Medium‐ and short‐term available organic matter,microbial biomass,and enzyme activities in soils under Pinus sylvestris L. and Robinia pseudoacacia L. in a sandy soil in NE Saxony,Germany

Total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities in a sandy soil under pine (Pinus sylvestris L.) and black locust (Robinia pseudoacacia L.) stands were investigated in a field study near Riesa, NE Germany. Samples of the organic layers (Oi and Oe‐Oa) and the mineral soil (0–5, 5–10, 10–20, and 10–30 cm) were taken in fall 1999 and analyzed for their contents of organic C and total N, hot‐water‐extractable organic C and N (HWC and HWN), KCl‐extractable organic C and N (C_org(KCl) and N_org(KCl)), NH ‐N and NO ‐N, microbial‐biomass C and N, and activities of β‐glucosidase and L‐asparaginase. With exception of the HWC, all investigated C and N pools showed a clear response to tilling, which was most pronounced in the Oi horizon. Compared to soils under pine, those under black locust had higher contents of medium‐ and short‐term available C (HWC, C_org(KCl)) and N (HWN, N_org(KCl)), mineral N (NH ‐N, NO ‐N), microbial‐biomass C and N, and enzyme activities in the uppermost horizons of the soil. The strong depth gradient found for all studied parameters was most pronounced in soils under black locust. Microbial‐biomass C and N and enzyme activities were closely related to the amounts of readily mineralizable organic C (HWC and C_org(KCl)). However, the presented results implicate a faster C and N turnover in the top‐soil layers under black locust caused by higher N‐input rates by symbiotic N₂ fixation.     

Comparison of a hot water and cold 0.01 M Cacl2 extraction procedures for the determination of boron in soil

Abstract

In 100 different soils, hot (100C) water extractable boron was determined and the results were compared with boron data after extraction of the same soil samples with cold (20C) 0.01 M CaCl₂. Since the boron concentrations in cold soil extracts are too low for direct determination, the extracted boron was converted into BF4‐ and subsequently extracted with a liquid anion exchanger, Aliquat 336, into xylene, and measured by ICP‐AES. A linear relation with R² = 0.74 was found between the two tested procedures. It is, therefore, concluded that with a cold 0.01 M CaCl₂ extraction equally valuable soil boron values can be obtained as with the more difficult to standardize hot water extraction procedure.     

Comparing concurrent in situ and batch‐extracted trace element concentrations

Different procedures to investigate dissolved trace element concentration at the transition from unsaturated to saturated zone in soils were compared by concurrent sampling of soil solution and solid soil material in this zone. The in situ sampled soil solution from the percolated water was used to measure in situ concentrations, while solid soil material was used to measure concentrations at two liquid–solid ratios using batch experiments on 250 sample pairs. The liquid–solid ratios were 2 L kg^–1 and 5 L kg^–1. At 5 L kg^–1, the ionic strength was adjusted with Ca(NO₃)₂ to a sample‐specific value similar to in situ, while at 2 L kg^–1, the ionic strength was not adjusted. The extracted concentrations of most trace elements exhibited a statistically significant but weak correlation (p value < 0.01) to the corresponding in situ concentrations. In the liquid–solid ratio of 2 L kg^–1 extracts, Pb and Cr showed very poor comparability with the in situ equivalent. A likely cause was the enhanced dissolved‐organic‐C release in the extract due to the lower ionic strength compared to in situ conditions in combination with effects from drying and moistening soil samples. For the other elements, correlation increased in the order As < Cu, Zn, Sb, Mo, V < Cd, Ni, Co where adjustment of the ionic strength led to slightly better results. In addition to the element‐specific shortcomings, it appeared that low concentration levels of in situ concentrations were generally underestimated by batch extraction methods. The liquid–solid ratio of 2 L kg^–1 extracts could only be used as a method to predict exceedance of thresholds if a safety margin of approximately one order of magnitude higher than the thresholds was adopted. The ability of the batch‐extraction methods to estimate in situ concentrations was equally limited.     

