Dynamics of microbial biomass in Cambisols under a three year succession fallow in North Eastern Saxony

The response of microbial biomass carbon (C_mic), nitrogen (N_mic), basal respiration, and the metabolic quotient to 3 years of a natural succession fallow were studied in a field experiment on sandy soil in Northeast Saxony/Germany from 1996 to 1998. Soil samples were taken from Eutric Cambisol and Mollic Cambisol every six weeks during the vegetation period at soil depths of 0—10 and 10—30 cm. The C_mic content in the topsoils increased with time of succession in both soil types. This trend was more distinct in the Mollic Cambisol (70.7 μg g^—1 in June 1996 to 270.9 μg g^—1 in October 1998 at 0—10 cm) than in the Eutric Cambisol (69.7 μg g^—1 in June 1996 to 175.0 μg g^—1 in October 1998 at 0—10 cm). By contrast, the N_mic content slightly decreased in the Eutric Cambisol from 18.9 μg g^—1 to 17.7 μg g^—1 during the same time period. In the Mollic Cambisol, the N_mic increased from 18.8 μg g^—1 in spring 1996 to 35.5 μg g^—1 in fall 1998, however to a lower extent than the C_mic. Subsequently, the (C:N)_mic ratio increased from 4.3 to 5.8 at soil depth of 0—10 cm and from 3.5 to 6.5 at 10—30 cm during the 3‐year‐study at the Eutric Cambisol. In the Mollic Cambisol, the enhancement of (C:N)_mic ratio was more pronounced (i.e. from 4.3 to 6.7 at 0—10 cm and from 3.5 to 7.2 at 10—30 cm). Most likely this results from a shift in microbial populations towards a dominance of soil fungi. The already low basal respiration of, on average, 0.26 mg CO₂ g^—1 (24h)^—1 (0—10 cm) in June 1996 decreased with time of succession fallow to 0.15 and 0.22 mg CO₂ g^—1 (24h)^—1 in October 1998 in the Eutric and the Mollic Cambisol, respectively. Thus, the metabolic quotient as an indicator for the efficiency of organic matter turnover in soil was very low in both soils. During the summer months, the metabolic quotients reached minimum levels of ≤ 0.1 μg CO₂ C (g C_mic)^—1 h^—1, probably because of low soil moisture contents. Correlation analyses revealed close relationships between N_mic and total N, N_mic and water content, and N_mic and pH values. These relationships became even more pronounced with the time period of natural succession. For the samples from fall 1998, highly significant correlations were determined between N_mic and total N (coefficients were r_s = 0.91^***), N_mic and water content (r_s = 0.91^***), and N_mic and pH value (r_s = 0.76^***). The values for all biological parameters studied were larger in the Mollic than in the Eutric Cambisol. This indicates higher turnover rates of different C and N fractions in the Mollic Cambisol. In general, set aside of formerly agricultural managed sandy soils resulted in greater C_mic : N_mic ratios and thus, in a change in the microbiological community structure as well as in reduced C and N turnover rates (i.e. low metabolic quotient) under the climatic conditions of the East German lowlands.     

Changes in total and labile carbon and nitrogen contents in a sandy soil after the conversion of a succession fallow to cultivated land

