Microbial biomass and mineralizable nitrogen distributions in no-tillage and plowed soils

Summary Distribution of soil microbial biomass and potentially mineralizable nitrogen (PMN) in long-term tillage comparisons at seven sites in the United States varied with tillage management and depth in soil. Microbial biomass and PMN levels of no-tillage soils averaged 54% and 37% higher, respectively, than those in the surface layer of plowed soil. Biomass and PMN levels were greatest in the surface 0 to 7.5-cm layer of no-tillage soil and decreased with depth in soil to 30 cm. Biomass and PMN levels of plowed soil, however, were generally greatest at the 7.5 –15 cm depth. Microbial biomass levels were closely associated with soil distributions of total C and N, water content, and water-soluble C as influenced by tillage management. Potentially mineralizable N levels in soil were primarily associated with distributions of microbial biomass and total N. Absolute levels of PMN and microbial biomass and the relative differences with tillage management were dependent on climatic, cropping, and soil conditions across locations. The additional N contained in soil biomass and PMN in the surface 0–7.5 cm of no-tillage compared with plowed soils ranged from 13 to 45 and 12 to 122 kg N/ha, respectively, for 6 of 7 locations. Fertilizer placement below the biologically rich surface soil layer and/or rotational tillage may improve short-term nitrogen use efficiency and crop growth on reduced-tillage soils.Contribution from USDA-ARS in cooperation with the Nebraska Agricultural Experimental Station, published as paper no. 8086, Journal Series, Nebraska Agricultural Experimental Station     

Microbial biomass and microbial C:N ratio in bulk soil and buried bags for evaluating in situ net N mineralization in agricultural soils

The in situ net nitrogen mineralization (N_net) was estimated in five agricultural soils under different durations of organic farming by incubating soil samples in buried bags. Simultaneously, soil microbial C and N was determined in buried bags and in bulk soil under winter wheat and after harvest. The aim was to check for variations in soil microbial biomass contents and microbial C:N ratios during the incubation period, and their importance for N_net rates. Microbial C and N contents were highest in soils that had been organically farmed for 41 years, whereas N_net rates were highest in a short‐term organically managed soil that had been under grassland use until 36 years ago. The mean coefficient of variation in the bulk soil for microbial C estimates ranged from 5 to 12 %. Microbial N contents were similar inside buried bags and in the bulk soil at the end of the incubation periods. Under winter wheat during the incubation period until harvest, microbial C contents and microbial C:N ratios (in 10—27 cm depth only) decreased more strongly inside buried bags than in the bulk soil. Following harvest of winter wheat and ploughing, microbial biomass increased while in situ N_net decreased, presumably due to N immobilization. The N_net rates were not correlated with microbial N contents or changes in microbial N contents inside buried bags. At the end of the vegetation period of winter wheat, N_net rates were negatively correlated with microbial C:N ratios. Because these ratios concurrently decreased more inside buried bags than in the bulk soil, the N_net estimates of the buried bag method may differ from the N_net rates in the bulk soil at that time.     

Effects of fertilizer application and fire regime on soil microbial biomass carbon and nitrogen,and nitrogen mineralization in an Australian subalpine eucalypt forest

The effects of a range of fertilizer applications and of repeated low-intensity prescribed fires on microbial biomass C and N, and in situ N mineralization were studied in an acid soil under subalpine Eucalyptus pauciflora forest near Canberra, Australia. Fertilizer treatments (N, P, N+P, line + P, sucrose + P), and P in particular, tended to lower biomass N. The fertilizer effects were greatest in spring and smaller in summer and late actumn. Low-intensity prescribed fire lowered biomass N at a soil depth of 0–5 cm with the effect being greater in the most frequently burnt soils. No interactions between fire treatments, season, and depth were significant. Only the lime + P and N+P treatments significantly affected soil microbial biomass C contents. The N+P treatment increased biomass C only at 0–2.5 cm in depth, but the soil depth of entire 0–10 cm had much higher (>doubled) biomass C values in the line + P treatment. Frequent (two or three times a year) burning reduced microbial boomass C, but the reverse was true in soils under forest burn at intervals of 7 years. Soil N mineralization was increased by the addition of N and P (alone or in combination), line + P, and sucrose + P to the soil. The same was true for the ratio of N mineralization to biomass N. Soil N mineralization was retarded by repeated fire treatments, especially the more frequent fire treatment where rates were only about half those measured in unburnt soils. There was no relationship between microbial biomass N (kg N ha^-1) and the field rates of soil N mineralization (kg N ha^-1 month^-1). The results suggest that although soil microbial biomass N represents a distinct pool of N, it is not a useful measure of N turnover.     