Dynamic of different C and N fractions in a Cambisol under five year succession fallow in Saxony (Germany)

The dynamic of different soil C and N fractions in a Cambisol under succession fallow was investigated from June 1996 until May 2001. Mineral soil samples (0 – 10 and 10 – 30 cm) were analyzed for their concentrations of organic C (C_org), total N (N_t), hot water extractable C and N (HWC and HWN), and KCl extractable C and N (C_org(KCl), N_org(KCl), NH₄⁺‐N, NO₃^–‐N). The values of all C and N fractions revealed a distinct depth gradient. While the concentrations of C_org increased after set aside significantly from 7.7 to 8.9 g kg^–1 at 0 – 10 cm, those at 10 – 30 cm depth decreased from 7.2 to 6.1 g kg^–1. N_t remained rather constant throughout the whole observation period. The HWC concentrations increased from 0.33 to 0.49 g kg^–1, while HWN decreased slightly at 0 – 10 cm with time. In contrast, both HWC and HWN increased at 10 – 30 cm soil depth. HWC showed close significant correlations to C_org, and HWN to N_t as well as to NH₄⁺‐N and NO₃^–‐N, respectively. In comparison to hot water‐extractable C and N, C_org(KCl) and N_org(KCl) accounted only about one tenth of those and showed a decreasing trend with time of succession. C : N ratio of the KCl fraction was in the same order of magnitude as the HWC : HWN ratio, except the last phase of the experiment where hot water extract values increased above 10.     

Rhizodeposition of maize: Short‐term carbon budget and composition

The aim of this study was to assess differences in rhizodeposition quantity and composition from maize cropped on soil or on 1:1 (w/w) soil–sand mixture and distribution of recently assimilated C between roots, shoots, soil, soil solution, and CO₂ from root respiration. Maize was labeled in ¹⁴CO₂ atmosphere followed by subsequent simultaneous leaching and air flushing from soil. ¹⁴C was traced after 7.5 h in roots and shoots, soil, soil solution, and soil‐borne CO₂. Rhizodeposits in the leachate of the first 2 h after labeling were identified by high‐pressure liquid chromatography (HPLC) and pyrolysis–field ionization mass spectrometry (Py‐FIMS). Leachate from soil–sand contained more ¹⁴C than from soil (0.6% vs. 0.4%) and more HPLC‐detectable carboxylates (4.36 vs. 2.69 μM), especially acetate and lactate. This is either because of root response to lower nutrient concentrations in the soil–sand mixture or decreasing structural integrity of the root cells during the leaching process, or because carboxylates were more strongly sorbed to the soil compared to carbohydrates and amino acids. In contrast, Py‐FIMS total ion intensity was more than 2 times higher in leachate from soil than from soil–sand, mainly due to signals from lignin monomers. HPLC‐measured concentrations of total amino acids (1.33 μM [soil] vs. 1.03 μM [soil–sand]) and total carbohydrates (0.73 vs. 0.34 μM) and ¹⁴CO₂ from soil agreed with this pattern. Higher leachate concentrations from soil than from soil–sand for HPLC‐measured carbohydrates and amino acids and for the sum of substances detected by Py‐FIMS overcompensated the higher sorption in soil than in sand‐soil. A parallel treatment with blow‐out of the soil air but without leaching indicated that nearly all of the rhizodeposits in the treatment with leaching face decomposition to CO₂. Simultaneous application of three methods—¹⁴C‐labeling and tracing, HPLC, and Py‐FIMS—enabled us to present the budget of rhizodeposition (¹⁴C) and to analyze individual carbohydrates, carboxylates, and amino acids (HPLC) and to scan all dissolved organic substances in soil solution (Py‐FIMS) as dependent on nutrient status.     

