Sulphur speciation and turnover in soils: evidence from sulphur K-edge XANES spectroscopy and isotope dilution studies

Sulphur K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to quantify S species in humic substance extracts from ten soils from the UK, China and New Zealand, which differ in land use and agricultural management. XANES spectroscopy showed the presence of most reduced (sulphides, disulphides, thiols and thiophenes), intermediate (sulphoxides and sulphonates) and highly oxidised S (ester sulphates) forms, with the three groups representing 14-32%, 33-50% and 22-53% of the organic S in the humic substance extracts, respectively. Land use had a profound influence on the relative proportions of S species. Well-drained arable soils generally had a higher proportion of organic S present in the most oxidised form than the grassland soils collected nearby, whereas paddy soils showed a more reduced profile due to episodic flooding. In the Broadbalk Classical Experiment at Rothamsted, reversion of an arable system to grassland or woodland in the 1880s resulted in an increase of the most reduced and intermediate S species at the expense of the most oxidised S species. Long-term applications of farmyard manure to an arable plot also shifted S species from the most oxidised to the intermediate and the most reduced species. Sulphur immobilisation and gross mineralisation were determined in seven soils using the ³⁵S isotope dilution method. Gross mineralisation during a 53-day incubation correlated more closely with the amounts of the most reduced and intermediate S species than with the most oxidised S species, suggesting that the former (C-bonded S) were the main source of organic S for mineralisation in the short-term.     

Soil sulphur speciation in two glacier forefield soil chronosequences assessed by S K‐edge XANES spectroscopy

In regions with little atmospheric input of sulphur (S) and S‐poor parent material, the bio‐availability of S, which is dependent on its speciation, may limit ecosystem production and succession. In our study, soil S speciation in two glacier forefield soil chronosequences (Hailuogou Glacier, Gongga Shan, China; Damma Glacier, Swiss Alps) was investigated for the first time. Different S species were quantified by synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy at the S K‐edge. Both chronosequences show similar patterns and pedogenetic trends of their topsoil S status. Topsoil concentrations of total S were correlated with the concentrations of organic carbon and pedogenic Fe/Al oxyhydroxides. Both moraine materials contained inorganic sulphides, which in the topsoil were oxidized within 30 (Hailuogou) or 75 years (Damma) of soil development after deglaciation. About 50% of total S in the fresh moraine material at Hailuogou and 75% of that in the 15 year‐old soil at Damma was organically‐bound. During initial soil development, the contribution of organic S to total S increased at the expense of inorganic sulphide and sulphate, resulting in organic S percentages > 90% of total topsoil S after 30 (Hailuogou) and 75 (Damma) years of pedogenesis. Organic S compounds with electronic oxidation states of the S atom > + 1.5 (sulphoxides, sulphones, sulphonates and ester sulphates) dominated the organic S pool in all soils. Hence, microbial degradation of non‐sulphide organic S (sulphonates and ester sulphates) is probably important to mitigate S scarcity caused by limited availability of SO₄^2?‐S in these soils. Changes in topsoil S speciation during initial stages of pedogenesis and ecosystem succession in glacier forefields under a cool, humid climate appear to be governed by combined effects of mineral weathering (oxidation of inorganic sulphides and formation of S‐adsorbing sesquioxides), accumulation and microbial turnover of soil organic matter and the type of vegetation succession.     

Sulfur speciation in well‐aerated and wetland soils in a forested catchment assessed by sulfur K‐edge X‐ray absorption near‐edge spectroscopy (XANES)

