Carbon and nitrogen mineralization in subarctic agricultural and forest soils

Summary C and N mineralization potentials were determined, in a 12-week laboratory incubation study, on soil samples obtained from recently cleared land which had been cropped to barley for 4 years (field soils) and from nearby undisturbed taiga (forest soils). Treatments for the cropped soils were conventional and no-tillage with and without crop residues removed. An average of about 3% of the total C was evolved as CO₂ from the field soils compared with > 10% and 4% for the upper (Oie) and lower (Oa) forest-floor horizons, respectively. Significantly more C was mineralized from the Ap of the no-till treatment with residue left on the surface than from the other field Ap horizons. Both forest-floor horizons showed rather long lag periods for net mineralization compared with the field soils, but at the end of the incubation, more mineral N was recovered from the forest Oie despite a rather wide C:N ratio, than from the field soils. After 12 weeks about 115, 200 and 20 g mineral N/g soil were recovered from the field Ap, the forest Oie and the forest Oa horizons, respectively. Very little C or N was mineralized from the B horizon of the forest or the field soils. Nitrification was rapid and virtually complete for the field soils but was negligible for both forest-floor O horizons.Paper no J-188 of the Journal Series of the Alaska Agricultural and Forestry Experiment Station     

The effects of feeding byOniscus asellus (Isopoda) on nutrient cycling in an incubated hardwood forest soil

Summary Oniscus asellus produced changes in the nutrients leached from Oie and Oa horizons of a hardwood forest soil. Soil with isopods lost more K⁺ (54%) from the Oie horizon and more Ca²⁺ (25%), Mg²⁺ (40%), and water-extractable S (23%) from the Oa horizon than soil without isopods. In contrast, soils with isopods lost less Ca²⁺ (39076) from the Oie horizon and less dissolved C-bonded S (33%) from the Oa horizon than soil without isopods. In addition, the Oia and Oa horizons exhibited different nutrient dynamics. When isopods were present, the Oa horizon leachates accumulated more Na⁺ K⁺, Ca²⁺, Mg²⁺, NO₃ ^– , water-soluble SO₄ ^2–, and dissolved C-bonded S, and the Oie horizon retained more of these nutrients. The type of leaching solution also had a major effect on nutrients. Leaching with a simulated soil solution resulted in smaller nutrient losses for K⁺ and Mg²⁺ in both horizons and for Na⁺, Ca²⁺, and NO₃ ^– in the Oa horizon than leaching with distilled water.     

Long-term organic phosphorus mineralization in Spodosols under forests and its relation to carbon and nitrogen mineralization

In forest soils where a large fraction of total phosphorus (P) is in organic forms, soil micro-organisms play a major role in the P cycle and plant availability since they mediate organic P transformations. However, the correct assessment of organic P mineralization is usually a challenging task because mineralized P is rapidly sorbed and most mineralization fluxes are very weak. The objectives of the present work were to quantify in five forest Spodosols at soil depths of 0-15 cm net mineralization of total organic P and the resulting increase in plant available inorganic P and to verify whether net or gross P mineralization could be estimated using the C or N mineralization rates. Net mineralization of total organic P was derived from the net changes in microbial P and gross mineralization of P in dead soil organic matter. We studied very low P-sorbing soils enabling us to use lower extractants to assess the change in total inorganic P as a result of gross mineralization of P in dead soil organic matter. In addition, to enable detection of gross mineralization of P in dead soil organic matter, a long-term incubation (517 days) experiment was carried out. At the beginning of the experiment, total P contents of the soils were very low (19-51 μg g⁻¹) and were essentially present as organic P (17-44 μg g⁻¹, 85-91%) or microbial P (6-14 μg g⁻¹; 24-39%). Conversely, the initial contents of inorganic P were low (2-7 μg g⁻¹; 9-15%). The net changes in the pool size of microbial P during the 517 days of incubation (4-8 μg g⁻¹) and the amounts of P resulting from gross mineralization of dead soil organic matter (0.001-0.018 μg g⁻¹ day⁻¹; 0.4-9.5 μg g⁻¹ for the entire incubation period) were considerable compared to the initial amounts of organic P and also when compared to the initial diffusive iP fraction (<0.3 μg g⁻¹). Diffusive iP corresponds to the phosphate ions that can be transferred from the solid constituents to the soil solution under a gradient of concentration. Net mineralization of organic P induced an important increase in iP in soil solution (0.6-10 μg g⁻¹; 600-5000% increase) and lower increases in diffusive iP fractions (0.3-5 μg g⁻¹; 300-2000% increase), soil solid constituents having an extremely low reactivity relative to iP. Therefore, soil micro-organisms and organic P transformations play a major role in the bioavailability of P in these forest soils. In our study, the dead soil organic matter was defined as a recalcitrant organic fraction. Probably because gross mineralization of P from this recalcitrant organic fraction was mainly driven by the micro-organisms’ needs for energy, the rates of gross mineralization of C, N and P in the recalcitrant organic fraction were similar. Indirect estimation of gross mineralization of P in dead soil organic matter using the gross C mineralization rate seems thus an alternative method for the studied soils. However, additional studies are needed to verify this alternative method in other soils. No relationships were found between microbial P release and microbial C and N releases.     