Labile organic matter fractions as early‐season nitrogen supply indicators in manure‐amended soils

Soil test indicators are needed to predict the contribution of soil organic N to crop N requirements. Labile organic matter (OM) fractions containing C and N are readily metabolized by soil microorganisms, which leads to N mineralization and contributes to the soil N supply to crops. The objective of this study was to identify labile OM fractions that could be indicators of the soil N supply by evaluating the relationship between the soil N supply, the C and N concentrations, and C/N ratios of water extractable OM, hot‐water extractable OM, particulate OM, microbial biomass, and salt extractable OM. Labile OM fractions were measured before planting spring wheat (Triticum aestivum L.) in fertilized soils and the soil N supply was determined from the wheat N uptake and soil mineral N concentration after 6 weeks. Prior to the study, fertilized sandy loam and silty clay soils received three annual applications of 90 kg available N (ha · y)^?1 from mineral fertilizer, liquid dairy cattle manure, liquid swine manure or solid poultry litter, and there was a zero‐N control. Water extractable organic N was the only labile OM fraction to be affected by fertilization in both soil types (P < 0.01). Across both test soils, the soil N supply was significantly correlated with the particulate OM N (r = 0.87, P < 0.001), the particulate OM C (r = 0.83, P < 0.001), and hot‐water extractable organic N (r = 0.81, P < 0.001). We conclude that pre‐planting concentrations of particulate OM and hot‐water extractable organic N could be early season indicators of the soil N supply in fertilized soils of the Saint Lawrence River Lowlands in Quebec, Canada. The suitability of these pre‐planting indicators to predict the soil N supply under field conditions and in fertilized soils from other regions remains to be determined.     

Sterols in soil organic matter in relation to nitrogen mineralization in sandy arable soils

Unusually low net N mineralization in soils relatively rich in total organic C and N was repeatedly reported for sandy arable soils in NW Europe. In order to adequately account for it in simulation models, it is necessary to know the involved substances and processes. Therefore, 9 arable top soils (< 6% clay) with a wide range of total organic C (1.1%–5.2%) and C : N ratios (12–35) were studied. The soils varied strongly in the mineralizability of soil organic N which was determined via long‐term laboratory incubations (> 200 d). It was hypothesized that mineralization was controlled by antioxidants, and the Trolox equivalent antioxidant capacity (TEAC) of the soils was measured. In addition, pyrolysis–field ionization mass spectrometry (Py‐FIMS) was applied to investigate the influence of the molecular‐chemical composition of soil organic matter. In these soils, the compound class of sterols from Py‐FIMS analysis was most closely, negatively correlated with the mineralizability of soil organic N (r² = 0.75, p = 0.003). This was probably not an antioxidative effect, because the TEAC values did not correlate sufficiently with the mineralizability and the sterol intensities. However, the negative relation with sterols could be causal, since the correlation was about as close with other components of the compound class of sterols and even closer with the main plant sterol beta‐sitosterol (r² = 0.84, p = 0.001). In addition, the variability among samples was strongly governed by the proportions of sterols, and sterols also had a high discriminating power in discriminant analysis. Furthermore, the proportions of sterols were extraordinary in those arable podzol soils that developed under previous heath‐ or woodland (up to 10.2% of total ion intensity from Py‐FIMS). In conclusion, the inhibitory effect of these compounds needs to be investigated in more detail in order to optimize parameterization of N as well as C simulation models especially for podzolized, sandy arable soils with former heath‐ or woodland vegetation.     

A simple wet chemical extraction procedure to characterize soil organic matter (Som): 3. Results of vegetation,crop litter,and forest litter in comparison to data as revealed with CPMAS 13NMR spectroscopy and pyrolysis‐field ionization mass spectroscopy

Abstract

Data on the organic matter composition of vegetation, crop litter, and forest litter of Oi (=L) horizons from several European locations available with the litter compound analysis (LCA) discussed in previous communications are presented. The wet chemical results are discussed in comparison to data as revealed by cross polarization magic angle spinning carbon‐13 nuclear magnetic resonance spectroscopy (CPMAS ¹³C‐NMR) and pyrolysis‐field ionization mass spectrometry (Py‐FIMS). The LCA of vegetation, crop litter, and forest litter provides much detailed information about the chemical composition of extractable organic matter. Normally, both CPMAS ¹³C‐NMR and Py‐FIMS confirm the wet chemical results. Our data suggest a moderate chemical variation between the fresh organic residue and the litter. NMR revealed structural information about the non‐extractable organic matter using a combination of wet chemical extraction and CPMAS ¹³C‐NMR of whole soil. In addition, Py‐FIMS identified specific SOM segments at a qualitative level.     

