Arylsulfatase activity of microbial biomass in soils as affected by cropping systems

 The impacts of crop rotations and N fertilization on different pools of arylsulfatase activity (total, intracellular, and extracellular) were studied in soils of two long-term field experiments in Iowa to assess the contibution of the microbial biomass to the activity of this enzyme. Surface-soil samples were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha^–1 before corn, and an annual application of 20 kg P ha^–1 and 56 kg K ha^–1. The arylsulfatase activity in the soils was assayed at optimal pH (acetate buffer, pH 5.8) before and after chloroform fumigation; microbial biomass C (C_mic) and N (N_mic) were determined by chloroform-fumigation methods. All pools of arylsulfatase activity in soils were significantly affected by crop rotation and plant cover at sampling time, but not by N fertilization. Generally, the highest total, intracellular, and extracellular arylsulfatase activities were obtained in soils under cereal-meadow rotations, taken under oats or meadow, and the lowest under continuous cropping systems.Total, intracellular, and extracellular arylsulfatase activities were significantly correlated with C_mic (r>0.41, P<0.01) and N_mic (r>0.38, P<0.01) in soils. The averages of specific activity values, i.e., of arylsulfatase activity of the microbial biomass, expressed per milligram C_mic, ranged from 315 to 407 μg p-nitrophenol h^–1. The total arylsulfatase activity was significantly correlated with the intracellular activity, with r values >0.79 (P<0.001). In general, about 45% of the total arylsulfatase activity was extracellular, and 55% was associated with the microbial biomass in soils, indicating the importance of the microflora as an enzyme source in soils. Received: 23 April 1998     

Soil microbial biomass carbon and nitrogen as affected by cropping systems

 The impacts of crop rotations and N fertilization on microbial biomass C (C_mic) and N (N_mic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. The C_mic and N_mic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The C_mic and N_mic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest C_mic and N_mic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, C_mic made up about 1.0% of the organic C (C_org), and N_mic contributed about 2.4% of the total N (N_tot) in soils at both sites and years of sampling. The C_mic values were significantly correlated with C_org contents (r≥0.41**), whereas the relationship between C_mic and N_tot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The C_mic : N_mic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater C_mic : C_org (1.1%) and N_mic : N_tot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively). Received: 9 February 1999     

Effect of cropping systems on phosphatases in soils

Phosphatases are widely distributed in nature and play a major role in phosphorus nutrition of plants. The effects of crop rotations and nitrogen fertilization on the activities of phosphatases (acid phosphatase, alkaline phosphatase, and phosphodiesterase) were studied in soils from two long‐term cropping systems at the Northeast Research Center (NERC) in Nashua and the Clarion Webster Research Center (CWRC) in Kanawha, Iowa, USA. Surface soils (0—15 cm) were taken in 1996 and 1997 from replicated field plots in corn, soybeans, oats, or meadow (alfalfa) that received 0 or 180 kg N ha^—1 before corn. Because of differences in organic C contents among soils of the two sites, the soils from the CWRC sites contained greater enzyme activity values than those from the NERC site. Plots under oats or meadow showed the greatest activity values, whereas those under continuous corn at the CWRC site and soybean at the NERC site showed the least activities. Analysis of variance indicated that the activities of the phosphatases were significantly affected by crop rotation (P < 0.001) in both years at the NERC site but not at the CWRC site. Nitrogen fertilization affected the activity of acid phosphatase in soils from the CWRC site in both years and alkaline phosphatase only in 1997; but it did not affect the activities of the phosphatases in the soils from the NERC site. With the exception of alkaline phosphatase (CWRC) and phosphodiesterase (NERC) in soils sampled in 1997, activities of alkaline phosphatase and phosphodiesterase were significantly correlated with microbial biomass C (C _mic) in soils from both sites and years, with r values ranging from 0.366* to 0.599***. Cropping systems and N fertilization affected the specific activities of phosphomonoesterases, especially acid phosphatase, but not of phosphodiesterase. Regression analysis showed that activities of phosphatases were significantly correlated with organic C contents of soils from the NERC site but not from the CWRC site.     

