European Journal of Soil Science

The response of soil respiration (R_s) and its components (autotrophic [R_a] and heterotrophic respiration [R_h]) to climate warming is one of the uncertainties in ecosystem carbon (C) models. Here we conducted a warming experiment in an alpine meadow dominated by Koresbia in the permafrost region of the Qinghai‐Tibet Plateau (QTP) to examine effects of warming on R_s and its components. Infrared heaters were used to simulate a 2°C warming of the surface soil temperature. Deep collars (50 cm to exclude root growth) were inserted into soil to measure R_h: R_a, which was calculated by subtracting R_h from R_s. Average R_s and its components (R_a and R_h) were significantly stimulated by 21.5, 27 and 15.6%, respectively, in warmed plots from January 2011 to October 2013. The contribution of R_h to R_s decreased in the warmed plots because of the smaller relative increase in R_h than in R_a. Annual soil C release increased by 263 and 247 g C m⁻² in 2011 and 2012, respectively. Stimulation in R_a and R_h was related to the significant increase in root biomass (0–50 cm) and in labile soil C in the deeper layer (40–50 cm). The temperature sensitivities (Q₁₀) of R_s and its components all increased with larger values in R_a, followed by R_s and R_h. Our results suggest a positive feedback between soil C release and climatic warming in the permafrost region of the QTP.

Mid‐infrared diffuse reflectance spectroscopy can provide rapid, cheap and relatively accurate predictions for a number of soil properties. Most studies have found that it is possible to estimate chemical properties that are related to surface and solid material composition. This paper focuses on prediction of physical and mechanical properties, with emphasis on the elucidation of possible mechanisms of prediction. Soil physical properties that are based on pore‐space relationships such as bulk density, water retention and hydraulic conductivity cannot be predicted well using MIR spectroscopy. Hydraulic conductivity was measured using a tension‐disc permeameter, excluding the macropore effect, but MIR spectroscopy did not give a good prediction. Properties based on the soil solid composition and surfaces such as clay content and shrink‐swell potential can be predicted reasonably well. Macro‐aggregate stability in water can be predicted reasonably as it has a strong correlation with carbon content in the soil. We found that most of the physical and mechanical properties can be related back to the fundamental soil properties such as clay content, carbon content, cation exchange capacity and bulk density. These connections have been explored previously in pedotransfer functions studies. The concept of a spectral soil inference system is reiterated: linking the spectra to basic soil properties and connecting basic soil properties to other functional soil properties via pedotransfer functions.

This work aimed to evaluate the potential of mid‐infrared reflectance spectroscopy (MIRS) to predict soil organic and inorganic carbon contents with a 2086‐sample set representative of French topsoils (0–30 cm). Ground air‐dried samples collected regularly using a 16 × 16‐km grid were analysed for total (dry combustion) and inorganic (calcimeter) carbon; organic carbon was calculated by difference. Calibrations of MIR spectra with partial least square regressions were developed with 10–80% of the set and five random selections of samples. Comparisons between samples with contrasting organic or inorganic carbon content and regression coefficients of calibration equations both showed that organic carbon was firstly associated with a wide spectral region around 2500–3500 cm⁻¹ (which was a reflection of its complex nature), and inorganic carbon with narrow spectral bands, especially around 2520 cm⁻¹. Optimal calibrations for both organic and inorganic carbon were achieved by using 20% of the total set: predictions were not improved much by including more of the set and were less stable, probably because of atypical samples. At the 20% rate, organic carbon predictions over the validation set (80% of the total) yielded mean R², standard error of prediction (SEP) and RPD (ratio of standard deviation to SEP) of 0.89, 6.7 g kg⁻¹ and 3.0, respectively; inorganic carbon predictions yielded 0.97, 2.8 g kg⁻¹ and 5.6, respectively. This seemed appropriate for large‐scale soil inventories and mapping studies but not for accurate carbon monitoring, possibly because carbonate soils were included. More work is needed on organic carbon calibrations for large‐scale soil libraries.

