Using perlite as a substrate carrier for measuring microbial available phosphorus by respiration kinetics in soils

The release of CO₂ by soil microorganisms after the addition of nitrogen and glucose in excess and calibration additions of phosphorus has successfully been used to assess microbial available P, assuming the native soil P pool is then limiting respiration. However, in P-fixing soils and soils with high P content, carbon can be exhausted before the available soil P pool. It is not possible to simply increase the amount of glucose as then the glucose concentration would be lethal for microorganisms. A modified method was tested where soil is mixed with perlite. It was hypothesised that perlite, having a high water holding capacity, would dilute the concentration of glucose, while maintaining the bioavailability of added nutrients, thus avoiding carbon limitation. Factorial combinations of amount of soil and perlite (both adjusted to −25 kPa water potential) were tested to examine if perlite as such had any effect on the respiration. Five tropical soil samples with a sharp gradient in P availability and one N-limited compost material were used. The method successfully reduced the risk of carbon limitation. Microbial indices, such as basal respiration, substrate-induced respiration and maximum P-limited respiration, were directly proportional to the amount of soil in the experiments but unrelated to the amount of perlite, showing that perlite did not affect microbial measurements.     

The Contribution of Various Components to pH Buffering Capacity of Acrisols in Southeastern Vietnam

This paper examined the contribution of various soil components to pH buffering capacity (pH_BC) of haplic Acrisols in an upland area of Southeastern Vietnam. Sampling was conducted in 2016 in Tan Bien district, Tay Ninh province at seven sites from the surface to 60-cm depth. Soils were very acidic (pH_H2O 4.53 ± 0.05). The pH_BC were very low, 0.46 ± 0.04 and 0.44 ± 0.05 cmol H⁺ kg^?1 pH^?1, respectively, for original samples and those from which soil organic carbon (SOC, 0.52 ± 0.09%) were removed. The contribution of Al³⁺ to pH_BC was remarkable while that of SOC was of little significance. The contribution of clay minerals to pH_BC was unclear due to the low (9.37 ± 0.76%) and kaolinite-dominated clay content. The current soil conditions indicated a potential for further soil acidification. Liming would be one of the measures to remediate soil acidity in the research area.     

Air Drying Affects Extractable Sulfate in Soils of Variable Charge: Test of Two Extraction and Two Quantification Methods

Abstract

Fertilizer recommendations need to be based on reliable soil sulfate determinations. Airdrying samples changes irreversibly many properties of soils with variable charge and might affect the extractable sulfate. In this study, sulfate extracted from air‐dry and field‐moist samples was compared. Two extracting solutions [water and 00.1 M Ca(H₂PO₄) ₂] and two quantification methods (turbidimetry and ion chromatography) were assayed on A and B horizon samples of five Humic Acrisols from southeast Mexico. Air drying increased water‐extractable sulfate in Ah horizons, whereas in Bt horizons, it increased the 00.1 M Ca(H₂PO₄)₂‐extractable sulfate. Airdrying increased dissolved organic carbon contents in all samples and increased soil acidity and oxalate extractable iron in 70 and 60% of the samples, respectively. Results showed larger coefficients of variation in air‐dried samples. Turbidimetry resulted less sensible than ion chromatography. To enhance sensitivity and reproducibility, particularly organic soil samples should be analyzed field‐moist and by ion chromatography.     

Impact of land use on soil organic carbon stocks in the humid tropics of NE Tanzania

