Total soil heavy‐metal concentrations and mobile fractions after 10 years of biowaste‐compost fertilization

The accumulation of heavy metals (HMs) in soils is the most often cited potential risk of compost application. As the ecological effects of metals are related to mobile fractions rather than to total concentrations in the soil, we measured the total (aqua regia–extractable) HM concentrations, the readily available water‐soluble and the potentially bioavailable LiCl‐extractable fraction of soil HMs in a field experiment after 10 y with total applications of 95, 175, and 255 t ha^–1 biowaste compost (fresh matter). Total soil concentrations of Cd, Cr, Cu, Ni, and Pb in the compost treatments were not significantly higher than in the unfertilized control. Total Zn concentrations increased in the treatment with the highest application rate, as expected from the calculation of the Zn load in the composts. In the mobile fractions, as measured in soil saturation extract and LiCl extract, Cd and Pb were not detectable. Concentrations of Cr, Ni, and Zn were in the range published for unpolluted soils in other studies and did not show any differences according to treatment. Easily exchangeable Cu (in LiCl extract) was increased with compost fertilization, most probably due to complexation with low‐molecular organic complexants. Except for Cd and Zn, the results of the mobile HM fractions in the soil were in good agreement with plant HM concentrations. In conclusion, fertilization with high‐quality biowaste compost at such rates and after 10 y of application gives no cause for concern with regard to both total HM concentrations and available HM fractions.     

绿化植物废弃物堆肥对城市绿地土壤的改良效果

针对城市中普遍存在的绿地土壤退化和有机废弃物处理处置难的现状,选择典型城市土壤进行绿化植物废弃物堆肥不同用量的绿地现场土壤改良实验.结果表明:绿化植物废弃物堆肥能提高土壤持水能力;降低土壤的酸碱度;增加土壤有机质、总N、总P、有效P、生物量C、N和微生物总量;堆肥对土壤全K的含量总体影响不大,但速效K的含量却成倍增加;说明绿化植物废弃物能改善土壤理化性质,提高土壤肥力,且随着堆肥用量的增加,对土壤的改良作用也越显著;鉴于绿化植物废弃物高C/N,其用量宜控制在13240 kg/hm~2以内.绿化植物废弃物堆肥就地利用即能改良城市退化土壤,又减少城市废弃物量,有利于提高城市生态环境质量.     

Effect of triple superphosphate and biowaste compost on mycorrhizal colonization and enzymatic P mobilization under maize in a long‐term field experiment

Phosphorus (P) fertilizers and mycorrhiza formation can both significantly improve the P supply of plants, but P fertilizers might inhibit mycorrhiza formation and change the microbial P cycling. To test the dimension and consequences of P fertilizer impacts under maize (Zea mays L.), three fertilizer treatments (1) triple superphosphate (TSP, 21–30 kg P ha^?1 annually), biowaste compost (ORG, 30 Mg ha^?1 wet weight every third year) and a combination of both (OMI) were compared to a non‐P‐fertilized control (C) in 2015 and 2016. The test site was a long‐term field experiment on a Stagnic Cambisol in Rostock (NE Germany). Soil microbial biomass P (P_mic) and soil enzyme activities involved in P mobilization (phosphatases and ß‐glucosidase), plant‐available P content (double lactate‐extract; P_DL), mycorrhizal colonization, shoot biomass, and shoot P concentrations were determined. P deficiency led to decreased P immobilization in microbial biomass, but the maize growth was not affected. TSP application alone promoted the P uptake by the microbial biomass but reduced the mycorrhizal colonization of maize compared to the control by more than one third. Biowaste compost increased soil enzyme activities in the P cycling, increased P_mic and slightly decreased the mycorrhizal colonization of maize. Addition of TSP to biowaste compost increased the content of P_DL in soil to the level of optimal plant supply. Single TSP supply decreased the ratio of P_DL:P_mic to 1:1 from about 4:1 in the control. Decreased plant‐benefits from mycorrhizal symbiosis were assumed from decreased mycorrhizal colonization of maize with TSP supply. The undesirable side effects of TSP supply on the microbial P cycling can be alleviated by the use of compost. Thus, it can be concluded that the plant‐availability of P from soil amendments is controlled by the amendment‐specific microbial P cycling and, likely, P transfer to plants.     