Effects of the addition of forest floor extracts on soil carbon dioxide efflux

Composition and effects of additions of fibric (Oi) and hemic/sapric (Oe + Oa) layer extracts collected from a 20-year-old stand of radiata pine (Pinus radiata) on soil carbon dioxide (CO₂) evolution were investigated in a 94-day aerobic incubation. The ¹³C nuclear magnetic resonance spectroscopy indicated that Oi layer extract contained greater concentrations of alkyl C while Oe + Oa layer extract was rich in carboxyl C. Extracts from Oi and Oe + Oa layers were added to a forest soil at two different polyphenol concentrations (43 and 85 μg g⁻¹ soil) along with tannic acid (TA) and glucose solutions to evaluate effects on soil CO₂ efflux. CO₂ evolution was greater in amended soils than control (deionized water) indicating that water-soluble organic carbon (WSOC) was readily available to microbial degradation. However, addition of WSOC extracted from both Oi and Oe + Oa layers containing 85 μg polyphenols g⁻¹ soil severely inhibited microbial activity. Soils amended with extracts containing lower concentrations of polyphenols (43 μg polyphenols g⁻¹ soil), TA solutions, and glucose solutions released 2 to 22 times more CO₂-C than added WSOC, indicating a strong positive priming effect. The differences in CO₂ evolution rates were attributed to chemical composition of the forest floor extracts.     

Temperature effects on N mineralization: changes in soil solution composition and determination of temperature coefficients by TDR

We studied the changes in composition of the soil solution following mineralization of N at different temperatures, with a view to using TDR to calculate temperature coefficients for the mineralization of N. Mineralization from soil organic nitrogen was measured during aerobic incubation under controlled conditions at six temperatures ranging from 5.5 to 30°C, and at constant water content in a loamy sand soil. We also monitored during the incubation the concentrations of SO₄^2–, Cl^–, HCO₃^–, Ca²⁺, K⁺, Mg²⁺ and Na⁺, and the pH and the electrical conductivity in 1:2 soil:water extracts. Zero‐order N mineralization rates ranged between 0.164 at 5.5°C and 0.865 mg N kg^?1 soil day^?1 at 30°C. There was a significant decrease in soil pH during incubation, of up to 0.6 pH units at the end of the incubation at 30°C. The electrical conductivity of the soil extracts increased significantly at all temperatures (the increase between the start and the end of the incubation was 4‐fold at 30°C) and was strongly correlated with N mineralization. The ratio of bivalent to monovalent cations increased markedly during mineralization (from 2.2 to 5.9 at 30°C), and this increase influenced the evolution of the electrical conductivity of the soil solution through the differences in molar‐limiting ion conductivity between mainly Ca²⁺ and K⁺. Zero‐order mineralization rate constants, k, for NO₃^– concentrations calculated from TDR varied between 0.070 (at 5.5°C) and 0.734 mg N kg^?1 soil day^?1 (at 30°C), which were slightly smaller, but in the same range, as the measured rates. Underestimation of the measured N mineralization rates was due, at least in part, to differences in cation composition of the soil solution between calibration and mineralization experiments. A temperature‐dependence model for N mineralization from soil organic matter was fitted to both the measured and the TDR‐calculated mineralization rates, k and k_TDR, respectively. There were no significant differences between the model parameters from the two. Our results are promising for further use of TDR to monitor soil organic N mineralization. However, the influence of changing cation ratios will also have to be taken into account when trying to predict N mineralization from measured electrical conductivities.     