The packing of elementary particles in soil largely determines the properties that depend on the textural soil pore space, but is studied little. The relations between packing and size and nature of soil particles were studied using fractions of clay, silt and sand, mixed when wet and then dried. Ternary mixtures (clay:silt:sand) were compared with binary mixtures (clay:silt, clay:sand). The pore space of the mixtures was studied using mercury porosimetry and scanning electron microscopy. In all the mixtures the textural pore space was divided into two compartments: (1) lacunar pores due to the presence of skeleton particles and to the shrinkage of the clay phase between these particles, and (2) the clay–fabric pores due to the packing of the clay. In the ternary mixtures, lacunar pores could be divided into two classes: (1) those due to sand particles within the clay–slit phase considered as a single phase, and (2) those due to silt particles within this same phase. For certain mixtures, lacunar pores, referred to as hidden lacunar pores, were not interconnected but were occluded. This occurred both for hidden pores caused by the presence of sand and occluded by the clay–slit phase, and for hidden pores caused by the presence of silt and occluded by the clay phase. The relations between these types of textural pores and the proportions of different size fractions in the mixtures provide guidelines for making optimum use of the particle‐size characteristics of the soil to determine its properties.

Although the effect of experimental warming on soil microorganisms has been well documented at surface horizons, less is known about its influence in subsurface horizons. An experiment was therefore carried out in an alpine meadow on the Qinghai‐Tibet Plateau to examine the responses of microbial communities to experimental warming at five soil depths (0–10, 10–20, 20–30, 30–40 and 40–50 cm). Plots were passively warmed for 3 years in open‐top chambers and compared with adjacent control plots at ambient temperature. Soil microbial communities were assessed by using phospholipid fatty acid (PLFA) analysis. Our results showed clearly that 3 years of experimental warming increased microbial biomass consistently and significantly throughout the upper 50‐cm soil profiles, as indicated by the changes in both microbial biomass carbon (C) and total PLFA contents. The composition of microbial communities was also affected significantly by warming, but its effect depended on soil depth. While warming induced a community shift towards bacteria at the 0–10‐cm depth, it tended to shift microbial communities towards fungi at the other, deeper, layers. These results indicate that warming had strong effects on soil microbial communities, including even those residing in subsurface horizons, which may help us to understand the microbial mediation of the feedback between terrestrial C cycling and climate warming.

In the detritusphere, particulate organic matter offers new sites for microorganisms, whereas soluble substrates are transported into the adjacent soil. We investigated how mechanisms of solute transport affect microbial abundance and function in the detritusphere. In a first experiment, transport was restricted to diffusion, whereas in a second experiment it was dominated by convection. Two soil moisture contents were established in each experiment. When diffusion was the exclusive transport mechanism, the addition of maize litter induced distinct gradients in enzyme activities, soil organic C content and microbial biomass to a depth of 1.5–2.8 mm. Convection enlarged these gradients to 2.5–3.0 mm. The moisture regime modified the temporal pattern of diffusive C transport, microbial growth and enzyme release by inducing faster transport at large water contents. Convective transport seemed to be unaffected by soil moisture content. Using a convective‐diffusive transport model with first‐order decay, it was possible to simulate the observed activity profiles. The results indicate that the spatial dimension of the detritusphere is governed by the ratio between decay rate of available substrates and transport rate. Bacteria and fungi showed differing utilization strategies as revealed by coupling phospholipid fatty acid (PLFA) analysis with stable isotope techniques. Fungi assimilated C directly in the litter, whereas bacteria took up the substrates in the soil and therefore depended more on transport processes than fungi. Our results demonstrate the impact of physicochemical conditions on the abundance and function of microorganisms in the detritusphere. Furthermore, the combination of enzymatic measurements and mathematical transport modelling may offer a new way to measure substrate decay rates in soil.

