Compositional Changes in Composts During Composting and Mushroom Growth: Comparison of Conventional and Environmentally Controlled Composts from Commercial Farms

Samples from conventional and environmentally controlled (EC) composts taken at various stages of composting and mushroom (Agaricus bisporus) growth were analyzed for changes in 80 percent ethanol and water extracts, monosaccharides in acid hydrolysates of polysaccharides, lignin concentrations and lignin structural features. The relative lignin content of all composts as measured by the acetyl bromide procedure increased, both during composting and mushroom growth. On the assumption that the absolute amount of lignin remains unaltered during composting and mushroom growth, the relative changes to the polysaccharide concentrations were calculated. Thus, during composting, 70, 53 and 58 percent of the initial wall polysaccharides for conventional, “cold” and “hot” EC, respectively, were consumed by compost microorganisms. During spawn running and fruiting, about 15 percent of wall polysaccharides were utilized from all types of composts. Thus, considerable amounts (17–31 percent) of polysaccharide remained at the end of mushroom production. During composting, there were changes in the degree of condensation and in the extent of oxidation of the lignins in all cases, but the rate and extent of these changes was dependent on the different composting regimes. During mushroom growth, further changes occurred, again with different patterns for the different compost types.     

Assessment of Fulvic Acid-Like Fractions during Tannery Waste Composting

Changes in the structural composition of fulvic acids were followed during composting of tannery solid waste after neutralization of its acidity by ammonium or by lime. Different techniques (elemental analysis, Fourier transform infrared (FTIR), ¹³C-NMR spectroscopy) were applied. During both trials of composting, a decrease of carbon and increase of oxygen occurred, which originated from strong microbial oxidation and preservation of some structures during composting. In fact, Nuclear Magnetic Resonance (NMR) spectra showed the preservation of anomeric or tannin structures around 105 ppm. The infrared (IR) spectral data showed a decrease in the intensity of COO^? group bands (1623 and 1399.5 cm^?1); this is correlated with a decrease of fulvic acid levels. The variations observed in NMR spectra could be explained by the involvement of COO^? groups of certain structures in polycondensing to form humic acids. The fulvic acids remain composed of less polycondensed structures with a –COOH extremity, as supported by a decrease of the C/H ratio. In contrast, in the trial with addition of ammonia a slight increase in N content occurred, a decrease in absorbance in the 1730–1000 cm^?1 regions compared to the absorbance around 3400 cm^?1, and a strong decrease of carboxyl carbon in the ¹³C-NMR spectra.     

Qualitative modifications of humic acid-like and core-humic acid-like during high-rate composting of pig faeces amended with wheat straw

Humic acid-like (HA-like) and core-humic acid-like (core-HA-like) were characterized during the high-rate composting process by CP-MAS ¹³C NMR, pyrolysis-gas chromatography (GC)/mass spectrometry (MS), and elemental analysis. Results obtained indicated that humification proceeded through a relative concentration of aromatic fractions due to the faster degradation of the O-alkyl and alkyl fractions. Core-HA-like, after purification of the parent material, showed a large reduction of the O-alkyl fraction in terms of HA-like. We concluded that HA-like consisted of refractory organic molecules, such as lignin and biopolymers, which formed a stable structure (core-HA-like) coated with degradable material associated with the core by weak physical association, ether or ester bounds.     

Nuclear Magnetic Resonance,Infra-Red and Pyrolysis: Application of Spectroscopic Methodologies to Maturity Determination of Composts

Compost maturity or stability reflects the degree of decomposition of the organic matter (OM). Since stability of natural OM is a relative term, defining it is not a trivial challenge. In addition, it requires a series of chemical, physico-chemical and spectroscopic determinations. Among the methods applied, ¹³C-NMR and FTIR (or DRIFT) and pyrolysis have been shown to be of significance and therefore this review will be dedicated to studies focusing on the application of these methods to composting research. In fact, solid-state ¹³C-NMR spectroscopy has become the most important tool for examining the chemical structure of natural OM (NOM) and the chemical changes associated with OM decomposition. Changes can be measured on the bulk OM either fresh or composted, on humic substances (HS) extracted from the compost or on dissolved organic matter (DOM). Recently, 2D ¹H NMR has been employed to study properties of HS extracted from MSW compost. In general, changes measured on decomposing OM are more distinct in the following order of tested materials: DOM > Bulk OM > HS > Core HS. In conclusion, compost HS which are “young” relative to soil HS were shown to differ from the latter mostly in their high levels of aliphatic and polysaccharide components, which tend to decompose during composting. ¹³C-NMR is the most effective instrument applied to date to structural studies of NOM.     