Soil microbial biomass and activity in organic tomato farming systems: Effects of organic inputs and straw mulching

Organic farming is rapidly expanding worldwide. Plant growth in organic systems greatly depends on the functions performed by soil microbes, particularly in nutrient supply. However, the linkages between soil microbes and nutrient availability in organically managed soils are not well understood. We conducted a long-term field experiment to examine microbial biomass and activity, and nutrient availability under four management regimes with different organic inputs. The experiment was initiated in 1997 by employing different practices of organic farming in a coastal sandy soil in Clinton, NC, USA. Organic practices were designed by applying organic substrates with different C and N availability, either in the presence or absence of wheat-straw mulch. The organic substrates used included composted cotton gin trash (CGT), animal manure (AM) and rye/vetch green manure (RV). A commercial synthetic fertilizer (SF) was used as a conventional control. Results obtained in both 2001 and 2002 showed that microbial biomass and microbial activity were generally higher in organically than conventionally managed soils with CGT being most effective. The CGT additions increased soil microbial biomass C and activity by 103-151% and 88-170% over a period of two years, respectively, leading to a 182-285% increase in potentially mineralizable N, compared to the SF control. Straw mulching further enhanced microbial biomass, activity, and potential N availability by 42, 64, and 30%, respectively, relative to non-mulched soils, likely via improving C and water availability for soil microbes. The findings that microbial properties and N availability for plants differed under different organic input regimes suggest the need for effective residue managements in organic tomato farming systems.     

Burning causes long-term changes in soil organic matter content of a South African grassland

The effects of burning a native grassland on soil organic matter status was investigated on a long-term (50 years) field experiment where different times and frequencies of burning were compared. Significant decreases in organic C were observed only in the surface 0-2 cm layer and only under annual and biennial winter burning and biennial and triennial autumn burning. Burning in spring did not significantly affect organic C content presumably because substantial amounts of litter decomposed and/or were incorporated into the soil by faunal activity prior to burning. Total N content was decreased substantially to a depth of 6 cm by all burning treatments and as a result, the C:N ratio of soil organic matter was widened. In addition, the amount of potentially mineralizable N, as measured by either aerobic incubation or plant N uptake in a pot experiment, was much reduced. Burning also induced a decrease in light fraction and hot water-extractable C in the 0-2 cm layer but an increase in these parameters, and in microbial biomass C and root density, in the 4-10 cm layer. This was attributed to burning causing a decrease in above-ground litter inputs but increased turnover of root material below the surface. Despite the decrease in organic C and total N content with increasing soil depth, potentially mineralizable N showed the opposite trend. This unexpected finding was confirmed at a nearby site under native grassland and contrasted with decreasing potentially mineralizable N with depth which was measured under a fertilized kikuyu grass dairy pasture. The wide C:N ratio of litter from native grassland, in association with the decreasing size and activity of the microbial biomass with depth results in greater N immobilization (thus less net mineralization) occurring in soil samples taken from close to the soil surface.     

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#>     

Long-Term Cultivation Effects on Biological C and N Fractions in a Fluvaquentic Haplustolls of Semi-arid Region of the Northern Turkey

Conversion of a native ecosystem can impact the nature and dynamics of organic carbon (C) fractions. The goal of this study was to determine the effects of cultivation and monoculture wheat production on soil organic C and biological C fractions compared to a previously flooded native pasture in northern Turkey. Soil samples were collected from four randomly selected locations of each management system. Some soil chemical [pH, calcium carbonate (CaCO₃), total nitrogen (N), and organic C], physical (sand, clay, and silt), and biological properties [microbial biomass carbon (MBC), mineralizable C, and mineralizable N] were measured. Conversion of pasture to cultivated land slightly increased soil pH, but CaCO₃, total organic C (TOC), and N contents were significantly (P < 0.05) decreased with cultivation. Total organic C and N contents were more than three times less in cultivated soils compared to pasture. Microbial biomass C was significantly decreased (P < 0.05) with long-term cultivation, and the greater seasonal fluctuations were measured at the surface of both ecosystems. The greatest level of potentially mineralizable C was observed in the pasture rather than the cultivated soil, but the proportional distribution of mineralized C to TOC was greater in the cultivated soil. These results suggested that the long-term cultivation (15 years) of previously flooded native ecosystems increased C mineralization and resulted in 72% C loss at the surface soil. Cultivated soils have a greater potential to restore atmospheric carbon dioxide (CO₂) if proper cultivation and management systems are used.     