Effect of cropping systems on nitrogen mineralization in soils

 Understanding the effect of cropping systems on N mineralization in soils is crucial for a better assessment of N fertilizer requirements of crops in order to minimize nitrate contamination of surface and groundwater resources. The effects of crop rotations and N fertilization on N mineralization were studied in soils from two long-term field experiments at the Northeast Research Center and the Clarion-Webster Research Center in Iowa that were initiated in 1979 and 1954, respectively. Surface soil samples were taken in 1996 from plots of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. N mineralization was studied in leaching columns under aerobic conditions at 30  °C for 24 weeks. The results showed that N mineralization was affected by cover crop at the time of sampling. Continuous soybean decreased, whereas inclusion of meadow increased, the amount of cumulative N mineralized. The mineralizable N pool (N _o) varied considerably among the soil samples studied, ranging from 137 mg N kg^–1 soil under continuous soybean to >500 mg N kg^–1 soil under meadow-based rotations, sampled in meadow. The results suggest that the N _o and/or organic N in soils under meadow-based cropping systems contained a higher proportion of active N fractions. Received: 10 February 1999     

Glycosidases in soils as affected by cropping systems

Glycosidases are a group of soil enzymes that play a major role in degradation of carbohydrates. This study was conducted to assess the impact of crop rotation and N fertilization on the activities of α‐ and β‐glucosidases and α‐ and β‐galactosidases in plots of two long‐term field experiments at the Clarion‐Webster Research Center (CWRC) and Northeast Research Center (NERC) in Iowa. Surface‐soil (0–15 cm) samples were taken in 1996 and 1997 in corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) plots that received 0 or 180 kg N ha^–1, applied as urea before corn, and an annual application of 20 kg P ha^–1 and 56 kg K ha^–1. Activities of the four glycosidases were significantly affected by crop rotations in both years at the two sites but not by nitrogen application. In general, higher activities were observed in plots under meadow or oat and the lowest in continuous corn (CWRC) and soybean (NERC). Four‐year rotation showed the highest activity, followed by 2‐year rotation and monocropping systems. Linear‐regression analyses indicated that, in general, the activities of the glycosidases were significantly correlated with microbial‐biomass C (r > 0.302, p ≤ 0.05) and microbial‐biomass N (r > 0.321, p ≤ 0.05), organic‐C (r > 0.332, p ≤ 0.05) and organic‐N (r > 0.399, p ≤ 0.01) contents of the soils. Results of this work suggest that multicropping stimulated the activities of the glycosidases. The specific activities of the glycosidases in soils of the two sites studied, expressed as g p‐nitrophenol released per kg of organic C, differed among the four enzymes. The lowest values were obtained for β‐galactosidase and α‐glucosidase, followed by α‐galactosidase and β‐glucosidase.     

Effects of increasing periods under intensive arable vegetable production on biological, chemical and physical indices of soil quality

 The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (C_org) content under long-term pasture (>60 years) was in the range of 55 g C kg^–1 to 65 g C kg^–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg^–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (C_mic) and basal respiratory rate. The microbial quotient (C_mic/C_org) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg^–1 to 15 g C kg^–1 but the microbial metabolic quotient (basal respiration/C_mic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg^–1. Soils with an organic C content above 45 g C kg^–1 had been under pasture for at least 30 years. At the same C_org content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. Received: 18 November 1997     