In forested catchments, retention and remobilization of S in soils and wetlands regulate soil and water acidification. The prediction of long‐term S budgets of forest ecosystems under changing environmental conditions requires a precise quantification of all relevant soil S pools, comprising S species with different remobilization potential. In this study, the S speciation in topsoil horizons of a soil toposequence with different groundwater influence and oxygen availability was assessed by synchrotron‐based X‐ray absorption near‐edge spectroscopy (XANES). Our investigation was conducted on organic (O, H) and mineral topsoil (A, AE) horizons of a Cambisol–Stagnosol–Histosol catena. We studied the influence of topography (i.e., degree of groundwater influence) and oxygen availability on the S speciation. Soil sampling and pretreatment were conducted under anoxic conditions. With increasing groundwater influence and decreasing oxygen availability in the sequence Cambisol–Stagnosol–Histosol, the C : S ratio in the humic topsoil decreased, indicating an enrichment of soil organic matter in S. Moreover, the contribution of reduced S species (inorganic and organic sulfides, thiols) increased systematically at the expense of intermediate S species (sulfoxide, sulfite, sulfone, sulfonate) and oxidized S species (ester sulfate, SO ). These results support the concept of different S‐retention processes for soils with different oxygen availability. Sulfur contents and speciation in two water‐logged Histosols subject to permanently anoxic and temporarily oxic conditions, respectively, were very different. In the anoxic Histosol, reduced S accounted for 57% to 67% of total S; in the temporarily oxic Histosol, reduced S was only 43% to 54% of total S. Again, the extent of S accumulation and the contribution of reduced S forms to total S closely reflected the degree of O₂ availability. Our study shows that XANES is a powerful tool to elucidate key patterns of the biogeochemical S cycling in oxic and anoxic soil environments. In contrast to traditional wet‐chemical methods, it particularly allows to distinguish organic S compounds in much more detail. It can be used to elucidate microbial S‐metabolism pathways in soils with different oxygen availability by combining soil inventories and repeated analyses of a sample in different stages of field or laboratory incubation experiments under controlled boundary conditions and also to study (sub)microspatial patterns of S speciation in aggregated soils.     

Sulfur K-edge XANES spectroscopy reveals differences in sulfur speciation of bulk soils, humic acid, fulvic acid, and particle size separates

X-ray absorption near edge structure (XANES) spectra at the sulfur (S) K-edge (E=2472 eV) were compared for bulk soil material, humic and fulvic acid fractions, and different particle size separates from Ah horizons of two arable Luvisols, from an O and a Bs horizon of a Podzol under Norway spruce forest, and from an H horizon of a Histosol (peat bog). In the bulk soil samples, the contribution of reduced organic S (organic mono- and disulfides) to total sulfur increased from 27% to 52%, and the contribution of ester sulfate and SO₄²⁻-S decreased from 39% to 14% of total S in the following order: arable Luvisols Ah—forested Podzol O—Histosol H. This sequence reflects the increasing organic carbon content and the decreasing O₂ availability in that order. Neither sulfonate nor inorganic sulfide was detected in any of the bulk soil samples. For all samples except the Podzol Bs, the XANES spectra of the bulk soils differed considerably from the spectra of the humic and acid fractions of the respective soils, with the latter containing less reduced S (16-44% of total S) and more oxidized S (sulfone S: 19-35%; ester sulfate S: 14-38% of total S). Also the S speciation of most particle size fractions extracted from the Ah horizon of the Viehhausen Luvisol and the Bs horizon of the Podzol was different from that of the bulk soil. For both soils, the contribution of oxidized S species to total S increased and the contribution of sulfoxides and organic mono- and disulfides decreased with decreasing particle size. Thus, sulfur K-edge XANES spectra of alkaline soil extracts, including humic and fulvic acids or of particle size separates are not representative for the S speciation of the original soil sample they are derived from. The differences can be attributed to (i) artificial changes of the sulfur speciation during alkaline extraction (conversion of reduced S into oxidized S, loss of SO₄²⁻ during purification of the extracts by dialysis) or particle size separation (carry-over of water-soluble S, such as SO₄²⁻), but also to (ii) preferential enrichment of oxidized S in hydrophilic water-soluble soil organic matter (ester sulfate) and in the clay fraction of soils (ester sulfate, adsorbed SO₄²⁻).     

Speciation of phosphorus in temperate zone forest soils as assessed by combined wet‐chemical fractionation and XANES spectroscopy