The conversion of tropical forests to agricultural land use is considered as a major cause for a decline in soil organic carbon (SOC) stocks. However, the extent and impact of different land uses on SOC stock development is highly uncertain, especially for tropical Africa due to a lack of reliable data. Interactions of SOC with the soil mineral phase can modify the susceptibility of SOC to become mineralized. Pedogenic Fe‐, Al‐oxides and clay potentially affect SOC stabilization in highly weathered soils typically found in the humid tropics. The aim of our study was to determine the impact of different land uses on SOC stock on such soils. For that purpose, 10 pedologically similar, deeply weathered acidic soils (Acrisols, Alisols) in the Eastern Usambara Mountains (Amani Nature Reserve, NE Tanzania) under contrasting land use were sampled to a depth of 100 cm. The calculated mean SOC stocks were 17.5 kg C m^?2, 16.8 kg C m^?2, 16.9 kg C m^?2, and 20.0 kg C m^?2 for the four forests, two tea plantations, three croplands, and one homegarden, respectively. A significant difference in mean SOC stock of 1.3 kg C m^?2 was detected between forest and cropland land use for the 0–10 cm depth increment. No further significant impacts of land use on SOC stocks were observed. All soils have a clearly clay‐dominated texture. They are characterized by high content of pedogenic oxides with 29 to 47 g kg^?1 measured for the topsoils and 36 to 65 g kg^?1 for the subsoils. No positive significant relationship was found between SOC and clay content. Statistically significant positive relationships existed between oxalate‐extractable Fe, Al, and SOC content for cropland soils only. Compared to data published in literature the SOC stocks determined in our study were generally high independent of the established land use. It appears that efficient SOC stabilization mechanisms are counteracting the higher disturbance regime under agricultural land use in these highly weathered tropical soils.     

Runoff,soil loss,and nutrient depletion under traditional and alternative cropping systems in the Transmexican Volcanic Belt,Central Mexico

In the Transmexican Volcanic Belt a traditional fallow system is practiced, called “ año y vez ” (AV), which does not benefit soil conservation due to its low level of nutrient recycling and because soil protection is poor during the cultivation year. The objective of the present work was to measure runoff and soil and nutrient losses during three annual cycles (2002–2004) in Central Mexico under AV rotation and two alternative systems: improved traditional (IT) and traditional organic (TO). Soil losses in the three systems were moderate (<1.2 Mg ha⁻¹ y⁻¹) except during 2002, in which significant soil losses were recorded in IT and TO due to the scarcity of plant cover (<20 per cent) that was present throughout the rainy season. During the resting period of the AV system (2003), the annual runoff increased from 19 to about 600 per cent, compared to IT and TO without grazing. The difference in runoff was attributed to an 18 per cent increase in bulk density of soil surface (0–5 cm) caused by cattle trampling while grazing. Nutrient losses in the three treatments were mainly of N, Ca²⁺, Mg²⁺, Na⁺, and K⁺. These results suggest that AV has a higher topsoil degradation effect during the resting year than during the cultivation period. The study shows that incorporating the maize/beans‐black oat rotation and residue cover causes a low runoff response that is important in reducing soil degradation. A spatial analysis is presented of erosion at watershed level for two soil management systems assessed. Copyright © 2009 John Wiley & Sons, Ltd.     

Modelling soil organic matter decomposition and rainfall erosion in two tropical soils after forest clearing for permanent agriculture

A soil organic matter turnover model has been developed to analyse soil carbon (soil organic-C) loss caused by organic matter decomposition and rainfall erosion in soils used for permanent cultivation. It has been used to build up model profiles of five soils, one occurring in temperate and four in tropical regions, on the basis of estimates for ‘natural’ organic matter input. Organic matter input data for different systems of cultivation were used to model the long-term decomposition of soil organic-C in these model profiles. The modelling results show that soil organic matter decomposition in the tropics is three to four times faster than in temperate regions, and that there is a marked influence of soil type and soil climate. Simulated losses of organic-C in the tropical soils, not accounting for erosion are 31 to 50 per cent after 50 years and 43 to 63 per cent after 100 years of continuous cultivation. The simulated loss of soil organic-C when rainfall erosion is also allowed for is 40 to 80 per cent. Erosion caused an extra loss of at least 7 per cent after 100 years. The initial input of charcoal from forest burning is lost through erosion at a rate of 50 to almost 100 per cent, depending on the severity of erosion. The sensitivity of modelling results to variations in input data was also analysed. The losses of soil carbon were also used to calculate the global flux of CO₂ from soils. Soils are probably a small but not negligible source of CO₂.