Plant litter and the microbial biomass are the major parent materials for soil organic matter (SOM) formation Plant litter is composed of complex mixtures of organic components, mainly polysaccharides and lignin, but also aliphatic biopolymers and tannins The composition and relative abundance of these components vary widely among plant species and tissue type Whereas some components, such as lignin, are exclusively found in plant residues, specific products are formed by microorganisms, eg amino sugars A wide variety of chemical methods is available for characterizing the chemical composition of these materials, especially the chemolytic methods, which determine individual degradation products and solid-state 13C NMR spectroscopy, that gives an overview of the total organic chemical composition of the litter material With the development of these techniques, an increasing number of studies are being carried out to investigate the changes during decay and the formation of humic substances An overview is given on the amount of litter input, the proportion of various plant parts and their distribution (below-ground/above-ground), as well as the relative proportion of the different plant tissues Major emphasis is on the organic chemical composition of the parent material for SOM formation and thus this paper provides information that will help to identify the changes occurring during biodegradation of plant litter in soils

The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter

A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia1. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known2. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients—of soil-age and of climate—in volcanic soil environments. During the first ∼150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between non-crystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments.

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Mineral control of soil organic carbon storage and turnover

The effects of lime, fertilizer and manure applications on soil organic matter status and soil physical properties are of importance to agricultural sustainability. Their effects are complex and many interactions can occur. In the short-term, liming can result in dispersion of clay colloids and formation of surface crusts. As pH is increased the surface negative charge on clay colloids increases and repulsive forces between particles dominate. However, at higher lime rates, Ca2+ concentrations and ionic strength in soil solution increase causing compression of the electrical double layer and renewed flocculation. When present in sufficient quantities, both lime and hydroxy-Al polymers formed by precipitation of exchangeable Al, can act as cementing agents bonding soil particles together and improving soil structure. Liming often causes a temporary flush of soil microbial activity but the effect of this on soil aggregation is unclear. It is suggested that, in the long-term, liming will increase crop yields, organic matter returns, soil organic matter content and thus soil aggregation. There is a need to study these relationships on existing long-term liming trials. Fertilizers are applied to soils in order to maintain or improve crop yields. In the long-term, increased crop yields and organic matter returns with regular fertilizer applications result in a higher soil organic matter content and biological activity being attained than where no fertilizers are applied. As a result, long-term fertilizer applications have been reported, in a number of cases, to cause increases in water stable aggregation, porosity, infiltration capacity and hydraulic conductivity and decreases in bulk density. Fertilizer additions can also have physico-chemical effects which influence soil aggregation. Phosphatic fertilizers and phosphoric acid can favour aggregation by the formation of Al or Ca phosphate binding agents whilst where fertilizer NH4
+ accumulates in the soil at high concentrations, dispersion of clay colloids can be favoured. Additions of organic manures result in increased soil organic matter content. Many reports have shown that this results in increased water holding capacity, porosity, infiltration capacity, hydraulic conductivity and water stable aggregation and decreased bulk density and surface crusting. Problems associated with large applications of manure include dispersion caused by accumulated K+, Na+ and NH4
+ in the soil and production of water-repellant substances by decomposer fungi.

Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: A review

Publisher Summary This chapter reviews the cycling of nutrients within pasture soils. The major emphasis is on the central role of the grazing animal in influencing soil fertility, particularly in the dung and urine patches. In addition, the progress is reviewed that has been made in modeling the cycling of nutrients in pasture systems. Nutrients are partitioned differently between dung and urine, with K being excreted mainly in urine; P, Ca, and Mg are excreted principally in dung and N and S are excreted in both forms. Nutrients returned in excreta can be in inorganic and organic forms, depending on the particular nutrient being considered. For some nutrients (e.g., P and S) significant mineralization of ingested organic forms occurs during passage through the animal, whereas much of the N is excreted in the readily available organic urea form. From the present knowledge of the major nutrient inputs and losses for the grazed pasture system and an understanding of the pathways of nutrient flux within the system and some key measurements, simple mass balance nutrient models for various pasture systems can be constructed. Such simple models have been used to calculate site-specific maintenance fertilizer requirements of pastures based on the amount of nutrient required to replace losses in the soil (e.g., through fixation and leaching) and losses by animal transfer and in animal products.