Soil organic matter quality as influenced by tillage,lime, and phosphorus

In Eastern Canada, cereal yields are often restricted by soil acidity and low fertility. Continuous cereal production can also lead to soil structural degradation. The addition of lime and fertilizers and the adoption of conversation tillage practices are proposed solutions which may have a positive impact on soil quality. The objective of the present work was to assess the impact of 3 years of different tillage practices and P additions, and of a single lime addition on organic C and total N, microbial biomass C, and on N mineralization at the surface layer (0–7.5 cm) of a Courval sandy clay loam (Humic Gleysol). The easily mineralizable N, total amount of N mineralized in 22.1 weeks, the rate of N mineralization, and microbial biomass C were significantly greater in the minimum tillage than in the moldboard plow treatment. Chisel plow treatment showed intermediate values. The ratios of potentially mineralizable N and of easily mineralizable to total soil N were also significantly larger under minimum tillage and chisel plowing than under moldboard plowing. The lime and P treatments had no significant effect on the measured soil quality parameters. The total amount of N mineralized per unit of biomass C decreased as the tillage intensity increased, suggesting a decrease in the efficiency of the biomass in transforming organic N into potentially plant-available forms and thus a loss in soil organic matter quality. The results of this study indicate that conservation tillage practices such as rototilling and chisel plowing are efficient ways of maintaining soil organic matter quality when old pastures are brought back into cultivation.     

黄土高原典型土壤全氮和微生物氮剖面分布特征研究

为阐明黄土高原典型土壤全氮和微生物氮含量随土壤类型、土层和土地利用方式变化规律,研究了从北向南依次分布的干润砂质新成土(神木)、黄土正常新成土(延安)和土垫旱耕人为土(杨陵)等典型土壤的全氮和微生物氮含量的变化特征。结果表明,不同土壤类型、不同土层全氮和微生物氮含量存在显著差异。从南到北,全氮和微生物氮含量显著下降(P0.05)。对同一土壤类型,全氮和微生物氮含量在060.cm随土层深度增加下降很明显,60120.cm有轻微下降,120.cm以下低而稳定。微生物氮含量随土壤类型的变化趋势与全氮完全相同,其与土壤全氮、有机碳及微生物碳含量均存在极显著正相关关系(P0.01)。土壤微生物氮与全氮比值变化在0.42%9～.44%之间。虽然土地利用对土壤全氮和C/N比影响不显著,但却显著影响微生物氮含量和微生物氮与全氮的比值;与农田土壤相比,草地土壤微生物氮含量和微生物氮与全氮比值均明显增加。这一结果说明微生物氮含量和微生物氮与全氮比值更能有效、快速地反映土壤质量的变化。     

Carbon and nitrogen relationships in the microbial biomass of soils in beech (Fagus sylvatica L.) forests

Soils from 38 German forest sites, dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of overlying organic layers. Microbial biomass N and C were measured by fumigation-extraction. The pH of the soils varied between 3.5 and 8.3, covering a wide range of cation exchange capacity, organic C, total N, and soil C:N values. Maximum biomass C and biomass N contents were 2116 g C m^-2 and 347 g N m^-2, while minimum contents were 317 and 30 g m^-2, respectively. Microbial biomass N and C were closely correlated. Large variations in microbial biomass C:N ratios were observed (between 5.4 and 17.3, mean 7.7), indicating that no simple relationship exists between these two parameters. The frequency distribution of the parameters for C and N availability to the microflora divided the soils into two subgroups (with the exception of one soil): (1) microbial: organic C>12 mg g^-1, microbial:total N>28 mg g^-1 (n=23), a group with high C and N availability, and (2) microbial:organic C12 mg g^-1, microbial:total N28 mg g^-1 (n=14), a group with low C and N availability. With the exception of a periodically waterlogged soil, the pH of all soils belonging to subgroup 2 was below 5.0 and the soil C:N ratios were comparatively high. Within these two subgroups no significant correlation between the microbial C:N ratio and soil pH or any other parameter measured was found. The data suggest that above a certain threshold (pH 5.0) microbial C:N values vary within a very small range over a wide range of pH values. Below this threshold, in contrast, the range of microbial C:N values becomes very large.     