The content of soil organic matter (SOM) can be considered as an important factor for evaluating soil fertility, crop yields, and environmental effects. Sensitive measurements for the assessment of quantitative changes in SOM shortly after the conversion of the management practice would be helpful to understand the SOM‐transformation cycle in more detail. Changes in SOM are reflected in modifications of total organic‐carbon (TOC) and total organic‐nitrogen (TON) contents. They are initially detectable in the readily decomposable fraction. We used hot water–extractable carbon (HWC) and nitrogen (HWN) as measurement of labile pools of SOM and aimed to quantify changes in contents of these C and N fractions in a sandy soil already few years after changing management strategy. In this context, we examined the impact of the conversion of a succession fallow (F) to organic (O) and intensive (I) agriculture on TOC, total N (TN), HWC, and HWN. The conversion of succession fallow to cultivated land resulted in a significant decrease of TOC, TN, and HWC at 0–10 cm soil depth. On average, TOC decreased approx. 0.70 g C kg^–1 (approx. 9% of initial TOC), TN decreased approx. 0.13 g N kg^–1 (approx. 17% of initial TN), and HWC decreased approx. 0.05 g C kg^–1 (approx. 12% of initial HWC) within 3 years. Relatively rapid changes in TOC and TN contents indicated comparatively high proportions of decomposable C and N. These were reflected in comparable high HWC (ranging from 0.37 to 0.59 g C kg^–1 at 0–30 cm soil depth) and HWN (ranging from 0.04 to 0.10 g N kg^–1 at 0–30 cm) contents. These high contents as well as the high HWC : TOC and organic hot water–extractable N (HWN_org) : TN ratios (both between 5% and 7%) implied that the soil investigated has a high ability to provide short‐term available organic C and N compounds. Long‐lasting applications of high quantities of organic fertilizer in the past and high quantities of rhizodepositions were assumed as reasons for the high capability of soil to provide short‐term to medium‐term available C and N. Changes in the HWN content due to the fertilization or crop rotation were mainly based on changes in its inorganic part. This ranged between 10% and 30% of HWN. By discriminant function analysis, it could be shown that the HWN represents a suitably sensitive measurement for the determination of management‐specific impacts in terms of the N, but also of the C cycle. In combination with other C and particularly with other N parameters, the HWN allowed a statistically significant separation of comparable sites varying in management practice already 2 years after the conversion of the management system.     

Mobile and readily available C and N fractions and their relationship to microbial biomass and selected enzyme activities in a sandy soil under different management systems

Within different land‐use systems such as agriculture, forestry, and fallow, the different morphology and physiology of the plants, together with their specific management, lead to a system‐typical set of ecological conditions in the soil. The response of total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities involved in C and N cycling to different soil management was investigated in a sandy soil at a field study at Riesa, Northeastern Germany. The management systems included agricultural management (AM), succession fallow (SF), and forest management (FM). Samples of the mineral soil (0—5, 5—10, and 10—30 cm) were taken in spring 1999 and analyzed for their contents on organic C, total N, NH₄⁺‐N and NO₃^—‐N, KCl‐extractable organic C and N fractions (Corg_(KCl) and Norg_(KCl)), microbial biomass C and N, and activities of β‐glucosidase and L‐asparaginase. With the exception of Norg_(KCl), all investigated C and N pools showed a clear relationship to the land‐use system that was most pronounced in the 0—5 cm profile increment. SF resulted in greater contents of readily available C (Corg_(KCl)), NH₄⁺‐N, microbial biomass C and N, and enzyme activities in the uppermost 5 cm of the soil compared to all other systems studied. These differences were significant at P ≤ 0.05 to P ≤ 0.001. Comparably high C_mic:C_org ratios of 2.4 to 3.9 % in the SF plot imply a faster C and N turnover than in AM and FM plots. Forest management led to 1.5‐ to 2‐fold larger organic C contents compared to SF and AM plots, respectively. High organic C contents were coupled with low microbial biomass C (78 μg g^—1) and N contents (10.7 μg g^—1), extremely low C_mic : C_org ratios (0.2—0.6 %) and low β‐glucosidase (81 μg PN g^—1 h^—1) and L‐asparaginase (7.3 μg NH₄‐N g^—1 2 h^—1) activities. These results indicate a severe inhibition of mineralization processes in soils under locust stands. Under agricultural management, chemical and biological parameters expressed medium values with exception for NO₃^—‐N contents which were significantly higher than in SF and FM plots (P ≤ 0.005) and increased with increasing soil depth. Nevertheless, the depth gradient found for all studied parameters was most pronounced in soils under SF. Microbial biomass C and N were correlated to β‐glucosidase and L‐asparaginase activity (r ≥ 0.63; P ≤ 0.001). Furthermore, microbial biomass and enzyme activities were related to the amounts of readily mineralizable organic C (i.e. Corg_(KCl)) with r ≥ 0.41 (P ≤ 0.01), suggesting that (1) KCl‐extractable organic C compounds from field‐fresh prepared soils represent an important C source for soil microbial populations, and (2) that microbial biomass is an important source for enzymes in soil. The Norg_(KCl) pool is not necessarily related to the size of microbial biomass C and N and enzyme activities in soil.<?show $6#>     

Soil organic matter,C and N accumulation during natural succession and reclamation in an opencast sand quarry (southern Poland)

