How increasing availabilities of carbon and nitrogen affect atrazine behaviour in soils

 The effect of increasing amounts of glucose and mineral N on the behaviour of atrazine was studied in two soils. One had been exposed to atrazine under field conditions (adapted soil), the other had not (non-adapted soil), resulting, respectively, in an accelerated degradation of atrazine in the adapted soil and in a slow degradation of the herbicide in the non-adapted soil. The dissipation of ¹⁴C-atrazine via degradation and formation of non-extractable "bound" residues was followed during laboratory incubations in soils supplemented or not with increasing amounts of glucose and mineral N. In both soils, glucose added at rates of up to 16 g C kg^–1 soil did not modify atrazine mineralization but increased the formation of bound residues; this was probably due to the retention of atrazine by the growing microbial biomass. Atrazine dealkylation was enhanced when a large amount of glucose was added. In both soils, the addition of the largest dose of mineral N (2.5 g N kg^–1 soil) decreased atrazine mineralization. The simultaneous addition of glucose and mineral N enhanced their effects. When the largest doses of mineral N and glucose were added, atrazine mineralization stopped in both soils, and the proportion of bound residues increased. Glucose and mineral N additions influenced atrazine mineralization to a greater extent in the adapted soil than in the non-adapted one, as revealed by ANOVA, although glucose addition had a greater effect than N. The competition for space and nutrients between atrazine-degrading microorganisms and the total heterotrophic microflora probably contributed to the decrease in atrazine mineralization. Received: 9 June 1998     

Biochemical properties and microbial community structure of five different soils after atrazine addition

Atrazine is one of the most used herbicides worldwide; however, consequences of its long-term agricultural use are still unknown. A laboratory study was performed to examine changes in microbial properties following ethylamino-¹⁵N-atrazine addition, at recommended agronomic dose, to five acidic soils from Galicia (NW Spain) showing different physico-chemical characteristics, as well as atrazine application history. Net N mineralization was observed in all soils, with nitrate being the predominant substance formed. The highest values were detected in soils with low atrazine application history. From 2% to 23% of the atrazine-¹⁵N was found in the soil inorganic-N pool, the highest values being detected after 9 weeks in soils with longer atrazine application history and lower indigenous soil N mineralization. The application of atrazine slightly reduced the amount of soil N mineralized and microbial biomass at short term. Soluble carbohydrates and β-glucosidase and urease activity decreased with incubation time, but were not significantly affected by the single application of atrazine. Microbial community structure changed as consequence of both soil type and incubation time, but no changes in the phospholipid fatty acid (PLFA) pattern were detected due to recent atrazine addition at normal doses. The saturated 17- to 20-carbon fatty acids had higher relative abundance in soils with a longer atrazine history and fungal biomass, as indicated by the PLFA 18:2ω6,9, decreased with the incubation time. The results suggested that the PLFA pattern and soil N dynamics can detect the long-term impact of repeated atrazine application to agricultural soils.     

Lumbricid macrofauna alter atrazine mineralization and sorption in a silt loam soil

Atrazine is a widely used herbicide and is often a contaminant in terrestrial and freshwater ecosystems. It is uncertain, however, how the activity of soil macrofauna affects atrazine fate and transport. Therefore, we investigated whether earthworms enhance atrazine biodegradation by stimulating herbicide degrading soil microflora, or if they increase atrazine persistence by facilitating herbicide sorption. Short (43 d) and medium term (86 d) effects of the earthworms Lumbricus terrestris and Aporrectodea caliginosa on mineralization, distribution, and sorption of U-ring-¹⁴C atrazine and on soil C mineralization was quantified in packed-soil microcosms using silt loam soil. A priming effect (stimulation of soil C mineralization) caused by atrazine supply was shown that likely lowered the earthworm net effect on soil C mineralization in atrazine-treated soil microcosms. Although earthworms significantly increased soil microbial activity, they reduced atrazine mineralization to ¹⁴CO₂-C from15.2 to 11.7% at 86 d. Earthworms facilitated formation of non-extractable atrazine residues within C-rich soil microsites that they created by burrowing and ingesting soil and organic matter. Atrazine sorption was highest in their gut contents and higher in casts than in burrow linings. Also, gut contents exhibited the highest formation of bound atrazine residues (non-extractable atrazine). Earthworms also promoted a deeper and patchier distribution of atrazine in the soil. This contributed to greater leaching losses of atrazine in microcosms amended with earthworms (3%) than in earthworm-free microcosms (0.003%), although these differences were not significant due to high variability in transport from earthworm-amended microcosms. Our results indicated that earthworms, mainly by casting activity, facilitated atrazine sorption, which increased atrazine persistence. As a consequence, this effect overrode any increase in atrazine biodegradation due to stimulation of microbial activity by earthworms. It is concluded that the affect of earthworms of atrazine mineralization is time-dependent, mineralization being slightly enhanced in the short term and subsequently reduced in the medium term.     