Soil microbial biomass and organic matter fractions during transition from conventional to organic farming systems

The aim of this study was to investigate the response of soil microbial biomass and organic matter fractions during the transition from conventional to organic farming in a tropical soil. Soil samples were collected from three different plots planted with Malpighia glaba: conventional plot with 10 years (CON); transitional plot with 2 years under organic farming system (TRA); organic plot with 5 years under organic farming system (ORG). A plot under native vegetation (NV) was used as a reference. Soil microbial biomass C (MBC) and N (MBN), soil organic carbon (SOC) and total N (TN), soil organic matter fractioning and microbial indices were evaluated in soil samples collected at 0–5, 5–10, 10–20 and 20–40 cm depth. SOC and fulvic acids fraction contents were higher in the ORG system at 0–5 cm and 5–10 cm depths. Soil MBC was highest in the ORG, in all depths, than in others plots. Soil MBN was similar between ORG, TRA and NV in the surface layer. The lowest values for soil MBC and MBN were observed in CON plot. Soil microbial biomass increased gradually from conventional to organic farming, leading to consistent and distinct differences from the conventional control by the end of the second year.     

Microbial biomass and mineralizable nitrogen in solonetzic soils: Influence of gypsum and lime amendments

Microbial biomass-C and -N and net mineralizable C and N were determined in the Ap horizon of several Solonetzic soils receiving gypsum (CaSO₄·2H₂O), lime, and NH₄NO₃ amendments. The main effect of gypsum, measured in long-term field experiments and laboratory studies, was to reduce soil microbial N by 10–43%, increase the C-to-N ratio of the biomass, and reduce mineralizable N by 10–54% and the resultant N mineralization potential. For the field study, a close relationship (r² = 0.82) was evident between microbial biomass N and the N mineralization potential.Both gypsum and NH₄NO₃ when applied alone or together changed soil chemical properties especially soil pH which decreased from approximately 6.0 to 5.0. Applications of NH₄NO₃ alone, however, did not influence microbial biomass or activity; while combined gypsum and NH₄NO₃ applied together had similar effects as gypsum alone. In the laboratory study, lime additions increased soil pH, microbial biomass and N mineralization. Small amounts of lime mixed with the gypsum, to prevent a drop in soil pH, corrected the deleterious effects of the gypsum on microbial biomass and mineralizable N for one of the Solonetzic soils.The results show the direct effect of gypsum on microbial activities and demonstrate inter-relationships between changes in soil chemistry of Solonetzic soils and microbial biomass and activity.     

Long‐term changes of aggregate‐associated and labile soil organic carbon and nitrogen after conversion from forest to grassland and cropland in northern Turkey

Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm⁻³) and cropland (1.14 g cm⁻³), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.     

Ecological study on the dynamics of soil organic matter and its related properties in shifting cultivation systems of Northern Thailand

