Mineralization of C and N from microbial biomass in paddy soil

Summary Soil samples of paddy fields with different fertilizer managements in Yamaguchi Agricultural Experiment Station, Japan were used to investigate the contribution of microbial biomass to the pool of mobile plant nutrients in paddy soil. The quantities of nutrients mobilized in soils which had been fumigated or dried were closely related to the quantities available in freshly killed biomass. A K_N-factor (28 days) of 0.24 for the proportion of total N mineralized from dead biomass in paddy soils was obtained. It was observed that the C to N ratio mineralized from freshly killed biomass by chloroform fumigation of paddy soils was nearly 10 under aerobic conditions. For an approximate calculation of biomass C from the flush-N by chloroform fumigation of paddy soils, the equations of (B=33 Fn, 10 days) and (B=26 Fn, 28 days) were indicated. In oven-dried (70°C, 24 h) and rewetted soils, about 66% of N was mineralized from the freshly killed biomass during 28 days of incubation and the remaining 34% was derived from non-biomass organic matter of paddy soils.     

Inorganic and organic P in soil solutions from three upland soils

Soil solutions from three P-deficient Cambisols were analyzed for inorganic orthophosphate (P_i), organically combined phosphorus (P_o), total phosphorus (P_t) and residual phosphorus (P_r=P_t–(P_o+P_i)). The solutions were obtained by centrifugation of soil samples wetted-up to 90% field capacity. Increasing the centrifugal force from 750 to 1400×g (for 60 minutes) increased the volume of soil solution obtained by 17–35%. Increasing the centrifugation period from 30 to 90 minutes (at 1000×g) increased the volume by 2–12%. The effect of the different centrifugation conditions on the P composition of soil solutions were not critical and had little effect on either P_t concentration or on the distribution of P between P_i, P_o and P_r fractions. Soil solutions were also obtained on a seasonal basis over a 2-year period. The soils, fresh from the field, were wetted-up to 90% field capacity and centrifuged at 1000×g for 60 minutes to isolate the soil solution. Although the soils were derived from contrasting parent rock, and had different Fe and Al sesquioxide contents, the P_t concentrations of the soil solutions and the distribution between the fractions were similar. Annual average P_t concentrations for the 3 soils ranged from 93 to 114 and 63 to 89 g dm^-3 during the first and second year, respectively. Seasonal changes were of a similar order as those resulting from differences in soil type. During May, June, August and October soil solutions had average P_t concentrations ranging from 82 to 111 and 51 to 119 g P dm^-3 in 1989 and 1990, respectively. P_o was a major P component in soil solution and exceeded the amount of P_i by about 5–20 times.     

Effect of soil solution phosphorus in Volcanic Ash soils on the response of tomato (Lycopersicum esculentum) to added phosphate

Summary Sorption isotherms of six Volcanic Ash soils were used to determine Optimum P, i.e. the amount of added P required to produce a soil solution concentration of 0.2 µg P per ml. In a pot test, application of these amounts of P to the soils led to maximum or near maximum growth of tomato plants, (Lycopersicum esculentum), harvested at 35 days. After harvest the soil solution concentration of P in all the soils had fallen below 0.2 µg per ml whatever the rate of P added initially. The amount of P required to restore the soil solution to 0.2 µg P per ml was linearly related to the fertilizer P originally applied to the soil; the higher the P applied originally the lower was the future P requirement. The regression coefficients suggested that the applied P would be twice as effective at reducing subsequent P requirement in a Recent Volcanic Ash soil than in the other more developed soils. Measurement of extractable Al (NH₄OAc at pH 4.2) showed that lack of growth at low levels of added P on a high P fixing soil could be caused by insufficient P due to high levels of Al, although with a soil of pH 6 this would not have been expected. re]19760217     

