添加黏土及植物残体混合物对沙质土壤中呼吸作用和微生物量碳、氮有效性的影响

Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil(7% clay) was amended with a clay-rich subsoil(73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control(sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg~(-1): mature wheat(Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean(Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions(0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C(MBC), available N, and p H on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency(cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil p H were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and p H were influenced by clay addition to the sandy soil studied.     

添加调节N和P浓度后的植物残体对土壤呼吸作用、微生物量和养分有效性的影响

Microbial activity and nutrient release are known to be influenced by organic matter properties,but it is difficult to separate the effect of C/N ratio from that of C/P ratio because in most plant residues both ratios are either high or low.An incubation experimeut was conducted to investigate the effects of reducing the C/N and C/P ratios of slowly decomposable plant residues (young eucalyptus leaves,mature wheat straw,and sawdust) to those of rapidly decomposable residues (young kikuyu shoots) on soil respiration,microbial biomass,and N and P availability.The C/N and C/P ratios of the former were adjusted to 15 and 89,respectively,by adding N as (NH4)2SO4,P as KH2PO4 or both and residues were added at 10 g C kg-1 to a silt loam.Soil respiration was measured over 21 d;microbial biomass C (MBC) and available N and P were measured on days 0,7,and 21.Compared to the unamended soil,addition of kikuyu increased cumulative respiration 20-fold,MBC concentration 4 to 8-fold,and available P concentration up to 4-fold,whereas the increase in available N concentration was small and transient.Cumulative respiration and MBC concentration were low in the sawdust-amended soil and were not influenced by reducing the C/N and C/P ratios.Cumulative respiration with original wheat and eucalyptus was 30％-40％ of that with kikuyu.Reducing the C/N ratio alone or both C/N and C/P ratios increased cumulative respiration and MBC concentration 2-fold compared to the original wheat and eucalyptus,whereas reducing the C/P ratio had little effect.Throughout the experiment,the available N concentration after addition of residues with reduced C/N ratio increased in the following order of eucalyptus ＜ wheat ＜ sawdust.By independently lowering the C/N and C/P ratios,microbial activity was more limited by C and N than P.However,lowering the C/N ratio of very slowly decomposable sawdust had no effect on soil respiration and MBC concentration,suggesting that other properties such as concentration of poorly decomposable C compounds limited decomposition.     

盐分对土壤呼吸和有机质动力学的影响与土壤电导率相关，而与渗透势的关系更为密切

Osmotic potential (OP) of soil solution may be a more appropriate parameter than electrical conductivity (EC) to evaluate the effect of salts on plant growth and soil biomass.However,this has not been examined in detail with respect to microbial activity and dissolved organic matter in soils with different texture.This study evaluated the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils with different texture.Four non-saline and non-sodic soils differing in texture (S-4,S-13,S-24 and S-40 with 4％,13％,24％ and 40％ clay,respectively) were leached using combinations of 1 mol L-1 NaC1 and 1 mol L-1 CaC12 stock solutions,resulting in EC (1:5 soil:water ratio) between 0.4 and 5.0 dS m-1 with two levels of sodicity (sodium absorption ratio (SAR) ＜ 3 (non-sodic) and 20 (sodic),1:5 soil:water ratio).Adjusting the water content to levels optimal for microbial activity,which differed among the soils,resulted in four ranges of OP in all the soils:from-0.06 to--0.24 (controls,without salt added),-0.55 to-0.92,-1.25 to-1.62 and-2.77 to-3.00 Mpa.Finely ground mature wheat straw (20 g kg-1) was added to stimulate microbial activity.At a given EC,cumulative soil respiration was lower in the lighter-textured soils (S-4 and S-13) than in the heavier-textured soils (S-24 and S-40).Cumulative soil respiration decreased with decreasing OP to a similar extent in all the soils,with a greater decrease on Day 40 than on Day 10.Cumulative soil respiration was greater at SAR =20 than at SAR ＜ 3 only at the OP levels between-0.62 and-1.62 MPa on Day 40.In all the soils and at both sampling times,concentrations of dissolved organic C and N were higher at the lowest OP levels (from-2.74 to-3.0 MPa) than in the controls (from-0.06 to-0.24 MPa).Thus,OP is a better parameter than EC to evaluate the effect of salinity on dissolved organic matter and microbial activity in different textured soils.     