Abstract

There is a large number of hill people in northern Thailand, who practices shifting cultivation. In order to analyze the soil ecological problems involved in the transition from traditional shifting cultivation to more intensive upland farming, the authors carried out comparative studies on the dynamics of organic matter and its related properties in soils both in the traditional shifting cultivation systems adopted by Karen people and more intensive upland farming practiced by Thai and Hmong people in the area. The contents of organic matter and available N in the surface 10 cm layers of soil from the fields continuously cultivated were lower than those in soils under prolonged fallow (more than 10 y) or natural forest. Based on the rate of soil respiration, the amount of organic matter decomposed within 1 y was estimated to reach nearly 10% of that stored in the upper 50 cm layers of the soil profile in the upland crop fields. These results indicate that the organic matter-related resources markedly decreased under continuous cropping. The contents of C, N, and P in the microbial biomass of the surface 10 cm layers of soil ranged from 0.37 to 2.09 mg C g^?l soil, from 22.7 to 188 µg N g^?l soil, and from 6.1 to 65.7 µg P g^?l soil, respectively. Since the contents of microbial C, N, and P in the surface soils were generally higher under prolonged fallow and natural forests than in the fields continuously cultivated, the microbial activity and/or the amounts of C, N, and P available for biological activity seemed to have declined under continuous upland farming. The incubation experiment to assess the N mineralization pattern showed two remarkable characteristics: 1) there was an initial time lag until active mineralization of N occurred in the soils from young fallow forest and 2) the soil burning effect was observed after burning in the fields under prolonged fallow. The active process of nitrification after N mineralization was always associated with a sharp fall in soil pH, suggesting that soil acidification was promoted and basic cations were lost from the soils. In conclusion, rapid deterioration of the soil organic matter-related properties in cropping fields can be considered to be one of the ecological reasons why upland fields must be returned to fallow again a few years after forest reclamation in traditional shifting cultivation systems. Therefore, in alternative farming systems with more intensive land use, it is essential to apply organic materials into soils to decrease the rate of soil degradation, or to improve the soil fertility, in avoiding soil acidification along with nitrification.     

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

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

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

Changes in amount of organic and inorganic fractions of nitrogen in an Eutrochrept soil after long‐term cropping with different fertilizer and organic manure inputs

The influence of 30 years of cropping with different fertilizer and farmyard manure (FYM) inputs on the contents and depth distribution of organic C, total N (N_t), soil mineralizable N, and organic and inorganic N fractions was investigated in an Eutrochrept. Continuous application of 100 %NPK(+S), 150 %NPK(+S), and 100 %NPK(+S)+FYM led to a marked increase in organic C, total N, hydrolyzable N (viz., amino acid N, hydrolyzable NH₄‐N, hexose amine N, and unidentified hydrolyzable N), and nonhydrolyzable N as compared to an adjacent fallow. The contents of the various organic N fractions were largest in surface soil and thereafter decreased with the depth. However, at 30 – 45 cm depth the content of organic C was not affected by the different treatments except 100 %NPK(+S)+FYM. On the other hand, continuous cropping without fertilization resulted in a depletion of total hydrolyzable N in control over fallow by 27.2 % (0–15 cm), 19.6 % (15–30 cm), and 4.7 % (30–45 cm). The incorporation of FYM with 100 %NPK(+S) resulted in greater contents of soil mineralizable N as compared to 100 %NPK(+S) (0–15, 15–30 cm). The proportion of hydrolyzable N (57–76 % of N_t) decreased and that of nonhydrolyzable N (22–40 % of N_t) increased with depth. The proportion of amino acid N (19–26 % of N_t), hexose amine N (2.1–3.5 % of N_t) and unidentified hydrolyzable N (17–27 % of N_t) decreased with depth. All organic soil N fractions including even nonhydrolyzable N in surface and subsurface soils were highly significantly correlated with soil mineralizable N derived from incubations under waterlogged and aerobic conditions. The best correlation to mineralizable N was found for amino acid N and the least significant correlation for nonhydrolyzable N.     

秸秆与地膜覆盖条件下旱作玉米田土壤氮组分生长季动态

研究不同覆盖措施下农田土壤全氮及其活性和半活性组分在作物生长季的动态变化,有助于深入理解农田土壤氮循环过程。基于黄土高原8年春玉米覆盖定位试验,系统分析了土壤全氮、矿质氮、微生物量氮、潜在可矿化氮以及颗粒有机氮在玉米不同生育期的动态特征。试验包括全生育期9 000kg/hm2秸秆覆盖、全生育期地膜覆盖和不覆盖对照3个处理。结果表明:(1)除硝态氮和铵态氮在苗期上升外,秸秆和地膜覆盖下土壤全氮及其组分含量在春玉米生育期基本呈苗期下降、拔节期上升、大喇叭口—抽雄期下降、灌浆和收获期回升的变化趋势;(2)与对照相比,秸秆覆盖提高了大多数生育时期0—40cm土层全氮和硝态氮含量及0—20cm土层铵态氮含量,提高各生育时期0—40cm土层微生物量氮、潜在可矿化氮以及颗粒有机氮含量;(3)地膜覆盖较对照提高大多数生育时期0—40cm土层硝态氮和0—20cm土层铵态氮含量,降低作物生育后期0—20cm土层全氮和0—40cm土层颗粒有机氮含量,降低大多数时期0—40cm土层微生物量氮和10—20cm土层潜在可矿化氮含量;(4)除了地膜覆盖下20—40cm土层颗粒有机氮相对含量在作物不同生育期差异不显著外,秸秆和地膜覆盖下0—40cm土层微生物量氮、潜在可矿化氮、颗粒有机氮对土壤全氮的动态均有重要贡献。总之,黄土高原的春玉米田秸秆覆盖具有明显的提升土壤全氮及其组分含量的作用,有助于培肥地力和土壤固氮;地膜覆盖则降低了作物生育后期土壤全氮及其组分含量,同时显著提高了土壤硝态氮水平,导致农田土壤氮素淋溶风险提高。     