Phosphorus availability in terrestrial ecosystems is strongly dependent on soil P speciation. Here we present information on the P speciation of 10 forest soils in Germany developed from different parent materials as assessed by combined wet‐chemical P fractionation and synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy. Soil P speciation showed clear differences among different parent materials and changed systematically with soil depth. In soils formed from silicate bedrock or loess, Fe‐bound P species (FePO₄, organic and inorganic phosphate adsorbed to Fe oxyhydroxides) and Al‐bound P species (AlPO₄, organic and inorganic phosphate adsorbed to Al oxyhydroxides, Al‐saturated clay minerals and Al‐saturated soil organic matter) were most dominant. In contrast, the P speciation of soils formed from calcareous bedrock was dominated (40–70% of total P) by Ca‐bound organic P, which most likely primarily is inositol hexakisphosphate (IHP) precipitated as Ca₃‐IHP. The second largest portion of total P in all calcareous soils was organic P not bound to Ca, Al, or Fe. The relevance of this P form decreased with soil depth. Additionally, apatite (relevance increasing with depth) and Al‐bound P were present. The most relevant soil properties governing the P speciation of the investigated soils were soil stocks of Fe oxyhydroxides, organic matter, and carbonate. Different types of P speciation in soils on silicate and calcareous parent material suggest different ecosystem P nutrition strategies and biogeochemical P cycling patterns in the respective ecosystems. Our study demonstrates that combined wet‐chemical soil P fractionation and synchrotron‐based XANES spectroscopy provides substantial novel information on the P speciation of forest soils.     

Sulphur speciation and biogeochemical cycling in long-term arable cropping of subtropical soils: evidence from wet-chemical reduction and S K-edge XANES spectroscopy

Agriculture has claimed a large share of terrestrial environments in the tropics and subtropics through cultivation of native grasslands or forests. The impact of this anthropogenic change on speciation, dynamics, and ecological significance of sulphur (S) compounds is still poorly understood. We combined degradative wet-chemical reduction and S K-edge X-ray absorption near edge structure (XANES) spectroscopy techniques to evaluate the impact of long-term agricultural management of native grassland soils in South African Highveld on the amount, form and dynamics of S species. Sulphur XANES in the humic substances extracted by 0.1 m NaOH/0.4 m NaF solution showed the presence of strongly reduced (polysulphides, disulphides, thiols, monosulphides and thiophenes), intermediate (sulphoxides and sulphonates) and strongly oxidized (ester sulphates) organic-S. It showed that strongly oxidized-S is the predominant form (39–54%) of the total organic-S in the humic substances, and organic-S in the intermediate oxidation state represented 30–37% (78–93% of which was attributed to sulphonates). The strongly reduced organic-S comprised only 17–24% of the total organic-S. We did not find a close correlation between the results of a degradative wet-chemical procedure and XANES spectroscopy conducted in both the bulk soils (ester SO₄-S from XANES versus HI-fractionation, r = 0.27; P < 0.05) and the humic substance extracts (ester SO₄-S from XANES versus HI-fractionation, r= 0.39; P < 0.05). The ratio of reduced-S to strongly oxidized-S (R-S/O-S) in the humic substances decreased from 0.61 to 0.21, while the ratio of intermediate-S to strongly oxidized-S (I-S/O-S) declined from 0.93 to 0.61 after 90 years of arable cropping of the native grassland soils. Hence, there was a shift in oxidation state towards strongly oxidized-S (+6) and, thereby, a change in the relative proportion of the organic-S moieties associated with each oxidation state following cultivation of the native grassland soils. Therefore, we conclude that changes in land-use practice brought about not only quantitative change but also altered the composition of organic-S functional groups in these native subtropical grassland soils.     

Speciation of sulphur in soils and soil particles by X-ray spectromicroscopy

Current wet chemical methods for the speciation of sulphur (S) in soils are inaccurate and do not allow one to assess the S speciation of individual soil particles and colloids. X-ray microscopy and Near Edge X-ray Absorption Fine structure Spectroscopy (NEXAFS) can be used to study individual species of S at the K-adsorption edge. We have used these techniques to identify and quantify S species in bulk soil, soil particles and colloids from Oh and Bh horizons of two forested Podzols. The partitioning of soil sulphur as determined on bulk samples of the Oh horizons by X-ray spectromicroscopy agreed fairly well with the results of a conventional S speciation for the soil at Schluchsee, and reasonably well for that at Rotherdbach. The NEXAFS analyses on individual soil particles revealed that they are richer in reduced organic sulphur than the bulk soil for the Schluchsee Oh and richer in sulphate for Rotherdbach Oh. The techniques can be used reliably to separate and quantify sulphur species with different oxidation states in the soil. The combination of X-ray transmission and sulphur fluorescence images with unfocused and focused NEXAFS spectra at the K-adsorption edge of sulphur at specific microsites allowed us to compare the distribution of S species in bulk soil with that of distinct soil particles and soil colloids. Moreover, we can use it to assess the spatial distribution of different S species on soil particles on a scale of a few hundred nanometres.     