Toxicity of Cu and Cd in Soils and Detoxication by Additives

Results from laboratory experiments indicated that the concentrations and toxicities of both water-soluble and 0.1 M HCl-extractable Cu and Cd from soils were in the order of red soil> yellow brown earth> black earth. The toxicity of soil varied with the concentrations of metals. The form, concentration and toxicity of Cu and Cd in soils were determined by cation exchange capacity, content of organic matter and composition of clay minerals in the soil. Addition of CaCO3 could significantly decrease the concentration and toxicity of water-soluble and 0.1 M HCl-extractable Cu or Cd from the red soil, and could notably transform the Cu and Cd from the water-soluble or exchangeable form into the organic, free oxides-occluded or sulfic form.     

聚合果属生物炭引起的咸水灌溉下土壤养分有效性和番茄生长变化

Thermally modified organic materials commonly known as biochar have gained popularity of being used as a soil amendment.Little information, however, is available on the role of biochar in alleviating the negative impacts of saline water on soil productivity and plant growth. This study, therefore, was conducted to investigate the effects of Conocarpus biochar(BC) and organic farm residues(FR) at different application rates of 0.0%(control), 4.0% and 8.0%(weight/weight) on yield and quality of tomatoes grown on a sandy soil under drip irrigation with saline or non-saline water. The availability of P, K, Fe, Mn, Zn and Cu to plants was also investigated. The results demonstrated clearly that addition of BC or FR increased the vegetative growth, yield and quality parameters in all irrigation treatments. It was found that salt stress adversely affected soil productivity, as indicated by the lower vegetative growth and yield components of tomato plants. However, this suppressing effect on the vegetative growth and yield tended to decline with application of FR or BC, especially at the high application rate and in the presence of biochar. Under saline irrigation system, for instance, the total tomato yield increased over the control by 14.0%–43.3% with BC and by 3.9%–35.6% with FR. These could be attributed to enhancement effects of FR or BC on soil properties, as indicated by increases in soil organic matter content and nutrient availability. Therefore, biochar may be effectively used as a soil amendment for enhancing the productivity of salt-affected sandy soils under arid conditions.     

中国湖南省主要水稻土类型的氨固定

The contents, affecting factors, seasonal changes and availability of fixed ammonium in major types ofpaddy soils derived from different parent materials in Hunan Province, China, were studied using the Silva-Bremner method by laboratory and pot experiments. Results showed that the content of fixed ammoniumin the plough horizons ranged from 88.3 mg kg-1 to 388.1 mg kg-1, with 273.2 ± 77.7 mg kg-1 on average,accounting for 11.2% of total soil N on average. Content of fixed ammonium decreased in the order of newlylacustrine clayey paddy soil ＞ alluvial sandy paddy soil ＞ purple clayey paddy soil ＞ newly alluvial sandypaddy soil ＞ yellow clayey paddy soil ＞ reddish-yellow clayey paddy soil ＞ granitic sandy paddy soil. Therewere four distribution patterns of fixed ammonium in the profiles to 1-m depth, i.e., increase with the depth,decrease with increasing depth, no distinct change with the depth, and abrupt increase or decrease in somehorizon. Percentage of fixed ammonium in total N increased with the depth in most of the soils. Fixationof NH4+ by soil was higher at 30 ℃ than at 20 ℃ and 40 ℃, and continuous submergence benefited thefixation of NH4+ in newly alluvial sandy paddy soil, purple clayey paddy soil and alluvial sandy paddy soil,while alternating wetting and drying contributed to the fixation of NH4+ in yellow clayey paddy soil mostly.Fixed ammonium content in the test paddy soils was significantly correlated with ＜ 0.01 mm clay content(P ＜ 0.05), but not with ＜ 0.001 mm clay content, total N, organic N and organic matter. Fixed ammoniumcontent varied with rice growth stages. Application of N fertilizer promoted fixation of NH4+ by soil, and Nuptake by rice plant promoted release of fixed ammonium from the soil. Recently fixed ammonium in paddysoil after N fertilizer application was nearly 100% available to rice plant, while native fixed ammonium wasonly partly available, varying with the soil type and rice type.     