Phosphorus depletion in the rhizosphere as influenced by soil moisture

To study the influence of soil moisture on phosphorus (P) depletion in the rhizosphere, maize (Zea mays cv. Trak) was pre-grown in vermiculite filled-PVC tubes for 9 days and then the plants with the tubes were transplanted into soil columns maintained at two soil moisture levels () of 0.14 and 0.20 cm³ cm^–3 for 10 days. The soil columns were separated at 1 cm depth by a nylon screen of 53 m inner mesh size, into 1 cm soil layer above and 3 cm soil column below screen. A root mat developed over the screen, but root hairs only could penetrate it. Regardless of the soil moisture level in the columns, and adequate and equal water and nutrients supply was maintained via wicks from an external nutrient solution to the plant roots in vermiculite. After 10 days, the soil columns were separated from the root mats, quickly frozen in liquid nitrogen and sliced into thin layers (0.2mm) using a refrigerated microtome to give soil samples at defined distances from the root mats for analyses. Lower soil moisture (=0.14) resulted in narrower and steeper depletion profile of 0.5 M NaHCO₃ extractable P (NaHCO₃-P_i) as compared to higher soil moisture (=0.20). Depletion of P in soil solution in the immediate vicinity of root mats did not differ much but the extension of the depletion zones was 0.10 cm at =0.14 and 0.20 cm at =0.20. The depletion up to 0.05cm with =0.14 and up to 0.07 cm with =0.20 was uniform, and may be attributed to the depletion in the root hair zone. Beyond the root hair zones, the theory of diffusion and mass flow was able to explain the observed differences in shape and extent of the P depletion profiles at the two soil moisture levels.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Determination of K forms in K-fertilized soils by electro-ultrafiltration

We confirmed the suitability of electro-ultrafiltration (EUF) for (a) determination of the distribution of potassium fertilizer among the various forms of potassium in soils with a predominance of micaceous minerals in their clay fraction, and (b) investigated the effects of the degree of openness of the dominant micaceous mineral and of incubation time on the kinetics of the EUF extraction of K from these soils.Samples of illitic, mixed-layer and vermiculitic soils from Galicia (N.W. Spain) were incubated at field capacity for 450 days with 0 (blank), 5,15 or 25 mg K (as KCl) per 100 g dry soil. After 1, 30, 150 and 450 days, subsamples were removed and repeatedly extracted using electro-ultrafiltration at low (20° C/200 V) and then high (80° C/400 V) temperature/voltage (6 and 10 five-minute extractions, respectively). Five different pools of K were identified: solution K (K_s), surface and internal K (collectively, K_p), slowly exchangeable K (K_e) and non-exchangeable K (K_i). The effects of increasing the incubation time depended on the dominant clay mineralogy: after 450 days, the K added to illitic soils was mostly solution K, whereas that added to vermiculitic soils was mostly internal K.For both low and high temperature/voltage EUF experiments, the extraction-time data were best fitted by the Elovich equation (extracted K=a+b ln t). The kinetic coefficient b depended on the incubation time and dominant clay mineral, and for given soil and incubation time increased linearly with the dose of added K.Abbreviations EUF Electroultrafiltration - K_s Solution potassium - K_p Easily exchangeable (surface + internal) potassium - K_e Slowly exchangeable potassium - K_i Non-exchangeable potassium     

Short-term effects of biological and physical forces on aggregate formation in soils with different clay mineralogy

The mechanisms resulting in the binding of primary soil particles into stable aggregates vary with soil parent material, climate, vegetation, and management practices. In this study, we investigated short-term effects of: (i) nutrient addition (Hoagland's solution), (ii) organic carbon (OC) input (wheat residue), (iii) drying and wetting action, and (iv) root growth, with or without dry–wet cycles, on aggregate formation and stabilization in three soils differing in weathering status and clay mineralogy. These soils included a young, slightly weathered temperate soil dominated by 2:1 (illite and chlorite) clay minerals; a moderately weathered soil with mixed [2:1 (vermiculite) and 1:1 (kaolinite)] clay mineralogy and oxides; and a highly weathered tropical soil dominated by 1:1 (kaolinite) clay minerals and oxides. Air-dried soil was dry sieved through a 250 m sieve to break up all macroaggregates and 100 g-subsamples were brought to field capacity and incubated for 42 days. After 14 and 42 days, aggregate stability was measured on field moist and air-dried soil, to determine unstable and stable aggregation respectively. In control treatments (i.e., without nutrient or organic matter addition, without roots and at constant moisture), the formation of unstable and stable macroaggregates (> 250 m) increased in the order: 2:1 clay soil < mixed clay soil < 1:1 clay soil. After 42 days of incubation, nutrient addition significantly increased both unstable and stable macroaggregates in the 2:1 and 1:1 clay soils. In all soils, additional OC input increased both unstable and stable macroaggregate formation. The increase in macroaggregation with OC input was highest for the mixed clay soil and lowest for the 1:1 clay soil. In general, drying and wetting cycles had a positive effect on the formation of macroaggregates. Root growth caused a decrease in unstable macroaggregates in all soils. Larger amounts of macroaggregates were found in the mixed clay and oxides soil when plants were grown under 50% compared to 100% field capacity conditions. We concluded that soils dominated by variable charge clay minerals (1:1 clays and oxides) have higher potential to form stable aggregates when OC concentrations are low. With additional OC inputs, the greatest response in stable macroaggregate formation occurred in soils with mixed mineralogy, which is probably a result of different binding mechanisms occurring: i.e., electrostatic bindings between 2:1 clays, 1:1 clays and oxides (i.e. mineral-mineral bindings), in addition to OM functioning as a binding agent between 2:1 and 1:1 clays.     