Differentiation between adsorbed and precipitated sulphate in soils and at micro-sites of soil aggregates by sulphur K-edge XANES

To investigate the potential of synchrotron‐based X‐ray Absorption Near‐Edge Structure spectroscopy (XANES) at the sulphur (S) K‐edge for a discrimination of adsorbed and precipitated sulphate in soils and soil particles, XANES spectra of ionic sulphate compounds and Al/Fe hydroxy sulphate minerals were compared with spectra of SO₄^2? adsorbed to ferrihydrite, goethite, haematite, gibbsite or allophane. Ionic sulphate and hydroxy sulphate precipitates had broader white‐lines (WL) at 2482.5 eV (full width at half maximum (FWHM) of edge‐normalized spectra, 2.4–4.2 eV; Al hydroxy sulphates, 3.0 eV) than SO₄^2? adsorbed to Al/Fe oxyhydroxides or allophane (FWHM, 1.8–2.4 eV). The ratio of the white‐line (WL) height to the height of the post‐edge feature at 2499 eV (WL/PEF) was larger for SO₄^2? adsorbed to Al/Fe oxyhydroxides or allophane (8.1–11.9) than for Al/Fe hydroxy sulphates and ionic sulphates (3.9–5.7). The WL/PEF ratio of edge‐normalized S K‐edge XANES spectra can be used to distinguish adsorbed from precipitated SO₄^2? in soils and also at microsites of soil particles. The contribution of adsorbed and precipitated SO₄^2? to the total SO₄^2? pool can be roughly quantified. Adsorbed ester sulphate may result in overestimation of precipitated SO₄^2?. The spectra of most soils could be fitted by linear combination fitting (LCF), yielding a similar partitioning between adsorbed and precipitated SO₄^2? as an evaluation of the WL/PEF ratio. The SO₄^2? pool of German forest soils on silicate parent material in most cases was strongly dominated by adsorbed SO₄^2?; however, in three German forest soils subject to elevated atmospheric S deposition, a considerable portion of the SO₄^2? pool was precipitated SO₄^2?, most likely Al hydroxy sulphate. The same is true for Nicaraguan Eutric and Vitric Andosols subject to high volcanogenic S input. In the subsoil of the Vitric Andosol, adsorbed SO₄^2? and Al hydroxy sulphate coexist on a micron scale.     

Iron speciation in soils and soil aggregates by synchrotron-based X-ray microspectroscopy (XANES,μ-XANES)

Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in soils (pyrite, ferrihydrite, goethite), soils in a forested watershed which constitute a toposequence with a hydrological gradient (Dystric Cambisol, Dystric Planosol, Rheic Histosol), and microsites of a dissected soil aggregate by X‐ray Absorption Near Edge Spectroscopy (XANES) at the iron K‐edge (7112 eV) to identify different Fe(II) and Fe(III) components. We calculated the pre‐edge peak centroid energy of all spectra and quantified the contribution of different organic and inorganic Fe‐bearing compounds by Linear Combination Fitting (LCF) conducted on the entire spectrum (E = 7085–7240 eV) and on the pre‐edge peak. Fe‐XANES conducted on organic and inorganic standards and on synthetic mixtures of pyrite, ferrihydrite and goethite showed that by calculating the pre‐edge peak centroid energy, the Fe(II)/Fe(III) ratio of different Fe‐bearing minerals (Fe sulphides, Fe oxyhydroxides) in mineral mixtures and soils can be quantified with reasonable accuracy. A more accurate quantification of the Fe(II)/Fe(III) ratio was possible with LCF conducted on the entire XANES spectrum. For the soil toposequence, an increased groundwater influence from the Cambisol to the Histosol was reflected in a larger contribution of Fe(II) compounds (Fe(II) silicate, Fe monosulphide, pyrite) and a smaller contribution of Fe(III) oxyhydroxides (ferrihydrite, goethite) to total iron both in the topsoil and the subsoil. In the organic topsoils, organically bonded Fe (33–45% of total Fe) was 100% Fe(III). For different microsites in the dissected aggregate, spatial resolution ofμ‐XANES revealed different proportions of Fe(II) and Fe(III) compounds. Fe K‐edge XANES andμ‐XANES allows an approximate quantification of Fe(II) and Fe(III) and different Fe compounds in soils and (sub)micron regions of soil sections, such as mottles, concretions, and rhizosphere regions, thus opening new perspectives in soil research.     