重复添加植物残体后土壤微生物活性和生物量对盐度的响应特征

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes,which requires high amounts of energy;however,a single addition of plant residues can only temporarily improve energy supply to soil microbes.Therefore,a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract（EC_e） of 0.6 dS m^-1.The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve EC_e of 12.5,25.0 and 50.0 dS m^-1.The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg^-1 soil on days 0,15 and29.The soils receiving no residues were included as a control.Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg^-1 soil and higher in the non-saline soil than in the saline soils.In the saline soils,the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with3.9 g C kg^-1 at EC_e of 50 dS m^₁.Though with the same amount of C added(7.8 g C kg^-1),salinity reduced soil respiration to a lesser extent when 3.9 g C kg^-1 was added twice compared to a single addition of 7.8 g C kg^-1.After the third residue addition,the microbial biomass C concentration was significantly lower in the soils with EC_e of 25 and 50 dS m^₁ than in the non-saline soil at3.9 g C kg^-1,but only in the soil with EC_e of 50 dS m^-1 at 7.8 g C kg^-1.We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity,which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.     

沙粒中添加Ca2+、粘土以及富粘土对水溶性有机碳呼吸作用及其吸附的影响研究

Clay addition to light-textured soils is used to ameliorate water repellency and to increase nutrient retention. However, clay addition may also increase the potential to bind organic matter and thus C sequestration. Divalent calcium ions （Ca2＋） play an important role in binding of organic matter to clay because they provide the bridge between the clay particles and organic matter which are both negatively charged. In the first experiment, quartz sand was mixed with clay isolated from a Vertosol at rates of 0, 50 and 300 g kg-1, finely ground mature wheat residues （20 g kg-1） and powdered CaSO4 at 0, 5 and 10 g kg-1. Soil respiration was measured over 28 d. Compared to the sand alone, addition of isolated clay at 300 g kg-1 increased cumulative respiration with a stronger increase than that at 50 g kg-1. Addition of CaSO4 increased electrical conductivity, decreased sodium adsorption ratio and reduced cumulative respiration. The latter can be explained by enhanced sorption of organic matter to clay via Ca2＋ bridges. In a second experiment, isolated clay or subsoil of the Vertosol without or with powdered CaSO4 at 10 g kg-1 were used for a batch sorption with water-extractable organic C （WEOC） from wheat straw followed by desorption with water. Addition of 10 g kg-1 CaSO4 increased sorption and decreased desorption of WEOC in both subsoil and isolated clay. In the third experiment, subsoil of the Vertosol was used for a batch sorption in which WEOC was added repeatedly. Repeated addition of WEOC increased the concentration of sorbed C but decreased the sorbed proportion of the added WEOC. This indicates that sorption of WEOC may be underestimated if it is added only once in batch sorption experaments.     

Temperature sensitivity of soil organic matter decomposition varies with biochar application and soil type

Biochar application has the potential to improve soil fertility and increase soil carbon stock, especially in tropical regions. Information on the temperature sensitivity of carbon dioxide(CO_2) evolution from biochar-amended soils at very high temperatures, as observed for tropical surface soils, is limited but urgently needed for the development of region-specific biochar management targeted to optimize biochar effects on soil functions. Here, we investigated the temperature sensitivity of soil respiration to the addition of different rates of Miscanthus biochar(0, 6.25, 12.5, and 25 Mg ha~(-1)) in two types of soils with contrasting textures. Biochar-amended soil treatments and their controls were incubated at constant temperatures of 20, 30, and 40℃. Overall, our results show that: i) considering data from all treatments and temperatures, the addition of biochar decreased soil CO_2 emissions when compared to untreated soils;ii) CO_2 emissions from biochar-amended soils had a higher temperature sensitivity than those from biochar-free soils; iii) the temperature sensitivity of soil respiration in sandy soils was higher than that in clay soils; and iv) for clay soils, relative increases in soil CO_2 emissions from biochar-amended soils were higher when the temperature increased from 30 to 40℃, while for sandy soils, the highest temperature responses of soil respiration were observed when increasing the temperature from 20 to 30℃. Together, these findings suggest a significantly reduced potential to increase soil organic carbon stocks when Miscanthus biochar is applied to tropical soils at high surface temperatures, which could be counteracted by the soil-and weather-specific timing of biochar application.     