Quantitative assessment of hydrolase production and persistence in soil

The aim of this work was to calculate indices of hydrolase production (Pr) and persistence (Pe) through simple arithmetical calculations. Changes in acid and alkaline phosphomonoesterase, phosphodiesterase, urease, protease, and β-glucosidase activities were monitored under controlled conditions in seven soils with a wide range of properties, in which microbial growth was stimulated by adding glucose and nitrogen. Glucose mineralization was monitored by CO₂–C evolution, and microbial growth was quantified by determining the soil adenosine triphosphate (ATP) content. Hydrolase Pr and Pe indices were numerically quantified by the following relationships: Pr = H / t _H and Pe = (r / H)Δt, respectively, where H indicates the peak value of each measured hydrolase activity, t _H is the time of the peak value, r indicates the residual activity value, and Δt is the time interval t _r − t _H, where t _r is the time of the residual activity value. Addition of glucose and N-stimulated soil respiration increased ATP content and stimulated the production of the measured hydrolase activities in all soils; the measured variable reached a maximum value and then decreased, returning to the value of the control soil. Apart from β-glucosidase activity, whose activity was not stimulated by glucose and N addition, the other measured hydrolase activities showed a trend that allowed us to calculate the Pr and Pe indices using the above-mentioned equations. Acid phosphomonoesterase and protease Pr values were significantly higher in soils under forest or set aside management; the alkaline phosphomonoesterase and phosphodiesterase Pr values were generally higher in the neutral and alkaline soils, and the urease Pr values showed no obvious relationships with soil pH or management. Concerning the persistence of enzyme activities, Pe values of the acid phosphomonoesterase activity were significantly higher in the acidic soils, and those of urease activity were higher in acidic soils and the Bordeaux neutral soil. No relationships were observed between Pe values of alkaline phosphomonoesterase, phosphodiesterase, or protease activities and soil pH or management. The different responses of hydrolases were discussed in relation to soil properties, microbial growth, and regulation at the enzyme molecular level.     

Temporal dynamics of soil nitrate-N after termination of forage phases in corn-forage rotation

 Efficient N-fertiliser management during the corn (Zea mays L.) phase in corn-forage rotation requires information on temporal dynamics of N release from forage biomass. The influence of forage phase, in corn-forage rotation, and no- versus conventional-till on (1) in situ temporal dynamics of soil nitrate-N (NO₃-N) during corn phase and (2) corn grain yield was investigated in this study. The data used were collected from a crop rotation (corn-forage) experiment, with superimposed tillage treatments, established on a silt loam soil in 1988 and continued until 1994. The cropping treatments were continuous conventionally cultivated (CT) corn, rotations involving corn and forages (alfalfa, Medicago sativa L.; and bromegrass, Bromus inermis L.) and continuous minimally tilled corn with under-seeded red clover (Trifolium pratense L.). The forages were grown for 6 years and corn was re-introduced in these plots under no- and conventional-till systems. Soil NO₃-N in the top 30 cm depth, determined six times during the corn phase, was significantly influenced by previous forage species and tillage system. Regression analysis indicated soil NO₃-N under continuous CT corn did not show significant temporal changes. In the rotations, soil NO₃-N after tillage or herbicide treatment, i.e. in no-till, increased with time until 45 days after tillage (DAT), reached a plateau between 45 and 65 DAT, and then decreased with time. During the plateau, soil NO₃-N in rotation plots ranged from 17 to 33 mg kg^–1 compared to 15.7 mg kg^–1 in the continuous CT corn. Tillage increased soil NO₃-N concentration in alfalfa plots whereas an opposite trend existed in the bromegrass plots. Soil NO₃-N in the rotation plots increased at rates ranging from 0.71 to 1.63 mg kg^–1 day^–1. The interaction involving forage species and the temporal dynamics of soil NO₃-N accounted for 68–77% of variability in corn grain yield. Received: 14 July 1998     

Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

 Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(¹⁵NO₃)₂ was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot^–1, 200 mg N pot^–1 or 300 mg N pot^–1 were applied at each N application date. At crop maturity, ¹⁵N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot^–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total ¹⁵N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha^–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (N_mic) was determined for cropped plots at GS 31. Although N_mic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in N_mic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low. Received: 6 July 1999     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Characterization of active nitrogen pools in soils under different cropping systems

 Studies were conducted to evaluate the relationships among different active N pools of organic matter in soils at two long-term cropping systems in Iowa. Results indicated that multi-cropping systems, particularly meadow-based systems, enhanced bioactivities of soils. Mono-cropping systems, particularly soybean, reduced soil microbial biomass and enzyme activities. The mineralizable N pool (potential N mineralization;N _o) was more sensitive to changes in the size of the microbial biomass N (N_mic) than to changes in organic N. One unit change in organic N did not lead to substantial changes in N _o, but 1 unit change in N_mic resulted in three or more units change in N _o. The active N pools and turnover rate were more sensitive to changes in organic C than to changes in microbial biomass C (C_mic). A unit change in organic C resulted in 10.6 units change in N _o, but a unit change in C_mic resulted in only 0.8 unit change in N _o. C_mic or N_mic are better indexes than organic C or N for the estimation of N _o or N availability, because biomass values are more highly correlated with cumulative N mineralized during 24 weeks of incubation, with r values ranging from 0.57 (P<0.001) to 0.88 (P<0.001). Received: 18 October 1999     