Nutrient cycling and soil fertility in the grazed pasture ecosystem

Organic Matter Characterization

Mineral Nitrogen in the Plant-Soil System

SUMMARY

Soil samples from long-term plots annually fertilized with superphosphate since 1952 and also unfertilized (control) plots of an irrigated and intensively grazed pasture in Canterbury, New Zealand, were subjected to a sequential extraction procedure followed by 31P nuclear magnetic resonance (NMR) analysis of the soil phosphorus (P) forms present.



Overall, 80% of the total organic P (as determined by ignition) was removed from the soil by the sequential extraction procedure. Most of the organic P (90%) detected by NMR was in the monoester fraction. Small quantities of diester and pyrophosphate were also found. Choline phosphate constituted a significant proportion (18–25%) of the monoester P in the acetylacetone and 0.5 M NaOH extracts. Long-term superphosphate additions resulted in almost all of the P accumulated in the monoester P fraction (up to 99%).

Nature and distribution of soil phosphorus as revealed by a sequential extraction method followed by 31P nuclear magnetic resonance analysis

SUMMARY

In this study a detailed sequential fractionation scheme was used to examine the effects of long-term annual applications of phosphatic fertilizer on the amounts and forms of phosphorus (P) in soils under intensively grazed irrigated pasture over a 25 year period (1952–1977). Results showed that increases in soil inorganic and organic P occurred in response to continued P fertilizer addition, although inorganic P decreased with time following the cessation of P fertilizer inputs in the residual fertilizer and control treatments. The rate of organic P accumulation in soils decreased with time, while a marked decline in soil organic P in all treatments between 1971 and 1974 was attributed to increased mineralization as a result of lime addition in 1972. Furthermore, changes in the forms of inorganic P which accumulated in the continually fertilized soils after 1971 were attributed to the combined effects of liming and changes in the chemical nature of applied fertilizer P.

Effects of long‐term phosphatic fertilizer applications on amounts and forms of phosphorus in soils under irrigated pasture in New Zealand

Ammonia (NH3) volatilisation from various nitrogenous (N) fertilisers broadcast at different times of year on to pasture at a rate of 3 g/m2 N (30 kg/ha N) was measured by an enclosure method. The average percent losses of NH3-N were: urea, 11.9; diammonium phosphate, 5.3; ammonium sulphate, 1.0; and calcium ammonium nitrate, 0.8. Loss from sulphur-impregnated urea was equal to, or slightly less than that from urea. The loss was positively related to the maximum soil surface pH produced by each of the fertilisers. No seasonal pattern was observed, except that NH3 loss tended to be lower in August–September. The loss from urea increased from 13 to 33% of N applied as the application rate increased from 3 to 20 g/m2 N (30–200 kg/ha N).

Effects of form of nitrogen, season, and urea application rate on ammonia volatilisation from pastures

Summary

Cross polarization carbon-13 nuclear magnetic resonance spectroscopy with magic angle spinning (CP-MASS) was used to analyse for various forms of carbon in soils, a moss peat, hydrochloric acid insoluble residues from soils and peat, and litter from beech and pine trees. The chemical composition of the litters was also investigated by conventional techniques.



The results show that hydrolysis with hydrochloric acid extraction removes nearly all oxygenated alkyl carbon from the soils used. It is shown that humification pathways in which carbohydrates are incorporated into humic substances via nonhydrolysable linkages are not important for the soils investigated in this work.



CP-MASS data suggest that the percentage of aromatic carbon in pine leaves increases with increase in ageing time. The results for the beech leaves are not sharply defined.

Studies of litter and acid insoluble soil organic matter fractions using 13C‐cross polarization nuclear magnetic resonance spectroscopy with magic angle spinning

K. M. Goh

Papers

Mineral Nitrogen in the Plant-Soil System

Nature and distribution of soil phosphorus as revealed by a sequential extraction method followed by 31P nuclear magnetic resonance analysis

Effects of long‐term phosphatic fertilizer applications on amounts and forms of phosphorus in soils under irrigated pasture in New Zealand

Effects of form of nitrogen, season, and urea application rate on ammonia volatilisation from pastures

Studies of litter and acid insoluble soil organic matter fractions using 13C‐cross polarization nuclear magnetic resonance spectroscopy with magic angle spinning