Microbial Activity Indices： Sensitive Soil Quality Indicators for Trace Metal Stress

Physicochemical properties, total and DTPA （diethylenetriaminepentaacetic acid）-extractable Cu, Zn, Pb and Cd contents, microbial biomass carbon （C） content and the organic C mineralization rate of the soils in a long-term trace metal-contaminated paddy region of Guangdong, China were determined to assess the sensitivity of microbial indices to moderately metal-contaminated paddy soils. The mean contents of total Cu, Zn, Pb and Cd were 251, 250, 171, and 2.4 mg kg^-1 respectively. DTPA-extractable metals were correlated positively and significantly with total metals, CEC, and organic C （except for DTPA-extractable Cd）, while they were negatively and highly significantly correlated with pH, totall Fe and Mn. Metal stress resulted in relatively low ratios of microbial biomass C to organic C and in remarkable inhibition of the microbial metabolic quotient and C mineralization rate, which eventually led to increases in soil organic C and C/N. Moreover, microbial respiratory activity showed a stronger correlation to DTPA-extractable metals than to total metal content. Likewise, in the acid paddy soils some “linked” microbial activity indices, such as metabolic quotient and ratios of basal respiration to organic C, especially during initial incubation, were found to be more sensitive indicators of soil trace metal contamination than microbial biomass C or basal respiration alone.     

Clay amendment to sandy soil—effect of clay concentration and ped size on nutrient dynamics after residue addition

Purpose

Crop growth on sandy soils can be increased by claying. In modified sandy soils, the added clay is in the form of peds ranging in size from millimetres to centimetres creating a highly non-uniform matrix where ped size could influence nutrient availability and organic C binding. The aim of the study was to determine the effect of clay addition rate and ped size in residue amended sandy soil on soil respiration, nutrient availability and organic C retention.

Materials and methods

In this study, clay peds of 1, 2 or 3 mm size derived from a clay-rich Vertosol (73 % clay) were added to a sandy soil (3 % clay) at clay addition rates of 10 and 20 % w/w. After the addition of ground faba bean residue (C/N 37) at 10 g kg^?1, the soils were incubated for 45 days at 80 % of water-holding capacity.

Results and discussion

Clay addition had no consistent effect on cumulative respiration, but reduced NH₄ ⁺ availability with a greater reduction at 20 % compared to 10 % clay and with 1 and 2 mm compared to 3 mm peds. Sandy soil with clay peds had a greater maximum NH₄ ⁺ and P sorption capacity than sandy soil alone, and sorption capacity was higher at 20 % compared to 10 % clay addition and greater with 1 mm compared to 3 mm peds. Retrieval of clay peds at the end of the experiment showed ped breakdown during the experiment but also the formation of larger peds. Compared to the <53 μm fraction added at the start of the experiment, the total organic carbon (TOC) content of the <53 μm fraction was increased up to nearly two fold, particularly in the smaller peds (1 and 2 mm).

Conclusions

When sandy soils are amended with clay, N availability and organic C binding depend on both clay addition rate and ped size.

     

Addition of a clay subsoil to a sandy topsoil changes the response of microbial activity to drying and rewetting after residue addition – a model experiment

The effect of drying and rewetting (DRW) on C mineralization has been studied extensively but mostly in absence of freshly added residues. But in agricultural soils large amounts of residues can be present after harvest; therefore, the impact of DRW in soil after residue addition is of interest. Further, sandy soils may be ameliorated by adding clay‐rich subsoil which could change the response of microbes to DRW. The aim of this study was to investigate the effect of DRW on microbial activity and growth in soils that were modified by mixing clay subsoil into sandy top soil and wheat residues were added. We conducted an incubation experiment by mixing finely ground wheat residue (20 g kg^–1) into top loamy sand soil with clay‐rich subsoil at 0, 5, 10, 20, 30, and 40% (w/w). At each clay addition rate, two moisture treatments were imposed: constantly moist control (CM) at 75% WHC or dry and rewet. Soil respiration was measured continuously, and microbial biomass C (MBC) was determined on day 5 (before drying), when the soil was dried, after 5 d dry, and 5 d after rewetting. In the constantly moist treatment, increasing addition rate of clay subsoil decreased cumulative respiration per g soil, but had no effect on cumulative respiration per g total organic C (TOC), indicating that the lower respiration with clay subsoil was due to the low TOC content of the sand‐clay mixes. Clay subsoil addition did not affect the MBC concentration per g TOC but reduced the concentration of K₂SO₄ extractable C per g TOC. In the DRW treatment, cumulative respiration per g TOC during the dry phase increased with increasing clay subsoil addition rate. Rewetting of dry soil caused a flush of respiration in all soils but cumulative respiration at the end of the experiment remained lower than in the constantly moist soils. Respiration rates after rewetting were higher than at the corresponding days in constantly moist soils only at clay subsoil addition rates of 20 to 40%. We conclude that in presence of residues, addition of clay subsoil to a sandy top soil improves microbial activity during the dry phase and upon rewetting but has little effect on microbial biomass.     