Long‐term application of biowaste compost versus mineral fertilization: Effects on the nutrient and heavy metal contents of soil and plants

In recent years the use of biowaste compost (BC) as a soil amendment is of increasing interest. The aim of the experiment was to investigate the influence of different fertilization systems: biowaste compost, annual average of 32 Mg ha^—1 BC (fresh matter) and mineral fertilizer (83:52:95 kg ha^—1 NPK fertilizer) on the nutrient and heavy metal contents of soil and plants. Soil samples (1997) and harvest products (1996—1998) from a field trial (initiated 1992) were analyzed for K, Mg, P, Cu, Mn, Mo, Zn, Cd, Ni, and Pb. The five‐year fertilization with composted biowaste did not influence the total contents of Cd, Mn, Mo, and Ni in soil. The total soil contents of Zn and Pb were significantly higher in soils of the BC treatment than in the unfertilized control. Both fertilized plots tended to have higher Cu and Zn contents in harvest products than the unfertilized control. The mineral fertilization inhibited the Mo uptake by plants. In 1998 the mineral fertilization led to higher, and the biowaste compost application to lower, Cd contents in potato tubers as compared to the control.     

Variation of intra‐aggregate organic carbon affects aggregate formation and stability during organic manure fertilization in a fluvo‐aquic soil

The aggregate formation and stability are controlled by the dynamics of soil organic matters (SOM), but how it is related to SOM chemical composition within different‐sized aggregates is largely unknown during manure fertilization. In this study, the variations of intra‐aggregate organic carbon (OC), including intra‐particulate organic matter (iPOM) and mineral‐associated organic matter, were quantitatively and qualitatively analysed, and then, their effects on aggregate formation and stability were assessed under four treatments: control (CK), mineral fertilizer (NPK), reduced manure (30%M) and manure fertilizers (M). Manure application (M) significantly increased macroaggregate proportion, mean weight diameter (MWD), and OC contents within different‐sized aggregates compared to CK, NPK, and 30%M. The OC accumulation of macroaggregate in M was attributed to OC content increase in silt plus clay subfraction rather than iPOM with more labile organic groups; oppositely, in microaggregate it was located in the relatively stable fine iPOM. The macroaggregate formation and stability were controlled by the fine iPOM within macroaggregates, whose abundant polysaccharide‐C and aliphatic‐C after manure fertilization advanced the microbial growth except for Gram‐positive bacteria, which further promoted macroaggregate formation and stability. The free silt plus clay fraction also affected macroaggregate formation and stability, and its polysaccharide‐C derived from microorganisms or decomposing SOM was positively associated with MWD and macroaggregate proportion. Because polysaccharide‐C can be easily associated with mineral particles, further improving micro‐ or macroaggregation. We conclude that continuous manure fertilization could increase labile SOM accumulation within aggregates and then facilitate microbial growth, which collectively are responsible for aggregate formation and stabilization.     

Crop nitrogen recovery and soil nitrogen dynamics in a 10‐year field experiment with biowaste compost

When fertilizing with compost, the fate of the nitrogen applied via compost (mineralization, plant uptake, leaching, soil accumulation) is relevant both from a plant‐production and an environmental point of view. In a 10‐year crop‐rotation field experiment with biowaste‐compost application rates of 9, 16, and 23 t ha^–1 y^–1 (f. m.), the N recovery by crops was 7%, 4%, and 3% of the total N applied via compost. Due to the high inherent fertility of the site, N recovery from mineral fertilizer was also low. In the minerally fertilized treatments, which received 25, 40, and 56 kg N ha^–1 y^–1 on average, N recovery from mineral fertilizer was 15%, 13%, and 11%, respectively. Although total N loads in the compost treatments were much higher than the N loads applied with mineral fertilizer (89–225 kg N_tot ha^–1 y^–1 vs. 25–56 kg N_tot ha^–1 y^–1; both on a 10‐year mean) and the N recovery was lower than in the treatments receiving mineral N fertilizer, soil NO ‐N contents measured three times a year (spring, post‐harvest, autumn) showed no higher increase through compost fertilization than through mineral fertilization at the rates applied in the experiment. Soil contents of N_org and C_org in the plowed layer (0–30 cm depth) increased significantly with compost fertilization, while with mineral fertilization, N_org contents were not significantly higher. Taking into account the decrease in soil N_org contents in the unfertilized control during the 10 years of the experiment, 16 t compost (f. m.) ha^–1 y^–1 just sufficed to keep the N_org content of the soil at the initial level.     