Long‐term effects of fertilization on some soil properties under rainfed soybean‐wheat cropping in the Indian Himalayas

This study aims to examine the effects of long‐term fertilization and cropping on some chemical and microbiological properties of the soil in a 32 y old long‐term fertility experiment at Almora (Himalayan region, India) under rainfed soybean‐wheat rotation. Continuous annual application of recommended doses of chemical fertilizer and 10 Mg ha^–1 FYM on fresh‐weight basis (NPK + FYM) to soybean (Glycine max L.) sustained not only higher productivity of soybean and residual wheat (Triticum aestivum L.) crop, but also resulted in build‐up of total soil organic C (SOC), total soil N, P, and K. Concentration of SOC increased by 40% and 70% in the NPK + FYM–treated plots as compared to NPK (43.1 Mg C ha^–1) and unfertilized control plots (35.5 Mg C ha^–1), respectively. Average annual contribution of C input from soybean was 29% and that from wheat was 24% of the harvestable aboveground biomass yield. Annual gross C input and annual rate of total SOC enrichment from initial soil in the 0–15 cm layer were 4362 and 333 kg C ha^–1, respectively, for the plots under NPK + FYM. It was observed that the soils under the unfertilized control, NK and N + FYM treatments, suffered a net annual loss of 5.1, 5.2, and 15.8 kg P ha^–1, respectively, whereas the soils under NP, NPK, and NPK + FYM had net annual gains of 25.3, 18.8, and 16.4 kg P ha^–1, respectively. There was net negative K balance in all the treatments ranging from 6.9 kg ha^–1 y^–1 in NK to 82.4 kg ha^–1 y^–1 in N + FYM–treated plots. The application of NPK + FYM also recorded the highest levels of soil microbial‐biomass C, soil microbial‐biomass N, populations of viable and culturable soil microbes.     

近三十年农田土壤磷分子形态的研究进展

农田土壤磷的赋存形态决定迁移、转化及归趋过程,单单通过全磷或有效磷含量并不能全面、准确、长效地评估土壤磷的养分供应能力和生态环境风险,探索可持续的农田磷素管理措施迫切需要能够科学表征、准确认识土壤磷形态。随着分析测试技术发展,农田土壤磷形态领域经历了以传统连续提取法为主的分级组分研究,到目前基于先进光谱技术的分子形态研究的发展历程。液相磷-31核磁共振技术(P-NMR)、基于同步辐射的X射线吸收近边结构谱技术(P-XANES)是当今土壤磷分子形态表征的主流技术,分别促进土壤多种有机磷和无机磷(铁磷/钙磷/铝磷)分子形态的有效识别。借助Histcite软件进行引文网络分析,梳理了近三十年(1990—2019年)土壤磷分子形态研究发展历程中具有重大借鉴意义的关键性成果,基于此综述了该领域的发展脉络,归纳发现农田土壤磷分子形态研究最初主要借助P-NMR技术侧重有机磷分子形态表征,而后过渡至与同步辐射XANES以及X射线微探针技术相结合,实现了土壤磷分子形态的全面认识。最后,对多谱学技术联用推动土壤磷分子形态研究的发展趋势进行了展望。     

Phosphorus in sequentially extracted fen peat soils: A K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy study