Nitrogen availability to grain sorghum from organic and inorganic sources on sandy and clayey soil surfaces in a greenhouse pot study

In a greenhouse pot study, we examined the availability of N to grain sorghum from organic and inorganic N sources. The treatments were¹⁵N-labeled clover residues, wheat residues, and fertilizer placed on a sandy clay loam and loamy sand soil surface for an 8-week period. Soil aggregates formed under each soil texture were measured after 8 weeks for each treatment. Significantly greater ¹⁵N was taken up and recovered by grain sorghum in sandy clay loam pots compared with loamy sand pots. Greater ¹⁵N recovery was consistently observed with the inorganic source than the organic sources regardless of soil texture or time. Microbial biomass C and N were significantly greater for sandy clay loam soil compared with the loamy sand. Microbial biomass ¹⁵N was also significantly greater in the sandy clay loam treatment compared to the loamy sand. The fertilizer treatment initially had the greatest pool of microbial biomass ¹⁵N but decreased with time. The crop residue treatments generally had less microbial biomass ¹⁵N with time. The crop residues and soil texture had a significant effect on the water-stable aggregates formed after 8 weeks of treatments. Significantly greater water-stable aggregates were formed in the sandy clay loam than the loamy sand. Approximately 20% greater water-stable aggregates were formed under the crop residue treatments compared to the fertilizer only treatment. Soil texture seemed to be one of the most important factors affecting the availability of N from organic or inorganic N sources in these soils.Contribution from the MissouriAgricultural Experiment Station, Journal Series No.12131     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

水溶性有机碳在各种粘土底土中的吸附：土壤性质的影响

Clay-rich subsoils are added to sandy soils to improve crop yield and increase organic carbon (C) sequestration; however, little is known about the influence of clay subsoil properties on organic C sorption and desorption. Batch sorption experiments were conducted with nine clay subsoils with a range of properties. The clay subsoils were shaken for 16 h at 4 ^oC with water-extractable organic C (WEOC, 1 224 g C L^-1) from mature wheat residue at a soil to extract ratio of 1:10. After removal of the supernatant, the residual pellet was shaken with deionised water to determine organic C desorption. The WEOC sorption was positively correlated with smectite and illite contents, cation exchange capacity (CEC) and total organic C, but negatively correlated with kaolinite content. Desorption of WEOC expressed as a percentage of WEOC sorbed was negatively correlated with smectite and illite contents, CEC, total and exchangeable calcium (Ca) concentrations and clay content, but positively correlated with kaolinite content. The relative importance of these properties varied among soil types. The soils with a high WEOC sorption capacity had medium CEC and their dominant clay minerals were smectite and illite. In contrast, kaolinite was the dominant clay mineral in the soils with a low WEOC sorption capacity and low-to-medium CEC. However, most soils had properties which could increase WEOC sorption as well as those that could decrease WEOC sorption. The relative importance of properties increasing or decreasing WEOC sorption varied with soils. The soils with high desorption had a low total Ca concentration, low-to-medium CEC and low clay content, whereas the soils with low desorption were characterised by medium-to-high CEC and smectite and illite were the dominant clay minerals. We conclude that WEOC sorption and desorption depend not on a single property but rather a combination of several properties of the subsoils in this study.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [14C(U)]glucose and [15N](NH4)2So4 under different moisture regimes