Degradation and restoration of sandy soils under different agricultural land uses in northeast Thailand: a review

Conversion of natural forest to agricultural land use has significantly lowered the soil organic matter (SOM) content in sandy soils of northeast Thailand. This paper reviews the findings of comparative studies on contents of SOM pools (labile, i.e. microbial biomass and particulate organic matter—POM and stable, i.e. humic substance) and related soil aggregate formation, in natural forest plots and cultivated fields (monocrops of cassava, sugarcane and rice) in sites representative of northeast Thailand from the viewpoints of terrain (i.e. undulating), soils (sandy) and land use and discusses the restoration of SOM and fertility (nitrogen) in these degraded soils. Monocultural agriculture brings about the degradation of all SOM pools and associated soil aggregation as compared to the forest system because of decreased organic inputs and more frequent soil disturbance. The build‐up of SOM was achieved through the continuous recycling of organic residues produced within the system. Low‐quality residues contributed the largest SOM build‐up in whole and fractionated SOM pools, including POM and humic substance. However, to restore N fertility, high quality residues, (i.e. with low C/N ratios, lignin and polyphenols) were also needed. Timing of N release to meet crop demand was achieved by employing a mixture of high and low quality residues. Selection of appropriate residues for N sources was affected by environmental factors, notably soil moisture regimes, which differed in upland field and lowland paddy subsystems. Copyright © 2007 John Wiley & Sons, Ltd.     

Comparing NaOH‐extractable organic matter of acid forest soils that differ in their pedogenic trends: a pyrolysis‐GC/MS study

Soil organic matter (SOM) in Alu‐andic Andosols and Alu‐humic Umbrisols is believed to accumulate because of the protection caused by binding to aluminium (Al). We investigated soils that differed in the abundance of organo‐Al complexes to determine the effect of such binding on SOM chemistry. For this, the surface horizons of three types of acid soils in the Basque Country (northern Spain) under forest stands were studied: (i) Alu‐andic Andosols (AND soils) on basalts and trachytes, (ii) Umbrisols or so‐called ‘aluminic’(ALU) soils also on basalts and trachytes and (iii) soils with a podzolizing trend (POD), on quartzites. Values of Al extractable with sodium pyrophosphate (Al_p) in the surface horizons of these soils ranged between 8.5 and 13.1, 1.9 and 9.3, and 0.8 and 3.7 g kg^?1 dry weight, for the AND, ALU and POD soils respectively. For POD and ALU soils, surface horizons were sampled at two depths, 0–5 and 5–20 cm, whereas the AND soils were sampled at different depths down to the B horizon. NaOH‐extractable SOM from three AND soils, 12 ALU soils and 12 POD soils was studied by pyrolysis‐gas chromatography/mass spectrometry. The POD soils had the largest loads of plant‐derived markers (lignin, long‐chain alkanes and alkenes, methyl ketones, fatty acids); SOM of the AND soils had the smallest amounts of plant‐derived SOM and the largest amounts of microbial products (microbial sugars and N‐compounds) of the soils studied. ALU soils had an intermediate pattern, as expected. The results indicate that the SOM of Alu‐andic Andosols, developed from basalt and trachyte rocks, is essentially dissimilar to that of soils derived from quartz‐rich parent material, under the same climate conditions and similar forest stands. The dominance of secondary (microbial‐derived) SOM in Alu‐andic Andosols, also observed in previous research on Sil‐andic Andosols (these are dominated by short‐range ordered Si compounds in contrast to the dominance of organo‐Al complexes in Alu‐andic Andosols), reveals the small contribution of primary (plant‐derived) material to SOM in soils with andic properties.     