Drying and rewetting effects on C and N mineralization and microbial activity in surface and subsurface California grassland soils

Rewetting a dry soil has long been known to cause a burst of respiration (the “Birch Effect”). Hypothesized mechanisms for this involve: (1) release of cellular materials as a result of the rapid increase in water potential stress and (2) stimulating C-supply to microbes via physical processes. The balance of these factors is still not well understood, particularly in the contexts of multiple dry/wet cycles and of how resource and stress patterns vary through the soil profile. We evaluated the effects of multiple dry/wet cycles on surface and subsurface soils from a California annual grassland. Treatments included 4, 6, and 12 cycles that varied the length of the drying period between rewetting events. Respiration was monitored after each wetting event while extractable C and N, microbial biomass, and microbial activity were assayed initially, after the first rewetting event, and at the end of the experiment. Initially, microbial biomass and activity (respiration, dehydrogenase, and N mineralization) in subsurface soils were ca. 10% and 20% of surface soil levels. After multiple cycles, however, subsurface soil microbial biomass and activity were enhanced by up to 8-fold, even in comparison to the constantly moist treatment. By comparison, surface soil microbial biomass and activity were either moderately (i.e. 1.5 times increase) or not affected by wetting and drying. Drying and rewetting led to a cascade of responses (soluble C release, biomass growth, and enhanced activity) that mobilized and metabolized otherwise unavailable soil carbon, particularly in subsurface soils.     

Effect of long-term liquid pig manure application on atrazine mineralization in a soil cultivated with maize

Soil samples were collected in plots from a field experiment in maize monoculture receiving 0, 60 and 120 m³ ha^-1 liquid pig manure (LPM) for 19 years. Soils were sampled from the 0- to 20-cm layer in August and October 1997 and in June, July and September 1998. Subsurface samples were also evaluated in September 1998. Laboratory soil radiorespirometry was used to evaluate atrazine mineralization using [U-ring-¹⁴C]-atrazine mixed with commercially available product. The effect of atrazine dose (50, 100 and 500 mg atrazine kg^-1 soil) was evaluated on soils sampled in August 1997. For the other sampling dates, the soils were spiked with 50 mg atrazine kg^-1 soil. No LPM dose effect on atrazine mineralization was obtained in the different experiments. Increasing atrazine dose to 500 mg kg^-1 decreased significantly the mineralization rate (R_i) and the maximum of atrazine mineralized (MAX), while the time needed to mineralize 50% of MAX (DT-50%) was not significantly affected. Sampling time had a significant effect on atrazine mineralization. Atrazine mineralization in the soils sampled in June 1998 showed lower R_i and MAX than in the soils sampled at the other dates. Atrazine mineralization in subsurface soils (20–60 cm) was very variable and quite high in some samples. This may be due to atrazine pre-exposure in subsoils resulting from atrazine deep movement by preferential flow.     

Atrazine mineralization in bulk soil and maize rhizosphere

An experiment was carried out in the greenhouse in order to compare atrazine mineralization in bulk soil and maize rhizosphere at different development stages. After 4, 8 and 12 weeks, we have (1) measured the soil microbial biomass C, (2) characterized the C substrate utilization profiles of the culturable microflora, and (3) analyzed atrazine mineralization. Microbial growth was stimulated in planted soil and different C substrate utilization patterns were obtained in bulk and rhizosphere soils during the first 2 months. During this period, laboratory tests for atrazine biodegradation revealed a lower mineralization potential in bulk than in planted soil. Atrazine mineralization was stimulated to a greater extent after atrazine application in the greenhouse but again the presence of plants had a favorable effect. After 12 weeks of cropping, the atrazine mineralization potential decreased in planted soil with or without prior atrazine application.     