Two soils, one a sandy loam and the other of relatively high clay content, were incubated with [¹⁴C(U)]gtucose and [¹⁵N](NH₄)₂SO₄ for 101 days, either under continuously moist conditions, or with intermittent drying of soils. Rates of evolution of ¹⁴CO₂, decline in residual organic ¹⁴C, and net immobilization and mineralization of N and ¹⁵N in the sandy loam soil were more rapid than in the clay soil. First order decay rates for the decomposition of residual ¹⁴C, after 10 days, were consistently twice as fast in the sandy loam soil. By contrast, the efficiency with which glucose was utilized within the first few days, and the amounts of C, ¹⁴C, N and ¹⁵N present as soil biomass throughout the incubation, were greater in the clay soil than in the sandy loam. Biomass ¹⁴C as a percentage of residual organic ¹⁴C, was consistently 1.5 times greater in the clay soil. Compared with soils held continuously moist, soils which were intermittently dried and remoistened contained smaller amounts of isotope-labelled biomass C and N, but overall similar amounts of total residual organic ¹⁴C and ¹⁵N. Remoistening of dried soils caused a temporary (4 days) flush in C and N mineralization rates.A simulation model describes C and N behaviour in the two soils. Three features of the model are proposed to expain short-term differences between soils in the rates of C and N turnover, viz. the clay soil (a) has a greater capacity to preserve biomass C and N (b) holds a higher proportion of microbial decay products in the near vicinity of surviving cells, and, to a lesser extent, (c) utilizes glucose and metabolic products more efficiently for biosynthetic reactions.     

Clay Addition to Sandy Soil Reduces Nutrient Leaching—Effect of Clay Concentration and Ped Size

Leaching of nutrients, particularly in sandy soil with low nutrient and water holding capacity (WHC), is a major threat to marine and fresh water pollution. Addition of clay soil to sandy soil could be an option to increase water and nutrient holding capacity of sandy soils, but the effect of clay soil addition may depend on the form in which the clay soil is added and the addition rate. Clay soil was added to sandy soil at rate of 10 or 20% (w/w) finely ground (<2 mm) or 2 and 5 mm peds with and without nitrogen (N) and phosphorus (P) fertilizer equivalent to 60 kg N ha^?1 and 15 kg P ha^?1. The clay sand mixture for each treatment was weighed (30 g) in cores with nylon mesh at the bottom. The soils were incubated at 80% WHC for 7 weeks. To obtain leachate, 20 mL reverse osmosis (RO) water was added every week to each core. Leachate was analysed for inorganic N, P, and pH. Soil was analyzed for N, P, and pH before and after the leaching. Clay addition significantly reduced the leaching of N and P compared to sandy soil alone, with greatest reduction by finely ground clay soil and least with 5 mm peds. Compared to sandy soil alone, 83% more N was retained in clay-amended soil and P retention was doubled. This study showed that addition of finely ground clay soil can substantially reduce N and P leaching and thereby increase fertilizer retention compared to sandy soil alone.     

Soil properties following cultivation and non-grazing of a semi-arid sandy grassland in northern China

Cultivation and overgrazing are widely recognized as the primary causes of desertification of sandy grassland in the semi-arid region of northern China. Very little is known about the effect of cultivation and overgrazing on soil physical, chemical and biological properties in this region. The objective of this study was to quantitatively evaluate the magnitude of changes in soil properties due to 3 years of cultivation (3CGS) and 5 years of ungrazed exclosure (5RGS) in a degraded grassland ecosystem of the semi-arid Horqin sandy steppe. Short-term cultivation resulted in a 18–38% reduction in concentration of soil organic C, and total N and P in the 0–15 cm plow layer. Cultivation had a significant influence on N and P availability and soil biological properties, with lower basal soil respiration (BSR) and enzyme activities than the grassland soils. This was mostly due to strong wind erosion when sandy grassland was cultivated. Data indicated a considerable difference in soil particle size distribution between the cultivated and grassland soils, and fine fraction (<0.1 mm) in the cultivated soil was lower than that in the grassland soils. Moreover, grassland vegetation recovery in the 5RGS resulted in significant improvement in soil properties measured at the 0–7.5 cm depth. From the perspective of soil resource management and environmental conservation, a viable option for these sandy grasslands would be to stop conversion of grassland to cropland and adopt proper fencing practices to limit overgrazing.