Relative importance of substrate type and previous soil management in synthesis of microbial biomass and substrate mineralization

Our aim was to determine whether the soil microbial biomass, which has developed naturally over many years in a given ecosystem, is specially adapted to metabolize the plant‐derived substrate C of the ecosystem within which it developed or whether the nature of recently added substrate is the more important factor. To examine this, soils from three sites in close proximity (woodland, grassland and arable from the Broadbalk Experiment at Rothamsted Research, Harpenden, UK) were each amended with air‐dried wheat straw (Triticum aestivum), ryegrass leaves (Lolium perenne) or woodland leaf litter (mainly Quercus robur and Fagus sylvatica) in a fully replicated 3 × 3 factorial laboratory experiment. The initial mineralization rates (evolved CO₂‐C) were determined during the first 6.5 hours and again, together with the amount of microbial biomass synthesized (microbial biomass C), at 7, 14, 21, 30 and 49 days of incubation. The hourly rate of CO₂‐C production during the first 6.5 hours was slowest following leaf litter addition, while the added grass gave the fastest rates of CO₂‐C evolution both within and between soils. Ryegrass addition to the arable soil led to approximately four times more CO₂‐C being evolved than when it was added to the woodland soil, at an overall rate in the arable soils of 41 μg C g^?1 soil hour^?1. In each soil, the net amounts of CO₂‐C produced were in the order grass > straw > leaf litter. In each case, the amount produced by the added leaf litter was significantly less (P < 0.05) than either the added grass or straw. Overall, the trend was for much slower rates of mineralization of all substrates in the woodland soil than in either the arable or grassland soils. During 49 days of incubation in the woodland and grassland soils, the net total amounts of CO₂‐C evolved differed significantly (P < 0.01), with grass > straw > leaf litter, respectively. In the arable soil, the amounts of CO₂‐C evolved from added grass and straw were significantly larger (P < 0.01) than from the leaf litter treatment. Our findings indicated that the amounts of CO₂‐C evolved were not related to soil management or to the size of the original biomass but to the substrate type. The amount of biomass C synthesized was also in the order grass > straw > leaf litter, at all stages of incubation in the woodland and grassland soil. In the arable soil, the same effect was observed up to 14 days, and for the rest of the incubation the biomass C synthesized was in the order grass > straw > leaf litter. Up to three times more biomass C was synthesized from the added grass than from the other substrates in all soils throughout the incubation. The maximum biomass synthesis efficiency was obtained with grass (7% of added C). Overall, the woodland soil was most efficient at synthesizing biomass C and the arable soil the least. We conclude that substrate type was the overriding factor that determined the amount of new soil microbial biomass synthesized. Mineralization of substrate C by soil microorganisms was also influenced mainly by substrate type and less by soil management or size of original biomass.     

Soil respiration is not limited by reductions in microbial biomass during long-term soil incubations

Declining rates of soil respiration are reliably observed during long-term laboratory incubations. However, the cause of this decline is uncertain. We explored different controls on soil respiration to elucidate the drivers of respiration rate declines during long-term soil incubations. Following a long-term (707 day) incubation (30 °C) of soils from two sites (a cultivated and a forested plot at Kellogg Biological Station, Hickory Corners, MI, USA), soils were significantly depleted of both soil carbon and microbial biomass. To test the ability of these carbon- and biomass-depleted (“incubation-depleted”) soils to respire labile organic matter, we exposed soils to a second, 42 day incubation (30 °C) with and without an addition of plant residues. We controlled for soil carbon and microbial biomass depletion by incubating field fresh (“fresh”) soils with and without an amendment of wheat and corn residues. Although respiration was consistently higher in the fresh versus incubation-depleted soil (2 and 1.2 times higher in the fresh cultivated and fresh forested soil, respectively), the ability to respire substrate did not differ between the fresh and incubation-depleted soils. Further, at the completion of the 42 day incubation, levels of microbial biomass in the incubation-depleted soils remained unchanged, while levels of microbial biomass in the field-fresh soil declined to levels similar to that of the incubation-depleted soils. Extra-cellular enzyme pools in the incubation-depleted soils were sometimes slightly reduced and did not respond to addition of labile substrate and did not limit soil respiration. Our results support the idea that available soil organic matter, rather than a lack microbial biomass and extracellular enzymes, limits soil respiration over the course of long-term incubations. That decomposition of both wheat and corn straw residues did not change after major changes in the soil biomass during extended incubation supports the omission of biomass values from biogeochemical models.     