Metabolism and persistence of atrazine in several field soils with different atrazine application histories

To assess the potential occurrence of accelerated herbicide degradation in soils, the mineralization and persistence of (14)C-labeled and nonlabeled atrazine was evaluated over 3 months in two soils from Belgium (BS, atrazine-treated 1973-2008; BC, nontreated) and two soils from Germany (CK, atrazine-treated 1986-1989; CM, nontreated). Prior to the experiment, accelerated solvent extraction of bulk field soils revealed atrazine (8.3 and 15.2 μg kg(-1)) in BS and CK soils and a number of metabolites directly after field sampling, even in BC and CM soils without previous atrazine treatment, by means of LC-MS/MS analyses. For atrazine degradation studies, all soils were incubated under different moisture conditions (50% maximum soil water-holding capacity (WHC(max))/slurried conditions). At the end of the incubation, the (14)C-atrazine mineralization was high in BS soil (81 and 83%) and also unexpectedly high in BC soil (40 and 81%), at 50% WHC(max) and slurried conditions, respectively. In CK soil, the (14)C-atrazine mineralization was higher (10 and 6%) than in CM soil (4.7 and 2.7%), but was not stimulated by slurried conditions. The results revealed that atrazine application history dramatically influences its degradation and mineralization. For the incubation period, the amount of extractable atrazine, composed of residues from freshly applied atrazine and residues from former field applications, remained significantly greater (statistical significance = 99.5 and 99.95%) for BS and CK soils, respectively, than the amount of extractable atrazine in the bulk field soils. This suggests that (i) mostly freshly applied atrazine is accessible for a complex microbial community, (ii) the applied atrazine is not completely mineralized and remains extractable even in adapted soils, and (iii) the microbial atrazine-mineralizing capacity strongly depends on atrazine application history and appears to be conserved on long time scales after the last application.     

Short-Term Effect of Nitrogen Intensification on Aggregate Size Distribution, Microbial Biomass and Enzyme Activities in a Semi-Arid Soil Under Different Crop Types

There is a lack of quantitative assessments available on the effect of agricultural intensification on soil aggregate distribution and microbial properties. Here, we investigated how short-term nitrogen(N) intensification induced changes in aggregate size distribution and microbial properties in a soil of a hot moist semi-arid region(Bangalore, India). We hypothesised that N intensification would increase the accumulation of macroaggregates 2 mm and soil microbial biomass and activity, and that the specific crop plant sowed would influence the level of this increase. In November 2016, surface(0–10 cm) and subsurface(10–20 cm) soil samples were taken from three N fertilisation treatments, low N(50 kg N ha~(-1)), medium N(75 and 100 kg N ha~(-1) for finger millet and maize, respectively),and high N(100 and 150 kg N ha~(-1) for finger millet and maize, respectively). Distribution of water-stable aggregate concentrations,carbon(C) and N dynamics within aggregate size class, and soil microbial biomass and activity were evaluated. The high-N treatment significantly increased the concentration of large macroaggregates in the subsurface soil of the maize crop treatment, presumably due to an increased C input from root growth. Different N fertilisation levels did not significantly affect C and N concentrations in different aggregate size classes or the bulk soil. High-N applications significantly increased dehydrogenase activity in both the surface soil and the subsurface soil and urease activity in the surface soil, likely because of increased accumulation of enzymes stabilised by soil colloids in dry soils. Dehydrogenase activity was significantly affected by the type of crop, but urease activity not. Overall, our results showed that high N application rates alter large macroaggregates and enzyme activities in surface and subsurface soils through an increased aboveground and corresponding belowground biomass input in the maize crop.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

阿特拉津对我国东北半干旱区黑钙土微生物活性影响研究

按照0.5～800 μg·g^-1 soil 施入量添加阿特拉津到黑钙土中培养54 d进行实验室培养试验,研究了阿特拉津对我国东北半干旱区黑钙土微生物量碳、土壤碳及氮矿化量、脲酶和脱氢酶活性影响。结果表明,阿特拉津添加到土壤中后,显著提高了（P〈0.05）土壤微生物量碳,增加了土壤碳及氮矿化量,提高土壤脱氢酶活性;多数情况下,各培养时间添加阿特拉津各处理间土壤脲酶活性的差异未达到显著水平（P〈0.05）。由此得出结论：土壤脱氢酶活性、土壤微生物量碳和土壤碳矿化量及氮矿化量是对阿特拉津处理土壤较敏感的生物学指标,适合作为半干旱区黑钙土微生物活性对阿特拉津响应的参数;而土壤脲酶活性不适合作为半干旱区黑钙土土壤微生物活性的指标,因为它不能敏感地反映阿特拉津作用下土壤脲酶活性差异。     