The speciation of phosphorus (P) in native and degraded peat soils is an analytical challenge, and synchrotron‐based P K‐edge X‐ray absorption near‐edge structure (XANES) is a suitable method to gain information on P species in soils and organic materials. The objective of the present study was to test if P K‐edge XANES reflected differences in P fractions in fen peat due to sequential extraction and peat degradation. We investigated each one top‐ and subsoil sample of a Fibric Histosol, which differed in the degree of humification (H8 vs. H5) and concentration of total P (P_t) (1944 mg kg^–1 vs. 436 mg kg^–1). In the topsoil, residual P, H₂SO₄‐P, and NaOH‐P accounted for roughly the same proportions of P_t (≈30%). In the subsoil, residual P (64% of P_t) was more abundant than NaOH‐P (21% of P_t) and H₂SO₄‐P (10% of P_t). Among many different P reference standards, the P XANES spectra reflected differences in mineral P more distinctive than in organic P compounds. Phosphorus XANES spectra of the residues after each sequential extraction step all showed a prominent white‐line peak at around 2152 eV. Stepwise removal of resin‐P, NaHCO₃‐P, and NaOH‐P were reflected mainly by the peak intensity but scarcely by distinct spectral features. Extraction with H₂SO₄ led to the disappearance of spectral features of Ca and Mg phosphates which is a first direct hint to these compounds in the peat. In conclusion, a combined sequential fractionation and spectroscopic (³¹P NMR, P K‐ and L‐edge XANES with linear‐combination fits) approach is proposed to overcome limitations of the present study and gain more insight into the P species in peat soils.     

The role of clay, organic carbon and long-term management on mouldboard plough draught measured on the Broadbalk wheat experiment at Rothamsted

Despite a high energy requirement, the mouldboard plough remains the dominant tillage tool in northwest Europe. The aim of this work was to evaluate the relative influences of soil texture (clay content), soil organic carbon (SOC) and long‐term management on soil‐specific draught (S), where S is the force per cross‐sectional area of worked soil. Measurements were made during autumn 2000 on the then 157‐year‐old Broadbalk wheat experiment at Rothamsted, UK, where clay contents vary from 19 to 39% and the different cropping history, mineral and organic fertilizer treatments lead to SOC values of 0.7–3.2%. Minimum SOC values increased with increasing clay and were associated with zero or low mineral N inputs, while higher SOC values (>2%) were associated with long‐term applications of farmyard manure (FYM), despite these being on the lighter (<24% clay) soils. S values ranged between 52 and 142 kPa, with higher values co‐located in areas with high clay contents. Contour maps were generated to illustrate the spatial variability of S and show similarity to those for clay. Where FYM had been added, S was 66 kPa compared with 74 kPa where only mineral or no fertilizer was applied on soils of the same texture. Increasing applications of mineral N resulted in relatively small increases in SOC but up to 12% reduction in S.     

Phosphorus speciation of forest‐soil organic surface layers using P K‐edge XANES spectroscopy

The phosphorus (P) speciation of organic surface layers from two adjacent German forest soils with different degree of water‐logging (Stagnosol, Rheic Histosol) was analyzed by P K‐edge XANES and subsequent Linear Combination Fitting. In both soils, ≈ 70% of the P was inorganic phosphate and ≈ 30% organic phosphate; reduced P forms such as phosphonate were absent. The increased degree of water‐logging in the Histosol compared to the Stagnosol did not affect P speciation.     

Long‐term effects of fertilizer and manure applications on soil quality and yields in a sub‐humid tropical rice‐rice system

Widespread yield stagnation and productivity declines in the rice–rice cropping system have been reported and many of the associated issues are related to soil quality. A long‐term experimental study was initiated in 1969 to assess the impact of continuous cultivation of rice as a single crop grown in wet as well as dry seasons using varying levels of chemical fertilizer and manure applications on soil quality indicators (physical, chemical and biological), a sustainable yield index (SYI) and a soil quality index (SQI). The treatments comprised chemical fertilizers and farmyard manure (FYM) either alone or in combination viz. control, N, NP, NK, NPK, FYM, N+FYM, NP+FYM, NK+FYM and NPK+FYM, laid out in a randomized complete block design with three replications. Soil samples were collected after the wet season rice harvest in 2010 and were analysed for physical, chemical and biological indicators of soil quality. A SYI based on long‐term yield data and SQI using principal component analysis (PCA) and nonlinear scoring functions were calculated. Application of NPK fertilizers in combination with FYM significantly increased the average grain yield of rice in both wet and dry seasons and enhanced the sustainability of the system compared to the control and plots in receipt of fertilizers. The SYI for the control was higher in the wet season than in the dry one, whereas the reverse was true for NPK+FYM treatment. The value of the dimensionless SQI varied from 1.46 in the control plot to 3.78 in the NPK+FYM one. A greater SYI and SQI in the NPK+FYM treatment demonstrated the importance of using a chemical fertilizer in combination with FYM. For the six soil quality indicators selected as a minimum data set (MDS), the contribution of DTPA‐Zn, available‐N and soil organic carbon to the SQI was substantial ranging from 59.4 to 85.7 per cent in NPK+FYM and control plots, respectively. Thus, these soil parameters could be used to monitor soil quality in a subhumid tropical rice–rice system.     