Effects of antibiotics in soil on the population dynamics of transposon Tn5 carrying Pseudomonas fluorescens

The effects of kanamycin and streptomycin added to soil on the survival of transposon Tn5 modified Pseudomonas fluorescens strain R2f were investigated. Kanamycin in high (180 g g^-1 dry soil) or low (18 g g^-1) concentration or streptomycin in low concentration in Ede loamy sand soil had no noticeable effect on inoculant population dynamics in soil and wheat rhizosphere, whereas streptomycin in high concentration had a consistent significant stimulatory effect, in particular in the wheat rhizosphere. Streptomycin exerted its effect by selecting P. fluorescens with Tn5 insertion whilst suppressing the unmodified sensitive parent strain, as evidenced by comparing the behaviour of these two strains in separate and mixed inoculation studies.Soil textural type influenced the effect of streptomycin on the Tn5 carrying inoculant; the effect was consistently detected in rhizosphere and rhizoplane samples of wheat grown in Ede loamy sand after 7 and 14 days incubation, whereas it was only apparent after 7 days in rhizoplane or rhizosphere (and bulk soil) samples of wheat grown in two silt loam soils. Modification of soil pH by the addition of CaCO₃ or bentonite clay resulted in an enhancement of the selective effect of streptomycin by CaCO₃ and its abolishment by bentonite clay.The addition to soil of malic acid or wheat root exudate, but not of glucose, enhanced the streptomycin selective effect on the Tn5-modified P. fluorescens strain. Neither the streptomycin producer Streptomyces griseus nor two non-inhibiting mutants obtained following UV irradiation affected the dynamics of P. fluorescens (chr::Tn5) in soil and wheat rhizosphere.The effect of streptomycin in soil on inoculant Tn5 carrying bacteria depends on conditions such as soil type, the presence of (wheat) root exudates and the type of available substrate.     

Clay Mineralogy of Central Victorian (Creswick) Soils: Clay Mineral Contents as a Possible Tool of Environmental Indicator

The clay mineralogy and heavy metal/metalloid (As, Pb and Cu) contents of soils developed on the various rock units in a central highlands environment in Victoria (Creswick, Australia) have been investigated. The clay minerals identified showed an order of abundance as: kaolinite ? illite > smectite > mixed-layer (ML) ≈ vermiculite. The soil clay mineralogy did not change systematically with depth (0～ 10, 10～ 20 and 20～ 30 cm) and showed large variations spatially. The high proportion of kaolinite was probably due to the removal of 2:1 phyllosilicates by the formation of 1:1 kaolinite through weathering, which also reduced the cation exchange capacity (CEC) and electrical conductivity (EC, soil: water ratio of 1:5) of soils by aging. Soils were classified as silty loam to loam with a low clay size (≤ 2μ m) fraction. The soils were acidic to moderately acidic with pH ranging from 4.5 to 7.1, averaging 5.7. Concentrations of As, Pb, and Cu (average values 24.3, 16.7 and 11.0 mg/kg, respectively) did not show an association with the clay mineral contents except vermiculite. The occurrence of smectite and mixed-layer clay contents, although far lower than kaolinite and illite, contributed significantly to CEC of soils. The study area was affected by mining, high natural background As values dominate the area and the role of clay minerals in fixation of metalloid/metals was found to be less significant. Low organic matter content (average ～ 6.5%), low soil surface area (average ～ 1.0 m²/g) and the high proportion of kaolinite mineral content result in a limited ability to fix heavy metals. The role of Fe oxides appeared to be a key influence in the fixation of As and other potentially toxic metals, rather than the clay minerals, and therefore requires further research. This work highlighted the importance of the determination of types and amounts of clay minerals of natural soils in environmental management.     