Clear-cutting of a Norway spruce stand: implications for controls on the dynamics of dissolved organic matter in the forest floor

Clear‐cutting of forest provides a unique opportunity to study the response of dynamic controls on dissolved organic matter. We examined differences in concentrations, fluxes and properties of dissolved organic matter from a control and a clear‐cut stand to reveal controlling factors on its dynamics. We measured dissolved organic C and N concentrations and fluxes in the Oi, Oe and Oa horizons of a Norway spruce stand and an adjacent clear‐cutting over 3 years. Aromaticity and complexity of organic molecules were determined by UV and fluorescence spectroscopy, and we measured δ¹³C ratios over 1 year. Annual fluxes of dissolved organic C and N remained unchanged in the thin Oi horizon (～ 260 kg C ha^?1, ～ 8.5 kg N ha^?1), despite the large reduction in fresh organic matter inputs after clear‐cutting. We conclude that production of dissolved organic matter is not limited by lack of resource. Gross fluxes of dissolved organic C and N increased by about 60% in the Oe and 40% in the Oa horizon upon clear‐cutting. Increasing organic C and N concentrations and increasing water fluxes resulted in 380 kg C ha^?1 year^?1 and 10.5 kg N ha^?1 year^?1 entering the mineral soil of the clear‐cut plots. We found numerous indications that the greater microbial activity induced by an increased temperature of 1.5°C in the forest floor is the major factor controlling the enhanced production of dissolved organic matter. Increasing aromaticity and complexity of organic molecules and depletion of ¹³C pointed to an accelerated processing of more strongly decomposed parts of the forest floor resulting in increased release of lignin‐derived molecules after clear‐cutting. The largest net fluxes of dissolved organic C and N were in the Oi horizon, yet dissolved organic matter sampled in the Oa horizon did not originate mainly from the Oi horizon. Largest gross fluxes in the Oa horizon (control 282 kg C ha^?1) and increased aromaticity and complexity of the molecules with increasing depth suggested that dissolved organic matter was derived mainly from decomposition, transformation and leaching of more decomposed material of the forest floor. Our results imply that clear‐cutting releases additional dissolved organic matter which is sequestered in the mineral soil where it has greater resistance to microbial decay.     

Hot water extracted organic matter: chemical composition and temporal variations in a long-term field experiment

Hot water-soluble organic matter was extracted from soil samples collected weekly between April and October in untreated and NPK+farmyard manure-fertilized plots in the 88-year-old Static Experiment (Loess Chernozem) at Bad Lauchstädt, Germany. As shown by solid-state ¹³C-nuclear magnetic resonance spectroscopy (¹³C-NMR) combined with pyrolysis-field ionization mass spectrometry this organic matter fraction was largely composed of carbohydrates and N-containing compounds, in particular amino-N species and amides. This composition and the low pyrolysis temperatures (mainly between 300 and 500°C) indicated its origin from soil biomass and root exudates and lysates, and its presence in the soil solution or weakly adsorbed by soil minerals and humic macromolecules. Long-term fertilization with NPK+farmyard manure resulted in larger mean concentrations of hot water-extracted C and N (0.933 and 0.094 g kg^-1) than soil management without fertilization (0.511 and 0.056 g kg^-1). The C and N extracted by hot water were in the range of 3–5% of total soil C and N. In the two treatments distinct temporal changes were observed, which appeared to be related to population dynamics of soil organisms, root growth and decomposition, and climatic influences on soil.     

Analyzing organic matter composition at intact biopore and crack surfaces by combining DRIFT spectroscopy and Pyrolysis‐Field Ionization Mass Spectrometry#

In the clay‐illuvial horizons (Bt) of Luvisols, surfaces of biopores and aggregates can be enriched in clay and organic matter (OM), relative to the bulk of the soil matrix. The OM composition of these coatings determines their bio‐physico‐chemical properties and is relevant for transport and transformation processes but is largely unknown at the molecular scale. The objective of this study was to improve the interpretation of spectra from Fourier transform infrared spectroscopy in diffuse reflectance mode (DRIFT) by using thermograms and released ion intensities obtained with pyrolysis‐field ionization mass spectrometry (Py‐FIMS) for a more detailed analysis of the mm‐scale spatial distribution of OM components at intact structural surfaces. Samples were separated from earthworm burrow walls, crack coatings, uncoated cracks, root channels, and pinhole fillings of the Bt‐horizons of Luvisols. The information from Py‐FI mass spectra enabled the assignment of OM functional groups also from spectral regions of overlapping DRIFT signal intensities to specific OM compound classes. In particular, bands from C=O and C=C bonds in the infrared range of wave numbers between 1,641 and 1,605 cm^?1 were related to heterocyclic N‐compounds, benzonitrile, and naphthalene. The OM at earthworm burrow walls was composed of chemically labile aliphatic C‐rich and rather stable lignin and alkylaromatic compounds whereas the OM of thick crack coatings and pinholes was dominated by heterocyclic N and nitriles and high‐molecular compounds, likely originating from combustion residues. In combination with Py‐FIMS, DRIFT applications to intact samples seem promising for generating a more detailed mm‐scale spatial distribution of OM‐related sorption and wettability properties of crack and biopore surfaces that may serve as preferential flow paths in structured soils.     