Short‐term effect of biochar and compost on soil fertility and water status of a Dystric Cambisol in NE Germany under field conditions

Crop growth in sandy soils is usually limited by plant‐available nutrients and water contents. This study was conducted to determine whether these limiting factors could be improved through applications of compost and biochar. For this purpose, a maize (Zea mays L.) field trial was established at 1 ha area of a Dystric Cambisol in Brandenburg, NE Germany. Five treatments (control, compost, and three biochar‐compost mixtures with constant compost amount (32.5 Mg ha^–1) and increasing biochar amount, ranging from 5–20 Mg ha^–1) were compared. Analyses comprised total organic C (TOC), total N (TN), plant‐available nutrients, and volumetric soil water content for 4 months under field conditions during the growing season 2009. In addition, soil water‐retention characteristics were analyzed on undisturbed soil columns in the laboratory. Total organic‐C content could be increased by a factor of 2.5 from 0.8 to 2% (p < 0.01) at the highest biochar‐compost level compared with control while TN content only slightly increased. Plant‐available Ca, K, P, and Na contents increased by a factor of 2.2, 2.5, 1.2, and 2.8, respectively. With compost addition, the soil pH value significantly increased by up to 0.6 (p < 0.05) and plant‐available soil water retention increased by a factor of 2. Our results clearly demonstrated a synergistic positive effect of compost and biochar mixtures on soil organic‐matter content, nutrients levels, and water‐storage capacity of a sandy soil under field conditions.     

Effects of compost on the chemical composition of SOM in density and aggregate fractions from rice–wheat cropping systems as shown by solid‐state 13C‐NMR spectroscopy

The 4‐year application of pig‐manure compost (PMC) to crop fields in Jiangsu significantly increased organic‐C and total N concentrations compared to chemical fertilization and control treatment. To identify the soil processes that led to these changes, ¹³C cross‐polarization magic‐angle spinning nuclear‐magnetic resonance (¹³C CPMAS NMR) and dipolar‐dephasing nuclear‐magnetic‐resonance spectroscopy (DD NMR) were conducted on soil organic matter (SOM) fractions separated by wet‐sieving and density fractionation procedures. This allowed characterization of the SOM quality under three contrasting fertilizer regimes. The results indicate that PMC application can alter the distribution of functional groups and improve alkyl C‐to‐O‐alkyl C ratios compared to chemical‐fertilizer treatment (CF). Alkyl C contents were increased from macroaggregate fractions (> 2 mm) to microaggregate fractions (0.05–0.25 mm) for all treatments, suggesting that recalcitrant material accumulates in the microaggregate fractions. The O‐alkyl C contents were decreased from macroaggregate fractions (> 2 mm) to microaggregate fractions (0.05–0.25 mm) under CF and PMC treatments, while no consistent trend was found for the control (NF) treatment. The alkyl C‐to‐O‐alkyl C ratios in macroaggregates were lower than those in microaggregates, indicating that the degrees of SOM decomposition were lower in macroaggregates compared to microaggregates. In all aggregate‐size classes, the amount of organic matter appeared to depend on the fertilization regime. This study provides useful information regarding the buildup of organic material in soil from long‐term manure‐compost enrichment.     

Changes in soil aggregate‐associated enzyme activities and nutrients under long‐term chemical fertilizer applications in a phosphorus‐limited paddy soil

Paddy soils in subtropical China are usually deficient in phosphorus (P) and require regular application of chemical fertilizers. This study evaluated the effects of chemical fertilizers on the distribution of soil organic carbon (SOC), total nitrogen (N) and available P, and on the activity of the associated enzymes in bulk soil and aggregates. Surface soils (0–20 cm) were collected from a 24‐yr‐old field experiment with five treatments: unfertilized control (CK), N only (N), N and potassium (NK), N and P (NP), and N, P and K (NPK). Undisturbed bulk soils were separated into >2, 1–2, 0.25–1, 0.053–0.25 and <0.053 mm aggregate classes using wet sieving. Results showed that both NP‐ and NPK‐treated soils significantly increased mean weight diameter of aggregates, SOC, available P in bulk soil and aggregates, as compared to CK. Most SOC and total N adhered to macro‐aggregates (>0.25 mm), which accounted for 64–81% of SOC and 54–82% of total N in bulk soil. The activities of invertase and acid phosphatase in the 1–2 mm fraction were the highest under NPK treatment. The highest activity of urease was observed in the <0.053 mm fraction under NP treatment. Soil organic carbon and available P were major contributors to variation of enzyme activities at the aggregate scale. In conclusion, application of NP or NPK fertilizers promoted the formation of soil aggregates, nutrient contents and activities of associated enzymes in P‐limited paddy soils, and thus enhanced soil quality.     

