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Little is known about the effect of addition of plant residues from which water-extractable organic C (WEOC) was partially removed on soil microbial activity and growth. Two incubation experiments were conducted in a sandy clay loam amended with barley (Hordeum vulgare L.) residues (ground to 0.5-1 mm) from which WEOC was removed by extraction (Experiment 1) or leaching (Experiment 2). For the first experiment, the residues were extracted 5 times to maximise WEOC removal. There were three treatments: unamended control soil, soil amended with original residue or with extracted residue. Cumulative respiration increased rapidly within the first 5 days with the original residues, whereas it started more slowly with extracted residues. By the end of the experiment (day 18) compared to the original residues, cumulative respiration was 18% lower with extracted residues, but the microbial biomass C (MBC) concentration was 27% greater. For the second experiment, the residue was leached 1 to 8 times to reduce WEOC concentrations by 29 and 80%, respectively. There were four treatments: unamended control, soil amended with original residue, residue leached once and eight times. Cumulative respiration was reduced by up to 21% by partial removal of WEOC compared to the original residues in the first 10 days, but by day 28 (end of experiment) it was greatest with residues leached 8 times where it was 5% greater than with the original residues. This treatment also had a 16% higher MBC concentration on day 28 than soil with the original residue. It is concluded that nearly complete removal of WEOC reduced soil respiration compared to the original residues in the first week after residue addition. However, when removal is incomplete and a small proportion of WEOC (20%) remains in the residues, respiration may increase later above that of the original residues. [ABSTRACT FROM AUTHOR]

Understanding the factors determining cumulative respiration upon rewetting of dry soil is critical for predicting C efflux from soils. The response of respiration to drying and rewetting may be influenced by land management due to its effect on the soil organic C pool and differ between soil size fractions. An incubation experiment was conducted with soils collected from two plots with a long history of different management (wheat-fallow and permanent pasture). The soils were sieved to 4-10 mm and <2 mm to obtain two size factions. There were five moisture treatments with the same length (48 days). The constantly moist control (CM) was maintained at 50% of WHC throughout. In the drying and rewetting (DRW) treatments, the number of dry and moist days was equal but the number of DRW events ranged from one to four (1-4DRW). Respiration was measured daily, microbial biomass C (MBC) was determined six days after rewetting in each DRW cycle and on day 48 (end of the experiment). The proportion of soil in the 4-10 mm size fraction decreased over time with a greater decrease in pasture than in wheat soil and in the DRW treatments compared to the constantly moist treatment (CM). Cumulative respiration at the end of the experiment was greater in the <2 mm than in the 4-10 mm fraction in both soils and was highest in CM and 1DRW. In wheat soil, cumulative respiration decreased from 1DRW to 3DRW, whereas it decreased only between 2 and 3DRW in pasture soil. In treatments with two to four DRW, the proportion of total cumulative respiration was lowest in the last cycle. In 2DRW, cumulative respiration was smaller in the second than in the first moist period whereas the reverse was true for 3DRW and 4DRW. Cumulative respiration in the second moist period was greater in 3DRW than in 2DRW (8 and 12 prior moist days) whereas cumulative respiration in the third moist period was greater in 4DRW than in 3DRW (12 and 16 prior moist days). At the end of the experiment, the MBC concentration in the 4-10 mm fraction was unaffected by moisture treatment, whereas in the <2 mm fraction, it was greatest in CM and lowest in 4DRW. We conclude that the response of respiration to drying and rewetting is more strongly influenced by management than size fraction. In a given soil, the cumulative respiration upon rewetting is influenced not only by the number of DRW cycles but also the number of moist days prior to rewetting. [ABSTRACT FROM AUTHOR]

Abstract: Addition of clay-rich subsoils to sandy top soils is an agricultural management option to increase water and nutrient retention and may also increase organic carbon sequestration by decreasing the decomposition rates. An incubation experiment was carried out in a loamy sand top soil mixed with a clay-rich subsoil (84% clay) at 0, 10 and 30% (w/w) amended with finely ground mature shoot residues of two native perennial grasses and annual barley individually or in 1:1 mixtures of two residues. Extractable C, microbial biomass C, available N and soil pH were analysed at days 0, 3, 14 and 28. Cumulative respiration after 28days was highest with barley residue and lowest with Wallaby grass at all clay soil addition rates; 30% clay soil addition reduced cumulative respiration, especially with barley alone. In the mixture of native grasses and barley, the measured respiration was lower than expected at a clay soil addition rate of 10%. A synergistic effect (higher than expected cumulative respiration) was only found in mixture of Kangaroo grass and barley at a clay soil addition rate of 30%. Clay soil addition also decreased extractable C, available N and soil pH. The temporal change in microbial biomass C and available N in residue mixtures differed among clay addition rates. In the mixture of Wallaby grass and Kangaroo grass, microbial biomass C (MBC) decreased from day 0 to day 28 at clay soil addition rates of 0 and 10%, whereas at 30% clay MBC increased from day 0 to day 3 and then decreased. Our study shows that addition of a clay-rich subsoil to a loamy sand soil can increase C sequestration by reducing CO2 release and extractable C which are further modulated by the type of residues present individually or as mixtures. [Copyright &y& Elsevier]