Effects of the addition of forest floor extracts on soil carbon dioxide efflux

Composition and effects of additions of fibric (Oi) and hemic/sapric (Oe + Oa) layer extracts collected from a 20-year-old stand of radiata pine (Pinus radiata) on soil carbon dioxide (CO₂) evolution were investigated in a 94-day aerobic incubation. The ¹³C nuclear magnetic resonance spectroscopy indicated that Oi layer extract contained greater concentrations of alkyl C while Oe + Oa layer extract was rich in carboxyl C. Extracts from Oi and Oe + Oa layers were added to a forest soil at two different polyphenol concentrations (43 and 85 μg g⁻¹ soil) along with tannic acid (TA) and glucose solutions to evaluate effects on soil CO₂ efflux. CO₂ evolution was greater in amended soils than control (deionized water) indicating that water-soluble organic carbon (WSOC) was readily available to microbial degradation. However, addition of WSOC extracted from both Oi and Oe + Oa layers containing 85 μg polyphenols g⁻¹ soil severely inhibited microbial activity. Soils amended with extracts containing lower concentrations of polyphenols (43 μg polyphenols g⁻¹ soil), TA solutions, and glucose solutions released 2 to 22 times more CO₂-C than added WSOC, indicating a strong positive priming effect. The differences in CO₂ evolution rates were attributed to chemical composition of the forest floor extracts.     

Drying–rewetting cycles release phosphorus from forest soils

Drying–rewetting cycles (D/W) occur frequently in topsoils and may mobilize phosphorus (P). We investigated the effect of repeated D/W on the release of dissolved inorganic (DIP) and organic P (DOP) from forest floors and A horizons. Samples were taken from 3 European beech sites and from 3 Norway spruce sites. Soils were desiccated up to pF 6 (–100 MPa) in three D/W cycles in the laboratory, while the controls were kept permanently at 50% water holding capacity. After each drying, P was extracted from the soils in water. D/W caused the release of DIP and DOP especially from O layers. There was no general difference in response to D/W between samples from beech and spruce. The net release of DIP after D/W was largest from the Oe horizons (average 50–60 mg P kg^?1) for both beech and spruce forest soils. The net release of DIP from Oi layers was on average 7.8 mg P kg^?1 and from spruce Oa layers 21.1 mg P kg^?1. In the A horizons, net DIP release was similar in beech and spruce soils with 0.4 mg P kg^?1. The release of DOP was less than the release of DIP except for the A horizons. Repeated cycles did not increase the release of DIP and DOP. The release of DIP and DOP was positively correlated with the microbial biomass in Oe and Oa layers but not in Oi layers. Our results suggest that D/W may significantly influence the short term availability of dissolved P in both beech and spruce forest soils.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture

A sandy loam soil was mixed with three different amounts of quartz sand and incubated with (¹⁵NH₄)₂SO₄ (60 g N g^-1 soil) and fresh or anaerobically stored sheep manure (60 g g^-1 soil). The mineralization-immobilization of N and the mineralization of C were studied during 84 days of incubation at 20°C. After 7 days, the amount of unlabelled inorganic N in the manure-treated soils was 6–10 g N g^-1 soil higher than in soils amended with only (¹⁵NH₄)₂SO₄. However, due to immobilization of labelled inorganic N, the resulting net mineralization of N from manure was insignificant or slightly negative in the three soil-sand mixtures (100% soil+0% quartz sand; 50% soil+50% quartz sand; 25% soil+75% quartz sand). After 84 days, the cumulative CO₂ evolution and the net mineralization of N from the fresh manure were highest in the soil-sand mixutre with the lowest clay content (4% clay); 28% fo the manure C and 18% of the manure N were net mineralized. There was no significant difference between the soil-sand mixtures containing 8% and 16% clay, in which 24% of the manure C and -1% to 4% of the manure N were net mineralized. The higher net mineralization of N in the soil-sand mixture with the lowest clay content was probably caused by a higher remineralization of immobilized N in this soil-sand mixture. Anaerobic storage of the manure reduced the CO₂ evolution rates from the manure C in the three soil-sand mixtures during the initial weeks of decomposition. However, there was no effect of storage on net mineralization of N at the end of the incubation period. Hence, there was no apparent relationship between net mineralization of manure N and C.     

Phosphate additions have no effect on microbial biomass and activity in a northern hardwood forest

High rates of atmospheric nitrogen (N) deposition have raised questions about shifting patterns of nutrient limitation in northern hardwood forests. Of particular interest is the idea that increased supply of N may induce phosphorus (P) limitation of plant and microbial processes, especially in acid soils where P sorption by Al is high. In this study, we established field plots and plant-free laboratory mesocosms with P and Ca additions to test the hypotheses that 1) microbial biomass and activity are limited by P in the northern hardwood forest soils at the Hubbard Brook Experimental Forest in NH USA; 2) elevated Ca increases inherent P availability and therefore reduces any effects of added P and 3) P effects are more marked in the more carbon (C) rich Oie compared to the Oa horizon. Treatments included P addition (50 kg P ha⁻¹), Ca addition (850 kg Ca ha⁻¹) and Ca + P addition (850 kg Ca ha⁻¹ and 50 kg P ha⁻¹). The P treatments increased resin-available P levels and reduced phosphatase activity, but had no effect on microbial biomass C, microbial respiration, C metabolizing enzymes, potential net N mineralization and nitrification in the Oie or Oa horizon of either field plots or plant free mesocosms, in either the presence or absence of Ca. Total, prokaryote, and eukaryote PLFA were reduced by P addition, possibly due to reductions in mycorrhizal fungal biomass. These results suggest that increased N deposition and acidification have not created P limitation of microbial biomass and activity in these soils.     

A method for determining microbially available N and P in an organic soil

Summary A bioassay of microbially available soil N and P is described. It is based on the addition of glucose together with N or P to soil, followed by monitoring of the respiration rate. The addition of glucose + N resulted in an immediate increase in the soil respiration rate followed by a short period of exponential increase, reflecting the growth of microorganisms on the added substrate. The exponential phase levelled off, when lack of P prevented further growth of the soil microorganisms. The soil respiration rate then remained constant for several hours before decreasing, when glucose became limiting. The addition of glucose + P resulted in a lower plateau of the soil respiration rate, indicating that microbial growth was more limited by N than P in this forest soil (0.28 and 0.79 mg CO₂ g^-1 organic matter h^-1, respectively). Additions of the limiting nutrient resulted in a proportional increase in the constant level of the soil respiration rate. This was used to calculated the increase in the soil respiration rate per mg N (0.71 mg CO₂ h^-1) or mg P (4.6 mg CO₂ h^-1) added to this particular soil. Microbially available N was then calculated in two ways from the regression equation (0.15 or 0.40 mg g^-1 organic matter) and P (0.13 or 0.17 mg g^-1 organic matter). A comparison with 2 M KCl extraction showed that in nutrient-poor forest soils the microbially available N was 6.3 or 18.5 times higher than the KCl extractable N.     