Nitrogen mineralization from mature bio‐waste compost in vineyard soils. III Simulation of soil mineral‐nitrogen dynamics

A model for nitrogen (N) dynamics in compost‐amended vineyard soils was tested for its predictive power. A soil–mineral N data set from a 3‐year field study on four different vineyard sites was used for model evaluation. The soils were treated with mature bio‐waste compost (30 and 50 Mg ha^–1 fresh matter, respectively). The model calculated soil mineral‐N contents at all sites with an overall mean bias error of –2.2 kg N ha^–1 for layers of 0.1 m thickness and an overall mean absolute error of 7.4 kg N ha^–1 layer^–1. Modeling efficiencies for the simulations of the respective treatments ranged from –0.05 to 0.41, and Willmott's Index of Agreement showed values of between 0.41 and 0.81. Acceptable model predictions as defined by the observed variability of mineral‐N contents in the respective soils ranged from 40% to 72%. A strong increase in soil mineral‐N concentration following the compost application at all sites could not be reproduced with the model, thereby reducing the prediction accuracy significantly. The model performance confirms that previously derived N‐mineralization parameters are suitable to describe the N release from soil‐applied mature bio‐waste compost under the environmental conditions of vineyards in Germany.     

Relationship between soil neutral sugar composition and the amount of labile soil organic matter in Andisol treated with bark compost or leaf litter

 The effect of short-term bark compost (Ba) and leaf litter (Li) applications on the labile soil organic matter (SOM) status was investigated. The SOM status studied in this paper includes soil microbial biomass, soil available N, hot water extractable C (HwC) and N (HwN) and soil neutral sugar-C composition. The soil microbial biomass C (MBC) and N (MBN), soil available N, HwC and HwN increased upon application of Ba and Li. No quantitative relationship was observed between application of organic material and MBC, MBN or soil available N. A positive linear correlation was observed between MBN and HwC but not between MBN and soil available N. Among the various soil neutral sugar C, xylose C (Xyl) content in Ba plots showed a remarkable increase but mannose C (Man) did not differ among Fer (fertilizer), Ba or Li plots. Soil neutral sugar C had a positive linear correlation with soil available N, MBN and HwC. The proportion of MBN : TN is positively correlated with the Xyl/Man ratio. The increase in the proportion of MBN in SOM seems to occur with the increase of SOM derived from plant debris. Received: 20 October 1997     

Repeated freeze–thaw cycles changed organic matter quality in a temperate forest soil

Under temperate climate, the frequency of extreme weather events such as intensive freezing or frequent thawing periods during winter might increase in the future. It was shown that frost and subsequent thawing may affect the fluxes of C and N in soils. In a laboratory study, we investigated the effect of frost intensity and repeated freeze–thaw cycles on the quality and quantity of soil organic matter (SOM) in a Haplic Podzol from a Norway spruce forest. Undisturbed soil columns comprising O layer and top mineral soil were treated as followed: control (+5°C), frost at –3°C, –8°C, and –13°C. After a 2‐week freezing period, frozen soils were thawed at +5°C and irrigated with 80 mm water at a rate of 4 mm d^–1. Lignin contents were not significantly affected by repeated freeze–thaw cycles. Phospholipid fatty acid (PLFA) contents decreased in the mineral soil, and PLFA patterns indicate that fungi are more susceptible to soil frost than bacteria. Amounts of both plant and microbial sugars generally decreased with increasing frost intensity. These changes cannot be explained by increased mineralization of sugars or by leaching with DOM nor by a decreased microbial activity and, thus, sugar production with increasing frost intensity. Also physical stabilization of sugars due to frost‐induced changes in soil structure can be ruled out as sugar extraction was carried out on ground bulk soil. Therefore, the only possible explanation for the disappearance of plant and microbial sugars upon soil freezing are chemical alterations of sugar molecules leading to SOM stabilization.     