Long-term winter cover cropping effects on corn (Zea mays L.) production and soil nitrogen availability

 This study was conducted to determine effects of long-term winter cover cropping with hairy vetch, cereal rye and annual ryegrass on soil N availability and corn productivity. From 1987 to 1995, with the exception of the first year of the study, the cover crops were seeded each year in late September or early October after the corn harvest and incorporated into the soil in late April or early May. Corn was seeded 10 days to 2 weeks after the cover crop residues had been incorporated, and N fertilizer was applied as a side-dressing at rates of 0, 67, 134, or 201 kg N ha^–1 each year. While the average annual total N input from the above-ground biomass of the cover crops was highest for hairy vetch (72.4 kg N ha^–1), the average annual total C input was highest for cereal rye (1043 kg C ha^–1) compared with the other cover crops. Hairy vetch was the only cover crop that significantly increased pre-side-dressed NO₃ ^–-N (N_i) corn biomass and N uptake at 0 N. At an N fertilizer rate of 134 kg N ha^–1 or higher, the cover crops had a minimal effect on corn biomass. This indicated that even after 9 years of winter cover cropping, the effect of the cover crops on corn growth resulted primarily from their influence on soil N availability. The amount of available N estimated from the cover crops (N_ac) was significantly correlated with relative corn biomass production (r ²=0.707, P<0.001). The total amount of available N, comprising N_ac and N added from fertilizer (N_f), was strongly correlated (r ²=0.820, P<0.001)) with relative corn biomass production. The correlation was also high for the available N comprising N_i and N_f (r ²=0.775, P<0.001). Although cereal rye and annual ryegrass did not improve corn biomass production in the short term, they benefited soil organic N accumulation and gradually improved corn biomass production compared with the control over the long term. Received: 10 August 1999     

Effects of Cd,Zn, or both on soil microbial biomass and activity in a clay loam soil

We investigated Cd, Zn, and Cd + Zn toxicity to soil microbial biomass and activity, and indigenous Rhizobium leguminosarum biovar trifolii, in two near neutral pH clay loam soils, under long-term arable and grassland management, in a 6-month laboratory incubation, with a view to determining the causative metal. Both soils were amended with Cd- or Zn-enriched sewage sludge, to produce soils with total Cd concentrations at four times (12 mg Cd g⁻¹ soil), and total Zn concentrations (300 mg Zn kg⁻¹ soil) at the EU upper permitted limit. The additive effects of Cd plus Zn at these soil concentrations were also investigated. There were no significant differences in microbial biomass C (B _C), biomass ninhydrin N (B _N), ATP, or microbial respiration between the different treatments. Microbial metabolic quotient (defined as qCO₂ = units of CO₂–C evolved unit⁻¹ biomass C unit⁻¹ time) also did not differ significantly between treatments. However, the microbial maintenance energy (in this study defined as qCO₂-to-μ ratio value, where μ is the growth rate) indicated that more energy was required for microbial synthesis in metal-rich sludge-treated soils (especially Zn) than in control sludge-treated soils. Indigenous R. leguminosarum bv. trifolii numbers were not significantly different between untreated and sludge-treated grassland soils after 24 weeks regardless of metal or metal concentrations. However, rhizobial numbers in the arable soils treated with metal-contaminated sludges decreased significantly (P < 0.05) compared to the untreated control and uncontaminated sludge-treated soils after 24 weeks. The order of decreasing toxicity to rhizobia in the arable soils was Zn > Cd > Cd + Zn.     