Abstract

The work presents study results on the formation of humus horizons, the accumulation of organic carbon and nitrogen as well as humus composition in successional and reclaimed soils in a sand mine cast in southern Poland. Research plots were designed in chronosequence: 5, 17, 20 and 25 years. Increased thickness of humus horizon and accumulation of organic carbon was reported both in successional soils and in reclaimed soils. However, in corresponding age groups of reclaimed soils these characteristics were two times as high as in successional soils. The estimated accumulation rate of organic carbon (C_org) was three times higher and total nitrogen (N_t) five times higher in reclaimed soils than in successional soils. In both types of soils there was an increase in the amount of carbon trapped with humic and fulvic acid (C_Ha + C_Fa) and structure of humic acids. Studies indicated that reclamation treatment significantly accelerated soil-formation in opencast sand mine.     

Effects of fallow or planting wheat (Triticum aestivum L.) and fertilizing P or fertilizing P and N practices on soil carbon and nitrogen in a low-organic-matter soil

Soil carbon (C) and nitrogen (N) are important for maintaining soil fertility, and they are considerably affected by soil use and management. In the present study, we conducted an 8-year ?eld experiment on loessial dryland soil (Eum-Orthic Anthrosol, Food and Agriculture Organization of the United Nations (FAO)) in the southern Loess Plateau, China. We tested four soil management regimes—i.e., winter wheat (Triticum aestivum L.) cultivation with phosphorus (P) fertilization (WP), winter wheat cultivation with N and P fertilization (WNP), natural fallow (NF) and bare fallow (BF)—to evaluate their effects on soil C and N fractions. After 8 years, compared with the WNP treatment, the total soil organic nitrogen (SON) in the WP treatment decreased by 14.6% and 36.8%, and microbial biomass nitrogen (MBN) by 35.6% and 61.1%, at 0–20 and 20–40 cm soil depths, respectively. The soil heavy fraction nitrogen (HFN) and light fraction nitrogen (LFN) in the WP treatment also decreased by 36.6% and 39.4%, respectively. Furthermore, BF treatment decreased total soil organic carbon (SOC), heavy fraction carbon (HFC), LFN and MBN at both soil depths with average reductions of 43.4%. The NF treatment decreased light fraction carbon (LFC) by 17.0% at 0–20 cm soil depth, as well as MBN by 24.8% and 71.2%, and inorganic C by 29.1% and 23.8%, at 0–20 and 20–40 cm soil depths, respectively. There was no significant difference of microbial biomass C concentration among the WP, NF and BF treatments. These results confirmed that a lack of N fertilization decreased SON, BF reduced both SOC and SON, and NF decreased soil inorganic C. Therefore, the managements of a recommended rate of N fertilizer application and shortened time of bare fallow are critical for maintaining or increasing SON fraction sequestration, and natural fallow management is not a useful method for maintaining soil fertility in dryland in the Loess Plateau in China.

Abbreviations: HFC: heavy fraction carbon; HFN: heavy fraction nitrogen; LFC: light fraction carbon; LFN: light fraction nitrogen; MBC: microbial biomass carbon; MBN: microbial biomass nitrogen; SOC: soil organic carbon; SON: soil organic nitrogen     

C and N accumulation in arable soils of West Germany and its influence on the environment — Developments 1970 to 1998

During the last three decades, large amounts of soil organic matter (SOM) and associated nutrients have been accumulated in arable soils of Western Germany (former FRG) due to deepening of the plough layers (from < 25 to > 35 cm) and to fertilizer application rates which have exceeded the amounts of nutrients removed in harvested crops. Organic carbon and total nitrogen balances (1970—1998) on 120 plots from 16 farms in southern Lower Saxony yielded a cumulative increase of up to 16 t C ha⁻¹ and 1 t N ha⁻¹ in loess soils used for cash crop production and up to 26 t C ha⁻¹ and 2.4 t N ha⁻¹ in sandy soils under livestock production. The buffering capacity for reactive compounds, particularly of C, N, S and P and of other (organic or inorganic) pollutants will reach its limits in the near future, after organic matter ”︁equilibria” have been re‐established. An immediate adaptation of the current fertilizer application rates to the nutrient export by field crops is therefore urgently needed.     