NaOH‐extractable organic matter of andic soils from Galicia (NW Spain) under different land use regimes: a pyrolysis GC/MS study

The objective of this study was to determine to what extent the attenuation or loss of andic soil properties caused by land use change – from forest (FOR, average C content 118.2 ± 23.7 g kg^?1) to agricultural land (AGR, average C content 55.7 ± 16.7 g kg^?1) use – is reflected in soil organic matter (SOM) at the molecular level. For this, NaOH‐extractable SOM of A horizons from 17 soils developed on amphibolitic parent material in NW Spain was studied by pyrolysis gas chromatography spectrometry (Py‐GC/MS). We also included two buried andic A horizons (PAL, 2200 cal yr BP in age) on the same parent material, as a reference for the molecular composition of SOM from soils without recent litter additions. Organic matter of PAL soils had a composition largely different from that of superficial soils (FOR and AGR), with an important relative contribution of microbial polysaccharides and N‐compounds, and an absence of compounds that characterize fresh plant litter (e.g. lignins). In the superficial soils, the relative contribution of lignin‐derived compounds was greater in AGR than in FOR soils. Differences were also observed in the relative contribution of aliphatic compounds, FOR soils being enriched in this type of components compared with AGR soils. The results indicated that land use change from FOR to AGR, which was accompanied by a decrease in total SOM, resulted in an enrichment in primary SOM. The smaller relative abundance of primary SOM derivatives in andic FOR soils indicates that these compounds were quickly degraded in Andisols.     

Biochar‐compost substrates in short‐rotation coppice: Effects on soil and trees in a three‐year field experiment

Both biochar and compost may improve carbon sequestration and soil fertility; hence, it has been recommended to use a mixture of both for sustainable land management. Here, we evaluated the effects of biochar–compost substrates on soil properties and plant growth in short rotation coppice plantations (SRC). For this purpose, we planted the tree species poplar, willow, and alder in a no‐till field experiment, each of them amended in triplicate with 0 (= control) or 30 Mg ha^?1 compost or biochar–compost substrates containing 15% vol. (TPS15) and 30% vol. biochar (TPS30). For three years running, we analyzed plant growth as well as soil pH, potential cation exchange capacity (CEC), stocks of soil organic carbon (SOC), total N, and plant‐available phosphate and potassium oxide.Biochar‐compost substrates affected most soil properties only in the topsoil and for a limited period of time. The CEC and total stocks of SOC were consistently elevated relative to the control. After three years the C gain of up to 6.4 Mg SOC ha^?1 in the TPS30 plots was lower than the added C amount. Especially in the case of TPS30 treatment, C input was characterized by the greatest losses after application, although the black carbon of the biochar was not degraded in soil. Additionally, tree growth and woody biomass yield did not respond at all to the treatments. Overall, there were few if any indications that biochar–compost substrates improve the performance of SRC under temperate soil and climate conditions. Therefore, the use of biochar for such systems is not recommended.     

Molecular changes in organic matter of a compost-amended soil

The molecular changes of organic matter in a cultivated soil after compost amendments was followed by off-line-pyrolysis-TMAH GC-MS. Thermochemolysis of soil and compost provided a detailed molecular characterization of soil organic matter (SOM) by releasing a large amount of different molecules mainly derived from plant biopolymers such as lignin, waxes and bio-polyesters. No significant differences were found before and after 1 year of cultivation in the pyrolytic products released by control soil, which were mainly fatty acids, oxidized forms of lignins, and minor amounts of microbial bio-products and biopolyesters derivatives. Conversely, significant qualitative and quantitative variations were found in the molecular characteristics of SOM between control and compost-amended soils after 1 year of cultivation. Increasing amounts and diversified components of fatty acids, n -alkanes and various biopolyesters derivatives such as hydroxy-alkanoic and alkandioic acids were found in the compost-amended soil. These results indicate that a significant amount of exogenous compost-derived organic molecules were incorporated into SOM after 1 year of cultivation. The organic structural indexes derived from these results indicated direct inputs of undecomposed lignin residues and hydrocarbon waxes from compost material. When compared with the control soil, small but significant amounts of plant biomarkers, such as cyclic di- and triterpenes derivatives, were found only in the compost-amended soil. These findings suggest that the molecular changes of SOM brought about by amendment with biomass residues can be followed by using thermochemolysis of bulk soil samples.     