Ryegrass straw component decomposition during mesophilic and thermophilic incubations

The decomposition of perennial ryegrass straw was examined under mesophilic and thermophilic temperatures. Thermophilic conditions were used to define the composting process. The change in lipids, sugars, soluble polysaccharides, cellulose, and lignin was determined during a 45-day incubation. C, H, O, and N steadily decreased in both temperature treatments. The lignin content, as measured by the Klason or 72% H₂SO₄ method, decreased by 10% under mesophilic and 29% under thermophilic conditions. The Klason lignin C loss was 25 and 39% under mesophilic and thermophilic incubations, respectively. The changes in element (C, N, H, and O) ratios indicated that 94% of the lignin fraction was altered during both low- and high-temperature incubations. The changes in the lignin-like fraction as shown by elemental ratios were more extensive than those indicated by the Klason method, showing that this lignin determination has limited value in describing plant residue decomposition. The decomposition of the straw components and the concomitant degradation of the lignin fraction represent an important decomposition process that facilitates the composting of ryegrass straw with a high C:N ratio.     

Degradation of lignin in wheat straw during growth of the oyster mushroom (Pleurotus ostreatus) using off-line thermochemolysis with tetramethylammonium hydroxide and solid-state (13)C NMR.

The oyster mushroom (Pleurotus ostreatus) is widely cultivated on wheat straw (Triticum aestivum); however, there is a need to better understand the relationship between the chemical composition of the compost and mushroom growth. Wheat straw was degraded over a period of 63 days by P. ostreatus during which time it was sampled at weekly intervals. Off-line thermochemolysis with tetramethylammonium hydroxide and solid-state (13)C NMR were then used in the molecular characterization of the undegraded wheat straw and the degraded samples. The degraded wheat straw samples had a lower proportion of syringyl- to guaiacyl-derived moieties and cinnamyl- to guaiacyl-derived moieties than the undegraded control. There were increases in both guaiacyl and syringyl acid to aldehyde ratios with composting time, which showed that side-chain oxidation has been mediated by P. ostreatus. The (13)C NMR spectra confirmed the increase in carboxyl content but indicated that the overall lignin and methoxyl contents remained relatively constant, although some nonsystematic variations were observed. The spectra also showed a decrease in amorphous noncellulosic polysaccharides in relation to the crystalline cellulose upon degradation.     

Decomposition of lignin in wheat straw in a sand-dune grassland

The degradation of organic macromolecules, including lignin, in plant-derived soil organic matter, is important to the global carbon cycle. In grasslands, saprotrophic (decomposer) fungi are major decomposers of such organic material. The aim of this study was to characterise lignin degradation, particularly with respect to lignin oxidation typical of white-rot basidiomycete fungi. Lignin breakdown products, analysed by gas chromatography–mass spectrometry (GC–MS) with TMAH thermochemolysis, in initial wheat (Triticum aestivum var. Swatham) straw samples were compared with those in samples which had been buried as a “model” resource for 46 months in a sand-dune grassland at Ainsdale National Nature Reserve, Lancashire, UK.Our results showed that lignin oxidation occurred in the straw over the 46 month period, as there were general increases in the [Ac/Al]_S and [Ac/Al]_G ratios and a clear decrease in the [S/G] ratio. These data provide tentative support for the theory that white-rot basidiomycete fungi are involved in the degradation of lignin in grasslands.     