Nutrient concentrations and NH4+‐N mineralization under different soil types and fallow forms in southern Cameroon

To evaluate the soil‐fertility sustainability of the fallow systems, nutrient concentrations and NH₄⁺‐N mineralization were determined in different soil and fallow types in the humid forest zone of southern Cameroon. Two experiments were conducted, the first comprised planted leguminous tree Calliandra calothyrsus, planted leguminous Pueraria phaseoloides, and regrowth mainly composed of Chromolaena odorata on the Typic Kandiudult. The second experiment made up of a fallow dominated by C. odorata, a fallow with C. odorata removed, and a P. phaseoloides fallow on the Rhodic Kandiudult, Typic Kandiudult, and Typic Kandiudox. In the first experiment, available P, Ca²⁺, K⁺ concentrations and effective CEC under C. calothyrsus were, respectively, 40%, 22%, 45%, and 15% lower when compared to P. phaseoloides but no differences were found between soils under P. phaseoloides and C. odorata. Mineralization of NH₄⁺‐N was higher under C. calothyrsus than under C. odorata‐ and P. phaseoloides‐fallow types, indicating the impoverishment of organic material under the former. In the second experiment, the beneficial effect of P. phaseoloides was found in the Rhodic Kandiudult in the 0–10 cm layer throughout its low NH₄⁺ release from mineralization. In the Typic Kandiudult, no differences in NH₄⁺‐N mineralization were found between C. odorata and P. phaseoloides fallows. In the Typic Kandiudox, there was no difference in NH₄⁺ mineralization between the three fallow types. According to the nutrient concentrations and NH₄⁺ mineralization, the fertility sustainability of the different fallow types may be ranked as follow: P. phaseoloides ≥ C. odorata > C. calothyrsus > fallow without C. odorata.     

Hot water extractable C and N in relation to microbiological properties of soils under beech forests

Hot water extraction is sometimes recommended as an easy method to estimate the readily mineralizable fractions of total C (C_t) and total N (N_t) in arable soils. However, the usefulness of this method for forest soils has not been adequately studied. The objectives of this study were to relate the hot water extractable C (C_hw) and N (N_hw) to microbiological and chemical properties of the forest soils under beech (Fagus sylvatica L.) stands and to test the ability of near infrared spectroscopy (NIRS) to predict chemical and microbial properties of these soils. Soils differing in humus type, soil type and soil texture were collected from five locations and five depths. In all soils the amount of C_hw was higher than the microbial biomass C (C_mic) indicating that a considerable part of C_hw was of non-microbial origin. The amount of C_hw in mineral soil correlated significantly (r =–0.30–0.53) with C_mic, basal respiration (BAS) and C_t/N_t ratio but was not related to C_mic/C_t ratio. The amount of N_hw was correlated with C_mic, BAS, C_mic/C_t ratio, and C_t/N_t ratio (r =–0.59–0.78). However, C_t and N_t values showed better relationships (r =–0.42–0.88) with all the parameters, indicating no advantage in using C_hw and N_hw in forest soils. NIRS predicted satisfactorily C_t, N_t, C_hw, N_hw, C_mic, C_mic/C_t ratio and BAS in the mineral soils [the regression coefficients (a) of linear regression (measured against predicted values) ranged from 0.84 to 1.17 and the correlation coefficients (r) ranged from 0.86 to 0.94] indicating the applicability of NIRS to estimate these properties.     

有机物料输入稻田提高土壤微生物碳氮及可溶性有机碳氮

土壤微生物量碳、氮和可溶性有机碳、氮是土壤碳、氮库中最活跃的组分,是反应土壤被干扰程度的重要灵敏性指标,通过设置相同有机碳施用量下不同有机物料处理的田间试验,研究了有机物料添加下土壤微生物量碳（soil microbial biomass carbon,MBC）、氮（soil microbial biomass nitrogen,MBN）和可溶性有机碳（dissolved organic carbon,DOC）、氮（dissolved organic nitrogen,DON）的变化特征及相互关系。结果表明化肥和生物碳、玉米秸秆、鲜牛粪或松针配施下土壤微生物量碳、氮和可溶性有机碳、氮显著大于不施肥处理（no fertilization,CK）和单施化肥处理,分别比不施肥处理和单施化肥平均高23.52%和12.66%（MBC）、42.68%和24.02%（MBN）、14.70%和9.99%（DOC）、22.32%和21.79%（DON）。化肥和有机物料配施处理中,化肥+鲜牛粪处理的微生物量碳、氮和可溶性有机碳、氮最高,比CK高26.20%（MBC）、49.54%（MBN）、19.29%（DOC）和32.81%（DON）,其次是化肥+生物碳或化肥+玉米秸秆处理,而化肥+松针处理最低。土壤可溶性有机碳质量分数（308.87 mg/kg）小于微生物量碳（474.71 mg/kg）,而可溶性有机氮质量分数（53.07 mg/kg）要大于微生物量氮（34.79 mg/kg）。与不施肥处理相比,化肥和有机物料配施显著降低MBC/MBN和DOC/DON,降低率分别为24.57%和7.71%。MBC和DOC、MBN和DON随着土壤有机碳（soil organic carbon,SOC）、全氮（total nitrogen,TN）的增加呈显著线性增加。MBC、MBN、DOC、DON、DOC+MBC和DON+MBN之间呈极显著正相关（P<0.01）。从相关程度看,DOC+MBC和DON+MBN较MBC、DOC、MBN、DON更能反映土壤中活性有机碳和氮库的变化,成为评价土壤肥力及质量的更有效指标。结果可为提高洱海流域农田土壤肥力,增强土壤固氮效果,减少土壤中氮素流失,保护洱海水质安全提供科学依据。     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.     