Effects of Alfalfa on Aggregate Stability,Aggregate Preserved‐C and Nutrients in Region Mountain Agricultural Soils 1 Year After its Planting

In mountain areas, water erosion plays an important role on soil structure and can strongly affect its functions. Suitable management practices, namely choice of crops, may be able to improve aggregate stability and reduce soil loss by surface runoff. To study the effects of cover crops on aggregation in mountain soils, we investigated a soil planted with wheat (site C) compared with two soils under alfalfa (sites A and B). All investigated soils were Typic Ustorthents. The soil aggregates were isolated by a physical‐functional procedure defined on the basis of aggregate resistance against specific breakdown mechanisms such as slaking and water abrasion. One year after alfalfa plantation, its effect on wet aggregate stability and the amount of organic carbon (OC) and nutrients retained by the isolated aggregates were measured. In the less degraded soils (site B), organic matter decomposition was stimulated, and after 1 year, the total OC (TOC) and TOC/N declined. Consequently, a reduction in aggregate stability and ability to preserve elements occurred, as shown by the decrease of wet aggregate stability indexes and total nutrients (e.g. N, P and K) retained by stable aggregates. In more degraded soil (site A), alfalfa generally positively affected the investigated properties. The effect of alfalfa in the studied crop rotation depends on the degree of land degradation due to water erosion. As regards the investigated breakdown mechanisms, our data showed that the aggregate resistance to water abrasion was related to TOC, while the resistance against fast wetting breakdown is correlated to the microbial biomass. Copyright © 2017 John Wiley & Sons, Ltd.     

Organic matter stabilization in a Xanthic Ferralsol of the central Amazon as affected by single trees: chemical characterization of density,aggregate, and particle size fractions

Not only the amount of organic carbon in soil is important for soil organic matter (SOM) stability, but also its physical and chemical properties. The appropriate technique for the assessment of SOM dynamics can vary between soil types, and information about this is lacking for Ferralsols of the central Amazon basin. First, this work identified SOM pools which are sensitive to land-use changes on the terra firme in the central Amazon. In a second step, the effects of single trees on SOM properties were evaluated in a mixed tree crop plantation in comparison to secondary and primary forest sites. Thus, the processes of organic matter stabilization could be studied in the highly aggregated soils. A combination of aggregate and density fractionation was found to be most suitable for physical SOM characterization. The particulate organic matter (POM, density less than 1.6 Mg m⁻³) varied by one order of magnitude between sites and could be used as a sensitive indicator of land-use changes. Aggregate stability was not related to SOM contents or bulk SOM properties. The incorporation of plant material into stable SOM, however, was enhanced by aggregation. Among aggregate separates, the fraction, 0.25–0.5 mm, showed single-tree effects the most. SOM replenishment was higher under tree species with low quality litter, i.e. high C-to-N and polyphenol-to-N ratios. High quality litter from a leguminous ground cover, however, showed low soil nitrogen and carbon replenishment but increased nitrogen concentrations in light fractions. Litter with a high quality may improve soil nitrogen availability but not amounts of total SOM, which could only be shown for low quality litter. The results indicate the importance of aggregation and POM dynamics for SOM stabilization in the studied Xanthic Ferralsols of the central Amazon basin.     

Soil‐microbial response to sugarcane filter cake and biogenic waste compost

An incubation experiment was carried out to examine the N‐immobilizing effect of sugarcane filter cake (C : N = 12.4) and to prove whether mixing it with compost (C : N = 10.5) has any synergistic effects on C and N mineralization after incorporation into the soil. Approximately 19% of the compost‐C added and 37% of the filter cake–C were evolved as CO₂, assuming that the amendments had no effects on the decomposition of soil organic C. However, only 28% of the added filter cake was lost according to the total‐C and δ¹³C values. Filter cake and compost contained initially significant concentrations of inorganic N, which was nearly completely immobilized between day 7 and 14 of the incubation in most cases. After day 14, N remineralization occurred at an average rate of 0.73 µg N (g soil)^–1 d^–1 in most amendment treatments, paralleling the N mineralization rate of the nonamended control without significant difference. No significant net N mineralization from the amendment N occurred in any of the amendment treatments in comparison to the control. The addition of compost and filter cake resulted in a linear increase in microbial biomass C with increasing amounts of C added. This increase was not affected by differences in substrate quality, especially the three times larger content of K₂SO₄‐extractable organic C in the sugarcane filter cake. In most amendment treatments, microbial biomass C and biomass N increased until the end of the incubation. No synergistic effects could be observed in the mixture treatments of compost and sugarcane filter cake.