Decomposition of Zn-rich Arabidopsis halleri Litter in Low and High Metal Soil in the Presence and Absence of EDTA

Hyperaccumulating plants are increasingly investigated in combination with EDTA addition to soil for phytoremediation of heavy metal contaminated soils. A 60-day incubation experiment was carried out to investigate the effects of heavy metal release during the decomposition of Zn-rich (15.7 mg g^?1 dry weight) Arabidopsis halleri litter on C mineralization, microbial biomass C, biomass N, ATP, and adenylate energy charge (AEC). These effects were investigated in two soils with different Zn, Cu, and Pb levels, with and without EDTA addition to soil. The sole addition of Zn-rich A. halleri litter to the two soils did not increase the contents of NH₄NO₃ extractable Zn, only with the combined additions of EDTA and litter was there a considerable increase, being equivalent to three times the added amount in the low metal soil and to 50% in the high metal soil. Litter amendment increased the CO₂ evolved; being equivalent to 44% of the added C in the two soils, but EDTA addition had no significant effect on CO₂ evolution. Litter amendment resulted also in an 18% increase in microbial biomass C, 27% increase in ATP and 6% increase in AEC in the two soils, but EDTA had again no effect on these indices at both metal levels. In contrast, the sole addition of litter had no effect on microbial biomass N, but EDTA addition increased microbial biomass N on average by 49%. The application of EDTA for chelate-assisted phytoextraction should in the future consider the risk of groundwater pollution, which is intensified by resistance of EDTA to microbial decomposition.     

Adenosine triphosphate and microbial biomass in soil

Two methods for measuring adenosine 5'-triphosphate (ATP) in soil were compared, one based on extraction with NaHCO₃-CHCl₃ and thel other on extraction by a trichloracetic acid-phosphate-paraquat reagent. Recoveries of added ATP were greater with the NaHCO₃-CHCl₃ reagent but the extraction of “native” soil ATP by NaHCO₃-CHCl₃ was only about a third of that by TCA-phosphate-paraquat.Microbial biomass C and ATP were measured in 8 contrasting English soils, using the fumigation method to measure biomass C and the TCA-phosphate-paraquat method to measure ATP. Except in one acid woodland soil, the ratio (ATP content of the soil)/(biomass C content of the soil) was relatively constant, with a mean of 7.3 mg ATP g^?1 biomass C for the different soils. This value is very similar to that obtained earlier in a range of 11 grassland and arable soils from Australia. Taking the English and Australian grassland and arable soils together, there is a close (r = 0.975) linear relationship between ATP and microbial biomass C that holds over a wide range of soils and climates. From this relationship, the soil biomass contains 7.25 mg ATP g^?1 biomass C, equivalent to an ATP-to-C ratio of 138, or to 6.04 μmoles ATP g^?1 dry biomass.The acid woodland soil (pH 3.9) contained much less biomass C, as measured by the fumigation method, than would have been expected from this relationship. This, and other evidence, suggests that the fumigation method for measuring microbial biomass C breaks down in strongly acid soils.The ATP content of the biomass did not depend on the P status of the soil, as indicated by NaHCO₃-extractable P.     