Medium-term effects of a single application of mustard residues on soil microbiota and C content of vertisols

 A study of the effects of different qualities (fresh and composted) and rates (equivalent to 120, 240, and 360 kg N ha^–1) of mustard meal application on wheat yields on humid tropical vertisol was started in 1990 at Ginchi Research Station in Ethiopia. After continuous wheat cropping for 7 years and without any further fertilisation, soil microbial parameters (basal respiration, microbial biomass-C and N, organic-C, and ecophysiological quotients) were studied during one growth period. After 7 years of application, mustard meal still exerted a significant positive effect on microbial biomass, basal respiration, organic-C, C_mic : N_mic ratio, and metabolic quotient (qCO₂). Organic-C, qCO₂ and C_mic : N_mic ratios were higher for the compost-amended plots than plots amended with fresh mustard meal. Basal respiration, C_mic, and C_mic : N_mic ratio showed a clear seasonality, but only in manured plots. The data indicate shifts in microbial community structure (from bacteria to fungi and from r to K strategists) and suggest positive medium-term effects of mustard meal on humid tropical vertisol biological qualities. Received: 25 May 1999     

Simultaneous effects of plants and earthworms on mineralisation of 15N-labelled organic compounds adsorbed onto soil size fractions

 The simultaneous impact of three successive crops of wheat (Triticum aestivum L.) and of the earthworm (Lumbricus terrestris L.) on the mineralisation of ¹⁵N-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues >50 μm; silt 50–2 μm; coarse clay 2–0.2 μm and fine clay <0.2 μm) was studied under controlled conditions in the greenhouse. Unplanted soils (UPS) were used as controls. In planted soils without earthworm (PS) total plant biomass decreased with each cropping by up to 50%. However, in planted soils with earthworms (PES) the total plant biomass loss was only 17%. This pattern was explained by the earthworm effect. Compared to the unplanted soils, the planted soils had an increased (mean +37%) mineralisation of ¹⁵N adsorbed onto fine clays and a partial transfer of ¹⁵N to silt and coarse clay. The quantities of ¹⁵N mineralised and transferred were higher in the planted soils with earthworms, indicating an amplification of the phenomenon in the presence of earthworms. The simultaneous effect of the rhizosphere and the drilosphere did not lead to increased mineralisation of N adsorbed onto coarse clays and silts but instead a greater transfer of N associated with the fine fractions towards the coarser fractions. Received: 25 April 2000     

Activity and stability of jack bean urease in the presence of peat humic acids obtained using different extractants

 The effects of two humic acid extractants, 0.1 M Na₄P₂O₇ plus 0.1 M NaOH (NaPP) and 0.1 M NaOH (NaOH), on the activity and stability of a humic-urease complex were studied. The two humic acids isolated (HA_NaPP and HA_NaOH) exhibited different elemental compositions, metal concentrations and structural modifications in the FT-IR spectra. Depending on the pH, HA_NaPP and HA_NaOH influenced both the urease activity and urease kinetic parameters (V _max and K _m) in the same way. They inhibited urease activity between pH 6 and 7 and reduced the V _max of the reaction at pH 6 and 7. The presence of humic acids improved the affinity of the enzyme for the substrate (K _m). The stability of the urease with time, and in the presence of pronase, was improved by HA_NaPP and HA_NaOH with respect to free enzyme. These results confirm the importance of the interaction of urease with humic acids as a fundamental gateway for extracellular urease stabilisation. Since no difference in the extent of urease inhibition and urease stabilisation was observed for the two humic acids, it may be concluded that neither urease activity nor stability are influenced by the humic acid extractant used. Received: 5 November 1999     

Recovering decomposing plant residues from the particulate soil organic matter fraction: size versus density separation

 A detailed size separation of particulate organic matter (POM) from soils amended with straw from Hordeum vulgare or Vicia sativa revealed that the loss of C during the first 56 days of incubation mainly occurred from particles >2,000 μm, without a concomitant reduction in the size of these large particles. Preliminary studies of POM from non-amended soil had shown that the stable heavy (>1.4 g cm^–3) POM fraction was mainly (>80%) composed of particles <400 μm, whereas the light fraction was dominated by larger particles (>80%). Therefore we decided to compare the POM <1.4 g cm³ with POM >400 μm. There was a very close relationship between POM>400 μm and POM <1.4 g cm^–3 with regard to amounts of C and N, as well as the appearance of these fractions under the microscope. Similarly there was a close relationship between changes in the C content of the POM fractions and the CO₂ respired, and this was also the case when comparing changes in POM-N with net N mineralization. This indicated that the biological activity during decomposition was actually localized in the POM. Due to the lighter workload and lower expenditure for reagents in connection with size separation of POM, we recommend the size separation procedure in connection with studies of residue decomposition in arable systems. Received: 23 May 2000     