Soil heterogeneity influences microbial access to substrates and creates habitats varying in substrate concentrations, thus leading to local variations in carbon (C) dynamics. Based on theoretical considerations, we expected that higher heterogeneity would decrease microbial activity. To test this hypothesis, we modified substrate spatial heterogeneity using 3D-printed cylinders with four compartments (either preventing or allowing diffusion between compartments). The same total amount of glucose (1.5 mg glucose C per cylinder) was added either to one compartment (highest local concentration, 2.0 mg glucose C g−1 soil, and highest heterogeneity), to two (medium concentration, 1.0 mg glucose C g−1 soil, and intermediate heterogeneity), or to four compartments (lowest local concentration, 0.5 mg glucose C g−1 soil, and equivalent to homogeneous conditions). Thus, we experimentally created a gradient of substrate spatial heterogeneity. The 3D cylinders containing soil were transferred into standard calorimetry ampoules and were incubated in isothermal calorimeters to monitor soil heat dissipation rates as a proxy of soil microbial activity over 51 h at 18 °C. When diffusion among compartments was prevented, the most heterogeneous treatment showed the lowest heat dissipation rates, despite having the highest local substrate concentration. Compared to homogeneous conditions, the heat dissipation rate from the most heterogeneous treatment was 110% lower at the beginning of the experiment (12.7 μJ g−1 soil s−1) and 50% lower when heat dissipation rates reached a peak (72.6 μJ g−1 soil s−1). Moreover, the peak was delayed by approximately 2 h compared to the most homogeneous treatment. When diffusion among compartments was allowed, the effect of substrate spatial heterogeneity on microbial activity was strongly diminished. Our findings emphasize the influence of substrate spatial heterogeneity on soil microbial dynamics, highlighting the importance of including it in C cycling models for a better understanding of soil C dynamics. • A gradient of substrate spatial heterogeneity is created experimentally. • Increasing degree of heterogeneity decreases microbial activity. • Allowing substrate to diffuse diminishes the effect of heterogeneity. • Effect of heterogeneity is explained by scale transition theory in C cycling model. • Substrate spatial heterogeneity provides evidence for the delay in C decomposition. [ABSTRACT FROM AUTHOR]

Soil function, such as decomposition of organic materials, is of crucial importance to sustain soil fertility and may be enhanced through soil management. We hypothesized that manure amendment would increase soil functional stability more effectively than mineral fertilization when soil nutrients were not limited. By using a 22-yr field experiment, the objectives were 1) to determine the effects of manure substitution and reduction of mineral fertilizers on soil physio-chemical properties, soil microbial community structure, and soil biological functional stability; 2) to isolate the effects of organic amendment from those of mineral fertilization on soil biologic functional stability; and 3) to elucidate the controlling mechanisms on the soil functional stability. Soils were sampled from the field treatments, no fertilization (CK), mineral N, P and K (NPK), two doses of NPK (2NPK), manure amendment (OM) and OM in combination with NPK (NPK + OM). The nutrient inputs were similar in treatments OM and 2NPK. The functional stability was quantified by measuring the decomposition rate of crop litter added to the soils following Cu addition and heating. Soil nutrients, organic carbon and pH increased due to mineral fertilization and organic amendment. The principal component analysis of phospholipid fatty acid (PLFA) profiles demonstrated that the structure of soil microbial communities shifted between the mineral-fertilized soils and manure-amended soils and the shifts were not due to nutrient limitation because the soil microbial communities were not separated between the treatments of NPK and 2NPK. The manure amendment enhanced the resistance and resilience to Cu and heating more than the mineral fertilization, to a larger extent in treatment NPK + OM than in treatment OM. The resistance and resilience to Cu addition was positively correlated with soil organic matter, soil aggregate stability, while only the resistance to heating was positively correlated to soil aggregate stability. Moreover, the resistance and resilience were correlated with the shifts of functional and physiological structure of soil microbial communities due to long-term manure amendment and mineral fertilization. In conclusion, the partial substitution of mineral fertilizers with manure (NPK + OM) increased soil functional stability to heavy metal pollution and global warming through altered structure and physiology of soil microbial communities due to improved soil aggregation with higher soil organic matter. [ABSTRACT FROM AUTHOR]