Effect of long‐term fertilization and manuring on potassium release properties in a Typic Ustochrept

A long‐term fertilizer experiment, over 27 years, studied the effect of mineral fertilizers and organic manures on potassium (K) balances and K release properties in maize‐wheat‐cowpea (fodder) cropping system on a Typic Ustochrept. The treatments consisted of control, 100% nitrogen (100% N), 100% nitrogen and phosphorus (100% NP), 50% nitrogen, phosphorus, and potassium (50% NPK), 100% nitrogen, phosphorus, and potassium (100% NPK), 150% nitrogen, phosphorus, and potassium (150% NPK), and 100% NPK+farmyard manure (100% NPK+FYM). Nutrients N, P, and K in 100% NPK treatment were applied at N: 120 kg ha^—1, P: 26 kg ha^—1, and K: 33 kg ha^—1 each to maize and wheat crops and N: 20 kg ha^—1, P: 17 kg ha^—1, and K: 17 kg ha^—1 to cowpea (fodder). In all the fertilizer and manure treatments removal of K in the crop exceeded K additions and the total soil K balance was negative. The neutral 1 N ammonium acetate‐extractable K in the surface soil (0—15 cm) ranged from 0.19 to 0.39 cmol kg^—1 in various treatments after 27 crop cycles. The highest and lowest values were obtained in 100% NPK+FYM and 100% NP treatments, respectively. Non‐exchangeable K was also depleted more in the treatments without K fertilization (control, 100% N, and 100% NP). Parabolic diffusion equation could describe the reaction rates in CaCl₂ solutions. Release rate constants (b) of non‐exchangeable K for different depth of soil profile showed the variations among the treatments indicating that long‐term cropping with different rates of fertilizers and manures influenced the rate of K release from non‐exchangeable fraction of soil. The b values were lowest in 100% NP and highest in 100% NPK+FYM treatment in the surface soil. In the sub‐surface soil layers (15—30 and 30—45 cm) also the higher release rates were obtained in the treatments supplied with K than without K fertilization indicating that the sub‐soils were also stressed for K in these treatments.     

Effects of humus content,farmyard manuring,and mineral‐N fertilization on yields and soil properties in a long‐term trial

Investigations carried out at Field F3 of the Halle long‐term fertilization trials using data from 1974 to 1983 showed that with adequate supply of mineral N‐fertilizer soil organic matter (SOM) had no significant effects of yield. Similarly enhanced SOM did not justify a reduction of mineral N (Stumpe et al., 2000). The studies presented here examine the effects of the SOM differences existing after the termination of those trials in 1986 up until 1997 (then mainly differences of hardly decomposable SOM) in comparison to farmyard manuring with enhanced mineral N application (3‐factor‐experiment). As with total SOM, hardly decomposable SOM did not directly affect yields. The effects of FYM treatment observed at lower mineral‐N levels were compensated for by enhanced mineral‐N supply. The direct effect of FYM (40 t ha^—1) corresponded to a mineral‐N supply of about 60 kg ha^—1 and the residual effect to about 20 kg ha^—1. The differences of the C‐content in the soil at the beginning of the present studies continued throughout the experimental period of 12 years. In addition, significant differentiation has been caused by FYM and N fertilization in comparison to unfertilized treatments. The major finding is that differences in SOM content do not lead to yield differences on physically good soils (chernozem‐like soils) if appropriate compensation by mineral‐N fertilization takes place.     