Inorganic nitrogen source utilization byFagus crenata on different soil types

The nitrogen source utilization by Fagus crenata distributed on soils with different forms of inorganic nitrogen in a cool-temperate deciduous forest in central Japan was determined by measuring foliar ¹⁵N. Two soil habitat types along a slope were delineated based on nitrogen transformation patterns, i.e., soils with high net nitrification rates and with no or low net nitrification, respectively. Despite differences in soil types, the study species, F. crenata, was distributed along the entire slope. The foliar ¹⁵N value of F. crenata from the lower slope area was significantly lower than that from the upper slope. Given the finding of a previous study that the ¹⁵N of NO₃^– was lower than that of NH₄⁺, our results indicate that reliance on NO₃^– as a nitrogen source was greater in the lower slope area than in the upper slope area. Differences in the values of foliar ¹⁵N were about 1, which is far less than the 10 ¹⁵N value of soil inorganic N reported in the previous study. This discrepancy might suggest that the study species utilized NO₃^– even in the upper site where net nitrification had not been detected. Measurements of nitrate reductase activity, an index of NO₃^– uptake, also supported this interpretation. Nitrate reduction occurred in leaves and roots at both the lower and the upper sites. Thus, the study species may be able to use NO₃^– even in soils with no net nitrification, a factor that could allow the distribution of F. crenata along the entire length of the slope.     

Soil phosphorus levels needed for equal P uptake from four soils with different water contents at the same water potential

Soil volumetric water contents, , at –33 kPa potential may vary with soil from 0.06 to 0.70. Because P diffusion depends on , most economic P fertilizer rates required for different soils may require adjusting according to their soil-water relationships. The objective of this study was, after experimentally verifying a mechanistic nutrient uptake model on a series of soils varying in at –33 kPa potential, to use the model to predict labile P levels needed for each of these soils to achieve equal P uptake by maize (Zae mays L.) and verify these predictions. Maize was grown in a pot experiment using four soils having of 0.13, 0.20, 0.26, and 0.40 at –33 kPa each at 0, 200, and 400 mg kg^-1 of added P. When root parameters obtained experimentally were used, predicted P uptake with the uptake model agreed with observed P uptake, y=0.99x+9.08 (r²=0.98). When P uptake was plotted vs. soil solution P, C_li, the relation varied with soil. The higher the the lower the C_li needed for equal P uptake. A similar relation was found between P uptake and diffusible soil P, C_si. Differences between the two plots occurred because of differences among soils in buffer power, C_si/C_li. The C_si vs. P added relation was used to calculate differences among soils in the C_si needed to obtain equal P uptake. The C_si values ranged from 1.3 to 4.0 mmol kg^–1. The calculated values were used in a second pot experiments to verify the predictions. No significant difference (=0.05) in P uptake occurred. The results of this research indicate that the mechanistic nutrient uptake model can be used to predict the degree of adjustments in C_si needed to obtain the most economic P fertilizer rates among soils varying in .Journal Paper No. 13072. Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907.     

The capacity of soils to preserve organic C and N by their association with clay and silt particles

Although it has been recognized that the adsorption of organics to clay and silt particles is an important determinant of the stability of organic matter in soils, no attempts have been made to quantify the amounts of C and N that can be preserved in this way in different soils. Our hypothesis is that the amounts of C and N that can be associated with clay and silt particles is limited. This study quantifies the relationships between soil texture and the maximum amounts of C and N that can be preserved in the soil by their association with clay and silt particles. To estimate the maximum amounts of C and N that can be associated with clay and silt particles we compared the amounts of clay- and silt-associated C and N in Dutch grassland soils with corresponding Dutch arable soils. Secondly, we compared the amounts of clay- and silt-associated C and N in the Dutch soils with clay and silt-associated C and N in uncultivated soils of temperate and tropical regions.We observed that although the Dutch arable soils contained less C and N than the corresponding grassland soils, the amounts of C and N associated with clay and silt particles was the same indicating that the amounts of C and N that can become associated with this fraction had reached a maximum. We also observed close positive relationships between the proportion of primary particles < 20 m in a soil and the amounts of C and N that were associated with this fraction in the top 10 cm of soils from both temperate and tropical regions. The observed relationships were assumed to estimate the capacity of a soil to preserve C and N by their association with clay and silt particles. The observed relationships did not seem to be affected by the dominant type of clay mineral. The only exception were Australian soils, which had on average more than two times lower amounts of C and N associated with clay and silt particles than other soils. This was probably due to the combination of low precipitation and high temperature leading to low inputs of organic C and N.The amount of C and N in the fraction > 20 m was not correlated with soil texture. Cultivation decreased the amount of C and N in the fraction > 20 m to a greater extent than in the fraction < 20 m, indicating that C and N associated with the fraction < 20 m is better protected against decomposition.The finding of a given soil having a maximum capacity to preserve organic C and N will improve our estimations of the amounts of C and N that can become stabilized in soils. It has important consequences for the contribution of different soils to serve as a sink or source for C and N in the long term.     

Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils

We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500–5000 mm y^–1; 350–1050 masl) and along topographic gradients (ridge to valley, 150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O₂ availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O₂ concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O₂. Cross correlation analyses showed that soil O₂ concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O₂ concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes. We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O₂ depletion along the rainfall gradient, but less sensitive to the variable soil O₂ environment exhibited at lower elevations along topographic gradients.     

A technique for studying rhizosphere processes in tree crops: soil phosphorus depletion around camellia (Camellia japonica L.) roots

Rhizosphere studies on tree crops have been hampered by the lack of a satisfactory method of sampling soils at various distances in the rhizosphere. A modified root study container (RSC) technique developed for annual crops, grasses and legumes was used to study the mechanisms by which camellia plants (Camellia japonica L.) utilise soil P in the glasshouse and field. Plants belonging to the Camellia family (e.g. tea) have the ability to utilise P from relatively unavailable native P sources and for this reason camellia plants were selected for this study.In the glasshouse trial, the RSCs were filled with a Recent soil, treated with P fertilisers; North Carolina phosphate rock (NCPR), diammonium phosphate (DAP), mono calcium phosphate (MCP) and single superphosphate (SSP) at 200 g P g^-1 soil. A planar mat of roots was physically separated by a 24 m polyester mesh and the soil on the other side of this mesh was cut into thin slices parallel to the rhizoplane and analysed for pH, and different forms of P (organic, P_o and inorganic, P_i) to understand P depletion at different distances from camellia roots. In the field trial this technique was modified and used to study the rhizosphere processes in mature camellia trees fertilised with only SSP and NCPR.In both field and glasshouse trials, all P fertilisers increased all the bulk soil P fractions except NaOH-P_o over unfertilised soil with the greatest increases being in the H₂SO₄-P_i fraction in the NCPR treatment and NaOH-P_i in the SSP treatment. Resin-P, NaOH-P_i and H₂SO₄-P_i were significantly lower in the rhizosphere soil compared to the bulk soil whereas NaOH-P_o was higher in the rhizosphere soil than in the bulk soil. Plant and microbial P uptake were thought to be the major causes for the low resin-P rather than P fixation by Fe and Al because the NaOH-P_i fraction which is a measure of Fe-P and Al-P, also decreased in the rhizosphere soil. The rhizo-deposition of NaOH-P_o suggests that labile inorganic P was immobilized by rhizosphere microbes which were believed to have multiplied as a result of carbon exudates from the roots. A marked reduction in pH (about 0.2–0.4 in the glasshouse and 0.2 in the field trial) was observed near the rhizoplane compared to that in the bulk soil in all treatments. The pH near the rhizoplane as well as in the bulk soil was highest for NCPR treated soil. The increase in pH in the NCPR treatment over the control was consistent with the number of protons consumed during the dissolution of NCPR. In both trials, the dissolution of NCPR in the rhizosphere was higher than in the bulk soil due to lower pH and plant uptake of solution P in the rhizosphere. The RSC technique proved to be a viable aid to study the rhizosphere processes in tree crops.     