Chemical composition of the organic matter in forest soils: The humus layer

Decomposition and humification were studied within three types of forest humus (mull, moder, and mor) by means of CPMAS ¹³C NMR spectroscopy combined with degradative methods. The NMR data show that O-alkyl carbon decreases in all soils, and alkyl as well as carboxyl carbon increase as depth and decomposition increase; the percentage of aromatic carbon remains constant at about 25%. With increasing depth the amount of carbon that can be identified as belonging to specific compound classes by wet chemical methods decreases from 60% to 40%. Microbial polysaccharides and the proportion of non polysaccharide O-alkyl carbon increase with depth. A selective preservation of recalcitrant, condensed lignin structural units is also observed. In order to relate the spectroscopic and chemical data from investigations of whole soils with studies of humification, samples were fractionated into fulvic acid, humic acid, and humin fractions. The fulvic acid fraction contains large concentrations of carbohydrates irrespective of the soil horizon. The humic acid fraction contains less polysaccharides, but high amounts of alkyl carbon and aromatic structures. The percentage of aromatic carbon existing in the humic acid fraction increases with depth, probably reflecting the amount and degree of oxidative decomposition of lignin. A loss of methoxyl and phenolic groups is evident in the ¹³C NMR spectra of the humic acid fraction. The humin fraction resembles relatively unchanged plant-derived materials as evident from the lignin parameters and carbohydrate contents. All the observed data seem to indicate that humic acids originate form oxidative degradation of humin or plant litter.     

Structural Changes of Humic Acids During Olive Mill Pomace Composting

The aim of this work was to characterize the humic acids during composting using both chemical and spectroscopic analytical methods. Humic acids (HA) were extracted from olive mill pomace (OMP) after 5, 10, 21, 32, 48, 60, 84, 95, 109 and 153 days of composting and their elemental composition, functional groups, molecular weight, ¹³C CP-MAS NMR and FTIR spectra were determined. Elemental composition of HA showed a decrease of carbon content from 56.8% to 47.1% and an increase of oxygen content from 32.7 % to 43.3%. A slight increase of carboxylic groups was observed with no variation of phenolic groups. The molecular weight of HA determined by gel chromatography showed that after 95 days of composting the molecular weight of about 40% of the polymers was greater than 10² KDalton. The ¹³C CPMAS NMR and FTIR spectra of HA showed compositional and structural changes indicating a slight increase of polysaccharides and aromatic groups and a decrease of long aliphatic chains during the composting.     

Comparison of humic acids derived from city refuse with more developed humic acids

Abstract

The humic acids (HA) from composted and uncomposted city refuse (CR) were characterized by degradative (oxidation with persulphate and permanganate) and non-degradative techniques (FT-IR and ¹³C-NMR) in order to analyze the effect of the composting process on these HA. They were also compared with commercial HA extracted from leonardite. The carboxyl and carbonyl group content of the HA from CR increased slightly during composting. Since the HA from the composted CR showed a lower N and H content, the FT-IR spectra showed a lower intensity in the bands corresponding to peptides and carbohydrates. Differences were revealed when the HA from both CR were compared with those from leonardite which showed a much lower N and H content and a less aliphatic character. The percentage of degraded products by persulphate was higher for the HA from uncomposted CR. For the HA from both CR the major components among the oxidation products were dicarboxylic acids and normal fatty acids. In the leonardite HA, the major components consisted of benzene polycarboxylic acids. ¹³C-NMR revealed an attenuation of the aliphatic character of the HA from CR with composting.     

城市污泥与调理剂混合堆肥过程中有机质组分的变化

【目的】研究城市污泥堆肥过程中各项有机质组分及碳、 氮在堆肥过程中的形成与转化,以期改善堆肥的生物有效性,促进其土地利用。【方法】在工厂规模化下,以城市污泥、 蘑菇渣锯末以及返混料按照6∶3∶1的质量比混合形成堆肥物料,辅以强制通风措施和翻抛,进行为期18 d的高温堆肥试验。堆肥期间定期采样,测定指标包括温度、 C/N值、 pH、 含水率、 有机质降解率、 水溶性组分、 半纤维素、 纤维素和木质素,研究堆肥期间不同阶段堆肥物料中有机质组分的动态变化。【结果】堆体温度随着发酵时间的延长呈现先升高后降低的趋势,最高温度达到71.3℃; 含水率由60.7%降低到51.4%,pH呈现先升高后降低的趋势,总体处于6.0~7.5之间; 总有机碳含量持续下降,氮素含量表现为高温期持续下降随后呈上升的趋势; 初始阶段,堆肥物料中四种成分含量分布为: 水溶性组分纤维素半纤维素木质素,至堆肥结束变化为: 纤维素水溶性组分木质素半纤维素,经过堆肥之后水溶性组分及半纤维素含量分别由39.5%和20.1%下降为27.9%和14.4%,纤维素含量由初始的21.8%上升至29.5%,木质素含量相对稳定不变。物料经过堆肥化处理后达到腐熟标准,水溶性组分和半纤维素含量分别降低了38.6%和38.8%,纤维素和木质素含量在高温期分别降解了11.7%和18.5%; 物料总量降低了9.8%。水溶性组分的主要降解阶段为高温期,期间降解部分占总降解量的65.5%; 半纤维素的主要降解阶段为稳定期,稳定期降解部分占总降解量的69.1%,且有继续降解的趋势; 纤维素和木质素仅在高温期有少量降解; 氮素则表现为高温期铵态氮的损失及稳定期硝态氮的积累。【结论】堆肥化处理在实现污泥减量化基础上,污泥中有机质得到了稳定化,有利于城市污泥的土地利用。     