Effects of decreasing water potential on gross ammonification and nitrification in an acid coniferous forest soil

Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the ¹⁵N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of ¹⁵N, we compared different extraction and incubation times.T₀ times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg⁻¹ soil d⁻¹ and from 15.4 to 5.6 mg N kg⁻¹ soil d⁻¹ when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg⁻¹ soil d⁻¹ when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation.     

Fulvic acid composition in degraded fenlands

Fulvic acids (FAs) from topsoil and ground water solutions were investigated to discover effects of land use and peat degradation on their molecular chemical composition and thermal properties. The FAs were extracted from three Gleysols under arable land, intensive and extensive grassland, and from three Histosols under alder forest, extensive grassland, and a natural succession in a long‐term (> 200 years) cultivated fen area. Functional groups and molecular subunits of the FAs were investigated by ¹³C Nuclear Magnetic Resonance (¹³C NMR) spectroscopy. Thermal properties and structural molecular subunits were investigated by off‐line pyrolysis, and Pyrolysis‐Field Ionization Mass Spectrometry (Py‐FIMS). The ¹³C NMR spectra showed that the FAs from topsoil solutions had smaller proportions of alkyl C (mean: —8 %) and more aromatic C (mean: + 6 %) than FAs from ground water. This clear differentiation of dissolved FAs in the soil profiles is consistent with Py‐FIMS data which have shown enrichments of lipids in ground water FAs. Furthermore, Py‐FIMS revealed that the FAs from topsoils were richer in phenols + lignin monomers, carbohydrates, as well as mostly aromatic N‐containing compounds. These molecular subunits of FAs, relatively enriched in topsoil, were also the main indicators of land use and peat degradation. For topsoil solutions, the proportions of phenols + lignin monomers and carbohydrates increased stepwise with peat degradation in Gleysols and Histosols. Correspondingly, the thermal properties indicated the incorporation of these compounds into FAs by chemical bonds of larger thermal stability. Statistical evaluation by principal component analysis of Py‐FIMS clearly supported the differentiation of FAs according to the origin from topsoils and ground water, different soil types, and land use and peat degradation. Hence, it is concluded that water soluble FAs can be utilized as objective ecological indicators for soil effects on adjacent ground and surface waters.     

Rate of soil‐aggregate formation under different organic matter amendments—a short‐term incubation experiment

To improve soil structure and take advantage of several accompanying ecological benefits, it is necessary to understand the underlying processes of aggregate dynamics in soils. Our objective was to quantify macroaggregate (> 250 μm) rebuilding in soils from loess (Haplic Luvisol) with different initial soil organic C (SOC) contents and different amendments of organic matter (OM) in a short term incubation experiment. Two soils differing in C content and sampled at 0–5 and 5–25 cm soil depths were incubated after macroaggregate destruction. The following treatments were applied: (1) control (without any addition), (2) OM₁ (addition of OM: preincubated wheat straw [< 10 mm, C : N 40.6] at a rate of 4.1 g C [kg soil]^–1), and (3) OM₂ (same as (2) at a rate of 8.2 g C [kg soil]^–1). Evolution of CO₂ released from the treatments was measured continuously, and contents of different water‐stable aggregate‐size classes (> 250 μm, 250–53 μm, < 53 μm), microbial biomass, and ergosterol were determined after 7 and 28 d of incubation. Highest microbial activity was observed in the first 3 d after the OM application. With one exception, > 50% of the rebuilt macroaggregates were formed within the first 7 d after rewetting and addition of OM. However, the amount of organic C within the new macroaggregates was ≈ 2‐ to 3‐fold higher than in the original soil. The process of aggregate formation was still proceeding after 7 d of incubation, however at a lower rate. Contents of organic C within macroaggregates were decreased markedly after 28 d of incubation in the OM₁ and OM₂ treatments, suggesting that the microbial biomass (bacteria and fungi) used organic C within the newly built macroaggregates. Overall, the results confirmed for all treatments that macroaggregate formation is a rapid process and highly connected with the amount of OM added and microbial activity. However, the time of maximum aggregation after C addition depends on the soil and substrate investigated. Moreover, the results suggest that the primary macroaggregates, formed within the first 7 d, are still unstable and oversaturated with OM and therefore act as C source for microbial decomposition processes.     