Soil and total ecosystem respiration in agricultural fields: effect of soil and crop type

A study was made of the effect of soil and crop type on the soil and total ecosystem respiration rates in agricultural soils in southern Finland. The main interest was to compare the soil respiration rates in peat and two different mineral soils growing barley, grass and potato. Respiration measurements were conducted during the growing season with (1) a closed-dynamic ecosystem respiration chamber, in which combined plant and soil respiration was measured and (2) a closed-dynamic soil respiration chamber which measured only the soil and root-derived respiration. A semi-empirical model including separate functions for the soil and plant respiration components was used for the total ecosystem respiration (TER), and the resulting soil respiration parameters for different soil and crop types were compared. Both methods showed that the soil respiration in the peat soil was 2–3 times as high as that in the mineral soils, varying from 0.11 to 0.36 mg (CO₂) m^–2 s^–1 in the peat soil and from 0.02 to 0.17 mg (CO₂) m^–2 s^–1 in the mineral soils. The difference between the soil types was mainly attributed to the soil organic C content, which in the uppermost 20 cm of the peat soil was 24 kg m^–2, being about 4 times as high as that in the mineral soils. Depending on the measurement method, the soil respiration in the sandy soil was slightly higher than or similar to that in the clay soil. In each soil type, the soil respiration was highest on the grass plots. Higher soil respiration parameter values (R_s0, describing the soil respiration at a soil temperature of 10°C, and obtained by modelling) were found on the barley than on the potato plots. The difference was explained by the different cultivation history of the plots, as the potato plots had lain fallow during the preceding summer. The total ecosystem respiration followed the seasonal evolution in the leaf area and measured photosynthetic flux rates. The 2–3-fold peat soil respiration term as compared to mineral soil indicates that the cultivated peat soil ecosystem is a strong net CO₂ source.     

Soil Enzyme Activities and Their Relationship with Soil Physico-Chemical Properties and Oxide Minerals in Coastal Agroecosystem of Puducherry

The objectives of our research were to assess the soil enzyme activities in relation with soil physicochemical and oxide minerals in the coastal agroecosystem of Puducherry region, India. Soils from nine farms in organic (ORG), sustainable (SUS), and conventional (CON) farming were sampled. Organically managed farming system soils contain significantly higher amounts of soil total N, organic carbon, and a higher level of microbial biomass C and N. Urease, protease, β-glucosidase, cellulose, saccharase, xylanase, and alkaline phosphatase enzyme activities were higher in organic farming system soils compared to sustainable and conventional farming soils. In addition, silt, clay, Al₂O₃, CaO, Fe₂O₃, K₂O, MgO, MnO, Na₂O, and P₂O₅ oxides were higher in organic farming soil and they showed a significant positive correlation with soil enzyme activities. Our study revealed that soil enzyme activities and soil minerals were significantly affected by farm management practices. The organic farming system had improved the soil health, enzyme activities, and plant available nutrients in coastal agro-ecosystem.     

Variation in Phosphorus Sorption with Soil Particle Size

Phosphorus (P) is considered a primary cause for surface water eutrophication that leads to anoxia. Understanding the relationships between soil particle size and P sorption helps devise effective best management practices (BMPs) to control P transport by erosion, leaching, and overland flow from agricultural land. Consequently, this study examined the effect of surface soil particle size on the sorption of P in five soil series (four Ultisols and one Entisol) from the Mid-Atlantic region. The sorption of P in each soil was assessed by equilibrating (after shaking for 24?h) 5?g soil containing varied amounts of KH₂PO₄ in 20?mL of 0.01?M KCl solution. Phosphorus in solution was determined by the molybdate blue method of Murphy and Riley. The P adsorption characteristics of these soils were described using the Langmuir isotherm. Results indicated that variability in P sorption was related to particle size and soil type. Soil organic matter content contributed a great deal to P sorption in the Entisol. However, soil clay had influence on the P sorption characteristics of each soil. The maximum P retentive capacities of soils (as determined by Sm from Langmuir equation) and P sorbed at 500?mg P kg^?1 addition showed a linear relationship (r2 = 0.94). Therefore, based on the results obtained, the single point method of Bache and Williams may be appropriate to describe the maximum P sorption capacity of non-sandy soils, as observed in this study.     

Effect of clay,organic matter and CaCO3 content on zinc adsorption by soils

Summary Zinc adsorption was studied in suspensions of six soils of different physicochemical characteristics in dilute ZnSO₄ solutions. At low concentrations, Zn²⁺ adsorption was described by the Langmuir adsorption equation. The calculated Langmuir adsorption maxima were related positively to clay and carbonate content and negatively with organic matter content of soils. Multiple regression analysis revealed that zinc adsorption maxima can be predicted with good precision from information in soil survey reports. When the added Zn²⁺ exceeded the adsorption maximum, the solid phase of zinc controlling its concentration in solution was either zinc hydroxide or carbonate so long as soil carbonates were present. The values of zinc potential also indicated that soils retain Zn²⁺ more strongly than Zn(OH)₂ or carbonate. Postgraduate student Professor of Soils. Professor of Soils.     