Microbial mineralization of atrazine and 2,4-dichlorophenoxyacetic acid in riparian pasture and forest soils

Microbial biomass and mineralization of atrazine [2-chloro-4(ethylamino)-6(isopropylamino)s-triazine] and 2,4-D (2,4-dichlorphenoxyacetic acid) were examined in the top 10 cm of riparian pasture soils and in the litter layer and top 10 cm of mineral soils of riparian forest ecosystems. The riparian forest litter had higher levels of active and total fungal biomass than forest or pasture mineral soils in winter, spring, and fall. Active bacterial biomass was higher in forest litter than in forest and pasture mineral soils in spring and autumn, and higher in forest mineral soils than in pasture soils in summer. Total bacterial biomass was higher in forest mineral soils than in pasture soils during all seasons. In spring, it was also higher in forest litter than in pasture soils. Atrazie and 2,4-D mineralization in pasture soils was exceeded by that in forest litter in spring and autumn and by that in forest mineral soils in summer and autumn. There was no correlation between either active or total fungal and bacterial biomass with pesticide degradation.     

Phosphatase activity does not limit the microbial use of low molecular weight organic-P substrates in soil

Plant roots and soil microorganisms contain significant quantities of low molecular weight (MW) phosphorylated nucleosides and sugars. Consequently, upon death these can represent a significant input of organic-P to the soil. Some of these organic-P substrates must first be dephosphorylated by phosphatases before being assimilated by the soil microbial community while others can be taken up directly from soil solution. To determine whether sorption or phosphatase activity was limiting the bioavailability of low MW organic-P in soil we compared the microbial uptake and C mineralization of a range of ¹⁴C-labeled organic-P substrates [glucose-6-phosphate, adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP)] to that of the parent compounds (adenosine and glucose). In a fertile grassland soil we showed that at low organic-P substrate concentrations (<0.5 mM) phosphatase activity did not limit microbial uptake or mineralization in comparison to their non-phosphorylated counterparts. However, at high substrate concentrations (1-10 mM) the mineralization of the organic-P compounds was significantly lower than that of the non-phosphorylated compounds suggesting that phosphatase activity or microbial transporter capacity limited bioavailability. Sorption to the solid phase followed the series glucose<adenosine<G-6-P<AMP<ADP=ATP. However, sorption of the organic-P compounds to the solid phase did not appear to greatly affect bioavailability. The high adenosine mineralization capacity of the microbial biomass suggests that nucleosides may represent a significant source of C and N to the soil microbial biomass. We conclude that at low organic-P substrate concentrations typical of those in soil, neither phosphatase activity nor sorption greatly limits their bioavailability.     

Tillage effects on microbial biomass and nutrient distribution in soils under rain-fed corn production in central-western Mexico

Quantifying how tillage systems affect soil microbial biomass and nutrient cycling by manipulating crop residue placement is important for understanding how production systems can be managed to sustain long-term soil productivity. Our objective was to characterize soil microbial biomass, potential N mineralization and nutrient distribution in soils (Vertisols, Andisols, and Alfisols) under rain-fed corn (Zea mays L.) production from four mid-term (6 years) tillage experiments located in central-western, Mexico. Treatments were three tillage systems: conventional tillage (CT), minimum tillage (MT) and no tillage (NT). Soil was collected at four locations (Casas Blancas, Morelia, Apatzingán and Tepatitlán) before corn planting, at depths of 0–50, 50–100 and 100–150 mm. Conservation tillage treatments (MT and NT) significantly increased crop residue accumulation on the soil surface. Soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were highest in the surface layer of NT and decreased with depth. Soil organic C, microbial biomass C and N, total N and extractable P of plowed soil were generally more evenly distributed throughout the 0–150 mm depth. Potential N mineralization was closely associated with organic C and microbial biomass. Higher levels of soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were directly related to surface accumulation of crop residues promoted by conservation tillage management. Quality and productivity of soils could be maintained or improved with the use of conservation tillage.     