Content and composition of free and occluded particulate organic matter in a differently textured arable Cambisol as revealed by solid‐state 13C NMR spectroscopy

The composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse‐grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS ¹³C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O‐alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O‐alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O‐alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O‐alkyl C ratio.     

Concurrent increase in 15N and radiocarbon age in soil density fractions

Differences in the isotopic signature of organic matter between soil fractions are indicative of transformation and ageing processes. Here we show that with increasing microbial transformation measured by δ¹⁵N, there is a concomitant increase in carbon age as measured by ¹⁴C. The age of the soil's heavy fraction further increases with microbial utilization, indicating that stabilized OM ages yet continues to be reused.     

High biochar rates may suppress rice (Oryza sativa) growth by altering the ratios of C to N and available N to P in paddy soils

Although most studies have indicated that biochar can boost rice (Oryza sativa) growth, the material may also suppress it, depending on ratios of carbon (C) to nitrogen (N) and available N to available phosphorous (P). The current study sought to examine the impacts of biochar on rice growth and to identify underlying mechanisms. A pot experiment was conducted using two soils of high (3.05%) and low (0.54%) organic carbon (OC) content, mixed with 0, 1.5, 3, 6, and 12% biochar and planted with rice. Rice growth components, five rice tissue nutrients, and nine soil properties were measured. The results showed that the response of rice growth to biochar rates could be described using an exponential-growth function in high-OC soil but an inverted U-shaped curve in low-OC soil. In high-OC soil, the 12% biochar rate led to the greatest total biomass, increased by 47%, whereas in low-OC soil, the 3 and 6% rates exhibited the highest total biomass, increased by 44%, compared to the no-biochar added soils. Biochar elevated the C:N ratio from 11.5 to 39.1, with an optimal range of 20–30 corresponding to the highest rice growth. Biochar declined the ratio of NH₄-N to Mehlich-1 P, causing N deficiency. In brief, high biochar rates may suppress rice growth when the soil C:N ratio exceeds 30. The applied biochar rate should be considered based on soil properties typically OC and N content to obtain the C:N ratio between 20 and 30 for optimal rice growth.     

C and N NMR spectroscopy as a tool in soil organic matter studies

Nuclear magnetic resonance (NMR) is a valuable tool for the characterization of soil organic matter and humification processes in soils. This review highlights soil organic matter studies based mainly on solid-state ¹³C and ¹⁵N NMR spectroscopy and some emerging applications, that may provide significant progress in our knowledge on soil organic matter. A major advantage of Nmr spectroscopy is that it can be used as a non-invasive method for solid soil samples or soil fractions. Although resolution is limited, one can obtain an overview on the organic matter structures present in the soil sample. Application of ¹³C and ¹⁵N NMR to soils has, for a long time, been confined to the study of bulk soils or humic extracts for structural characterization. The transformations of soil organic C and N are now being investigated after addition of ¹³C- and ¹⁵N-labelled parent materials to the soil and following their evolution in different C and N pools. With labelling techniques it is also possible to study the interaction of organic pollutants with soil organic matter. Contamination of a soil with man-made additives, such as soot or brown coal dust, can also be detected in soils or individual soil fractions.     