Influence of storage on soil microbial biomass estimated by three biochemical procedures

The effects of 28 and 56 days' storage at 25°, 4° and ?20°C on the microbial biomass content of four soils from tussock grasslands were studied by three biochemical procedures. Two of the procedures involved measurement of CO₂ and mineral-N (Min-N) production by chloroform-fumigated and unfumigated soil, and consequent estimation of biomass C and Min-N flush respectively. In the third, adenosine 5'-triphosphate (ATP) content was determined.Patterns of CO₂ production were often influenced by storage treatment. The use of fixed incubation periods for estimating the CO₂ flush of fumigated soil and the steady rate of CO₂ production by unfumigated soil did, however, give biomass C estimates that were generally similar to those calculated from individually determined incubation periods for each treatment and soil.Biomass C values could change significantly at all storage temperatures, but generally least at ?20°C. Storage at ?20°C was also the most suitable for retaining ATP contents, whereas 4°C was best for values of Min-N flush. Values of Min-N flush after storage of soil at ?20°C decreased significantly in two of the soils but increased in another. No storage temperature was thus satisfactory for all three indices of microbial biomass. Generally, however, 4°C was adequate for short periods, and 25°C the least suitable.     

Microbial biomass estimations in soils from tussock grasslands by three biochemical procedures

Microbial biomass was determined by three biochemical procedures in nine topsoils from a climosequence in tussock grasslands. The pH values of the samples ranged from 4.4 to 6.2 and organic C contents from 2.5 to 20.0%. When determined by a chloroform-fumigation procedure, contents of biomass C and mineral-N (Min-N) flush ranged from 530–2780 and 59–167 μgg^?1 dry soil respectively. Adenosine 5'-triphosphate (ATP) content ranged from 2.2 to 10.7 μg g^?1 dry soil. All three estimates were significantly correlated with each other and with several soil properties, including organic C and total N contents and CO₂ production. They were not significantly correlated with any climatic factor.In spite of these significant correlations, the ratios of the biomass estimates varied appreciably in the different soils. The ratios of biomass C/Min-N flush ranged from 7.8 to 22.8 (average 12.5), biomass C/ATP from 163 to 423 (average 248) and Min-N flush/ATP from 12 to 35 (average 22). These ratios were mostly higher than those found elsewhere for Australian and English soils. The high biomass C/ATP and Min-N flush/ATP ratios did not appear to originate from inefficient extraction of “native” ATP or from the soils' P status. Based on these results, care in the use of factors for obtaining soil microbial biomass content from Min-N flush or ATP values is indicated.     

Relation between respiration,ATP content,and Adenylate Energy Charge (AEC) after incubation at different temperatures and after drying and rewetting

Temperature, drying, and rewetting are important climatic factors that control microbial properties. In the present study we looked at the respiration rates, adenosine 5′‐triphosphate (ATP) content, and adenylate energy charge (AEC) as a measure for energy status of microbial biomass in the upper 5 cm of mineral soils of three beech forests at different temperatures and after rewetting. The soils differed widely in pH (4.0 to 6.0), microbial biomass C (92 to 916 μg (g DW)^—1) and ATP content (2.17 to 7.29 nmol ATP (g DW)^—1). The soils were incubated for three weeks at 7 °C, 14 °C, and 21 °C. After three weeks the microbial properties were determined, retaining temperature conditions. The temperature treatment did not significantly affect AEC or ATP content, but respiration rates increased significantly with increasing temperature. In a second experiment the soils were dried for 12 hours at 40 °C. Afterwards the soils were rewetted and microbial properties were monitored for 72 hours. After the drying, respiration rates dropped below the detection limit, but within one hour after rewetting respiration rates increased above control level. Drying reduced AEC by 16 % to 44 % and ATP content by 47 % to 78 %, respectively. Rewetting increased AEC and ATP content significantly as compared to dry soil, but after 72 hours the level of the controls was still not reached. The level of AEC values indicated dormant cells, but ATP content increased. These results indicate that the microbial carbon turnover was not directly linked to microbial growth or microbial energy status. Furthermore our results indicate that AEC may describe an average energy status but does not reflect phases of growing, dormant, or dying cells in the complex microbial populations of soils.     