中国亚热带稻田土壤碳氮含量及矿化动态

Dynamics of soil organic matter in a cultivation chronosequence of paddy fields were studied in subtropical China. Mineralization of soil organic matter was determined by measuring CO2 evolution from soil during 20 days of laboratory incubation. In the first 30 years of cultivation, soil organic C and N contents increased rapidly. After 30 years, 0-10 cm soil contained 19.6 g kg^-1 organic C and 1.62 g kg^-1 total N, with the corresponding values of 18.1 g kg^-1 and 1.50 g kg^-1 for 10-20 cm, and then remained stable even after 80 years of rice cultivation. During 20 days incubation the mineralization rates of organic C and N in surface soil （0-10 cm） ranged from 2.2% to 3.3% and from 2.8% to 6.7%, respectively, of organic C and total N contents. Biologically active C size generally increased with increasing soil organic C and N contents. Soil dissolved organic C decreased after cultivation of wasteland to 10 years paddy field and then increased. Soil microbial biomass C increased with number of years under cultivation, while soil microbial biomass N increased during the first 30 years of cultivation and then stabilized. After 30 years of cultivation surface soil （0-10 cm） contained 332.8 mg kg^-1 of microbial biomass C and 23.85 mg kg^-1 of microbial biomass N, which were 111% and 47% higher than those in soil cultivated for 3 years. It was suggested that surface soil with 30 years of rice cultivation in subtropical China would have attained a steady state of organic C content, being about 19 g kg^-1.     

Soil quality response to tillage and crop residue removal under subarctic conditions

Little is known about the long-term effects of tillage and crop residue management on soil quality and organic matter conservation in subarctic regions. Therefore, we quantified wet aggregate stability, bulk density, pH, total organic C and N, inorganic N, microbial biomass C and N, microbial biomass C:N ratio, microbial quotient, and potential C and N mineralization for a tillage/crop residue management study in central Alaska. Soil from no-till (NT), disked once each spring (DO), and disked twice (DT, spring and fall) treatments was sampled to 20 cm depth in spring and fall of the 16th and 17th years of the study. Crop residues were either retained or removed after harvest each year. Reducing tillage intensity had greater impact on most soil properties than removing crop residues with the most notable effects in the top 10 cm. Bulk density was the only indicator that showed significant differences for the 10–20 cm depth, with values of 0.74 Mg m⁻³ in the surface 10 cm in NT compared to 0.86 in DT and 1.22 Mg m⁻³ in NT compared to 1.31 in DT for the 10–20 cm depth. Wet aggregate stability ranged from 10% in DT to 20% in NT. Use of NT or DO conserved soil organic matter more than DT. Compared to measurements made in the 3rd and 4th years of the study, the DT treatment lost almost 20% of the soil organic matter. Retaining crop residues on the soil conserved about 650 g m⁻² greater C than removing all residues each year. Soil microbial biomass C and mineralizable C were highest in NT, but the microbial C quotient, which averaged only 0.9%, was not affected by tillage or crop residue treatment. Microbial biomass C:N ratio was 11.3 in DT and 14.4 in the NT, indicating an increasing predominance of fungi with decreasing tillage intensity. Barley grain yield, which averaged 1980 kg ha⁻¹ over the entire 17 years of the study, was highest in DO and not significantly different between NT and DT, but weeds were a serious problem in NT. Reduced tillage can improve important soil quality indicators and conserve organic matter, but long-term NT may not be feasible in the subarctic because of weed problems and build up of surface organic matter.