. )n press). Many case studies on long-term changes of SOM in natural and agricultural systems worldwide confirmed that native (vs. arable) systems have greater SOM, i.e. carbon (C) storage, and that conversion from native to arable cropping significantly influence soil C (Soussana et al. 2004). Conversion from grassland and forest to cropland usually results in a decrease in SOM content, and the opposite conversion increases SOM content (Ondrasek 2008 and references therein). Higher SOM content in grassland and forest vs. cropland may be due to many factors: greater return of plant residues, return of dung during grazing, absence of soil disturbance and therefore restricted aeration causing decreased mineralisation.

Increased concerns for healthy food production and environment protection, i.e. increased emphasis on sustaining the productive capacity of natural resources, have raised interest in the maintenance and increasing of SOM for various land uses and management practices. For example, conservation tillage systems in which >30% of the crop residues remains on the soil surface after planting (IPCC 2000) , mainly with the aim to restrict soil erosion, result in reduced soil compaction, disturbance and energy consumption, i.e. they conserve plant-available water and SOM. Land management practices from (1) reduced (minimum) tillage, where ploughing replaces surface tillage and/ or strip tillage, to (2) complete absence (zero) tillage are increasingly used worldwide on >70 millions ha (e.g. Cerri et al. 2004). There are also other agricultural practices (crop selection, green manure, (fert)irrigation, etc.) which may improve/ conserve SOM in arable topsoils.

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