Dynamics of 13C‐labeled mustard litter (Sinapis alba) in particle‐size and aggregate fractions in an agricultural cropland with high‐ and low‐yield areas

The application of ¹³C‐labeled litter enables to study decomposition processes as well as the allocation of litter‐derived carbon to different soil C pools. ¹³Carbon‐labeled mustard litter was used in order to compare decomposition processes in an agricultural cropland with high‐yield (HY) and low‐yield (LY) areas, the latter being characterized by a finer texture and a lower organic‐C (OC) content. After tracer application, ¹³C concentrations were monitored in topsoil samples in particulate organic matter (POM) and in fine mineral fractions (silt‐ and clay‐sized fractions). After 568 d, approximately 5% and 10% of the initial ¹³C amount were found in POM fractions of LY and HY areas, respectively. Higher amounts were found in POM occluded in aggregates than in free POM. Medium‐term (0.5–2 y) storage of the initial ¹³C in fine silt‐ and clay‐sized fractions amounts to 10% in HY and LY soils, with faster enrichment but also faster disappearance of the ¹³C signal from LY soils. Amounts of 80%–90% of the added ¹³C were mineralized or leached in the observed period. Decomposition of free POM was faster in HY than in LY areas during the first year, but the remaining ¹³C amounts in occluded‐POM fractions were higher in HY soils after 568 d. High‐yield and low‐yield areas showed different ¹³C dynamics in fine mineral fractions. In LY soils, ¹³C amounts and concentrations in mineral‐associated fractions increased within 160 d after application and decreased in the following time period. In HY areas, a significant increase in ¹³C amounts did not occur until after 568 d. The results indicate initially faster decomposition processes in HY than in LY areas due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote a faster aggregate formation and turnover, leading to a closer contact between POM and mineral surfaces in this area. This favors the OC storage in fine mineral fractions in the medium term. Lower aggregate formation and turnover in the coarser textured HY soil leads to a delayed C stabilization in silt‐ and clay‐sized fractions.     

Effect of burning on soil organic matter content and N mineralization under shifting cultivation system of Karen people in Northern Thailand

In the traditional shifting cultivation system practiced by the Karen people in northern Thailand, the effects of burning on the content of extractable organic matter, microbial biomass, and N mineralization process of the soils were studied. Five plots (5×5 m² quadrat) with 0, 10, 20, 50, and 100 Mg ha^-1 of slashed materials were arranged and burned. Ten to 20 Mg ha^-1 of slashed biomass corresponded to the amount commonly burned by the Karen people. During the burning process, the soil temperature at the depth of 2.5 cm in the 100 Mg ha^-1 plot almost evenly increased to 300°C while the temperature in the 10 to 50 Mg ha^-1 plots increased with large variations from 50 to 300°C. Burning caused a conspicuous increase in the contents of organic C and (organic + mineral)-N extracted at room temperature and a simultaneous decrease in the contents of microbial biomass C and N, especially in the soil of the 100 Mg ha^-1 plot. In the rainy season, the values of the changes induced by burning reverted to the values recorded before burning, except for the microbial biomass in the 100 Mg ha^-1 plot, which still remained lower. Based on an incubation experiment, N mineralization rate was higher in the soils taken just after burning, especially in the 100 Mg ha^-1 plot, than in the soils taken during the rainy season. However, the soil in the 100 Mg ha^-1 plot was considered to have the lowest ability to supply mineral N among the soils in the rainy season. Burning of 10 to 20 Mg ha^-1 biomass corresponding to the values recorded in Karen peoples' shifting cultivation system was more compatible with soil ecology in terms of N supply at the initial stage of crop growth and of microbial biomass recovery during the rainy season, compared to the burning of 100 Mg ha^-1 biomass corresponding to the value recorded in a natural forest. Thus, the shifting cultivation system implemented by the Karen people can be considered to be a well-balanced agricultural system.     