Determination of ATP in soils: Effect of soil treatments

The adenosine triphosphate (ATP) contents of seven soil samples were determined after air-drying, freeze-drying, storage, incubation with glucose or water. The amount of ATP extracted was rapidly reduced after air-drying of the field moist soils, but a short period of wetting of the air-dried soils increased their ATP contents significantly. Addition of an ATP-uncoupler to the air-dried soils indicated that the additional amount of ATP extracted after wetting may not be due to synthesis during the wetting, but from some other processes. Freeze-drying of moist soils reduced the amounts of ATP extracted from soils to a lesser extent than air-drying. Storage of the freeze-dried soils at 25° and — 15°C led to substantial losses of ATP.Incubation of soils with and without glucose rapidly increased the ATP contents of soils, particularly those which had been air-dried previously.Biomass C: ATP ratios in two soils declined during the first few days of incubation and then became relatively constant as incubation proceeded, although significantly different for a loam and a clay soil.     

Microbial biomass dynamics in recently air-dried and rewetted soils compared to others stored air-dry for up to 103 years

Our aim was to compare the soil microbial biomass concentration and its activity (measured as CO₂-C evolved) following the rewetting and aerobic incubation of soils which have previously been stored air-dry for different periods. Some of the soils have been stored in the Rothamsted sample archive for 103 years, others were comparable freshly sampled soils following air-drying and rewetting and other soils were stored air-dry for 2 years then rewetted for the work described here. Following air-drying, soil ATP concentrations were variable in recently air-dried soil, comprising about 10-35% of the initial ATP concentrations in fresh soil. Following rewetting, the percentage recovery of ATP increased in all soils by 7 days, then declined to between 73% and 87% of the original ATP concentration in the air-dried soils by day 12. Storage of air-dried soils decreased the ability of the microbial biomass to restore its ATP concentrations. For example, the ATP concentration in a soil sampled from stubbed (i.e. tree seedling, saplings and bushes cut frequently to ground level) grassland of the Broadbalk continuous wheat experiment at Rothamsted then air-dried for 2 years was only about 14% of that in the fresh soil at 2 days after rewetting. In other soils from the Hoosfield Barley Experiment, also at Rothamsted, previously given NPK or FYM since 1852, and sampled then stored air-dry for between 13 and 83 years, from 52% to 57% of the ATP in the comparable fresh soils was measured at two days after rewetting. The soil ATP concentration then changed little more up to 12 days. One of the most interesting findings was that while the microbial biomass ATP concentration in the above NPK soils only ranged from about 2 to 4 μmol ATP g⁻¹ biomass C, in the FYM soil the microbial biomass ATP concentrations (range 11.5-13.6 μmol ATP g⁻¹ biomass C) were the same as we repeatedly measure in fresh moist aerobic soil. We do not yet know the reasons for this. More than twice as much CO₂-C was evolved from the long-term stored soils than from freshly sampled ones. However, the specific respiration of the microbial biomass did not change much after the first 12 years of storage, indicating that loss of viability mainly occurred in the earlier years.     

Adenylate energy charge and ATP-to-microbial biomass C ratio in soils differing in the intensity of disturbance

We carried out an 8-days' incubation experiment with three different intensities of soil disturbance to analyse the effects on the ATP-to-microbial biomass C ratio and on the adenylate energy charge (AEC=(ATP+0.5×ADP)/(AMP+ADP+ATP). Single mixing of soil at 50% water holding capacity with a spatula during weighing of the samples into extraction jars at the end of the 8-days' incubation or 8-times repeated daily mixing for 2 min triggered the immediate formation of ATP, increasing both AEC and the ATP-to-microbial biomass C ratio. The energy for this extra ATP produced seems to be mainly derived from an accelerated turnover of C within the microbial biomass. In contrast, 8-days' continuous mixing led to a significant decrease in AEC and ATP-to-microbial biomass C ratio.     