Molecular changes in particulate organic matter (POM) in a typical Chinese paddy soil under different long‐term fertilizer treatments

A combination of solid‐state CPMAS‐¹³C‐NMR and TMAH thermochemolysis‐GC/MS was applied to investigate the molecular composition of particulate organic matter (POM) separated from a Chinese paddy soil, from the Tai Lake region, under a long‐term field experiment with different fertilizer treatments. The treatments were: (i) no fertilizer application (NF), (ii) chemical fertilizers only (CF), (iii) chemical fertilizer plus pig manure (CFM) and (iv) chemical fertilizer plus crop straw (CFS). CPMAS‐¹³C‐NMR spectra showed that POM from all treated plots was rich in O‐alkyl‐C compounds, followed by alkyl‐C and aromatic‐C compounds. However, as compared with a control (NF), POM under CFM and CFS treatments exhibited a smaller relative O‐alkyl‐C content and a larger contribution of aromatic‐C and alkyl‐C, thus increasing both aromaticity and hydrophobicity and, hence, recalcitrance of POM samples. Thermochemolysis of POM from all treatments demonstrated a dominance of aliphatic and lignin‐derived compounds. However, the distribution of lignin monomers (p‐hydroxyphenyl, P, guaiacyl, G, and syringyl, S) revealed significant differences among the treatments. The relative distribution of lignin P, G and S monomers in NF, CF and CFS indicated a preferential contribution of annual crops and maize straw, as compared with that found for CFM. Concomitantly, a larger content of aliphatic thermochemolysis derivatives was found for CFS and CFM. The relative increase of aliphatic molecules in CFS was attributed to hydrophobic polyesters from higher plants. In the CF and CFM systems, the presence of aliphatic components of microbial origin suggested a greater microbial activity in comparison with NF and CFS. The combined application of solid state CPMAS‐¹³C‐NMR and TMAH thermochemolysis‐GC/MS can be used to assess effectively the accumulation of recalcitrant organic compounds in soil POM under long‐term fertilizer application with organic biomass. It is thus inferred that soil organic matter stabilization by molecular recalcitrance contributes to carbon sequestration in Chinese paddy soils under long‐term managements.     

餐厨垃圾与菌糠混合好氧堆肥效果

分别使用玉米秸秆与菌糠作为餐厨垃圾的堆肥调理剂,进行堆肥1次发酵对比试验,旨在考察菌糠作为餐厨垃圾堆肥调理剂的可行性。通过对2种调理剂与餐厨垃圾堆肥过程中理化性质及腐熟度变化的分析,表明餐厨垃圾好氧堆肥时,菌糠是一种优于玉米秸秆的良好调理剂,餐厨垃圾与菌糠混合堆肥时升温速度快、高温期持续时间长,含水率与有机质分别下降19.6%和20.2%。同时,其混合堆料在堆肥过程中散发臭气较少,1次堆肥处理后发芽指数较高（55.6%）,基本实现腐熟。     

Spectroscopic (IR,NMR) characterization of water-soluble organic substances extracted from straw,straw incubated with Pleurotus ostreatus,and straw compost

Water extracts from fresh wheat and barley straw, straw incubated with Pleurotus ostreatus, and straw compost were studied by IR, ¹H NMR, and ¹³C NMR spectroscopy. During incubation lignin was degraded, water extractability increased, and water extracts were rich in polysaccharides. After composting solubility decreased and the water extracts were rich in aromatic, methoxyl, and carboxyl C, but poor in O-alkyl C indicating that during composting mainly polysaccharides had been mineralized. GPC revealed that water extracts from straw compost contained polysaccharides, peptides, C- and O-substituted aromatics, and alkyl compounds.     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.     