Sustainable phosphorous management in two different soil series of Pakistan by evaluating dynamics of phosphatic fertilizer source

Phosphorous (P) plays the prominent role to promote the plants storage functions and structural roles, as it is recognized as a vital component of ADP, ATP, Cell wall as well as a part of DNA. Soils acts as the sink to supply P to plants because soil pH and its physical condition are the main factor which regulate the solubility and availability P element. Phosphorus is not deficient in Pakistani soils but its availability to plants is the serious matter of concern. A pot experiment was conducted to evaluate P dynamics in two different soil series of Pakistan (Bahawalpur and Lyallpur) using Maize as test crop. The treatments applied were T0: Control (without any fertilizer), T1: Recommended DAP @648 mg pot^?1, T2: Half dose DAP @324 mg pot^?1, T3: Recommended rate of TSP @900 mg pot^?1, T4: Half dose TSP @450 mg pot^?1. Soil analysis showed that Bahawalpur soil has sandy clay loam texture with 33% clay and Lyallpur series has sandy loam texture with 15.5% clay; furthermore, these soil contain 4.6 and 2.12% CaCO₃ respectively. Results showed an increase in P concentration in roots (23 mg kg^?1) with the application of half dose of TSP in Lyallpur series and lowest in Bahawalpur series (14.6 mg kg^?1) at recommended dose of DAP. Concentration of P in shoots responded the same; increase at half dose of TSP (16.7 mg kg^?1) and lowest at full dose of DAP in Bahawalpur series as (15.58 mg kg^?1). Adsorbed P (17 mg kg^?1) was recorded highest in Bahawalpur soil with more clay amount in pot with DAP application but lower in Lyallpur soil series (14 mg kg^?1) with the application of applied TSP. The PUE was recorded highest in Lyallpur series with the application of half dose of TSP and it was 61% more than control and was Highest in Bahawalpur series was with the application of recommended dose of DAP is 72% more than control treatment. On estimation; results showed that applied sources made an increase in P availability than control, but TSP gave better P uptake than DAP unless of rates applied. Soil of Lyallpur series showed better uptake of P and response to applied fertilizers than Bahawalpur series which showed more adsorption of P by high clay and CaCO₃ amount. Conclusively, the study suggested that soil series play a crucial role in choosing fertilizer source for field application.     

The heterogeneous nature of microbial products as shown by solid-state13C CP/MAS NMR spectroscopy

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 m fractions of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabelled and uniformly¹³C-labelled (99.9% atom) glucose were incorporated into the artificial soils to study the effects of clay types, exchangeable cations and clay contents on the mineralization of glucose-carbon and glucose-derived organic materials. Chemical transformation of glucose-carbon upon incorporation into microbial products and metabolites, was followed using solid-state¹³C CP/MAS NMR spectroscopy.There was a significant influence of exchangeable cations on the mineralization of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than those with kaolinite clay. Statistical analysis of data showed no overall effect of clay type on mineralization of glucose-carbon. However, the interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-soils with Georgia kaolinite. For Ca-soils this difference was not significant. Due to the increased osmotic tension induced by the added glucose, mineralization of glucose-carbon was slower in soils with 5% clay than soils with 25% clay.Despite the differences in the chemical and physical characteristics of soils with Ca-, Na- and Al-clays, the chemical composition of organic materials synthesised in these soils were similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly¹³C-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24–25%), O-alkyl (56–63%), carbonyl (11–15%) and small amounts of aromatic and olefinic carbon (2–4%). However, as decomposition proceeded, the chemistry of synthesised material showed some changes with time. In the Ca- and Na-soils, the proportions of alkyl and carbonyl carbon decreased and that of O-alkyl carbon increased with time of incubation. However, the opposite trend was found for the Al-soil.Proton-spin relaxation editing (PSRE) subspectra clearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T₁(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. These subspectra were very similar to those obtained for microbial and fungal materials and were probably microbial tissues attached to clay surfaces by polysaccharide extracellular mucilage. Subspectra of fast-relaxing (short T₁(H)) domains comprised mostly O-alkyl and carbonyl carbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.