Surface and subsurface microbial biomass, community structure and metabolic activity as a function of soil depth and season

Microbial biomass, size and community structure along with an estimate of microbial activity and soil chemical parameters were determined at three depths in two soils (e.g. sandy loam Ultic Hapludalf and silt loam Mollic Hapludalf) replicated three times under one winter and summer season. Microbial biomass and community structure were estimated from phospholipid-PO₄ content and fatty acid methyl ester (FAME) measurements. Microbial activity and assimilative capacity were estimated using a ³H-acetate incorporation into phospholipids and by incubating the soil samples at the average winter and summer temperatures, 3 and 20 °C, respectively. We found that the size of the microbial biomass in both the surface and the subsurface soils was not significantly affected by the seasonal variation but activity increased by as much as 83% at the summer temperatures in the surface soil. We demonstrated using FAME analysis that for both soils seasonal changes in the subsurface microbial community occurred. These findings suggest that winter conditions will shift the population activity level in both the surface and subsurface systems and the biochemical structure of the community in the subsurface. In all cases, the inorganic chemical properties of the soil, as a function of season, remained constant. The greatly increased activity of microbial population at the higher temperature will favor the capacity of the system to utilize nutrients or organic materials that may enter soil. During low temperature seasons the capacity of either surface or subsurface soils to assimilate materials is generally diminished but the reduction reflects changes in metabolism and not a reduced biomass size.     

Impact of tillage on microbial activity and the fate of pesticides in the upper soil

The impact of two tillage systems, plow tillage (PT) and no-tillage (NT), on microbial activity and the fate of pesticides in the 0–5 cm soil layer were studied. The insecticides carbofuran and diazinon, and the herbicides atrazine and metolachlor were used in the study, which included the incubation and leaching of pesticides from untreated soils and soils in which microorganisms had been inhibited. The mineralization of ring¹⁴C labeled pesticides was studied. The study differentiated between biotic and abiotic processes that determine the fate of pesticides in the soil. Higher leaching rates of pesticides from PT soils are explaned by the relative importance of each of these processes. In NT soils, higher microbial populations and activity were associated with higher mineralization rates of atrazine, diazinon and carbofuran. Enhanced transformation rates played an important role in minimizing the leaching of metolachlor and carbofuran from NT soils. The role of abiotic adsorption/retention was important in minimizing the leaching of metolachlor, carbofuran and atrazine from NT soils. The role of fungi and bacteria in the biodegradation process was studied by selective inhibition techniques. Synergistic effects between fungi and bacteria in the degradation of atrazine and diazinon were observed. Carbofuran was also degraded in the soils where fungi were selectively inhibited. Possible mechanisms for enhanced biodegradation and decreased mobility of these pesticides in the upper layer of NT soils are discussed.     

Rapid development of enhanced atrazine degradation in a Dundee silt loam soil under continuous corn and in rotation with cotton

Mississippi Delta cotton (Gossypium hirsutum L.) production in rotation with corn (Zea mays L.) was evaluated in field experiments from 2000 to 2005 at Stoneville, Mississippi. Plots maintained under minimum tillage were established in 2000 on a Dundee silt loam with treatments including continuous cotton or corn and alternate cotton-corn rotations. Mineralization and dissipation of 14C [ring]-labeled atrazine were evaluated in the laboratory on soils collected prior to herbicide application in the first, second, third, and sixth years of the study. In soils collected in 2000, a maximum of 10% of the atrazine was mineralized after 30 days. After 1 year of herbicide application, atrazine-treated soils mineralized 52-57% of the radiolabeled atrazine in 30 days. By the sixth year of the study, greater than 59% of the atrazine was mineralized after 7 days in soils treated with atrazine, while soils from plots with no atrazine treatment mineralized less than 36%. The data also indicated rapid development of enhanced atrazine degradation in soils following 1 year of corn production with atrazine use. Atrazine mineralization was as rapid in soils under a rotation receiving biannual atrazine applications as in soils under continuous corn receiving annual applications of atrazine. Cumulative mineralization kinetics parameters derived from the Gompertz model (k and ti) were highly correlated with a history of atrazine application and total soil carbon content. Changes in the soil microbial community assessed by total fatty acid methyl ester (FAME) analysis indicated significant interactions of cropping system and sampling date, with FAME indicators for soil bacteria responsible for differences in community structure. Autoclaved soil lost all ability to mineralize atrazine, and atrazine-mineralizing bacteria were isolated from these plots, confirming the biological basis for atrazine mineralization. These results indicate that changes in degradative potential of a soil can occur rapidly and some changes in soil properties may be associated with cropping systems, which can contribute to enhanced atrazine degradation potential.     