In the Congo, near Pointe‐Noire, Pinus and Eucalyptus were planted on the savanna for 30 years. We have characterized the effects of this change on land‐use on the composition of carbohydrates in whole soil and particle‐size fractions of the soil. Carbohydrates represent variable proportions of the total soil organic carbon (TOC) of various particle size fractions. The largest proportions of sugar‐C were found in the savanna soil with as much as 250 mg g⁻¹ TOC in the coarsest plant remains and approximately 190 mg g⁻¹ TOC in the finest organo‐mineral fractions, whereas there was always less sugar in plantation soils. The monosaccharide xylose and mannose have different distributions: xylose appears to be the marker of the vegetal inheritance, whereas the dominance of mannose in the clay fraction bears the signature of current microbial sugar synthesis.

The quantitative and qualitative evolution of the whole soil carbohydrates was studied as a function of plantation age. Carbohydrate‐C represents 131 mg g⁻¹ of the soil organic carbon in the savanna soil, but decreases to an average value of 75 mg g⁻¹ in plantations more than 6 years old. This appears to be due mainly to the stimulation of the mineralization of the glucose, which represented 60% of the total sugars in savanna soil and only 45–48% in tree plantations. The ratio [arabinose + galactose + fucose]/[rhamnose + xylose], which is the largest in the oldest plantations, is significant for evaluating the replacement of carbohydrates of the original grass savanna by those of the trees.

Soil from Eutrochrept A horizons under long‐term spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp) and arable rotation (Ar) was fractionated according to particle size and analysed for contents of C, N, lignin‐derived phenols and carbohydrates.

Whole soil from Sf, Df, Gp and Ar contained 84, 59, 73 and 25 g C kg⁻¹ soil, respectively. For all sites, the C content declined and C/N ratio increased in the order: clay (<2 μm), silt (2–20 μm), sand (20–2000 μm). Clay and silt were significantly lower in C in Ar than in Sf, Df and Gp, C associated with sand being substantially lower under arable rotation.

The yield of lignin‐derived phenols decreased and carboxyl functionality and methoxyl demethylation of lignin derivatives increased with decreasing particle size, indicating a progressive lignin alteration. Whole soil from Sf and Gp was substantially higher in vanillyl (V), syringyl (S) and cinnamyl (C) units (VSC) than soil from Df and Ar. Compared to whole soil, clay was depleted and sand enriched in VSC. Only sand appeared to be affected significantly by land use. Sand from Ar and Df was more enriched in VSC than sand from Gp and Sf.

Whole soil carbohydrates decreased in the order: Gp>Ar>Df>Sf. Sand‐ and clay‐sized separates were enriched in carbohydrates compared to silt. Carbohydrates in sand were mainly of plant origin whereas microbially‐derived sugars accounted for a larger proportion in the clay. Compared to Sf, Df and Gp, clay from Ar was enriched and sand depleted in microbial sugars.

Lignin and carbohydrate distribution patterns indicate that organic matter was in a more advanced stage of decomposition in the sand separates from forest than from agricultural A horizons. The forest soils also show a higher degree of oxidative changes in lignin associated with clay. In contrast, differences between silt from the four A horizons were small.

Particulate organic matter (POM) is a labile fraction of soil organic matter which is thought to be physically protected from biodegradation when within soil aggregates. We have developed a fractionation method to separate POM located outside stable soil macroaggregates (> 200 μm) and microaggregates (50–200 μm) from that within them, and applied it to a cultivation sequence of humic loamy soils. The natural abundance of ¹³C was used to determine the amounts of POM derived from forest and that derived from crop in the free and occluded fractions. In the forest soil the free and occluded POM fractions had the same composition, morphology and isotopic signature. On cultivation the amounts of POM decreased sharply. The loss of C in the POM from forest was mainly from POM outside the aggregates. The POM occluded within microaggregates was found to turnover slowly. This may be due either to its recalcitrant chemical nature or to its physical protection within microaggregates