不同C/N比对双孢蘑菇培养料发酵过程温室气体排放的影响

针对双孢蘑菇培养料发酵过程中物质能量转化效率低、CO_2和CH_4等温室气体排放量大等问题,采用自制强制通风发酵箱装置研究了不同碳氮比对培养料发酵过程中温室气体排放和碳氮物质转化的影响。结果表明:发酵过程中温室气体的排放主要以CO_2为主,CH_4、N_2O和NH_3的排放量较少,并且CO_2、N_2O和NH_3的累积排放量均随C/N比的增加而降低。C/N分别为28,33,38,43处理以温室气体形式损失的碳分别为46.16,37.44,33.04,31.76 g/kg,损失的氮分别为4.72,3.49,1.76,1.65 mg/kg。C/N为33的处理更适合微生物活性,有机物降解率高于其它处理。到发酵结束时,C/N分别为28,33,38,43处理的碳物质损失分别为36.55%,45.5%,37.22%,32.71%,氮物质损失分别为5.41%,13.84%,7.59%,16.33%;但随发酵的进行,全氮相对含量因有机物降解而浓缩,呈现增加趋势。综合考虑温室气体排放和培养料发酵质量两个方面的因素,在实际生产过程中可适当提高C/N比,采用C/N为33:1~38:1的培养料配方,在不影响发酵质量的同时可减少温室气体排放量。另外,由于温室气体的排放主要集中在高温期,高温期越长,排放的温室气体越多,因此在发酵过程中应加强管理,及时翻堆,以控制发酵温度和温室气体排放。     

Decomposition and distribution of N labelled mustard litter (Sinapis alba) in physical soil fractions of a cropland with high- and low-yield field areas

High-yield (HY) areas of an agricultural cropland were characterized by different positions on a slope and lower silt and clay contents, compared to low-yield (LY) areas, and this was associated with differences in water regime and C and N turnover. To understand differences in N flows of HY and LY areas, a combination of ¹⁵N tracer techniques and physical fractionation procedures was applied. Within 570 d after application of ¹⁵N labelled mustard litter to an agricultural cropland, the distribution of ¹⁵N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 d, only 2.5% of the initial ¹⁵N amount was found in POM fractions, with higher amounts in POM occluded in aggregates than in free POM. After this period, stabilization of the initial ¹⁵N in fine silt- and clay-sized fractions amounts to 10% in HY, but 20% in LY soils. 70% to 85% of the added ¹⁵N were lost. Initial decomposition of labelled material was faster in HY than in LY areas during the first year, but the remaining ¹⁵N amounts in POM fractions of the different areas were similar after 570 d. ¹⁵N amounts and concentrations in mineral-associated fractions increased within 160 d after application. From 160 to 570 d, HY and LY areas showed different ¹⁵N dynamics, resulting in a decline of ¹⁵N amounts in HY, but constant ¹⁵N amounts in LY soils. The results indicate faster decomposition processes in HY than in LY areas, due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote N stabilization in fine mineral fractions. As a whole, N flows were higher in HY compared to LY areas, thus supporting higher yields and accelerated organic matter degradation due to higher N supply.     

Organic‐carbon and nitrogen stocks and organic‐carbon fractions in soil under mixed pine and oak forest stands of different ages in NE Germany

The objective of this work was to evaluate the C and N stocks and organic‐C fractions in soil under mixed forest stands of Scots pine (Pinus sylvestris L.) and Sessile oak (Quercus petraea [Matt.] Liebl.) of different ages in NE Germany. Treatments consisted of pure pine (age 102 y), and pine (age 90–102 y) mixed with 10‐, 35‐, 106‐, and 124‐y‐old oak trees. After sampling O layers, soils in the mineral layer were taken at two different depths (0–10 and 10–20 cm). Oak admixture did not affect total organic‐C (TOC) and N stocks considering the different layers separately. However, when the sum of TOC stocks in the organic and mineral layers was considered, TOC stocks decreased with increasing in oak age (r² = 0.58, p < 0.10). The microbial C (C_MB) was not directly correlated with increase of oak age, however, it was positively related with presence of oak species. There was an increase in the percentage of the C_MB‐to‐TOC ratio with increase of oak‐tree ages. On average, light‐fraction C (C_LF) comprised 68% of the soil TOC in upper layer corresponding to the highest C pool in the upper layer. C_LF and heavy‐fraction C (C_HF) were not directly affected by the admixture of oak trees in both layers. The C_HF accounted on average for 30% and 59% of the TOC at 0–10 and 10–20 cm depths, respectively. Despite low clay contents in the studied soils, the differences in the DCB‐extractable Fe and Al affected the concentrations of the C_HF and TOC in the 10–20 cm layers (p < 0.05). Admixture of oak in pine stands contributed to reduce topsoil C stocks, probably due to higher soil organic matter turnover promoted by higher quality of oak litter.