Salinity-induced changes in the microbial use of sugarcane filter cake added to soil

A laboratory experiment was carried out to prove the hypothesis that the decomposition of a complex organic substrate is reduced by the lower content of fungal biomass in a saline soil in comparison to a non-saline soil under acidic conditions. Three different rates (0.5, 1.0, and 2.0%) of sugarcane filter cake were added to both soils and incubated for 63 days at 30 °C. In the saline control soil without amendment, cumulative CO₂ production was 70% greater than in the corresponding non-saline control soil, but the formation of inorganic N did not differ between these two soils. However, nitrification was inhibited in the saline soil. The increase in cumulative CO₂ production by adding filter cake was similar in both soils, corresponding to 29% of the filter cake C at all three addition rates. Also, the increases in microbial biomass C and biomass N were linearly related to the amount of filter cake added, but this increase was slightly higher for both properties in the saline soil. In contrast to microbial biomass, the absolute increase in ergosterol content in the saline soil was on average only half of that in the non-saline soil and it also showed strong temporal changes during the incubation: a strong initial increase after adding the filter cake was followed by a rapid decline. The addition of filter cake led to immobilisation of inorganic N in both soils. This immobilisation was not expected, because the total C-to-total N ratio of the filter cake was below 13 and the organic C-to-organic N ratio in the 0.5 M K₂SO₄ extract of this material was even lower at 9.2. The immobilisation was considerably higher in the saline soil than in the non-saline soil. The N immobilisation capacity of sugarcane filter cake should be considered when this material is applied to arable sites at high rations.     

Biochemical properties of forest soils as affected by a fire retardant

Synthetic polymers are currently being used as water additives to control wildfires and prescribed burns. This laboratory study examines the effects of one of these acrylic-based polymers (Firesorb) on some biochemical properties (microbial biomass C, hydrolysis of FDA, #-glucosidase, urease and N mineralization) of two coarse textured soils (loamy sand and sandy loam) under pinewood located at Galicia (NW Spain). Firesorb was added to unheated and heated soil samples at two levels of application (1 and 3 times the recommended dose) and measurements were made after 6 and 12 weeks of aerobic incubation. The results obtained for both soils at different incubation times were found to be comparable. Except for N mineralization, which was reduced by Firesorb addition, in both unheated and heated soils, the Firesorb-treated samples showed similar or significantly higher values for the biochemical parameters analyzed than those in the untreated control soils. This finding suggests that under these assay conditions the synthetic polymer used as a fire-fighting chemical had no adverse effects on soil microbial communities.     

Microbial biomass and activity in soils amended with glucose

Four contrasting soils were amended with glucose at concentrations up to 10 mg g^?1 soil. The soils were incubated at 22°C for 14 days and the biomass determined at various times by chloroform fumigation or substrate-induced respiration. The adenosine triphosphate (ATP) content or the amylase and dehydrogenase activities were also determined. The size of the increases in biomass, ATP content and the enzyme activities was generally related to the amount of glucose added. The initially higher ATP levels quickly declined, and apparent substrate conversion figures up to 84% indicated that substrate-induced respiration overestimated the biomass. There were generally no significant correlations between ATP, biomass or enzyme activities.     

Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass

Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass.     

Interference from plant roots in the estimation of soil microbial ATP,C, N and P

Excised, solution-grown roots of maize or ryegrass added to two pasture soils at the rate of 6.0mg g^?1 and 13.8 mg g^?, respectively, increased the flush (fumigated minus control values) of CO₂-C by up to 1.89-fold, KCl extractable N by up to 1.88-fold, and NaHCO₃ extractable P by 3.28-fold. The ATP content of the soil was increased by up to 1.42-fold. Because of high variability the effect of the roots on the C and N flushes was not significant at P < 0.05.Incubation of the root-amended soils for 7 days at 25°C prior to fumigation much decreased the contribution from the roots to the C and N flush, and to the ATP content. There was, however, still a large significant effect of the roots on the P-flush, this being up to 3 times greater than the equivalent soil without roots.In soil samples with a high viable root density (> 6mg g^?1) such as may occur in dense pastures, greenhouse pot experiments or rhizosphere soil samples, it is recommended that they be incubated for 7 days prior to fumigation and analyses. Without such prior incubation there is the risk that root material may be included in the “microbial” biomass estimations.     