Chemical and biochemical properties as potential indicators of disturbances

 The response of a series of soil microbial, chemical, and enzymatic chacacteristics to two different tillage systems (conventional and no-tillage) and two crop rotations (continuous corn and soybean–corn) was evaluated in a long-term field experiment in the Rolling Pampas Region, Marcos Juárez, Córdoba, Argentina. The parameters studied included: oxidizable, soluble and respiration C, total and hydrolyzable N, microbial activity, ammonifiers, nitrifiers, cellulolytic and total microflora, protease and urease activities. After 15 years, oxidizable C, total N and hydrolyzable N showed no differences regarding no-tillage systems. However, differences were found regarding control soil. While soluble C did not exhibit any significant differences, respiration C and microbial activity were indicators sensitive to different treatments. Enzymatic activities, protease in particular, did reflect changes due to management. Fluctuation in microbial population counts were more related to plant residues than to tillage systems. Received: 29 May 1999     

Amino acid composition of soil organic matter

This study investigated the amino acid composition of soil organic matter extracted from ten surface soils in addition to surface soils from two long-term cropping systems [continuous corn (CCCC), corn-soybean-corn-soybean (CSCS), and corn-oats-meadow-meadow (COMM)] at two sites in Iowa: the Clarion-Webster Research Center (CWRC) and the Galva-Primghar Research Center (GPRC). Results showed that, with the exception of asparagine pluse aspartic acid and glutamine plus glutamic acid, the other 13 amino acids studied, expressed as perecentages of total amino acids extracted, were generally very uniform among the soils. The total amino acids extracted from the ten soils were significantly correlated with organic carbon (C) ( and clay content (, but not with total nitrogen (N), pH, or sand content. Expressed as percentages or organic C and N in soils, the amounts extracted ranged from 10.9% to 32.4% and from 12.0% to 27.4%, respectively. The amino acid N identified, expressed as percentages of organic N extracted, ranged from 32% to 50% and the C/N ratios of the extracted organic matter ranged from 10.1 to 14.9. The type of rotation did not significantly affect the total amino acid content of the soils from the same N treatment, but it did affect the total amino acid content of soils from the control plots. The total amino acids measured under the different crop rotations at the CWRC site were in the order: COMM>CCCC>CSCS. The order for the GPRC site was: CSCS>COMM>CCCC. The amino acid N identified, expressed as percentages of organic N extracted from soils at the CWRC site, ranged from 33.1% to 50% and for the GPRC site ranged from 26.5% to 51.4%. The C/N ratios of the organic matter extracted ranged from 10.4 to 14.1 and from 6.5 to 14.3 for the soils from CWRC and GPRC sites, respectively. Received: 26 May 1997     

Influence of nitrogen on cellulose and lignin mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and active fungal biomass, cellulose degradation and lignin degradation at 5, 10 and 15 weeks in soils from blackwater and redwater wetlands in the northern Florida panhandle. Blackwater streams contain a high dissolved organic C concentration which imparts a dark color to the water and contain low concentrations of nutrients. Redwater streams contain high concentrations of suspended clays and inorganic nutrients, such as N and P, compared to blackwater streams. Active bacterial and fungal biomass was determined by direct microscopy; cellulose and lignin degradation were measured radiometrically. The experimental design was a randomized block. Treatments were: soil type (blackwater or redwater forested wetlands) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, the active bacterial biomass in redwater soils was lower than in blackwater soils; the active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in redwater wetland soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. Cellulose and lignin degradation was higher in redwater than in blackwater soils. After 10 and 15 weeks of incubation, the addition of 200 or 400 kg N as NH₄NO₃ ha^–1 decreased cellulose and lignin degradation in both wetland soils to similar levels. This study indicated that the addition of N may slow organic matter degradation and nutrient mineralization, thereby creating deficiencies of other plant-essential nutrients in wetland forest soils. Received: 7 April 1999