Structure-function relationship of vermicompost humic fractions for use in agriculture

Purpose

The use of humic substances (HS) in agriculture is beneficial and has positive environmental impacts. However, to optimize the use of HS possible links between their structural characteristics and bioactivity must be shown. The goal of this study is to evaluate the bioactivity of different humic fractions extracted from vermicompost (VC) in rice plants and to shed light to possible structure-function relationships.

Materials and methods

Humic-like fractions were obtained from cattle manure vermicompost processed by African nightcrawlers (Eudrilus eugeniae spp.). Humic-like acid fraction using only water as extractor (HLAw), HLA fraction extracted following the International Humic Substances Society (IHSS) recommended method, and the solid residue (humified residual (HR)) after extraction of HLA were characterized using complementary chemical, physic, and spectroscopic technics (elemental composition, UV-Vis and Fourier transform infrared spectroscopy (FTIR) spectroscopies, ¹³C-CP MAS NMR, and MEV). Biological activity of the three HS was conducted in growth chambers and measured in roots using WinRhizo Arabidopsis software. Principal component analysis (PCA) was used to find a grouping pattern between the structural variables evaluated and the obtained root parameters.

Results and discussion

Differences were found in elemental composition among HS with larger C/N ratio in HR than in HLA and HLAw. HLA and HLAw FTIR spectra showed carboxyl band at 1714.66 cm^?1 better resolved than in HR. Bands at 1642 cm^?1 (amide I) and 1510 cm^?1 (lignin), were better resolved in HLA. ¹³C-NMR showed the following order of aromaticity: HLA > HLAw > HR. For HLAw bioactivity, the structures C_Alkyl-H,R, C_C=O, and C_COO-H,R correlated with the number and growth of smaller root. The aromatic C_Ar-H,R, C_Ar-O,N, and aliphatic C_Alkyl-O,N, C_Alkyl-O, and C_Alkyl-di-O structures in HLA, correlated with larger roots growth. HR also stimulated root growth and development in rice plants.

Conclusions

Aliphatic and oxygenated structures in HLAw showed a relation with induction of initial root emissions, whereas the presence of aromatic compounds in HLA was related with root growth stimulation activity. Higher concentration of HLAw was necessary to produce an equivalent stimulus compared with HLA; it could indicate that, although both fractions showed similar types of structures in their composition, differences in the predominant structures may be determining different effects on the root.

     

Lignin degradation during a laboratory incubation followed by 13C isotope analysis

Recent in situ ¹³C studies suggest that lignin is not stabilised in soil in its polymerised form. However, the fate of its transformation products remains unknown. The objective of the present research was to provide the first comprehensive picture of the fate of lignin-derived C across its transformations processes: (1) C remaining as undecomposed lignin molecules, (2) C in newly formed humic substances, i.e. no longer identifiable as lignin-polymer C, (3) C in microbial biomass, (4) C mineralised as CO₂, and (5) dissolved organic C. To achieve this objective, we designed an incubation experiment with ¹³C-labelled lignin where both elementary and molecular techniques were applied. Lignin was isolated from ¹³C labelled maize plants (¹³C-MMEL) and incubated in an agricultural soil for 44 weeks. Carbon mineralisation and stable isotope composition of the released CO₂ were monitored throughout the incubation. Microbial utilisation of ¹³C-MMEL was measured seven times during the experiment. The turnover rate of the lignin polymer was assessed by ¹³C analysis of CuO oxidation products of soil lignin molecules. After 44 incubation weeks, 6.0% of initial ¹³C-MMEL carbon was mineralised, 0.8% was contained in the microbial biomass, and 0.1% was contained in dissolved organic C form. The compound-specific ¹³C data suggest that the remaining 93% were overwhelmingly in the form of untransformed lignin polymer. However, limited transformation into other humic substances potentially occurred, but could not be quantified because the yield of the CuO oxidation method proved somewhat variable with incubation time. The initial bacterial growth yield efficiency for MMEL was 31% and rapidly decreased to plateau of 8%. A two-pool first-order kinetics model suggested that the vast majority (97%) of MMEL lignin had a turnover time of about 25 years, which is similar to field-estimated turnover times for soil-extractable lignin but much longer than estimated turnover times for fresh plant-residue lignin. We conclude that natural lignin structures isolated from plants are rather unreactive in soil, either due to the lack of easily available organic matter for co-metabolism or due to enhanced adsorption properties. The data also suggest that fairly undecomposed lignin structures are the main reservoir of lignin-derived C in soils.     