Phosphate additions have no effect on microbial biomass and activity in a northern hardwood forest

High rates of atmospheric nitrogen (N) deposition have raised questions about shifting patterns of nutrient limitation in northern hardwood forests. Of particular interest is the idea that increased supply of N may induce phosphorus (P) limitation of plant and microbial processes, especially in acid soils where P sorption by Al is high. In this study, we established field plots and plant-free laboratory mesocosms with P and Ca additions to test the hypotheses that 1) microbial biomass and activity are limited by P in the northern hardwood forest soils at the Hubbard Brook Experimental Forest in NH USA; 2) elevated Ca increases inherent P availability and therefore reduces any effects of added P and 3) P effects are more marked in the more carbon (C) rich Oie compared to the Oa horizon. Treatments included P addition (50 kg P ha⁻¹), Ca addition (850 kg Ca ha⁻¹) and Ca + P addition (850 kg Ca ha⁻¹ and 50 kg P ha⁻¹). The P treatments increased resin-available P levels and reduced phosphatase activity, but had no effect on microbial biomass C, microbial respiration, C metabolizing enzymes, potential net N mineralization and nitrification in the Oie or Oa horizon of either field plots or plant free mesocosms, in either the presence or absence of Ca. Total, prokaryote, and eukaryote PLFA were reduced by P addition, possibly due to reductions in mycorrhizal fungal biomass. These results suggest that increased N deposition and acidification have not created P limitation of microbial biomass and activity in these soils.     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

Nitrification in two coniferous forest soils after different fertilization treatments

The effects of seven different fertilization treatments on nitrification in the organic horizons of a Myrtillus-type (MT) and a Calluna-type pine forest in southern Finland were studied. No (NO^?₃ + NO^?₂)-N accumulated in unfertilized soils during 6 weeks at 14 or 20°C in the laboratory. Net nitrification was stimulated by urea in both soils (but more in the MT pine forest soil) and to a lesser degree by wood ash but not by ammonium nitrate or nitroform (ureaformaldehyde). Nitrification was not detected in nitroform fertilized soils although ammonium accumulation was high during incubation. In the MT pine forest soil, net nitrification appeared to be stimulated by apatite, biotite and micronutrients. Nitrapyrin inhibited nitrification indicating that it was carried out by autotrophic nitrifiers. In the urea-fertilized MT pine forest soil, nitrification took place at an incubation temperature of 0°C. Accumulation of (N0^?₃ + NO^?₂)-N was highest in soil sampled at < 10°C.     

Phosphorus fractions and dynamics as affected by land-use changes in the Central Mexican highlands

Land-use change from forest to agriculture in the volcanic ash-derived soils of Mexico has increased over recent decades. It is likely that land uses and management practices, particularly fertilizer use have affected phosphorus (P) distribution and availability. The objective of this study was to evaluate the effects of land-use types (native forest and maize mono-cropping), and the related P addition, on the forms and distribution of soil P and their isotopic exchangeability. An Andisol, sampled from a cropping site, along with the contiguous area under native forest was treated with ³²P-labelled potassium phosphate (KH₂³²PO₄). The soil samples were extracted after incubation times of 7, 21, 35 and 49 days. Phosphorus content and ³²P recovery in fractions sequentially extracted were assessed for each incubation time. Total soil P was dominated by inorganic fractions (79 to 86%) in both land-use types. Resin-Pi, bicarbonate extractable inorganic P (Bic-Pi) and sodium hydroxide extractable inorganic P (NaOH_0.1-Pi) were all raised with P addition. However, the proportion of organic P fraction was reduced under cropped soil. The recovery of ³²P in soils with P addition indicates that resin-Pi, Bic-Pi and NaOH_0.1-Pi comprised nearly all the exchangeable P. In native soils with no P addition, more than 19% of the ³²P was recovered in Bic-Po and NaOH_0.1-Po forms. This finding indicates that organic P cycling is crucial when soil Pi reserves are presented in an inadequate amount. Ecologically based management has to be designed for replenishment and succeeding maintenance of soil organic P compounds to increase sustainable agricultural production.     