Long‐term fertilization impacts on corn yields and soil organic matter on a clay‐loam soil in Northeast China

A long‐term fertilization experiment with monoculture corn (Zea mays L.) was established in 1980 on a clay‐loam soil (Black Soil in Chinese Soil Classification and Typic Halpudoll in USDA Soil Taxonomy) at Gongzhuling, Jilin Province, China. The experiment aimed to study the sustainability of grain‐corn production on this soil type with eight different nitrogen (N)‐, phosphorus (P)‐, and potassium (K)–mineral fertilizer combinations and three levels (0, 30, and 60 Mg ha^–1 y^–1) of farmyard manure (FYM). On average, FYM additions produced higher grain yields (7.78 and 8.03 Mg ha^–1) compared to the FYM0 (no farmyard application) treatments (5.67 Mg ha^–1). The application of N fertilizer (solely or in various combinations with P and K) in the FYM0 treatment resulted in substantial grain‐yield increases compared to the FYM0 control treatment (3.56 Mg ha^–1). However, the use of NP or NK did not yield in any significant additional effect on the corn yield compared to the use of N alone. The treatments involving P, K, and PK fertilizers resulted in an average 24% increase in yield over the FYM0 control. Over all FYM treatments, the effect of fertilization on corn yield was NPK > NP = NK = N > PK = P > K = control. Farmyard‐manure additions for 25 y increased soil organic‐matter (SOM) content by 3.8 g kg^–1 (13.6%) in the FYM1 treatments and by 7.8 g kg^–1 (27.8%) in the FYM2 treatments, compared to a 3.2 g kg^–1 decrease (11.4%) in the FYM0 treatments. Overall, the results suggest that mineral fertilizers can maintain high yields, but a combination of mineral fertilizers plus farmyard manure are needed to enhance soil organic‐matter levels in this soil type.     

Physical properties of a Luvisol for different long‐term fertilization treatments: I. Mesoscale capacity and intensity parameters

The physical properties of a Luvisol derived from loess near Bonn, Germany, under different long‐term fertilization treatments were examined. For the investigation of the impact of farmyard manure (FYM) on soil strength at the mesoscale (100 to 300 cm³ soil cores), undisturbed samples were taken from two different depths (10 and 40 cm), either with no fertilization at all, with full mineral fertilization, with FYM only, and with both mineral and organic fertilization. We investigated hydraulic and mechanical parameters, namely precompression stress, pore‐size distribution, saturated hydraulic and air conductivity, and calculated pore connectivity. Long‐term organic fertilization resulted in significantly more and coarser pores which in addition were more conductant and mechanically stronger by trend. Mineral fertilization also increased pore volume by trend but not pore functionality. Mechanical strength generally increased with fertilization by trend, however, was reduced again when organic and mineral fertilization were combined. Nonetheless, FYM led to relatively higher soil strength as the FYM‐treated plots with lower bulk density attained similar soil strength as the unfertilized but denser plots and thus supported the soil‐improving impact of organic amendments. The subsoil physical properties were rather unaffected by fertilization, but were dominated by texture.     

Influence of Long‐Term Sodic‐Water Irrigation,Gypsum, and Organic Amendments on Soil Properties and Nitrogen Mineralization Kinetics under Rice–Wheat System

Abstract

Influence of long‐term sodic‐water (SW) irrigation with or without gypsum and organic amendments [green manure (GM), farmyard manure (FYM), and rice straw (RS)] on soil properties and nitrogen (N) mineralization kinetics was studied after 12 years of rice–wheat cropping in a sandy loam soil in northwest India. Long‐term SW irrigation increased soil pH, exchangeable sodium percentage (ESP), and sodium adsorption ratio (SAR) and decreased organic carbon (OC) and total N content. On the other hand, application of gypsum and organic amendments resulted in significant improvement in all these soil properties. Mineralization of soil N ranged from 54 to 111 mg N kg^?1 soil in different treatments. Irrigation with SW depressed N mineralization. In SW‐irrigated plots, two flushes of N mineralization were observed; the first during 0 to 7 d and the second after 28 d. Amending SW irrigated plots with GM and FYM enhanced mineralization of soil N. Gypsum application along with SW irrigation reduced cumulative N mineralization at 56 days in RS‐amended plots but increased it under GM‐treated, FYM‐treated, or unamended plots. Nitrogen mineralization potential (N_o) ranged from 62 to 543 mg N kg^?1 soil. In the first‐order zero‐order model (FOZO), the easily decomposable fraction ranged from 5.4 to 42 mg N kg^?1 soil. Compared to the first‐order single compartment model, the FOZO model could better explain the variations in N mineralization in different treatments. Variations in N_o were influenced more by changes in pH, SAR, and ESP induced by long‐term SW irrigations and amendments rather than by soil OC.