Extraction of water‐soluble organic matter from mineral horizons of forest soils

Dissolved organic matter (DOM) is involved in many important biogeochemical processes in soil. As its collection is laborious, very often water‐soluble organic matter (WSOM) obtained by extracting organic or mineral soil horizons with a dilute salt solution has been used as a substitute of DOM. We extracted WSOM (measured as water‐soluble organic C, WSOC) from seven mineral horizons of three forest soils from North‐Rhine Westphalia, Germany, with demineralized H₂O, 0.01 M CaCl₂, and 0.5 M K₂SO₄. We investigated the quantitative and qualitative effects of the extractants on WSOM and compared it with DOM collected with ceramic suction cups from the same horizons. The amounts of WSOC extracted differed significantly between both the extractants and the horizons. With two exceptions, K₂SO₄ extracted the largest amounts of WSOC (up to 126 mg C kg^–1) followed by H₂O followed by CaCl₂. The H₂O extracts revealed by far the highest molar UV absorptivities at 254 nm (up to 5834 L mol^–1 cm^–1) compared to the salt solutions which is attributed to solubilization of highly aromatic compounds. The amounts of WSOC extracted did not depend on the amounts of Fe and Al oxides as well as on soil organic C and pH. Water‐soluble organic matter extracted by K₂SO₄ bore the largest similarity to DOM due to relatively analogue molar absorptivities. Therefore, we recommend to use this extractant when trying to obtain a substitute for DOM, but as WSOM extraction is a rate‐limited process, the suitability of extraction procedures to obtain a surrogate of DOM remains ambiguous.     

Minor response of gross N turnover and N leaching to drying,rewetting and irrigation in the topsoil of a Norway spruce forest

Forest floors in the temperate climate zone are frequently subjected to strong changes in soil moisture, but the consequences for the soil N cycle are poorly known. In a field experiment we tested the hypotheses that soil drying leads to a decrease of gross N turnover and that natural rewetting causes a pulse of gross N turnover and an increase of N leaching from the forest floor. A further hypothesis was that optimal water availability induced by irrigation causes maximum N turnover and N leaching. Replicated control, throughfall exclusion and irrigation plots were established in a Norway spruce forest to simulate different precipitation patterns during a growing season. Gross N turnover rates were determined in undisturbed soil cores from Oi + Oe and Oa + EA horizons by the ¹⁵N pool dilution technique. Forest floor percolates were periodically collected by suction plates. After 142 mm throughfall was excluded, the median soil water potential at the throughfall exclusion plots increased from pF 1.9 to 4.5 in the Oi + Oe horizon and from pF 1.8 to 3.8 in the Oa + EA horizon. Gross ammonification ranged from 14 to 45 mg N kg⁻¹ soil day⁻¹ in the Oi + Oe horizon and from 4.6 to 11.4 mg N kg⁻¹ soil day⁻¹ in the Oa + EA horizon. Gross ammonification of both horizons was smallest in the throughfall exclusion plots during the manipulation, but the differences between all treatments were not statistically significant. Gross nitrification in both horizons was very small, ranging from 1.6 to 11.1 mg N kg⁻¹ soil day⁻¹. No effects of decreasing water potential and rewetting on gross nitrification rates were observed because of the small rates and huge spatial variations. Irrigation had no effect as the differences from the control in soil water potential remained small. N leaching from the forest floor was not affected by the treatments. Our findings suggest that ammonification in forest floors continues at considerable rates even at small water potentials. The hypotheses of increased N turnover and N leaching following rewetting of dry forest floor or irrigation were not confirmed.     