The influence of nitrogen on atrazine and 2,4-dichlorophenoxyacetic acid mineralization in grassland soils

The influence of fertilizer N on the mineralization of atrazine [2-chloro-4(ethylamino)-6(isopropylamino)-s-triazine] and 2,4-D (2,4-dichlorophenoxyacetic acid) in soils was assessed in microcosms using radiometric techniques. N equivalent to 0, 250, and 500 kg N as NH₄NO₃ ha^-1 was added to three grassland soils. Compared to the control, the 250- and 500-kg treatments suppressed mineralization of atrazine by 75 and 54%, respectively, and inhibited mineralization of 2,4-D by 89 and 30%, respectively. Active fungal biomass responded to the N treatments in an opposite manner to herbicide mineralization. Compared to the control, the 250- and 500-kg treatments increased the active fungal biomass by more than 300 and 30%, respectively. These results agree with other observations that N can suppress the decomposition of resistant compounds but stimulate the primary growth of fungi. The degree of suppression was not related to the amount of N added nor to the inherent soil N levels before treatment. The interaction between the N additions and the active fungal biomass in affecting herbicide mineralization suggests that N may alter microbial processes and their use of C sources and thus influence rates of herbicide degradation in the field.     

Short-term effects of tillage systems on active soil microbial biomass

 Conservation tillage, and especially no-tillage, induce changes in the distribution of organic pools in the soil profile. In long-term field experiments, marked stratification of the total soil microbial biomass and its activity have been observed as consequence of the application of no-tillage to previously tilled soils. Our objective was to study the evolution of the total and active soil microbial biomass and mineralized C in vitro during the first crop after the introduction of no-tillage to an agricultural soil. The experiment was performed on a Typic Hapludoll from the Argentinean Pampa. Remaining plant residues, total and active microbial biomass and mineralized C were determined at 0–5 cm and 5–15 cm depths, at three sampling times: wheat tilling, silking and maturity. The introduction of no-tillage produced an accumulation of plant residues in the soil surface layer (0–5 cm), showing stratification with depth at all sampling dates. Active microbial biomass and C mineralization were higher under no-tillage than under conventional tillage in the top 5 cm of the profile. The total soil microbial biomass did not differ between treatments. The active soil biomass was highly and positive correlated with plant residues (r ²=0.617;P<0.01) and with mineralized C (r ²=0.732;P<0.01). Consequently, the active microbial biomass and mineralized C reflected immediately the changes in residue management, whereas the total microbial biomass seemed not to be an early indicator of the introduction of a new form of soil management in our experiment. Received: 23 February 1999     

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.     

Shifting microbial community structure across a marine terrace grassland chronosequence, Santa Cruz, California

Changes in the biomass and structure of soil microbial communities have the potential to impact ecosystems via interactions with plants and weathering minerals. Previous studies of forested long-term (1000s - 100,000s of years) chronosequences suggest that surface microbial communities change with soil age. However, significant gaps remain in our understanding of long-term soil microbial community dynamics, especially for non-forested ecosystems and in subsurface soil horizons. We investigated soil chemistry, aboveground plant productivity, and soil microbial communities across a grassland chronosequence (65,000-226,000 yrs old) located near Santa Cruz, CA. Aboveground net primary productivity (ANPP) initially increased to a maximum and then decreased for the older soils. We used polar lipid fatty acids (PLFA) to investigate microbial communities including both surface (<0.1 m) and subsurface (≥0.2 m) soil horizons. PLFAs characteristic of Gram-positive bacteria and actinobacteria increased as a fraction of the microbial community with depth while the fungal fraction decreased relative to the surface. Differences among microbial communities from each chronosequence soil were found primarily in the subsurface where older subsurface soils had smaller microbial community biomass, a higher proportion of fungi, and a different community structure than the younger subsurface soil. Subsurface microbial community shifts in biomass and community structure correlated with, and were likely driven by, decreasing soil P availability and Ca concentrations, respectively. Trends in soil chemistry as a function of soil age led to the separation of the biological (≤1 m depth) and geochemical (>1 m) cycles in the old, slowly eroding landscape we investigated, indicating that this separation, commonly observed in tropical and subtropical ecosystems, can also occur in temperate climates. This study is the first to investigate subsurface microbial communities in a long-term chronosequence. Our results highlight connections between soil chemistry and both the aboveground and belowground parts of an ecosystem.