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT

Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at ?8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH₄ uptake and from 70 to 90% WFPS for N₂O and CO₂ emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N₂O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g^?1 C. Cumulative soil CO₂ emissions and CH₄ uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N₂O, CO₂, and CH₄ fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.     

Relationship between rate of carbon dioxide evolution,microbial biomass carbon,and amount of dissolved organic carbon as affected by temperature and water content of a forest and an arable soil

Abstract

Using an Ochrept soil of a forest at climax stage or of an arable site at Kita‐Ibaraki, a city in central Japan, the rates of carbon dioxide (CO₂)‐carbon (C) evolution, the amounts of microbial biomass carbon (MBC) and the amounts of dissolved organic carbon (DOC) were measured in a laboratory with special reference to the incubation temperature and the soil water content. The rates of CO₂‐C evolution increased exponentially with increase in the incubation temperature in the range of 4–40°C. The temperature coefficients (Q₁₀) were 2.0 for the forest and 1.9 for the arable soil. The amounts of MBC were almost constant of 980 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 340 μg g^‐1 soil in the incubation temperature up to 31 °C for the arable soil. The amounts of DOC in soil solutions were almost constant at 3.1 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 3.8 μg g^‐1 soil in the incubation temperature up to 31°C for the arable soil. The rates of CO₂‐C evolution and the amounts of DOC increased with increase in soil water content (% of soil dry weight) up to 91% for the forest or up to 26% for the arable soil. However, the rates of CO₂‐C evolution and the amounts of DOC were almost constant within soil water content in the range of 91–160% or 26–53%, respectively. The amounts of MBC of the forest or arable soil were almost constant over a wide range of soil water content in the range of 41–220% or 8–73%, respectively. The rates of CO₂‐C evolution of both the forest and the arable soils were highly correlated with the amounts of DOC, but not with the amounts of MBC, under laboratory conditions in the case that the amounts of DOC were changed by various treatments. The regression equation,     

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing,China

Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO₂ concentrations and global warming through soil respiration. We conducted a comparative study to determine NH₄⁺ and NO₃⁻ concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO₃⁻-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH₄⁺-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg⁻¹ d⁻¹) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH₄⁺ and, NO₃⁻ concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.     

Energetic eco‐physiology of the soil microbiota in two landscapes of southern and northern Germany

Interactions between microbial communities and organic matter were analyzed for soils from the project regions ’︁Ecosystem Research in the Agricultural Landscape/FAM, Munich’ in southern Germany and ’︁Ecosystem Research in the Bornhöved Lake district’ from northern Germany using ratios between microbial biomass content (C_mic), microbial metabolic quotient (qCO₂) and organic carbon content (C_org). In the agricultural soils in southern Germany, the qCO₂/C_org ratio differed significantly with respect to agricultural management in contrast to ecophysiological C_mic/C_org ratio. In addition, C_mic/C_org ratio decreased from 39 to 21 mg C_mic g^—1 C_org and qCO₂/C_org ratio increased from 72 to 180 mg CO₂‐C g^—1 C_mic h^—1 (g C_org g^—1 soil)^—1 with increasing soil depth. For the upper soil horizons from the landscape in northern Germany the two quotients differed significantly with reference to land use showing highest microbial colonization under grassland and lowest under beech forest. In contrast, C use efficiency was lowest in arable field under maize monoculture and highest in a wet grassland having a high organic C content.     