Incorporation studies of NH+4 during incubation of organic residues by l5N-CPMAS-NMR-spectroscopy

This study focuses on the processes occurring during incorporation of inorganic nitrogen into humic substances. Therefore rye grass, wheat straw, beech saw dust, sulphonated lignin and organosolve lignin were incubated together with highly ¹⁵N-enriched ammonium sulphate in the laboratory for 600 days. Samples from the incubates were periodically analysed for weight loss, and carbon and nitrogen contents. The samples were also analysed by solid-state ¹³C- and ¹⁵N-CPMAS-NMR-spectroscopy to follow the turnover of the materials during incubation. Most of the detectable N-signals was assigned to amide - peptide structures. The remaining intensities could be ascribed to free and alkylated amino groups, and those on the low field side of the broad amide-peptide signal to indole, pyrrole and nucleotide derivatives. Abiotic reactions of ammonia with suitable precursors and the formation of pyridine, pyrazine or phenyloxazone derivatives were not observed. Signals from ammonia and nitrate occurred only at the end of the incubation.     

Characterization of cultivation effects on soil organic matter

Humus properties in various Ap horizons from field plots, that have been cultivated in long-term experiments under different management conditions, were investigated by pyrolysis-field ionization mass spectrometry (Py-FIMS) and ¹³C-NMR spectroscopy. The results of Py-FIMS were evaluated by correlation and principal component analysis from reproducible data sets of bulk soil samples and extracted humic substances, and allowed a distinct discrimination on the basis of humus quality and composition. The chemical subunits suitable for discrimination are the major plant constituents carbohydrates, lignin, and proteinaceous materials as well as their humification products. The contribution of these compound classes to soil organic matter decreased with the intensity of management. CPMAS and solution ¹³C NMR spectra of soils and humic substances demonstrated that with more intense management, both the intensities of the phenolic region (140–160 ppm) and the aromatic region (110–140 ppm) decreased. The combination of both independent methods MS and NMR, together with microbiological and biochemical data, yields the general result that intensive soil management leads to a less active humus.     

Savanna-derived organic matter remaining in arable soils of the South African Highveld long-term mixed cropping: Evidence from C and N natural abundance

Sustainable agriculture requires the formation of new humus from the crops. We utilized ¹³C and ¹⁵N signatures of soil organic matter to assess how rapidly wheat/maize cropping contributed to the humus formation in coarse-textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) cropped for lengths of time varying from 0 to 98 years, after conversion from native grassland savanna (C4). We performed natural ¹³C and ¹⁵N abundance measurements on bulk and particle-size fractions. The bulk soil δ¹³C values steadily decreased from −14.6 in (C4 dominated) grassland to −16.5‰ after 90 years of arable cropping. This δ¹³C shift was attributable to increasing replacement of savanna-derived C by wheat crop (C3) C which dominated over maize (C4) inputs. After calculating the annual C input from the crop yields and the output from literature data, by using a stepwise C replacement model, we were able to correct the soil δ¹³C data for the irregular maize inputs for a period of about one century. Within 90 years of cropping 41-89% of the remaining soil organic matter was crop-derived in the three studied agroecosystems. The surface soil C stocks after 90 years of the wheat/maize crop rotation could accurately be described with the Rothamsted Carbon Model, but modelled C inputs to the soil were very low. The coarse sand fraction reflected temporal fluctuations in ¹³C of the last C₃ or C₄ cropping and the silt fraction evidenced selective erosion loss of old savanna-derived C. Bulk soil ¹⁵N did not change with increasing cropping length. Decreasing δ¹⁵N values caused by fertilizer N inputs with prolonged arable cropping were only detected for the coarse sand fraction. This indicated that the present N fertilization was not retained in stable soil C pool. Clearly, conventional cropping practices on the South African highlands neither contribute to the preservation of old savanna C and N, nor the effective humus reformation by the crops.