Distribution of phosphorus in size fractions of sandy soils with different fertilization histories

A major challenge in sustainable crop management is to ensure adequate P supply for crops, while minimizing losses of P that could negatively impact water quality. The objective of the present study was to investigate the effects of long‐term applications of different levels of mineral fertilizers and farmyard manure on (1) the availability of P, (2) the relationship between soil C, N, and P, and (3) the distribution of inorganic and organic P in size fractions obtained by wet sieving. Soil samples were taken from the top 20 cm of a long‐term (29 y) fertilization trial on a sandy Cambisol near Darmstadt, SW Germany. Plant‐available P, determined with the CAL method, was little affected by fertilization treatment (p < 0.05) and was low to optimal. The concentration of inorganic and organic P extracted with a NaOH‐EDTA solution (P_NaOH‐EDTA) averaged about 350 mg (kg dry soil)^–1, with 42% being in the organic form (P_o). Manure application tended to increase soil C, N, and P_o concentrations by 8%, 9%, and 5.6%, respectively. Across all treatments, the C : N : P_o ratio was 100 : 9.5 : 2 and was not significantly affected by the fertilization treatments. Aggregate formation was weak due to the low clay and organic‐matter content of the soil, and the fractions > 53 μm consisted predominantly of sand grains. The different fertilization treatments had little effect on the distribution of size fractions and their C, N, and P contents. In the fractions > 53 μm, P_NaOH‐EDTA ranged between 200 and 300 mg kg^–1, while it reached 1260 mg kg^–1 in the fraction < 53 μm. Less than one third of P_NaOH‐EDTA was present as P_o in the fractions > 53 μm, while P_o accounted for 70% of P_NaOH‐EDTA in the smallest fraction (< 53 μm). Therefore, 16% and 28% of P_NaOH‐EDTA and P_o, respectively, were associated with the smallest fraction, even though this fraction accounted for < 5% of the soil mass. Therefore, runoff may cause higher P losses than the soil P content suggests in this sandy soil with a weak aggregate formation. Overall, the results indicate that manure and mineral fertilizer had similar effects on soil P fractions.     

Effects of residue location on soil organic matter turnover: results from an incubation experiment with 15N‐maize

Differences in the mechanisms of storage and decomposition of organic matter (OM) between minimum tillage (MT) and conventional tillage (CT) are generally attributed to differences in the physical impact through tillage, but less is known about the effects of residue location. We conducted an incubation experiment at a water content of 60% of the maximum water‐holding capacity and 15°C with soils from CT (0–25 cm tillage depth) and MT fields (0–5 cm tillage depth) with ¹⁵N‐labeled maize straw incorporated to different depths (CT simulations: 0–15 cm; MT simulations: 0–5 cm) for 28 d in order to determine the effects of the tillage simulation on (1) mineralization of recently added residues, (2) the dynamics of macroaggregate formation and physical protection of OM, and (3) the partitioning of maize‐derived C and N within soil OM fractions. The MT simulations showed lower relative C losses, and the amount of maize‐C mineralized after 28 d of incubation was slightly but significantly lower in the MT simulations with maize added (MT_maize) than in the respective CT (CT_maize) simulations. The formation of new water‐stable macroaggregates occurred during the phase of the highest microbial activity, with a maximum peak 8 d after the start of incubation. The newly formed macroaggregates were an important location for the short‐term stabilization of C and N with a higher importance for MT_maize than for CT_maize simulations. In conclusion, our results suggest that a higher amount of OM in MT surface soils compared with CT surface soils may not only result from decreased macroaggregate destruction under reduced tillage but also from a higher efficiency of C retention due to a more concentrated residue input.