CO2 efflux by rapid decomposition of low molecular organic substances in soils

Decomposition rates of the [2-¹⁴C]-glucose and [2-¹⁴C]-glycine in four different soils of the long-term field trial of Moscow were investigated in a 3-months laboratory experiment in which ¹⁴CO₂ respiration was measured. A model with three decomposition components and two distribution parameters was developed and validated with the data of the experiment. The decay rate constants of free [2-¹⁴C]-glucose (4–32 day^-1) were slower than those of [2-¹⁴C]-glycine (16–44 day^-1). The calculated use efficiency for microbial biosynthesis of the second carbon atom was 47% for glucose and 31% for glycine. The potential half-life of labelled carbon in the microbial soil biomass ranged from 0.6 to 4.4 days, depending on the soil type and the initial amount of added substrate. The calculated total utilisation of carbon by the soil biomass from glycine was about 2–5 times lower than that of glucose.The modelled ¹⁴C incorporation into the microbial soil biomass reached its maximum on the first day of the incubation experiment and did not exceed 22% of the ¹⁴C input. Both of the investigated substances decomposed most rapidly in the soil samples from sites that have not being fertilised with organic or mineral fertilisers during an 81-years period.     

Determination of the fate of regularly applied 13C‐labeled‐artificial‐exudates C in two agricultural soils

Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of ¹³C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO₂‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO₂‐C concentrations and the respective δ¹³C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO₂‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC << MBC < CO₂‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO₂‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.     

Soil moisture and the rates of biodegradation of diallate and triallate

Degradation experiments were combined with biomass measurements and adsorption tests to determine how soil moisture content influences the rates of degradation of ⁴¹C-labelled diallate and triallate. In soils treated with 1 μg^?1 herbicide and incubated at constant temperature and moisture, degradation rates were regulated by two variables: the quantity of microbial biomass in the soil; and the quantity of herbicide dissolved in the soil solution. The quantity of biomass was influenced by soil water content and the duration of incubation. The amounts of herbicide in solution were determined by the amount of water present and the total quantity of herbicide in the soil. In all soil samples, the rates of degradation increased with increasing water content but decreased with prolonged incubation. The factors responsible for decrease with time were the loss of biomass during incubation and the decline in herbicide concentration in the soils as degradation proceeded.     

Mineralization and nitrification in three Malaysian forest soils

Examination of three forest soils from Malaysia using the soil incubation technique suggests that nitrification was not inhibited in these oligotrophic soils. Nitrification rates were between 40 and 750 ngN produced g^?1 dry weight soil day^?1 of incubation. Addition of phenolic metabolites (tannic acid) and leaf filtrates from hill and lowland forest litter did not significantly inhibit nitrification. Addition of sucrose (1% w/w carbon source) decreased nitrification but not ammonification.     

冬小麦-夏玉米轮作体系中不同施氮水平对玉米生长及其根际土壤氮的影响

在河北衡水潮土上进行田间试验,以当地习惯高氮用量(小麦季施N 300 kg/hm2,玉米季施N 240 kg/hm2)为对照,研究冬小麦-夏玉米轮作体系中减少氮肥用量对玉米季植株生长、氮素吸收及根际土壤中无机氮与微生物量氮的影响。结果表明,两季作物氮肥施用量减少25%和40%,对玉米产量、生物量及植株体内氮累积量未产生明显影响,氮肥利用率提高。不同氮肥施用量对根际和非根际土壤铵态氮含量的影响不显著;减少氮肥施用量,对玉米根际土壤硝态氮含量也没有明显影响。在玉米苗期、抽雄期和成熟期,习惯高施氮量处理的非根际土壤硝态氮含量较高,其中抽雄期,非根际土壤硝态氮含量较氮肥减施40%用量处理高出近一倍,但非根际土壤微生物量氮水平含量明显降低。氮肥减施未影响根际土壤微生物量碳、氮含量,反而增加了非根际土壤微生物量碳、氮水平。在高肥力的潮土上,冬小麦/夏玉米轮作体系中适当减施氮肥并未影响玉米根际土壤氮素水平,可保证玉米稳产,实现减氮增效。