Nitrogen and phosphorus uptake by maize as affected by particulate organic matter quality, soil characteristics, and land-use history for soils from the West African moist savanna zone

 The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g^–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg^–1) and lowest under natural fallow (6.3 mg kg^–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg^–1) and lowest in continuously cropped plots (107 mg N kg^–1). After addition of all nutrients except N, a highly significant linear relationship (R ²=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R ² increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability. Received: 9 April 1999     

Effects of deforestation on phosphorus pools in mountain soils of the Alay Range, Khyrgyzia

 The amount, quality and turnover of soil P is heavily influenced by changes in soil management. The objective of this study was to investigate the effects of deforestation and pasture establishment on the concentrations, forms and turnover rate of soil P in mountain soils of the Alay Range, Khyrgyzia. A sequential extraction was applied to distinguish soil P pools. We used particle-size fractionation to follow the dynamics of different P pools in soils under forest and pasture and ³¹P-NMR spectroscopy to investigate the structure of alkali-soluble P forms. In the A horizons of the forest soils, total soil P concentration was 1093 mg kg^–1, organic P (P_o) representing 46% of the total P. Deforestation followed by pasture establishment not only increased significantly (P<0.01) the total P concentration (1560 mg kg^–1) but also the contribution of P_o to total P was increased by 17%. Pasture soils had significantly higher P pools than forest soils except highly labile inorganic P (P_i NaHCO₃) and primary P_i (P_i HCl_dil). Both in forest and pasture soils stable P increased with decreasing particle size (coarse sand 50%, clay 80% of total P) and primary P decreased with decreasing particle size. Phosphate monoesters and diesters represented 80% of P identified by ³¹P NMR. Low monoester to diester ratios in the alkali extracts of forest and pasture soils indicate low microbial activity. This is consistent with high C/P_o ratios and high stable P_o concentrations in the fine earth of forest and pasture. Received: 10 March 1999     

Phosphorus Management of Lucerne Grown on Calcareous Soil in Turkey

ABSTRACT

Lucerne or alfalfa (Medicago sativa L.) is grown as a forage crop on many livestock farms. In calcareous soils in eastern Turkey, lucerne production requires phosphorus (P) additions as the soils are naturally P deficient. Phosphorus sorption isotherms were used to estimate P fertilizer needs for lucerne grown for two years in a 3-cut system on a calcareous P deficient Aridisol in eastern Anatolia, Erzurum province, Turkey. Annual P applications ranged from 0–1200 kg P ha^?1. The Langmuir two-surface adsorption equation was used to derive the maximum P sorption capacity of unamended soil and to determine soil solution P, maximum buffer capacity (MBC), equilibrium buffer capacity (EBC), and P saturation at the optimum economic P rate (OEPR) for dry matter (DM) production. Soils were tested for Olson P at the onset of the study and after two years of P applications. In both years, tissue was analyzed for P content at flowering prior to first cutting. The OEPR (2-year average) was 754 kg P ha^?1 yr^?1 corresponding with a soil solution P concentration of 0.30 mg L^?1, a DM yield of 8725 kg DM ha^?1, and $528 ha^?1 annual profit. The P content of leaves at flowering increased linearly with P application beyond 100 kg P ha^?1 and was 3.2 g kg^?1 P at the OEPR. The unfertilized soil had an EBC, MBC, P saturation, and X_max of 3304 mL g^?1, 3401 mL g^?1, 6%, and 1086 mL g^?1, respectively, whereas two years of fertilization to the OEPR decreased EBC and MBC to 358 mL g^?1 and 540 mL g^?1, and increased P saturation and Olsen P to 56% and 32 mg kg^?1, respectively. These results suggest a P saturation >50% or Olsen P >30 mg kg^?1 are needed to maintain an optimum soil solution concentration of 0.30 mg L^?1 in this calcareous Aridisol. Similar studies with different soils and initial soil test P levels are needed to conclude if these critical soil test values can be applied across the region.